Abstract:
A low-cost absorber panel for a solar hot water heating system is provided. An aluminum absorber plate, under exposure to solar radiation, makes an efficient transfer of heat to water circulating within the collector panel by direct contact with the medium in an optimized area-to-volume configuration. An innovative use of fasteners makes it possible to accommodate expansion for freezing and to allow for disassembly and maintenance.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to solar energy collectors, and more specifically to low pressure solar hot water heater absorbers. 
       BACKGROUND OF THE INVENTION 
       [0002]    With the potential of future capacity shortages in the aging infrastructure of the electricity grid in the United States, not to mention the adverse-weather-prone supply of natural gas in the nation, solar energy represents an alternative to looming energy crisis&#39;s, as well as rising energy costs in the present. Next to space heating, water heating is the most significant factor in residential energy consumption and accounts for 13% of the household energy bill. What is needed is a solar hot water heating installation that can be accomplished on a budget by the average homeowner. 
         [0003]    Flat-plate solar collectors for water heating are present in the art. They are comprised of a heat-absorption panel in contact with circulating water and are typically housed under a transparent covering and over an insulating bed. The solar collectors are part of a system including a water supply and/or storage means, a circulation pump, and a heat-exchanger means. The heat transfer efficiency is a function of reflective, conductive, and convective heat losses, as well as the temperature differential between the absorption panel and the incident water. The convective losses are affected by the insulating bed and air envelopes, and the reflective losses depend on the reflectivity of facing surfaces, including the transparent covering. 
         [0004]    Commercially available types of flat-plate collectors are typically constructed with rows of riser tubes placed in contact with an absorber plate exposed to radiant heat from the sun. Higher pressure is required to circulate the water at a rate up to 2 gpm through the tubing. Since the water is not in direct contact with the collector plate, some loss of conductive heat efficiency occurs. The cost of materials for a copper-tube collector is about $5.00-$10.00/sq ft, and the retail cost can range up to $15.00/sq ft. 
         [0005]    A preferred type of collector is a low pressure system wherein the water circulates through relatively wide passageways between thinly-sandwiched absorber and companion plates and wherein the water is in direct contact with the absorber plate. This is more thermally efficient in two ways: First, the water is intimate with the radiantly heated surface of the absorber plate, so conductive losses are less; second, the absorber plate operates with a lower thermal gradient with respect to the environment, so convective losses are less. The passageways are sufficiently broad as to allow a 1.5 psi pressure for a 2 gpm circulation; consequently, seal integrity is less of a problem. Typically, there is a serpentine path of water flow created by internal baffles which lengthen the contact time and create a certain amount of turbulence for the even distribution of heat. The present invention addresses a simple and cost-effective means for constructing such a system from readily-available materials. 
         [0006]    The prior art discloses different methods of construction for low pressure collectors. U.S. Pat. No. 4,103,675 to Bar-On, for example, uses an assembly of extrusions with channels therein to form passageways between inlet and outlet manifolds. The passageways do not follow a tortuous path in this case. Insufficient dwell time in contact with a radiantly heated surface might be a problem with this construction, which problem is overcome by the preferred serpentine path. Such a construction, additionally, involving welded joints, cannot be taken apart for cleaning and maintenance. Furthermore, the welding of metal parts, and particularly aluminum parts, is not a fabrication process in common usage in the home. 
         [0007]    Another method is exemplified by U.S. Pat. No. 4,182,308 to Reynolds. Reynolds teaches a construction wherein two sheets of rubber-like material are bonded along the periphery to create a water-tight interior and in alternating parallel strips with unbonded portions at the ends to create a serpentine flow path. Channels are formed by expansion of the material between the bonds when water is introduced to the system. Again, the bonded construction makes disassembly impossible, and the thermal efficiency of the elastic material is furthermore sub-optimal. Rubber, for example, has a thermal conductivity of 1.6 W/m-K, whereas the thermal conductivity of aluminum, by comparison, is 237. 
         [0008]    U.S. Pat. No. 4,315,499 to Shonerd demonstrates a third alternative for channel construction. Shonerd discloses baffles formed as a part of a base extending from sidewalls to create the flow path which alternates from one sidewall to the other. The sidewalls and the baffles are the spacers which form the narrow-depth passageways. While Shonerd uses a thermally conductive metal absorber plate over the base, and while the system is conceptually capable of disassembly, Shonerd&#39;s construction, on the other hand, does not use simplified materials readily available to the average homeowner. 
