Note: Descriptions are shown in the official language in which they were submitted.
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FIELD OF THE INVENTION
This invention relates to an evacuated solar
collector tube in which the vacuum is degradable under
certain conditions to limit the stagnation temperature
of the tube.
BACKGROUND_OF THE INVENTION
Collector tubes conventionally comprise an inner
glass or metal tube, through which a heat exchange fluid
is passed, and an outer glass tube enveloping at least a
portion of the length of the inner tube. The space
between the two tubes is evacuated and the outer sueface
of the inner tube is coated with a solar selective
surface coating. The selective surface coating is
chosen to provide for hlgh absorption of solae radiation
and low emittance of thermal radiation, so that the
collection of incident solar radiation is maximised and
the quantity of ene~gy lost by infra-red radiation is
minimised. A high level vacuum, in the order of 10
Torr surrounding the selective surface coating virtually
eliminates heat loss from the surface by conduction and
convection processes.
A selective surface coating which has been found to
be particularly efficient comprises a base coating of
copper and an outer iron-chromium-nickel-carbon cermet.
The copper is sputtered onto the tube by a non-reactive
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process and the metal-carbide coating is deposited by
sputtering the metal from a stainless steel electrode
onto the tube surface in the presence of a reactive gas
such as acetylene. The reactive sputtering process is
controlled such that the resultant cermet is graded from
a metal-rich coating at the interface with the copper to
a carbon-rich coating at the outside.
The combination of an efficient selective surface
coating and a high vacuum insulation results in high
quality collectors which stagnate (i.e., reach an
equilibrium condition where losses equal energy gain) at
temperatures as high as 300 C in non-concentrated
sunlight. Thus, the collector elements are particularly
useful in thermal systems wh;ch are designed to operate
at quite high temperatures. The collectors are also
very suitable for use in systems which are intended to
operate at relatively low temperatures, because the low
losfies and high collection efficiency permit high energy
collection under various conditions, including when
relatively heavy cloud cover exists.
However, in some low temperature systems, such as
those which are used for producing domestic hot water,
tubes which stagnate at very high temperatures (i.e., in
the order of 300 C) can create significant problems.
; 25 The systems must be designed to prevent the high
temperature fluids from entering the system reservoir,
where the heat may cause permanent damage to the
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reservoir lining. Moreover, the high temperature fluid
may pre~ent a potential safety hazard to users of the
system. Therefore, it is desirable under certain
circumstances to limit the capability of collector tubes
to reach very high temperatures, but any process which
is employed to do so should not effect the high
performance of the collectors at low temperatures.
One method of achieving high performance at low
temperatutes and of reducing the stagnation temperature
has been suggested in a paper entitled "Thermal
Conduct;on in Evacuated Concentric Tubular Solar Energy
Collectors Degraded by l.ow Pressure Gas" by G.L. Harding
and B. Window in Solar Energy Material 4 (1981) 421-434,
North-Holland Publishing Company. In page 434 of this
paper brief reference has been made to the feasibility
of using a gas in the evacuated space of a collector
tube to limit the stagnation temperature.
S~MMARY OF THE INVENTION
The present invention extends this basic concept by
proposing the employment of a gas which exhibits
hydrophobic characteristics, which adsorbs onto the
selective surface coating at low temperatures (e.g., at
temperatures less than about 150C) and which desorbs
into the otherwise evacuated space at higher
temperatures.
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Thus, the present invention provides a collector
element for use in a solar energy collector system and
which comprises an inner (glass or metal) tube into
which a heat exchange fluid may be directed, an outer
glass tube enveloping at least a portion of the length
of the inner tube, an evacuated space between the two
tubes, a solar selective coating in thermal contact with
the inner tube, and a gas ~which may comprise a mixture
of two or more gases) which is admitted into the
evacuated space for the purpose of limiting the
stagnation temperature of the collector element. The
gas is admitted to the space between the tubes following
evacuation of the space and before removal of the
collector element from a vacuum pump. The gas exhibits
hydeophobic characteristics and is selected to adsorb
onto the ~elective surface coating at temperatures less
than a predetermined temperature, to desorb into the
evacuated space at temperatures greater than the
predetermined temperature but to not adsorb
significantly onto the outer glass tube.
The ~olar selective coating may be deposited on a
plate which is in thermal contact with the inner tube,
or it may be deposited directly onto the outer surface
of the inner tube.
