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Description  |
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FIELD OF THE INVENTION
The present invention relates to the collection and concentration of solar
energy as thermal energy, and the transfer of that thermal energy by means
of a heat collection fluid.
BACKGROUND OF THE INVENTION
Solar energy collection devices have been developed which are oriented to
receive solar radiation and to collect and transform solar radiation to
thermal energy using a circulating or recirculating heat collection fluid.
The collection fluid flows to a solar radiation collection station where
it is heated by the rays of direct or reflected sunlight. The fluid is
pumped from the solar collection station to a heat exchanger, where the
thermal energy collected is utilized. In one type of heat exchanger for
use with solar collectors in which, for example, liquid sodium is the heat
collection fluid, the heat exchanger may include coils carrying the liquid
sodium proximately positioned to coils carrying water. In this manner the
water is heated by the liquid sodium, thereby effectuating transfer of the
thermal energy acquired by solar collection to the water.
Solar radiation which is entrapped as thermal energy may be used directly
for the purpose of heating water. Alternatively or in addition to the use
of a heat exchanger, the heat collection conduit may carry the heat
collection fluid to a heat sink. The heat sink may be an insulated storage
tank in some applications, such as where the heat collection fluid is
water. The tank, in such instances, resembles a tank used in conventional
gas or electric hot water heaters. The water heated by solar energy is
dispensed from the heat sink as required for use. One embodiment of such a
system provides heated water for household use or for a heated swimming
pool.
Several different forms of solar energy collection systems are utilized in
which a heat transfer fluid is circulated or recirculated to acquire
thermal energy and to carry this energy from the solar collector for later
release. The most widely used type of solar collector is the panel or flat
plate collector. In this type of solar energy collection device a heat
collection fluid follows a serpentine path through a tubing system lying
substantially in a flat plane. A transparent sheet or membrane may be
stretched across the upper surface of the collector and reflectors are
sometimes located beneath distinct portions of the tubing to entrap
radiation from the sun as thermal energy within the collector and to
transfer the incident heat of radiation to a fluid flowing through the
tubing. Another type of solar energy collector is a concentrator. In a
concentrator, a highly reflective parabolic trough is directed to face the
sun. A linearly aligned conduit is positioned to extend along the focal
axis of the parabolic trough so that solar radiation incident to the
trough is reflected to the linear conduit and is absorbed as thermal
energy by a heat transfer fluid circulating therein. While numerous other
types of solar energy collection devices have been developed, the flat
plate collector and the concentrator are of the greatest commercial
significance.
One problem present in connection with solar energy collection devices that
employ a heat transfer fluid is the problem of retaining heat collected
within the fluid. Once the temperature of the heat collection fluid has
been raised to exceed ambient temperature, there is a tendency for the
heat collection fluid to radiate thermal energy, thereby losing the solar
energy that it has acquired. To compensate for this, some systems have
jacketed the heat collection tubing used with a transparent vacuum jacket.
The objective of such an arrangement is to allow solar radiation to pass
as incident light energy through the transparent vacuum jacket to enter
the fluid transfer medium. By transforming the incident solar radiation to
thermal energy, rather than maintaining it in the form of light energy,
outward radiation from the heat collection fluid is inhibited by the
surrounding insulating vacuum. However, several defects exist in
connection with this approach. One principal disadvantage of conventional
systems is that oftentimes the vacuum jacket fails to maintain a good
vacuum seal. This is because the transparent material, usually glass, is
sealed to the fluid transfer tubing, typically formed of copper pipe with
a blackened outer surface. Because of the differences in coefficients of
thermal expansion between the glass jacket and the metallic tubing, the
vacuum seal formed therebetween is easily broken when the device is in
use. Thus, the vacuum surrounding the fluid transfer tubing is frequently
lost or of such a low differential from surrounding ambient pressure that
it is ineffective to adequately prevent radiation of thermal energy from
the interiorally located tubing.