         [0009]    What is missing in the prior art is a simplified means to form spacing and channeling in a low-pressure, thermally-efficient, easily-maintainable, solar collector with the use of standard hand tools. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    In view of the above-mentioned unfulfilled needs, the present invention embodies, but is not limited by, the following objects and advantages: 
         [0011]    A first objective of the present invention is to achieve a reduction of cost per BTU per square foot of 30%-45%. 
         [0012]    A second objective of the present invention is to achieve an operating pressure not exceeding 2 psi, and preferably, not exceeding 1 psi, for a circulation of 1.5-2.0 gpm. 
         [0013]    A third objective of the present invention is to provide an absorber plate material optimizing thermal efficiency with cost. 
         [0014]    A fourth objective of the present invention is to provide an absorber plate with a thermal conductivity exceeding that of stainless steel at 16.3 W/m-K. 
         [0015]    A fifth objective of the present invention is to provide a means for creating spacing and sealing between collector plates which is simple to construct with readily available materials. 
         [0016]    A sixth objective of the present invention is to provide a means for creating channeling and flow paths which is simple to construct with readily available materials. 
         [0017]    A seventh objective of the present invention is to provide a means for disassembling and cleaning the solar collector for easy maintenance. 
         [0018]    An eighth objective of the present invention is to provide a water-tight means for sealing which is adjustable under varying climatic and wear conditions. 
         [0019]    A ninth objective of the present invention is to provide a means for expansion to accommodate the freezing of water during winter night-time temperatures. 
         [0020]    In a preferred embodiment of the present invention, a low-cost absorber panel for solar hot water systems is comprised of a rectangular aluminum absorber plate combined with a matching aluminum companion plate in a sandwich-like composite. The two plates are held together with a means for fastening, the means also providing a shaping function. A means for spacing is inserted between the plates to form a boundary there around and create a water-tight internal space for a flow of water there through. Included in the internal space is a means for channeling defining a serpentine flow path. The water enters the internal space through a port in one of the absorber and companion plates and exits through another port in one of the absorber and companion plates, the two ports located so as to maximize the serpentine flow path. The water entering the internal space and moving there through under low-pressure is heated by contact with the absorber plate, which is in turn exposed to solar radiation. The water exits the space to transfer the heat to an external system. 
         [0021]    In a particularly preferred embodiment, the means for fastening and shaping is comprised of machine screws and nuts. The machine screws and nuts are individually adjustable to shape the plate surfaces and the space in between. The machine screws and nuts are optionally provided with o-ring seals when positioned in the internal space. 
         [0022]    In a particularly preferred embodiment, the means for spacing is an elastic gasket of substantially circular cross-section which is arrayed peripherally and adjustably tensioned to provide a compression seal by means of the machine screws and nuts. The machine screws and nuts straddle the gasket at regular intervals along its extent. 
         [0023]    The same elastic gasket material is used for the means for channeling. In this case, lengths of gasket extend in alternating fashion from one end or the other of the periphery gasket, beginning with the end closest to the entry port and ending with the end closest to the exit port. The lengths stop short of the opposite end to provide passages between adjacent channels, and the passages at alternating ends create a serpentine path of water flow from entry to exit. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
         [0024]    Various other objects, features and attendant advantages of the present invention will become fully appreciated as the same becomes better understood through the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views, and wherein: 
           [0025]      FIG. 1  is a perspective view of the assembled solar hot water heater; 
           [0026]      FIG. 2  is an exploded perspective view of the solar hot water heater; 
           [0027]      FIG. 3  is a perspective view of the absorber panel; 
           [0028]      FIG. 4  is a perspective view of the absorber panel with the absorber plate removed; 
           [0029]      FIG. 5  is a partial sectional view of the absorber panel showing the gasketing and channel; 
           [0030]      FIG. 6  is a partial cutaway plan view showing fastener and o-ring details; 
           [0031]      FIG. 7  is a partial sectional view showing fastener and o-ring detail. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    For the purposes of this description and for the claims following, the term “o-ring” can mean both toroidal and flat-shaped, and connotes compressibility such that sealing is achieved. 
         [0033]    A solar hot water heater  1  is shown in  FIGS. 1 and 2 . Solar hot water heater  1  is comprised of a frame  2 , which houses an absorber panel  10 , and a transparent cover  3 , which provides an enclosure. The air space  5  within the enclosure and above the absorber panel  10  functions to reduce convective heat losses. Solar radiation  4 , passing through the transparent cover  3  and the air space  5 , is incident upon the absorber panel  10  where the radiation heats the contents therein by conduction through its absorption. Insulation material may optionally be added between frame  2  and absorber panel  10 . Coatings may also be optionally added to both the absorber panel  10  and the transparent cover  3  to respectively enhance absorption and decrease reflective losses. 
