Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
10~155
The invention relates to a heating device for
supplying heat to a heat user, comprising a closed space with-
in which one or more closed reservoirs are arranged which in
operation contain a heat-accumulating material which can be
heated by means of at least one primary heat source, the said
heat-accumulating material being adapted to act as a secondary
heat source for exchanging heat, via one or more heat-transmit-
ting reservoir walls, with a heat transport medium which is
present in the closed space and which transports heat from
the reservoirs to the heat user by an evaporation/condensation
cycle.
Heating devices of the kind set forth are known
from Canadian Patents 944,742 and 958,290.
The heat-accumulating material may be a material
which remains in the solid state in any operating condition
~for example, A1203; BeO; TiO, MgO2; SiO2), so that only
sensible heat is stored, or a material in which the storage
of hea~ is achieved mainly by utilizing the transition from
the solid to the liquid phase (melting heat). Examples of the
latter materials are: LiF; CaF2; SrF2; NaCl and other metal
salts or mixtures thereof.
If a plurality of reservoirs are present for the
heat-accumulating material, they may be arranged to be in-
dependent of each other or be connected in series with or
parallel to each other by way of one or more common filling
and/or discharge ducts.
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~ cjJ
Because the heating device is operated over a
wide temperature range (from room temperature to temperatures
beyond 1500C), the reservoirs cannot be completely filled,
because the fact that the volume of the heat-accumulating
S material increases as the temperature increases should be
taken into account.
The volume increase of melting heat-accumulating
metal salts generally amounts to 20% to 30%, going from room
temperature to operating temperature.
In practice, this fact is usually taken into
account by choosing a suitable filling degree for the
reservoir, even though other solutions are also feasible,
for e~ample, the use of an overflow reservoir for liquid
metal salt.
The following problem occurs in the known heating
devices.
In order to ensure that the evaporation/
condensation process of the heat transport medium ln the
closed space can properly taken place, this space is normally
evacuated. A substantial vacuum then prevails in the closed
space at room temperature. As the operating temperature of
the device increases, the vapour pressure of the heat
transport medium in the closed space strongly increases. This
means that the reservoir walls on the outside are subject to
strongly varying pressures, the largest pressure occurring
at the highest operating temperature.
Inside the partly filledreservoires however, usually
a very low pressure prevails, because the reservoirs are
e usually also evacuated after provision of the h~at-
i(~51~155
accumulating material and before closing, notably to prevent oxidation of theheat-accumulating material by oxygen present in the air.
As the temperature of the heat-accumulating material increases,
the pressure inside the reservoir remains constantly low. This i9 not only
applicable to solid heat-accumulating materials, but also to materials which
change over to the liquid phase when heated. This is because the vapour
pressures of the commonly used heat-accumulating metal salts in liquid form
are very low at the temperature levels occurring (below 1 Torr (= 1 mm
mercury pressure)).
As a result, the reservoir walls are subjected to strongly varying
and large mechanical stresses, because of the variable pressure difference
between the variable heat transport medium pressure in the closed space on
the one side, and the substantially constant, very low pressure in the
reservoir on the other side.
The reservoir walls must thus be thick, since they have to be
capable of withstanding the largest pressure differences occurring at the
highest operating temperatures. Thick reservoir walls, however, have a high
thermal resistance and lead to a heavy construction of the device.
The present invention provides an improved heating device in which
the described dra~backs have been eliminated in a simple manner.
The invention provides a heating device for supplying heat to a
heat user, comprising a closed space, at least one closed reservoir having
one or more heat transmitting walls being arranged in the closed space and
adapted to contain a heat-accumulating material, means for heating the heat-
accumulating material, and an evaporable heat transport medium in the closed
space for the transfer of heat from the heat-accumulating material to the
heat user, wherein an evaporable heat transport medium is also present in
each said reservoir for equalizing or substantially equalizing the pressure
in said reservoir and the pressure in the closed space outside of the said
reservoir, chemical reaction between the said heat-accumulating material and
the said evaporable heat transport medium in each said reservoirs being
avoided.
