Patent Application
20190063787
Solar thermal panels capture solar radiation in the form of heat. An absorber, typically a metal plate or foil coated with a specialized solar absorber or black paint, converts incident solar radiation into heat through the process of solar absorption. The absorbed heat can be used in many ways, including to directly heat air in a forced air heating system or water in a forced hot water system, or to evaporate a fluid in an absorption chiller of a refrigeration cycle. The amount of solar energy available for use is described by what is known as the solar constant, which is a measure of unit energy per unit time per unit area of incoming solar electromagnetic radiation. The solar constant is typically represented in kilowatts per square meter (kW/m2), and the average value of the solar constant has been determined to be about 1.361 kW/m2. Accordingly, over the span of an hour, a one square meter solar panel with 100% conversion efficiency would generate approximately 1.361 kWh, which is equivalent to about 4435 British Thermal Units (BTUs).
All solar thermal panels have less than 100% collector efficiency due to energy and power loss and dissipation. Solar thermal panels that are regarded as efficient (e.g., having peak efficiencies of at least about 80%) typically have a rigid design fabricated from an aluminum or copper metal layer coated with a solar absorbent material, a rigid metal frame, a rigid insulator, and rigid glazing comprised of an expensive low-iron solar glass. Because a solar thermal panel typically requires air gaps between the glazing and absorber layers and the absorber and insulation layers, rigid solar glass and rigid metal support structures are often used to provide and maintain such air gaps. As a result, a large solar thermal panel, particularly with glass glazing, would weigh too much for economical fabrication, shipping, and installation. Because of weight and size limitations of the rigid glass and frame materials, the largest practical solar thermal panels have generally been less than 32 square feet, and panels of that size have weighed several hundred pounds. Even with a flexible glazing (e.g., optical film) in place of solar glass, the rigid frame of a typical solar thermal panel is heavy and limited in length, restricting the panel length to just a few feet. There exists a need for high-efficiency solar thermal panels that are more light-weight and cost-efficient.
The present invention provides for solar thermal panels made from thin flexible materials that can be lighter weight, rigid, and highly efficient. Methods of manufacturing rigid solar thermal panels from flexible materials are also provided.
A rigid solar thermal panel comprises a solar absorber layer comprising a metal sheet coated with a solar absorbent material, a tensioned optical film above an upper surface of the solar absorber layer, and an insulation layer. The insulation layer is located below and spaced apart from a lower surface of the solar absorber layer to form a cavity in the rigid solar thermal panel. The metal sheet includes dimples that project into the cavity. At least one of the dimples is in contact with the insulation layer, providing rigid support for the rigid solar thermal panel.
A method of manufacturing a rigid solar thermal panel includes forming dimples in a metal absorber sheet and forming side walls along at least two edges of the metal absorber sheet to form a solar absorber layer and placing an optical film on a surface of the solar absorber layer. The method further includes applying a force substantially perpendicular to the surface of the solar absorber layer to cause the metal absorber layer to bow and to secure the optical film on either side of the solar absorber layer proximate to the side walls. The bowing can facilitate the mechanical joining of solar thermal panel components, including the optical film to the solar absorber layer. The force is released to permit the solar absorber layer to return to its pre-bowed state and to tension the optical film across the surface of the metal absorber layer.
Embodiments of the present invention have many advantages. Rigid solar panels as described herein provide high-efficiency performance panels with lower manufacturing, transportation, and installation costs as a result of lighter component materials, which can result in lower weight panels.
The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention.
A description of example embodiments of the invention follows.
Solar absorber layer 8 can include dimples of varying dimensions. As shown in
While
The number of dimples included in a solar thermal panel can be dependent on the desired overall size of the thermal panel. For example, a panel of 2.2 m2 can include about 27 larger dimples each having a diameter of about 0.21 m and about 32 smaller dimples each having a diameter of about 0.1 m. In addition, or alternatively, the solar absorber layer can include a dimpled area of, for example, at least about 70%, at least about 80% or at least about 90% of a total exposed area of the solar absorber layer. Smaller dimples can be interspersed among the larger dimples, such that the solar absorber layer includes an adequate number of larger dimples to provide structural support to the solar thermal panel while maximizing use of the area surrounding the larger dimples by including smaller dimples that provide additional air baffling.
Solar absorber layer 8 can be formed from a sheet of metal, such as aluminum, copper or other metal, that is coated with a solar absorber, such as silicon dioxide or titanium dioxide. The silicon dioxide coating can be formed from, for example, quartzite gravel or crushed quartz that has been purified. Dimples can be formed in a flexible material, such as sheets of aluminum or copper, with known metal forming techniques.
