SOLAR ENERGY COLLECTOR SYSTEM

Abstract: A solar energy collector system comprising a thermal storage tank (43) and a plurality of evacuated solar collectors (42) in which the open end of each evacuated collector (42) is connected directly into a heat exchanger element (41) attached to the exterior surface of the storage tank (43). The evacuated collectors (42) are positioned with the open ends of the absorber tubes located at the same level or at a higher level than the closed ends, and the heat exchanger element (41) or elements and evacuated collectors (42) are filled with fluid.

Description: SOLAR ENERGY COLLECTOR SYSTEM BACKGROUND This invention relates to a solar energy collector system comprising a plurality of evacuated solar collectors and an adjacent thermal storage tank which incorporates a device for the transfer of heat from the evacuated collectors to the storage tank. Glass tubular evacuated solar collectors are of basic construction similar in concept to an elongated thermos flask with the region between inner and outer glass tubes being evacuated and permanently sealed off. The inner tube (absorber tube) is coated on its outside with a selective surface. Sunlight passes through the outer envelope and is absorbed by the selective surface, thus heating the inner tube and its contents. Thermal losses due to conduction and convection are eliminated by vacuum insulation of the absorber tube, and thermal losses due to reradiation are minimised by the low emittance of the selective surface. To eliminate problems .resulting from differential thermal expansion of the hot absorber tube relative to the cold glass envelope, the collectors are normally of single-ended design. A number of evacuated collectors are assembled side by side to form a "collector panel". Extraction of heat from a collector panel is achieved by the circulation of a heat transfer fluid (normally water) through the evacuated collectors. The device incorporated into the collector panel to provide for fluid circulation through the single-ended absorber tubes of the evacuated collectors in the panel is termed the "collector manifold". In one type of manifold (water-in-glass) the water is circulated directly through the glass absorber tubes of the evacuated collectors. Each collector is sealed into a "cup" by means of an 0-ring, the "cups" being soldered to two header pipes which connect to either a storage tank or heat exchanger. Water is introduced into the absorber tubes of the collectors by means of a glass feed tube which is sealed into the copper cup by means of an additional 0-ring. The header pipes are insulated, with the insulation protected by metal or plastic cover s ins. In the alternative type of manifold (water-in-metal) the water is circulated through copper U-tubes which are inserted into the absorber tubes of the evacuated collectors in the panel. Heat is transferred from the glass wall of the absorber tube to the copper U-tube by means of an attached split copper or aluminium cylinder which makes a snug fit with OMPI the glass (corrosion problems are associated with both types of fin). The copper U—tubes are interconnected outside the collectors by soldering or welding into larger diameter header pipes which are connected to a storage tank or heat exchanger. The header pipes are insulated, with the insulation protected by metal or plastic cover skins. The main advantages of the water-iu-glass manifold relative to the water-in-metal manifold are: (i) Lower material and labour costs. - (ii) No corrosion problems. (iii) Higher efficiency as a result of the direct contact between the heat transfer fluid and the glass absorber tube. For the water-in-glass manifold the absorber tubes operate at a 10-20 C lower temperature than for a water-in-metal manifold operating under similar conditions. (iv) The low thermal losses from the evacuated collectors combined with the large volume and high specific heat and latent heat of fusion of the water in the evacuated collectors result in excellent resistance to freezing. As a consequence, freezing has only occurred in the header pipes or connecting cups of installed collectors employing water-in-glass manifolds. The main disadvantages of the water-in-glass manifold relative to the water-in-metal manifold are: (i) Breakage of one collector tube results in the total loss of water from the header pipes, with the consequence that heat transfer ceases from the collectors to the storage tank or heatexchanger. (ϋ) The collectors are subject to thermal shock and breakage during refilling of the collectors and manifold with cold water following a period of collector stagnation (the absorber tubes "stagnate" at a temperature of 250-300 C during periods of water circulation failure), (iii) The collectors must be connected to a mains pressure storage tank indirectly via a heat exchanger (however, employing a heat exchanger has the advantage of substantially reducing the accumulation of particulates and corrosion products in the manifold and collectors). (iv) The high thermal mass of water in the collectors results in a significant fraction of the absorbed solar energy being stored in the collector tubes rather than the storage tank at sundown (typically 5%) , with the subsequent loss of some of this heat due to thermal losses from the tubes during the night. The objects of the present invention are the construction of a low cos water-in-glass type manifold which continues to operate following the breakage of one or more evacuated collectors, which substantially eliminates the efficiency reduction resulting from the high thermal mass of water in the collectors while maintaining the associated resistance to freezing, which eliminates the problem of thermal shock during a "hot-fill" of the collectors, and which eliminates the requirements for separate