By appropriately selecting the gas or, possibly, a
mixture of two or more gases, the adsorption and
desorption of the gas (or gas mixture) onto and from the
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surface coating may be varied to meet the needs of
different systems. For example, in the case of a
domestic hot water system it may be decided that the
collector tube should stagnate at 80C whilst in another
system it may be desirable that the tube stagnate at
150C. With the approach which is taken by the present
invention, the high temperature performance of the
collector tube may be degraded significantly without
affecting the low temperature performance of the tube.
lo The temperature at which desorption commences and
the rate at which desorption proceeds is determined by a
number of factors, including the molecular structure of
the gas. It is important that the gas should not adsorb
to any significant extent onto the relatively cold outer
glass tube. This is achieved by use of a hydrophobic
gas, by which is meant a gas which exhibits a signi-
ficantly greater adsorption affinity to the selective
surface coating than to the glass from which the outer
tube of the collector element is formed. The outer
glass tube has an inner surface which presents to the
desorbed gas an atomic structure which is similar to
that of water, in the sense that it comprises many OH
radicals and a gas which exhibits a low interactive
energy wlth the glass may therefore be regarded as
hydrophobic. The gas preferably is selected such that
the selective surface coating is geometrically accom-
modating to the gas molecules, in the sense that a
packing affinity will exist between the molecules of the
gas and the surface coating.
In the case of a metal-carbide surface coating
which is composed predominantly of carbon at its outer-
most surface, it has been found that hydrocarbon gases
such as alkanes and aromatics are suitable. These
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species include the simple alkanes, such as pentane,
hexane and heetane, and their isomers, such as
isopentane. Suitable aromatics include benzene and its
der;vatives, for example toluene. Substituted side
group or groups may be chosen to permit an increase in
the interaction of the gas molecule with the selective
surface coating. The isomers and homologues of the
abovementioned species may also be suitable.
If it is found that the volume or surface area of
the selective surface coating is not sufficient to fully
adsorb the gas or gas mixture at low temperatures and
the vacuum is unacceptably degraded by free molecules at
the low temperatures, then one of two possible
approaches may be taken. The volume of gas which is
admitted to the evacuated space may be reduced, although
this may result st;ll in an unacceptably high stagnation
temperature, or an adsorbing agent may be located in the
evacuated space. Such agent may comprise, for example,
a layee of charcoal cloth oe a deposit of carbon which
20 i8 exce6sive to the requieements of the selective
fiurface coating. The cloth or carbon deposit may be
applied in a localised band to the outer sueface of the
inner tube, preferably at one end of the evacuated space
so as not to significantly reduce the collection
efficiency of the selective surface coating.
In most applications of the invention it is
desirable that a sharp transition occur between the low
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temperature, high efficiency and the high temperature,
low efficiency operating conditions, and it has been
found that this may be achieved by maintaining the solar
selective surface coating ;n close-spaced relationship
to the outer tube. However, if the size of the outer
tube is reduced to such an extent as to achleve the
close-spaced relationship over the entire area of the
surface coating, it is likely that normal manufacturing
tolerences will result in interference between the outer
tube and the substrate (i.e. the inner tube or a plate)
which carr;es the surface coating. Therefore, in one
embodiment of the invention it is proposed that the
selective surface coating be positioned adjacent the
outer tube by locating the inner tube eccentrically
within the outer tube. Alternatively, the inner tube
may be positioned concentrically within the outer tube
if the fiurface coating is deposited on a plate which is
shaped to lie adjacent the outer tube over at least a
portion of the area of the plate.
The invention will be fully understood from the
following description which is provided with reference
to the accompanying drawings.
BRIEF DESCRIPTION OP THE DRAWINGS
In the drawings:
Figure 1 shows a side elevation view a collector
element to which the invention has application,
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Figure 2 shows a graph of eesults obtained when
adding a hydrophobic gas to the evacuated space within
the collector element,
F;gure 3 shows a side elevation view of an
alternative collector element,
Fiqure 4 shows a vertical end elevation view of the
element which is illustrated in Figure 3, the view being
taken in the direction of sect;on plane 4-4 shown in
Figure 3, and
Figures 5 to 6 show sectional end elevation views
of alternative elements.
