Among the obstacles in the way of the triumph of renewable energy, one stands out: the lack of cheap, efficient means to store power. Now, it could just be starting to crumble. Matt Scotland and Caspar Henderson report.
Solar, wind and other renewables are booming as never before. On the face of it, there's nothing to stop them becoming the dominant source of our electricity sometime during the course of this century.
Except for one big, black fly in the ointment. Our limited ability to store the electricity they generate. Energy storage is the holy grail for renewables, since many of the most promising can only produce power on an intermittent basis (the sun doesn't always shine, the wind doesn't always blow). But to date, no technology has proven cheap - or practical - enough to provide this on the (massive) scale required.
But this could be starting to change. Several new or modified technologies are coming close to market, while a number of governments, with an eye to achieving energy security, are putting financing and policy mechanisms in place to encourage them.
Storing power generated intermittently is nothing new. The first commercial electric system, set up by Thomas Edison in New York City in the 1890s, depended on batteries to balance its somewhat erratic generators of direct current. And in the 1950s and 60s, during what was seen at the time as the white heat of a new industrial revolution, major investments in nuclear power in Britain, Canada and other countries were complemented by large-scale construction of pumped hydropower storage. This was seen as a necessary standby for when such supposedly reliable 'base-load' generators went offline.
Pumped storage uses electricity to send water uphill to a reservoir, from where it can be released as needed to produce power via hydro turbines when there's a 'generation gap'. One of Britain's largest ever construction projects, at Dinorwig in North Wales, hollowed out a large mountain for this purpose.
It's a tried and tested technology, with around 127GW of installed capacity worldwide. But it's limited by topography. Pumped storage relies on being able to site two adjacent reservoirs at substantially different heights. And there are only so many locations where this is possible - particularly in Europe.
But if not water, what about air?
Compressed air energy storage (CAES), which usually entails injecting air into underground reservoirs for release when needed, was first developed in Germany in the 1970s, and is in the spotlight again thanks to investments by companies such as Dresser-Rand and start-up General Compression. As with pumped storage, CAES is limited by the availability of suitable sites - it requires very specific geological formations. But now US company Sustain X has developed an above ground system, with a full-scale demo due in 2013. While issues over efficiency, safety and spatial requirements remain, it has injected fresh interest in the sector.
Then there's thermal storage: this typically uses heat to melt salts or similar substances stored in tanks, with the heat then used to drive steam generators. It's ideal for concentrated solar power (CSP) facilities, helping them generate electricity long after the sun's gone down. So far, though it has found few other applications. Now a British company, Highview Energy Storage, is pioneering a liquid nitrogen thermal system that it claims can store hundreds of megawatts of electricity cheaply and efficiently. It harnesses surplus power to liquify air, which is stored in a tank. The process of bringing it back to ambient temperature releases high pressure gases which can be used to drive a turbine. Backed by the UK's Department of Energy and Climate Change, it's already won a clutch of awards, and Highview now plans to build a commercial scale system within the next year. At present, the system is only 50% efficient (that is, it can recover around half the electricity fed into it), but this could rise to 70% if a source of waste heat is available to reheat the air to higher than ambient temperatures.
Most homes, of course, are full of simple energy storage devices: the humble battery. Now there's an array of innovations coming on stream which is bringing batteries into contention for larger scale storage. Some are old technologies with revamped components, such as large advanced lead-acid batteries (smaller ones are found under car bonnets); some are the new breed of lithium-ion (Li-ion) batteries most commonly associated with mobile phones and laptops; and others, more recent battery technologies such as flow batteries, metal-air batteries and sodium-nickel chloride ones.
Energy storage expert Dr Ali Nourai of DNV KEMA believes these could play a major role in the sort of 'Third Industrial Revolution' advocated by Jeremy Rifkin. This envisages an electricity grid composed of interconnected small-scale generation and storage systems, which would function like an 'energy internet' [see 'Are we on the cusp of a third industrial revolution?']. "Both GE and Fiamm have developed a sodium-nickel chloride battery that is cheaper, denser, smaller, safer and less expensive than traditional grid-scale ones", he says. "They can be shipped, dropped on site and connected in a fraction of the time of larger systems." These units, which may only be capable of storing kilowatts, can essentially be assembled like building blocks to store tens to hundreds of megawatts. The more units bought, the greater the economies of scale.
For the moment, costs remain dauntingly high, but as the various competing systems start to mature, so energy-conscious governments are starting to intervene.
Across the gamut of technologies, nations seeking first mover advantage are breaking new ground. South Korea is notably ambitious. In 2011, it announced a multi-year $5.4 billion energy storage investment programme, with the objective of making the country the global leader in the field. About a third of the total was allocated to research, with the remainder going to demonstration projects and the development of a nationwide storage infrastructure. A Smart Grid test bed on Jeju Island off the south coast of the country, where each house has either a wind turbine or photovoltaic array, is being touted as a national pilot for energy storage and management at micro-grid scale.The German Government is also looking to help integrate its rapidly expanding renewable capacity with new storage technologies. Federal ministries allocated €200 million in 2011 to energy storage, with wind turbines linked to hydrogen storage or CAES seen as among the most promising avenues.
And we mustn't forget the US, which is supporting several storage start-ups through grants and loans. This has led to sharp criticism for profligacy during turbulent economic times, following the bankruptcy of energy storage companies Ener1 and Beacon Power. Despite this, Energy Secretary Steven Chu recently announced a new Energy Innovation Hub that would fund $120 million in advanced research over the next five years.
At present, there's no silver bullet solution to energy storage. Yet there is much to suggest we are getting closer to a viable solution - whether it is because the likes of GE are backing battery development, the sheer number of start-ups out there researching and testing the next generation of storage technologies, or the emergence of government backing for energy storage development.
It's too early to pick winners for certain, but it does seem to be just a matter of time until, out of the turbidity, a technology emerges with the right cost and technical performances to sweep away the barriers to energy storage - and so trigger a surge in renewable power.
Matt Scotland is a freelance journalist specialising in the fields of energy, renewable energy, climate change and sustainability. Caspar Henderson is a writer and journalist. He lives in Oxford.