header pipes with the associated insulation and cover skins, SUMMARY OF THE INVENTION. The present invention is a solar energy collector system comprising a thermal storage tank and a plurality of evacuated solar collectors in which the open end of each evacuated collector is connected directl into and is in fluid passage communication with a heat exchanger element attached to the exterior surface of the storage tank. The evacuated collectors are positioned with the open ends of the absorber tubes located at the same level or at a higher level than the closed en and the heat exchanger element or elements and evacuated collectors are filled with a fluid. Heat transfer from the collectors to the heat-exchanger element/s is due to thermosiphon circulation of the fluid in the absorber tubes. The fluid in the heat exchanger elements is in direct contact with the storage tank wall, so the efficiency of heat transfer from the heat exchanger elements to the storage tank is a function of the area and position of the heat exchanger elements. In accordance with the objects of the invention: (i) A number of heat exchanger elements attached to the storage tank allows a design having independent operation of the heat exchanger elements, and thus breakage of one or more collectors attached to one element does not effect the operation of the remaining collector - heat exchanger element combinations. (ii) For the case of water as the heat transfer and thermal storage fluid (solar hot water system), typical daily hot water usage patterns exhibit peak consumption in the morning and evening periods with the associated injection of cold water into the bottom of the storage tank. As a consequence, a major fraction of the heat stored in the collectors at sundown is transferred to the heat excha-.ger element/s and hence to the storage tank due to the reconjnencement of thermosiphoning in the collectors. Also, the water in an evacuated collector w.iich is connected to a storage tank in accordance with the invention, will not freeze, even in an extreme low temperature environment. The probable mechanism preventing freezing of the water column in the absorber tube is an "inverse thermosiphon". That is, the ice particles forming in the water column have lower density than the surrounding water and thus rise through the absorber tube water column into the heat exchanger element and melt. (iii) The rate of delivery of fluid to the heat exchanger elements can be reduced to a level which minimises thermal shock during filling of a collector - heat exchanger element combination following collector stagnation (by a relatively high impedance at the heat exchanger element - fluid delivery pipeconnection, for example), (iv) The storage tank thermal insulation also insulates the heat exchan element/s, thus eliminating the requirement for separate manifold insulation and cover skins. BRIEF DESCRIPTION OF THE DRAWINGS. The invention will be more fully understood from the following detailed description, which is given with reference to the accompanying drawings wherein: Figure 1 snows a sectional end-elevation view of a prior art water-in-met manifold, and Figure 1A shows a sectional view of an e vacuated collector with the manifold inserted. Figure 2 shows a side elevation view of a prior art water-in-glass manifold. Figure 3 shows a perspective (partly schematic) view of a solar collector system to which the present invention relates. Figure 4 shows a side elevation of an evacuated collector connected to a heat exchanger element attached to the atorage tank wall in accordance with a first method of the present invention. Figure 5 shows a schematic view of a solar collector system with a number of heat exchanger elements attached to the exterior -urface of the storage tank in accordance with the present invention. Figure 6 shows a side elevation of an evacuated collector connected to . a heat exchanger element attached to the stora 0 e tank wall in accordance with a second method of the present invention. Figure 7 shows a plan view of an inter eαiate gasket suitable for the second method of the present invention. ^»UK.£ ' OMP Figure 8 shows a plan view of another intermediate gas.-cet suitable for the second method of the present invention. igures 7A and 8A show sections of parts of these gaskets. DETAILED DESCRIPTION. As shown in Figures 1 and 1A, one prior art collector system compris a twin header pipe manifold 12 to which a U-tube 13 is connected for channelling heat transfer fluid through a single-ended absorber tube 1 The fluid is not in direct contact with the absorber tube 14 but one side of the U-tube is maintained in thermal contact with the inner wall of the absorber tube via a resilient metal retaining sleeve 15. As shown in Figure 2, a second prior art collector system comprises a distributor element 21 to which axe connected header pipes 22 and a glass conduit 23 and evacuated collector 24. The glass conduit 23 functions to convey fluid from the evacuated collector 24 and into a header pipe 22. The inflowing fluid is directed into the evacuated collector from the manifold header and passes along the annulus between the glaas conduit 23 and the inner wall of the absorber tube of the evacuated collector 24• A significant feature of the prior art solar collector systems is that the evacuated collectors are connected indirectly to a storage tank or heat exchanger by means of one or two header pipes. According to the invention the open ends of individual evacuated collectors in a collector panel are connected directly to heat exchange elements attached to the exterior wall of the storage tank, and the evacuated collectors are positioned with the open ends of the absorber tubes located at a highes level than the closed ends. The heat exchange elements and evacuated collectors are filled with heat