EP2220442A1

EP2220442A1 - Solar thermal energy collector

Info

Publication number: EP2220442A1
Application number: EP08855928A
Authority: EP
Inventors: Randy C. Gee; Roland Winston
Original assignee: University of California
Current assignee: University of California; Solfocus Inc
Priority date: 2007-12-03
Filing date: 2008-11-21
Publication date: 2010-08-25
Also published as: US20090139515A1; CN101910749A; WO2009073416A1; EP2220442A4

Abstract

A solar thermal energy collector includes a receptacle and a tube positioned within the receptacle and having a closed end. The tube includes a divider cross-sectionally bifurcating the tube. The divider is spaced apart from the closed end of the tube to allow fluid communication between two bifurcated portions of the tube. A fluid is circulated through the two bifurcated portions of the tube for transferring of the solar thermal energy.

Description

SOLAR THERMAL ENERGY COLLECTOR

BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to the field of solar thermal energy. In particular, the present invention relates to solar thermal energy collectors. [0002] Solar thermal collectors have been utilized for over 20 years. The designs have varied from flat plate, box, air, integral, unglazed more commonly to parabolic troughs and dishes and full power towers. Though they have been commercially available for over 20 years, recent designs of evacuated tubes have become more efficient and less costly, allowing them to be both commercially and domestically available as well as more widely utilized. Some devices contain heat removal inserts that are placed within the tubes that serve the purpose of transferring the collected energy to a heat-transfer fluid and are used to transfer heat to a manifold located at the end of the tubes or in connection with the inserts.

[0003] Conventional designs are limited in their ability to transfer heat from the collector. It is desirable to improve the efficiency with which such heat is transferred.

SUMMARY OF THE INVENTION

[0004] In one aspect, the invention includes a solar thermal energy collector comprising a receptacle and a tube positioned within the receptacle and having a closed end. The tube includes a divider cross-sectionally bifurcating the tube. The divider is spaced apart from the closed end of the tube to allow fluid communication between two bifurcated portions of the tube. A fluid is circulated through the two bifurcated portions of the tube for transferring of the solar thermal energy. [0005] In one embodiment, the fluid for transfer of solar thermal energy is mineral oil.

In another embodiment, the fluid for transfer of solar thermal energy is an antifreeze solution.

[0006] In one embodiment, the tube is coupled to a manifold and the manifold is coupled to a pump that circulates the fluid through the manifold and the two bifurcated portions. The manifold may be coupled to additional tubes.

[0007] In one embodiment, the divider is a plate which cross-sectionally divides the tube into the two bifurcated portions. The tube may have a circular cross section, and the divider may form two bifurcated portions with semi-circular cross sections.

[0008] In one embodiment, the tube is an integral part of a manifold.

[0009] In one embodiment, the receptacle is a dewar, the dewar having an outer wall and an inner wall, the dewar having a vacuum drawn between the outer wall and the inner wall, and wherein the dewar is all glass. A region between an outer surface of the tube and the inner wall of the dewar may be filled with a second fluid to facilitate heat transfer. The second fluid may be mineral oil. In one embodiment, the dewar has a thermal absorption coating on an outer surface of the inner wall. The coating may be aluminum nitride cermets.

[0010] In one embodiment, absent a solar tracker component and in combination with an external reflector component, the fluid has a temperature above 280 degrees

Fahrenheit when the fluid exits the receptacle.

[0011] In one embodiment, the collector further includes an external reflector for reflecting sun rays onto the receptacle. The external reflector may be a compound parabolic concentrator (CPC).

