Note: Descriptions are shown in the official language in which they were submitted.
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WO 99/55795 PCT/EP99/O1B09
The invention relates to a latent heat storage
body having a paraffin-based latent heat storage
material and to methods for producing a latent heat
storage body. .
It i.s known to use latent heat storage bodies
to temporarily decouple the generation of heat or cold
and subsequent consumption of heat or cold. They enable
efficiency to be improved, in that the latent heat
storage material contained in them stores heat during a
phase transition, for example from solid to ~.iquid,
caused by the supply of heat, and is able to release
heat during an opposite phase transition at a different
time. The temporal decoupling of the supply and release
of heat makes it possible to achieve long, continuous
running times for heat or cold generators combined with
high levels of efficiency and low start-up, shut-down
and standstill costs. Latent heat storage bodies are
used, for example, in installations for generating heat
from solar energy or from fossil energy carriers, and
also in cooling circuits. For the prior art, reference
i.s made, for example, to PCT/EP93/03396 and
PCT/EP98/01956, and to the further documents referred
to therein. In particular, PCT/EP98/01956 has disclosed
a latent heat body with paraffin-based latent heat
storage material held in a carrier material which has
holding spaces. In the known latent heat body, it is
provided that the carrier material is assembled from
individual support-material elements, for example by
adhesive bonding, capillary-like holding spaces for the
latent heat storage material being formed at least
between the support elements. This arrangement leads to
a latent heat body which is easy to produce and is
highly effective, having a high heat storage capacity
together_with sufficient structural strength even in
the heated state, and with carrier material which is as
far as possible automatically filled with the latent
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heat storage material. Depending on the selected
dimensions and fields of use, the advantages of the
abovementioned latent heat storage bodies, as wel.1 as
other known latent heat storage bodies, are
counterbalanced by the undesirably long time intervals
which are required to supply or store thermal energy.
Excessively long heating times result in particular
when the~thermal energy has to be supplied exclusively
by means of thermal conduction from the surface into
the interior of a latent heat storage body and if
barriers to thermal conduction are present, which
barriers may, for example, exist between sub-bodies
which loosely adjoin one another inside a latent heat
storage body.
Therefore, it has already been attempted to
introduce microwave energy into latent heat storage
bodies containing a large amount of paraffin as latent
heat storage material and, in this way, to heat this
material. It is known that microwaves are able to
penetrate through bodies which are to be heated at very
high speed and to heat microwave-active substances
contained therein by exciting molecular vibrations by
means of kinetic energy, without thermal conduction
being required. Therefore, by heating a body by means
of microwave radiation, it is in principle possible to
achieve considerably shorter heating times compared to
heat transfer by means of thermal conduction. F~owever,
a fundamental difficulty is that in industrial
applications, microwave-passive substances, for example
paraffin-based latent heat storage material, whose
molecules cannot be heated, or cannot be heated to a
sufficient extent for industrial application, by the
microwave radiation, are frequently of importance in
addition to microwave-active substances. While the
microwave-active property of water and some carbon
compounds is now assumed to be known, in many technical
fields problems are caused by numerous other
substances, e.g. cotton, some plastics, wood and
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paraffins, having an insufficient or unforeseeable
microwave activity. To solve this problem, microwave
antennae, for example in the form of carbon, OH groups
in the form of soot, glycerol or alcohols, are added to
these substances. For example, in PCT/EP98/01956, which
has already been referred to in the introduction, it is
proposed for the latent heat body to contain a
microwave-active substance, in particular from one or
more of the groups of materials consisting of glass
materials, plastics materials, minerals, metals, coal
or ceramic. The result is that, depending on the
arrangement or distribution of the microwave-active
substance in the latent heat body, numerous heat nests
are formed under the influence of. microwave radiation,
and because of the temperature difference which exists,
these nests release their thermal energy to the
adjoining paraffin-based latent heat storage material,
which is predominantly microwave-passive. The shortened
thermal conduction path in this way in principle leads
to acceleration of the heating operation.
However, a general drawback of adding'microwave
antennae is that these added substances are frequently
undesirable from ,a use aspect, require increased
vigilance when they are used, may be irrevocably
consumed or, for example, entail the risk of
segregation and therefore dangerous differences in
concentration, which may lead to local overheating and
to a material composite comprising microwave-passive
and microwave-active material being "burnt through". In
general terms, therefore, the use and the range of
applications of many microwave-passive materials has
hitherto been restricted by the addition of
microwave-active substances.
Even with latent heat storage bodies, for
example in the case of heat cushions or panels, with a
high level of paraffin as latent heat storage material,
it has not hitherto been possible to achieve a
satisfactory solution enabling microwave energy to be
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introduced and, in this way, the latent heat storage
body to be heated. Over and above the difficulties
mentioned above, previous attempts were made more
difficult by the fact that a high vapour pressure may
build up in a hermetically closed enclosure For a
paraffin' packing containing, for example, a liquid
microwave-active material, microwave-active substances
can often only be incorporated separately from the
paraffin in microencapsulated form, involving
considerable technical outlay (metered extrusion), and
this in turn requires relatively high levels with
respect to the paraffin. However, over the course of
time the microwave-active additions which have been
incorporated in this way are also irreversibly
volatilized or at least tend to appear. In turn,
different layers of the microwave-active and/or the
microwave-passive material lead to considerable
temperature fluctuations. Overall, therefore, there are
still considerable technical problems involved i_n the
production, the use properties and the operational
reliability of microwave-passive materials dbped with
microwave-active substances.
Therefore, working on the basis of the
abovementioned PCT/EP98/01956, it is an object of the
present invention to provide a latent heat storage body
which can be heated by microwaves, contains a
paraffin-based latent heat storage material and,
compared to the body in the above document, is easier
to produce, has more advantageous use properties and a
higher operational reliability. A further part of the
object consists of providing a simplified production
method for a latent heat storage body containing a
paraffin-based latent heat storage material. Moreover,
the object includes providing a method for producing a
latent heat storage body which can be heated by
microwaves and contains a paraffin-based latent heat
storage material.
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According to the invention, the first part of
the object is achieved by a latent heat storage body
having the features of Claim 1, advantageous
configurations of which are given in Claims 2 to 21. In
the latent heat storage body according to the invention
having a paraffin-based latent heat storage material,
it is provided that the latent heat storage body
contains a hygroscopic material. The hygroscopic
material has a pronounced capacity for_ taking up
moisture from its environment and binding this moisture
to itself.
Particularly suitable hygroscopic materials
include calcium chloride (CaClz*6H20), iron chloride
(FeCl3), copper sulphate (CuS09*5H20), magnesium
chloride (MgCl2*6Hz0), potash (potassium carbonate,
KZC03), silica gel and numerous other substances.
The moisture may in particular be water-based
liquid, including, of course, pure water, which can be
taken up from the environment by a hygroscopic material
even in the vapour phase, that is to say in gaseous
form. The hygroscopic action is based partially on
adsorption and, in addition to other - frequently
subordinate - effects, in fine-pored materials, is also
often based on capillary condensation. Furthermore, the
hygroscopic action may also be based on the moisture
being contained in the hygroscopic material as a salt
solution (water of crystallization). The capillary
condensation is important if the vapour pressure which
i_s approximately described by the Gibbs-Thomson
equation above a liquid surface which is concavely
curved in the pores or capillaries of a body is reduced
to such an extent that it becomes lower than the vapour
pressure in the surrounding gas. By taking up moisture,
in particular water-based moisture, according to the
invention the hygroscopic material contained in the
latent heat storage body brings about automatic doping
of a relatively microwave-passive latent heat storage
material with a highly microwave-active material, the
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high efficiency of which is based on the extremely
dipolar character of water. The inclusion of
hygroscopic material allows the latent heat storage
body according to the invention, which may, for
example, be a paraffin-containing heat cushion, to be
heated successfully in a standard domestic microwave.
Furthermore, the hygroscopic material overcomes the
difficulties which have previously been involved when
the aim was to use water as microwave-active material,
namely its extreme paraffin phobicity (segregation),
its ready volatility and the associated increase in
vapour pressure at elevated temperatures.
A further advantage is that after_ it has been
heated or after the latent heat storage body has been
used, the microwave-active moisture always returns to
those locations in the latent heat storage body at
which the hygroscopic material is contained in the
latent heat storage body, and that the hygroscopic
material has no tendency to become segregated from the
latent heat storage material. In this way, in addition
to automatic regeneration of the latent heat storage
body through moisture uptake, the further advantage is
achieved that the moisture always reproducibly adopts
the originally provided distribution in the latent heat
storage body, so that segregation and undesirable
concentration differences are not possible.
Consequently, local overheating of the latent heat
storage body or "burning through" is effectively
prevented, so that there is no risk of explosion or
fire even in the event of_ the latent heat storage body
being used incorrectly. Overall, therefore, the
operational reliability of the latent heat storage body
is also increased considerably compared to known
designs.
Further advantages of the latent heat storage
body according to the invention are that the thermal
conductivity is also increased considerably on account
of the content of water of crystallization and the
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extremely finely distributed condensation water, so
that for the first time it is realistically possible to
achieve greater layer thicknesses. Moreover, there is
no need to apply a vacuum with a view to achieving the
hygroscopic properties, and there is no risk of leaks.
The latent heat storage body according to the invention
is also distinguished by a pari:icularly great
versatility, since in addition to the preferred option
of heating by microwaves, as an alternative or in
addition heating rnay also take place using conventional
methods, for example in a water bath or in an oven. An
additional advantage of heating by microwaves i.s that
only minimal outlay on energy is required, since the
microwave energy can be introduced very efficiently
into the moisture bound in the hygroscopic material, in
particular including into the water of crystallization.
In addition, numerous hygroscopic materials are very
inexpensive and are also relatively or even altogether_
non-toxic, and in many cases do not cause any chemical
change to the paraffin-based latent heat storage
material.
In a preferred possible configuration, the
latent heat storage body is held i_n a sheath which is
permeable to vapour diffusion and may, for example, be
a film which at its edges or connecting regions and/or
within surface regions has openings which are permeable
to vapour diffusion and lead to the environment
surrounding the latent heat storage body. In this "open
system", vapour exchange takes place between the
interior, of the latent heat storage body and its
surrounding environment, so that moisture which is
present in the environment can be taken up by the
hygroscopic material contained in the latent heat
storage body. If the latent heat storage body is
irradiated with microwaves, this leads to heating and
subsequent evaporation of the microwave-active
moisture., in particular water, stored in the
hygroscopic material. At the locations where it is
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formed, the heated steam is in direct and immediate
heat exchange with the adjoining heat storage material,
with the result that the latter can also be heated
within a short time. The evaporation of the moisture
emerging from the hygroscopic material leads to an
increase in the volume of the microwave-active
moisture, so that the volume of the latent heat storage
body enclosed in the sheath also rises. The pressure
which is formed in the sheath in this way allows some
of the vapour to escape from the sheath which is
permeable to vapour diffusion into the environment, so
that it is advantageously possible to prevent the
sheath from being destroyed by an unacceptably high
internal pressure. The heated latent heat storage body
can then be supplied for its intended use. The loss of
moisture from the latent heat storage body as a result
of at least some of the vapour escaping is
automatically compensated for by the fact that the
hygroscopic material contained in the latent heat
storage body binds the moisture which is still present
to itself as cooling of the latent heat storage body
progresses, and a vapour pressure drop leads to ambient
moisture flowing back through the openings in the
sheath, which are permeable to vapour diffusion, into
the interior of the latent heat storage body until an
equilibrium is established as a result of a large
quantity of moisture once again being stored in the
hygroscopic material. In a further variant, the latent
heat storage body may also be accommodated in a sheath
which is impermeable to vapour diffusion, for example
in a plastics film or aluminium foil (closed system).
In this case, destruction caused by vapour pressure can
be prevented, for example, by a suitable amount of
spare material in the sheath, which may also consist of
an expandable material, and/or by a suitably adapted
quantity. of moisture in the latent heat storage body.
Furthermore, it is also possible for the hygroscopic
material for its part to be accommodated in a sheath
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which is permeable to vapour diffusion. In this case,
the hygroscopic material together with this sheath may
be separated from the adjoining latent heat storage
material in such a manner that it is permeable to
vapour diffusion, so that its surface cannot be
passivated by liquefied paraffin.
The latent heat storage body according to the
invention may have capillary spaces which open up paths
to the hygroscopic material. By way of example, it is
possible for the paraffin-based latent heat storage
material to have a solidification structure which is
modified by additives, in particular with cavities
which are in the form of hollow cones, as described in
PCT/EP93/03346.
This makes it possible to significantly improve
the response of the latent heat storage material when
heat is supplied. As a result, the paraffin-based
latent heat. storage material adopts, as it were, a
porous structure. When heat is supplied, it is easier
for constituents of the latent heat storage material
which melt to flow through the hollow structures which
are provided in the material itself. It is possible, i.f
appropriate also in view of air inclusions which are
present, for a type of microconvection to be
established. This also results in a high mixing action.
