Evaluation of the performance of air heaters of conventional designs

Abstract

Air heaters, because of their simplicity, are more amenable to theoretical investigations than the other solar thermal systems like solar stills, etc. In air-heating collections various parameters on which its performance depends can be identified and their effect can be studied theoretically. In this paper the transient equations of three types of air heater are written and their solutions are attempted. In Type I air heater there is one air channel above a metallic plate, in Type II, the air channel exists between two metallic plates and in Type III, there are two air channels one above the plate and another between two metallic plates. The transient equations of the model are solved within the framework of periodic analysis. The temperature of any component of the air heater is represented by a Fourier series in time, the coefficients being coordinate dependent. The effect of flow rate, plate length, selective coating, etc. are studied. It is shown that for a given rate of flow, there is an optimum value of plate length for which efficiency is maximum. The optimum value of plate length in Type II air heater increases as rate of flow increases. For a given plate length, the efficiency increases with rate of flow. The selective coating will be useful only when plate length is large and rate of flow is small. For a flow rate of 100 kg m −2hr −1 and plate length of 7 m, the difference between outlet air temperature from a selectively-coated plate and that of black-painted plate, at midday, is 33°C. The selectively-coated air heater is 21 per cent more efficient than the black pained one at the same condition. It is found that an efficient configuration has lower glazing temperature.