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105~5
Substantially the same temperature prevails on both sides of the
reservoir walls in any operating condltion. Because a quantity of heat
transport medium has also been added to the reservoirs, the vapour pressure
on both sides of the reservoir walls is also substantially the same at any
temperature. The reservoir walls may now be thin walls having a low thermal
resistance, which on the one hand reduces the weight of the device and makes
it cheaper, whilst on the other hand there exists improved transfer of heat
between the heat-accumulating material in the reservoirs and the heat trans-
port medium outside the reservoir in the closed space.
This is achieved without special aids (for example, a pressure
control device reacting to the pressure in the closed space) being used.
It may occur that the heat transport medium added to the reservoir
tends to enter into a chemical reaction with the heat-accumulating material,
so that the proper operation of the device might be disturbed.
In order to avoid such a possibility, a preferred embodiment of the
heating device according to the invention is characterized in that in each of
the reservoirs, a flexible, movable partition is provided between the heat-
accumulating material and the evaporable heat transport medium.
The partition may be, for example, a metal foil which is made~ for
example, of the same material as the reservoir walls.
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.l ri ~
The invention furthermore relates to a hot-gas
reciprocating machine, such as a hot-gas engine and a hot-
gas turbine, comprising a working medium which performs a
thermodynamic cycle in a closed working space, heat being
applied to the said working medium from without. According
to the invention, the hot-gas reciprocating machine is
provided with a heating device as described.
The invention will be described in detail hereinafter
with reference to the drawing which is diagrammatic and not
to scale.
Figure 1 is a longitudinal sectional view of a~-heating
device in which a small quantity of heat transport medium
is present above the heat-accumulating material in the res-
ervoir.
Figure 2 is a longitudinal sectional view of a heating
device in which the heat transport medium and the heat-
accumulating material inside the reservoir are separated
from each other by a flexible, movable partition.
Fig. 3 is a longitudinal sectional view of a hot-gas
reciprocating engine, comprising a heating device which is
equipped with three reservoirs for heat-accumulating material
The reference 1 in Fig. 1 denotes a closed ~ube which
bounds a closed space 2 in which a closed reservoir 3 is
arranged which is mainly filled with a heat-accumulating
material 4, for example, a mixture of the metal salts NaF
and MgF2. Heat can be applied to this meltable salt mixture
by means of the electrical heating elements 5 and 6 which
are connected in series in the present embodiment and which
comprise electrical supply wires 7 and 8 which are passed
through the wall of tube l.
The reservoir 3 comprises heat-transmitting
walls 3a-3b-3c wherethrough the heat-accumulating material
can exchange heat with a heat transport medium 9, for example,
sodium, in the closed space 2. The inner walls of tube l
and the outer sides of the reservoir walls 3a-3b-3c are
covered with a capillary structure lO, which is formed, for;
example, by one or more layers of metal gauze. Tube l
furthermore comprises a heat-transmitting wall ll, wherethrough
heat can be transferred to a heat user not shown. The
~-remainder of tube 1 is thermally ~nsulated from the surround-
ings by means of a thermal-insulating material layer 12.
During operation, when the heat-ac~umulating material 4
has been "charged" by the heating elements 5 and 6 as the
primary heat source, this material transfers heat as a
secondary heat source, via the reservoir walls 3a-3b-3c,
to heat transport medium 9 in space 2 which, consequently,
evaporates and flows in the vapour phase to the heat-trans-
mitting wall 11, because a lower temperature and pressure
prevail at this wall. The heat transport medium vapour
condenses on wall 11 while giving off heat thereto. The
condensate is subsequently returned to reservoir 3 through
the capillary structure 10 because of capillary forces
while utilizing the surface tension of the condensate, the
condensate subsequently spreading over the entire surface
of the walls 3a-3b-3c via the capillary structure. Complete
and uniform wetting of the reservoir walls is thus ensured.
