Solar collector comprising an opaque cover

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to and the benefit of U.S. Provisional Application Ser. No. 61/811,495, filed on 12 April 2013 (12/04/2013).

TECHNICAL FIELD

[0002] The present invention relates to solar collectors, and more particularly to solar thermal collectors designed to heat a working fluid to temperatures within a range of about 25°C to 180°C.

BACKGROUND ART

[0003] The flat plate solar collector has not fundamentally changed since the 1970s. U.S. Pat. No. 4,01 1,856 to Gallagher (1977) teaches an exemplary design that fairly represents the vast majority of glazed flat plate solar collectors in use today. Such solar collectors typically employ tempered glass with reduced iron oxide content as the cover plate material. The glass must be thick enough to prevent sagging of unsupported spans. The glass must also have impact resistance and, in high wind zones, must be attached to the solar collector assembly with a frame and fasteners capable of withstanding the dynamic air pressures generated by severe windstorms. Consequently, the glass cover plate is typically either 3.2 or 4 millimeters thick, with corresponding weights of about 8 and 10 kg/m ² . The use of glass makes flat plate solar collectors difficult to handle and expensive to ship. Also, tempered glass is not impervious to breakage. While tempering produces excellent flat surface impact resistance, the edge strength is poor. A sheet of tempered glass can shatter when lateral compressive force (for example, the force from a solar collector being dropped on its side during handling) drives the head or length of an adjacent glazing frame screw into the edge of the glass. [0004] Translucent plastics, including acrylics and polycarbonates, have been used as solar collector glazing in an attempt to reduce weight and cost. Unfortunately, these materials suffer significant reductions in transmittance over time due to discoloration and structural degradation, caused by accumulated exposure to ultraviolet radiation and stagnation temperatures. Plastic glazing materials also tend to experience unacceptable levels of outward bowing when exposed to the dynamic pressures associated with severe windstorms.

[0005] An additional problem for both transparent and translucent cover plate materials is that airborne dust and grime can accumulate on the glazing surface, which reduces solar energy transmittance. In many climates, the frequency and intensity of rainfall are not sufficient to remove accumulated dust and grime. Further, it is a practical reality that periodic manual glass cleaning is problematic for solar collectors on residential rooftops and in large commercial arrays.

[0006] The side walls of a typical glazed flat plate solar collector are formed of four elongated, straight aluminum extrusions, with 45-degree beveled ends and various extruded appendages extending outward from the vertical wall portions. The four side walls are joined at 90-degree angle corners, often reinforced with L-shaped brackets on the interior side of each corner. Screws, bolts, or rivets tighten the side walls to the L-shaped reinforcing brackets. These sharp angle corner joints have two shortcomings. First, the solar collector side walls expand and contract with daily temperature variations. Over time, repeated expansion and contraction can lead to small gaps at the corner joints. This problem may be exacerbated by a difference in the upper horizontal side wall and the lower horizontal side wall temperatures, leading to small but meaningful differences in the longitudinal expansion and contraction of the upper and lower horizontal side walls. With even the smallest corner gaps, moisture and particulate laden air infiltration into the solar collector interior will inevitably occur, urged by pressure differences between the air inside the solar collector and ambient air. Moisture and particulate intrusion eventually end up as a grimy film on the underside of the glazing, reducing solar energy transmittance. The corner joints discussed are seldom, if ever, sealed with an elastomeric material that might help prevent moisture intrusion over time. The lack of effective corner sealing is due in part to the practical difficulty of sealing the abrupt, planar and pointed surface intersections at the top and bottom of each corner joint. While the corner joints could be welded, this strategy imposes unacceptable labor costs and is incompatible with the most common frame material and finish, which comprises an anodized finish of an aluminum frame.

[0007] During severe windstorms, square corners on a solar collector increase dynamic pressure on the cover plate. Comparative tests of roof gravel scouring showed that aerodynamic corners can double the damage threshold wind speed compared to conventional square corners. Pressure measurements showed up to 75% reduction in uplift pressures in a roof corner test for an aerodynamic corner, when compared with a square corner. See Lin, et al., "Aerodynamic Devices for Mitigation of Wind Damage Risk," 4th International Conference on Advances in Wind and Structures. AWAS 08. Jeju, Korea, May 29-31, 2008.

[0008] A problem closely related to the wind uplift pressure issue discussed above is that any structure comprising a blunt windward edge with square corners promotes formation and shedding of vortex currents along the edges disposed downwind from such corners. See Okamoto, S. and Uemura, N. "Effect of rounding side-corners on aerodynamic forces and turbulent wake of a cube placed on a ground plane," Experiments in Fluids, 11, 58-64. Springer- Verlag. 1991.) If the frequency of the vortex shedding happens to match the resonance frequency of the structure, the structure will begin to resonate and the structure's movement can become self-sustaining. Vortex shedding on a solar collector perimeter during a severe windstorm can literally shake roof shingles loose and start progressive undermining of the entire roof structure, leading to eventual catastrophic loss. Roof deck failure is the leading cause of catastrophic residential building damage during severe windstorms. Once a building loses one or more pieces of roof deck, damage increases exponentially as vast amounts of wind-driven water enter the structure. Insurance claim data show that damage escalates quickly once a roof deck starts to fail. Even if the walls remain intact and the roof trusses do not fail, loss of the roof deck typically results in losses greater than 50% of building insured value. See Applied Research Associates, Inc. "Development of Loss Relativities for Wind Resistive Features of Residential Structures," Florida Department of Community Affairs (DCA Contract 02-RC-l 1-14-00-22-003), Version 2.2, March 28, 2002.

[0009] The evacuated tube solar collector is an alternative to the glazed flat plate solar collector. U.S. Pat. No. 4,067,315 to Fehlner and Ortabasi (1978) teaches an exemplary evacuated tube solar collector. The evacuated tubes of such solar collectors typically comprise 1.6 millimeter thickness borosilicate glass cylinders. While such glass cylinders do have a degree of impact resistance, breakage can nevertheless occur as a result of impacts by hail, wind-driven storm debris, errant golf balls and baseballs, and dropped tools. Breakage can also occur during shipping and handling.

