IT WAS while hang-gliding that Jonathan Howes, a British aeronautical engineer, came up with an idea for a meteorologically powered engine to fly him between thermals, the columns of rising air which glider pilots use to gain altitude. It would use a thermal’s updraught to turn a small turbine which captured some of the thermal’s energy, for storage. Then, when in level flight, the pitch of the turbine blades would change and it would become a propeller driven by that stored energy.

A neat thought. But what was truly novel was that the captured energy would be stored not as electricity in a battery, but as heat—or, rather, heat and cold—in two small reservoirs. Mr Howes has, however, yet to put such a device on his hang-glider because he is too busy building a far bigger version as part of the search for a cheap and reliable way to store electricity on the grid, a breakthrough which would transform the use of renewable energy.

At the moment, grid managers match demand by generating just enough electricity at any given time. But power from renewable sources, particularly the wind and the sun, is intermittent. Nevertheless, as costs come down and efficiency increases, renewable energy is being used more widely. Solar power is already the cheapest form of electricity in most sunny climes, and in America the Department of Energy wants it to provide 27% of the country’s electricity by 2050, up from less than 1% today. But without efficient grid-scale storage, costly backup generation will be needed to keep the lights on. In many poor countries that means running a smoky diesel generator.

A number of technologies are being developed to store energy on the grid, such as flow batteries which can accumulate energy in liquids and discharge rapidly. Giant flywheels and supercapacitors are also being explored. But Mr Howes thinks his invention can beat them all.

His system is based on a heat pump—a device like an air-conditioner that transfers heat from one place to another. In his case, though, the device is reversible, and when the heat flows back it works like a heat engine, converting thermal energy to mechanical power like a car engine.

Isentropic, a company Mr Howes co-founded with two colleagues, James Macnaghten and Mark Wagner, has developed several prototypes of what the trio call pumped-heat electricity storage (PHES). The firm is now completing a demonstration unit with an output of 1.5 megawatts for further testing at its factory near Fareham, on Britain’s south coast.

The PHES system consists of two silos, each filled with gravel. The silos are connected top and bottom by a series of pipes filled with argon, an inert gas. When electricity is in surplus, it runs a motor that operates a compressor to squeeze the argon. Doing work to compress a gas heats it, and in this case the argon reaches 500°C. As it is pumped through the first silo it transfers its heat to the gravel. The cooler gas that emerges is then expanded, which lowers its temperature to -150°C. When the gas is pumped through the second silo it cools the gravel in it. The resulting difference between the silos can be used to generate electricity by reversing the process. Gas heated by the hot silo flows to the cold one, driving the cylinders of what had been the pump to turn what had been the motor as a generator.

There is always some loss in energy storage. But Isentropic says PHES is able to store and discharge electricity with a “round-trip” efficiency of 72-80%. This compares with 74% for pumped-hydro, the way 99% of grid-scale storage happens at the moment. Pumped-hydro uses surplus electricity to pump water up to a mountain lake and let it flow down again to drive a hydroelectric plant when required. It is efficient, but you need a mountain nearby to make it work.

The efficiency of PHES comes from the design of the pump/engine, which employs a number of advanced hydrodynamic principles, including a novel sliding valve built into the head of the pistons. It is low-cost, compact and has a long service life, says Mr Wagner.

Isentropic calculates that the “levelised” cost of storage of a PHES system, based on its capital cost amortised over 25 years and its running costs, is $50 a megawatt hour. The price of pumped-hydro varies a lot, as installations differ, but is typically around $65 a megawatt hour.

Though Mr Howes has no plans yet to scale the system down for his hang-glider, he does think a smaller version could be made for domestic use. This would let householders store surplus electricity from rooftop solar panels or a local wind turbine, offering some people the prospect of energy independence.