Effect of standing time after usage on the performance of storage-type domestic electrical water-heaters

Abstract

In many developing countries, it is the custom to switch storage-type domestic electrical water-heaters (EWHs) 'on' for a few hours just before their use. This makes it difficult to apply utility cyclic control on EWHs in these countries, since the water may cool down and there may be a demand for hot water during the shut-down period. An investigation was carried out on a 121 l EWH in order to study the cooling behaviour and the availability of hot water for different time intervals ( referred to as the standing times) between the initial use, defined as a typical shower period, and the consecutive use. It is observed that, as the standing time is increased, the initial temperature gradient in the storage tank decreases and the thermocline is lost completely after 24 h. However, as the second discharging process is started, the temperature gradient is formed again due to incoming cold fluid and its mixing with the hot water at the bottom of the tank. Furthermore, although the temperature of the hot water at the outlet port decreases as the standing time is increased, the volume of water discharged for the temperature to drop to 40 degrees C is approximately the same for each standing time. The more the second draw-off is delayed, the smaller the fraction of water discharged at constant temperature, ranging from 40 to 60% of the total storage volume between 0 to 24 h of standing time.