Another problem that has existed in connection with conventional vacuum
jacketing of solar collection conduits is the positioning of the
surrounding vacuum jacket in direct contact with the fluid transfer
tubing. Thus, while thermal radiation from fluid within the tubing may be
inhibited by the surrounding vacuum, the contact of the jacket with the
tubing provides a path for conducting heat away from the fluid. That is,
heat is transferred by conduction through the structure of the jacket in
addition to any heat losses which may exist by virtue of radiation as a
result of poor vacuum sealing.
Accordingly, it is an object of the present invention to provide a solar
collection conduit which inhibits radiation loss from a heat transfer
fluid within a solar energy collection device while at the same time
guarding against loss of heat by conduction. This objective is achieved by
surrounding the heat collection tubing with a vacuum jacket within an
insulation space. This insulation space is typically filled with dead air
which is prevented from circulating. Thus, to escape the heat transfer
fluid, thermal energy must be conducted from the fluid to the surrounding
copper tubing, and must radiate through a dead air space and subsequently
through an evacuated chamber before it is lost as a source of energy. This
arrangement markedly decreases thermal losses from the fluid transfer
medium when contrasted with conventional devices.
A further object of the invention is to provide a conduit construction in
which heat transfer fluid tubing is surrounded by an evacuated chamber
which avoids vacuum seals between the fluid transfer tubing and the
material of which the vacuum jacket is constructed. A vacuum jacket
construction according to the present invention thereby avoids the
metal-glass interfaces that are so unsatisfactory for maintaining vacuum
seals and which have been so prevalent in the prior art. Rather, the
present invention employs a vacuum jacket comprised of an inner glass
sleeve and an outer glass sleeve. These sleeves may be of several
configurations, depending upon the heat transfer fluid tubing
configuration employed. Where the heat transfer fluid tubing is
constructed to allow fluid to traverse from one side or end of a solar
collection panel or reflector trough and to travel across the collector
surface to exit at an opposite end, the vacuum jacket is preferrably
configured as a longitudinally elongated pair of concentric cylinders
joined at either end by opposing halves of a toroidal surface. The length
of the cylinders is commensurate with the length of the heat collection
fluid tube sections which are surrounded thereby.
A further object of the invention is to provide a fluid transfer conduit
for use in a solar energy collection system which is equipped with a
vacuum chamber surrounding central fluid conducting tubing located within
the interior confines of the vacuum jacket within an insulation space
between the inner wall of the jacket and the fluid transfer tubing.
Moreover, this heat conserving conduit is preferrably protected externally
of the solar radiation receiving section by armored sheathing located
thereabout. Such armor sheathing may take the form of steel or copper
tubing positioned to coaxially envelop the vacuum jacket. An additional
vacuum layer may be provided in such an embodiment where the steel or
copper tubing is itself constructed of multiple walls which define an
additional evacuated area therebetween. Again, sealing between disimiliar
materials is avoided, since no seal exists between the glass vacuum jacket
and the surrounding metal vacuum jacket. Instead, the glass walls of the
interior glass jacket are sealed together while the metal walls of the
exterior metal jacket are either sealed to each other or to interface
connections.
An additional object of one embodiment of the invention is a solar energy
fluid transfer conduit system in which fluid transfer tubing has a
transparent vacuum jacket that includes an intermediate partition between
spaced inner and outer walls. The vacuum jacket is thereby divided to
define a pair of concentric vacuum chambers on either side of the
partition.
Another object of one embodiment of the invention is to provide a collector
tube which maximizes the efficiency or reflected solar radiation
collection and retention. This is achieved by providing coaxial tubes for
the circulating fluid used to collect the reflected solar energy. Thus,
the fluid may flow through the innermost tube from one end to other, and
then back around the outside of the innermost tube within a second outer
coaxial tube. Alternatively, flow may proceed in the opposite direction.
