CA1112528A

CA1112528A - Evacuated solar collector

Info

Publication number: CA1112528A
Application number: CA311,982A
Authority: CA
Inventors: Everett M. Barber, Jr.
Original assignee: SUNTHONE A PARTNERSHIP
Current assignee: SUNTHONE A PARTNERSHIP
Priority date: 1977-09-26
Filing date: 1978-09-25
Publication date: 1981-11-17
Also published as: JPS5723870B2; DE2838883A1; AU6493680A; FR2457450A1; GB2071305B; IT1206551B; AU4010478A; ES473385A1; JPS5461339A; IT7851235A0; GB2005402A; FR2404184A1; GB2071305A; JPS5798758A; AU518738B2; GB2005402B

Abstract

ABSTRACT
This invention relates to solar energy collectors, and more especially to evacuated solar energy collectors comprising a housing forming a chamber. The chamber has a transparent top member and bottom member which are joined together along the tims, thereby providing an airtight seal, the top and bottom members each having a surface convex to the outside whereby the chamber resists the external atmos-pheric pressure while having a relatively large absorber area to collector area ratio. The bottom member of the collector can have a reflective surface to reflect the solar energy upon the underside of the heat absorbing unit, which is a spiralled tubular element retained within the collector.
Furthermore, a system of solar energy collectors can be constructed so that the top members of the chambers receive solar radiation during the day, and mirrors situated in the interstices between adjacent collectors direct solar energy to transparent lower surfaces of the collectors, the low profile of each chamber facilitating the architectural integration of the collectors in close-packed arrays.

Description

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S P E C I F I C A T I O N

This invention relates to solar energy collectors and more specifically to evacuated solar collectors.
~ enerally, the higher the temperature of heat avail-able for a given thermo-dynamic process, the more efficiently the process can be carried out. The higher the heat temperature, for instance, the smaller the auxiliary equipment, e.g., heat exchangers, pumps, fans, pipes and ducts can be, and a commensurate reduction in the amount of energy required to operate the auxiliary equipment can be realized.
When conventional energy sources such as fossil fuels or electrical resistance heat are used, very high temperatures can be obtained and maintained. In solar energy applications, however, the temperature of the heat available from commercially available collectors is relatively low.
One way to maximize the temperature of the heat obtained ~rom a solar energy collector is to minimize the heat losses therefrom. There are three primary sources of ~eat loss in solar collectors. First, heat is lost by conduction from ` the absorber to the housing. Second, reflection and radiation from the absorber panel reduce the heat retained by the collector. ~inally, heat is lost through convection currents built up in the collector between the front panel and the absorber panel. The first problem can be obviated by ~ suspending the absorber within the collector housing using a : 25 thermal insulative support. The second problem can be minimized by providing a selective coating on the absorber.
The last problem can be greatly reduced by providing means ,A` ~'' . .

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for minimizing convection currents, or by evacuating the inside of the collector, thereby eliminating the air that forms the convection currents. Then much higher temperatures may be obtained and retained by the absorber.
The convection heat losses may be minimized by plural transparent covers, and other devices which tend to retard convection currents. The most promising technique of - avoiding losses due to the convection of air within the collectors appears to be evacuating the collector housing to eliminate the air from within the housing.
A primary problem with evacuated collectors, however, is the ever-present atmospheric pressure which provides a force of about fifteen pounds per square inch of surface area. Thus, for a collector approximately three feet in diameter, the pressure on the collector cover is on the order of seven and one half tons. Since at least the surface ~;; facing the sun must be transparent, and preferably glass, ., extreme care must be taken to provide an optimum shape to guard against implosion.
Tubular evacuated collectors are known. Tubular . .
collectors normally have rounded ends. ~owever, these collectors have a poor net absorber area to gross collector :,~.,.
area ratio. They are also relatively small in net absorber ` area, therefore a relatively large number of piping A;', 25 connections must be made to achieve a given net collector area. If a spherical shape is used, the collectors would be large in volume compared to the available absorber surface area. Spheres do not lend themselves well to use in an array of closed packed collectors.
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Accordingly, the present invention provides an evacuated 'i.

