Energy Com'. & M,qmt Vol. 21, pp. 121 to 123. 1981 0196-8904 81 020121-03S02,00 0 Prinled in Great Britain. All rights reserved Copyright © 1981 Pergamon Press Lid PERFORMANCE STUDY OF FORCED CIRCULATION SOLAR WATER HEATERS USING PACKED-BED COLLECTORS C. B. MISHRA and A. K. BHAT Department of Mechanical Engineering, Birla Institute of Technology, Mesra, Ranchi, Bihar 835215, India (Receired 6 October 1980} Abstract--A performance study of forced circulation solar water heaters using packed-bed solar collec- tors is presented. Iron chips, gravel and stones have been used as packing materials. Thermal energy stored in the tank, system overall efficiency and pay-back capital for these solar water heaters are compared with those for solar water heaters using a plane collector. It is observed that the performance of the solar water heater is improved appreciably by packing its collector with packing material. A solar water heater using an iron chip, packed-bed collector shows the best performance. Solar energy utilisation Solar water heater Solar energy collectors Packed-bed collectors INTRODUCTION Solar energy is a resource for the future. At a time when the world is facing problems of energy supply, increased pollution and mounting costs, the promise of this clean source of energy deserves the serious attention of scientists, technologists, social scientists, policy makers and planners. For a tropical country such as India, which is especially favoured by nature as far as insolation goes, the potential for harnessing solar energy is enormous. Thus, there is clearly a need for systematic and exhaustive studies of the perform- ance behaviour of solar energy systems of various designs. Solar water heating is one of the most widely used applications of solar energy. Some attempts[I-5] have been made to improve the performance of solar water heaters. However, no work seems to have been done on solar water heaters using packed-bed collec- tors. Hence, in the present investigation the perform- ance behaviour of solar water heaters using packed- bed collectors has been analysed and compared with that using plane collectors. Iron chips, gravel and stones have been used as packing materials. EXPERIMENTAL DETAILS Three solar water heaters with packed-bed collec- tors having blackened iron chips, gravel or stone as packing materials and one with a plane collector were fabricated and tested. The overall size of each collec- tor was 95 x 95 x 15cm. The absorber plate was a G.I. sheet painted with blackboard paint. The gap between the absorber plate and the single glass pane used was kept at 5 cm to achieve minimum gap con- ductance [6]. The pipe grid consisted of 2.54 cm dia headers at either side of the collector and 1.25 cm dia connecting pipes. An electrically driven water pump, having a capacity of 2401/hr under a head of 15 m was used to circulate water through the system. The collectors were installed at 30 with the horizontal facing south, this being the optimum tilt angle for the site [7]. RESULTS AND DISCUSSION Thermal energy stored in the tank with time of the day has been plotted in Fig. 1 for the 4 solar water heaters, 1 having a plane collector and 3 having packed-bed collectors using iron-chips, gravel and stones as packing materials. The thermal energy stored in the tank is calculated with reference to the ambient temperature at any instant when the samo water is circulated in the system. Hot water is not removed from the tank during the test period. The plotting has been done for one complete day and night starting from 7.00 a.m. on a particular day and ending on next day at 7.00 a.m. Thermal energy stored in the tank increases from 7.00 a.m. until 2.00 a.m. at night and then decreases in all types of packed-bed solar water heaters. In the case of the plane collector solar water heater, thermal energy stored in the tank increases up to 9.00 p.m. and then decreases, the de- crease being rapid after 12.00 midnight. Due to pack- ing material, the heat absorbing capacity of the collec- tor improves. As a result, the thermal energy stored in all packed-bed solar water heaters is higher than that in the plane collector solar water heater. Moreover, packed-bed collectors also have the capacity to store heat, which is then given out to circulating water dur- ing non-insolation periods. On the whole, iron-chip, packed-bed solar water heaters store maximum ther- mal energy throughout, followed by gravel, stone packed-bed and plane collector solar water heaters. 121