Water-energy Nexus of Residential Rainwater Tank Systems at a Micro-component Level: Evidence from Australia

Abstract

Water demand in many parts of the world continues to rise with population growth. The combination of rainfall unreliability, limited water sources and increases in water demand have led to an increased focus on water demand management and source substitution. Installation of rain water tanks supplying water for toilet flushing, cold water for washing machines and external irrigation taps, has become a popular potable source substitution measure in Australia. While there has been a plethora of studies attempting to understand the potential water savings from rain tanks, there is little knowledge on the energy intensity implications of using pumps for these specific end uses. This paper methodology utilises high resolution smart water meters (0.014 L/pulse) and energy meters (0.1 Wh/pulse) with aligned 5s data collection intervals to establish the energy intensity of common pumping configurations for the four end use event categories supplied by the rain tank (i.e. clothes washer, half flush toilet, full flush toilet, and irrigation) for a sample of 19 households located in Gold Coast City, Australia for a period of 6 month. The paper findings indicated that half flush and full flush events had the highest energy intensity at 1.8 Wh/L and 1.6 Wh/L, respectively. Toilet flushing had high energy intensities mainly due to the short duration of these events and the flow rate of cistern filling being considerably lower than the optimal pumping flow rate of the fixed speed pumps. The average clothes washer energy intensity value was lower than for toilet flushing at 1.3 Wh/L. The energy intensity of clothes washing events are highly dependent on the washing machine type (e.g. front or top loader). Lastly, irrigation events had the lowest average energy intensity (i.e. 1.0 Wh/L). Irrigation event energy intensities were typically lower since they predominately had mode flow rates that operated within the pump optimal operating. This detailed study has implications for rainwater tank policy and design practices.