Investigation on water cooling and heating performance of an unglazed metallic roof panel solar water heater

Thailand is a tropical country therefore the cooled water and hot water are necessary for every season. In Ubon Ratchathani province, most of the roof of house is made by metallic roof. The unglazed metallic roof panel solar water heater is possible to apply to the house. During the day time, the water heater is produced by solar radiation while the water is cooled by nocturnal radiative cooling. Although this system has not widely used, it is possibly to be promoted to use in the near future because of the oil crisis. Therefore, the performances of the system should be investigated in detail. This paper presents the performance of water cooling and heating of an unglazed metallic roof panel solar water heater. The effects of solar radiation on water heating performance and the effects of water mass flow rate and wind velocity on water cooling performance were investigated. To construct this system, seven copper tubes with diameter 12.7 mm are welded to a 2.5 x 0.67 m² metallic roof panel. The water tank capacity is 60 liters. Water is circulated from upper storage tank to lower tank by gravity and from lower tank to upper tank by pumping. The experimental results showed that the system constructed in this study yields water temperature of 51°C in the clear sky day for water heating and 2.3-2.5°C for water cooling. The economical analysis revealed that cost of hot water is 0.03 Baht/liter. For water cooling, it was found that at the water flow rate 0.01 and 0.02 kg/s the system can released energy 1.15 and 1.24 MJ and COP was 4.6 and 2.5 consequently.