Patent Publication Number: US-2010108054-A1

Title: Optically efficient and thermally protected solar heating apparatus and method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to solar heating devices and methods, and more particularly, to an optically efficient solar heater that is thermally protected. 
     2. Description of the Related Art 
     Solar heaters for heating water or other liquids or aerosols are useful for heating water for providing building heating, hot tap water supply, swimming pool heating and other uses such as providing heat for thermo-chemical reactions. 
     A common form of solar heater uses a stationary connected system of pipes, without solar tracking. The pipes are attached to an optically absorbent (black) backing, which is typically thermally isolated from the mounting structure to which it is attached, typically by a thermal insulator provided beneath the backing. The system of pipes contains a liquid medium, typically water, which is heated by direct solar radiation. The system is covered by a glazing glass window, which is optically transparent in the visible spectrum, but is opaque to thermal infrared radiation that is emitted by the heated absorber. The presence of the glazing glass and insulator permit the liquid in the system to reach thermal equilibrium at a much higher temperature than would be possible with a set of pipes in open air. 
     However, there are several disadvantages to the typical solar heater described above. First, no solar concentration is employed, which makes the heating process slow, reduces the usable “Sun time”, and reduces the achievable liquid temperature. In water heating applications, the lower the output water temperature, the less the available hot water capacity, since less cold water can be mixed with the heated water during use. Therefore, large water tanks are required to store heated liquid, approaching 100% of the maximum demand amount for systems designed to provide as much hot water as possible at the end of the available “Sun time.” 
     Second, the piping system and the absorber form a large linked thermal mass, and therefore the thermal response time of the liquid medium to the onset of a solar radiation cycle is slow, causing additional loss of available “Sun time.” Also, the hot portion of the system—the piping system and the absorber—has a large surface area, which increases system losses. Third, while no overheating protection is necessary for typical non-concentrating solar heating systems, such systems are susceptible to damage in freezing conditions, typically requiring preventative draining of the system during cold weather conditions, or making the system more complicated and less efficient by separating the exposed heating loop from the main water supply by using a heat exchanger and filling the exposed heating loop with an anti-freeze liquid mixture. 
     Fourth, the typical solar heating system has significant weight, raising structural support and installation issues. The above-described solar water heaters are typically heavy and installed in large sections, making installation difficult for a solitary installer or homeowner. Finally, typical collectors are fabricated from large quantities of expensive metals (e.g., copper) that are heavy, difficult to recycle, and involve carbon dioxide emission in their manufacture. 
     Therefore, it would be desirable to provide a more efficient stationary solar heating system, with a fast heating response and elevated water temperature. It would further be desirable to provide such a solar heating system that is lightweight, has low manufacturing and installation cost, which is protected against freezing and overheating, and which has a lowered environmental impact. 
     SUMMARY OF THE INVENTION 
     The objective of providing an efficient, lightweight, thermally protected solar heating system having low manufacturing and installation cost, and which has lowered environmental impact, is provided in a solar heater apparatus and method of heating a first liquid or aerosol medium. The method is a method of manufacturing the solar heater. 
     The solar heater comprises a stationary concentrating light collector including transparent tubes through which a liquid or aerosol flows and is heated. The light collector has multiple parallel concave reflector sections each for containing one of the tubes, and the tubes are interconnected via manifolds at each end. The collector is closed by a transparent top and on the ends by end walls, which may incorporate the manifolds. The top may be formed in two separate layers, providing an insulating air gap between the layers, with the topmost layer serving as glazing. Similarly, side walls may be formed having an air gap to insulate the sides of the final assembly. 
     The transparent tubes contain an absorbing material or structure through which the first liquid or aerosol is permitted to flow. The absorbing material may also form a catalytic surface for enhancing a reaction between substances in the first liquid or aerosol medium. Under normal heating conditions, the concentrating light collector is completely filled with a second liquid, which may be of the same composition as the medium in the tubes. 
     The collector, the tubes, the top, the side walls and air gap can be extruded as a single recyclable transparent plastic unit forming multiple parallel collector reflectors, which are coated from the back (bottom surface) of the collector portion to form the reflective collector. Similarly, end walls forming the manifolds can be extruded as whole units. The tubes can be filled with a granular absorbing material, or an absorbing structure or sponge-like absorbing material inserted into the tubes during the assembly process. Extruding most or all of the solar heater from the same recyclable plastic material simplifies both manufacture and recycling of the system components. A bottom panel may be made from the same material as the collector, and attached to the bottom and side walls to form a complete thermally insulating housing. 