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collector having at least one transparent cover member which is formed as a section of a sphere to withstand atmospheric pressure when the collector is evacuated. The section permits the collector to have a large absorber area to S collector area ratio in comparison to the more conventional tubular evacuated collectors. It also has a low profile to facilitate the architectural integration of these ; collectors in close-packed arrays.
Briefly stated, the invention in one form thereof comprises an evacuated solar collector housing formed of two members having surfaces that are sections of a sphere.
The members are joined at their rims to form a chamber ; around an absorber. Means are provided to counteract , . . .
' stresses at the rim of the evacuated housing. Additionally, both members may be transparent to extend the time of effective collection during daylight hours.
It is an object of the invention to provide a new and .
`; improved evacuated solar collector.
It is a further object of the invention to provide an evacuated solar collector having front and back transparent cover members to enhance collection of solar energy.
; It is a further object of the invention to provide a new and improved evacuated solar collector having transparent upper and lower cover members formed as sections of a . .
~m 25 sphere to resist the compressive forces of the ambient , atmospheric pressure.
;~ c The features of the invention which are believed to be novel are particularly pointed out and distinctly claimed in the concluding portion of this specification. The ;~
~` 30 invention, however, both às to its organization and operation, '; .
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2~3 together with further objects and advantages thereof may best be appreciated by reference to the following detailed description taken in conjunction with the drawings, wherein:
FIG. 1 is a top view of a plurality of solar collectors according to the invention showing one form of array thereof;
FIG. 2 is a view seen in the plane of lines 2-2 of Fig. l;
FIG. 3 is an enlarged view of a portion of Fig. 2;
FIG. 4 is a detail in section of the bottom center o the collector as shown in Fig. 2;
FIG. 5 is the top view of a collector shown in Fig. 1 showing one form of absorber therefor;
FIG. 6 is the top view of an alternative form of absorber to that shown in Fig. 5;
FIG. 7 is a view seen in the plane of lines 7-7 of Fig. 6;
FIG. 8 is a view of another embodiment of the collector shown in Figs. 2 and 4;
FIG. 9 is a view in section of another embodiment of the invention to that shown in Figs. 2 and 3;
FIG. 10 is a view in section of another embodiment of the invention to that shown in Figs. 2 and 3;
FIG. 11 is a side elevation of an array of collectors embodying the invention;
FIG. 12 is a front elevation of the array of Fig. 11;
~-~ FIG. 13 is a view seen in the plane of lines 13-13 of Fig. 11; and FIG. 14 is a sectional vlew seen in the plane of - lines 14-14 of Fig. 12~
Fig. 1 shows a plurality of solar collectors 10 mounted - ` ~ 523 ,~
^ on a support surface 11 in a hexagonal close-packed array.
The hexagonal array provides the densest nesting for collectors having circular configurations. Mirrors (not shown) can be situated in the interstices between three ' 5 adjacent collectors to direct energy to transparent lower surfaces of the collectors. A square or other quadilateral ~ array may also be used if dictated by architectural ;~ considerations.
':; As exemplified in Fig. 2, a collector 10 embodying the ~` 10 invention comprises a transparent cover member 12 and : .
a preferably transparent lower cover member 13. The cover members are formed as sections of a sphere with a low . profile and having peripheral rims 14 and 15, respectively.
~: Members 12 and 13 can be formed by sag molding sheets of glass in a well known technique. The rims 14 and 15 are - fitted to a tension ring 16 in seats 17 and 18 provided thereon. Adhesive seals 19 and 20 provide an airtight bond between seats 17 and 18, respectively, and rims 14 and 15, and cushion the rims in the respective seats. A preferred sealing ... . .
: 20 material for all glass to metal seals is a low vapor pressure ~, epoxy marketed by Varian Associates under the trademark TORR-SEAL.
Disposed within chamber 21 defined by cover members 12 . and 13 is an absorber 22. As shown in Fig. 5, absorber 22 .::,;....
:~ ` 25 is in the form of a closely-wound parallel double spiral.
Inlet and outlet lines 23 and 24, respectively, are provided ~` and ~xpansion elbows 25 and 26 may be provided in the inlet , and outlet lines respectively.
The inlet and outlet lines 23 and 24 extend through a closure member 27 which closes a bottom opening 28 in lower ....

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' member 13.
~- Absorber 22 is supported within chamber 21 by means of .
a plurality of support members 29 which may be of a ceramic or other highly heat resistant material. As exemplified - S in Fig. S, absorber 22 may include a plurality of radial stiffening members 30. The absorber has a selective coating on its top and also on its bottom if member 13 is transparent.
.. ~. . , The spiral is close wound to present maximum absorber area and adjacent turns may be in contact with each other.
Where the convolutes of the spiral are not in contact, as shown in Fig. 5, the interior surface of back member 13 is coated to be reflective to reflect solar radiation to the underside of the absorber. The absorber tubing and lines 23 and 24 may be glass or metal.
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; 15 Member 27 is a metallic member having passageways 30 and ,.:
' ; 31 for inlet and outlet lines 23 and 24 respectively.
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The inlet and outlet lines are braised to the plug to provide an airtight seal. Plug 27 is bonded to the edge defining ,: , opening 28 by means of an adhesive 32. A check valve 33 is fitted into a third passage 34 in plug 27 to permit collector 10 to be evacuated.
' Member 27 and opening 28 are preferably circular and ~: centrally located on member 13 to prevent thermal and , pressure stress concentrations from developing due to high temperatures along lines 23 and 24.
, Collector 10 i5 supported by a plurality of legs 35 having supporting feet 36. Legs 35 may be fastened to ring 16 by means o a bolt 37. Generally, three legs 35 will be sufficient.
A heat exchange medium such as ethyl glucol may be .

