Impact of grey water heat recovery on the electrical demand of domestic hot water heaters

Grey water heat exchangers (GWHE) are used to recuperate part of the energy contained in grey waters. The configuration used in this study recuperates part of the energy contained in the grey water from showers to pre-heat domestic hot water. Previous simulations studies have shown that this configuration can recuperate part of the energy that would otherwise be lost and allow the use of smaller electric domestic hot water (DHW) tanks. This paper focuses on the impact that GWHE have on peak electrical demand from electric DHW tanks. Simulations are performed using TRNSYS with a standard DHW tank model and a special GWHE model. A total of ten different yearly water draw profiles are statistically generated at 1 minute intervals. This small time step is required in order to capture the transient effects in the GWHE. It is shown that the aggregated effect of these profiles corresponds to the electrical consumption measurements performed on 600 residential electric DHW tanks. Simulation results show that GWHE have an impact on the peak electrical demand with reductions of 119.4 Watts (10.4% reduction) at 8:00 and 184.0 Watts (21.5% reduction) at 22:00. On an annual basis, the energy required for DHW heating is 4501 and 5299 kW-hr with and without a GWHE, respectively.