     The top may include a bottom (inside) surface that is shaped and has a refractive index greater than that of air, such that when reflective collector is completely filled with the second liquid medium, incident light is transmitted through the top, but when the liquid is not in contact with the bottom surface of the top, incident light is reflected. Thus, in order to prevent damage due to overheating in extreme temperature conditions, the second liquid inside the collector may be at least partially drained, preventing most or all of the incident light from reaching the transparent tubes. The draining process may be initiated manually or performed automatically in response to a thermal sensor built into the system. The shaped top can be manufactured in the same single extrusion process described above. 
     The foregoing and other objectives, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiment of the invention, as illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein like reference numerals indicate like components, and: 
         FIGS. 1A-1B  are an end view and an isometric view, respectively, of a solar collector unit in accordance with an embodiment of the invention. 
         FIG. 2  is a partial sectional view of the solar collector of  FIGS. 1A-1B . 
         FIG. 3A-3C  are a side view, an end view and an isometric view, respectively, of a manifold  30  in accordance with an embodiment of the invention that can be attached to the ends of the solar collector of  FIGS. 1A-1B . 
         FIGS. 4A-4B  are a partial sectional view and a partial top view, respectively, showing details of a completely assembled solar heater in accordance with an embodiment of the invention. 
         FIGS. 5A and 5B  are pictorial drawings illustrating the thermally protective action of transparent top  17  of  FIGS. 1A-1B ,  2 , and  4 A- 4 B. 
         FIGS. 6A  and  FIG. 6B  are pictorial drawings illustrating a shape of transparent tubes  14  of  1 A- 1 B,  2 , and  4 A- 4 B during normal operating conditions and during a freezing condition, respectively. 
     
    
    
     DESCRIPTION OF ILLUSTRATIVE EMBODIMENT 
     The present invention encompasses solar heating systems that employ concentrating reflective collectors to provide high efficiency over a wide collection angle and that provide thermal protection by using flexible (elastic) structures and by providing a top that is substantially reflective when a second liquid medium that normally fills the space between the top and the reflective collector surface is drained or otherwise displaced by a gas. Heating of a first liquid or aerosol medium is provided by flowing the first liquid or aerosol medium through a transparent tube containing a porous absorbing material or structure. The medium being heated may be a liquid, such as water, or an aerosol, such as a chemical mixture that reacts under thermal agitation. The absorbing material may be coated with a catalytic surface, enhancing a reaction between substances in the first liquid or aerosol medium. 
     Since the second liquid medium has a refractive index greater than that of air, the interface between the air and the second liquid medium “bends” incident light rays towards the angle normal to the interface, providing concentration for the full angular range of light receivable by the aperture formed by the top of the collector (i.e., substantially 180 degrees), including direct light at the beginning and end of the “Sun time”, as well as collecting diffuse light that reaches the aperture, such as light reflected from clouds. The concentration ratio of such a system is equal to the refractive index of the second liquid medium, which is sufficient to considerably improve the solar heating system&#39;s efficiency by elevating the temperature of the heated medium. 
     The present invention also encompasses solar heating systems that employ a solar collector that is extruded as a single piece, including reflective collector(s) and a transparent top. Optionally, the tubes through which the medium to be heated is conducted, a top glazing cover forming an air gap above the transparent top, and side panels including air gap channels may also be formed in the same single extrusion. Manifolds for attachment at ends of the collector unit can also be extruded or molded from the same material, providing simplicity of manufacture, thermal/chemical compatibility of materials, and ease of recycling. The manifolds may be glued, ultrasonically welded, or attached in another suitable manner to provide a liquid-tight seal. The attachment may be performed at the time of manufacture, or on-site during installation. The plastic material employed in the extrusion is generally an elastic material, such as LEXAN or APEC, rather than other more brittle plastics or other materials such as glass. The collector assembly is also extruded as a thin-wall structure, so that under internal or external pressure, such as will occur during freezing, the assembly changes shape but does not fracture. Moreover, when the internal or external pressure is removed, the collector assembly will recover its original shape. As such, the plastic collector assembly is highly resilient and can be certified for use in outside walls, windows and tiles in weather conditions including hurricane level forces. 