[0010] Snow accumulation is also a problem for evacuated tube collectors. The vacuum that eliminates convective heat losses from inside the glass tube to ambient air also allows snow to accumulate on the glass tubes. The glass cover plate of a flat plate solar collector is warmed during sunny conditions by continual heat transfer from the hotter air on the underside of the glass to the cooler ambient air. However, the glass of an evacuated tube solar collector is only warmed by the small percentage of incident solar energy absorbed by the glass, and this heat is quickly lost to the cold ambient air that accompanies a snowfall. Thus, absent manual snow removal, evacuated tube solar collectors are rendered useless for some period of time after a snowfall.

[0011] Evacuated flat plate solar collectors have been proposed. U.S. Pat. Nos. 4,332,241 to Dalstein, et al. (1982), 7,810,491 to Benvenuti (2010) and 8,161,965 to Palmieri (2012) are exemplary designs. The Benvenuti '491 patent provides an excellent discussion of both tube and flat plate evacuated solar collectors. The Dalstein '241 patent teaches a complex double-walled frame, including an inner frame wall comprising four pieces of square tubing welded at 45-degree beveled sharp corners. The Dalstein '241, Benvenuti '491 and Palmieri '965 patents all teach various approaches to soldering or fusing metal to glass. These complex and relatively expensive processes are conceived to address the dissimilar materials and dissimilar rates of expansion and contraction of the glass cover plate and the metal frame walls. However, the Dalstein '241, Benvenuti '491 and Palmieri '965 patents do not solve the problems associated with flat plate solar collector glazing or sharp side wall corners, or the problem of snow accumulation on the outer glass surface of an evacuated solar collector.

[0012] The unglazed solar collector is another alternative to the glazed flat plate solar collector. Unglazed plastic solar collectors, with carbon black added to the plastic resin to enhance solar energy absorptance and combat the effects of prolonged exposure to ultraviolet radiation, are widely used for swimming pool heating. U.S. Pat. No. 3,934,323 to Ford, et. al. (1976) and U.S. Pat. No. 4,060,070 to Harter (1977) teach examples of unglazed plastic solar collectors. While swimming pool heating requires temperatures between 25°C and 32°C, unglazed plastic solar collectors have been employed for potable water heating, which requires temperatures between about 45°C and 60°C.

[0013] Unglazed solar collectors are capable of delivering 60°C water when the flow rate of the working fluid is reduced. Unfortunately, though, unglazed solar collectors suffer much greater convective heat losses than glazed flat plate solar collectors when the ambient air temperature falls below the solar collector fluid inlet temperature. Wind compounds the problem. Poor cold weather performance is acceptable for solar swimming pool heating because the goal in most climates is only to extend the swimming season by a few extra months. Other water heating applications require hot water on a year-round basis.

DISCLOSURE OF INVENTION

[0014] The present invention is directed to a solar collector comprising an opaque cover. Solar energy is absorbed by the opaque cover and converted to heat, which is transferred from the opaque cover by conduction, convection, and infrared emittance across a gap within an at least substantially airtight enclosure to an absorber containing a working fluid. The exterior surface of the opaque cover has high solar energy absorptance and the interior surface has high infrared emittance. The exterior surface preferably also has low infrared emittance. The composition, structure and thermal mass of the opaque cover and the composition, structure and thermal mass of the absorber are selected to promote an operating relationship whereby the interior surface of the opaque cover is hotter than the absorber surface.

[0015] One embodiment is directed to a solar collector comprising a continuous side wall having a hollow cross-section and rounded corners. The continuous side wall is economical to fabricate yet unusually strong compared to current art solar collector frame walls. The rounded corners of the continuous side wall reduce windload stresses during hurricane-force winds, which permits a reduced thickness of the opaque cover and can reduce potential catastrophic roof damage. The form and inherent strength of the continuous side wall also allow alternatives for reducing convective heat loss to the ambient air. In a most economical embodiment of a continuous side wall, convective heat loss is reduced by the dead air space in the hollow portion of the continuous side wall. In a second alternative embodiment, convective heat loss can be further reduced by injecting mass insulation, such as, for example, polyurethane foam, into the hollow portion of the continuous side wall. In a third alternative embodiment, the inherent strength of the continuous side wall and, in particular, the inherent strength of the rounded corners combine with the simplicity and ease of airtight sealing a solar collector of the present invention to permit evacuation of the solar collector interior. Evacuating the solar collector interior virtually eliminates convective heat losses.

[0016] In higher performance embodiments, the opaque cover preferably has a selective coating on the exterior surface with high solar energy absorptance and low infrared emissivity, and preferably has a coating on the interior surface with high infrared emissivity. The absorber preferably has a selective coating with high infrared absorptance..

[0017] While not wishing to be bound to a single theory as to the optimal structure for promoting heat transfer from the opaque cover to the absorber, one preferred embodiment is directed to a solar collector that achieves a greater absorber wetted surface exposed to infrared energy transfer by providing a spaced apart relationship above, below and between the fluid passageway tubes of the absorber, thereby providing for infrared energy transfer not only directly from the opaque cover to the absorber but also indirectly from a highly reflective surface below the absorber to the underside surfaces of the absorber.

BRIEF DESCRIPTION OF DRAWINGS

[0018] FIG. 1 illustrates an exterior perspective view of a prototype solar collector comprising one exemplary embodiment, installed on a roof.

[0019] FIG. 2 illustrates a cross-sectional view of one basic embodiment of an opaque cover, a housing, and an absorber. The absorber configuration depicted is not critical to the invention.

[0020] FIGS. 3A, 3B, 3C, 3D and 3E illustrate cross-sectional views of several exemplary embodiments of an opaque cover of the present invention.

[0021] FIG. 4 and FIG. 5 illustrate cross-sectional views showing three exemplary fluid passageway tubes of one preferred embodiment comprising an absorber having fluid passageway tubes in a spaced apart relationship.

