Obviously there are times when the sun don’t shine and the wind don’t blow. Such times are called ‘interruptions of service’ for solar and wind energy generation. It limits their effectiveness as alternative energy sources. The solution is to store energy when the wind blows and the sun shines and release it during the non-productive periods. Trouble is, such storage is not easily accomplished, or more to the point, accomplished inexpensively. Huge batteries, the image of the Energizer bunny writ large, are far too expensive. However, here’s an idea developed by a team of engineers at Cambridge University (UK):

Two insulated silos are built (size depends on capacity desired) and filled with pulverized rock or gravel. As a wind turbine generates electricity during a windy period, the electricity is converted to heat. The heat is distributed through the gravel in one silo by argon gas so that the gravel reaches a temperature of about 500C. From there the now cooler argon gas is pumped into the second silo and allowed to expand, which sucks the heat out of that gravel to a temperature of about -160C.

To generate electricity from this stored energy, the gas reverses course and comes back as heat to turn turbines.

This approach, developed with the company Isentropic (UK), claims up to 80% efficiency and because the silos with gravel are relatively cheap, the cost per kilowatt hour of storage is roughly $10-$55.

Like so many innovations for alternative energy, this one is in need of some real-world testing. Isentropic plans to build a pilot plant with silos approximately 7 meters tall and 7 meters in diameter (20 x 20 ft). Support for the project is coming from Britain’s Technology Strategy Board.

David Bott, director of innovation programmes at the Technology Strategy Board, one of the sponsors of the 2010 Clean and Cool trade mission said: “Isentropic have done something very exciting, by revisiting scientific theory and coming up with a new technology that answers the need to match the generation of electricity with its use. For instance, the system could enable the more efficient use of wind power, by storing the energy generated by a turbine until it is needed. We need ways to store the energy we generate when we have a surplus, so that it can be used when we need extra and this innovative new system could provide the answer.”

[Source: The Guardian]

Indeed, it could provide an answer – with mostly natural materials too. We shall see.