Optical film 4 can be a flexible, transparent film that permits a high percentage (e.g., at least about 80%, at least about 85%, at least about 90%, or at least about 95%) of incident solar radiation to pass through the film. For example, optical film 4 can be composed of a thin fluoropolymer material, such as a sheet of polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVDF), which is capable of withstanding high temperatures and is optically transparent to about 90% to about 95% of incident solar radiation. Examples of such optical films include Tefzel® (Dow Chemical, Marlborough, Mass.), Kynar® (Arkema, King of Prussia, Pa.) and Norton® (Saint-Gobain, Worcester, Mass.). Films formed from PTFE or PVDF are also durable, flexible, tear-resistant, and lightweight. An optical film 4 formed from PTFE or PVDF film can have a thickness of about 0.5 mil (0.013 mm) to about 50 mil (1.27 mm), or of about 1 mil (0.025 mm).
Heat transfer in a solar thermal panel is a function of radiation, conduction, and convection. Radiation exchange, or the transfer of heat through a vacuum or gas, is the primary method used to capture solar radiation and convert it to heat in a solar thermal panel. Solar radiation can pass directly through any optically transparent material, such as glass or PTFE or PVDF film. An ideal solar absorber would have 100% absorptivity and 0% emissivity, indicating that 100% of solar radiation is captured and converted to heat. However, typical high-performing solar absorbers have specifications that indicate ranges of about 85% to about 95% absorptivity and about 5% to about 10% emissivity. Solar thermal panels of the present invention, which include optical films of PTFE or PVDF as a glazing layer, are configured to enable absorption of about 90% to about 95% of incident solar radiation.
Once the radiation is converted into heat at the surface of the solar absorber layer (e.g. solar absorber layer 8), the heat can be lost via conduction or convection. The glazing layer located above the solar absorber surface minimizes convection heat losses. Conduction heat losses occur when the glazing and solar absorber surfaces are in direct physical contact. To eliminate or reduce conduction losses, a solar thermal panel can include an air gap between the absorber surface and the glazing. The dimples (e.g., dimples 13, 14) in solar thermal panels advantageously create air gaps between the glazing layer (e.g. optical film 4) and the surface of the solar absorber layer (e.g., solar absorber layer 8), thereby reducing conduction heat losses while simultaneously providing air baffles and structural support to the solar absorber thermal panel.
Following absorption, heat transfers from the surface of the solar absorber to a space to be heated (e.g., cavity 16 located between a bottom surface of solar absorber layer 8 and an upper surface of insulation layer 12) via convection with a transfer fluid of either liquid (e.g., water) or gas (e.g., air). The larger the heat transfer surface area in a solar thermal panel that the transfer fluid contacts, the more efficient the heat transfer process becomes. In an air-based solar thermal panel, the air typically transverses the length of the bottom side of the solar absorber layer. Air enters at the bottom of the panel through an inlet duct and out of the top of the panel through an outlet duct (e.g. air ducts such as air connection 19 as shown in
Solar thermal panels can be formed from flexible, lightweight materials, minimizing the use of heavy materials, such as heavy structural frames and solar glass, and can be less expensive to manufacture and install. Structural rigidity in solar thermal panels of the present invention is provided, at least in part, by dimples that extend from an upper surface of a solar absorber layer to an upper surface of an insulation layer, thereby preventing deformation of the panel and, further, supporting the glazing layer of the panel.
Prior attempts at reducing the cost of solar thermal panels have included replacing low-iron solar glass with rigid sheets of transparent, polycarbonate plastic, such as Lexan™ (Sabic, Pittsfield, Mass.), or ultra-thin, high-temperature optical films, including polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVDF) optical films. For example, as described in U.S. Pat. No. 9,243,815 to Sylvan, the entire contents of which are incorporated herein by reference, the rigid glass glazing of a solar panel is replaced with a lightweight optically transparent film. Additionally, as also described in U.S. Pat. No. 9,243,815 to Sylvan, materials, such as metal foils and thin ceramic oxide paper insulator layers have been used to create flexible solar thermal panels. However, for some applications it can be advantageous to use a rigid panel.
Solar thermal panels of the present invention are rigid, yet lightweight. For example, a 24 ft2 rigid solar thermal panel of the present invention can weigh about 7 lbs to about 12 lbs. A similarly sized conventional rigid solar thermal panel weighs about 90 lbs.
To support the glazing layer and prevent optical films from sagging or tearing, solar thermal panels of the present invention can include a tensioning mechanism to tension the optical film over the upper surface of the solar absorber layer.
To provide extra rigidity to the vertical sides 3 of the solar absorber layer 8, L-shaped indentations 17 or embossed right angle indentations can be incorporated into the metal absorber layer 8 during the manufacturing process. This rigidity enhancing technique can be further understood with reference to metal-forming operations, such as in the manufacturing of angle irons and sheet metal enclosures. As applied to solar thermal panels, the right angle indentations 17 strengthen the vertical wall 3 of the metal absorber layer 8 and can assist with maintaining a precise right angle between flange 5 and the vertical wall 3, further maintain the structural integrity of the solar thermal panel 1.