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circulated through absorber 22, entering through inlet line 23 and exiting to a storage medium or energy utilization device through outlet 24. Inlet and outlet lines 23 and Z4 ; will be suitably insulated to prevent the collected heat from being lost to the ambient atmosphere. Inlet and outlet lines 23 and 24 may be connected to suitable manifolds or headers to serve all the collectors 10 shown in Fig. 1.
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When the collector is evacuated, compressive atmospheric forces will tend to force the surfaces 12 and 13 towards a planar shape. This will create radial compressional forces ; and tangential tensional forces in the members. The tangential forces will increase in magnitude from the center toward .
the edges. The compressive forces are counteracted by tension ring 16 which also counteracts the tangential forces tending ` 15 to diametrically enlarge rims 14 and 15.
., The tension ring is shown as being formed in one piece.
., :, However, it is to be understood that it may be formed of two butt welded bands which are joined together, the inner .....
ba~d being narrower than the outer band to provide seats 17.
'~ 20 Figs. 6 and 7 exemplify an alternate absorber 40 which may be used in the collector shown in Figs. 1 through ,` 5. Absorber 40 includes a panel 41 having a heat exchange , grid in intimate heat exchange relationship therewith.
The grid comprises headers 42 and 43 and a plurality of ; 25 conduits 44 extending therebetween. Inlet and outlet lines 45 and 46 are taken from headers 41 and 42 respectively.
Fig. 8 exemplifies an alternate sealing member 47 for opening 28. A formed glass sealing member 47 is attached to lower member 13 by glass solder indicated at 48.
Tubes 23 and 24 extend through member 47 and plugs 49 : ' ~%~
- and 50 provide a vacuum seal therefor. A vacuum closure pull 51 is provided to be sealed after the collector 10 is evacuated. Plugs 49 and 50 can be any suitable glass-to-metal sealing material. An example of such a material is an alloy sold under the trademark KOVAR by Carpenter Technology Corporation of Reading, Pennsylvania.
A second embodiment of the invention, shown in Fig. 9, comprises a collector 52 formed of members 53 and 54 which are generally sections of a sphere. Members 53 and 5~ are provided with rims 55 and 56, respectively, which are short ` cylindrical sections. Members 53 and 54, including rims - 55 and 56, may each be die cast in a manner similar to the way the screen portions of cathode ray tubes and television picture tubes are made. Rims 55 and 56 are joined together at their ends by a glass solder 57. An annular tension ring 58 is attached to the outer surfaces of rings 55 and 56 through an adhesive seal 59. The seal also acts as a cushion between the metal and the glass.
As exemplified in Fig. 10, a further embodiment includes -` 20 a collector 60 having an upper member 61 and lower member 62.
Both have surfaces defined on sections of a sphere.
Member 61 includes a relatively thick rim 63 and member 62 -~ includes a rim 64. Rim 64 is dimensioned to telescope into - rim 63. A glass solder seal 65 provides an airtight seal 25 between concentric rims 63 and 64. A ledge 66 may be provided projecting from rim 63 to provide a uniform stop to abut the end of rim 64. The increased thickness of rim 63 obviates the metallic tension ring.
A ring 67 having a lower flange or support ears 68 is provided around rim 63 to provide support for collector 60.

~ `-Ring 67 merely acts as a support means for the collector.
` Ring 67 is attached to rim 63 through an epoxy seal 69.~` Members 61 and 62, including rims 63 and 64 respectively, can each be die formed and then assembled in a manner similar to the way some television picture tubes are made.
The collector housings shown in Figs. 9 and 10 may be - formed in an elliptical or rectangular configuration as seen - from the top. In such case, the absorber shape would follow that of the housing and could take the form of Fig. 5 or 6 or any other suitable form.
Collectors embodying the invention may be mounted -` to the structure they serve or may be mounted to a free ` standing support as exemplified in Figs. 11-14. The `:, ' ' arrangement of Figs. 11-13 exemplifies a support structure 70 ,;~ , which comprises a plurality of parallel support members 71-75 ~, of genera~ I-cross-section defining channels 76 to receive the edges of collectors 80. The members 71-75 are disposed at an angle to the vertical dependent on the latitude of installation.
The collectors 80 are shown as having a generally rectangular configuration, and are of the structure shown .
. in Fig. 9. The structure of Fig. 10 may also be used for . ~:
the rectangular collector.
;~ The collectors 80 have a tension or support band 81, .
previously exemplified, which are received in channels 76 .
~`; of the members 71-75. Bottom members 82-85 retain the columns of collectors. I~ desired, cross members such as 82-85 may be provided for each collector. A top cross - member 86 is provided to rigidize support member 70.
In accordance with another aspect of the invention, ~.. ' 9 : -',"

