Water Journal : Water Journal May 2011
demand management 72 MAY 2011 water technical features Abstract This paper presents the results of a detailed study into the energy consumption of domestic rainwater tank systems. The study included eight different types and brands of common rainwater supply pumps combined with seven different types and brands of pump controllers and rain-to-mains switches. Detailed energy and water use logging provided a clear understanding of the four underlying factors that combined to make up the total system energy consumption. The study presents several recommendations. These include applying holistic systems thinking to rainwater systems (e.g. fast-filling toilet cisterns, header tanks and washing machines that use fewer starts/ stops per cycle); maximising the energy efficiency of pumps across end uses, and introducing an "Energymark" type rating system akin to the Watermark to raise awareness of the energy efficiency issue among homeowners. Introduction There is a large amount of public interest in rainwater harvesting. Water retailers such as South East Water in Victoria have responded to this popularity with incentive programs to support and promote rainwater harvesting as an alternative to traditional water supplies. Adoption of domestic rainwater harvesting is generally considered to be more energy efficient than alternative water sources, such as large desalination plants. Recently, however, studies examining the energy to deliver rainwater from the tank to the end use have been conducted. These studies have shown a surprisingly high energy use. They include a study (Hauber- Davidson et al., 2010) for South East Water involving 30 houses. Similar studies have been conducted by the Institute for Sustainable Futures (Retamal et al., 2009) and by Sydney Water. These studies yielded widely varying specific energy requirements for domestic rainwater harvesting systems, from under 1kWh to over 10kWh to supply 1kL of rainwater. These figures are far higher than theoretically calculated values of well below 0.5kWh/kL. Indeed, some of the higher figures exceed the energy need for large scale desalination at 3.5kWh/kL--4.5kWh/kL. Australian Bureau of Statistics data states that 21% of the approximately 7,900,000 Australian households are equipped with a rainwater tank. From this, it can be extrapolated that there may be 1.6 million rainwater tanks with the potential to supply some 50GL/a, or about 10% of the planned capacity of all Australia's seawater desalination plants. Their energy consumption would be in the vicinity of 100GWh/yr or 2% of a large 1GW coal-fired power station. Associated carbon emissions are 100,000t.CO2-eq. Efficiency improvements across the board of just 20%, such as those readily identified in this study, could lead to energy savings of 20GWh/yr or emissions reductions of 20,000t. CO2-eq/year -- the equivalent of taking 4,000 cars off the road. Study Aims The broad objectives of this study were to investigate and understand the specific energy consumption of a range of domestic rainwater harvesting components and systems over a range of end uses, and then draw from this a number of specific recommendations regarding how to maximise the energy efficiency of domestic rainwater harvesting systems. It was performed by Water Conservation Group Pty Ltd on behalf of South East Water, with co-funding from Davey Water Products P/L and Tankworks Australia. Methodology The energy use of a rainwater harvesting is due to two basic components: 1. The pump that transfers water from the rainwater tank to the desired end uses; and 2. The pump controller and switch. It determines when the rainwater pump starts and stops and when it switches over to supply mains water instead of rainwater -- for example, when the tank is empty or in case of power or pump failure. Figure 1 shows how these components are typically configured in domestic rainwater harvesting systems. In total, eight rainwater pumps and seven control/switch devices were included in the study. The matrix in Table 1 (overleaf) shows the combinations tested. The approach was to monitor the energy consumed by the rainwater pump and the controller/switch device separately. Energy consumption was recorded for the pumping of water under a range of different scenarios as well as the energy use caused by the controller/ switch and its control logic. The tests were undertaken in a residential house in Sydney. The house was vacated during the testing period so that each test could be performed in isolation from any other water use. A standard suite of tests consisting of simple flow tests (both for a variety of flow rates and run times), switch standby power tests, and toilet flushing tests were conducted on all pumps. In addition, a number of tests were run on a limited number of systems to test the specific effects of each end use. These consisted of tests on washing machine use, indoor and outdoor garden taps (both upstairs and downstairs), shower use, and a variety of toilets with different flushing mechanisms. G Hauber-Davidson, J Shortt Why supplying rainwater uses more energy than it should ENERGY CONSUMPTION OF DOMESTIC RAINWATER TANKS Figure 1: Typical process flow diagram for a rainwater harvesting system.
Water Journal April 2011
Water Journal July 2011