Furthermore, there are also advantages with regard to
the expansion performance in the event of a phase
change. The structural additive is preferably
homogeneously dissolved in the latent heat storage
material. In detail, structural additives such as those
based on polyalkyl methacrylates (PA-MA) and polyal.kyl
acrylates (PAA) have proven suitable as individual
components or in combination. Their crystal-modifying
action is brought about by the fact that the polymer
molecules are also incorporated into the growing
paraffin crystals, preventing this crystal form from
growing further. Because the polymer molecules are also
present in associated form in the homogeneous solution
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i_n paraffin, it is possible for paraffins to grow onto
the special associates. Hollow cones which are no
1_onger able to form networks are formed. Because of the
synergistic action of this structural additive on the
crystallization behaviour of the paraffins, cavities
are formed and, as a result, the pervi;ousness of the
heat storage medium paraffin (for example to air or
water vapour enclosed in the latent heat storage body
or to liquefied phases of the latent heat storage
material, i.e. the paraffin itself) is improved
compared to paraffins which are not compounded in this
way. In general terms, suitable structural additives
include ethylene/vinyl acetate copolymers (EVA),
ethylene%propylene copolymers (OCP), diene/styrene
copolymers, both as individual components and in a
mixture, as well as alkylated naphthalenes (Paraflow).
The proportion of structural additives starts at a
fraction of. a percent by weight, realistically at
approximately O.Olo by weight, and presents distinct
changes, in the sense of an improvement, in particular
up to a level of approximately to by weight. The
capillary spaces on the one hand make it easier for the
hygroscopic material to take up moisture, in particular
from the environment surrounding the latent heat
storage body, and, on the other hand, following the
evaporation of the moisture, assist with heat transfer
to the latent heat storage material as a result of
improved flow of the heated vapour through the latent
heat storage body. Furthermore, to accelerate and make
more uniform the heating of the latent heat storage
body, it is preferable for the hygroscopic material to
be disposed in distributed manner in the latent heat
storage body.
With a view to the possibility of achieving a
uniform and rapid flow of the microwave-active moisture
through the latent heat storage body, the hygroscopic
material preferably forms 50 or less by mass of a
latent heat storage body, so that it is also possible
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to achieve the desired, short heating times. The small
amounts added, and also the small amount, of
microwave-active moisture which is required, therefore
do not significantly reduce the amount of
paraffin-based lat=ent heat storage material, so that
the volume-specific or weight-specific heat storage
capacity is not significantly impaired. According to a
preferred refinement of the latent heat storage body,
this body contains hygroscopic material of differing
efficiency. Very strongly hygroscopic materials can be
used as "water extractors" and can be used in
combination with less strongly hygroscopic substances
which are more difficult to heat, as product,
performance and temperature regulators in a latent heat
storage body. The combination of hygroscopic material
of differing efficiency enables moisture to be
evaporated during heating over a range of temperatures
which can be influenced by the composition of the
material. In addition to high operating reliability,
compared to sudden evaporation, this also results in
more favourable heat transfer to the latent heat
storage material.
According to another advantageous configuration
of the latent heat storage body, the latter may have a
carrier material with capillary-like holding spaces
which hold latent heat storage material. In the first
instance, consideration is given to forming the
capillaries in such a way that the holding spaces have
the effect of sucking in automatically, in particular
with respect to the latent heat storage material. A
latent heat storage body of this type is distinguished
by a desired dimensional stability even with liquefied
latent heat storage material, preventing the latent
heat storage material from being sweated out. With
regard to the microwave-active moisture which is
additionally present and cannot be mixed with the
latent heat storage material, in particular water,
separation of the two components is also prevented.
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Moreover, the bodies comprising carrier material and
latent heat storage material, on account of their high
specific surface area, together with the openings of
the capillary holding spaces, act as condensation cores
or nuclei for the vapour phase of the heated
microwave-active material, which has a positive effect
on the heat transfer from the vapour to the latent heat
storage material. Furthermore, it is possible for the
capillary-like holding spaces also to be adapted for a
self-sucking action with regard to the microwave-active
moisture.
Preferably, it is provided that the latent heat
storage body contains a number of individual
support-material bodies, which may be in platelet-like
or grain-like form. With regard to the use of carrier
material with capillary-like holding spaces which hold
latent heat storage material, reference is also made to
PCT/EP98/01956, which is incorporated in its entirety
into the present application, partly with a view to
incorporating features into claims. Moreover, the
carrier material may be commercially -available
packaging fillers, suction agents for chemicals, in
particular for oil, fire-retarding agents,
viscosity-increasing agents, carrier materials - in
particular for chemical waste - and micro-nonwovens or
suction mats. In this context, reference is made in
particular to the products supplied in different
specifications by Rench Chemie GmbH, for example under
the protected trade names Rench-Rapid 'R', Rench-Rapid
'G', Perleen 222, Perleen 444, Rapon 5090, Rapon 5092
and Rapon 5093. The high apparent density which is
inherent to suitable oil binders results in an
additional and significant heat storage effect.
Furthermore, it is preferred for the
hygroscopic material to be in the form of flakes,
grains or granules, or to be contained as a powder in
the latent heat storage material. In particular, it is
possible for the hygroscopic material to be disposed on
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one or more of the individual support-material bodies.
In addition to being disposed on the surface of the
individual support-material bodies, it is also
conceivable for the hygroscopic material to be disposed
in the interior of the individual support-material
bodies. In a further preferred embodiment, the
individual support-material body arid the sheath of the
latent heat storage body are disposed spaced-apart by a
gas-containing space. This gas-containing space can be
used in particular to apply microwave-active moisture
from the environment to the latent heat storage
material, and may furthermore be provided as a moisture
store and/or as an expansion vessel.
Alternatively, or in combination with the
hygroscopic material being disposed on an individual
support-material body, it is possible for the
hygroscopic material to be disposed on a distribution
body which extends in two or three dimensions in the
latent heat storage body. A distribution body of this
nature may have capillary spaces which open up paths to
the hygroscopic material for the microwarve-active
moisture and, as a result, distribute the moisture
within the latent heat storage body. In this case, the
intention is to split roles, so that the distribution
body which has capillary spaces distributes the
microwave-active moisture in liquid form inside the
latent heat storage body, so that it can be taken up by
the hygroscopic material which is preferably likewise
disposed in distributed manner thereon. After the
moisture, in use, has evaporated and escaped from the
hygroscopic material and/or directly from the
distribution body with capillary spaces, the
hygroscopic material fulfils the role of binding the
microwave-active moisture back again as completely as
possible in a uniform distribution. If complete
rebinding is not possible, for example as a result of
vapour having escaped into the environment:, the
moisture deficit is compensated for as a result of
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microwave-active liquid flowing back in through the
branching capillaries of the distribution body.
Therefore, the form of the capillaries of the
distribution body is preferably such that they are
aimed at achieving the maximum possible passage of
microwave-active liquid, while the capillaries of the
hygroscopic material, in order to reinforce the
hygroscopic property, are preferably formed or
dimensioned in such a way that they also bring about
capillary condensation of microwave-active vapour.
Furthermore, it is also possible for the distribution
body itself to be formed from a hygroscopic material.
furthermore, consideration may be given to the sheath
of the latent heat storage body having a closeable
opening, by means of which, particularly in the case of
a sheath which is impermeable to vapour diffusion, it
is possible where necessary to influence a supply or
removal of moisture. In a specific configuration, the
distribution body with the capillary spaces for the
microwave-active liquid extends from the closeable
opening in the sheath into the latent heat storage
body. An advantageous configuration of the
distribution body provides for the capillary spaces
contained therein to exert an automatic sucking action
only on the microwave-active liquid, but not on the
latent heat storage material, thus preventing the
capillaries from becoming blocked with latent heat
storage material. This may be achieved on the basis of
the different viscosities of paraffin-based latent heat
storage material and of water, for example by suitably
adapting the dimensions of the capillary spaces, or in
some other suitable way. In this respect, attention
should also be paid to ensuring that the pores in the
hygroscopic material are in a suitable form so that
they act as capillaries only with regard to the
microwave-active moisture. In addition, or as an
alternative, it is also possible for the hygroscopic
distribution body to be surrounded by a sheath which is
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impermeable to the latent heat storage material. As a
result of the sheath, latent heat storage material is
also prevented from penetrating into pores in the
hygroscopic material and blocking these pores. In
particular, a construction in which the hygroscopic
material extends in the manner of a wick inside the
sheath may offer advantages, in which case the sheath
may consist, for example, of a film of very small wall
thickness. At any rate, the bodies made from
hygroscopic material have a self-cleaning action to the
extent that, at least with a sheath of latent heat
storage material which is not yet sealed against vapour
diffusion, they take up water again on their own and
melt open again on the next occasion of use.
In further detail, it is also preferred for an
additive which leads to a high viscosity to be added to
the latent heat storage material. For this additive, it
is possible to use a standard agent with thixotropic
properties. Even in the heated state, in which the
latent heat storage material is usually liquefied, a
high viscosity, in the sense of a jelly-like
consistency, is still present. Even in the event of a
carrier material impregnated with paraffin-based latent
heat storage material being unintentionally cut
through, latent heat storage material does not run out,
or at least does not do so to a significant extent.
It is also possible for the paraffin-based
latent heat storage material to contain a proportion of
mineral oil and/or polymers and/or elastomers. The
rubbers and/or elastomers predominantly result in a
higher flexibility, which may also be retained in the
solidified state of the latent heat storage material
and offers advantages, for example when used for seat
cushions or bandages. These materials are preferably
present in an amount of less than 5%. If the polymers
are not elastomers, they do not increase the
flexibility and simply prevent the latent heat storage
material from running out, if appropriate as an
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additional measure. Highly refined mineral oil i.s
preferably used. By way of example, this may be a
mineral oil which is usually referred to as white oil.
The polymers are crosslinked polymers produced by
copolymerization. The crosslinked polymers form a
gel-like structure with the mineral oil by forming a
three-dimensional network or by their physical
crosslinking (nodular structure). These gels have a
high flexibility combined with simultaneous stability
when subjected to mechanical forces. The paraffin is
included in this structure in the liquid state. When
the phase change, the crystallization, takes place, the
paraffin crystals which form are surrounded by the gel
structure, resulting i.n a flexible overall mixture.
In one possible application, a latent heat
storage material which contains paraffin with a melting
temperature of 50° Celsius and a copolymer with a
melting temperature of 120° Celsius can be heated to a
temperature of 125° Celsius, so that firstly
homogeneous mixing of the two components is achieved,
and the low-viscosity mixture can be taken tip by the
carrier material on account of the capillary forces
acting therein until it is completely saturated. During
subsequent cooling, the paraffin crystals formed are
surrounded by the copolymer. In an example of a
conceivable upper operating temperature of the latent
heat storage body of 80° Celsius, only the paraffin
fraction, but not the copolymer, is liquefied. This has
the advantageous effect that the paraffin cannot escape
from the copolymer and remains in the carrier material
together with the latter. It is pertinent to the
invention that the desired paraffin retention capacity
in the latent heat storage body when using the carrier
material described above can be achieved even when the
copolymer forms less than 5s by mass of the latent heat
storage material.
Examples of polymers used are
styrene/butadiene/styrene (SBS), styrene/isoprene/styrene
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(SIS) or styrene/ethylene/butylene/styrene (S-EB-S). In
particular, an agent known under the trade mark
"KRATON G", marketed by Shell Chemicals, is used. Since
"KRATON G" contains hydrogenated copolymers, this agent
has a high thermal stability and is therefore eminently
suitable for the application proposed here. The
"KRATON G" rubbers are known to be compatible with
paraffin and naphthene oils. The tr_iblock copolymers
are said to be able to take up more than twenty times
their weight in oil, so that it is possible to produce
products whose consistency - depending on the grade and
concentration of the rubber - can be varied within
broad limits. Optionally mixed diblock polymers contain
the AB type, for example styrene-ethylene propylene (S-
EP) and styrene/isoprene (SI). The ABA structure of
Kraton rubber molecules contains polystyrene end blocks
and elastomeric middle blocks. Furthermore, however, it
is also possible to use other known Kraton variations.
This block copolymer is preferably suitable as a
thickening agent for increasing the viscosity and/or as
a flexibilizing agent for increasing the elasticity.
Kraton G is a thermoplastic plastics material; a number
of types of copolymers of the Kraton G series exist,
these copolymers differing in their structure. The
Kraton rubber polymers have elastomeric properties and
an unusual combination of a high strength and a low
viscosity. Moreover, they have a molecular structure
comprising linear diblock, triblock and radial
copolymers, the molar weight of which varies and which
have a differing ratio of styrene content to elastomer
content. Of the known Kraton G grades, it is preferably
possible to use the grades known as G 1650, G 1651 and
G 1654. Each molecule of the Kraton rubber may comprise
block segments of styrene monomer units and rubber
monomer and/or comonomer units.
Furthermore, it is also possible to use
copolymers, such as for example HDPE (high-density
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polyethylene), PP (polypropylene) or HDPP (high-density
polypropylene).