[0022] FIG. 6 illustrates a bird's-eye view of an embodiment directed to a continuous side wall, with the opaque cover removed to reveal the continuous side wall and a plurality of transverse frames. The specific configuration of the generic absorber structure depicted is not critical to the invention. [0023] FIGS. 7A, 7B and 7C illustrate cross-sectional views of various embodiments comprising a continuous side wall and transverse frames. The generic absorber depicted is not critical to the invention and is shown only to illustrate a spaced apart relationship between the absorber and the opaque cover, between the absorber and the continuous side wall, and between the absorber and the back plate.

[0024] FIG. 8 illustrates a perspective view of one preferred embodiment with the opaque cover and back plate removed to reveal the continuous side wall having rounded corners, the transverse frames, and an absorber having substantially parallel fluid passageway tubes in a spaced apart relationship.

[0025] FIG. 9 illustrates a bird's-eye view of an exemplary embodiment directed to an evacuated solar collector comprising a plurality of heat pipes connected to an internal heat exchange manifold.

MODES FOR CARRYING OUT THE INVENTION

[0026] Turning now to FIG. 1, the present invention is directed to a solar collector comprising an opaque cover (10) disposed to receive incident solar energy and coupled to an upper portion of a housing (12). A cross-sectional view of one embodiment, taken at section line 2-2 of FIG. 1, is illustrated in FIG. 2. The opaque cover (10) preferably comprises a planar structure but can also be dome-shaped, corrugated, or any other shape suitable for receiving incident solar energy and providing for structural integrity under anticipated loads. The housing (12) may be of any shape and construction suitable to isolate an absorber (26) from ambient air, provided that coupling of the opaque cover (10) to an upper portion of the housing (12) defines an at least substantially airtight enclosure (14). The absorber (26) is disposed within the enclosure (14) in an opposing spaced apart relationship with the opaque cover (10).

[0027] The present invention is directed to a solar collector that operates by a fundamentally different principle than glazed flat plate solar collectors of the current art. Instead of transmitting incident solar energy through a transparent or translucent glazing material, the opaque cover (10) absorbs solar energy at its exterior surface (22), the absorbed radiant energy heats the opaque cover (10), then, in addition to any heat transfer from the opaque cover (10) to the absorber (26) via conduction or convection, infrared energy is emitted from the interior surface (24) to the absorber (26). The exterior surface (22) has solar energy absorptance greater than about 0.80 and the interior surface (24) has infrared emittance greater than about 0.80. For improved thermal performance, the exterior surface (22) preferably has infrared emittance of less than about 0.30.

[0028] In a single-glazed, flat plate solar collector circulating a working fluid at temperatures of 40-60°C, the glass cover plate temperature is about 50-70% of the absorber mean surface temperature. In stark contrast to a glazed flat plate solar collector, the interior surface (24) of the opaque cover (10) is hotter than the absorber (26) during normal operation, which facilitates net radiant energy transfer from the opaque cover (10) to the absorber (26). Without being bound to any particular theory, it is believed that the temperature of the interior surface (24) of the opaque cover (10) should preferably be as hot as possible during normal operation because infrared emittance increases to the fourth power of the temperature of an emitting surface. The temperature of the interior surface (24) can be maximized by material selection and thickness directed to high thermal conductivity and low thermal mass such as, for example, an interior surface (24) comprising a 0.813 millimeter thickness aluminum sheet.

[0029] The opaque cover (10) can employ a variety of strategies to meet a range of performance goals and cost considerations. FIG. 3A illustrates a basic embodiment of the opaque cover (10) comprising a central region (20), the exterior surface (22), and the interior surface (24) as a homogeneous structure and composition. One exemplary homogeneous opaque cover (10), directed to low cost, comprises a molded polymer structure incorporating carbon black in the resin. Such a composition has high absorptance at the exterior surface (22) and high emissivity at the interior surface (24). While emissivity of the exterior surface (22) in this example is undesirably high, the reduction in performance can be acceptable for an embodiment of the present invention directed primarily to a very low cost solar collector.

[0030] FIG. 3B illustrates an embodiment of the opaque cover (10) wherein each of the exterior surface (22) and the interior surface (24) comprises a selective coating. The term "selective coating" as used herein refers to paints, plating, resins, sheets, laminates, films, vacuum deposited metals, and other coatings that increase or reduce the radiant energy absorptance of a surface, increase or reduce the radiant energy reflectance of a surface, increase or reduce the radiant energy emittance of a surface, deliver a preferred range of absorptance and emittance, or deliver a preferred range of the ratio of absorptance to emittance. Exemplary selective coatings include oxides of titanium nitride or other metals, anodized aluminum, black chromium electroplated over a nickel substrate, ceramic matrices, quartz-encapsulated bi-metallic alloys, layered compositions of silicone polymers and metal oxides, black lacquer, urethane paints, silicone paints, compositions of polypropylene polymers and carbon black, compositions of butyl or EPDM rubber incorporating carbon black, and individual sheets or multilayer stacks of polymer films such as, for example, stacks of transparent cross-stretched birefringent sheets.

[0031] In embodiments comprising selective coatings, the exterior surface (22) is preferably selective for enhanced absorption of incident solar energy and reduced emittance of infrared energy, and the interior surface (24) preferably has high infrared emittance. A variety of commercial selective coatings are available that offer solar energy absorptivity ranging from 0.90 to 0.96, with simultaneous infrared emissivity ranging from 0.05 to 0.30. The selection of selective coatings can be based upon cost and thermal performance targets. In an exemplary low cost embodiment, a selective coating for the exterior surface (22) might have absorptivity of about 0.87 and emissivity of about 0.90; while an exemplary high thermal performance embodiment might have absorptivity of about 0.95 and emissivity of less than about 0.30. [0032] The central region (20) of the opaque cover (10) can be solid such as, for example, a sheet of aluminum, molded polymer, fiberglass, or carbon fiber or, alternatively, comprise a complex structure such as, for example, a honeycomb sandwich structure having honeycomb core walls disposed in a perpendicular sandwiched relationship between each of the exterior surface (22) and the interior surface (24). In such a honeycomb sandwich structure, each of the exterior surface (22) and the interior surface (24) comprise facing sheets such as, for example, aluminum sheets having selective coatings applied on the sides distal to the honeycomb core walls. The honeycomb sandwich structure combines high compressive strength and weight savings. Further, the honeycomb sandwich structure reduces convective heat loss by providing air spaces within the honeycombs that dampen convection because of very low volume. While a honeycomb sandwich structure reduces convective heat loss, energy transfer via direct infrared energy emittance from the exterior surface (22) to the interior surface (24) across the air spaces in the honeycombs is enhanced by conduction via thermal bridging through the honeycomb core walls, which are in direct contact with each of the exterior surface (22) and the interior surface (24). Alternatively, the air spaces in the honeycomb structure may be filled with material having thermal conductivity less than about 2 W/m-°C at 25°C or greater than about 15 W/m-°C at 25°C.