With spring forms 6 and gaskets 7, the metal solar absorber layer 8 and optical film 4 are held tightly against insulation layer 12. Insulation layer 12 can be, for example, a ceramic or fiberglass mat. As it is unsafe to use fiberglass or ceramic fiber directly as an insulation layer (i.e., without a cover layer) in a forced air system due to the potential dispersal of ceramic or glass fibers into the airflow, insulation layer 12 can include metal foil bonded to the top surface, or to the top and bottom surfaces, of the mat. With a metalized material added to the surface(s) of the fiberglass or ceramic mat (e.g., aluminum foil or other thin metal material), the insulation layer 12 can function as the bottom layer of the solar thermal panel 1. Thus, a cover layer of metal, in addition to providing an additional layer for insulation, functions to prevent the dispersal of contaminants from the mat into the airflow.
To increase assembly speed and decrease manufacturing costs, solar thermal panels can be substantially formed from materials that are available as roll stock. Roll stock is typically a thin material cut to a specific width and rolled onto a hollow or solid core. In production, the materials are unwound from the core as continuous sheets and are then processed in that format.
In an embodiment of this invention, an automated manufacturing system feeds lightweight, absorber roll stock (e.g., metal sheets coated with solar absorbent material), spring roll stock (e.g., stainless steel sheets), wear strips (e.g. aluminum foil), and optical film (e.g., PTFE or PVDF film) into a metal former, metal sealer and slitter. The metal former shapes the flat absorber roll stock into a panel having three-dimensional indentations (e.g., metal solar absorber layer 8 with dimples 13, 14). The manufacturing system can also, optionally, form side walls (e.g. walls 3) and flanges (e.g., flanges 5) from the flat metal absorber roll stock. Alternatively, side walls and flanges can be created from separate materials that are welded or otherwise affixed to the solar absorber layer. The manufacturing system can also add indentations (e.g. indentations 17) to the vertical sides and flanges (e.g., sides 3 and flanges 5) of the solar absorber layer to increase rigidity and support a right angle configuration between the flanges and vertical sides of the solar absorber layer.
Because the tensioned optical film can experience significant wear to wind and temperature expansion and contraction, two wear strips (e.g., metal strips 9, 10) formed of, for example, aluminum, can be applied to the edges of the optical film, protecting the film from directly contacting either the aluminum solar absorber top surface or the tensioning spring bottom surface, or both. The use of wear strips is further shown and described in U.S. Pat. No. 9,243,815 to Sylvan, the entire contents of which are incorporated herein by reference.
The wear strips may be mechanically bonded to both sides of the optical film by ultrasonic welding techniques or metal clinching systems, or the wear strips may be chemically bonded by adhesives, thereby creating a unified optical film and metal assembly. The optical film with applied wear strips can then be interposed beneath a tensioning spring (e.g., spring forms 6) and a flange (e.g., flange 5) of the formed solar absorber panel (e.g., metal solar absorber layer 8). The optical film assembly can then be mechanically clamped to the flange by welding, riveting, and/or by other metal-joining techniques to generate a tight bond. The joining can form a substantially airtight seal.
During the manufacturing process, it may be desirable or necessary to bow the solar absorber layer and flanges to facilitate the mechanical joining of these elements. Advantageously, the flexible nature of the absorber material (e.g., low gauge metal of, for example, 0.3 or 0.4 mm) permits bowing the materials during manufacturing of the panel. Bowing to facilitate assembly of a panel can be performed by applying pressure substantially perpendicular to the panel surface. When in the bowed state, automated assembly equipment can mechanically bond the optical film with attached wear strips to the two horizontal flanges of the solar absorber panel. Upon release of the perpendicular force, the elements return to their pre-bowed state and the optical film is tensioned over the upper surface of the solar absorber layer. Because the solar absorber material is typically of a light metal gauge, the solar thermal panel may not be fully tensioned until the entire assembly is attached to a support surface (e.g., insulation layer 12 and/or support base 18) and the angle between the vertical sides of the panel are adjusted to be at approximately 90 degrees with the flanges.
Constructing a solar thermal panel from flexible materials permits the panel to be incorporated into a low cost, non-rigid base, such as, for example, a thermoformed composition or similar technology (e.g., support base 18). With rigid solar panels that incorporate glass as a glazing, the support frame must be rigid enough to prevent flexing of the solar panel. Any flexing of the panel would cause the glazing to crack. By making a rigid solar panel with film glazing and flexible materials, the solar thermal panel can be less expensive and simpler to deploy. In place of rigid and expensive mounting frame, the rigid solar thermal panel can be mounted on a low cost thermoformed support panel. This type of support panel can then be mounted on a wall or roof of a structure, or attached to a support stand for ground installation. With a stand-based support, the panel's angle to the sun can be adjusted over the solar year to optimize solar thermal performance. For example, in the winter, the panel can be oriented to a more vertical position, and in the summer, a more horizontal position.
The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety.
While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.
