Experimental and theoretical analysis of heat transfer in a solar collector storage

Abstract

The current research includes a practical study of the thermal performance of solar collector integral storage system (ISCS) in which different types of heat exchangers are immersed in its enclosure for comparison. The first type is a straight tube heat exchanger (ST) and the other is a heat exchanger in the form of a coiled tube (CT). The effect of single and double glass layers and fluid flow rates inside the heat exchanger on the natural convection heat transfer of ISCS system is experimentally evaluated by determining the temperature difference of inlet and outlet water through the heat exchanger immersed into the ISCS enclosure, the temperature distribution inside the enclosure, the ISCS efficiency, and the amount of thermal storage during evening times. Three sets of experiments are performed for a different water flow rates (1.0, 1.5, 2.0, and 2.5 Lpm) inside the heat exchanger that is immersed in the thermally insulated enclosure and different glassing layers. The first set represents the presence of a heat exchanger in the form of a straight tube with a single glass was layer on the top face of the enclosure that is called (STSG). The second set uses a coiled tube heat exchanger with a single glass on the upper side of the enclosure and it’s named as (CTSG). The last set of experiments is represented by a heat exchanger in the form of a coiled tube with a double glass on the upper face of the enclosure and it’s abbreviated as (CTDG). At water flow rate of 1 Lpm, the CTDG exhibited higher temperature difference (28 °C and 19.9 °C than (19.4 and 11.4 °C) for CTSG and (12 and 7.3°C) for STSG during day and night times, respectively. The results reveal that the thermal efficiency (84.5 - 77.8 %) of the (CTDG) case at water flow rate of 2.5 Lpm was higher than the efficiency (68.8 - 56.6%) of (CTSG) case and that (41.7 - 39.7%) of STSG) case during day and night times, respectively. It was also observed that the internal energy exceeds the amount of solar radiation during the day and provides a thermal storage at night due to the complete isolation of the collector. The temperature measurements near the tube allow the calculation of the Rayleigh number during the heat transfer process, and empirical relationships haves been deduced for each set of experiments between the Nusselt number and the Rayleigh number.