[0033] The exterior surface (22) of the opaque cover (10) is directly exposed to the ambient air. Atmospheric moisture generally and acidic rain in particular can attack some selective coatings by wet corrosion. If the exterior surface (22) comprises a selective coating susceptible to wet corrosion, it is advantageous to apply a transparent protective coating (32) over the selective coating as illustrated in FIG. 3C. The transparent protective coating (32) can be a simple transparent "hardcoat" layer or a multilayer polymer stack film that further comprises adhesive or polymer layers spectrally selective to reflect ultraviolet wavelengths. It is well known in the art of polymer films for sun control that adhesives having additives to selectively absorb ultraviolet radiation, or film sheets that selectively reflect ultraviolet radiation, can be placed in a layer proximal to a surface receiving incident solar energy to enhance longevity of both the stack and structures and surfaces for which sun control is sought.

[0034] As disclosed above, the interior surface (24) of the opaque cover (10) preferably has high emittance of infrared energy. Since the interior surface (24) is preferably hotter than the absorber (26) during normal operation, absorptance by the interior surface (24) of infrared energy reradiated from the absorber (26) is not believed to be a significant concern. Many coatings, such as, for example, black paints (including compositions comprising silicones and urethanes), and paint, plastic, acrylic, and rubber compositions incorporating carbon black combine economy, durability, and high emissivity. See Clatterbuck, Caroll H. and Scialdone, John J. "NASA Technical Memorandum 100768: An Evaluation of Two Flat-Black Silicone Paints for Space Application," National Aeronautics and Space Administration, Goddard Space Flight Center, Greenbelt, MD, December 1990, for an overview of high emissivity paints. Any film applied to the interior surface (24) will have enhanced longevity due to the absence of exposure to ultraviolet radiation.

[0035] FIG. 3D illustrates another alternative embodiment of the present invention comprising a transparent layer (34) applied to the interior surface (24) of the opaque cover (10). The transparent layer (34) has selective reflectivity in the infrared wavelengths. An exemplary transparent layer (34) comprises a cross- stretched stack of birefringent polymer film sheets. It should be noted that such cross-stretched stacks are available in assembled form such as, for example, multilayer optical films manufactured by 3M.

[0036] As disclosed above, the temperature of the interior surface (24) is preferably as hot as possible during normal operation. In stark contrast, the temperature of the exterior surface (22) is preferably as cool as possible to reduce both infrared energy emittance to the sky and convective heat loss to the ambient air. Such an ideal differential relationship is difficult to achieve in practice. However, FIG. 3E illustrates yet another alternative embodiment of the present invention comprising a laminated structure of the opaque cover (10) that minimizes convective heat loss from the exterior surface (22) while simultaneously maximizing the temperature and consequent infrared energy emittance of the interior surface (24) by employing different materials for an exterior layer (36) and an interior layer (38) of the central region (20). For example, and in addition to selective coatings and protective or reflective coatings comprising the exterior surface (22) and the interior surface (24), the exterior layer (36) comprises an insulating layer with low thermal conductivity and the interior layer (38) comprises a layer with high thermal conductivity. The insulating aspect of the exterior layer (36) can be further enhanced by high thermal mass. The high thermal conductivity aspect of the interior layer (38) can be further enhanced by low thermal mass. Without being bound to any particular theory, it is believed that the insulating exterior layer (36) of the exemplary laminated structure, while reducing convective heat loss, only minimally impacts conductive heat transfer of absorbed solar energy to the interior surface (24) because the insulating aspect of the exterior layer (36) increases not only resistance to convective heat loss at the exterior surface (22) but also the thermal capacitance of the opaque cover (10).

[0037] The alternative embodiments of the opaque cover (10) disclosed above are intended to be merely illustrative examples and not exhaustive. Within the scope of the present invention, many more materials, alternative combinations of the various aspects, and modifications in addition to those described above are possible without departing from the inventive concepts disclosed.

[0038] The absorber (26) is disposed within the enclosure (14) in an opposing spaced apart relationship with the opaque cover (10). At least inlet and outlet conduits (28), shown in FIG. 6, are provided to supply and extract, respectively, the working fluid to and from the solar collector. The specific design, flow configuration, and construction aspects of the absorber (26) and conduits (28) are not critical to the present invention. The present invention can be used with any absorber design, including absorbers comprising, for example, integral storage, fin structures, opposing and generally planar sheets joined to define a vessel, serpentine (serial) or harp (parallel) interconnected tubes, heat pipes, and air ducts, so long as the interior surface (24) of the opaque cover (10) is hotter than the surface of the absorber (26) during normal operation. In one alternative embodiment comprising integral storage for the working fluid, the integral storage capacity of the absorber (26) is preferably between about 20 and 100 liters per square meter of planar surface area of the opaque cover (10).

[0039] The preferred differential temperature relationship of the interior surface (24) and the absorber (26) can be enhanced by increasing the ratio of wetted surface area of the absorber (26) to the total planar surface area of the opaque cover (10) exposed to incident infrared energy. A higher wetted surface area of the absorber (26) allows the working fluid to more completely cool the absorber (26) as heat is extracted therefrom and transferred into the working fluid. The absorber (26) may also be alternatively or further cooled by selection of an absorber (26) material with lower thermal conductivity, especially in combination with a fully wetted surface of the absorber (26) because thermal conductivity is not critical to radiant energy transfer.

