Advanced Flat Plate Collectors

Abstract

The main drawback of the conventional flat plate collector is its inability to operate with reasonable collection efficiencies at temperature around 80 °C, thus limiting their applications largely for providing hot water and space heating. Therefore, the major concern of solar collector designers is the reduction of heat losses from the solar energy collector to the surroundings. Designers seek economic alternatives to minimize these thermal losses, which result from conductive, convective and thermal radiative heat exchanges and also the reflection losses at the interfaces. For the utilization of solar energy for producing cold, for power generation and for pumping of water, fluid temperatures in the range of 110–150°C are required. The use of moderate vacuum between the cover glass and the absorber plate can suppress the natural convection losses from the absorber plate of the collector and will give the desired temperature and efficiency. The affect of the vacuum environment is especially pronounced when it is used in conjunction with a selective black surface on the absorber plate, making it possible to operate at a temperature of 150°C with a daily energy collection efficiency in excess of 40 percent. Because of the pressure of the atmosphere, however, evacuating the space between flat plates is technically not possible. Honeycombs, or glass supports could offer good physical support if the space were evacuated but many sealing problems remain and this approach is so far impractical. For these reasons, evacuated collectors have generally involved tubular designs, ,which have inherently higher strength to withstand external pressure and are termed as advanced collectors in this chapter. On the other hand there is much technology available in evacuating glass tubes for fluoroscent lighting and TV electron tubes applications, and collectors based on using evacuated tubes have been successfully developed. Therefore, the evacuated tubular collectors which can withstand high vacuum and can be mass produced are preferred. Their high temperature effectiveness is essential for the efficient operation of solar air conditioning systems, power generation and process heat systems. In these tubular or advanced collectors high performance is achieved by the use of the following advanced features: i) Vacuum insulation ii) Selective black absorber coatings iii) Anti reflective coatings films iv) Heat mirror coating v) Highly efficient removal of the absorbed heat from the solar collector by the principle of heat pipe.