Effect of selective coating on thermal performance of flat plate solar air heaters

Abstract

A transient mathematical model was presented for a single pass flat plate solar air heater. This model was based on an analytical solution of the energy balance equations for various elements of the heater. The flowing air temperature was assumed to vary only in the flow direction. The thermal performance of the heater was investigated by computer simulation using the climatic conditions of Jeddah (lat. 21° 42′ N, long. 39° 11′ E), Saudi Arabia. Effects of solar radiation intensity, mass flow rate of the flowing air $\left({\dot{m}}_{\text{f}}\right)$ and the length (L) and width (b) of the absorber plat on the flowing air outlet temperature (T_fo) and the heater instantaneous (η_inst) and daily (η_d) efficiencies were studied. To improve the heater performance, effect of using absorber plates coated with various selective coating materials on the heater performance was also investigated. The best performance was achieved using nickel–tin as a selective coating material with a daily average of the instantaneous efficiency of 0.46. To validate the proposed mathematical model, the simulated results were compared with the measurements that had been performed for the heater with a black painted absorber plate under Tanta, lat. 30° 47′ N (Egypt), weather conditions. It was found that the proposed model is able to predict the T_fo accurately with a daily average relative percentage error of 7.7%. It was also inferred that the annual average of η_d with a nickel–tin selectively coated absorber is higher than that with a black painted absorber by 29.23%.

Introduction

Solar air heaters can be used for many applications including crop drying and space heating. Solar air heaters have many attractive advantages over liquid heaters regarding the problems of corrosion, boiling, freezing and leaks. There are about two review papers in the area of solar air heaters published in recent years. The first was proposed by Chandra and Sodha [1]. These authors have provided a fundamental understanding of testing procedures for solar air heaters. The second was proposed by Ekechukwu and Norton [2]. They have classified solar air heaters broadly into two types: bare plate and cover plate solar energy collectors. Based on these classifications, these authors have summarized various designs of solar air heaters. A review of the mathematical models performed for predicting thermal performances of different designs of solar air heaters systems was recently published [3]. It was indicated that the major governing equations in the models were based on the first law of thermodynamics and most of the models have to be solved numerically.

One of disadvantages of solar air heaters is the decreased rate of heat transfer from the absorber plate to the flowing air due to the lower heat transfer coefficient of air. Therefore, many attempts had been carried out to increase the amount of heat that would be transferred to the flowing air by using fins attached over and under the absorber plate [4], recycling of the flowing air in double-path solar air heaters [5], using absorber plates with different types of roughness [6], [7], [8], [9], [10], [11], using packed bed materials above or under the absorber plate [12], [13], using absorber plates with V-grooves [14], [15], etc. Saini and Verma [16] indicated that the heat transfer coefficient between the absorber plate and air can be considerably increased by using artificial roughness on the underside of the absorber plate. All these methods require high velocity for the flowing air which results in higher efficiency but also in higher pressure drop. This increases the pumping power required to force the air within the heater's duct(s) which increases the cost. Another method that may be used to improving the thermal performance of flat plate solar air heaters is by using selected coatings, which have a very high absorptivity of solar radiation but a very small emissivity of thermal radiation, on the absorber and glass cover [14], [15]. Detailed heat transfer and sensitivity analysis of unglazed selective absorber solar air collector were performed by Njomo [17] and Njomo and Daguenet [18]. They investigated theoretically effects of the change in operation and meteorological parameters on the thermal efficiency and the fluid temperature rise between entrance and exit of the heater.

Inspections of the published papers about solar air heaters indicated that, a detailed study on the effect of selective coatings of the absorber plate on the year round thermal performance of solar air heaters in view of energy losses, useful energy and instantaneous and daily efficiencies is not present in the literature. Therefore, the main objective of this paper was to investigate, by computer simulation, effect of using different selective coating materials on the thermal performance of a double glass single pass flat plate solar air heater. Numerical calculations have been carried out on typical summer and winter days in Jeddah (Saudi Arabia) for the heater with black painted and selectively coated absorbers for the purpose of comparison. To verify the theoretical model proposed for the heater, comparison between measured and calculated results was performed and some conclusions were drawn.