Giant gravel batteries could make renewable energy more reliable

Wind and solar power are often criticised for being too intermittent, but Cambridge researchers could change that

Alok Jha
guardian.co.uk, Monday 26 April 2010 12.33 BST
Newly designed giant gravel batteries could be the solution to the on-off nature of wind turbines and solar panels. By storing energy when the wind stops blowing or the sun stops shining, it is hoped the new technology will boost to renewable energy and blunt a persistent criticism of the technology - that the power from it is intermittent.
Electricity cannot be stored easily, but a new technique may hold the answer, so that energy from renewables doesn't switch off when nature stops playing ball. A team of engineers from Cambridge think they have a potential solution: a giant battery that can store energy using gravel.
"If you bolt this to a wind farm, you could store the intermittent and relatively erratic energy and give it back in a reliable and controlled manner," says Jonathan Howe, founder of Isentropic and previously an engineer at the Civil Aviation Authority.
The Labour government committed to cutting the country's carbon emissions by 34% by 2020 and 80% by 2050, both relative to 1990 levels. To achieve this, ministers outlined plans to build thousands of wind turbines by 2020. 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.
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.
Howe 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.
To demonstrate how much less infrastructure his system requires, Howe uses the example of the Bath County Pumped Storage hydro-electric dam in Virginia, US. This is the biggest energy-storage system in the world, with two reservoirs covering 820 surface acres can store up to 30 GWh storage capacity. An Isentropic gravel battery of the same capacity would occupy 1/300th of the area, according to Howe.
John Loughhead, executive director of the UK Energy Research Centre, said that the novelty of the Isentropic system lay in using cheap materials as the heat store, thus making a normally expensive and mechanically complex process very simple. But he said demonstrators would need to be built to prove the idea actually functions. "The question is, does it work? From an engineering standpoint, the temperature differences they mention, +550C to -150C are initially credibility-stretching for a single-pass cycle, and the potential for gravel particles to pass through the engine and damage or clog the inevitable cooling and lubricating systems seems high."
Howe is in the process of designing a small pilot plant that could store 16MWh at full capacity - enough for the electrical needs of thousands of homes. That energy could be stored in two silos of gravel that are 7 metres tall and 7 metres in diameter. There is no reason why multiple units could not be connected together to store much more power, Howe says several gigawatt hours.
Howe says he is in talks with what he refers to as "a large utility company" to sponsor the construction of a full-storage demonstrator system, something around the 100 kilowatt scale.
Isentropic was selected recently by the government-sponsored Technology Strategy Board for a trade mission to meet Silicon Valley investors, one of around 20 of the Britain's most promising clean technology startup companies.
David Bott, director of innovation programmes at the Technology Strategy Board, one of the sponsors of the 2010 Clean and Cool trade mission said: "Isentropic have done something very exciting, by revisiting scientific theory and coming up with a new technology that answers the need to match the generation of electricity with its use. For instance, the system could enable the more efficient use of wind power, by storing the energy generated by a turbine until it is needed. We need ways to store the energy we generate when we have a surplus, so that it can be used when we need extra and this innovative new system could provide the answer."