Making good use of gravel

Electricity cannot be stored easily, and that has been the hurdle of intermittent renewable energy. But a new technique that uses gravel and argon gas may hold the answer.

So far the only economically viable way of storing large amounts of energy is through pumped hydro – where excess electricity is used to pump water up a hill. The water is held back by a dam until the energy is needed, when it is released down the hill, turning turbines and generating electricity on the way. The problem is that this needs a water source nearby as also the expenses.

The company 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 pulverized rock such as gravel. Electricity would be used to heat and pressurize 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 minus160C. 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. The energy in the hot silo (which is insulated) can easily be stored for extended periods of time – as long as three years.

Sounds perfect? Goes to show how all it needs is some elementary thinking and some waste material to achieve big things.