     Referring now to  FIGS. 1A-1B , a solar heating collector unit  10  in accordance with an embodiment of the present invention is shown. A number of transparent tubes  14 , in which the first liquid or aerosol medium flows and is heated, terminate on the ends of collector unit  10 . Transparent tubes  14  are disposed within corresponding collector channels  12  within collector unit  10 , which can be extruded as a single thin-walled polymeric structure and then back-coated to provide a reflective surface  13  on the bottom side of collector channels  12 . Alternatively, the reflective surface may be formed on either side of, or internal to the material forming collector channels  12 , or collector channels may be made of a reflective material in embodiments in which a single extrusion of collector unit  10  is not performed. A top cover  15  of collector unit  10  includes an air gap channel  16  formed between a liquid-retaining top  17  of collector channels  12 , and an outer top surface  21  of collector unit  10 . Air gap channel  16  provides thermal insulation for the outer top surface  21  of collector unit and outer top surface  21  serves as a glazing surface. 
     Referring now to  FIG. 2 , details of collector unit  10  of  FIG. 1B  are shown in cross-section. In addition to the air gaps provided between liquid-retaining top  17  and outer top surface  21 , side walls  11  also include air gap channels  19  to provide thermal insulation at the sides of collector unit  10 . Transparent tubes  14  are elliptical in cross section, rather than circular. If pressure increases within transparent tubes  14  due to boiling or other overpressure condition, transparent tubes  14  will deform to become more circular, preventing transparent tubes  14  from bursting. Further, if a second liquid  20  disposed within collector channels  12  freezes, then transparent tubes  14  accommodate the increased external pressure by flattening (i.e., becoming more elliptical), which provides protection against uncontrolled deformation and cracking that would otherwise be caused in more rigid tubes of circular cross-section that are commonly used in solar water heating systems. Similarly, collecting channels  12 , as well as the liquid-retaining top  17 , top cover  15 , and side walls  11 , due to their thin-wall structure, will also deform rather than crack or burst under such conditions. 
     Transparent tubes  14  contain an absorbing material or structure  18  that absorbs light, whether directly incident on transparent tubes  14  or reflected by reflective surface  13  of collector channels  12 . The light striking absorbing material  18  is converted to thermal energy by absorption. Absorbing material or structure  18 , which may be composed of granules, fins, threads or a sponge-like porous material, allows the first liquid or aerosol medium to flow or percolate through absorbing material or structure (absorber)  18 , and provides a large effective surface area with respect to the incident light due to multiple reflections within absorber  18 . Absorber  18  may be extruded out of a thermally-conductive light-absorbing plastic material having a profile with fins and through holes for passing the liquid or aerosol medium, and cut into small segments of the extrusion. The material may have a density close to that of a liquid medium, so that the segments will float in the liquid medium, allowing absorber  18  to readily adapt to changes in the shape of transparent tubes  14  under differing pressure conditions. Absorber  18  provides a large contact area with the first liquid or aerosol medium for efficient heat transfer, while providing a relatively small outer surface area via which thermal energy is radiated outward. 
     Collector unit  10  is supported, and is thermally isolated on the bottom side of collector unit  10 , by a bottom insulator  26 , which may be foam layer, air gap or other suitable thermally-insulating structure disposed around the bottoms of collector channels  12 , and that provides sufficient structural support for collector unit  10  when collector channels  12  are filled with second liquid medium  20 . Thermally isolating air gap channel  16 , side air gap channels  19 , and bottom insulator  26  reduce thermal losses from the system by thermally isolating the warmer portions of collector unit  10  from their surroundings. 
     Transparent tubes  14  can be filled with absorbing material either during manufacture or installation, generally on a building rooftop. Mesh plugs may be inserted to retain granular absorber  18 , which may be provided in bags for on-site assembly, to provide for lightweight transport of the system components to the point of installation. Alternatively, manifold structures as described below may incorporate perforations or plugs that retain absorber  18 . Liquid-retaining top  17  of collecting unit  10  includes a structured bottom surface  24  that causes liquid-retaining top  17  to re-direct incident light to the outside of collecting unit  10  when second liquid medium  20  is drained or otherwise displaced by gas. Such draining can be automatically performed by opening an appropriately-placed thermal valve if the system is overheated, which may occur during conditions of extreme external temperatures during full sun and low flow conditions. By re-directing incident light outside of collecting unit  10 , an effective shutdown of the heating action is accomplished, which provides automatic overheat protection. Once the overheat conditions cease to exist, the thermal valve shuts, collector channels  12  are refilled with second liquid medium  20 , and normal heating operation is thereby resumed. 
