Performance of a collector-cum-storage type of solar water heater

Abstract

An inexpensive solar water heater of about 701 capacity, combining collection and storage, has been tested. The blackened plate (1.5 m 2) of the collector-cum-storage unit of this heater absorbs solar energy and transfers it to the water stored in its enclosure (140 × 90 × 5.5 cm), the water being in direct contact with the absorber plate. The collector-cum-storage unit is enclosed in a wooden box with 10 cm thick glass wool insulation at the bottom and one glass cover. Experiments have been carried out to test the performance of the water heater under four different modes of operation: (a) water circulation with a small pump (b) natural convection conditions (c) water draw-offs taking place when the water is around 50–60°C (d) water flowing continuously past the absorber plate with flow rates of 38, 60, and 75.9 kg/hr. The day-long collection efficiency under the first two modes has been ascertained to be around 50–53 per cent for a rise in water temperature of 57-50°C. For water temperatures between 60 and 70°C, the collection efficiency is around 65-58 per cent. No appreciable difference in the collection efficiencies has been observed under the first two modes of operation. The average collection efficiency under the third mode of testing has been found to be 64.8 per cent with 202.61 of water heated from 38.5 to 58°C. In continuous flow of water past the absorber plate, a collection efficiency as high as 71.8 per cent was attained at the mass flow rate of 75.9 kg/hr, when tested under steady flow conditions. If no water is drawn off during the day, temperatures between 50 and 60°C are reached at about 11 a.m.–12 noon, 60–70°C at 12 noon–1 p.m., and 70–80°C at about 1–2 p.m., the maximum being as high at 86°C by about 3.30 p.m. In addition a theoretical calculation based on Hottel and Woertz equation for the overall heat loss coefficient between the absorber plate and the surroundings for the hourly rise in water temperature shows a very good agreement with the experimentally measured values of water temperatures.