Giant batteries made out of gravel could save green grid

Did I speak too soon? This is a nice concept that may work well. The efficiency is a potential red herring, since the input energy is "free" in the sense that once the plant is set up, wind just keeps on turning up. So the important figure is the final cost per unit energy. This is not the only storage option which might be scalable, but it looks potentially cheap. A new form of battery using molten metals and their salts was featured a week ago in New Scientist, then there are elctrolyte flow batteries and hydrogen cells. A hydrogen cell could generate and then re-use hydrogen in a closed cycle and have essentially unlimited capacity.

PopSci -  GreenPower  can be incredibly inconsistent. A new gravel-battery storage scheme could cheaply store excess power when the wind is strong to supplement wind turbines when the gusts die down.

Wind and solar are such promising technologies for the hydrocarbon-free energy sources of tomorrow, but intermittent, inconsistent output renders them unfeasible as anything other than secondary power sources. But UK firm Isentropic thinks it may have solved the problem as it pertains to wind power; all we need to stabilize the energy flow from turbines are giant batteries made out of gravel.

The battery consists of two large silos filled with crushed rock. Electricity generated by the turbine heats and pressurizes argon gas and feeds it into the first silo. The gravel is heated to more than 900 degrees as the hot, pressurized argon passes through, though by the time the argon leaves the chamber it has cooled to ambient temperature.

The process isn't a perfect closed energy loop, but Isentropic claims a complete trip through the cycle retains up to 80 percent of the original electricity. Even better, gravel is cheap; the cost per kilowatt-hour falls somewhere between $10 and $55, depending on the costs of other materials. Isentropic also claims the batteries are highly durable; according to the company's founder, a 164-foot tall silo with an equal diameter would retain half its energy even if left untouched for three years.

[Guardian, Isentropic]