Maximization of Solar Hot Water Production Using a Secondary Storage Tank

Abstract

The use of passive thermosiphon solar water heating (SWH) is a cost-effective renewable source of energy for hot water production in Mauritius, given the relatively high solar yield year round. However, unavailability of sufficiently hot water for early morning use has been reported in households, confirmed by a survey undertaken, which may be due to improperly sized tank and/or collectors. Backup gas or electric heating is normally installed to cater for this problem. This paper presents the research work performed to analyze the dynamics of hot water production, consisting mainly of monitoring the temperature of the water inside the tank of an actual SWH in operation with the objective of characterizing the factors affecting hot water production. The temperature of the hot water inside the storage tank was found to fluctuate during the day and with consumption of hot water, and the mixing of mains cold water with the hot water was deemed to be a major factor leading to the shortage of hot water the next morning. Based on the analysis, a passive thermosiphon SWH in use in a household was modified by adding a second hot water tank with associated solenoid-actuated valves modulated by a microcontroller to regulate flow between the two tanks and to the end use point according to an appropriate rule set. The results show that hot water production can be optimized in currently installed SWH systems with the proposed design without back-up heating systems, thus providing an effective retrofit solution to systems in use.