transfer fluid, and the heat exchanger elements can be designed for independent operation so that breakage of one or more collectors does not affect th operation of the remaining collector tube - heat exchanger element combinations. The principle design features of the invention are illustrated in Figures 3 and 4, which show a relation of the heat exchanger elements 31 and 41 to the evacuated collectors 32 and 42, the storage tank, wall 33 and 43» and the fluid delivery path 44 and air bleed path 4 • The heat exchanger element or elements 31 and 4 and evacuated collectors 32 and 4 are filled with a heat transfer fluid 46, with heat transfer from the evacuated collectors to the heat exchanger elements due to theriaosiphoπing. Fluid levels in the heat exchanger element or elements f OMPI 31 and 41 are maintained via the fluid delivery pipe 44. An 0-ring can be employed to seal the evacuated collectors to the heat exchanger e lement or elements. The fluid in the heat exchanger elements 31 and 41 is in direct contact with the storage tank wall 33 and 43» so the efficiency of heat transfer from the heat exchanger elements 31 and 41 to the storage tank wall 35 and 43 is a function of the area and position of the heat exchanger elements. The storage tank insulation 38 and 48 and outer jacket 39 and49 also insulates the heat exchanger elements. Figure 5 shows a relation of the heat exchanger elements 51 to the evacuated collectors 52, the storage tank 53i and the fluid delivery path 54 and air bleed path 55 which permits independent operation of individual heat exchanger element 51 - evacuated collector 52 combinatio For a water supply rate to the header tank 57 and water delivery pipe 54 having a value which is many times higher than the water delivery rate to an individual heat exchanger element 5 1 the water delivery pipe 54 remains filled with water even if several collectors are broken, and thus the water levels in the heat exchanger element 51 - evacuated collector combinations are maintained. Also, the rate of delivery of water to the heat exchanger elements 51 can be reduced to a level which minimises thermal shock during filling of a heat exchanger element 51 - evacuated collector 52 combination following collector stagnation (by a relatively high impedance at the heat exchanger element - water deli ery pipe connections, for example). Figure 6 shows a modification of the design which employs an intermediate gasket 61 to connect the heat exchanger element 62 to the storage tank wall 63. The fluid delivery path 63 and the air bleed path 64 for the heat exchanger element 62 can be incorporated into the gasket 61. Figures 7 and 8 show plan views and Figures 7A, 8A show sections 3B 1 and CC of intermediate gaskets 71 and 81 which incorporate a main water delivery path 72 and 82, water delivery paths 73 and 83 to individual heat exchanger element areas 74 and 84, a main air bleed path 75 and 85 , air bleed paths for the individual heat exchanger element areas 76 and 86, an air bleed path 77 and 87 for the main water delivery inlet 78 and 88 and air bleed outlet 79 and 89 for the gasket. The heat exchanger element can be molded in plastic as a single unit and attached to the storage tank wall via a rubber OMPI gasket tne ruooer gasκeι compensaces xor tne α iereni tnerinai expansion rates of plastic and metal).

Claims: STATEMENT OF CLAIMS. The claims defining the invention are as follows: 1. A solar energy collector system comprising a thermal storage tank and a plurality of evacuated solar collectors in which the open end of each evacuated collector is connected directly to and is in fluid passage communication with a heat exchanger element attached to the exterior surface of the storage tank; the evacuated collectors being positioned with the open ends of the absorber tubes located at the same level or at a higher level than the closed ends, and the heat exchanger element or elements and the evacuated collectors being filled with a fluid. 2. The solar energy collector system of Claim 1 in which the evacuated collectors are all-glass evacuated collectors. 3. The solar energy collector system of Claim 1 in which one heat exchanger element is attached to the exterior surface of the storage tank. 4. The solar energy collector system of Claim 1 in which a plurality of heat exchanger elements are attached to the exterior surface of the storage tank. 5. The solar energy collector system of Claim 4 in which a substantial length of each individual heat exchanger element is not in direct fluid passage communication with adjacent heat exchanger elements. 6. The solar energy collector system of Claim 1 in which the heat exchanger element or elements are attached to the storage tank exterior surface via an intermediate gasket. 7» The solar energy collector system of Claim 1 in wnich the heat exchanger element or elements are in fluid passage communication with a fluid delivery pipe and/or an air bleed pipe. 8. The solar energy collector system of Claim 6 in which fluid del ivery passages are incorporated into the intermediate gasket. 9« The solar energy collector systems of Claims 7 and 8 in which the fluid delivery pipe/passage is in fluid passage communication with a fluid storage tank. 10. The solar energy collector system of Claim 1 in which the evacuated collectors are spaced and a reflector coextensive with the plurality o collectors is positioned-on the rear side of the collectors. 11. The solar energy collector system of Claim 1 in which the exterior surface of the storage tank coextensive with the heat exchanger element or elements has a protective coating. Dated this 11th day of January, 1 84. STUART CRAIG. NAME OF APPLICANT.

Inventors: CRAIG STUART, CRAIG, STUART
Applicants: CRAIG STUART, CRAIG, STUART
Application year: 1984
Main classification: F24J3
IPC codes: F24J2