More and more renewable energy sources are being plugged into Australia’s electricity grids. South Australia, for example, will get 40% of its electricity from wind and solar once the Snowtown wind farm is completed later this year.

But if renewable energy is ultimately to dominate the market, we will need ways to store the energy so we can use it round the clock. The good news is that it is easy to store energy. All you need is two small reservoirs – one high, one low – and a way to pump water between them.

This technique, called “off-river pumped hydro energy storage”, can potentially provide the energy storage that Australia needs to embrace renewables fully. It’s cheap, too.

How pumped hydro works

When there is excess electricity, water is pumped through a pipe or tunnel, to the upper reservoir. The energy is later recovered by letting the water flow back down again, through a turbine that converts it back into electricity. Efficiencies of 90% in each direction are possible.

Pumped hydro is by far the most widely used form of energy storage, representing 99% of the total. Worldwide, pumped hydro storage can deliver about 150 gigawatts, mostly integrated with hydroelectric power stations on rivers.

In an “off-river” system, the same water circulates in a closed loop between the upper and lower reservoirs, eliminating the need for the facility to be built on a river. The amount of energy stored is proportional both to the elevation difference between the upper and lower reservoirs (typically between 100 and 1000 m), and to the volume of water stored in the upper reservoir.

Electricity storage systems need to be able to deliver instant power output for periods of a few hours. This covers short-term fluctuations in wind and solar outputs, peaks in consumer demand (such as very hot summer afternoons), and unplanned outages of generation and transmission infrastructure. Using stored energy also helps to keep power lines from wind and solar facilities in use for more of the time.

Of the available electricity storage options, such as batteries and flywheels, pumped hydro is by far the cheapest. It has no standby losses while the water waits in the reservoir, and can reach full power in 30 seconds.

Time to go off-river

There is little opportunity for Australia to develop on-river hydroelectric power, because of environmental and other constraints. But, there are vast opportunities for short-term off-river energy storage. A typical site would comprise a pair of small reservoirs connected by a pipe through which water would be cycled daily, together with a pump and turbine, powerhouse and power lines.

Australia has thousands of excellent potential sites in hilly areas outside conservation reserves, with typical elevation differences of 750 m. They don’t need to be near a wind or solar farm.

Off-river electricity storage has several advantages over typical on-river facilities:

There are vastly more potential sites

Sites can be selected that do not clash with environmental and other values

The upper reservoir can be placed on top of a hill rather than in a valley, allowing the elevation difference to be maximised

No provision needs to be made for floods (typically a major cost).

A system comprising twin 10-hectare reservoirs, each 30 m deep, with a 750 m elevation difference, can deliver about 1,000 megawatts for five hours.

Between 20 and 40 of these systems would be enough to stabilise a 100% renewable Australian electricity system.

How much does it cost?

As the reservoirs are tiny (just a few hectares) compared with typical hydro reservoirs, they are a minor component of the cost. Most of the cost is in the power components (pipes, pumps, turbines, transformers and transmission). Initial estimates suggest that the cost of an off-river system at a good site is around A$1,000 per kilowatt of installed capacity.

Here is a hypothetical case study. A 200 megawatt solar power facility delivers a maximum of half of its power output to the grid in real time, and stores the rest for the evening. Now, instead of peaking at the sunniest time of day, the solar power output extends from 8am to 10pm (depending on season and cloud cover), with a maximum power output to the grid and the pump each being 90 megawatts (after allowing for losses). The reservoir can be recharged at night using wind energy to cover the morning demand peak.

The stand-alone costs of the solar power system and the short-term hydro storage system are A$2,000 and A$1,000 per kilowatt, respectively. After accounting for storage losses balanced by savings from sharing of the transformer and transmission costs between the two systems, and the fact that the hydro storage rating is half that of the PV system, that puts the total system cost at about A$2500 per kilowatt.

In other words, using pumped hydro storage to smooth out the peaks in output from a solar power station only adds an extra 25% to the cost. That’s much cheaper than using batteries.

Location, location, location

Spend some time with a map or Google Earth and you can spot dozens of excellent potential sites, in hilly farmland or along existing powerline routes. Australia has thousands of candidate sites throughout most inhabited parts of the country.

For example, the Tumut 3 hydroelectric station has Australia’s largest pumped hydro storage capacity (1500 megawatts), an elevation difference of 151 m, and a substantial lake that must cope with major floods. But a small off-river system could be built nearby, comprising twin 13-hectare reservoirs with an altitude difference of 700 m, connected by a 5 km pipe traversing a powerline route. This system would store enough water to deliver 1,500 megawatts for three hours, and would cost much less.

* This article was co-authored by Roger Fulton from Jacobs/SKM, who has worked in the hydroelectric industry since 1975 as an engineer and project manager.