Rainwater Tanks & Energy

Rainwater tanks can save a lot of water, but preliminary Australian research warns pumping water from them can be energy intensive.

We should ensure we don't create an energy problem while trying to solve a water problem, says Professor Stuart White of the Institute for Sustainable Futures at the University of Technology in Sydney.

"The message is that we need to be really careful about this because of the growth in the use of rainwater tanks," says White, co-author of a new report on energy and rainwater use.

"The message is not that rainwater tanks are a problem." He says Australia has the widest application of rainwater tanks in the world, especially in urban areas, encouraged by government rebates.

White says gravity is a useful way to deliver rainwater to taps, but it is not always feasible in urban areas because tanks need to be higher than the roof to provide sufficient pressure for showers. As a result most households use small pumps that switch on every time you turn the tap on, even if it's just briefly to clean your teeth or flush the toilet. "It means you have a pump running at full revs to do a tiny amount of work," says White.

In what is believed to be the first study of its kind, White and colleagues, monitored the rainwater consumption and associated energy consumption of 8 households in Sydney and Newcastle.

They calculated the "energy intensity" of household rainwater systems, which is the amount of energy used to get each kilolitre of water to the tap. They found in most cases pumping water from rainwater tanks is more energy intensive than getting it from mains water, although lower than getting it from desalination.

White says the standard suburban system rainwater tank with pump had an energy intensity of 1.5 kilowatt-hours per kilolitre. "The water that you get out of a tap would be less than 1 [kilowatt-hour per kilolitre]," he says. White says the task now is to work out how to reduce the energy intensity of rainwater systems. He says one option is to use pressure vessels, which can be fitted to a standard pump.

These store pressure and avoid using the pump when only a small amount of water is needed.

Another option is to use variable speed pumps, which match the amount of energy used to the amount of water needed.

Still another option are rainwater switches, which uses mains water for small usages, only turning the pump on for larger usages. White says, in general, the energy intensity of water is increasing due to the construction of desalination plants and new dams.

"$30 billion worth of water infrastructure over the next 10 years is being constructed and most of it is very energy intensive and will increase the amount of energy we use to supply water in towns and cities," he says.

The research was done jointly with the CSIRO under their Water for a Health Country Flagship program.

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