DETAILED DESCRIPTION OF THE INVENTION
As illustrated in Figure l, the collector element
comprises a single-ended ;nner tube 10 which normally
would be formed from glass but which could equally be
formed f~om metal if a different type of construction
were to be employed. An outer glass tube 11 envelopes
the inner tube and is sealed to the inner tube adjacent
its open end. An evacuated space 13 exists between the
two tubes and the outer surface of the inner tube 10 is
coated along a major portion of its length with a solar
selective surface coating 14. The coating may take
various forms but it preferably comprises a copper base
layer and a metal-carbide cermet outer layer as
hereinbefore described.
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In manufacture of the collector element, the
selective surface coating is first deposited on the
inner tube 10 and the inner tube is then assembled
(joined) to the outer tube 11 using conventional glass
fabrication ~echniques. Thereafter, the collector
element is connected to a vacuum pump and the complete
element i~ subjected to a high temperature bakeout
whilst the space 13 between the tubes is being
evacuated. The seace would normally be evacuated to a
pressure in the order of 10 Torr. The purpose of
this bakeout is to remove gas from the selective surface
coating 14, from the glass and from any other components
such as tube spacers (not shown) which are located
within the space 13.
The above described manufacturing technique is
applicable to prior art collector elements as well as
that of the prefient invention, and the bakeout process
is effected in order to remove gas which would give rise
to (uncontrolled) permanent degradation of the vacuum.
In prior art manufacturing techniques, the collector
element is permanently sealed after the bakeout and
before removal of the element from the vacuum pump.
However, in the manufacture of the collector
element of the present invention a small quantity of gas
such as benzene is admitted to the space 13 between the
tubes immediately following bakeout but before sealing.
Benzene, like other hydrophobic hydrocarbon materials
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adsorbs onto the solar selective sureace coating at
relatively low temperature6 (i.e., below loo C) but
desorbs ;nto the space 13 to degeade the vacuum at
elevated temperatures without adsorbing onto the outer
glass tube to any significant extent. The volume of
benzene which is admitted to the evacuated space 12 will
be dependant on the volume of the space itself, on the
effective surface area of the surface coating and on the
degree of vacuum degradation required. However. the gas
would normally be admltted in an amount sufficient to
reduce the vacuum level to a pressure in the order of
to 10 3 Torr when the temperature of the solar
selective surface coating is sufficiently high that most
of the gas is desorbed.
ln operation of the collector element, when the
fielective fiurface coating ;s at a low temperature, the
benzene i~ adfiorbed onto the selective surface coating
and very little would exist in a gaseous state. Thus,
the vacuum within the space 13 will exist at a high
level and heat losfies from the selective surface to the
fiurroundings will be very low. Therefore, the
performance of the tube will be similar to that of a
conventional evacuated tube.
However, as the temperature of the selective
2S surface coating increases, a proportion of the adsorbed
gas will be desorbed from the surface coating, without
adsorbing onto the relatively cold outer glass tube to
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any signi~icant extent and will result in de~radation of
the vacuum. At suf~iciently high pressures (greater
than 10 Torr) thermal conduction through the partial
vacuum w;ll give rise to a significant heat loss process
foe the collector. Such heat loss will limit the
max;mum temperature which the selective surface coating
can reach under stagnation conditions.
Then, when the temperature of the selective surface
coating decreases, either due to the absence of incident
solar energy or because heat is extracted from the
collector element, the gas will re-adsorb onto the
selective surface coating and the level of vacuum in the
tube will increase. Thus, the performance of the tube
will eeturn to its original high level.
The graph of Figure 2 shows curves of heat
conduction loss and radiation loss against temperature
for a collector element of the type shown in Figure 1.
The radiation loss increases at an insignificant rate
with increasing temperature and such loss is contained
as a function of the composition and structure of the
selective surface coating. However, it can be seen that
the conduction loss increases significantly with
increasing temperature above approximately 60C and
this results from desorption of the benzene gas and
degradation of the vacuum.
It normally would be desirable that the stagnation
temperature reduce sharply when the operating
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temperature increases above a predetermined level of.
say, 80OC and th;s may be achieved by constructing the
collector element~ with a small space between the tubes
or in one or other of the ways shown in Figures 3 to 6.
However. before referring specifically to these ~igures
it should be explained tbat heat transport in gases at
low ~ressures is characterised by two distinct domains.
