Pergamon Renewable Energy, Vol. 14, Nos. 1-4, pp. 77-82, 1998 0 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain PII: SO960-1481 (98) 00050-O 0960-1481/98 $19.00+0.00 FORCED VERSUS NATURbL CIRCULATION SOLAR WATER HEATERS: A COMPARATIVE PERFORMANCE STUDY RSDUL-JABBAR N. KHALIFA Solar Energy Research Center, Jadiriya, P,O. Box 13026, Baghdad, IRAQ RBSTRACT An experimental study has been carried out to compare the performance of natural and forced circulation domestic solar water heaters. Several measurments have been made for the two cases which includedI the collector water inlet and outlet temperatures, the mass flow rate, the tank temperature, the ambient temperature and the solar insolation. The main paraneters for the solar collector are calculated for the natural and forced circulation systems, These included; the top, back, and overall loss coefficients, the heat removal factor, the efficiency factor, the useful energy gain and the instantaneous efficiency. The comparison showed that the efficiency of the forced circulation system could be 35 to 60% higher. 0 1998 Published by Elsevier Science Ltd. All rights reserved. KEYWORDS Solar heaters; natural circulation; forced circulation; system performance. INTRODUCTION In a natural circulation system, the thermal storage tank has to be located above the collector, and water will circulate by natural convection due to density difference. There is no requirement in the forced circulation system for location of the tank above the collector. A pump is used to circulate water between the collector(s) and the storage tank which can be located in this case at any convenient place. As each system has its own advantages and disadvantages, a comaprison between the performance of both systems is found to be of interest. THEORY The thermal analysis of solar collectors is covered in many solar thermal engineering publications, (for example; Duffie et al -'1 1980; Howell et al -.I 1982; Lunde, 19801, The equations which describe the main parameters and their importance are shown below: Top LossCoefficient (UT) A coefficient which accounts for the heat loss from the top surface of the collector due to convection and radiation, W/m'K, 77