Effect of selective coating on thermal performance of flat plate solar air heaters
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.
Section snippets
Thermal analysis
A schematic diagram of the conventional flat plate solar air heater is shown in Fig. 1a. The heater has an area of 1 m2 and a flat plate absorber fabricated from a black painted galvanized iron sheet of thickness 2 mm. The bottom and sides of the heater are insulated by 3 cm layer of sawdust contained in a wooden frame of 1 cm thickness. Two glass covers (3 mm thick) with distance separation of 2.5 cm are used as the cover for the heater. The heater is oriented to face south making an angle of
Numerical calculations and experiment
Fig. 2 shows the hourly variations of horizontal solar radiation I(t) and ambient temperature Ta on typical summer (18/7/05) and winter (17/1/05) days for Jeddah that were used for numerical calculations. A computer program, based on Liu and Jordan isotropic model [20], was prepared in Pascal language by writing subroutines for calculation of global solar radiation incident on the heater cover using the hourly measured values of solar radiation incident on a horizontal surface shown in Fig. 2a.
Results and discussions
Fig. 3 shows hourly variations of temperatures of different elements of the heater with the black painted absorber on a typical summer day (as an example). The temperatures of the heater elements increase with time as solar radiation increases until they achieve their maximum values at 13:00 h. Afterwards, the temperatures of all elements decrease as solar radiation and ambient temperature decrease. The shift of peaks positions of temperature–time curves compared to that of solar radiation (see
Conclusions
A transient mathematical model was presented for a single pass flat plate solar air heater. The model was validated by comparing the simulated results with the measurements that had been performed in a previous work [24]. The model was found to be able to predict the heater performance with good accuracy. The thermal performance of the heater was investigated by computer simulation for various black painted and selectively coated absorbers under Jeddah prevailing weather conditions. The best
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