At veey low pressures, the average distance moved by a
gas molecule between collisions with other molecules is
considerably greater than the dimensions of the
container enclosing the gas. ~ost of the coll;sions
made by molecules therefore occur with the container
walls. In an evacuated solar collector tube operating
at a very low pressure, gas molecules transport heat
from the hotter selective surface to the colder outer
glass tube and undergo a negligible number of collisions
with other gas molecules on the way. ~nder these
conditions it can be shown that the rate of heat
transport per unit area between the two surfaces H is
proportional to pressure p:
H = Kp(Ti-To)
where Ti and To are the temperatures of the inner and
outer surfaces respectively. In addition, it can be
shown that the rate of heat transport between the
surfaces is only weakly dependent on the geometry of the
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system and, in particular, is not strongly a~fected by
the average distance between the surfaces.
~ t relatively high pressures, the mean ~ree path of
gas molecules for molecule molecule collision is quite
small compared with distance between the surfaces.
Under these conditions, it can be shown that the rate of
heat transport between the surfaces is essentially
independent of pressure. The heat transport in a tube
operating under these conditions can be characterised by
a conventional thermal conductivity, K, of the gas, and
written as
H = ~ - ~T, or
Q
H = 2~ L ~T
Qn(r2/rl)
for plane parallel, and cylindrical geometry
respectively. In these expressions, A i8 the area of
the surfaces, and Qthe distance between them, for plane
parallel geometry; r2 and rl. are the radii of the outer
and inner cylinders, and L is the length of the
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cylinders, for cylindrical geometry. ~T i6 the
temperature difference between the surfaces.
As above ment;oned, in an evacuated solar collector
tube which is required to stagnate at a low temperatuee,
it is highly desirable that the transition from the high
vacuum, negligible loss case to the poor vacuum, larqe
conduction loss case occurs over as narrow a temperature
range as possible. ~ first requirement which must be
met if this is to be achieved is that the gas pressure
in the tube must increase rapidly as the temperature
increases. The gas pressure in the tube is determined
(among other things) by the rates at which gas molecules
desorb from, and resorb on the selective surface
coating. ~s a generalisation, the probability per unit
15 time of desorption of a molecule from a surface can be
written:
-EotkT
Ye = Yeo e
where Eo is an energy characteristic of the molecule and
the surface, and is referred to as the activation energy
for the adsorption/desorption process, and Yeo is a
constant.
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If this probability o~ desorption increa6es rapidly
with temperature it can be shown that the gas pressure
will also exhibit a similar strong temperature
dependence, at least in the low pressure domain. In
order to achieve a rapid increase of pressure with
temperature, it is therefore necessary to choose a
molecule and a surface with as high an activation
energy, Eo, as possible.
The dependence of pressure on temperature is
directly reflected in the temperature dependence of heat
losses from the tube at low pressures. As the pressure
increases, however, a point is reached at which the
mean-free-path for molecule^molecule collisions is
comparable with the distance between the hot selective
surface and the colder outer wall of the tube. At
higher temperatures, further increases in gas pressure
are not accompanied by more rapid heat transfer and as
previoufily mentioned, heat losses in this domain are
virtually independent of pressure,
In order to increase the sharpness of the
transition from efficient to inefficient collector
operatian, it is highly desirable for the evacuated tube
to operate in the low pressure domain since the rapid
variations in pressure with temperature are reflected in
rapid changes in heat loss. The range of pressures (and
temperatures) over which the tube operates in this
domain can be increased by increasing the pressure at
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which the mean-f~ee-path for molecule-molecule
collifiions become~ comparable with the container
dimensions. This can be achieved by reducing the
distance between the selective surface and the glass
wall of the vacuum vessel.
A decreased distance between selective surface and
outer glass tube has no signif;cant effect on the heat
losses at low ~ressures. Heat losses are effectively
independent of container dimensions in this pressure
range. It can also be shown that the qas pressure is
not strongly affected by container dimensions in this
range, at least in the situation when most of the gas
molecules in the system are adsorbed on the selective
surface (as occurs here). However, a decreased
surface-to-glass distance permits the temperature range
to be increased over which the pressure (and therefore
the heat losses) increase rapidly with temperature.
This ha~ the overall effect of decreasing the range of
temperatures between the point at which significant heat
losses commence and the point at which all the energy
ab~orbed by the tube is transferred by thermal
conduction through the gas.