[0012] In another aspect of the invention, a method for collecting solar thermal energy includes positioning one or more reflectors external to one or more receptacles, the reflectors being adapted to direct solar thermal energy to the one or more receptacles; positioning a manifold having one or more tubes adapted to fit within the one or more receptacles, each tube having a closed end and having a divider cross-sectionally bifurcating the tube, the divider being spaced apart from the closed end of the tube to allow fluid communication between two bifurcated portions of the tube; and circulating a fluid through the two bifurcated portions of the tube for transferring of the solar thermal energy.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 illustrates a solar thermal energy collector according to an embodiment of the present invention; and

[0014] Figures 2A and 2B illustrate cross-sectional views taken along II-II of Figure 1 of solar thermal energy collectors according to embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Embodiments of the present invention provide devices, methods and systems for collection and/or transferring of solar thermal energy. In this regard, embodiments of the present invention may provide inexpensive and efficient manners for collection of solar thermal energy.

[0016] Referring to Figures 1, 2A and 2B, a solar thermal energy collector according to an embodiment of the present invention is illustrated. In the illustrated embodiment, a collector 100 includes one or more receptacles 120 coupled to a manifold 110. The manifold 110 includes an inlet pipe 112 and an outlet pipe 114 for circulating fluid through the manifold 110 and the collector 100. A pump 116 is provided to circulate the fluid 110. The dimensions of the inlet pipe 112, the outlet pipe 114 and the pump 116 may be selected according to the requirements of the specific implementation of the collector 100.

[0017] The manifold 110 is coupled to one or more receptacles 120. The number of receptacles 120 may be selected from any practical number dependant on the size of the collector system desired. Further, the manifold may be coupled to a plurality of receptacles in a serial manner, a parallel manner or any combination thereof. [0018] Each receptacle 120 is preferably an all-glass dewar having a double-wall configuration, as most clearly illustrated in Figures 2A and 2B. Of course, in other embodiments, various other types of receptacles may be used. In one embodiment, the receptacles are cylindrical borosilicate glass bottles with a closed end, as exemplarily illustrated in Figure 1. Each dewar 120 is provided with an inner wall 122 and an outer wall 124. The region between the inner wall 122 and the outer wall 124 is evacuated. The vacuum region results in low heat loss. The level of evacuation of the region between the inner wall 122 and the outer wall 124 may be varied to either increase efficiency (e.g., reduce heat loss) or improve cost-effectiveness. The vacuum space may also incorporate either a passive or active mechanism to prohibit or mitigate effects of permeation of space by other gases such as hydrogen or helium. [0019] In one embodiment, an outer surface of the inner wall 122 (i.e., the surface facing the vacuum region) is coated with a thermal absorption coating 126, such as aluminum nitride cermets. In other embodiments, other commercially available coatings may be used. The thermal absorption coating 126 facilitates absorption of solar thermal energy by the receptacle 120.

[0020] Each receptacle 120 is provided with a tube 130 adapted to fit within the receptacle 120. In one embodiment, on one end, the tube 130 is inserted into the receptacle 120 and has a closed end 139. The tube 130 is provided with a divider 134 which divides, or bifurcates, the cross section of the tube 130, as most clearly illustrated in Figures 2 A and 2B. In the illustrated embodiment, the tube 130 has a circular cross section, and the divider 134 bifurcates the tube 130, resulting in two bifurcated portions 136, 138, each having a semi-circular cross section. Of course, in other embodiments, the cross-sectional shape of the tube 130 or the bifurcated portions may be different. In preferred embodiments, the cross-sectional area of the bifurcated portions 136, 138 is substantially similar to each other. As illustrated in Figure 1, the end of the divider 134 is spaced apart from the closed end 139 of the tube 130. The amount of space between the divider 134 and the closed end 139 of the tube is sufficient to allow fluid to flow freely around the divider 134.

