WIND farms and solar-energy plants have the advantage that their fuel is free, but the disadvantage that the availability of that fuel may change from minute to minute. If they are to become the large-scale contributors to power generation that their boosters suggest, then cheap and reliable means of smoothing their output, by storing surpluses for use during times of scarcity, need to be developed.
At the moment, there is only one good way of saving surplus grid electricity, regardless of how it is generated. This is pumped storage. It requires two reservoirs at different elevations, linked by tunnels and pumps in order to create a head of water whose pressure, when released, can drive the pumps backward, to act as generating turbines.
Pumped storage is cheap to run, but needs convenient geography to build in the first place. Or, rather, it did. For a pair of alternatives to the two-reservoir model, both of which still exploit the power-generating potential of a head of water by pumping fluids around, are now being investigated. One is a year old this month. The other is about to start trials.
The one-year-old project is in Toronto, Canada—or, rather, just offshore, at the bottom of Lake Ontario. It was designed and built by Hydrostor, a company founded by Cameron Lewis, who developed the technology after working in the oil industry. The plant is operated by Toronto Hydro, a local power utility.
In this case the working fluid is air rather than water. The air is compressed on land and pumped through 2.5km of pipes to a station on the lake bed 55 metres below the surface, a head of water that generates a pressure five atmospheres above normal atmospheric pressure. Here, the air is stored in six spherical bags, known as accumulators, made of a proprietary material. Each accumulator has a capacity of 100 cubic metres.
Compressing air heats it, and the heat thus generated is also stored for later use. This is done by melting a material with a high heat capacity (exactly which, remains a trade secret—though paraffin wax is often used in similar, commercially available heat-storage devices). Then, when the time comes to generate electricity from the energy stored in the compressed gas, the process is simply put into reverse. The air is released into the pipes, travels back to the onshore plant, and its expansion there as it returns to normal pressure drives a turbine. Just as compressing air heats it, so expansion cools it. To stop the machinery freezing, therefore, the compressed air entering it is first warmed up using the stored heat from the original compression.
According to Hydrostor, the Ontario plant can regenerate 60-70% of the electricity put into it, and produces around 400kW of power. The firm now plans, in partnership with AECOM, an American engineering company, to build a 1.75MW plant in Goderich, Ontario, on the shores of Lake Huron. It has also signed an agreement with Aruba’s electricity provider, WEB Aruba, to build a plant to be connected to wind farms there.
The newcomer, which will begin operating on November 11th, is a system called StEnSea (“Storing Energy at Sea”). This is being developed by the Fraunhofer Institute for Wind Energy and Energy System Technology in Kassel, Germany. In its case the working fluid is water itself, but, like Hydrostor’s system, the pressure head is created by putting the storage vessels underwater—in this case, 100 metres down in Lake Constance, a depth that creates an excess pressure of ten atmospheres.
Balls of fire
Unlike Hydrostor’s system, StEnSea uses rigid pressure vessels, made of concrete, that have a volume of 12 cubic metres (see diagram). This gives them an energy-storage capacity at this depth of 3kWh each. When the system is charging up, the water in these vessels is pumped out of them into the surrounding lake. When it is generating, the water is let back in, turning turbines as it travels. StEnSea’s advantage over Hydrostor’s system is that no pipework is needed to connect the storage vessels to the land (though it does need cables, to carry generated power). Its disadvantage is that all the machinery is underwater, and thus harder to inspect and service.
The plant in Lake Constance is a pilot. If it works, the plan is to build a commercial version at sea. Jochen Bard, the project’s boss, has his eye on the Norwegian trench, which is over 600 metres deep. Combining that depth with spheres 1,000 times the volume of the pilot’s would create a system that stored 20MWh per sphere, and supplied 5MW of power.
Whether this could compete with conventional pumped storage remains to be seen. The Cruachan pumped-storage station in Scotland, for example, has a capacity of 7GWh. StEnSea would need 350 spheres in the Norwegian trench to match that. But both StEnSea and the Hydrostor system have the advantage over plants like Cruachan that you can start small and add extra units as needed—rather like wind and solar energy themselves.