L'urthermore, it is possible to add a mixture
which at least contains various copolymers selected
from the group consisting of diblock copolymers,
triblock copolymers, radial block copolymers and
multiblock copolymers, to the paraffin-based latent
heat storage material, the mixture preferably
containing at least one diblock copolymer and at least
one triblock copolymer, while the diblock copolymer and
the triblock copolymer may contain segments comprising
styrene monomer units and rubber monomer units.
It is relevant that the abovementioned
additives on the one hand be distributed homogeneously
within the paraffin or be penetrated homogeneously by
the paraffin and, secondly, that there is no chemical
interaction between the additives and the paraffin.
Furthermore, it is particularly important for the
selection to be made in such a way that there are
virtually no differences in density between the
additives and the paraffin, so that as a result it is
also impossible for any physical segregation to take
place.
The second part of the object is achieved by
the provision of a production method having the
features. according to Claim 22, in respect of which
advantageous procedures are specified in subclaims 23
to 28.
To this end, Claim 22 provides a method for
producing a latent heat storage body with
paraffin-based latent heat storage material held in a
carrier material which has holding spaces, in which
method the latent heat storage material is liquefied
and is supplied in liquefied form to automatically
sucking, capillary-like holding spaces in the carrier
material, in which method it is provided that the
liquefied latent heat storage material is supplied to a
plurality of individual support-material bodies of a
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latent heat storage body. This supplying may, for
example, be effected by pouring l:he liquefied latent
heat storage material over the carrier material.
Particularly for the production of relatively large
latent heat storage bodies, i_t is recommended for
individual support-material bodies, which are
prefabricated in relatively large numbers and i.n
dimensions which are smaller than the latent heat
storage body, to be impregnated with latent heat
storage material. Compared to the known, reverse
procedure, in which firstly a cohesive support-material
body of any size is impregnated with latent heat
storage material and partial latent heat storage bodies
are only cut from the support-material body in the
impregnated state, the method according to the
invention results in quicker and therefore less
expensive impregnation of the carrier material. As in
the known method using the reverse sequence of
operations, it is possible to use numerous relatively
small partial or individual bodies of virtually any
desired shape and/or size for a latent heat storage
body, so that the impregnated carrier material provides
virtually unlimited, options for forming the latent heat
storage body. Furt=hermore, the method according to the
invention may particularly advantageously also be used
for producing a microwave-active latent heat storage
body with a paraffin-based latent heat storage material
by applying a hygroscopic material_ to the surface of
the carrier material. In practice, to do this the
procedure may be such that the paraffin-based latent
heat storage material to be used is initially worked up
so as to. form a molten material, the viscosity of which
can be adjusted, and preferably thereby increased, by
the addition of additives, for example of Kraton, in a
concentration of up to ten percent, preferably of up to
two percent. In a subsequent method step, this molten
material is supplied to automatically sucking,
capillary holding spaces in the individual
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support-material bodies, for example by dipping the
latter into the molten material or pouring the molten
material over the individual latent heat storage
bodies, with the additional option of assisting the
sucking action by controlling the temperature in a
specific way and/or by supplying mechanical energy, for
example by agitation. In a further method step, the
hygroscopic material can then be applied to the surface
of the carrier material. To do this, it is preferable
to add a hygroscopic material which is in the form of
grains and/or granules and/or flakes and/or powder to
the impregnated individual support-material bodies and
to achieve intimate mixing, for example by kneading or
stirring, as a result of which the hygroscopic material
covers the surface of the individual support-material
bodies as uniformly as possible. In this case, it has
proven advantageous that, particularly given complete
impregnation; there is a layer of molten paraffin-based
latent heat storage material on the partial
support-material elements, which 7_ayer forms again
during the cooling process and to which, in particular
in the molten state, hygroscopic material adheres
particularly well, .thus simplifying its homogeneous
distribution. As a modification of the procedure
described, the hygroscopic material may also be applied
to the individual support-material bodies before they
are impregnated with latent heat storage material.
Particularly in the case of a hygroscopic material in
powder form, this makes it possible for this material
to enter into the holding spaces in the carrier
material. together with the latent heat storage material
when the latter is sucked in, so that microwave
activation also takes place in the interior of the
individual support-material bodies. From this, it
becomes clear that the proposed use of individual
support-material bodies of preferably small dimensions
by the method according to the invention in order to
suck up. latent heat storage material additionally
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offers particular advantages for the production of a
7_atent heat storage body which is microwave-activated
by means of hygroscopic material. Tf microwave-active
properties are not required, it is, of course, possible
to dispense with the addition of the hygroscopic
material f_or producing a latent heat storage body with
a paraffin-based latent heat storage material using the
method according to t=he i.nvent_ion, a 1. though the
advantages of the abovementioned production technology
over known production methods for latent heat storage
bodies are maintained as a result of the latent heat:
storage material being sucked into individual
support-material bodies of preferably small dimensions
and therefore relatively large numbers.
Furthermore, it has proven suitable for
commercially available oil binders to be used as
individual support-material bodies, in particular the
products supplied by Rench Chemie GmbH under the brand
names Rench-Rapid R, Rench-Rapid G, Per_l.een 222.,
Perleen 444, Rapon 5090, Rapon 5092 and Rapon 5093. If
an oil binder in grain form is used to suck up the
latent heat storage material which has been prepared to
form a high-viscosity molten material, the result is a
bed of spheres with pulverulent fractions, in which the
latent heat storage material is so strongly bound in
the individual suction elements or individual latent
heat storage bodies that it does not escape even at
temperatures which are 20 to 30° above the melting
point of paraffin. In this case too, a shiny layer of
molten paraffin forms on the suction elements, and this
layer forms once again during the cooling process,
making an adhesion surface for pulverulent elements of
the microwave-active, hygroscopic material. Until after
the cooling process, this form of the bed remains
freely mobile internally, i.e. it does not become a
hard mass, this mobility being desirable in particular
for heat cushions. Furthermore, other materials with
structures which are capable of exerting a sucking
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action, such as for example fibres made from mineral or
ceramic materials, organic materials, such as cotton or
wool, glass, phenolic resins, plastics, in all the
forms in which they can be processed, such as spinning,
foaming, granulating, pulverizing, braiding, weaving,
etc., can be used as carrier material .for sucking up
the latent heat storage material. Therefore, the
carrier material can be used, for example, as a
material in the form of grains and/or granules and/or
flakes. Furthermore, it may also be in the form of
platelets of a desired strength or in the form of a
nonwoven. Furthermore, the method according to the
invention may also be used to obtain additional
features of a latent heat storage body which are
mentioned in C7_aims 1 to 21 or the associated
description. It also follows from the above description
of the production method that a latent heat storage
body according to t=he invention may contain any desired
combination, with regard to its components, of the
materials proposed for the production method in the
specifications which are in each case taken into
consideration or are similar.
To achieve, the further part of the object
according to Claim 29, the invention proposes a rnethod
for producing a latent heat storage body with
paraffin-based latent heat storage material held in a
carrier material which has holding spaces, in which
method the latent heat storage material is liquefied
and is ,supplied in liquefied form to automatically
sucking, capillary-like holding spaces in the carrier
material, in which method it is provided that a
hygroscopic material is supplied to a surface of the
carrier material. Accordingly, to produce a latent heat
storage body, as an alternative to a plurality of
individual support-material bodies it is also possible
to use a cohesive carrier material. One example of a
possible application of this method is the production
of latent heat storage bodies of small dimensions or
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layer thicknesses and/or of simple geometric form, in
which it is possible both to make up a cohesive carrier
material without problems and also for this material to
be fully impregnated within sufficiently short periods
of time.
The invention also relates to. a method for
heating a solid or liquid heat storage material which
on its own cannot be heated by microwave beams or c:an
be heated to a lesser extent than water, and to a heat
storage device having a solid or liquid heat storage
material which on its own cannot be heated by microwave
radiation or can be heated to a lesser extent than
water.
E3ecause of the possible time and energy savings
compared to heating techniques which were previously
customary, heating liquids arid solids by microwave
radiation has gained increasing importance i.n recent
years. Microwave radiation (microwaves for short)
generally involves electromagnetic waves in a frequency
range between 1 GHz and 1 THz, corresponding to a
wavelength range of between about 0.3 mm and 39 cm. One
use of microwave radiation which has by now become very
widespread is the heating of foodstuffs in a microwave
oven in which energy is extracted by the foodstuffs
which have been placed in the oven from the microwave
field at frequencies between 2.425 and 2.475 GHz as a
result of dielectric losses, leading to the foodstuffs
being heated. In industrial applications, a frequency
of 5.8 GHz is also in widespread use. On account of the
possible time and energy savings involved in microwave
heating, it is desirable for a large number of liquids
and solids other than foodstuffs to be warmed or heated
by microwave radiation. However, of the materials which
come under consideration for this heating, many are not
inherently heated in a microwave field, and many
further materials are only heated to a much lesser
extent or more slowly than water. Where, in the latter
group, heating is so weak or slow as to be unacceptable
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for industrial applications or in domestic use, the
corresponding materials are grouped together with the
materials which inherently cannot be heated at all by
microwave radiation to form the category of so-called
"microwave-passive" materials. The group of
"microwave-passive" materials also includes those which
on their own can be heated to a considerably lesser
extent than water by microwave radiation, water being
one of the strongly microwave-active materials. For.
this reason, the high water content of many foodstuffs
means that they also belong to the group of
"microwave-active" materials which on their own can be
heated by microwave beams to an extent or within a
period which is technically useful. A particular
drawback is that a range of packaging materials, in
particular based on paper, wood and plastics, which are
frequently also used for foodstuffs, and in addition a
large number. of predominantly organic liquids on their
own cannot be heated by microwave radiation or. can only
be heated to a considerably lesser extent than water by
such radiation. Particularly in the fast-food- sector,
the packaging material for foodstuffs, in addition to
having a protective function, also has the function of
keeping the food hot during transport. However, if the
heat storage material used for the packaging cannot on
its own also be heated by microwave radiation along
with the foodstuffs during the heating of the
foodstuffs, the foodstuffs lose some of their heat as a
result of subsequent thermal conduction to the
packaging which is at a lower temperature.
In view of the above, it also counts as an
object of the invention to provide a method for heating
a heat storage material which, in the context of the
invention, is microwave-passive, by microwave
radiation, in such a manner that it is advantageous for
use, and also a heat storage device which is suitable
for this purpose. In this context, a heat storage
material is in principle understood as meaning any
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material which is able to store heat at least for a
short time and to a limited extent.
The first part of tine object in this respect is
achieved by the method specified in Claim 31,
advantageous procedures for this method being specified
in Claims 32-37 which are dependent on.Clairn 31. With
regard to the heat storage device, the object set is
achieved by the subject-matter of Claim 38.
According to Claim 31, to achieve the object,
it is provided that, a hygroscopic material is added to
the heat storage material for heat exchange with the
heat storage material in a quantitative proportion
according to which, starting from a moisture
equilibrium of the hygroscopic material at 50o relative
atmospheric humidity and 20°C, an amount of 500 g of
the heat storage material is heated by at least 50°C
starting from 20°C when exposed to microwave radiation
with a power of 400-600 watts over a period of from 2-
10 min, and that irradiation of_ the hygroscopic
material with microwave radiation is effected under
corresponding conditions. By way of example,' in this
respect, consideration is given to using a domestic
microwave oven, into whose cooking chamber the heat
storage material and the hygroscopic material which has
been added to it for heat exchange can be introduced.
Alternatively, it is possible to allow the microwave
radiation to act on the hygroscopic material in some
other way. The hygroscopic material has a pronounced
ability to take up moisture from its environment and
bind this moisture to itself. In particular, it is also
able to remove the moisture contained in the air in the
chamber, in the form of water vapour, under standard
conditions and to take up this moisture. Furthermore,
it is also possible to promote the uptake of water by
increasing the water vapour content in the air. In
addition, water which is present in liquid form is also
taken up by the hygroscopic material within a very
short time, until a saturated state is reached. The
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water which is stored in the hygroscopic material
itself represents a very microwave-active liquid in the
sense of the invention, so that when placed in a
microwave field the water is heated up very quickly and
intensively, and so is the hygroscopic material. The
heat storage material, which by contrast to this is
microwave-passive, on the other hand, is not heated or
is heated to only an insignificant extent. On account
of the hygroscopic material having been added to the
heat storage material, heat exchange then commences, in
such a manner that thermal energy is transferred from
the heated water or water vapour directly and, after
the hygroscopic material has been heated, also from the
latter to the heat storage material. The heat transfer
may take place as thermal conduction, by convection, by
thermal radiation or in, any desired combinations of
these transfer mechanisms. For heat exchange, it is
possible to add the hygroscopic material to the heat
storage material, by way of example, by disposing the
hygroscopic material on one or more surfaces of the
heat storage material. If this is not possible or not
desirable, the hygroscopic material may also be
disposed in distributed manner at a suitable distance
from the heat storage material. In any case, it is
advantageous if the hygroscopic material, however it is
disposed, has a high ratio of surface area to volume or
mass, in order to provide the largest possible
heat-exchange surface area for the abovementioned heat
transfer mechanisms. Suitable hygroscopic materials for
the invention are all those in the broader sense which
are able to take up the amount of water required for
the proposed method within a relatively short time.
