Patent Application
20100108054
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.
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.
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:
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'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
Referring now to
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
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
Referring now to
Referring now to
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
As illustrated in
While absorber 18 of
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.
