Performance of black-painted solar air heaters of conventional design

Abstract Although selective surfaces can significantly improve the performance of solar heaters, black-painted solar heaters are still almost exclusively used, due to the difficulty of producing selective surfaces by the inexperienced. This paper is concerned primarily with black-painted collectors, although in principle the results also apply to collectors with selective surfaces. The object of this paper is four-fold: 1. a) To demonstrate how a rational heat-transfer analysis of solar air heaters of conventional design leads to performance equations from which the useful heat collection can be accurately predicted for any operating condition. 2. b) To prove that the resistance to heat transfer between the air being heated and the solar absorbing plate (black painted or selectively-treated absorber plate) is a major limiting factor in solar air heater performance, and to demonstrate how performance can be significantly improved by use of screens or roughened plates without adding greatly to the cost of construction. 3. c) To compare the performance of glass and Tedlar (DuPont PVF film) covered solar air heaters. (The comparison is restricted to collectors with black-painted absorbing surfaces; selective surfaces are not considered.) 4. d) To show that for heating air from ambient temperature to about 50°F above ambient the economically-optimum construction of solar heater employing black-painted and not selective surfaces comprises a Tedlar cover about one inch above a blackened mesh gauze screen, with the air to be heated flowing primarily beneath the gauze screen. A subsequent paper will deal with other designs of solar air heater.