     Incident light is collected by collector channels  12 , which act as concentrators to direct all incident light into a region occupied by transparent tubes  14 . U.S. Pat. No. 4,002,499 to Winston, incorporated herein by reference, describes the design of a reflective concentrator suitable for use as concentrator channels  12 . However, in the present embodiment, the size of transparent tube  14  is made larger by a factor of approximately 10% from the optimal reflector design described by Winston, as such increase provides that light rays reaching transparent tube  14  will not be at grazing incidence at the tube perimeter, and therefore will better penetrate transparent tube  14 , reaching absorber  18 . As a result, a small sacrifice concentration ratio, an approximately 10% reduction, provides for a greater efficiency of light absorption, and since high concentration rations are not generally needed for applications such as solar water heating, the increase in size of transparent tube  14  provides superior operation. 
     Referring now to  FIGS. 3A-3C , details of a pair manifolds  30 , one for each end of collector unit  10 , are shown. Manifolds  30 , serve as end walls for collector unit  10 , as well as providing the manifold function, and are generally extruded or molded from the same material as collector unit  10 . Manifold  30  has two chambers, a manifold chamber  36  and an air-gap chamber  38 . The outer wall of manifold  30  that abuts collector unit  10  in the final assembly is perforated to provide for flow of both the first liquid/aerosol medium and second liquid medium  20  into/from manifold chamber  30 . Hole pattern  32  abuts the ends of transparent tubes  14  and acts as a mesh to retain absorber  18  if needed (e.g., if absorber  18  is a granular material), while hole pattern  34  aligns with the upper portion of collectors  12  to provide for introduction and drainage of second liquid medium  20 . 
     Since, for overheat protection, drainage of second liquid medium  20  is only required to the degree that removes contact between second liquid medium  20  and structured bottom surface  24 , and further, since collector unit  10  will generally be installed at an inclination away from horizontal and drained from its lower end, the placement of hole pattern  34  is not critical, and drainage of even a small amount of second liquid medium  20  will provide thermal protection. In one particular embodiment of the invention, second liquid medium  20  is the same medium as the first (liquid) medium, which are both water being heated by the system, and the two media are allowed to communicate with each other in a common manifold chamber. Because of the directional flow of water through the system and the size of the holes in hole patterns  32 , 34 , little intermixing occurs between second liquid medium  20  and the first liquid medium. It is noted that when second liquid medium is a different material from the first liquid/aerosol medium, then two separate manifold chambers  36  will generally be required to align with hole patterns  32 , 34  and therefore hole patterns  32 , 34  will generally have different placements in such embodiments of the invention that provide communication with each of hole patterns  32 , 34  with corresponding separate manifold chambers. Even in embodiments in which second liquid medium  20  and the first medium are the same, separate manifold chambers  36  for each medium can be provided to further improve the efficiency of the system. 
     In the embodiment of the invention including manifolds as depicted in  FIG. 3A-3C , second liquid medium  20  is the same tap water as the first liquid medium. During overheating conditions, the thermal valves will disconnect the system from the main water supply and open outlets connected to the manifold, emptying collector channels  12 , and thereby shutting down the heating action. When the overheating condition ceases, the thermal valves close the drain and reconnect the system to the water supply, automatically filling the collectors with water and resuming normal function. 
     Referring now to  FIGS. 4A-4B , details of a portion of a completed solar heater assembly are shown in accordance with an embodiment of the invention. A bottom plate  40  and insulating foam  26  are added beneath collector unit  10 . Manifold  30  has been attached to an end of collector unit  10  as illustrated by the top view of  FIG. 4B  (and similarly another manifold  30  has been attached to the other end that is not visible in the Figure). As shown, collector unit  10  includes side panels  19  providing air gaps  11 , which in combination with the top air gap channel, provide a rigid thermally-insulating outer housing. Side channels are constructed as hollow structures providing thermal insulation without adding significant weight. As an alternative, bottom plate  40 , in combination with additional structural support members (e.g., vertical struts) provided on the bottom side of collector channels  12  or extending upward from bottom plate  40 , can be attached without including insulating foam  26  and can provide thermal isolation via the air gap formed between bottom plate  40  and the bottoms of collector channels  12 . 