[0040] FIG. 4 illustrates one preferred embodiment wherein the absorber (26) comprises fluid passageway tubes (40) without fins, in a spaced apart relationship with a highly reflective surface (42) disposed beneath the tubes. Infrared energy first emitted (44) from the interior surface (24) of the opaque cover (10) that passes between the tubes (40) of the absorber (26) can be reflected (46) back to the undersides of such tubes (40). The highly reflective surface (42) may be a sheet of unpolished or polished aluminum, an aluminum foil, a reflective paint composition, or a polymer or metalized polymer film spectrally reflective for infrared wavelengths.

[0041] Infrared energy emitted from a planar surface is diffuse. Accordingly, the spaced apart relationship of the tubes (40) and reflective surface (42) exposes the full 360-degree circumference of each tubes (40) to infrared energy first emitted (46) by the interior surface (24) of the opaque cover (10). Further, as illustrated in FIG. 5, when the average centerline spacing (50) of substantially parallel aligned tubes (40) is between about 0.75 and 1.00 times the average circumference of a tube (40), total circumferential outer surface area of the tubes can approximate 100% of the planar surface area of the opaque cover (10). In another aspect, the full 360-degree circumference of each fluid passageway tube (40) and the full 360-degree circumference of any connecting U-bends, or manifold structures in a parallel flow configuration, preferably have a selective coating (48) with high absorptance and low emittance. Alternatively, high absorptance may be incorporated into a polymer absorber (26) such as, for example, by incorporating carbon black in the polymer resin. The selective coating (48) preferably has infrared absorptance greater than about 0.90 and infrared emittance of less than about 0.30.

[0042] The spaced apart relationship of the absorber (26) and the opaque cover (10) preferably defines a gap (52) of at least about 3 millimeters, and the spaced apart relationship of the absorber (26) and the highly reflective surface (42) preferably defines a gap (54) of at least about 3 millimeters. In one preferred alternative embodiment of the present invention, the spaced apart relationships between the absorber (26) and the opaque cover (10) and between the absorber (26) and the highly reflective surface (42) are enabled by a plurality of transverse frames (70), which are illustrated in FIG. 6 and more fully described below.

[0043] The disclosed fully wetted surface geometry is less expensive and involves less material than a nominally fully wetted surface absorber comprising the same material, tube diameter and wall thickness, either without spacing between fluid passageway tubes or with substantially less spacing between fluid passageway tubes.

[0044] A wide variety of absorber structures and materials can be employed. The fluid passageway tubes (40) can be copper, bent or brazed to U-shaped copper fittings to create a serpentine (serial) fluid flow pattern. Alternatively, substantially parallel aligned copper tubes (40) may be brazed to copper header manifold tubes, creating a parallel flow pattern. Tubes (40) can also be aluminum. Aluminum is less expensive than copper, even where a greater tube (40) wall thickness is required for a given fluid pressure, to compensate for the lower tensile strength of aluminum. Aluminum has excellent formability characteristics for bending and is lighter and less expensive than copper. Copper has higher thermal conductivity than aluminum but the difference is trivial in an absorber (26) comprising a 100% wetted surface, where radiant heat transfer is a significant component of total heat transfer.

[0045] The interconnected fluid passageway tubes (40) of the absorber (26) can also be a plastic tubing such as, for example, polypropylene or cross-linked, high density polyethylene ("PEX") tubing, or a PEX-AL-PEX variant wherein the tube (40) walls comprise an aluminum layer sandwiched between PEX inner and outer layers. Any alternative tube (40) material selected should be capable of withstanding stagnation temperatures associated with the particular convective heat loss control strategy employed (e.g., dead air space, mass insulation, or evacuation). For example, PEX tubing would not be suitable in an embodiment directed to an evacuated solar collector, or in embodiments comprising mass insulation. The tubes (40) can also be a synthetic rubber such as, for example, ethylene propylene diene monomer ("EPDM") rubber, which has a higher operating temperature range than PEX tubing provided; however, that EPDM rubber may not be suitable where potable water containing chlorine will circulate through the absorber (26). It should be noted that the present invention is directed to a solar collector wherein all materials and surface coatings of the absorber (26) are protected from exposure to ultraviolet radiation by the opaque cover (10), which is not the case in glazed flat plate and evacuated tube solar collectors.

[0046] FIG. 6 illustrates a birds-eye view of one embodiment of the present invention directed to a solar collector comprising a continuous side wall (60) having a hollow cross-section (62) and rounded corners (64). Transverse frames (70) support the absorber (26) and create the spaced apart relationship between the absorber (26) and the opaque cover (10) (not shown), between the absorber (26) and the continuous side wall (60), and between the absorber (26) and a back plate (66) (not shown). The generic absorber (26) structure illustrated in FIG. 6 is not critical to the invention. The outside surface radius of each rounded corner 64, which outside surface is exposed to ambient air, is preferably at least about 8 centimeters.

[0047] A cross-sectional view taken at section line 7-7 of FIG. 6 is illustrated in FIG. 7A. The opaque cover (10) is coupled to an upper portion of the continuous side wall (60) and a back plate (66) is coupled to a lower portion of the continuous side wall (60). The continuous side wall (60), the opaque cover (10), and the back plate (66) define an enclosure (14). The method of coupling, which may comprise fasteners, adhesives, brackets or parts that snap together by the temporary deformation of one or more mating surfaces, is not critical to the invention. However, it is preferable to provide a coupling method whereby the opaque cover (10) can be removed and replaced, for inspection and servicing of the absorber (26), without causing permanent damage or deformation. It is also preferable that the method of coupling promote preservation of the at least substantially airtight character of the enclosure (14) over the anticipated service life of the solar collector. A gasket material (68) may be interposed at the contact perimeter between the continuous side wall (60) and the opaque cover (10), and at the contact perimeter between the continuous side wall (60) and the back plate (66), to provide a seal that serves to at least substantially reduce the intrusion of moisture, dirt, and particulates into the enclosure (14). The contact perimeter gasket material (68) is illustrated in FIGS. 7B and 7C. The contact perimeter gasket material (68) may be eliminated in alternative embodiments directed to low manufacturing cost.