Preferably, consideration may be given to the use of
calcium chloride, iron chloride, copper sulphate,
magnesium chloride, potash and silica gel, while in
other applications the use of blotting paper or
hygroscopic woven fabrics, nonwovens and the like may
offer advantages. In principle, the method according to
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the invention can be used to heat all solid or liquid
microwave-passive heat storage materials whose
introduction into a microwave field is known not to
entail danger; for the heat storage material,
consideration may be given to any material which has at
least a limited ability to store heat for a short time.
With a view to the problems set out above,
consideration is given in particular to the use of
paper-based or cardboard-based, wood-based or
plastics-based packaging materials.
In a preferred method of application, a heat
storage material which lets through microwave radiation
is used. Furthermore, it is preferable to use a
hygroscopic material whose hygroscopic property is not
changed by heating caused by microwave radiation. This
means that the hygroscopic material, even after the
method according to the invention has been employed
numerous times, still has the unchanged property of
taking up moisture from the environment and releasing
this moisture to the environment during evaporation
caused by heating. The method according ' to the
invention may advantageously be embodied by the
hygroscopic material being disposed in sandwich form
between two panel-like heat storage elements made from
heat storage material, preferably from a solid heat
storage material. In this case, two or more of the
panel-like heat storage elements may be disposed
substantially parallel to one another and the
hygroscopic material may be distributed in the
corresponding interspaces, resulting in the formation
of a multilayer system. In practice, the procedure may
be such that firstly the hygroscopic material is
disposed on the surface of a heat storage element made
from microwave-passive heat storage material, and
subsequently a further heat storage element is placed
onto the hygroscopic material, whereupon these working
steps may be repeated until the desired layer structure
is achieved. Alternatively, or in combination, it is
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also possible for recesses, for example in the form of
holes, grooves, notches or geometrically indeterminate
three-dimensional surfaces for holding the hygroscopic
material, to be made in the microwave-passive heat
storage material_ or in the heat storage elements formed
from this material. It is then possible for the
hygroscopic material to be introduced into the recesses
and fixed inside them by further heat storage material.
8y way of example, it is possible to provide a surface
of a heat storage element with a ribbed profile, to
fill the valleys of this profile with a hygroscopic
salt and then to attach a further heat storage element
to the filled surface. Furthermore, it is preferable
for a panel-like heat storage element to be provided
with cavities which extend continuously from a surface
of the heat storage element which faces towards the
hygroscopic material to a surface of the heat storage
element which exchanges moisture with the environment.
In particular, consideration is given to forming the
cavities by spaced-apart punctures or cuts, for example
made with a needle. The cavities also make it 'possible
to use vapour-impermeable, microwave-passive heat
storage material, ir1 that they themselves provide flow
paths for the desired exchange of vapour with the
environment. Furthermore, in the case of
vapour-permeable, microwave-passive heat storage
material, the cavities still allow the ability of this
material to diffuse microwave-active moisture to be
improved considerably. Furthermore, it is advantageous
if, when carrying out the method according to the
invention, capillary-like holding spaces for holding a
latent heat storage material, in particular a
paraffin-based heat storage material, are provided in a
solid heat storage element. With regard to the
capillary-like holding spaces, reference is made to
PCT/EP98/01956, the disclosure content of which is
incorporated into the present application in its
entirety. According to a further preferred application
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of the method, a heat storage element is formed from
poplar wood.
It is known treat it is not possible to achieve
a fully uniform distribution of the microwave radiation
intensity i_n the interior of the cooking cl-iamber of
microwave ovens. This leads to uneven heating of the
microwave-active materials contained therein and may,
depending on conditions, lead to local overheating and
to damage. Therefore, for an advantageous configuration
of the method according to the invention, it is
proposed for the three-dimensional distribution of the
microwave radiation intensity to be made more uniform
by a foil which reflects the microwaves in the region
where the radiation intensity is high in relative
terms. The procedure for this purpose may be such that
. prelimi.nary tests are used to determine the temperature
distribution within a microwave-active material which
is spread out substantially flat in the microwave oven,
and that the position and distribution of_ regions which
are at relatively high temperatures, corresponding to
the regions of_ relatively high radiation intensity, are
marked. Then, in a subsequent step, a foil which
reflects the microwaves can be cut out in such a way
that its shape corresponds precisely to the surface
regions of relatively high radiation intensity. The
reflective foil which has been cut out can then
preferably be disposed beneath the material to be
heated during further usages of the microwave oven. In
the present case, therefore, it is possible for the
foil which has been cut out to be disposed beneath the
hygroscopic material and, if appropriate, in addition
beneath the microwave-passive heat storage material.
The relatively high-intensity microwave radiation which
is incident on the foil is reflected on incidence and
is deflected into regions with a lower radiation
intensity, so that overall the radiation intensity is
made more uniform, leading to more uniform heating of
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the microwave-active moisture and therefore of the
hygroscopic material and of the heat storage material.
Furthermore, it is proposed for the
three-dimensional distribution of the microwave
radiation intensity to be made more uniform by a
homogenizing mask which reflects and/or diffracts
and/or refracts the microwaves, in which case the
homogenizing mask may preferably be disposed i_n the
region of relatively high radiation intensity. In the
context of the invention, a homogenizing mask is
understood as meaning a device which, as a result of
its materials properties and/or design features, brings
about preferential reflection and/or diffraction and/o.r
refraction of microwave beams in a microwave field. To
make the radiation intensity more uniform, it is
possible for the homogenizing mask to be disposed
inside and/or outside the heat storage material in a
microwave field or in a cooking chamber of a microwave
oven, it being possible for the homogenizing mask to
comprise a plurality of individual parts which may be
active on their own or in combination with one another
and/or by interacting with internal fittings of the
microwave, for example a turntable or even the boundary
walls of the cooking chamber. By making the radiation
intensity more uniform, the homogenizing mask makes it
possible to prevent partial overheating caused by
increased microwave radiation concentration, and the
mask ma,y consist of different materials. In this
context, the dielectric loss factor plays a subordinate
role. It is possible for the microwaves which impinge
on the body to be heated to be scattered by optical
deflection. As a result, excessively high radiation
concentrations at individual locations, in particular
in the centre of the microwave, where the object to be
heated, which is situated on a turntable, for example,
is relatively stationary, are avoided. The homogenizing
mask primarily utilizes the optical properties of the
microwaves in order to achieve deflection and partial
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extinction. For this application, it is possible to
give consideration to uniform, continuous glass bodies
with non-homogeneous compositions of the glass material
or glass of a uniform structure with a diverging lens
surface (either machined directly into the glass or
applied, for example by adhesive bonding). The glass
may also be in the form of a bed of pounded glass
("glass crunch") or regular geometric bodies, e.g.
spheres, rhombi, pyramids and other suitable bodies or
mixtures of such bodies with one another. At the phase
boundaries which are formed, the microwaves are
deflected in undetermined directions, so that a diffuse
wave field is formed. If a plurality or parts made from
glass or~from another suitable material of this nature
are used jointly as a homogenizing mask, it is
possible, depending on the distribution of the
microwave radiation intensity, which is known from
preliminary tests, for example, to achieve a
particularly uniform radiation intensity by distributed
disposition of the parts in the microwave field or
microwave oven, disposition in the region where the
radiation intensity is relatively high being preferable
effected.
In another variant, it is possible for the
homogenizing mask used or provided to be a metal grid.
In this case, the extinction and/or deflection and/or
diffraction of the microwave beams can be influenced by
the selection of mesh size and/or wire thickness and/or
material composition of the metal grid. In this
context, the percentage of the area covered by the mesh
grid with respect to the largest possible free
irradiation area of the microwave transmitters within
the microwave appliance constitutes a decisive
parameter. The blocking effect ("Faraday cage") is
controlled by selecting the wire thickness and mesh
width. The tighter the mesh of the grid, the stronger
the screening effect becomes. In the case of total
screening from above, the object to be heated is then
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only heated from the sides and from below by the beams
reflected inside the microwave appliance. In this case,
consideration is also given to introctuc:i.ng a
tight-meshed metal grid between the heat storage
material and the microwave radiation source, in order
to screen the microwave radiation in_ the principal
direction of incidence. Furthermore, it is possible for
the two variants of the homogenizing mask explained
above to be used in combination with one another, so
that it is possible to control the microwave radiation
intensity in virtually all regions. In this case, the
effects of diffraction, refraction and extinction are
combined with one another and can be appropriately
combined with one another by materials combinations and
arrangements for the particular application. By means
of the homogenizing mask, for example, local
overheating of a heat cushion which has been introduced
into a microwave can be prevented, which would
otherwise lead to the cushion being destroyed.
In addition to the abovementioned materials,
(glass, metal) and the body shapes speEifically
mentioned, other expedient configurations of a
homogenizing mask are also conceivable. A practical
configuration will be based on the desired reflection
and/or_ diffraction and/or refraction properties and on
not impairing the operation and reliability of the
microwave oven. Furthermore, it is also possible for
the homogenizing mask to be used independently of the
proposed method for heating a solid or liquid heat
storage material which on its own cannot be heated by
microwave radiation or can be heated to a lesser extent
than water. For this purpose, the homogenizing mask may
be provided or used in any desired microwave field,
said mask exhibiting the advantageous effects referred
to above.
Furthermore, it is also possible for the
temperature distribution within the heat storage
material and/or the hygroscopic material and/or between
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hygroscopic material and heat storage material to be
made more uniform by heat-conducting sheets made from
material with good thermal conductivity in the
transition region between different temperatures. By
way of example, sheets made from copper, aluminium or
the like, which may be cut into strips or any other
suitable shapes, are suitable. The heat-conducting
sheets are preferably brought into simultaneous
contact, over a large area, with regions at higher and
lower temperatures, so that more rapid temperature
balancing can be achieved as a result of the good
thermal conductivity of these sheets.
To achieve the further part of the object, the
independent Claim 90 proposes a heat storage device
having a solid or liquid heat storage material which on
its own cannot be heated by microwave radiation or can
be heated to a lesser extent than water, it being
provided that the heat storage device contains a
hygroscopic material for heat transfer to the heat
storage material. In this context, consideration is
preferably given to one of the arrangements described
above in connection with the method according to the
invention for heating a microwave-passive heat storage
material. Furthermore, the heat storage device may
additionally also have any desired individual features
or combinations of features as have likewise been
described in connection with the abovementioned method.
The invention also relates to another heat
storage device with a solid or liquid heat storage
material which on its own cannot be heated by microwave
radiation or can be heated to a lesser extent than
water, which, with regard to the heating of
microwave-passive heat storage material in a microwave
field, is based on its own solution idea compared to
the heat storage device mentioned above. The starting
point for these considerations is that in a number of
specific applications, for example medical technology
or space travel, it may be of interest to avoid or
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reduce as far as possible a vapour phase which is
present in the environment. Therefore, under these
conditions, for heating microwave-passive material in a
microwave field there is a need for a suitable heat
storage device in which it is possible to dispense with
hygroscopic material and which, if appropriate, can
also be combined with the above-described heat storage
device with hygroscopic material. Therefore, a further
object of the present invention is to specify a heat
storage device of the generic type in which, in order
to heat a microwave-passive heat storage material in a
microwave field, it is possible to dispense with i:he
use of hygroscopic material. In the context of the
present invention, a heat storage material is any
material which is able to store heat at least for a
short time and to a limited extent.
According to Claim 49, this object is achieved
by a heat storage device with a solid or liquid heat
storage material which on its own cannot be heated by
microwave radiation or can be heated to a lesser extent
than water, it being provided that the heat, storage
device contains an absorption body with a high
dielectric loss index for heat transfer to the heat
storage material in a microwave field, and that the
length (L, L') of the absorption body in one direction
of extent corresponds to at least half the wavelength
of microwave radiation selected for supplying energy.
In the context of the invention, an absorption body is
understood as meaning a body which, on account of its
materials properties and the characteristic ratio of
its length in at least one direction of extent to the
wavelength of the microwave radiation, undergoes
preferential heating in a microwave field as a result
of dielectric losses. Further details on dielectric
losses are to be found, for example, in "Werkstoffkunde
[Materials Science], H J Bargel, G Schulze, VDI-Verlag,
Dusseldorf, 1999, 6th edition". According t:o these
explanations, dielectric losses arise if a dielectric
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has a low conductivity or if it is not of entirely
homogeneous structure. In an alternating field (under
AC voltage), in the case of non-polar plastics, the
time delay in the pole change, and, in the case of
polar materials, the vibration of the dipoles which
then occurs, leads to further energy losses. These
effect a change in the phase shift between current and
voltage. If the phase angle in the loss-free capacitor
is 90°, the losses of energy im the alternating field
result in it being reduced by the angle delta as the
complementary angle to 90°. The tangent of this loss
angle is referred to as the dielectric loss factor tan
delta. This represents the ratio between active current
and reactive current and therefore also i:he ratio
between active power (= loss) and reactive power of the
capacitor. The dielectric loss index is then:
Epsilon'r = epsilonr * tan delta,
Epsilonr - dielectric constant
This is material-dependent, and its magnitude
changes with frequency and temperature, having an
increasing effect in particular at high frequencies.
