Control optimization of PV powered electric storage and heat pump water heaters

Domestic rooftop photovoltaic (PV) systems are typically installed without energy storage and power generated in excess of the building electric load must be exported to the grid or curtailed. In this paper the use of excess PV electricity for water heating is investigated, with the hot water storage tank acting as a low-cost thermal battery. A 3.6 kWp PV system installed on a domestic house with a 315 L storage hot water heater is analysed. It is shown that advanced control of the water heater is critical to make effective use of local intermittent PV power. Without appropriate controls the excess PV power can only reduce the grid energy consumption of an electric storage water heater by approximately 13%. However, with appropriate control, savings of purchased grid electricity above 80% can be achieved for electric storage water heaters. Similarly, using a heat pump, the consumption of purchased grid electricity for water heating can be reduced by close to 90% by making use of excess PV power. The use of excess PV electricity for water heating purposes can also significantly reduce export PV electricity, thereby also assisting the voltage and frequency stabilisation of the electrical distribution network.