The present invention relates to solar collectors. More particularly, the present invention relates to a solar energy collector comprising an array of light transmitting cells positioned adjacent a solar energy absorber.
The solar energy absorber can take many forms. For example, it can be of a flat plate collector that heats fluids for residential or commercial use. Flat plate solar collectors usually comprise an optically black solar absorbing surface in conjunction with a series of fluid containing tubes and/or a fluid containing tank. The black solar absorbing surface converts the light from the sun into energy and then transmits that energy to the fluid, usually water, in the tubes or tank. The hot water can be used for cleaning; e.g., personal clothing and vehicles or, in some cases, for heating other bodies of fluid such as the water in a swimming pool or air in a room.
The solar energy absorber can also be all or part of an interior or exterior structure wall. In this approach the wall would be constructed from a material with a high heat capacity and low thermal conductivity. The wall can be constructed either with or without black, heat absorbing material on its surface. Light from the sun would be absorbed by the wall and converted into heat energy which would then be transmitted into the structure to heat the fluid; i.e., air, within the rooms of the structure. Alternatively, the sun's energy can be absorbed by a flowing wall or body of water or fluid containing phase change materials immediately adjacent the structure wall. By either of these means the resultant heat energy can be readily transported and delivered to any part of the structure. Similarly, the solar energy absorber can be the very contents of a room such as a green house; i.e., the solar energy absorber could be the plants and earth, etc. within the room.
Whatever form the solar energy absorber may take, it is inherently inefficient and is continually losing a significant portion of the absorbed energy by the well known mechanisms of convection, conduction and radiation. The interaction of these heat loss mechanisms limits both the amount of energy transmitted by the absorber to the adjacent fluid and the peak temperature attainable by that fluid. Moreover, as in any complex system, action to control one variable often has a detrimental effect on other variables.
For example, in the context of a flat plate solar collector, convection losses generated by surrounding environmental factors can be controlled to a limited extent by containing the solar absorber within a confined space such as a box. Typically, the solar absorber will form the bottom of the box with the sides being insulated and the top cover being constructed of glass, plastic or other materials which are transparent to solar radiation. The box prevents external air currents from interacting with the solar absorber. However, this "solution" to forced convection losses necessarily brings the solar absorber into intimate contact with other surfaces, gases and materials. This contact, in turn, permits conduction losses. Moreover, while the side walls and top cover of the box do reduce the effects of forced convection caused by externally generated air currents, they have no impact on natural convection and thermal instabilities within the box.
The most difficult energy loss mechanism to control has been radiation. The solar energy absorber is heated by radiant energy from the sun and can lose a significant portion of that energy by re-radiation to the atmosphere. The typical solution in flat plate solar collectors is to incorporate one or more infrared absorbing plates, that is a plate which will transmit in the visible spectrum, parallel to the solar absorber. For example, a glass or plastic cover used alone or in conjunction with other glass or plastic plates or films. Such plates will absorb the infrared energy radiated to it from the solar absorber. Some of this energy will be re-radiated outward and lost, but in the process the rate of heat loss is slowed substantially. The difficulty of the problem is compounded by the fact that the very absorbing materials which have a salutary effect on re-radiation losses also reduce the transmission of incident sunlight to the solar absorber.
Transmissivity of a solar collection system, defined as the ability of a system to transmit incident sunlight to the solar energy absorber, is a function of two principal variables: The materials of construction of the system and the number of surfaces between the solar energy absorber and the atmosphere. Each plate has two surfaces. The lowest transmission loss produced in a plate to date as a result of reflection is about 8 percent; i.e., and inherent loss of 4 percent per surface for a total transmissivity for the plate of 92 percent. If two plates of this material are used in a solar collection system, one plate to form the top cover of the box and one plate within the box to reduce radiation losses, the transmissivity of the system is 0.92.times.0.92 or 84.64 percent. Thus, even when the very best materials are used in the most sparing fashion possible, the inherent reduction in transmissivity is dramatic.
A partial solution to this problem is found in the work "Design Considerations for Solar Collectors with Cylindrical Glass Honeycombs" by Buchberg, et.al. reported in Solar Energy, Volume 18, Pages 193-203 and published by Pergamon Press, 1976. The flat plate solar collector shown therein has a glass cover and incorporates a hexagonal or square packed honeycomb array of cylindrical glass tubes in a plane perpendicular to the solar absorber plate. The honeycomb of tubes acts as a baffle to suppress the most damaging form of convection by confining the fluid motion to walled cells. In a tilted array, heated from the bottom, upward flowing hot air in the honeycomb is retarded by viscous shear on the cell walls and thermal energy in the air is drained by conduction to the cell walls. Lateral conduction through the cell walls and radiation exchange across the cells redistribute this energy to the colder regions and from there to the colder down flowing air. This, in turn, lessens the buoyancy difference between the relatively hotter and colder air. Viscous shear created by the cell walls also acts to slow the down flowing colder air.
Thermal radiation transfer from the hot solar absorber to the relatively cold cover glass is also reduced significantly by the presence of the cell walls. Emitted photons from the solar absorber which encounter the cell walls are absorbed strongly because of their infrared wavelengths. The re-radiation from the cell walls is directed both upwardly and downwardly. The downward directed infrared radiation reduces the net difference between the upward and downward infrared radiant fluxes at the solar absorber and thereby reduces the net infrared re-radiation loss.
The Buchberg, et.al. approach is, however, only a partial solution to the problems of convection, conduction and re-radiation losses in solar collectors. Even with very thin cell walls and length-to-diameter (L/D) ratio for the tubes in the optimum range, the R-value (the measure of the ability of the system to retain absorbed energy) for the Buchberg, et.al. collector is only about 1.60. While this does represent an improvement over thermal resistance of other prior systems, it does not approach the R-value of at least 2 required to generate steam or an R-value of at least 3 required before a solar collector can be used as the basis for an air conditioning system.
Moreover, Buchberg, et.al. is restricted to a honeycomb of closely packed hexagonal or square packed arrays wherein the tubes are in contact with each other over their full length. As Buchberg, et.al. indicates, the cost of the honeycomb, the solar energy reaching the absorber and the heat loss by conduction are all influenced by the amount of glass in the cross-section of the honeycomb; i.e., as the amount of glass in the cross-section increases, the energy transmitted to the absorber decreases and the heat loss due to conduction increases. All of these are negative effects both to the economy and the effectiveness of a solar collector.