[0048] Alternative embodiments of the continuous side wall (60) can be formed of extruded plastic, shaped composites, or other suitable materials. In one preferred embodiment, a single piece of hollow aluminum square tube is bent within a rotary compression bending machine to form rounded corners (64) of a generally rectangular continuous side wall (60). For example, a 6 meter x 25 millimeter square piece of 1.575 millimeter gauge thickness 6063-T52 aluminum square tube can be bent to form a continuous side wall (60) measuring approximately 0.61 meters x 2.44 meters with about 8 centimeter radius rounded corners, the radius being measured at the outside exterior surface of the aluminum square tube. Exemplary rotary compression bending machines are manufactured by Pedrick Tool and Machine Company of Cinnaminson, New Jersey. The two ends of the square tube are welded and smoothed at a single point along a remaining straight section of the square tube, to form the continuous side wall (60) without corner joints.

[0049] Holes can be drilled through the continuous side wall (60) to accommodate fluid inlet and outlet passageways, for fitting ancillary devices or mounting hardware, and for injecting foam insulation into the hollow cross- section (62). It is also noteworthy that some holes, such as, for example, holes for mounting hardware or for injecting foam insulation, may only penetrate the exterior portion of the continuous side wall (60), advantageously limiting the number of penetrations through the interior portion of the continuous side wall (60) for the purposes of convective heat loss reduction an, in an evacuated solar collector embodiment, vacuum preservation. Steps unrelated to the bending operation such as, for example, drilling holes in the continuous side wall (60), can be performed prior to the bending operation. This advantageously allows such unrelated tasks to be performed on straight stock of different lengths at the same work station. Thus, a space-efficient work station arrangement can be employed for processing straight stock for solar collectors of varying sizes. It is not necessary to have a different workstation for the non-bending operations associated with fabricating each different size of the continuous side wall (60).

[0050] The structure and composition of the opaque cover (10) are determined in part by the anticipated dynamic air pressures to which the solar collector may be subjected. In this regard, the rounded corners (64) reduce dynamic air pressure on the opaque cover (10) during high winds. Consequently, the opaque cover (10) can have less mass than might otherwise be required under a given windload requirement. In addition to material cost savings, an opaque cover (10) with reduced mass will generally attain higher temperatures when exposed to incident solar energy, which increases infrared emittance by the opaque cover (10) to the absorber (26).

[0051] The transverse frames (70) will now be discussed in greater detail. As discussed above, one embodiment of the present invention is directed to a solar collector comprising the absorber (26) supported within the enclosure (14) by transverse frames (70). The transverse frames (70) position the absorber (26) such that a spaced apart relationship is created between the absorber (26) and each of the opaque cover (10), the continuous side wall (60), and the back plate (66). The transverse frames (70) are preferably seated against the interior walls of the continuous side wall (60), as illustrated in FIG. 6. In embodiments wherein the transverse frames (70) are the same height as the continuous side wall (60), the transverse frames (70) can float freely or can be rigidly coupled to the continuous side wall (60). The transverse frames (70) preferably comprise 90- degree bends or extruded "I-beam" sections at each end, to provide mating surfaces for the continuous side wall (60) and added strength against deformation by external loads.

[0052] FIG. 8 illustrates a perspective view of one embodiment of the present invention with the opaque cover (10) and the back plate (66) removed to reveal the continuous side wall (60), transverse frames (70), and an absorber (26) having substantially parallel fluid passageway tubes (40) in a spaced apart relationship.

[0053] The transverse frames (70) perform a variety of functions. In a first aspect, the transverse frames (70) isolate the absorber (26) from contact with interior surfaces of the opaque cover (10) and the housing (12), reducing the potential for conductive heat loss and promoting radiant energy transfer, which requires a spaced apart relationship between opposing, radiant energy exchanging surfaces. In a second aspect, the transverse frames (70) reduce potential convective heat transfer within the enclosure (14) by separating the enclosure (14) into a plurality of smaller spaces, which reduces the potential temperature differences within upper and lower portions of each such smaller space (temperature difference in a fluid is the driver of natural convection). In an alternative embodiment of the present invention comprising an evacuated solar collector, the transverse frames (70) provide interior structural support against compressive forces applied by atmospheric pressure to the opaque cover (10) and the back plate (66). The opaque cover (10) and the back plate (66) may each be rigidly coupled to the transverse frames (70) to help resist the dynamic pressures associated with hurricane force winds, an option that is not practical with sheet glass. Alternatively, the transverse frames (70) may not be rigidly coupled to any portion of the opaque cover (10), the continuous side wall (60), or the back plate (66), and are instead held in a fixed position by ridges or indentations in the opaque cover (10), the continuous side wall (60), and the back plate (66), or by an alternative combination thereof. Each transverse frame 70 may comprise two parts: an upper portion and a lower portion, to facilitate placement of the absorber (26) during assembly of the solar collector. The precise number of transverse frames (70) is not critical to the invention, but rather is based upon the structural requirements of a particular solar collector model. Such requirements include, for example, the maximum unsupported spans possible for a particular combination of material, thickness, and construction of each of the opaque cover (10) and the back plate (66), and the maximum unsupported length of fluid passageway tubes (40) possible without deformation of the fluid passageway tubes (40) to a degree that would reduce infrared reflectance from the highly reflective surface (42) to the undersides of the fluid passageway tubes (40).

[0054] The transverse frames (70) may be selected from a variety of materials based upon factors such as, for example, ease of machining and forming, cost, service temperature range, strength, weight, and strength-to-weight ratio. Two such exemplary materials include aluminum and carbon fiber. It is desirable to isolate the transverse frames (70) from the absorber (26) (e.g., with butyl or EPDM rubber rings, or a non-metallic coating) where the absorber (26) and the transverse frames (70) are different metals or comprise different metals and metallic coatings, to reduce galvanic corrosion potential. In one embodiment, at least the portions of the transverse frames (70) in direct contact with the absorber (26) may comprise an insulating material to prevent thermal bridging from the absorber (26) to the opaque cover (10), to the continuous side wall (60), or to the back plate (66), via the transverse frames. Alternatively, transverse frames (70) with high thermal conductivity may be coated or isolated with an insulation.

