COMPRESSED AIR UNDERGROUND
On farms near Huntorf, Germany, about 100 miles southwest of Hamburg, a nondescript industrial plant makes an unusual feature: when the electricity demand in the local network is low, the plant uses the excess energy to compress air and pump it into two caves of salt which is combined volume of 300,000 cubic meters. Then in times of high demand, leaving the compressed air is expanded through turbines located on the surface to regenerate electricity.
Huntorf Central, in operation since 1978, can supply nearly 300 megawatts of reserve power for three hours and starts operating around 100 times a year. But it can not be said to have had precisely a legion of imitators. In 1991 began operating a plant similar but somewhat smaller, in McIntosh, Alabama, and in 2002 began planning the construction of another similar system in Iowa right now is about to buy the land for test excavations .
The problem is that these energy storage compressed air (CAES) are considerably more complex in practice than in theory. When compressed, the gas heats up, limiting the amount of air that can be pumped underground without getting too hot to be safely stored. Moreover, the longer you let the hot air in one place, more heat, which is an important part of the input energy-dispersed through the walls of the cave. And when again released, the air expands it cools. In Huntorf facilities and McIntosh, in fact, the released air passes through a standard gas turbine to increase its efficiency, so the overall effect of air compression system is to raise the efficiency of a central more or less conventional fueled by natural gas.
In the short term, this type of hybrid system is very reasonable, "said Haresh Kamath, a researcher at the Electric Power Research Institute (EPRI) in Palo Alto, California, especially as more and more electricity is produced from renewable energy that can recharge the system night. However, if we think of the future, EPRI and other agencies are considering improvements to turn in a CAES energy storage system with no real need for fossil fuels. A system "advanced adiabatic" of that kind would capture and stock the heat of compression and would use it to reheat the air after release, which would trigger the turbine directly without fuel. Metal smelters and blast furnaces have spent years capturing the waste heat in piles of refractory bricks or similar materials, said Christoph Jakiel researcher MAN Turbo in Oberhausen, Germany. Applying this technique to the compressed air storage should be simple.
Calculate that the efficiency of such a system would be almost 80% which is comparable to the systems of hydroelectric pumped storage. The total cost of construction and operation would also be about the same. It should not be difficult to find suitable locations almost everywhere in the world, adds Jakiel. The salt caves are not uncommon and, once constructed, the project of the Iowa Stored Energy Park compressed air pumped to an aquifer.