Bottling sunshine

Roger Highfield outlines possible solutions to the problem of storing solar power

The World Today Published 12 February 2016 Updated 25 January 2019 2 minute READ

To create one of the biggest solar farms of its kind, 300 hundred workers are currently banging in posts and laying miles of cables on a former wartime airfield at Wroughton, the storage hub of the four museums in the Science Museum Group. When finished in March, the array near Swindon will generate close to 62 gigawatts of power each year, pumping more than four times the electricity the museums use back into the national grid.

Of course, unlike fossil fuels or nuclear, electricity generated this way is intermittent ‘so we are looking at the possibility of storing energy produced when the sun is shining and released to the national grid when needed’, said Matt Moore, the head of sustainable development with the Science Museum Group.

Energy storage methods come in many forms, each with different limitations. Some work by converting electricity to kinetic, or potential, energy and then discharging that energy when required. But big devices – such as flywheels or pumped storage hydroelectricity where water is pumped up to reservoirs at off-peak periods – do not store much energy per unit volume and are limited when it comes to where they can be sited.

Electrochemical methods, on the other hand, can efficiently store electricity in chemical form. They are like household batteries, only bigger, and use various materials such as aluminium, carbon, ceramics and salt. Since they were commercialized by Sony in the early 1990s, lithium-ion batteries have become dominant.

Lithium is light and offers high ‘energy density’, that is the energy that can be stored in a given volume, typically twice that of nickel-cadmium, making it ideal for mobile uses. And lithium holds charge well compared with lead, zinc or nickel-cadmium. More efficient variants are evolving, such as a lithium-air battery under development by teams worldwide.

To make enough lithium batteries to power 500,000 cars by 2020, Elon Musk, the billionaire founder of the electric car-maker Tesla, is building a $5 billion ‘Gigafactory’ in Nevada with Panasonic. Some will also be offered to households, for whom Tesla has developed the 7 kWh ‘Powerwall’.

There are drawbacks. Lithium is highly reactive so overcharging and manufacturing flaws can cause the battery to heat up and sometimes catch fire. As alternatives, EOS Energy Storage, a New York start-up, has developed container-sized, zinc-based batteries for utilities, while Aquion Energy, a spin-out from Carnegie Mellon University, in Pittsburgh, is marketing a salt-water battery.

Flow batteries offer one alternative. External tanks contain liquid electrolytes that store energy and, when there is demand, electrolytes are pumped from the tanks into a stack where electricity is produced by an electrochemical reaction. To cut costs, researchers want to use inexpensive organic molecules.

There are many more ways to solve the intermittency problem. Summer heat gathered in rooftop solar collectors can be stashed in rocks and used for heating in winter. Excess or low-cost electricity could be used to make ice, which would be used later for cooling when the price of electricity is high. During low-demand hours, excess electricity could be used to create hydrogen, which could be stored in fuel cells.

Molten salt batteries are among the more exotic options. Gemasolar, in Fuentes de Andalucía, near Seville, was the first commercial-scale plant to use this approach. More than 2,600 mirrors over 185 hectares reflect sunlight on to the top of a tower, where potassium and sodium nitrate salts are heated to 565C and then pass through a heat exchanger to drive turbines. When there is excess heat, hot salt is stored in a tank. This battery allows electricity to be generated for up to 15 hours in darkness.

Why use storage at all? Selling electricity from a solar farm in Texas to a household in a northern state would be cost-effective – while cutting emissions – if the US was plugged into a single power grid, rather than a patchwork of regional grids, according to modelling by the National Oceanic and Atmospheric Administration and University of Colorado, Boulder.

‘The wind is always blowing somewhere, or the sun shining,’ says Alexander MacDonald, co-lead author. Central to the scheme would be the construction of a high-voltage, direct current transmission network (costing $200 billion), which can send electricity more efficiently over great distances. Similarly, he says, ‘an HVDC network for all of Europe would be far better than a piecemeal approach’.