[0021] On the other end, the tube 130 is coupled to the manifold 110, which is coupled to a tube corresponding to each of the other receptacles of the collector 100. The tube 130 may be coupled to the manifold 110 in a variety of manners including, but not limited to, welding. In one embodiment, the coupling of the manifold 110 and the tube 130 includes use of screw-type threads formed on the manifold 110 and the tube 130, similar to those found on conventional plumbing joints, that may use a thread seal. In a particular embodiment, as illustrated in Figures 1, 2A and 2B, the tube 130 is an integral part of the manifold 110. In this regard, the tube 130 may be formed as an integral part of the manifold and does not include any joints, connections or seals. Thus, once created, the tube 130 may not easily be removed from the manifold 110. The integral configuration of the tube 130 and the manifold 110 reduces the number of parts required, thereby reducing the time and effort required for installation and assembly of the collector 100 in the field. Thus, during assembly, the receptacle 120 only needs to be positioned around the tube 130. A seal (not shown) may be provided to secure the tube 130 to the receptacle 120. Further, the integral configuration eliminates a potential leakage point for fluid flowing through the receptacle, as described below. [0022] In order to facilitate transfer of heat from the receptacle 120 to the tube 130, a region 132 between the receptacle and the tube 130 may be filled with a heat transfer fluid. In one embodiment, this heat transfer fluid is mineral oil. The use of the fluid reduces heat loss when compared to empty space (e.g., air or vacuum) therein. In order to retain the heat transfer fluid in the region 132, a seal (not shown) may be provided between the receptacle 120 and either the tube 130 or the manifold 110. Such seals or seal arrangements are well known to those skilled in the art. [0023] In accordance with embodiments of the present invention, assembly and maintenance of the collection 100 is simplified. With the tube 130 integrally formed (or otherwise pre-assembled) with the manifold 110, only the receptacle 120 needs to be connected. Thus, for maintenance purposes, individual receptacles that may become damaged can be replaced without replacing the entire collector 100. Further, use of appropriate seals between the receptacle 120 and the manifold 110 can make such replacement of receptacles simple, time-efficient and effective. A worker in the field can accomplish such maintenance without expending substantial time and effort. [0024] The receptacle 120 and the manifold 110 are positioned such that an external reflector 140 concentrates solar thermal energy (or solar irradiance) onto the receptacle 120. The shape of the reflector 140 may be selected from a variety of shapes. In some embodiments, the reflector 140 may operate in conjunction with a solar tracking component. Preferably, the reflector 140 is adapted to operate in the absence of such a tracking component. In one embodiment, the external reflector 140 is a compound parabolic concentrator (CPC). Such reflectors are well known to those skilled in the art. [0025] Figures 2 A and 2B illustrate two embodiments of an external reflector 140a, 140b for use with embodiments of the present invention. Referring first to Figure 2 A, the external reflector 140a has two concave, parabolic components joined by a central convex, v-shaped component. Each concave component forms substantially half of a parabola.

[0026] Referring now to Figure 2B, the external reflector 140b includes two concave, parabolic segments joined to each other. In this embodiment, each concave component forms substantially more than half of a parabola. In this regard, the two concave segments join to form an inverted "v" shape.

[0027] Thus, the shape of the reflector 140 directs substantially all sunlight incident on the reflector 140 within a predetermined angle of incidence onto the receptacle 120 and, more specifically, onto the thermal absorption coating 126 on the outer surface of the inner wall 122 of the dewar 120. In this regard, sunlight is concentrated efficiently onto the receptacle 120 while minimizing heat loss. Further, the evacuated, double-wall configuration of the dewar 120 and the use of the heat transfer fluid in the region 132 between the dewar 120 and the tube 130 facilitate minimizing of the heat loss. Thus, sufficient efficiency of the collector 100 can be achieved in the absence of a solar tracking component, thereby resulting in significant cost reduction. The combination of the reflector 140, the receptacle 120 and the tube 130 is preferably configured to have a large acceptance angle. For example, in one embodiment, an acceptance angle of at least ±35 degrees. Thus, sunlight within at least a 70-degree range is captured, and the associated solar thermal energy is collected. [0028] To facilitate collection of solar thermal energy, the reflector 140 may be configured specifically to capture energy within the solar spectrum. In this regard, the reflector 140 may be formed of a material optimized for the solar spectrum of energy. In some embodiments, the reflector 140 may be coated with a material for such optimization.