At the start of the heat transport to the wall 11, when
the heat-accumulating material 4 in the reservoir 3 and
the heat transport medium 9 ln space 2 are both at their
highest and mutually substantially equal temperature, the
vapour pressure in space 2 also has its highest value. The
vapour pressure of the mixture of metal salts in the reser-
voir, however, ls then still below l Torr. In order to
ensure that at any temperature the pressures on both sides
of the reservoir walls 3a-3b-3c are equal or substantially
equal, a quantity of evaporable heat transport medium,
denoted by the reference numeral 9', has also been added
to reservoir 3. At any temperature level of reservoir
3 and space 2, this heat transport medium 9' produces a
vapour pressure which is equal to the vapour pressure of
heat transport medium 9 occurring at the relevant tempera-
ture level. Because of the negligibly small partial vapour
pressure of the heat-accumulating material 4, this means
that in substantially any operating condition substantially
equal operating pressures prevail on both sides of the res-
ervoir walls 3a-3b-3c. The said walls are thus constantly
subjected to a low load (only the weight of the heat-accumu-
lating material is of importance in this respect), so that
the reservoir walls are thin and represent low thermal
resistances in the heat exchange process.
The heating device shown in Fig. 2 comprises a tube 20
having a heat-transmitting wall 21, and a heat insulating
layer 22. Inside the tube 20 there is arranged a reservoir
23 having heat-transmitting walls 23a-23b-23c which
communicate, by way of a capillary structure 24, with tube
wall 21. Space 25 inside tube 20 again contains a quantity
of evaporable heat transport medium 26, in this
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case calcium. Inside reservoir 23 there is arranged a
heating element 27 as a primary heat source in heat-accumu-
lating material 28, in this case NaF, which is separated
from a quantity of evaporable calcium 29 by a thin metal
wall 30 which is connected to the inner wall of reservoir
23 along its circumference, but which is constructed to be
flexible such that the contraction and expansion of the
NaF can be comp~etely followed. Thus on both sides of the
partition 30 always the same pressure prevails. The
partition ensures that the calcium cannot react with the
sodium fluoride to form calcium fluoride and free sodium.
The operation of the device is the same as that of the
device dèscribed with reference to Fig. 1.
Fig. 3 shows a combination of ahot-gas engine and a
heating device.
Tube 40 bounds a closed space 41 in which on the one
s~de three reservoirs 42, 43 44 are arranged, and on the
other side the heater parts 45 of a hot gas engine 46, are
arranged.
The walls of tube 40 are covered on the inside with a
capillary structure 47, whilst the heat-transmitting walls
of the reservoirs 42, 43, and 44 are covered with a
capillary structure 47 on the outer side. Inside the
reservoirs a heat-accumulating material 48 is present, and
above this material a small quantity of evaporable heat
transport medium 49 is provided, the said medium being of
the same kind as the heat transport medium present in space
41 (not shown).
The reservoirs 42, 43 and 44 are in open communic~tion
with each other and are connected to a common filling and
discharging duct 50 which is closed by means of a valve 51.
For the primary heat source use is made of an electrical
heating element 52 which applies heat to heat accumulating
material 48 directly as well as by way of evaporation of
the heat transport medium in space 41. When this material
gives off heat to the heat transport medium in space 41
via the heat-transmitting reservoir walls, the heat trans-
port medium evaporates, flows to the heater pipes 45 and
condenses thereon while giving off heat through the heater
pipe walls to the working medium in the engine which flows
through these pipes (for example, helium or hydrogen). The
condensate formed on the heater pipes is returned, via the
capillary structure 47, to the reservoirs 42, 43 44 so as
to be evaporated again.
The heat transport medium 49 inside the reservoirs
42, 43, 44 again ensures that at~any operating temperature
the pressure level in the reservoirs is at least substan-
tially equal to that in the space 41.
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