Exemplary Convective Heat Loss Reduction Strategies

[0055] For embodiments comprising an air-filled space between the opaque cover (10) and the absorber (26), it would be advantageous to provide alternative and combination strategies for reducing convective heat loss to ambient air, especially during cold weather operation. As disclosed above, convective heat loss through the opaque cover (10) can be reduced with each of the honeycomb or laminated sandwich structures previously disclosed. Alternatively, or in combination with such structures, the gap (52) between the opaque cover (10) and the absorber (26) can be divided with a transparent planar sheet (74) in a parallel spaced relationship with the opaque cover (10), as illustrated by FIG. 7B. One exemplary planar sheet would be a thin film of PTFE. The transparent planar sheet (74) is preferably disposed in a spaced apart relationship at least about 3 millimeters apart from each of the opaque cover (10) and the absorber (26). In one exemplary configuration, the transparent planar sheet (74) is stretched across the upper surfaces of the transverse frames (70) and either adhered to transverse frames (70') or pinned between transverse frames (70') and the transverse portions of the continuous side wall (60). It should be noted that the durability and longevity of a polymer film such as, for example, PTFE, will be significantly increased in a solar collector of the present invention because the polymer film will not be exposed to ultraviolet radiation, in stark contrast to such a polymer film deployed in a current art glazed flat plate solar collector. FIG. 7B also illustrates foam insulation (76), which can be injected into the hollow cross- section (62) of the continuous side wall (60), and mass insulation (78) such as, for example, rock wool or a closed cell polyisocyanurate board, which can be disposed between the back plate (66) and the lower portions of the transverse frames (70). In alternative embodiments comprising mass insulation (78), the highly reflective surface (42) may be foil facing, reflective paint, or aluminum sheet applied, adhered to, or loosely coupled to the mass insulation (78) on the side proximal to the absorber (26). The highly reflective surface (42) may also comprise a planar sheet disposed between the mass insulation (78) and the lower portions of the transverse frames (70).

[0056] Convection currents within a closed and heated air space increase with the depth of the air space. For example, heated air spaces tend to have few if any convection currents up to a depth of about 1.25 centimeters. A dead air space (in other words, a space with no meaningful convection currents) reduces convective heat transfer. Thus, for cold climate operation, it might be preferable to limit the air gap between the opaque cover (10) and the transparent planar sheet (74) to a depth of less than about 1.25 centimeters.

Exemplary Evacuated Solar Collector Embodiments

[0057] The inherent strength of the disclosed hollow, continuous side wall (60) with rounded corners (64) is well suited for an embodiment comprising an evacuated flat plate solar collector. Further, the absence of abrupt corner joints in the continuous side wall (60) results in a solar collector wherein the only coupled perimeter surfaces are easily sealed, two-dimensional planar surfaces between the continuous side wall (60) and the opaque cover (10), and between the continuous side wall (60) and the back plate (66).

[0058] In an alternative embodiment of the present invention directed to an evacuated solar collector, it would be advantageous for the opaque cover (10), continuous side wall (60), and back plate (66) to have similar coefficients of expansion, thus eliminating the longstanding technical issues of dissimilar rates of expansion and contraction of a glass cover plate and metal side walls, and the related difficulty of achieving a durable vacuum seal between the dissimilar materials of flat plate evacuate solar collector glass cover plates and metal side walls. The opaque cover (10), continuous side wall (60), back plate (66), and transverse frames (70) of an evacuated embodiment of the present invention may be formed of metals, carbon fiber, ceramics, plastics, silicates, laminates, composites, and combinations thereof, it being preferred that structures employed in longitudinally coupled relationships, such as, for example, the opaque cover (10) and continuous side wall (60), have substantially similar coefficients of expansion.

[0059] The material composition, structure and thickness of the opaque cover (10), continuous side wall (60), back plate (66) and transverse frames (70) of an evacuated embodiment are preferably selected to withstand external atmospheric compressive forces when the vacuum pressure inside the evacuated airtight enclosure is less than about one torr, in an alternative embodiment of an evacuated solar collector that effectively eliminates convective heat losses; and preferably less than about 10 ^"2 torr, and more preferably less than about 10 ^"4 torr, in an alternative embodiment of an evacuated solar collector that effectively eliminates both convective and conductive heat losses.

[0060] In another aspect of an evacuated solar collector, it is advantageous to provide a gasket material (68) for airtight sealing of the contact perimeter between the opaque cover (10) and the continuous side wall (60), the contact perimeter between the continuous side wall (60) and the back plate (66), and within each opening through the airtight enclosure. A variety of suitable gasket materials may be provided such as, for example, silicone, PTFE, butyl rubber, or EPDM rubber. In particular, butyl rubber has excellent characteristics as a gasket material for sealing an airtight vacuum enclosure. Even very high-grade butyl rubber gasket seals manufactured to aerospace standards for human spaceflight will likely be more economical and more easily maintainable than the complex methods and structures proposed in the current art for soldering or fusing metal ribbons or skirts to glass cover plates.

