A battery of options: the storage solution

Energy from renewable sources has to be stored as production is unpredictable. But, says Peter J Hal

We have a continuous water supply because water companies use huge reservoirs to trap water when it is raining to provide a supply during dry periods. But what is true for water is definitely not true for electricity, where utilities pretty much try to produce energy at the same rate at which we use it. As you read this, people are monitoring how much electricity is being consumed and adjusting the supply accordingly. This relatively efficient system has served developed countries for 60 years.

However, as concerns about global warming and energy sustainability increase, a growing proportion of our energy may well come from renewable sources such as wind, wave and tides. By their nature these produce energy intermittently or unpredictably - we cannot control wind or wave - and it is not possible to guarantee energy production when we need it. The answer is to develop storage facilities, or "reservoirs", from which energy can be tapped when required.

There is a bewildering array of technologies to store energy, none of which, it must be said, is as yet either desirable or viable. They range from the extraordinary - such as Superconducting Magnetic Energy Storage, a cryogenically-cooled coil storing magnetic energy - through the interesting - flywheels which use rotational energy - to the mundane - compressed air.

The simplest, and the only technology that is widely deployed, is one that pumps water uphill when energy is plentiful, then allowing it to flow through turbines to generate electricity when there is a shortage. The only problem is that this method is fully utilised in all OECD countries and supplies a very small amount of the UK's energy.

Probably the best energy storage technology is one with which we are most familiar: batteries. They are a necessity when instant transportable power is needed; but can also be frustrating, because of their long recharging times. So while they do act as energy reservoirs, they are reservoirs that take annoyingly long to replenish.

Research is taking place to develop batteries large enough to absorb the majority of the energy output from wind farms. Experiments have taken place with so-called flow batteries, which contain large chemical tanks of liquids that can absorb and release electricity due to electrochemical reactions. Unfortunately, they are best described as environmental nightmares and are engineering headaches to build and operate. Most technologies rely on bromine (the most dangerous element for ozone depletion) or vanadium (thought to be toxic to fish) compounds or contain large volumes of highly corrosive liquid. Moreover, flow batteries are also relatively inefficient: less than 80 per cent of the energy stored can be re-released. If they do need to be developed, they are best viewed as a stop-gap technology.

Much ink has been spilt about the highly-vaunted hydrogen economy. All things considered, this is probably the least acceptable solution from social, technical and economic points of view. Socially, because hydrogen is a colourless, odourless and highly explosive gas that has no use outside of chemical plants. Technically, because despite over 50 years of intensive research there is no good way of storing hydrogen. Economically, because the whole process of producing hydrogen in an electrolyser and using it in a fuel cell is less than 50 per cent efficient. The widest impact of hydrogen will probably be in the transport sector where it will power fuel cells, which in turn will recharge batteries.

The ideal solution to energy storage in the longer term may well lie in the humble Li-ion battery, which probably powers your mobile telephone and laptop computer. The technology used today is almost 20 years old, but new nano-structures offer the prospect of batteries large enough to interface with wind turbines with storage efficiencies approaching 100 per cent and lifetimes of many hundreds of thousands of cycles.

But this is only half the picture. Batteries only take up and release energy slowly. When sudden surges of power are required, batteries of the future will be backed up by supercapacitors. These are very simple devices, in which energy storage involves no chemical changes, so they are able to store and release energy very quickly. As with Li-ion batteries, they have an efficiency of 100 per cent and very long cycling times.

There are no easy solutions to viable energy storage. But if we are fully to exploit renewable energy and make the most of its potential, we must develop a range of technologies to store it.

Peter J Hall is Professor of Chemical Engineering at the University of Strathclyde and leads the government-supported Supergen Energy Storage Consortium