A balancing act for renewables
Energy storage will play a key role in increasing the use of variable energy sources. Nonetheless, storage is not the only balancing option and the overall design of power systems will incorporate a range of flexible generation, storage and grid-balancing options of different types and scales.
Some renewable energy sources, such as wind and solar, are intermittent and so, it is claimed, require energy storage systems acting as reservoirs to allow constant and on-demand delivery of power. Contrary to the case of fossil fuels, where the actual energy source can be stored prior to electricity generation, wind or solar radiation cannot be stored; what needs to be stored in the case of renewables is the generated electricity itself.
Several types of storage system are already in wide use for conventionally generated electricity and can also be used for balancing the variable output from renewable sources. For example, on the large utility-power scale, pumped hydro systems use surplus grid electricity to pump water uphill into a hydro reservoir, so that it can be run out later through the hydro turbines to generate power when needed. At the smaller scale, batteries store electricity more directly, and are suited to use, for example, for home power. There are also many other options at various scales1.
Generally, energy storage systems are pricey because they only deliver energy for part of the time — so their capital cost per kWh output is high. They are thus economically viable only when there is no alternative energy source, for example, in the case of small portable batteries for torches or radios. For larger-scale energy supply, storage is usually only practical if the cost of the input energy is very low and/or the price that can be charged for the energy output is high.
In the context of renewable sources, that would be the case when energy demand is high and no other sources are available. In practice however, alternative sources — such as natural gas — are often available and low cost. Thus, we might conclude that storage of electricity from renewables cannot compete with cheap gas turbines, which already exist on the grid and which can be run to meet demand, when renewable inputs are low, using easily stored gas. Indeed, at present some argue that the flexible grid-balancing option offered by gas turbines is a better bet than storing electricity produced by wind or solar irradiation.