In both instances heat radiated from the inner tube is absorbed by fluid
in the outer tube. Thermal energy from fluid within the interior tube
segment thereby radiates into a surrounding layer of heat collection
fluid, thereby conserving thermal energy within the entire system.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross sectional view of one form of the conduit of the
invention.
FIG. 2 illustrates a parabolic concentrator system in which the improved
conduit of the invention is utilized.
FIG. 3 is a cross sectional view of the improved conduit taken along the
lines 3--3 of FIG. 1.
FIG. 4 is a sectional elevational view of the construction of an
alternative embodiment of the invention.
FIG. 5 is a sectional elevational view depicting an armored portion of a
conduit according to the invention.
FIG. 6 is a perspective view illustrating the interconnection of armored
portions of the conduit of the invention in flat plate collection system.
FIG. 7 is a cross sectional view of an alternative embodiment of the
invention as disposed in a flat plate collector such as that of FIG. 6.
DESCRIPTION OF THE EMBODIMENT
The improved solar collection conduit may be utilized either in association
with flat plate collectors or with concentrators, as well as with other
forms of solar radiation collection devices. The essential features of the
invention are apparent with reference to FIGS. 1 and 3. A conduit 15 is
provided having fluid transfer tubing segments 49 and 50 carrying a
circulating heat transfer fluid in opposite directions in a solar energy
collection device. The tubes 49 and 50 extend in parallel alignment and
are in communication with each other at their extreme right hand ends, as
depicted in FIG. 3. A vacuum jacket 53 having spaced inner and outer walls
123 and 124 respectively is positioned about and encompasses the tubes 49
and 50. The walls 123 and 124 of the vacuum jacket 53 define an evacuated
enclosure 125 therebetween. The fluid transfer tubes 49 and 50 are located
within the interior confines defined by the inner wall 123 and the
transverse end closures 45 and 47 within an insulation space 54.
The conduit 15 can be used in conjunction with a focusing parabolic
concentrator, as depicted in FIG. 2. The illustration of FIG. 2 depicts a
solar energy collection device 10 having a linearly extending reflector
trough 11 with a highly reflective interior surface which is of uniform
concave ruled parabolic cross section throughout. The reflector trough 11
is oriented to receive and concentrate solar radiation on the metal fluid
transfer tube sections 49 and 50 of the conduit 15. The conduit 15 is
located at the parabolic focal axis 12 of the trough 11.
Within the conduit 15, a first inner copper tube 49 is provided to carry
fluid from one end of the trough 11 to the other along the parabolic focal
axis 12. The second copper tube 50 has a blackened outer surface and is
coaxially positioned about the first tube 49 and is in communication
therewith at a single end of the trough, as depicted at the extreme
righthand end in FIG. 3. This end is preferrably elevated during use.
Thus, the tube 49 carries a heat collection medium such as water, in a
first longitudinal direction to the right in FIG. 3. The semi-circular
outer tube 50 carries the fluid back along the outside of the inner tube
49 in an opposite longitudinal direction to the left in FIG. 3. Relatively
cool fluid enters the central inner tube 49 through an axial inlet port 51
and is withdrawn through an outlet port 52 in communication with the
second or outer tube 50. Alternatively, fluid could be introduced through
the port 52 and circulate through the outer tube 50 before entering the
inner tube 49 at the elevated end of the trough 11. The fluid would then
be withdrawn through the port 51.
In addition to the two tubes for circulating the heat collection medium, a
vacuum jacket 53 is provided and is coaxially positioned about the tube 50
to minimize radiant heat loss therefrom. The volumetric area indicated at
54 is a dead air space surrounded by the interior wall of the cylindrical
inner glass sleeve 123 and the transverse end closures 45 and 47. An outer
cylindrical glass sleeve 124 is coaxially positioned about the inner
sleeve 123 to define an annular evacuated volume 125 therebetween. The
sleeves 123 and 124 are longitudinally elongated to a length commensurate
with the length of the tube sections 49 and 50. The sleeves 123 and 124
may be joined together at each end at junctions shaped in the form of
opposing halves of a torus, as depicted in FIG. 3. The extremities of the
sleeves 123 and 124 may be sealed together to define the annular evacuated
chamber 125 which encircles the tube segments 49 and 50.