Thus, the transition from negligible thermal losses
to maximum thermal losses can be made to occur over a
smaller range of temperatures if the gap between inner
and outer tubes is reduced. Reducing the gap has no
significant effect on the heat losses at low pressures.
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A tube with a smalle~ gap, however, exhibits a greater
thermal conductance in the high pres6ure region and
therefore dis6ipates energy under stagnation conditions
more effectively than a tube with a larger gap. ~ key
factor relating to the sharpness of the transition in
operating characteristics in the tubes is thus the gap
between the selective surface and the outer glass
envelope.
In a conventional type of evacuated collector tube,
such as shown in Figure 1 of the drawings, the minimum
size of gap between inner and outer tubes is determined
by dimensional variations of the glass tubes. These
tubes can vary in diameter, wall thickness and
fitraightness due to fluctuations in the drawing
process. In order to reduce the gap as required by the
prefient invention, it might be possible to specify tube
tolerances more closely than is conventionally done.
~lternatively, a comparative sizing operation might be
e~eected in order to match inner and outer tubes.
Finally a fitraightening operation might be formed on
elther or both tubes.
However, a significant reduction in the effective
gap between the inner and outer tubes may be achieved by
mounting tubes in a non-concentric manner, a shown in
Figures 3 and 4 of the accompany;ng drawings.
The collector element comprises a single-ended
inner tube 10 which would normally be formed from glass
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but which could equally be ~ormed from metal i~ a
different type of construction were to be employed. ~n
outer glass tube 11 envelopes the inner tube and is
sealed to the ;nner tube adjacent its open end ]2. An
evacuated space 13 exists between the two tubes and the
outer surface of the inner tube 10 is coated along a
major portion of its length with a solar selective
surface coating 14.
The significant factor to be noted in relation to
the ilustrated collector element is that the inner tube
is located eccentrically w;th respect to the outer tube,
so that a portion of the selective surface coating is
located closer to the outer tube than it would be if the
two tubes were disposed concentrically with respect to
one another. Since the thermal conductance at any point
of the tube is approximately inversely proportional to
the radial gap between the two tubes, the reduction in
heat flow at points where the gap is increased is more
than balanced by the increase at points where the gap is
decreased.
Pigures 5 and 6 of the accompanying drawings
illustrate alternative methods of implementing the
present invention. Figure 5 shows an end elevation view
of a collector element which has a small-diameter inner
tube or conduit 15 located eccentrically within a
relatively large outer glass tube 16, the space within
the outer tube and surrounding the inner tube being
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evacuated. An arcuately shaped substrate 17 is welded
or otherwise secured to the inner tube ]5 and the
substrate carries on lts outer surface a solar selective
surface coating 18. The substrate 17 extends arcuately
within the outer tube 16 and it extends longitudinally
for a substant;al portion of the length of the outer
tube.
The design shown in Figure 5 provides a very small
gap for heat transport between the inner and outer tubes
or, more particularly, between the selective surface
coating 18 and the outer tube 16 for heat transport. It
also has the advantage that, a substantially greater
area of selective surface is presented to absor~tion of
solar radiation than would be if the inner tube 15 alone
J.5 were to be coated on its outer surface with the
selective sur~ace coating.
The collector element fihown in Figure 6 also
compris0s a con~truction which has an inner tube l9
through which heat exchange fluid may be directed and a
slgnificantly larger outer tube 20 which defines an
evacuated space 21 surrounding the inner tube. However,
in cont~ast with the two previously described
embodiments, the collector element which is shown in
Figure 6 has the inner tube 19 located concentrically
within the outer tube. Then, in order to get close
proximity between the solar selective surface coating
23. wh;ch is carried by a substrate 22, the substrate is
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arranged to extend diametrically across the outer tube
and, by way of circumferentially extending legs 24, to
lie closely adjacent the outer tube.
In each of the above descri.bed embodiments the
space between the inner and outer tubes is subjected to
a high temperature bakeout whilst the space is being
evacuated, TheIeafter, a small quantity of a
hydrophobic hydrocarbon material such as benzene is
admitted to the space between the tubes. The material
adsorbs onto the solar selective surface coating at
relatively low temperatues and desorbs into the
evacuated space to degrade the vacuum at elevated
temperatures, as hereinbefore described.
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