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a mirror assembly 87 is disposed on ~he north side of collectors 80. Mirror assembly 87 comprises two mirror surfaces 88 and 89 at an angle A dependent on the latitude of .^
installation. A support exemplified by a strut 90 is provided for the mirror assembly.
- In Fig. 13, the east is to the right. Mirror surface 89 will reflect the morning sun rays to the back side of the collectors an~ to the absorbers therein. As the sun continues ` to rise, mirror surface 89 will reflect less light, and in the afternoon mirror surface 88 will reflect sunlight to the rear of collectors 80. At midday the mirror surfaces will ;: be shadowed by the collectors, except for the light that may pass therebetween.
This arrangement substantially extends the time of effective heat collection by the absorbers, and provides a - sun tracking effect to stationary collectors.
~; A collector embodying the invention has a high ratio of absorber to collector area and volume. The configuration and construction of the housing will withstand the atmosphere ; 20 pressure when evacuated, and will eliminate losses due to convection currents. With the elimination of convection currents, the collector may operate at very high temperatures, resulting in reduction in size of associated equipment in an overall solar heating system. Moreover, the disclosed ; 25 collector structure provides greater collecting efficiency ` with respect to time in a day through the ability to accept reflected solar energy through the rear cover member at low angles of azimuth of the sun.
While the cover members are preferably formed as sections of a sphere convex to the outside, any convex ,"~' , .
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shape such as paraboloid capable of withstanding atmospheric pressure may be utilized.
~ It may thus be seen that the objects of the invention - set forth as well as those made apparent from the foregoing description are efficiently attained. While preferred ,, embodiments of the invention have been set forth for . -. .
,~ purposes of disclosure, modification to the disclosed embodiments of the invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, t,.
~ ~ 10 the appended claims are intended to cover all embodiments ,.~................................ .
?,.~ of the invention and modifications to the disclosed embodiments which do not depart from the spirit and scope ` of the invention.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Solar energy collector comprising a housing forming a chamber, said housing comprising a transparent top member and a bottom member, said members having surfaces convex to the outside, each member having a rim and means continously joining and sealing the rims of said members together to provide an airtight seal; and a solar heat absorber disposed within said chamber, the chamber of said housing being evacuated to a pressure less than atmospheric, said convex surface being defined on continuous curvatures in two coordinate dimensions, said absorber being generally planar and substantially covering the area defined within said joining means, and means extending through said housing for removing heat from said absorber.

2. A collector as defined in claim 1 where said bottom member is also transparent.

3. A collector as defined in claim 1 where said surfaces of said members are formed as a section of a sphere and less than hemispheric.

4. A collector as defined in claim 1 further comprising valve means extending through one of said members by which said housing may be evacuated.

5. A collector as defined in claim 1 wherein said joining means comprises an annular ring to which each of said top member rim and said bottom member rim are joined.

6. A collector as defined in claim 5 in which said annular ring has two annular seats formed therein for receiving the top member rim and the bottom member rim.

7. A collector as defined in claim 1 in which each of said top member rim and said bottom member rim comprises a depending....

flange, the proximate edges of which are joined together, and a reinformcing band about said joined flanges and peripherally sealed to said flanges.

8. A collector as defined in claim 1 wherein said joining means comprises said top member rim and said bottom member rim dimensioned so that one of said top member rim or said bottom member rim telescopes into the other, and said rims are peripherally sealed together.

9. A collector as defined in claim 8 wherein the outer rim is substantially thicker than the inner rim.

10. A collector as defined in claim 8 wherein the outer rim has a ledge projecting therefrom to provide a uniform stop for the inner rim.

11. The collector of claim 1 wherein said top member and said bottom member have depending flanges defining said rims, and means sealing the edges of said rims together.

12. The collector of claim 1 wherein said absorber is in the form of a spiral of a tubular element arranged to conduct a heat transfer medium therethrough.

13. The collector of claim 12 wherein both of said top and bottom members are glass, and said joining means includes a reinformcing band extending about the seal of said rims.

14. The collector of claim 7 wherein said rims include depending flanges, the flange of one of said members telescopes within the flange of the other of said members, and the outer flange is of increased thickness.

15. A collector as defined in claim 12, 13 or 14 where said bottom member is also transparent.

16. A collector as defined in claim 1 where the absorber is of the liquid heat-exchange type, and liquid inlet and outlet conduits extend through said housing.

17. The collector as defined in claim 1 where said top member is substantially rectangular in plan view.