[0061] FIG. 9 illustrates one alternative embodiment of the present invention directed to an evacuated solar collector, wherein the solar collector is mounted at a non-horizontal angle of at least 10 degrees, and more preferably at least 20 degrees, and the absorber (26) comprises a plurality of parallel aligned heat pipes (90) wherein the condenser end (92) of each heat pipe (90) is elevated above the evaporator end (94). A horizontal heat exchange manifold (96) containing a second fluid is disposed inside and within the uppermost region of the enclosure (14), which is airtight and evacuated. The condenser end (92) of each heat pipe (90) is coupled in a heat exchange relationship with the heat exchange manifold (96), and conduits (98) are provided at each end of the heat exchange manifold (96) to supply and extract, respectively, the second fluid to and from the solar collector. The number of heat pipes (90), and the specific materials, designs and construction of the heat exchange manifold (96), the heat pipes (90), and any ancillary devices for vacuum loss detection, stagnation temperature control, or relief and restoration of the vacuum are not critical to the invention, so long as the interior surface (24) of the opaque cover (10) is hotter than the surface of the absorber (26) during normal operation. Further, the specific steps and parameters of any baking process to reduce future outgassing of materials inside the solar collector are not critical to the invention. However, in one preferred alternative embodiment consistent with FIGS. 4 and 5 and the associated detailed description therefor, the heat pipes (90) do not have fins, the full 360-degree circumference of each heat pipe (90) preferably has a selective coating with high absorptance and low emittance, and the heat pipes (90) are substantially parallel aligned in the spaced apart relationship of the fluid passageway tubes (40) in FIGS. 4 and 5 and the associated detailed description therefor. In sharp contrast to evacuated tube solar collectors of the current art, this configuration places the critical heat transfer connections between the heat pipes (90) and the heat exchange manifold (96) inside an evacuated airtight enclosure. Consequently, the substantial mass insulation typically required for a heat exchange manifold external to an evacuated tube of the current art is eliminated, and convective heat losses at the portion of the heat pipe disposed between the evacuated tube and the external heat exchange manifold, and from the heat exchange manifold, are virtually eliminated and greatly reduced, respectively.

Exemplary Low Cost Solar Collector Embodiments

[0062] The present invention can be directed to very low cost embodiments. For example, the opaque cover (10), housing (12), and transverse frames (70) can each be molded plastic structures enclosing a plastic absorber (26). Alternatively, the housing (12) comprises the hollow continuous side wall (60) formed of extruded plastic. Various combinations of the opaque cover (10), housing (12), and transverse frames (70), can be molded as single pieces. Plastic materials exposed to incident solar energy in such low cost embodiments are preferably formed with resins containing ultraviolet inhibitors or can employ an exterior surface coating, adhesive or film specifically reflective for, or absorptive of, ultraviolet wavelengths. Further, a plastic opaque cover (10) and plastic absorber (26) can be formed with resins containing carbon black to provide for greater absorptance of incident radiant energy. A plastic solar collector of the present invention will provide improved performance, relative to unglazed plastic solar collectors, by reducing convective heat losses during cold weather operation.

[0063] The present invention can also be directed to a low cost embodiment wherein the opaque cover (10), housing (12), and transverse frames (70) are each metal, such as, for example, aluminum, combined with a PEX, PEX-AL-PEX, or EPDM rubber absorber (26). While such a combination would pose stagnation temperature issues for the plastic tubing in a glazed flat plate solar collector of the current art, comprising mass insulation in the side walls and behind the absorber, a solar collector of the present invention in an alternative embodiment without mass insulation can be engineered to limit stagnation temperatures to levels well tolerated by a plastic or rubber absorber (26) by, for example, employing a coating of the exterior surface (22) having reduced solar energy absorptance and a coating of the interior surface (24) having reduced infrared emittance.

Weight Advantage of Opaque Cover

[0064] The density of aluminum, an exemplary opaque cover material, is actually slightly greater than the density of glass, and the modulus of elasticity of aluminum is about the same as glass. Thus, a brief explanation as to the nature of the weight advantage of a solar collector of the present invention, in contrast to a flat plate solar collector comprising a glass cover plate, is useful. Using aluminum for exemplary comparison, the theoretical tensile and compressive strengths of aluminum and glass are comparable. However, glass breakage is statistically unpredictable because surface imperfections in glass plates cause irregular stress concentrations. Glass plates are also negatively affected by the duration of a load: A glass plate might sustain a short-term load during testing that exerts more than twice as much pressure as a long-term load that causes failure. For structural applications, a multiplier factor reflecting an adjustment for the statistical probability of failure is used to calculate required thickness of glass plates. It is not unusual for a glass plate to be about twice as thick as its theoretical strength might suggest is sufficient to meet the negative pressure associated with a design windload, or the long-term compressive force associated with an evacuated solar collector. Thus, a 4 millimeter thickness glass plate might be required to sustain the same design load as a 2 millimeter thickness aluminum sheet. Given the similar densities of glass and aluminum, the aluminum sheet will weigh about half as much as the glass plate. The weight advantage of an opaque cover with respect to glass is even more pronounced in alternative embodiments directed to low weight such as, for example, opaque covers comprising plastics or carbon fiber, or honeycomb sandwich structures with aluminum facing sheets much thinner than the 2 millimeter thickness example above.

Conclusion

[0065] Visible light comprises about 44% of the energy content of the solar spectrum, so it is counterintuitive that an opaque cover can transfer solar energy to an absorber within a solar collector enclosure at levels which, in some embodiments, approach the performance of low-iron glass. However, it has been found that an opaque cover combining high solar energy absorptance by the exterior surface with high infrared emittance from the interior surface to the absorber can provide acceptable and economical thermal performance.

[0066] The present invention is directed to a solar collector that is more robust, more durable, has a simplified construction, and requires less periodic cleaning than a solar collector with a transparent or translucent cover plate. The present invention does not require the cost and complexity, or experience problems unique to transparent and translucent glazing materials. The performance of an opaque cover is less affected by dust and grime on either side of the cover than transparent or translucent glazing. In some embodiments, the present invention avoids the costs and shortcomings associated with mass insulation, absorber fins, cover plate mullions, and corner joinery.

[0067] The present invention is directed to a solar collector that is suitable for operation with a variety of working fluids such as, for example, water, air, glycols, water-glycol mixtures, hydrocarbon oils, eutectic phenyl mixtures, phthalate esters, refrigerants, salts, and silicones.

[0068] The foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been disclosed with reference to exemplary embodiments, the words used herein are intended to be words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as may be amended, without departing from the scope and spirit of the present invention in its aspects. While the present invention has been described with reference to particular materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures and uses, such as are within the scope of the appended claims. All terms used in this disclosure should be interpreted in the broadest possible manner consistent with the context.