The benefit derived from the conduit configuration depicted in FIG. 1 is
that while radiant energy entering the semi-cylindrical surface 55 of the
outer tube 50 is maximized, by striking the surface at substantially a
90.degree. angle, the radiant energy, once transmitted to the heat
collection fluid within the tubes 49 and 50, is trapped therein. That is,
thermal energy is able to radiate from the heat collection conduit from
the outer tube 50 through the dead air space 54 and through the evacuated
enclosure 125 to only a slight degree and with great difficulty. To the
contrary heat radiated from the inner tube 49 is absorbed in the outer
tube 50, thus further minimizing overall heat loss in the flow of fluid
circulating through the tubes 49 and 50. When utilized in conjunction with
the solar concentrator of FIG. 2, the semicircular surface 55 of tube 50
faces the concave reflector surface of the trough 11 while the opposing
flat surface 56 is parallel to the directrix of parabolic configuration,
indicated at 18.
In the embodiment of FIG. 2, the conduit 15 is held in position by a felt
packing which conforms to the outer surface of the sleeve 124. The felt
packing thereby enables the conduit 15 to be centrally located within a
bearing race. The bearing race and the conduit 15 rotate in synchronism
with the reflector trough 11. The bearing race is carried in a bearing
housing 59 which is welded to the upper extremities of stanchions 35 and
36 at the lower end of the solar collection mechanism 10. A similiar
bearing assembly is employed at the upper or elevated end of the trough
11, but it is to be understood that the tubes
The vacuum jacket 53 is coaxially positioned externally about the outermost
of the pair of metal fluid transfer tube segments 49 and 50 in spaced
relationship therefrom to minimize radiant heat loss. Other conduit
configurations are quite acceptable for some purposes, however. For
example, FIG. 7 discloses an arrangement in which a plurality of conduits
15' are disposed in lateral displacement from each other across the
surface of a flat plate collector, such as the collector 20 depicted in
FIG. 6. In this arrangement, a series of vacuum jackets 53 are provided
with end terminations along one edge of the flat plate collector 20 in the
end termination configuration of FIG. 8. At the opposite edge of the flat
plate collector 20 elongated U-shaped tubes 60 enter and leave the
collector panel. These tubes each include an elongated inlet segment 61
and a similiar parallel elongated outlet segment 62 joined at one end to
the segment 61. Heat transfer fluid enters the conduit 15' through the
tube segment 61, traverses the length of the conduit segment 15' and is
received in the outlet tube segment 62, from which it leaves the collector
panel at the same edge at which it entered.
The vacuum jacket 53 defines an evacuated chamber 125 as hereinbefore
described. However, the tube segments 61 and 62 are not coaxially oriented
relative to the vacuum jacket 53. Instead, the inlet tube 61 is joined
along its length to a semi-cylindrical heat absorbing sheet 63 which
delineates the lower portion of the dead air space 54' defined within the
interior surface of the glass sleeve 123. The heat absorbing sheet 63
receives reflected radiation from the corrugations or toughs 64 of the
corrugated aluminum sheet 65 located beneath the conduits 15' within the
collector panel 20. Heat is thereby maintained within the dead air space
54' and transmitted to the heat collection fluid flowing through the
parallel elongated tube segments 61 and 62.
The linearly extending reflector surfaces 64 are positioned externally of
the conduits 15' to reflect solar radiation towards the vacuum jackets 53
to promote concentration and collection of solar energy. While depicted in
semicircular form, it is to be understood that the reflector troughs 64
can assume a variety of configurations. An arcuate corrugated form is
depicted as representing a suitable economic balance between cost of
manufacture and efficiency of solar radiation collection in a flat plate
collector 20. It is to be understood that alternative forms, such as
V-shaped reflector troughs and parabolic reflectors are also quite
suitable for use in flat plate collectors.