[0029] In one embodiment, a protective cover 150 is positioned above the receptacles 120. The protective cover 150 may be sized to cover multiple receptacles 120. Alternatively, a single protective cover 150 may be positioned above each receptacle 120. The receptacle is preferably formed of a transparent glazing, such as soda lime glass, which does not interfere with the transmission of sunlight to the reflectors 140. [0030] To further prevent such interference, the protective cover 150 may be provided with an anti-reflective coating. Such anti-reflective coating ensures that sunlight is transmitted to the reflectors 140 without substantial reflecting of the sunlight away from the collector 100. The anti-reflective coating may be applied to either the inner surface of the protective cover 150 (i.e., the surface facing the reflector 140 and the receptacle 120) or the outer surface of the protective cover 150. In one embodiment, a similar anti- reflective coating may also be applied to a surface of the receptacle 120. The anti- reflective coating may be formed of any of a variety of materials. In one embodiment, the anti-reflective coating includes multi-layer, solgel texturing. Thus, collection of solar thermal energy is permitted while providing protection of the collector 100 from debris, for example.

[0031] In operation, a fluid is circulated through the manifold 110 via the pump 116. The flowrate of the fluid through the manifold 110 may be adjusted for particular conditions and particular implementations. The fluid circulates through the inlet pipe 112 and into the tube 130 within the receptacle 120. In embodiments in which the tube is integral with the manifold 110 (and the inlet pipe 112), no leakage issues are present. The positioning of the tube 130 within the receptacle 120 forms a circulation path within the tube 130 and within the receptacle 120. The circulation path includes the bifurcated portions 136, 138 of the tube 130. Thus, in one embodiment, the fluid is circulated first from the inlet pipe 112 through one bifurcated portion 136 and then through the second bifurcated portion 138. In this regard, while the fluid is flowing, it occupies substantially the entire volume within the tube 130 with one direction of flow occupying the volume in one bifurcated portion and the opposite direction of flow occupying the other bifurcated portion. The fluid then exits the tube 130 and the receptacle 120 to the outlet pipe 114. A seal (not shown) may prevent leakage of the fluid as it exits the receptacle 120. Those skilled in the art will understand that the circulation path (inlet pipe to first region to second region to outlet pipe) may be reversed in other embodiments, which are also contemplated within the scope of the present invention.

[0032] Thus, solar thermal energy is directed by the external reflector 140 onto the receptacle 120. The solar thermal energy is absorbed by the receptacle 120 and, more specifically, the absorption coating 126 on the outer surface of the inner wall 122 of the receptacle 120. As noted above, the evacuated region between the inner wall 122 and the outer wall 124 facilitates reduction in heat loss, thereby improving efficiency of the collector 100. While circulating through the two bifurcated regions 136, 138, the fluid is heated, thereby facilitating transfer of solar thermal energy from the collector 100. The fluid then carries the thermal energy out of the tube 130 and the receptacle 120 in the form of heat, whereby the fluid is heated by the thermal energy as it flows through the tube 130. The fluid circulated through the collector 100 may be selected from a variety of fluids. In one embodiment, the fluid is mineral oil. In another embodiment, the fluid is an antifreeze solution.

[0033] Embodiments of the present invention are capable of heating the fluid to temperatures of above 280 degrees Fahrenheit without the use of a solar tracker component. Certain embodiments are capable of heating the fluid to temperatures of above 300 degrees Fahrenheit as the fluid exits the receptacle 120. Thus, embodiments of the present invention can provide efficient collection of solar thermal energy in a cost- effective manner.

[0034] In one embodiment, the fluid is selected such that the boiling point of the fluid is higher than the maximum temperature reached by the fluid within the receptacle 120, typically at the point at which the fluid exits the receptacle 120. In this regard, the fluid does not boil while circulating within the receptacle 120 and, therefore, does not exert additional pressure on the walls of the receptacle 120. Accordingly, the receptacle 120 may be formed of a greater variety of materials. In a particular embodiment, the avoidance of additional pressure on the walls allows the receptacle 120 to be formed of glass.

