household monitoring
6.3 energy use estImAtIon
6.4.1 policy considerations for rainwater system energy use
Policy and legislation can play a significant role in the uptake and set-up of rainwater systems and subsequently on associated impacts such as energy needs. In Australia, severe drought and increase in demand on water resources due to population growth resulted in the introduction of demand management, water use efficiency and the diversification of water sources through policy and legislation (Tjandraatmadja et al. 2012). Across Australia, education campaigns, financial incentives and/or mandatory water demand reduction targets have driven the uptake of water efficient appliances and rainwater tanks (Tjandraatmadja et al. 2013).
Because adoption of rainwater tanks has focused on reducing mains water consumption, little consideration has been given to energy by industry, government or rainwater users. In addition, the awareness and expectations of tank owners regarding the operation of rainwater systems varies markedly in urban settings compared to rural settings where tanks are the sole source of water supply (Gardiner et al. 2008; Gardiner, 2009).
Overall, there is a lack of benchmarks and policy in Australia and around the world on energy efficiency for rainwater pumps and there is limited awareness and guidance to tank owners on how to minimise their energy usage or how to compare pump performance based on energy efficiency (Tjandraatmadja et al.
2012; Hauber-Davidson & Shortt, 2011). Typically, home dwellers rely on the knowledge of rainwater system and pump distributors and installers for system selection (Tjandraatmadja et al. 2012).
At the same time, the design of household pumps has not evolved at the same pace as water efficient appliances and demand management approaches. Whilst water requirements for end uses have decreased over time, the pumps sold for rainwater pumping are designed to achieve optimal performance at high flow rates, which is at odds with the low flow rates needed for most internal end uses as discussed in section 6.3.4.
The uptake of rainwater tanks in urban areas as a supplementary water source resulted in a large variety of pumps and system set-ups with large variance in the energy performance and many of the systems currently installed in dwellings may be oversized for their service requirements.
6.5 conclusIons
There is a need for a holistic perspective to assess the overall performance of rain supply systems, including cost and environmental implications such as energy consumption. Traditionally, rain tanks have been introduced as a water saving measure, but with little concern about optimising energy requirements until tens of thousands of systems had been installed (in Australia). Reducing the energy associated with rainwater supply will lead society closer to sustainable practices.
The current experience shows that:
• In assessing the energy efficiency of a rainwater supply system, it is important to consider not only the pump but the whole rainwater supply system including the intended end uses in a dwelling;
• Pumps are designed so that high flow rates correspond with lower specific energies. However the typical residential end uses often require low flow rates, causing pumps to operate well below their best efficiency point;
• Pump size, (i.e., kW), has a dominant effect on specific energy. Undersizing a pump can risk providing a poor level of service. However, more often, pumps in rainwater systems are oversized, leading to unnecessary energy consumption;
• Total energy use per day for a dwelling can be calculated for the various appliances connected to rainwater. Whilst the literature shows large variability in the energy associated with rainwater pumping, the median specific energy across Australian studies ranges from 1.4 to 1.8 kWh per kL.
Whilst this is in the upper range for mains potable water supply in Australian capital cities (0.06–
1.84 kWh/kL), it is much lower than the energy required for recycled water using reverse osmosis (2.8 kWh/kL) and desalinated sea water (3.5 kWh/kL);
• The energy use from internally plumbed rainwater systems represents only a small fraction of total household energy use (about 2% of average household use) (Gurung et al. 2012). However the overall energy and its variability could be reduced further. A number of simple measures can assist in reducing the energy requirements;
• Header tanks have the potential to significantly reduce specific energy. Laboratory studies in a simulated dwelling identified energy savings of 58% to 79% could be achieved by a 300 L header tank. However, their installation height in a dwelling needs to ensure that the hydrostatic water pressure will be sufficient to operate the solenoids on most domestic appliances. Alternatively the design of appliances could be modified to work at low water pressures. Header tanks do not seem to be viable in single story dwellings under current conditions;
• Pressure vessels reduce specific energy values by reducing the number of start-ups but more importantly because of the high flow rates on their refill cycle. In-situ tests undertaken at two households with pressure vessels reduced the overall energy intensity of rainwater supplied by 30–36% and lab studies have shown that energy savings will be proportional to the pump, vessel size and end uses in a dwelling, for example, the specific energy for filling a top loading washing machine using a 0.55 kWh pump is 1.47 kWh/kL, however this can be reduced by 7% and 50% respectively by the addition of vessels of 40 and 80 L capacity. This will further require life cycle cost analysis for comparison.
Overall, better education can reduce the installation of oversized pumps. It is important to advance the awareness and education among manufacturers, the plumbing industry, consumers and government on the energy associated with rainwater pumping systems. Such measures could lead to significant improvements in the design, and the overall efficiency and sustainability of rainwater systems and their components.
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