FIG. 4 depicts an alternative embodiment of the invention in which the
vacuum jacket includes an intermediate partion 127 between the spaced
inner and outer glass sleeves 123 and 124. The partition 127 thereby
divides the evacuated enclosure into a pair of concentric vacuum chambers
denoted as an inner evacuated chamber 129 and an outer evacuated chamber
130. The use of such multiple layers of glass finds particular utility
when the temperatures within the heat circulating fluid exceed 800.degree.
F. At such elevated temperatures, radiant heat loss becomes an
increasingly important factor. Accordingly, the provision of a plurality
of concentric surrounding evacuated enclosures represents a considerable
savings in conservation of thermal energy within the heat circulating
fluid. The use of plural evacuated chamber sections should be balanced
against the loss in heat transmission through the walls formed by the
concentric glass cylinders 123, 127, and 124, however. With each
additional layer of glass, a loss of radiant energy collection of
approximately 10% occurs. Accordingly, the suitability for the embodiment
of the invention of FIG. 4 should be empirically derived for particular
solar energy collection system configurations.
Still another embodiment of the invention embodies separate vacuum jacketed
conduit sections extending internally within a solar radiation receiving
section, such as within the solar collection panel 20 of FIG. 6, and also
conduits that extend externally thereof, as indicated at 21 and 22 in FIG.
6. In the embodiment of FIG. 6, fluid is transferred from the solar
collection panel 20 to a thermal sink indicated generally at 23. When a
demand for heat arises, the heat transfer fluid is passed through a
conduit section indicated at 22 to a heat exchanger 24. The use of vacuum
jacketed conduits in which fluid flows within tubing separated from
surrounding vacuum jackets by a dead air space serves to minimize heat
loss in the transfer of the circulating fluid throughout the entire
system. Preferably, however, the conduits are armored externally of the
solar energy receiving station. With specific reference to FIG. 5, it can
be seen that the armored portions 21 and 22 of the conduits of the
invention may be formed with multi-walled metal tubing, which may be
formed of any conventional structural metal, such as steel or copper.
Between the exterior cylindrical metal sheathing 25 and the interior metal
sheathing 24, a further evacuated region 26 is defined. No consideration
need be given to the solar radiation absorption characteristics of the
armored portions 21 and 22 of the conduit of the invention, as these
portion will exist only externally of the solar energy receiving station.
Consequently, the additional metal walls 24 and 25 serve not only to
further minimize the loss of radiant energy by providing an additional
vacuum enclosure 26, but also help to protect the conduit sections 21 and
22 from accidental damage. Such damage can easily occur during maintenance
operations, during positioning of the apparatus or by intermeddling which
might result at unattended locations.
The significance of the insulation space 54 surrounding the metal tubing in
all of the embodiments of the invention is extremely important. While
vacuum jackets have previously been used in association with solar energy
collection devices in which an evacuated chamber was defined immediately
externally of the fluid conducting tubes, there has heretofore been no
provision of a dead air space enclosing heat transfer fluid conducting
tubes within the confines of a vacuum jacket. Such an arrangement not only
provides an additional thermal insulating medium to maintain heat within
the circulating fluid, but also circumvents the requirement for
effectuating glass to metal seals in association with the provision of
evacuated chambers. Such seals have consistently failed to yield
satisfactory results, and the elimination of the requirement for such
seals by the invention herein allows solar energy collection systems to be
operated with far less maintenance and servicing than has heretofore been
possible.
It is to be understood that the various embodiments of the improved solar
energy collection heat transfer conduits depicted herein are illustrative
only, and are not intended to be complete. Various modifications and
alterations of the invention will undoubtedly occur to those familiar with
solar energy collection. Accordingly, the scope of the invention is
defined in the claims appended hereto.
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