Dielectric losses I reduce the performance of a
capacitor. They are converted into heat. Consequently,
plastics materials with a very low dielectric loss
index are excellent dielectrics. On the other hand, the
internal heating of plastics materials with a higher
epsilonr * tan delta can be utilized deliberately and
to good effect industrially, as is the case, for
example, in high-frequency welding. The heat storage
device proposed in this patent application uses the
preferred heating of an absorption body with a high
dielectric loss index to indirectly heat heat storage
material in a microwave field. The dielectric loss
factor for the various plastics materials is between
10-1 and .10-4. Plastics materials which are particularly
suitable for heating by high-frequency fields contain
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heteroatorns (thus not only hydrogen and carbon atoms,
but also, for example, nitrogen or chlorine atoms),
wloich cause permanent dipoles in the molecular
structure. Examples of plastics materials with a high
tan delta are polyamides (nylon), aminoplastics
(melamine) and PVC-P (plasticized PVC). Other
materials, e.g. water and certain types of glass, also
have high tan delta values.
In connection with the heat storage device
according to the invention, it is proposed for the
absorption body to be a glass body and/or to contain
polyamides and/or aminoplastics and/or PVC-P and/or
water. Alternatively, the absorption body may also
consist of another material with a dielectric loss
index of a suitable level. In particular, it is
possible for the dielectric loss index of the
absorption body to be between 10-1 and 10-9.
In a preferred configuration, the absorption
body is provided in the form of a sheet, in which case
the sheet length in one direction of extent corresponds
to at least half the wavelength of microwave radiation
which has been selected to supply energy.
Preferably, consideration is given to the
abovementioned direction lying inside the plane of the
sheet-like absorption body, for example glass body.
When microwave radiation impinges on the sheet-like
absorption body, for example glass body, it is absorbed
or totally absorbed. The microwaves are refracted in
the absorption body, for example glass body, and
transmitted in this body until they meet a surface or
dislocation, from which they are at least in part
reflected in the opposite direction of movement. The
reflected microwave radiation is transmitted in the
absorption body, for example g7.ass body, until it once
again reaches a surface or dislocation, from which it
is thrown back in the original direction of movement.
In the sheet-like absorption body, for example glass
body, the microwave beams are sent to and fro
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predominantly along the direction of extent which lies
in the plane of the sheet. As it passes through a
number of times, the wave energy is converted into
thermal energy, leading to desired heating of the
absorption body, for example glass body, in the
microwave field. In one direction of extent, along
which the length of the glass body corresponds to at
least half the wavelength of the microwave radiation
which has been selected to supply energy, a so-called
standing wave is formed, in that the microwave
radiation is reflected by surfaces which are opposite
one another and are oriented perpendicular to the
aforementioned direction of extent, in each case with
congruent phases and amplitudes. As a result of the
continuous introduction of further microwave beams and
resonance phenomena, wave energy becomes concentrated
in the standing waves, allowing a correspondingly
higher thermal energy yield to be achieved during the
energy conversion. If the length of the absorption
body, for example glass body, corresponds to at least
half the wavelength of the selected microwave radiation
even in only one of the directions of extent of the
absorption body, for example glass body, which lies in
the plane of the sheet, i.e. if at least one one-
dimensional standing wave is formed, it is already
possible to achieve substantial heating of the
absorption body, for example glass body, in the
microwave field within short times. By way of example,
a radiation frequency of 2.45 GHz gives a wavelength of
approximately 12.2 cm, so that an absorption body, for
example glass body, of a length of only approximately
6.1 cm is sufficient for formation of a standing wave.
Furthermore, it is also possible for the absorption
body, for example glass body, also to have a length
which corresponds to at least half the wavelength of
the selected microwave radiation in other directions of
extent, so that standing waves are formed- in a
plurality of spatial directions and the conversion of
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wave energy into thermal energy is still further
intensified. Preferably, consideration is given to a
configuration of the sheet-like absorption body, for
example glass body, in which this body has a
substantially planar structure, the length of the
absorption body, for example glass body, corresponding
to at least half the wavelength of the microwave
radiation which has been selected for supplying energy
only along a number of directions of extent oriented
inside the plane of the sheet. By contrast, the length
of the absorption body, for example glass body, in the
direction of extent perpendicular to the plane of the
sheet may be considerably shorter than half the
wavelength of the microwave radiation, in which case it
is nevertheless possible to achieve a level of
conversion of wave energy into thermal energy which is
extremely high and leads to rapid heating of the
absorption body, for example glass body. Given a
corresponding configuration of the absorption body, for
example glass body, as a planar sheet of small
thickness, it is possible to achieve a compact
arrangement, for example between sheets of
microwave-passive heat storage material which are
spaced apart from 'and substantially parallel to one
another. A corresponding sandwich-like layer composite
may also be composed of a plurality of_ absorption
bodies, for example glass bodies, disposed within one
sheet plane and/or substantially parallel to one
another, and a correspondingly larger number of sheets
of heat storage material. Alternatively, other
arrangements of the absorption body, for example glass
body, in relation to the heat storage material are also
possible. For example, it is possible for the
absorption body, for example glass body, to be immersed
in a vessel containing a microwave-passive liquid. The
relevant factor is that the absorption body, for
example glass body, should be heated more rapidly in a
microwave field than the microwave-passive heat storage
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material. On account of the temperature differences
which are established, heat transfer from the
absorption body, for example glass body, to the
microwave-passive heat storage material begins, so that
the heat storage material is also heated. The heat
transfer' may take place through thermal conduction,
convection, thermal radiation or any desired
combinations of these transfer mechanisms. The
absorption body, for example glass body, may itself be
made from extremely simple, inexpensive glass
materials, for example from window glass. Even with
such a simple absorption body, for example glass body,
the conversion of wave energy into thermal energy is
promoted by the fact that its length in one or more
directions of extent is selected to be equal to an
even-numbered multiple of a quarter of the microwave
radiation selected to supply energy, in which case the
even-numbered multiple must be at least double. It is
preferable for the heat storage material to let through
microwave radiation. This has the advantageous effect
that the entire surface of the absorption body, for
example glass body, can be utilized in order to
introduce the microwave radiation. An advantageous
refinement of the heat storage device according to the
invention may be effected by one or more surfaces of
the absorption body, for example glass body, being
formed to reflect microwave radiation which is incident
thereon from the interior of the absorption or glass
body. In this case, the "natural" reflection of the
microwave radiation from the inner sides of the
surfaces of the absorption body, for example glass
body, which only captures a certain proportion of the
radiation, can be considerably increased by a suitable
surface treatment, for example by a coating process.
Further advantages of the heat storage device in use
can be achieved by at least one surface of the
absorption body, for example glass body, having a
coating with a temperature-dependent transmission
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coefficient for microwave radiation. Preferably, at a
temperature of the absorption body, for example glass
body, which is initially still low, it is provided that
a coating of this nature has a transmission coefficient
which allows the microwave radiation to enter into the
absorption body, for example glass body,-with as little
interference as possible and which, as the temperature
of the absorption body, for example glass body, rises,
makes it more difficult for further microwave radiation
to enter. The way in which material layers of this
nature act is based on a temperature-dependent
structural transformation, for example from amorphous
(microwave-transmitting) to crystalline
(microwave-reflecting). A coating having a
temperature-dependent transmission coefficient for
microwave radiation makes it possible to form a
self-regulating system, the heating of which is
terminated automatically when set parameters are
reached, in particular when a desired heating
temperature is reached. Since heat transfer from the
absorption body, for example glass body, ' to the
microwave-passive heat storage material is only
possible in the direction of a temperature drop, the
heat storage material is also only heated up to a
maximum of that temperature at which the
temperature-dependent coating prevents further
microwave radiation from penetrating into the
absorption body, for example glass body. This has the
advantageous effect that even if the radiation
intensity and/or duration is unintentionally selected
at a high level, it is not possible for the heat
storage device and the microwave-passive heat storage
material contained therein to be overheated to an
unacceptable extent. The application of hygroscopic
material to the absorption-body or glass surfaces can
also serve to ensure harmonized heating/cooling of the
heat storage elements. For example, a simple, and
therefore inexpensive, yet effective heat storage
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device is achieved by forming the sheet-like glass body
as a flat parne of glass, the length of which in at
least one direction of extent lying in the plane of the
sheet corresponds to at least half the wavelength of
the microwave radiation which is selected to supply the
energy, by introducing this glass body into the cooking
chamber of a microwave oven, and by disposing the
microwave-passive heat storage material on the glass
body in distributed manner. Alternatively, it is
possible to provide a plurality of adjacent glass
sheets instead of a single glass sheet.
In a further preferred configuration, the
absorption body may be in the form of a film, film
packing or bundle of films, for example made from
plastics materials. The plastics materials can also be
used as a sheath for heating heat.-retention elements or
heat storage material in microwave appliances. In this
case, it may be important or even necessary for the
three-dimensional distribution of the microwave
radiation intensity to be made more uniform in a heat
storage device by a homogenizing mask, in which case
the homogenizing mask may have one or more of the
features of the homogenizing mask described above. By
way of example, it may be a foil which reflects the
microwaves and which may preferably be disposed in the
region of relatively high radiation intensity in order
to make the radiation intensity more uniform.
In addition, or as an alternative, it is also
possible for the temperature distribution within the
heat storage material and/or between heat storage
material. and absorption body, for example glass body,
to be made more uniform by at least one heat-conducting
sheet made from a material with a good thermal
conductivity in the transition region between different
temperatures. With regard to one possible specific
configuration, reference is made to the description of
such a configuration in connection with the heat
storage device containing a hygroscopic material. As a
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supplementary note, it is pointed out that in the heat
storage device containing hygroscopic material and in
that which has an absorption body, for example glass
body, in the form of a sheet, it is also possible to
provide one or more heat-conducting sheets whose
surfaces reflect and/or diffract and/or refract
microwave radiation incident thereon. Therefore,
corresponding heat-conducting sheets may be provided in
order to make the temperature distribution rnore uniform
both using the route of making the microwave radiation
intensity more uniform and by making the thermal energy
which has already been stored more uniform.
Furthermore, it is pointed out that the
absorption body described above may be provided or used
not only in a heat storage device with a solid or
liquid heat storage material which on its own cannot be
heated by microwave radiation or can be heated to a
lesser extent than water, but also, in very general
terms, may also be disposed or used in microwave
fields, where it leads to the advantageous effects
which have been explained.
The latent heat storage body according to the
invention and heat,storage devices according to the
invention are explained below, by way of example, with
reference to the appended drawings, in which:
Fig. 1 shows a latent heat storage body according to
the invention with a closed sheath, in
perspective view, partially cut away,
Fig. 2 shows a latent heat storage body according to
the invention with a perforated sheath, in a
perspective view, partially cut away,
Fig. 3a shows the interior of the latent heat storage
body in a regenerated state, as an enlarged
excerpt from Figs. 1 and 2,
Fig. 3b the interior of the latent heat storage body
following brief heating by microwaves, as an
enlarged excerpt from Figs. 1 and 2,
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Fig. 3c the interior of the latent heat storage body
following prolonged heating by microwaves, as
an enlarged excerpt from Figs. 1 and 2,
Fig. 4 a second exemplary embodiment of a latent heat
storage body, in a seci:ional view,
Fig. 5 a third exemplary embodiment of. a latent heat
storage body with a closeable opening, in a
sectional view,
Fig. 6 shows a distribution body which is connected to
a water vessel, has capillary spaces and has
hygroscopic material applied to it,
Fig. 7 shows a latent heat storage body with a
distribution body according to Fig. 6
incorporated therein, in an exploded view,
Fig. 8 shows a distribution body according to Fig. 6
without hygroscopi_c material. applied to it,
Fig. 9 shows a latent heat storage body with a
distribution body according to Fig. 8
incorporated therein, in an exploded view,
Fig. 10 shows a microwave-inactive latent heat storage
body with a microwave-active enclosure, in a
sectional view,
Fig. 11 shows a microwave-inactive latent heat storage
body with a microwave-active core,
Fig. 12 shows a sectional view through a first
embodiment of a heat storage device with
hygroscopic material contained therein and two
heat storage elements,
Fig. 13 shows a sectional view through a second
embodiment of a heat storage device with
hygroscopic material and two heat storage
elements with cavities passing through them,
Fig. 14 shows a sectional view through a third
embodiment of a heat storage device with
hygroscopic material and one heat storage
element,
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Fig. 15 shows a sectional view through a heat storage
device according to Fig. 14 with an additional
outer sheath,
Fig. 16 shows a perspective view of a heat storage
device which is formed as a container and has a
sheet-like glass body,
Fig. 16a shows a partial section through the container
base, along the section line XVI-XVI in Fig.
16, according to a first embodiment,
' 10 Fig. 16b shows a partial section through the container
base, along the section line XVI-XVI in Fig.
16, according to a second embodiment,
Fig. 17 shows a perspective view of a heat storage
device with homogenizing mask in the cooking
chamber of a microwave oven,
Fig. 18 shows a perspective view of a heat storage
device with an absorption body and with a
second embodiment of a homogenizing mask in the
cooking chamber of a microwave oven.
