https://specials.auma.com/en/profox?utm_source=engineerlive&utm_medium=onlinebanner&utm_campaign=profox_2024

Stockpiling electrons: key to take-up of renewable energy?

Paul Boughton
Large-scale use of renewable energy may depend on the ability to store electricity. Manel Romeu Bellés reports

Electricity is slippery stuff: invisible, and almost impossible to store in quantity except by converting it to some other form of energy. Yet many experts believe that the widespread take-up of wind and other renewables will depend on our being able to store electricity effectively.

Timescales for energy storage in electric power systems range from seconds to days. Wind power shows random variations over almost any timescale, points out Claus Nygaard Rasmussen, who researches energy storage at the Institute of Energy Technology, Aalborg University, Denmark. This means that for power companies, the ability to smooth out power dips of just a few seconds or minutes can be just as useful as for variations lasting hours or days.

The one-hour horizon is important because it is the shortest timescale over which electricity is traded, and because it is achievable with existing battery technology. "Our research shows that a storage capacity of 30 per cent of actual generation has a big effect," Rasmussen says. Such guarantees are worth a lot to the power companies, he says, because they make it possible to shut down fossil-fuel generating units.

Over even shorter timescales, storage allows wind power plants to increase their output briefly to cover 'spikes' in power demand. "Conventional power plants have a reserve that allows this 'up-regulation', whereas standard wind and solar plants don't," says Henrik Vikelgaard, an energy storage expert at Vestas. "Using storage to help maintain frequency stability on the grid will make wind power more acceptable to the generating companies."

Even for short-term storage, cost is an obstacle. "Either the cost of batteries will have to fall to half or one-third of its present level, or the value of reliable wind power will have to double," says Rasmussen. The next step up in storage, with capacity for six or eight hours to cover demand variations between day and night, would cost significantly more.

For immediate electricity storage, the answer lies in geology. Above ground, mountainous areas can use pumped storage hydro, which relies on off-peak or surplus electricity to move water up to a high reservoir. Pumped storage can achieve a 'round-trip' efficiency of 70-85 per cent, and hydro's ability to start generating within a few seconds makes it a good fit with other renewables. The US has around 20GW of pumped storage, and the EU around 32GW.

Below ground, surplus electricity can be used to pump air into caverns or rock fissures like those in which natural gas is stored (Fig. 1). To reclaim the energy, the compressed air drives turbines attached to generators. Because compression creates heat that must be removed and then replaced during the expansion phase, compressed air energy storage typically has an efficiency below 50 per cent, but the technology is proven.

R&D ideas in large-scale storage include underground pumped storage hydro based on mines and aquifers, huge plastic bags of compressed air tethered to the seabed, and heat storage to make compressed-air systems more efficient.

Batteries find the right chemistry

For shorter run times, batteries provide flexible electricity storage that does not rely on geology. Starting from the traditional lead-acid rechargeable battery, this fast-moving research area now includes a dozen or more battery types, including lithium ion, zinc-air, and sodium-sulphur.

Lead-acid batteries are competitive with fancier types for now, but feature poor power density and short working lives. Lithium ion batteries, as used in mobile phones and other small electronic gadgets, offer a larger number of charge-discharge cycles and the highest energy density of any commercially-available battery type.

A different type known as lithium iron magnesium phosphate (FeMgPO4) has lower energy density than lithium cobalt oxide, but longer life, especially in stationary applications.

Sodium-sulphur batteries operating at around 300°C have three times the energy density of lead-acid batteries and a predicted lifetime of 2500 cycles, says developer NGK Insulators of Japan (Fig. 2).

The company recently supplied a 1MW NaS system to a bus depot in New York, and has a 34MW demonstration plant running alongside a wind farm in Japan. A 1MW NaS unit with a capacity of 7MWh is as large as three 20-ft shipping containers, says Vestas' Henrik Vikelgaard.

Expanding the battery vision

Conventional batteries, whatever their chemistry, are self-contained units in which power is quite closely linked to capacity. Flow batteries, also known as redox batteries or reversible fuel cells, decouple power from capacity, potentially making very high capacities more affordable. They do this by storing charge in a liquid electrolyte that can be stored in large tanks and pumped through the battery as needed.

"At the moment, flow batteries are comparatively low-powered and expensive," says Claus Nygaard Rasmussen. The case of VRB, a pioneer in flow batteries, shows how challenging the new energy storage market can be. Despite its strong position in Japan, where several demonstration-scale flow batteries have been installed, the company laid off most of its staff last year and was recently acquired by Prudent Energy of Beijing.

Other storage devices that look a little like batteries, though they are actually very different, include ultracapacitors and superconducting magnetic energy storage (SMES) devices.

Ultracapacitors use electrodes made from nanoscale carbon to store electricity directly, rather than converting it to chemical energy, as batteries do. They are ideal for delivering large currents over short timescales, and, unlike batteries, they can operate for millions of cycles.

SMES devices store energy in the form of magnetic fields. They are robust, and can deliver large amounts of power for short periods.

Flywheels

Spinning flywheels that store kinetic energy offer robustness, high efficiency and an operating life measured in millions of cycles, says Damien Scott of UK company Williams Hybrid Power. Made from carbon fibre composite loaded with magnetic particles, Williams' flywheels spin in a vacuum at up to 40000rpm.

A version of the technology will appear in parent company Williams F1's Formula One cars.

"We have been looking into all these storage technologies, though we have not seen any breakthroughs yet, " says Henrik Vikelgaard, from Vestas. "We are working with energy storage researchers at Aalborg University and other institutions, including in the US.

"Vestas believes that energy storage will be important in improving the performance of wind power plants and making them behave more like traditional power stations," he continues. "It is also useful for postponing costly investments in grid capacity. Without storage, I think the world will have difficulty reaching its goals for renewable energy.m

Enter X www.engineerlive.com/ipe

Manel Romeu Bellés is with Vestas Wind Systems A/S, Randers, Denmark. www.vestas.com

Recent Issues