Pumped hydro: Storage solution for a renewable energy future

Of the various forms of renewable energy storage, pumped-storage hydropower is the most cost effective. Compressed air storage is another technology that is generally about 10 times smaller in capacity than hydro, much more expensive and requires large sealed underground caverns, which are rare. Although battery systems are, at most, 1 per cent of the capacity of typical pumped hydro and an order of magnitude more expensive, many dispersed batteries in residences, commercial premises and electric vehicles may also make a viable contribution to energy storage in future.

The main advantages of pumped hydro are relatively low levelised cost of energy (LCOE), large capacity (can be more than 1GW) and almost instantaneous dispatchability (response); the 1.7GW Dinorwig plant in Wales can ramp up from zero to maximum power in less than 60 seconds.

Hydro energy is generated by two-way turbines which can act as pumps and generators and is calculated by the formula: E = mghe, where m = mass of water, g = force of gravity and h = head (height), e = efficiency factor

Efficiency factors are applied twice – when water is pumped up and when it flows back, giving a round-trip efficiency of approximately 76 per cent, made up of pipe friction loss (6 per cent) and turbine efficiency loss (18 per cent). Pipe friction increases as diameter is reduced and distance increased. To keep friction losses low, short large-diameter pipes are required.

A recent Western Australian study into 100 per cent renewable energy for the SWIS electricity grid proposes solar and wind-powered scenarios with ‘over-built’ 10,800MW capacity to supply 5,500MW peak demand. Short-term backup is provided by pumped-storage hydro and molten salt storage. Longer duration backup is by biomass gasification and biomass co-firing of molten salt storage in concentrated solar thermal plants.

Stand-alone hydropower is not possible in WA and SA because flow is limited. There is limited potential for pump-back hydro on the existing small water supply dams. Pumped ocean storage hydro holds most potential for large-scale dispatchable power. Sites need to be within 2 km of the coast, >100 m in elevation and >100 ha in area, many of which have high conservation values. Nevertheless there a limited number of suitable sites within 100km north of Geraldton and east of Albany in WA, and the Fleurieu and Eyre Peninsulas and Port Augusta in SA.

A cliff-top salt water storage in WA would be located on land of 100-140m elevation about 1km from the shoreline. To provide 1500MW of capacity, 15-20 headrace tunnels or pipes about 5m in diameter feed hydro turbines, with intake/discharge into the ocean. To minimise environmental impacts and maximise cost effectiveness, two 200ha clay/membrane sealed cliff-top ponds with 120-140m head would be optimal; there are several sites fitting these criteria. Each power station would provide 700-800MW of fast response backup power for up to six hours and would stabilise the output of future wind and wave generators in the vicinity. Such as system would cost less than $5 billion.

Increasing the pond area fourfold would give a full day’s energy supply but would have to be weighed up against the environmental and cost impacts of the larger ponds. To put size in perspective, some natural coastal inlets on the SW coast are >2000ha.

From the tables below, we estimated the capital cost of pumped ocean storage in WA to be $2500/kW capacity, assuming a 10 per cent discount rate over 40 years, 7 per cent additional cost for operating and maintaining seawater systems and a life of 80 years. Energy cost would be about 8.6c/kWh (from IPCC table below). Given the excess wind energy stored would otherwise be wasted, it would cost only a few cents/kWh and it should be possible to supply low cost dispatchable / ‘peaking’ hydro energy at an LCOE of less than 14c/kWh. Currently the only commercial pumped ocean storage hydro power plant is a 30MW plant in Okinawa, Japan. A 480MW plant is planned for Glinsk, Ireland and another, 300MW in Hawaii.

It is regrettable that Australian state governments, faced with the huge dual problems of energy insecurity/high gas prices and global warming have not yet produced in-depth costed reports identifying optimal renewable energy electricity systems, including the transmission lines and smart distribution grids required to enable them. SEN hopes that its report will give them a ‘nudge along’ to do so.

Ben Rose is a member of Sustainable Energy Now and co-authored the report ‘100% Renewable Energy
on the SWIS Grid 2029’