Figure 1 describes a latent heat storage body 1
according to the invention which is formed as a heat
cushion. It has a sheath 2 which is impermeable to
vapour diffusion and in the exemplary embodiment shown
is formed from a film which is folded to form a double
layer along a wrap-around edge 3 and the side edges 4
of which are closed off by welding in such a manner
that they are impermeable to vapour diffusion. As can
be seen,from the partial cutaway in the sheath, the
latent heat storage body l contains in its interior a
number of individual support-material bodies 5 with
paraffin-based latent heat storage material 6 held in
capillary holding spaces. Furthermore, a hygroscopic
material 7 in the form of grains is disposed in
uniformly distributed manner over the surfaces of
individual support-material bodies, in the interior of
which hygroscopic material, water 8 is stored as
microwave-active material. For a detailed illustration
and description of the functioning, reference is made
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at this point to Figures 3a, 3b, 3c and the associated
description.
Figure 2 describes a latent heat storage body 1
which differs from that shown in Figure 1 by dint of a
sheaLln 2' which is permeable to vapour diffusion. It
has microperforations comprising openings 9 which are
permeable to vapour diffusion and are distributed over
the entire area. The size of the openings 9 i.s ideally
selected in such a way that the openings only allow
vapour to pass through. However, the bag remains fully
useable even if larger openings, for example in the
range of up to approximately 0.2 - 0.3 mm, are present.
In the "open system" shown in Figure 2, when heating
takes place the openings 9 allow vapour to escape into
the environment, and when cooling takes place the
openings allow the latent heat storage body i=o be
regenerated as required with moisture from the
environment. Therefore, with the ~~open system",
compared to the "closed system" illustrated in Figure
l, even small unsealed areas, as may occur during
welding, can be disregarded. This results in a
considerable reduction in the reject rate during
production, which also means that the costs of quality
control are reduced. Any excess paraffin can
effectively be prevented from blocking diffusion
openings, for example by controlled adaptation of the
ratio of the diameter of the openings to the surface
tension of the paraffin-based latent heat storage
material, so that sufficient diffusion openings are
always kept clear. In the example shown, the sheath is
a plastics film, but it is also possible to use films
or foils made from other suitable materials.
The latent heat storage body illustrated in
Figures 1 and 2 can be introduced into a microwave
which is then set in operation. The power level and
duration of action of the microwaves on the product are
dependent on the size and thickness, the desired
temperature and the intended heating time for the
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product. Depending on the parameters selected, the
latent heat storage body has been heated sufficiently
uniformly, after a few minutes, for all the paraffin
contained therein to have melted.
Figure 3a shows an enlarged view of some of the
individual support-material bodies 5 contained in the
latent heat storage body shown in Figures 1 and 2, with
paraffin-based latent heat storage material 6 stored in
capillary holding spaces in these bodies; the grains of
hygroscopic material 7 which are distributed uniformly
on the surfaces of the individual support-material
bodies 5 can be seen clearly. In further detail, the
dots indicated illustrate that, in a regenerated state
of the latent heat storage body 1, the microwave-active
water 8 is stored within the grains of hygroscopic
material 7.
Starting from this state, Figure 3b describes
how, even a .short time after the microwave 10 has been
switched, on, the microwave radiation 11 penetrating
into the latent heat storage body 1 leads to
evaporation caused by heating and consequently to the
water 8, initially in part, escaping from the
hygroscopic material 7. The escape of vapour from the
hygroscopic material is symbolically represented by the
dotted lines of escape. It can readily be seen in
Figure 3b that the heated water 8 in vapour form is
distributed in cavities 12 between the individual
support-material bodies and the hygroscopic material 7.
In the process, it passes over the surfaces of the
individual support-material bodies 5 or the latent heat
storage material 6, which is at a low temperature as a
result of being microwave-passive. The microwave
radiation 11 penetrating into the latent heat storage
body 1 leads to a uniform transfer of heat from the
water 8 in vapour form to the paraffin-based heat
storage material which is stored in the individual
support-material bodies 5 and is initially still at a
cold temperature. It can also be seen from Fig. 3b that
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the evaporation and the escape of vapour from the
hygroscopic material 7 causes the levels of water 8 in
the hygroscopic materia7_ 7 to be depleted compared to
the regenerated state illustrated in Figure 3a. This is
indicated by the fact that the dots inside the
hygroscopic material 7 are at a greater spacing from
one another. Furthermore, the transfer of heat from the
heated water 8 in vapour form to the relatively colder_
surfaces of the individual support-material bodies 5
filled with latent heat storage material 6 causes
partial condensation of the water 8 in vapour form,
with the result that water droplets 12 are formed on
the abovementioned surfaces and heat transfer is
promoted still further. As a result of this excellent
heat transfer, the paraffin contained in the latent
heat storage material 6 is melted quickly and
uniformly, and the latent heat storage body is heated
uniformly.
As illustrated in Fig. 3c, the water of
condensation, which is distributed in extremely fine
form and is delimited as droplets 12, is heated again
by the incoming microwave radiation, so that ultimately
the droplets are vaporized again, a cycle which is
repeated a number of times. At t:he same time, the
hygroscopic material 7 is being constantly heated
further without vapour condensing thereon. As a result,
the water vapour which has formed can take full effect
without being prematurely bound back into the
hygroscopic material 7. When the heating process has
ended and the condensation of the water vapour
progresses, the hygroscopic material 7 begins to bind
in water.8 again and to prepare this water for the next
heating operation.
If excessive water vapour is formed, for
example in the event of the microwave being incorrectly
operated or as a result of lack of care, the water
vapour which has already formed heats up to an ever
increasing extent and, in the latent heat storage body
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1 illustrated in Figure 2, escapes through the openings
9 of the microperforations, out of the sheath 2' which
is permeable to vapour diffusion, into the environment.
Because of the high binding forces, the remaining water
8 is expelled by heating ever more slowly, so that
there is no possibility of rapid or explosive evolution
of vapour (e. g. on account of retardation of boiling).
The water 8 which has escaped into the environment in
vapour form through the perforations is compensated for
again from the atmospheric humidity through the
perforations in the opposite direction by the
hygroscopic material 7 on account of diffusion
processes. This regeneration process always takes place
reproducibly and without hindrance.
If, in the extreme case, the microwave will no
longer switch off at all, the water 8 stored in the
hygroscopic material 7 is gradually expelled entirely
by heating. As soon as the water vapour has been
volatilized through the openings into the environment
(at temperatures of greater than 100°C), from this time
onwards microwave activation is no longer possible and
no further heating takes place. If any residual
moisture is still present, the risk of fire caused by
overheating to an impermissible extent is in addition
practically ruled out on account of the water vapour
atmosphere and residual water of crystallization which
is present, since the temperatures can rise to at most
200°C (temperature at which water of crystallization is
heated out of copper sulphate) and, secondly, the water
which is present (including in vapour form) serves to
"swallow" the ignition energy. During subsequent
cooling, the hygroscopic material 7 once again loads up
with water 8 from the atmospheric humidity, and after
some time (which depends on the atmospheric humidity
and the temperature), the concentration deficit of
water 8 which arose during heating is compensated for
once again, and the latent heat storage body has
automatically regenerated itself.
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The principle described in Figures 3a, 3b and
3c can also be extended, for example, to sheets,
pastes, mouldings and shaped parts of all kinds. for
example, it is also possible to produce heat-retention
elements, for example in the foodstuffs sector, which
do not firstly have to be heated for a prolonged period
in electric or steam ovens, but rather can be prepared
for use very quickly in a microwave appliance. The
result is a lower energy consumption and, furthermore,
lower holding capacities being required.
Figure 4 shows a sectional view of a latent
heat storage body 13 according to the invention which
contains a support-material body 7.4 in which
paraffin-based latent heat storage material 6 is held
in capillary holding spaces. In the specific example
shown, the support-material body 14 is a fibreboard
made from PAP material, reference also being made to
the content of PCT/EP98/01956 with regard to further
suitable carrier materials. The surface of the
support-material body 14 is covered by a film 15 which
contains the hygroscopic material 7. The filfri 15 may
itself be formed, for example, from a hygroscopic
material 7, but alternatively, or in combination, may
also be occupied by or coated with a hygroscopic
material 7. As shown in the cross section, the film 15
may be provided over the entire surface of the
impregnated support-material body 19, but alternatively
may also be disposed only in certain regions of t:he
surface and/or may have openings which are permeable to
vapour diffusion. The latent heat storage body 1
illustrated furthermore has a sheath 2, which in the
specific example shown is impermeable to vapour
diffusion and, by means of spacer elements (not shown),
is disposed at a distance from the support-material
body with the film 15, so as to form a gas-filled
interspace. In the state of the latent heat storage
body 13. described in Figure 4, the microwave=active
water 8 contained therein, following prior microwave
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heating, is still partially in the form of liquid water
8' stored in the film 15, partially in the form of
water_ 8" in the vapour phase which is stored in the
gas-filled interspace 16, and partially in the form of
liquid water 8"' which is condensed out of the vapour
phase on the sheath 2 which is impermeable to vapour
diffusion. The way in which the "closed system"
illustrated operates is based on evaporation of the
microwave-active water 8, 8', 8" , 8" ', which is
brought about by microwave energy, and subsequent
transfer of the heat from the vapour to the
microwave-passive and therefore initially colder latent
heat storage material 6. For this purpose, the
high-energy vapour may bring about heating of the film
15, which for its part transfers the heat to the latent
heat storage material 6 stored in the support-material
body 14. As an alternative, or in addition, the
high-energy vapour may come into direct contact with
the latent heat storage material 6 through openings in
the film 15 which are permeable to vapour diffusion or
via surface regions of the support-material body which
are not covered by the film 15, with the result that
heat transfer can ,take place particularly quickly.
Furthermore, it is possible for the latent heat storage
material 6 to have a modified crystal structure,
including with hollow structures, such as for example
hollow cones, brought about, for example, by additives,
providing flow paths with additional heat-exchange area
for the vapour, so that the heat transfer is
additionally accelerated. One advantage of the "closed
system" illustrated is that even after use in an
extremely dry external environment, it regenerates
itself quickly and can be used at virtually any time,
since the water 8, 8' , 8" , 8" ' which is present i.n
the system does not all have to be stored in the
hygroscopic material 7 in order for the system to be
used. Furthermore, a "closed system" requires only very
small amounts of microwave-active water, and for
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numerous applications - such as for example heat
cushions - even a few drops of water are sufficient for
rapid and uniform heating through the
evaporation/condensation processes, with equilibrium
states being reached very quickly. Moreover, the very
small amounts rule out unacceptable expansion and
therefore destruction of the microwave-active and
microwave-inactive material. As an alternative to the
film 15 illustrated in Figure 9, other suitable carrier
materials for the hygroscopic material 7 include woven
fabrics, knitted fabrics, braided fabrics, fibres and
papers made from microwave-active, and, if appropriate,
capillary materials, which preferably provide good
moisture, conduction (for example blotting paper). The
hygroscopic material 7 illustrated as a layer in
Figure 9 may, for example, be a layer of hygroscopic
powder or granules or .fine grains.
In Figure 5, there is shown a latent heat
storage body 17 which differs from the latent heat
storage body 13 shown in Figure 4 in that it has a
closeable opening 18. In the specific 'exemplary
embodiment shown, this opening is formed as a tab 19
which is made from, film material and can be pivoted
about a bending edge 20 of the sheath 2, the sheath
being impermeable to vapour diffusion. In the closed
position of the opening 18 which is shown in solid
lines, in the example illustrated an angled-off tab
section .21 engages over the outer side of the sheath 2
on the top side of the latent heat storage body 17,
adjoining the opening 18. A connection between the end
of the tab and the outer sheath 2 which is able to
withstand the vapour pressures which are permitted in
operation is created by a large-area closure which has
a high load-bearing capacity, for example by a
hook-and-loop connection. Together with the top outer
surface of the sheath 2, the seal 23 which is
integrated into the tab section 21 produces a
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connection which is at the same time impermeable to
vapour diffusion.
When the opening 18 is closed, the latent heat
storage body 17 illustrated in Figure 5, like the
latent heat storage body 13 shown in Figure 4, can be
used as a "closed system". In this case, however, there
is the additional option, by means of a planned
configuration of the large-area closure 22, in
particular by selecting a suitable closure principle
and/or suitable surface dimensions, of providing an
additional device protecting against undesirably high
vapour pressures in the interior of the 7_atent heat
storage body 17. If a suitable limit for the closure
force is provided, the large-area closure 22 is
automatically opened when a critical vapour pressure is
exceeded, so that the vapour escapes into the
environment and destruction of the latent heat storage
body is prevented. Even if the opening 18 is not opened
automatically, after the latent heat storage body has
been used the opening can be opened manually, in order
to effect a change, in particular an increase', in the
amount of microwave-active moisture contained therein.
