Water Journal : Water Journal May 2011
demand management technical features 76 MAY 2011 water should be replaced on a regular basis to minimise efficiency losses through them. Relative importance of components The relative importance of each component and the increasing importance of controller/switch standby energy use are shown in Figures 12 and 13. Figure 12 shows the energy use breakdown for a single full (6L) toilet flush (Test system: Davey HP45-05 with Rainbank). In contrast to the single-event analysis in Figure 12, Figure 13 provides a breakdown of the energy consumption for an entire day. It is assumed that the system is connected to a toilet only, which is flushed 15 times. This chart demonstrates the significance of switch standby energy consumption. From insignificant, it increases to 29%. While the pump operates for a few minutes per day only, standby power draw is 24/7. Long-term system comparison In addition to the short-term targeted testing that is the subject of the majority of this report, the long-term performance of a domestic rainwater harvesting system was observed for two different pumps and switch/controller setups. Observations were made for normal real life conditions in the test residence. The two systems were: • An Onga SMH55 equipped with a WaterSwitch; • A Davey HP45-05 equipped with a Rainbank. The overall specific energies of these systems were recorded for a number of weeks for a similar usage pattern consisting of the following end uses: • Toilet flushing -- both slow and fast cistern-filling valves; • Washing machine use; • Hand basin/kitchen sink tap use -- cold and hot; • Showering (with both hot and cold water connected); • Garden irrigation/hosing/car washing. The diversity of these uses meant that there was a mix of high flow, high volume use (e.g. the washing machine) and low flow, low volume use (e.g. hand basin taps). The rainwater connection to the hot water system and to the cold water of hand wash basins meant that there were frequent short water uses. This is not a typical setup, although connecting rainwater to hot water does provide a cost-effective way of connecting a large demand to a rainwater harvesting system to a suitable end use. It was found that for this longer- term monitoring period, the Onga system averaged 2.4kWh/kL, while the Davey system averaged 1.9kWh/ kL. This provides a practical example of the findings contained in this report. In accordance with the conclusions described above, this is attributable to the following factors: • The superior pumping energy efficiency of the Davey pump used compared to the Onga; • The lower system shutdown energy consumption of the Davey Rainbank compared to the Onga WaterSwitch; • While the Davey Rainbank has a higher standby energy consumption than the Onga WaterSwitch, the high overall usage of rainwater at the residence makes this negligible in the overall calculations. Recommendations and Conclusions As a result of this study the following recommendations are made: • Apply holistic systems thinking (e.g. fast-filling toilet cisterns should be chosen for rainwater supplied systems, header tanks could be considered, and washing machines should be designed with fewer fill start/stops per cycle). • Minimise pumping energy consumption by using an energy-efficient pump and by transferring water at a flow rate as high as possible. • Minimise system shutdown energy consumption (and pump start-up energy consumption) by pumping water for long pump runs -- and by using a switch/controller which minimises the run on time after water transfer is complete. • System shutdown energy consumption and pump start-up energy consumption could be further improved by developing switches that can work with pressure vessels, allowing small volumes of water (1L--2L) to be supplied without a pump start. • Minimise controller/switch standby energy consumption by designing these devices with a low or no standby energy use. • Minimise the relative contribution of controller/switch standby energy consumption by supplying as much water uses as possible -- in a waterwise manner. • Where rainwater is not expected to run out, or where only small amounts of top-up water are expected to be used (<7kL/year), a trickle top-up system should be used in lieu of an automatic rain-to-mains switch as the additional pumping cost for top-up mains water is less than the standby energy consumption of the switch. • Accept that for the currently available systems, frequent triggering of water use reduces the rainwater energy efficiency dramatically, even though it is water efficient. In relation to connecting rainwater to hand wash basins, or the hot water system, this means that the home owner will have to make a careful assessment of trading off additional water savings achieved vs. a higher specific energy use. • The industry should react to the findings of this study and provide more energy-efficient systems meeting the demands of typical domestic rainwater harvesting systems, such as more efficient rainwater pumps and controller/switches with lower standby energy consumption. • An "Energymark" type rating system (akin to the Watermark approval) should be introduced to readily indicate to the home owner exactly how energy efficient a particular rainwater harvesting system is. Acknowledgements Bridget Wetherall, a Project Manager at South East Water, organised the project and its funding. The Authors Guenter Hauber-Davidson (email: firstname.lastname@example.org) is the Managing Director of WCG Pty Ltd, a company which specialises in consulting and operations for integrated water efficiency solutions. Julian Shortt is a Senior Water Savings Engineer at WCG. References Hauber-Davidson G, Shortt J & Wetherall B, 2010: Energy Consumption in Domestic Rainwater Harvesting. Report available on request from South East Water. Retamal M, Turner R & White S, 2009: Energy Implications of Household Rainwater Systems. Water Journal 36, No.8 (December).
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