         [0034]    The assembled absorber panel  10  is best shown in  FIG. 3 . Referring to  FIG. 2 , absorber panel  10  is comprised of absorber plate  11  and companion plate  12  joined in a thinly-spaced-apart, sandwich-like, composite. Water is introduced to absorber panel  10  from a water supply  16  (not shown) through entry port  13 , and is distributed from the absorber panel to a heat exchanger  17  (not shown) through exit port  14 . Entry port  13  and exit port  14  can be positioned either on the absorber plate or the companion plate. 
         [0035]    In the preferred embodiment, absorber plate  11  and companion plate  12  are comprised of a 48 inch by 96 inch by 0.032 inch aluminum sheet of alloy 5052H32, or similar. 
         [0036]      FIG. 4  shows the absorber panel  10  with the absorber plate  11  removed. A means for spacing  30  is shown at the periphery edges of the exposed companion plate  12 , said means being a periphery gasket  32 , and, specifically, an elastic gasket  31  having a circular cross-section. Also shown in the figure, extending from alternating edges of periphery gasket  32 , is a means for channeling  40 , said means also being elastic gasket  31 . Means for spacing  30  and means for channeling  40  can be collectively described as a means for resiliently creating internal space and channels  200 . 
         [0037]    Periphery gasket  30  creates an internal space  15  for the containment of water therein. Lengths of elastic gasket  31 , each length shorter than one of the periphery edges to which it is placed in parallel, the lengths extending alternately from the two periphery edges which are perpendicular to the afore-mentioned periphery edge, collectively form a system of channels  43 , best shown in  FIG. 4 . Adjacent channels  43  are connected by alternating passages  42 , the passages being free spaces at the terminus of each wall section formed by the lengths of elastic gasket  31 . The alternation of passages  42 , from one perpendicular periphery edge to the other, creates a serpentine path  41  for the flow of water through the internal space  15 .  FIG. 5  shows one of the channels  43  defined by bordering sections of elastic gasket  31 , and particularly illustrates the extreme low-aspect ratio of the channel; that is to say, the short depth to the wide breadth. Such an aspect optimizes the contact surface area for heat transfer. 
         [0038]    It is preferred that the elastic gasket  31  have a circular cross-section. Because twisting is inevitable in any running-length of material, the constant height of a circular cross-section is an advantage when water-tight sealing is required, particularly when the seal is challenged by pressure. In the preferred embodiment, the elastic gasket is of a butyl rubber or silicone composition having a one-quarter inch diameter. This material, like the aluminum sheet material, is also a commodity-supply item. 
         [0039]    Turning to  FIGS. 6 and 7 , a means for fastening and shaping  20  is shown, said means being machine screws and nuts  21 . The means for fastening and shaping  20  is alternatively known as a means for adjustable fastening and shaping  100 . This feature is not shown in any of the previous figures, although positions are shown in several of them by screw holes. Machine screws and nuts  21 , shown in alternating positions on both sides of periphery gasket  30  in  FIG. 6 , serve both to anchor the gasket and to apply compressive force for sealing purposes. The staggered position applies the compressive force evenly around the gasket. The individually adjustable nature of each fastener makes it possible to stop any leaks which may result from the variation of materials and applied tension. In the same manner, machine screws and nuts  21  anchor the lengths of elastic gasket  31  forming the channels. 
         [0040]    The machine screws and nuts  21  can also provide a means for shaping surfaces and space. Because of the sheerness of absorber plate and the wideness of the channels, there will be bulging when water is moving there through under pressure. Through-bolted machine screws placed at selected locations intermediate the channels can be used to control the bulge, or alternatively, to allow a bulge for the expansion of freezing water. The positions for such intermediate placement are shown by screw holes in  FIG. 4 . All machine screws traversing internal space  15  are provided with O-rings  22  to seal the perforations of that space. Because the machine screws and nuts  21  are easily removable, it is a relatively simple matter to disassemble the absorber panel for cleaning and maintenance. 
         [0041]    In the preferred embodiment, the machine screws and nuts are placed at five inch intervals. Such placement optimizes the compression profile with the testing pressure of six psi. The machine screws of preference are one-half inch stainless steel, gauge 6-32. The o-rings are 0.25-0.60 inches in diameter. 
         [0042]    The fabrication process for the absorber panel could not be simpler. It merely requires drilling a pattern of holes, laying out the gasket material to form boundaries and channels, mounting the ports, and through-bolting to unify and seal. An assembled thirty-two square foot collector panel, made according to the present invention, could cost as little as $90. 
         [0043]    It is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the preceding description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of the description and should not be regarded as limiting.