For this purpose, it is also possible for tile latent
heat storage body 17 to be introduced into a microwave
with the opening 18 open, together with an amount of
water 8 which is held, for example, in a dish, and for
the microwave to be switched on. The water 8 which
evaporates from the dish is initially distributed in
the vicinity of the latent heat storage body 17 and
then passes through its opening 18 into the gas-filled
interspace 16, from which it is taken up in a desired
quantity by the hygroscopic material 7. Alternatively,
it is also possible for the latent heat storage body 17
to be used as an "open system", with the opening 18
continuously open.
In Figure 6, there is shown an arrangement
comprising a distribution body 29 and a container 26
which is connected thereto by means of a line 25 and
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contains water 8. The line 25 can be blocked or opened
for water to flow through by means of a valve 27. As is
also shown, grains of hygroscopic material 7 are
disposed in distributed manner on the distribution body
29. Furthermore, the distribution body 24 has capillary
spaces which open up or form paths to the hygroscopic
material 7 in the distribution body. Preferably, it is
provided that the capiJ_lary spaces are formed i.n such a
manner that they have a capillary action only for the
microwave-active water 8, but not f.or the latent heat
storage material 6, which is of_ a higher viscosity. The
distribution body 24 may preferably be in the form of a
"capillary network", in which the capillary spaces are
connected to one another in the manner of a network.
When the valve 27 is open, the water 8 is initially
distributed in the distribution body 24 approximately
in a star shape starting from the mouth of the line 25,
as a result of the capillary action, as illustrated by
the arrows. The incoming flow of water 8 only comes to
a standstill when there is no longer any concentration
gradient in the distribution body. Furthermore, the
hygroscopic material 7 disposed on the distribution
body 24 takes up water from the capillary spaces of the
distribution body 24 until its saturated state has been
reached.
Figure 7 describes a latent heat storage body
28 in which an arrangement as shown in Figure 6 is
incorporated. In the specific example shown, the
distribution body 24 is situated between two individual
support-material bodies 29 which are in panel form and
are spaced apart from and parallel to one another,
containing paraffin-based latent heat storage material
6 held in capillary-like holding spaces. The latent
heat storage body 28 is also surrounded by a sheath 2'
which is permeable to vapour diffusion and through
which the line 25 enters the interior of the latent
heat storage body from the vessel 26. In operation of
this latent heat storage body, the water 8 stored in
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the distribution body and in the hygroscopic material 7
is at least partially vaporized and, in the process,
flows along and, at the same time, releases heat to,
the latent heat storage material 6 and those surfaces
of the panels 29 which face towards the distribution
body 24. If, in addition, cavities are provided in the
latent heat storage material 6, there is also flow
through these cavities, thus accelerating heat
transfer. The excess steam escapes into the environment
through the openings 9 (not shown in the drawing) in
the sheath 2' which is permeable to vapour diffusion,
so that the latent heat storage body 28 gradually loses
water 8 while being heated rapidly and uniformly.
During the subsequent cooling process, the water 8 in
vapour form which is still present in the latent heat
storage body is preferably taken up by the hygroscopic
material 7. The loss of water which has occurred
compared to the starting state can be completely or
partially compensated for by opening the valve 27.
Compared with the embodiment illustrated, it is also
possible for the distribution body 24 itself to have
hygroscopic properties, so that it is possible to
dispense with providing separate hygroscopic material 7
on the distribution body 24.
The arrangement shown in Figure 8 also differs
from that shown in Figure 6 in that there is no
hygroscopic material 7 provided on the distribution
body in this case. This may be appropriate even if the
distribution body itself is not formed from a
hygroscopic material 7, but rather - as shown in
Figure 9 - hygroscopic material 7 is provided
distributed between the adjacent individual
support-material bodies 5 with latent heat storage
material 6 held therein. In the latent heat storage
body 30 shown in Figure 9, the individual
support-material bodies 5, which have sucked themselves
full of latent heat storage material 6, with the
hygroscopic material distributed between them, are
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formed into panels 29, between which the distribution
body 24 is located. '1'he bodies formed from hygroscopic
material 7 are enclosed by a film which is not shown in
the drawing but i.s impermeable to latent heat storage
material 6, the film containing a number of tiny holes
which are just visible macroscopically.. This results,
on the one hand, in the hygroscopic material 7 being
partitioned from the latent heat storage material 6, so
that the latter cannot penetrate into the pores of the
hygroscopic material 7. On the other hand, however, it
is possible for_~ moisture, in particular water, which
can be stored by the hygroscopic material 7 to pass
through the tiny holes in the film, so that the
hygroscopic material 7 can release moisture t.o the
environment or can take up moisture from the
environment. Due to the three-dimensional distribution
of the hygroscopic material 7 illustrated, automatic
regeneration, after heating is assisted by moisture
being taken up from the environment through the
sheath 2' which is permeable to vapour. diffusion.
In Figure 10, there is illustrated ~a latent
heat storage body 31 in which a continuous layer of
hygroscopic material 7 is disposed around a core region
of microwave-passive or microwave-active paraffin-based
latent heat storage material 6. Because of the
microwave-active moisture which is stored in the
hygroscopic material 7 and is not shown in the drawing,
microwave activation of the latent heat storage body 31
is achieved. As a reversal of this principle, Figure 11
shows a latent heat storage body 32 which has a core
region of a hygroscopic material ~7 inside the latent
heat storage material 6. In the exemplary embodiments
illustrated in Figures 10 and ll, it is also possible
for the latent heat storage material to be held in
capillary holding spaces in a support-material body.
Furthermore, consideration is given to using a
relatively large number of the latent heat storage
bodies illustrated in Figures 10 and 11 as partial
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latent heat storage bodies, by holding a plurality of
such partial bodies together in a latent heat storage
body of larger dimensions.
Figure 12 shows a sectional view of a heat
storage device 33 which is formed i_n a sandwich
structure from two heat storage elements. 34, 34' which
extend in panel form perpendicular to the plane of the
drawing and consist of heat storage material, and a
hygroscopic material 7 which is disposed between these
heat storage elements 34, 39' in the form of an
interlayer. In the exemplary embodiment illustrated,
the cohesion of the layer composite is based on the
heat storage device, as shown, ~ being disposed
horizontally and on the force of gravity acting
perpendicular thereto. As an alternative, cohesion can
also be assisted or brought about by fastening means,
the selection of which depends on the specific
materials used. If the heat storage elements 34, 39'
are, for example, plastics sheets and the hygroscopic
material 7 is blotting paper or a hygroscopic nonwoven,
cohesion may be brought about by adhesive- bonding
between the layers in certain areas or over the entire
surface. In a variant, it is possible to provide for
the heat storage elements 34, 34' to consist of wood,
for example of poplar wood, and for a salt in powder or
granular form to be used as the hygroscopic material 7.
In this case, i.t is possible to ensure that the layers
are held together by positive connecting elements, for
example rivets, which pass through them. In the
exemplary embodiment illustrated in Figure 12, the heat
storage elements 34, 34' are formed from a
water-impermeable material through which microwave
radiation can pass. In a microwave field indicated by
microwave beams 11, the microwave beams penetrate into
the hygroscopic material 7 through the heat storage
elements 34, 34' and, in a smaller amount corresponding
to the area ratio, also via the end faces. The- water
which is stored in the hygroscopic material 7 and is
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not illustrated in the drawing is heated through
dielectric losses and transmits tHis sensible heat to
the hygroscopic material 7 and, both directly and
indirectly via the hygroscopic material, to the
adjoining heat storage elements 34, 34'. For this
purpose, the heat storage device illustrated in Fig. 12
has the hygroscopic material disposed in such a way
with respect to the heat storage elements or heat
storage material that it is specifically ensured that
heat is transferred rapidly and without hindrance from
the heated water or heated hygroscopic material 7 to
the heat storage elements 34, 34', which are still at a
lower temperature, by thermal conduction as a result of
a large contact area being provided between the
individual layers. To a certain degree, the thermal
conduction is assisted by convective heat transfer as a
result of the water vapour formed during the heating
flowing through the hygroscopic material to the
surfaces of the heat storage elements 34, 34'. To a
certain extent, heat transfer also takes place through
heat being dissipated by radiation to the heat storage
elements. 34, 34' which are at a lower temperature. The
evaporation of the, water stored in the hygroscopic
material 7 caused by microwaves or heating is
associated with an increase in the volume of the water.
The increase in the volume leads to the pressure of_ the
water vapour in the cavities of the hygroscopic
material 7 rising, providing considerable impetus to a
flow of .water vapour towards the side edges 35, 35' of_
the heat storage device. The pressure drop leads to
water vapour escaping at the side edges 35, 35', as a
result of which the level of water vapour in the
hygroscopic material 7 temporarily drops. When the heat
storage device 33 is subsequently cooled, the
hygroscopic material 7 is able to extract atmospheric
humidity from the environment via its free surfaces at
the side edges 35, 35' . As a result of a corresponding
incoming flow 37 of water vapour, the loss of water is
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compensated for again, the water which is initially
taken up at the edges also passing into the interior_ of
the layer formed from hygroscopic material 7 as a
result of diffusion. After a certain time, tree
hygroscopic material 7 once again has an equilibrium
level of microwave-active moisture with,respect to the
environment, and the heat storage device 33 has been
fully regenerated. It is then available for further
heating operations in a microwave field.
Figure 13 describes in a sectional view a heat
storage device 38 which differs from the heat storage
device 33 shown in Figure 12 in that cavities 39 are
formed in the panel-like heat storage~elements 34, 34',
which cavities each extend continuously between the
inner surface 40, facing towards the hygroscopic
material 7, and the outer surface 41, which exchanges
moisture with the environment, of the same heat storage
element in each case. In this respect, it is to be
derived symbolically from the drawing that the cavities
39 form flow paths for water vapour between the
hygroscopic material and the environment. The escape 36
of water vapour and the incoming flow 37 of water
vapour are in this way intensified and take place in a
more uniform distribution along the surface of the
hygroscopic material 7. The result is shorter diffusion
paths and diffusion times of the water or the
microwave-active moisture used in the hygroscopic
material.7, so that it is advantageously possible for
the heat storage device to be regenerated more quickly
after it has been used in a microwave field. The
cavities 39 may be provided in a regular or irregular
two-dimensional distribution within the plane extending
perpendicular to the plane of the drawing.
In Figure 14, there is illustrated in a
sectional view a third embodiment of a heat storage
device 42 according to the invention, which likewise
contains a hygroscopic material 7 disposed suitably for
heat transfer to the heat storage material. Unlike the
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exemplary embodiments shown in Figures 12 and 13, the
heat storage device 42 has only a single heat storage
element 34 formed from heat storage material. This
element is connected over a large surface area to a
layer formed from hygroscopic material 7, in order in
this way to allow unimpeded transfer of.heat from the
water or water vapour which has been heated by
microwave radiation 11 and is not shown in the drawing
to the heat storage element 39 and from the water or
water vapour, via the hygroscopic material 7, to the
heat storage element 34. The omission of a second heat
storage element 34' results in a large exposed
regeneration surface 43. According:Ly~ compared to the
heat storage device 38, it is possible for the
hygroscopic material to be regenerated even more
quickly, and this process can be made still more rapid
by, in addition, a controlled increase in the partial
pressure of the water vapour in the environment.
In Figure 15, there is shown in a sectional
view a fourth embodiment of a heat storage device 44
with hygroscopic material 7 and a heat storage element
34, which differs from the heat storage device 42 shown
in Figure 14 through the presence of an additional
elastic or rigid, pressure-resistant sheath 45. In the
exemplary embodiment illustrated, the sheath 45 is
formed to be impermeable to water vapour, so that in
the event of heating and vaporization of the water
which is not shown in the drawing and is stored in the
hygroscopic material 7, brought about by microwave
radiation 11, it is not possible for any moisture to be
lost from the heat storage device 99. The moisture
which escapes from the hygroscopic material 7 when the
latter is heated is held by the storage space 46
enclosed. between heat storage element 34 and
hygroscopic material '7 by the sheath 45, so that the
hygroscopic material 7 can be regenerated rapidly from
this storage space. Alternatively, it is possible for
the sheath 95 to be formed to be permeable to vapour
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diffusion, so that it is possible for moisture to be
exchanged with the environment. With regard to the
selection of materials, and further possible
configurations of the sheath 45, reference is made to
the further description of these aspects contained in
this patent application.
Figure 16 shows a perspective view of a heat
storage device which is provided in the form of a
container and has a heat starage material made from
poplar wood, which on its own cannot be heated to a
significant extent by microwave radiation. The heat
storage device 47 is formed from a base element 48,
four side elements 99 and a cover elerrlent 50. The cover
element 50 is pivotably attached toy one of the side
elements 99 by means of a rotary hinge 51. 1'he
dimensions of the heat storage device 97 are selected
in such a way that this device can preferably be used
as a heat-storing container for a pizza or the like.
