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There is a boom in renewable energy sources coming online worldwide, but the predominant types – solar and wind – are problematic due to their variable nature. For most regions of the world, the sun cannot be expected to shine nor the wind blow when required.

What is needed is a way to capture that energy when available, perhaps in the middle of the night, when demand is low, and then store it until it can be used when demand rises. But this is not a trivial problem to solve.
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According to the European Wind Energy Association, at the end of 2013, the UK had 10.5GW of wind turbine capacity installed, with more in planning and construction. As the percentage of energy generated from renewables increases, the intermittency problem becomes more acute, as has been seen in countries like Germany or Ireland.

Germany, the country with the highest renewable capacity in Europe, has faced major technical problems due to the intermittency of renewable energy. The main issue is maintaining sufficient supply in the face of fluctuating levels of wind or sunshine. Back-up supply in the form of conventional power plants is required to meet demand. But as different types of power plant take time to come online – 48 hours for nuclear, 12 hours for coal-fired, down to a few hours for modern gas power plants, or ten seconds for the water released from a dam to start the turbines – having a back-up always available means having power plants running most of the time, which is inefficient and expensive.
Another problem is integrating renewable energy supplies into the high voltage electricity grid. For example, in Ireland given the fact that there is more wind at night when many businesses usage is low, a significant percentage of the energy produced may have been dumped, because the electricity produced cannot easily be transmitted across the grid.
So a means of storing energy is a vital part of any future energy system that includes a substantial amount of variable and uncontrollable renewable energy. Energy storage provides flexibility and reduces the need to rely on fossil fuel back-up power. Read on...