On Second-Law Efficiency of Solar Collectors

Abstract

Traditionally, the performance of solar collectors has been examined from the standpoint of energy-balance approach (Hahne, 1985; Francy et al., 1985; Lund, 1985; Satyamurty, 1985; Proctor, 1984a; Proctor; 1984b; Proctor, 1984c). It is important to note that this approach gives a very poor indication of how well a particular energy resource is being utilized by a given system to accomplish a specific objective. For example, it is difficult to examine the performance of solar thermal collectors versus solar-photovoltaic panels, strictly based on the first law of thermodynamics. A photovoltaic panel having ten percent first-law efficiency may be better than a solar collector converting 50 percent of useful energy by raising the water temperature from 30 to 50 C. Thus, it has been necessary to resort to the second law of thermodynamics for evaluation of solar systems, particularly collection devices. The second-law efficiency addresses the quality of energy. The quality of energy may be defined as its available portion; that portion which may be used for producing shaft work. It is important to note that the energy be conserved, but the manner in which energy is used must be examined. To use high quality energy for low-quality energy tasks is wasteful.more » This type of indication can only be examined from the second-law point of view. In this paper, measurements obtained from an experimental solar collector test facility are presented. Analysis of the performance of two flat-plate thermal solar collectors and one photovoltaic panel are analyzed based on both the first and second law of thermodynamics. The measurements presented, indicate very low second-law efficiency (maximum of 17 percent for photovoltaic panel and 11 percent for solar thermal flat-plate collectors), but are merely used as a vehicle for discussion.« less