The present invention relates to improved flat plate solar boiler configurations which are constructed to increase the maximum temperature of the water or steam produced in such boilers. The boilers of the present invention may be used in various solar applications and, in particular, can be used in systems of the type described in my prior U.S. Pat. No. 3,919,998, issued Nov. 18, 1975, for "Convection-Type Solar Heating Unit" and No. 4,076,025 issued Feb. 28, 1978, for "Solar Steam Boiler". The disclosures of these prior patents are incorporated herein by reference.
High temperature water or steam is needed in many applications such as, for example, absorption type air conditioners. In addition, the generation of high temperature water is highly desirable to facilitate the storage of heated water, e.g., inasmuch as boilers of known configuration are normally constructed to withstand pressures as high as 100 psi, water may be safely heated in such boilers to a temperature of over 300.degree. F. without boiling and, at this temperature, much more energy can be stored than in conventional household hot water heaters. By mixing the super heated water in such a boiler with, for example, cold water through a thermostatically controlled valve, water at any desired final temperature can be readily obtained.
Flat plate solar boilers of known configuration are capable of heating water to a temperature wherein the heat which is supplied to the boiler by solar radiation equals the heat which is removed from the boiler by radiation or otherwise. When a balance between the heat supplied to and removed from the boiler is achieved, further increase of temperature ceases. Heat is removed from the boiler by radiation, or by conduction, or by removal of hot water or steam from the boiler. Of these three mechanisms, heat loss by conduction is comparatively insignificant, and heat loss by removal of hot water or steam can be controlled; but the loss of heat from the boiler due to radiation is, for the most part, unavoidable and therefore represents a limiting factor which determines the maximum temperature to which water in the boiler may be heated.
In flat plate solar boilers which have been suggested heretofore, only one side of the boiler is heated by incident solar radiation from the sun. Radiation losses occur, however, from both sides of the boiler. The side which is remote from the sun, normally termed the dark side of the boiler, usually faces a heat reflective material which functions to make radiation heat loss from the dark side of the boiler less than that from the sun side of the boiler. Nevertheless, radiation heat losses from the dark side of the boiler are not negligible, and, in known boiler configurations, is determined by the quality of the heat reflecting material employed, e.g., aluminum foil, and is also determined by the quality of the thermal insulation material which is normally disposed behind the heat reflecting material to minimize radiation heat losses from the reflecting material itself resulting from the heating of said heat reflecting material during the operation of the boiler.
The present invention, in contrast to these known arrangements, minimizes radiation losses from a flat plate solar boiler, and increases the temperature to which the liquid in such boilers may be heated, by delivering solar radiation generated heat to both the sun side and the dark side of the solar boiler.