Influence of required tank water temperature on the energy performance and water withdrawal potential of a solar water heating system equipped with a heat pipe evacuated tube collector

Abstract Hot water is a key demand of many industrial and domestic heating systems. This demand is, however, variable, both in terms of when hot water is needed, and the water temperature required. The present work examines the use of solar energy as an alternative means of producing hot water. A solar water heating system (SWH) with a heat pipe evacuated tube collector (ETC) was designed, and the effect of the required tank water temperature (rTWT) on the energy performance of the system examined. The maximum quantity of withdrawable hot water was also determined. The results show that, as rTWT increases, the net energy that can be stored by the system falls, with differences of over 1000 W h m−2 d−1 between rTWTs of 40 °C and 80 °C at a solar radiation input of 8000 W h m−2 d−1 (system efficiency range 56–73%). This reduction is a consequence of the decreasing collector efficiency and increasing energy losses in the circuit’s tubing with increasing rTWT. The higher the rTWT, the more solar radiation is required for the first discharge of hot water to take place (6500 W h m−2 d−1 for 80 °C). In addition, the time between discharges increases, and therefore the number of discharges possible over the day decreases. As rTWT increases, the amount of hot water discharged falls, the consequence of falling collector efficiency and the greater energy content of the water, etc. This reduction fits a power curve for which R2 = 0.99: over 300 L m−2 d−1 are produced for an rTWT of 40 °C, but just 20 L m−2 d−1 for one of 80 °C (global efficiency 62% and 21% respectively). For an rTWT of 80 °C, an important percentage of the incoming energy would accumulate in the water tank as hot water, but without the required temperature being reached. Auxiliary systems would be required to profit from this energy, which should be used as soon as possible; certainly, nocturnal heat losses from the tank would considerably reduce the amount available if this water were not used until the next day. The viability of this type of installation as a sole provider of hot water is therefore reduced drastically as the rTWT increases.