[0035] In various embodiments, the fluid is selected such that the flash point of the fluid is higher than the maximum temperature reached by the fluid. In this regard, in the event of a leakage of fluid in the system (e.g., from the manifold in the region of a seal), the fluid does not ignite, thereby presenting a fire hazard. Accordingly, the system is made inherently fire-safe.

[0036] While particular embodiments of the present invention have been disclosed, it is to be understood that various different modifications and combinations are possible and are contemplated within the true spirit and scope of the appended claims. There is no intention, therefore, of limitations to the exact abstract and disclosure herein presented.

Claims

WHAT IS CLAIMED IS:

1. A solar thermal energy collector, comprising: a receptacle; and a tube positioned within the receptacle and having a closed end, the tube including a divider cross-sectionally bifurcating the tube, the divider being spaced apart from the closed end of the tube to allow fluid communication between two bifurcated portions of the tube, wherein a fluid is circulated through the two bifurcated portions of the tube for transferring of the solar thermal energy.

2. The collector of claim 1 , wherein the fluid for transfer of solar thermal energy is mineral oil.

3. The collector of claim 1 , wherein the fluid for transfer of solar thermal energy is an antifreeze solution.

4. The collector of claim 1 , wherein the tube is coupled to a manifold and the manifold is coupled to a pump that circulates the fluid through the manifold and the two bifurcated portions.

5. The collector of claim 4, wherein the manifold is coupled to additional tubes.

6. The collector of claim 1 , wherein the divider is a plate which cross- sectionally divides the tube into the two bifurcated portions.

7. The collector of claim 6, wherein the tube has a circular cross section and the divider forms two bifurcated portions with semi-circular cross sections.

8. The collector of claim 1 , wherein the tube is an integral part of a manifold.

9. The collector of claim 1, wherein the receptacle is a dewar, the dewar having an outer wall and an inner wall, the dewar having a vacuum drawn between the outer wall and the inner wall, and wherein the dewar is all glass.

10. The collector of claim 9, wherein a region between an outer surface of the tube and the inner wall of the dewar is filled with a second fluid to facilitate heat transfer.

11. The collector of claim 9, wherein the second fluid is mineral oil.

12. The collector of claim 9, wherein the dewar has a thermal absorption coating on an outer surface of the inner wall.

13. The collector of claim 12 wherein the coating is aluminum nitride cermets.

14. The collector of claim 1, wherein absent a solar tracker component and in combination with an external reflector component, the fluid has a temperature above 280 degrees Fahrenheit when the fluid exits the receptacle.

15. The collector of claim 1, further comprising an external reflector for reflecting sun rays onto the receptacle.

16. The collector of claim 15, wherein the external reflector is a compound parabolic concentrator (CPC).

17. A method for collecting solar thermal energy, comprising: positioning one or more reflectors external to one or more receptacles, the reflectors being adapted to direct solar thermal energy to the one or more receptacles; positioning a manifold having one or more tubes adapted to fit within the one or more receptacles, each tube having a closed end and having a divider cross- sectionally bifurcating the tube, the divider being spaced apart from the closed end of the tube to allow fluid communication between two bifurcated portions of the tube; and circulating a fluid through the two bifurcated portions of the tube for transferring of the solar thermal energy.

18. The method for collecting solar thermal energy of claim 17, wherein the fluid is mineral oil.

19. The method for collecting solar thermal energy of claim 17, wherein the fluid is an antifreeze solution.

20. The method for collecting solar thermal energy of claim 17, wherein the divider is a plate which cross-sectionally divides the tube into two bifurcated portions.

21. The method for collecting solar thermal energy of claim 17, wherein the receptacle is a dewar, the dewar has an outer wall and an inner wall, the dewar has a vacuum drawn between the outer wall and the inner wall, and the dewar is all glass.

22. The method of claim 21 , wherein a region between an outer surface of the tube and the inner wall o fthe dewar is filled with a second fluid to facilitate heat transfer.

23. The method of claim 21 , wherein the dewar has a thermal absorption coating on an outer surface of the inner wall.