SPITZENBELASTUNGSAUSGLEICH? PRESSLUFTSPEICHERUNG!

One can store a fuel indefinitely, and burn it when needed. Electric energy is more difficult to store; with certain exceptions, it must be consumed as soon as it is generated. That fact has always been a double headache - technical and financial - for electric utilities. Most of the investment in high capacity to meet the peak demand (usually in the afternoon) is idle during the rest of the day and, especially, the night. How can one produce extra power during the night and store it for use during the peak hours of the following afternoon?

At present, the only proven way is pumped storage: Water is pumped into higher reservoirs during the night, to be released through hydroelectric turbogenerators in the peak hour. The biggest US system of this type is run by Consumers Power Co. and Detroit Edison in Ludington, Michigan. But pumped storage has an overall efficiency of only 50%, and requires large land areas for the water storage. And most disastrous, Their Environmental Majesties don't like it. It is not radioactive, it does not pollute, and it provides recreational lakes - but who needs a reason? Con Ed's proposed 2,000 MW pumped storage project near Cornwall, N.Y., has been tied up in the courts by the environmentalists for 12 years. And whom does the Sierra Club sue when the power runs out during the peak hours of hot summer days? Con-Ed, naturally.

Most utilities, however, simply use stand-by equipment to tide them over the peak hour. Most often, this is done with gas turbines, which can be quickly put into and out of service. Basically, a gas turbine is the type of engine used by jet planes: Hot gases are produced in a combustion chamber, compressed, and allowed to expand in flowing through the blades of a turbine. But much of the turbine's energy is lost in compression (the compressor is on the same shaft), and the efficiency of gas turbine generators is low - not much more than 20%.

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Many exotic forms of storing electric energy, from superflywheels to large currents flowing in superconducting loops, have been suggested (for a survey, see Science, 17 and 24 May, 1974). One of these is the idea of pumping air into a cavity during the night, and releasing it through a gas turbine during the peak hour.

And that is just what Nordwestdeutsche Kraftwerke (NW German Power Plants) are going to do. The utility has invested $40 million in a pumped air plant near Bremen. Two salt caverns are to be enlarged to hold close to 300 MWh's worth of compressed air; construction is to begin in May, and the plant is scheduled to come on line in mid-1977. The air will be released at a pressure of about 1,000 psi into a combustion chamber, where it is mixed with natural gas and ignited to pass through a gas turbine in the usual way. The substantial difference is that the energy needed to compress the gas is supplied during the night, not, as in present gas turbines, during the peak hour.

It will take about 2 hours for the pressure in the caverns to drop to the point (between 700 and 800 psi) where it is insufficient to drive the turbine.

Apart from the three times higher efficiency (compared to a conventional gas turbine), there are some fat fringe benefits: Capital costs per kW are 13% lower, and operating costs will be cut by 40%.

In the US, the method is still in the study stage. Unlike pumped hydro-storage, pumped air storage (underground) does not spoil anybody's view from the window. Ah, but will it not deny some rats and worms a living?

[The two monsters in the heading are German for peak load shaving and compressed air storage.]