Two-phase water heater: Model and long term performance

Abstract

In the present study the model for the two-phase water heater proposed earlier is modified to account for diurnal variation of the heat transfer coefficient in the condenser. The present model is able to account for the observed temperature difference between the collector plate and storage water. The agreement with experimental results is good when liquid level in the collector is equal to or greater than 80%. This also ensures good performance of the heater. With lower liquid levels, as vapor superheating takes place, the condenser performance is affected resulting in poor performance of the water heater. The two-phase water heater was extensively tested over two years with and without hot water withdrawals. Its performance was compared with a thermosiphon water heater of identical collector area and storage volume. Weekly solar fractions for both the heaters are presented for a one year perios. It is found that the two-phase water heater is 4–8% less efficient than the thermosiphon water heater for clear sky and high radiation levels (22–28 MJ m −2 day −1). In monsoon, however, it is 40% less efficient due to low radiation level (12–15 MJ m −2 day −1). Low radiation level reduces vapor generation rate which significantly affects condenser performance.