Figure 16a uses a partial section through the
base element 48, along section line XVI-XVI in Figure
16, to illustrate the structure of this base element in
detail. Accordingly, the base element 98 furthermore
comprises a continuous, sheet-like glass body 52, the
plane of which run s perpendicular to the plane of the
drawing and which in the specific example illustrated
is formed as a planar pane of glass. Heat storage
elements 39, 34' made from poplar wood are provided
adjacent to and in contact with the principal surfaces
52', 52" of the glass body 52, which are parallel to
the plane of the sheet. The cohesion between the layers
is provided by an adhesive bond which is not shown in
the drawing and comprises an adhesive which is
permeable to microwave radiation. In a microwave
field, the microwave radiation 11, which is illustrated
symbolically and not to scale, in particular with
regard to its wave form, penetrates into the glass body
52 through the heat storage elements 39, 34' made from
poplar wood. In the process, the mir_rowave radiation 11
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is deflected and is shuttled back and forth in the
interior of the glass body 52 through repeated
reflection at the peripheral edge 53. In the example
shown, the illustrated length L of the glass sheet
should correspond to at least half the wavelength of
the microwave radiation 11 used. Consequently, in the
direction of extent of the length L, the conditions
required for the formation of a standing wave from the
microwave radiation 11 introduced are fulfilled. The
standing wave leads to accelerated conversion of wave
energy into thermal energy and as a result to the glass
body 52 being heated. As a result of the large contact
surfaces 52', 52" , the heated g~ass body 52 is
associated with the microwave-passive heat storage
elements 34, 39', which are colder in relative terms
and are made from poplar wood, in such a manner that
heat can flow into the heat storage elements virtually
without obstacle. This leads to the desired heating of
the microwave-passive heat storage elements in the
microwave field. With regard to Figure 16, it is
pointed out that the width B of the base element 48
preferably also corresponds to at least half the
wavelength of the microwave radiation 11, resulting in
the formation of a two-dimensional standing wave in the
glass body 52 and even more rapid conversion of wave
energy into thermal energy. In the case of the heat
storage device 47 shown in Figure 16, consideration is
also given to the possibility of the side elements 49
and the cover element 50 having the structure
illustrated in the sections 16a or 16b. In Figure 16,
the edge sides of the side elements 49 and of the cover
element .50 are each provided with a covering 54 which
may, for example, be strips of poplar wood or of strips
of an adhesive film or foil.
Figure 16b shows in a partial section along
line XVI-XVI in Figure 16 a second preferred embodiment
of the base element 48 or the side elements 99 - and of
the cover element 50 of the heat storage device 47.
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According to this, it is provided that a multiplicity
of sheet-like glass bodies 55 are arranged with their
side faces adjoining one another, so that the common
principal plane extends perpendicular to the plane of
the drawing. As shown in more detail, a coating 56
which has a temperature-dependent . transmission
coefficient for microwave radiation 11 is in each case
applied to the common top side 55' and the common
underside 55" of the glass bodies 55. Furthermore, the
outer edges 58 and the abutting edges 59 of the glass
bodies 55 are made almost completely reflective for
microwave radiation incident thereon from the interior
of the glass bodies by means of a surface treatment. A
heat-conducting sheet made from a thin aluminium foil
with a good thermal conductivity is adhesively bonded
to each of the principal outer surfaces of the coatings
56. For their part, the principal outer surfaces of the
heat-conducting sheets 57 are adhesively bonded over a
large area to heat storage elements 34, 34' made from
heat storage material. In the exemplary embodiment
illustrated, the heat storage elements 34, 34'- consist
of poplar wood and, like the heat-conducting sheets,
are permeable to microwave radiation 11. By contrast,
it is provided that the coating 56 is permeable to
practicaJ.ly all the microwave radiation 11 at a low
starting temperature and that, as the temperature
rises, there comes about a reduction in permeability.
Working on the basis of an arrangement as shown in
Figure 16b which has not yet been heated, in a
microwave field microwave beams 11 penetrate through
the heat. storage elements 39, 39', the heat-conducting
sheets 57 and the coatings 56 into the glass bodies 55,
with the microwave radiation 11 being deflected. As a
result of the reflective nature of the inside
peripheral surfaces 58 and abutting edges 59, the
microwave beams 11 which have been introduced into the
glass bodies 55 are preferably shuttled back and forth
in directions which are parallel to the plane of the
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sheet. In this case, it is provided that the length L'
of the glass bodies 55 in each case corresponds to half
the wavelength of t:he microwave radiation 11, and this
may also be the case in the direction of extent
extending perpendicular to the pJ_ane of the drawing.
With regard to the diagrammatically illustrated
microwave radiation, it should be noted that this
radiation is not to scale in terms of the wavelength
and wave amplitude compared to other dimensions shown.
In this way, a standing wave is developed in each
individual glass body 55 from the microwave radiation
11 introduced. As a result of wave energy being
converted into thermal energy in the~glass bodies 55,
the latter become heated, while the heat storage
elements 34, 34' which are made from a microwave-
passive heat storage material, in the specific example
from poplar wood, do not experience comparable heating.
The corresponding temperature drop causes thermal
conduction from the glass bodies 55, through the
coatings 56 and the heat-conducting sheets 57, into the
heat storage elements 34, 39', so that the latter also
become heated in the microwave field. If the microwave
radiation 11 is emitted by the radiation source with a
spatially uneven radiation intensity, the adjacent
glass bodies 55 may be heated unevenly. The temperature
difference which results from this is also compensated
for by the heat-conducting sheets 57 provided. As the
heating of the glass bodies 55 increases, the
temperature of the coatings 56 also rises. As a
reaction to this, the ability of the coatings 56 to
allow microwave radiation 11 to pass through them
decreases, so that the introduction of this radiation
into the glass bodies 55 is reduced and further heating
takes place more slowly. FinalJ.y, at a desired maximum
temperature, the coatings 56 are practically
impermeable to microwave radiation 11, so that there is
no further heating of the glass bodies 55 and therefore
of the heat storage elements 34, 34' made from
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microwave-passive material in the microwave field. The
result is a self-regulating systern which terminates the
heating when the set parameters are reached.
Fig. 17 shows a perspective view of a heat
storage device in the interior of a cooking chamber 60
of a microwave oven, which is not illustrated in more
detail in the drawing. A heat cushion 62 which contains
heat storage material is disposed on a turntable 61 in
the cooking chamber 60. A microwave emitter 64 is
integrated into the ceiling 63 of the cooking chamber
60, emitting microwave radiation 65, 65' which is
symbolically indicated as a wavy line. A comparatively
short lateral spacing between the w~vy lines of the
microwave radiation 65 indicates that a high radiation
intensity is achieved in this region of the cooking
chamber, while the relatively larger lateral distance
between the wavy lines of the microwave radiation 65'
illustrates a correspondingly lower field intensity.
The intensity of the microwave radiation 65 is above a
desired mean intensity, while the microwave radiation
65' has a lower intensity than the desired mean
intensity. As illustrated in further detail, the heat
cushion 62 is situated in the central region of the
turntable 61. Microwave radiation 65 of undesirably
high intensity impinges as so-ca7_led primary radiation,
which is illustrated by solid wavy lines, on a partial
region of the heat cushion 62 which covers the centre
of the turntable. It becomes clear that this partial
region of the heat cushion 62 cannot be moved out of
the region of undesirably high radiation intensity even
as a result of the turntable 61 being rotated in the
direction of rotation D, so that in that region there
is a risk of the heat cushion 62 being locally
overheated and burning through. Furthermore, it can be
seen that in its region which lies to the right, as
seen in. the in the direction of viewing, the heat
cushion is exposed to microwave radiation 65' of lower
radiation intensity than desired, so that in that
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region the heating will be undesirably low unless the
radiation intensity is made more uniform. To provide a
remedy to this, in accordance with Fig. 17 a
homogenizing mask 66 is provided, which has glass
bodies 67, 68, 69, 70 of different shapes. The glass
bodies 67 have a pyramid-shape, the glass body 68 is
formed as a rhombus, the glass body 69 has the shape of
a hemisphere and the glass bodies 70 have an irregular
outer contour and are referred to overall as "glass
crunch". It can be seen that some of the primary
radiation from the microwave radiation 65, 65' impinges
on surfaces of the homogenizing mask 66 or the glass
bodies 67, 68, 69, 70 which are d~,stributed on the
turntable 61 and, from there, following diffraction
and/or scattering and/or reflection, i.s transmitted
onwards in another direction as so-called secondary
radiation, which is shown as a broken wavy line. It is
also possible for the secondary radiation which has
been deflected by the homogenizing mask 66 firstly to
strike one or more of the walls 71 or the ceiling 63 of
the cooking chamber 60 and, from there, to impinge on
the heat cushion 62 as secondary radiation. In
particular, it can be seen that some of the microwave
radiation 65 which has been diverted by the
homogenizing mask 66 passes as secondary radiation into
a region of the cooking chamber 60 in which otherwise
only or predominantly primary radiation from the
microwave radiation 65' of undesirably low radiation
intensity is present. In this latter region, the
secondary radiation from the microwave radiation 65
also impinges on the surface of the heat cushion 62 and
brings about additional heating to supplement the
primary radiation from the microwave radiation 65'
which is incident in that region. Consequently, overall
the homogenizing mask 66 makes the radiation intensity
in the cooking chamber 60 more uniform and leads to the
heat cushion 62 being heated more uniformly. -If the
radiation intensity distribution in the cooking chamber
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60 is known, for example from preliminary tests, it is
possible to dispense with a rotary movement of_ the
turntable 61 and for the glass bodies 67 to 70 of the
homogenizing mask 66 to be disposed preferably in the
region of relatively high radiation intensity of the
microwave radiation 65, in order to make the radiation
intensity more uniform in a controlled, temporally
constant manner. In this case, depending on the
particular application, it is possible to optimize the
desired heating effects by a controlled selection of
glass bodies 67 to 70 of suitable shape, size,
thickness and type and by suitably adapting the way in
which they are disposed and the heat~ng time, as well
as the adjustable heating power of the microwave oven.
Instead of the glass bodies mentioned above, it is also
possible, by way of example, to use plastics bodies
which, compared to glass, have the advantages of
flexibility arid a low price. If, instead of the heat
cushion 62 illustrated, a liquid, for example, is to be
heated in the cooking chamber as heat storage material,
it is also possible for the homogenizing mask to be
disposed inside and/or outside the heat storage
material.
Fig. 18 shows a perspective view of a heat
storage device which is disposed in a cooking chamber
60 of a microwave oven and has a body 62' of heat
storage material which is to be heated, with a second
embodiment of a homogenizing mask 72 and with an
absorption body 73 which is wrapped around the body
62'. The body 62', together with the absorption body
73, which in the present example is in the form of a
film, is disposed on a turntable 61. In the example
illustrated, the absorption body is a plastics film
which is.wrapped several times around the body 62' and
is held together on the body by means of a piece of
string 7Q. The plastics material of the absorption body
73 has a high dielectric loss index, and consequently
it experiences very intense heating in the microwave
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field illustrated comprising the microwave radiation
65, 65'. Due to the wrapping around the body 62' and
the associated direct contact, the heat stored in the
absorption body 73 is quickly transferred to the body
62' predominantly through thermal conduction, so that
this body is likewise heated particularly uniformly. A
further detail illustrates that the homogenizing mask
72, in its second embodiment, has a tight-meshed wire
grid 75 which is disposed in the principal direction of
incidence of the primary radiation of the microwave
radiation 65, 65', i.e. between the microwave emitter
64 which is integrated in the ceiling 63 of the cooking
chamber 60 and the body 62' . In the e~ample shown, the
wire grid 75 is supported by four wire rods 76 of equal
length, which extend perpendicular to the wire grid 75,
at a distance from the turntable 61 which is such that
the body 62' together with the absorption body 73 finds
space beneath the wire grid 75 without coming into
contact with the latter. It is essential to the
exemplary embodiment illustrated that the dimensions
and small mesh width, which brings about a screening
action, of the wire grid 75 completely prevent prirnary
radiation of the microwave radiation 65, 65' from
impinging on the heat cushion 62. This prevents
excessive local heating of the absorption body 73 and
of the body 62' which has heat storage material
contained therein and exchanges heat with the
absorption body. Rather, the desired uniform heating is
achieved by the primary radiation being deflected by
the wire grid 75 of the homogenizing mask 72 and
impinging on the absorption body 73 with an intensity
which has been made more uniform, preferably in the
lateral direction, as secondary radiation, in some
cases only after a number of directional changes at
walls 71 and/or at the ceiling 63 and/or at further
internal. fittings in the cooking chamber. As a result,
the absorption body is heated uniformly and transfers
its uniform heat to the body 62'. The exemplary
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WO 99/55795 - 68 - PCT/EP99/01809
embodiments mentioned above make it clear that the
homogenizing mask is an essential means for utilizing
any microwave fields with differing field intensity
distribution, and any desired heating effec~:s can be
achieved in particular in conjunction with an
absorption body.
The features of the invention which are
disclosed in the preceding description, the drawings
and the claims may be of importance for r_ealizat.ion of
the invention both individually and in any desired
combination. All features disclosed are pertinent to
the invention. The content of the disclosure of the
associated/appended priority documen~.s (copy of the
prior application) and the contents of PCT/EP93/03346
and of PCT/EP98/01956 are hereby also fully
incorporated into the disclosure of the present
application.