     Referring now to  FIGS. 5A and 5B , operation of structured bottom surface  24  of transparent top  17  of collector channels  12  is illustrated.  FIG. 5A  illustrates paths of incident rays IR when second liquid medium  20  completely fills the volume between structured bottom surface  24  and the bottom of collector channels  12 .  FIG. 5B  illustrates operation of structured bottom surface  24  when collector channel  12  is drained of second liquid medium  20  and replaced by air (or another gas). In  FIG. 5B , the difference between the refractive indices at the interface is large and incident rays IR experience total internal reflection on the inclined features of structured bottom surface  24 . Most of incident rays IR are re-directed along and among the features of structured bottom surface  24  and are eventually emitted out of collector unit  10 , and therefore never reach transparent tube  14 . 
     It is noted that second liquid medium  20  need not be completely drained in order for collector  10  to become reflective. It is sufficient that a small air or water vapor gap is present between structured bottom surface  24  and the top surface of second liquid medium  20 . Structured bottom surface  24  provides a wider angle of shut-down operation than other structures such as retro-reflectors that operate over a very narrow angular range (e.g., 10 degrees). Using triangular shapes as depicted, with the faces of the triangular shapes inclined substantially 62 degrees to the left or the right from the primary plane of transparent top  17  (i.e., the bottom vertex of the triangles is substantially equal to 56 degrees), most of incident rays IR within a total angle of approximately 70 degrees are directed out of collector  10 , which is sufficient to provide thermal protection for a solar water heater. 
     Referring now to  FIGS. 6A and 6B , absorbing material  18  within transparent tubes  14  is shown. In  FIG. 6A , transparent tube  14  has a slightly elliptical shape, approximately 10-20% at nominal pressure, but in  FIG. 6B , transparent tube  14  has a further distorted shape due to pressure, which will occur when water outside of transparent tubes  14  has frozen. Side walls  19 , transparent top  17  and cover  15  may also be bent slightly, but not to the point of permanent deformation. Under high internal pressure conditions, such as due to boiling or line over-pressure conditions, the shape of transparent tubes  14  becomes more circular, absorbing the additional stress without bursting. Due to the elastic properties of the selected material, a nominal shape will be recovered after any freezing or over-pressure conditions are removed. 
     As illustrated in  FIG. 6A  by the arrows, light entering transparent tubes  14  is effectively trapped by absorber  18 . Any light which passes between granules (or between fins, pores or other structures formed by absorber  18 ) will generally strike another granule within transparent tubes  14 . Moreover, any small amount of light not absorbed at the first incidence on absorber  18  will be absorbed on subsequent incidences within absorber  18 . Further, thermal re-radiation in the infrared spectrum, a critical issue with surface-type absorbers, is reduced by the granular structure, as thermal (black body) radiation from sides of the granules that are not facing transparent tubes  14  will also generally strike another granule. Effectively, thermal re-radiation takes place only from the outer perimeter of absorber  18 , which has a much smaller surface area than the light-absorbing and liquid-contacting surface areas of absorber  18 . Therefore, the granular structures shown in  FIGS. 6A and 6B , provide an optically efficient absorber that also transfers heat efficiently and minimizes losses. The granule size can be non-uniform to pseudo-randomize the optical paths through the absorbing material  14 , or may be uniform. 
     While absorber  18  of  FIGS. 6A and 6B  is a granular structure in accordance with one embodiment of the present invention and provides high efficiency, other light-trapping thermally conductive structures may be used, as long as the liquid or aerosol can flow through transparent tubes  14 . One such alternative structure is a sponge-like structure, in which porosity is provided for conducting the liquid or aerosol, while still providing an external surface that can trap incident light. The sponge-like structure is substantially continuous as opposed to the granules, although it may be provided in pieces smaller than the entire length of transparent tubes  14 . As mentioned above, absorber  18  can be coated with or made from a catalyst, which may be a thermo-chemical or photo-chemical catalyst for bio-chemical, thermo-chemical or photo-chemical processes. Another alternative is a structural absorber, such as a black anodized metal or black thermally conductive plastic structure that has multiple fins to increase the optical and contact surface area of absorber  18 . 
     While the invention has been particularly shown and described with reference to the preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in form, and details may be made therein without departing from the spirit and scope of the invention.