INVESTIGATION OF PERFORMANCE OF SOLAR AIR HEATER WITH LOW POROSITY METAL MATRIX ABSORBER

Abstract

A solar air heater is a simple device to heat air by utilizing solar energy. The efficiency of a conventional solar air heater has been found to be low because of low convective heat transfer coefficient between the absorber plate and air. To enhance the efficiency of solar air heater, a number of methods have been proposed. Porous packing of the air duct is one of them. From literature review it has been found that the volumetric heat transfer coefficient is generally higher for beds with lower porosity. Presence of wire screen matrices in the solar air heater duct results in an entirely different flow arrangement because air flow is parallel to the plane of screens. Previous work in low porosity shows substantial improvement in performance, which attracts further investigation in low porosity. In view of the above facts, the present investigation has been carried out with the following objectives: 1. Collection of experimental data of heat transfer and friction in a packed bed solar air heater using low porosity wire screen matrices system under actual outdoor conditions. 2. Development of correlations for heat transfer coefficient and friction factor on the basis of experimental data in terms of geometrical parameters of the bed. 3. Investigation of the enhancement of thermal performance of a packed bed solar air heater compared to that of a conventional solar air heater. 4. Investigation of the optimum bed parameters for a given set of operating conditions on the basis of thermo-hydraulic and thermal performance. Therefore an experimental set-up was designed, fabricated and used for data collection for the packed bed and conventional collector ducts. Experimental data on heat transfer and friction has been used to determine the values of Colburn factor Jh, and friction factor, fp, as function of system (bed) and operating parameters. The analysis of experimental data on heat transfer and friction characteristics revealed that the heat transfer coefficient and friction factor are strong functions of these parameters. Based on experimental data, the following correlations have been developed. Colburn J factor 0.772 YRe -o.62 = 0.505 C (1nP)(pf dw)] P 11 Friction factor YReP_ o.sz= 0.602 1 nP) a.1 J 1.224 It has been found that the thermal efficiency of packed bed solar air heater improves appreciably over that of a conventional collector as a result of packing the collector duct with wire screen matrices. The enhancement in thermal performance has been found to be between 38% and 87.5% in the range of the parameters investigated. This enhancement in the thermal efficiency appears to be due to the creation of turbulence and the absorption of energy in depth resulting in better heat transfer. The efficiency of the matrix collector increases with increase in mass flow, rate. This occurs, because at higher flow rate, thermal losses are decreased as the collector operates at lower temperatures. In the range of lower flow rate, the rate of increase in efficiency with an increase in flow rate is higher, whereas in the higher range of flow rate it decreases. This is due to a corresponding decrease in the rate of -increase in; heat transfer as the mass flow rate increases. The thermo-hydraulic performance parameter called "effective efficiency", has been employed to evaluate the net useful thermal energy gain, taking into account the equivalent thermal energy required to produce the work energy necessary to overcome the additional friction or hydraulic losses as a result of packing the solar air heater duct with absorber matrices. It is observed that the thermo-hydraulic efficiency decreases with the increase in the values of bed depth to element size ratio and bed porosity, but it increases with an increase in mass flow rate of air, attains a maximum and subsequently decreases with further increase in mass flow rate. A system which is thermo-hydraulically superior i.e. its thermo-hydraulic efficiency is high, is considered for design under given operating conditions. On the basis of thermo-hydraulic performance the optimum values of bed parameter i.e. porosity, pitch and wire diameter have been found for a range of operating conditions. An optimization procedure to determine the optimum values of the bed parameter has been proposed.