Now this is interesting! IF this system works as advertised, this could prove to be a very useful energy storage mechanism.
Isentopic claims its gravel-based battery would be able to store equivalent amounts of energy but use less space and be cheaper to set up. Its system consists of two silos filled with a pulverised rock such as gravel. Electricity would be used to heat and pressurise argon gas that is then fed into one of the silos. By the time the gas leaves the chamber, it has cooled to ambient temperature but the gravel itself is heated to 500C.
After leaving the silo, the argon is then fed into the second silo, where it expands back to normal atmospheric pressure. This process acts like a giant refrigerator, causing the gas (and rock) temperature inside the second chamber to drop to -160C. The electrical energy generated originally by the wind turbines originally is stored as a temperature difference between the two rock-filled silos. To release the energy, the cycle is reversed, and as the energy passes from hot to cold it powers a generator that makes electricity.
Isentropic claims a round-trip energy efficiency of up to 80% and, because gravel is cheap, the cost of a system per kilowatt-hour of storage would be between $10 and $55.
Howes says that the energy in the hot silo (which is insulated) can easily be stored for extended periods of time – by his calculations, a silo that stood 50m tall and was 50m in diameter would lose only half of its energy through its walls if left alone for three years.
Professor Barry Brooks reviews it saying:
eclipsenow, I hope it works out – sounds interesting in principle and the energy density doesn’t look too bad. From the figures given, 540 cubic metres stores 16 MWh of energy, or 30 kWh per cubic metre. To store one day of output of a 1 GW power station would require 1,500 of those 7m tall, 7 m diameter twin silos. Let say you stacked them in a 15 x 15 m square (for each silo), that would require about 1 square km of silos (ignoring interconnections and local generators etc.) Could be useful for storing cheap baseload nuclear-generated electricity for peaking purposes.
$/kW is generating capacity (peak, in the case of wind), whereas $/kWh is energy storage. To store 1 day of energy from a 1 GW power plant requires 24 million kWh. At $55 per unit, that’s a storage cost of $1.3 billion for 24 hours of storage. Not ridiculously expensive by any means, but neither is it cheap as chips as you might have thought it to be, given the implicit comparison you made above. I’ve used the upper price figure cited, since we’ve not even seen a demonstration unit yet.
Regarding rise in steel and concrete prices, these will effect the cost of renewables at least 10 times more than nuclear power, see: http://bravenewclimate.com/2009/12/06/tcase7/