We seem to have an ever-increasing need for more energy. Both households and industries require large amounts of power. At the same time our means of production face new problems. International treaties aim to limit the levels of pollution, global warming prompts action to reduce the output of carbon dioxide and several countries have decided to decommission old nuclear power plants and not build new ones. Such development brings about the need to replace old energy production methods with new ones. While several are in development, including the promising but ever-so-distant nuclear fusion power, other production methods are already in commercial use. These often rely on the weather or climate to work, and include such methods as wind power, solar power and hydroelectricity in its many forms . These new sources of energy have some indisputable advantages over the older methods. At the same time, they present new challenges. The output of the traditional methods is easy to adjust according to the power requirements. The new energy sources are based more directly on harnessing the power of the nature and as such their peak power outputs may not match the power requirements. They may exhibit large fluctuations in power output in diurnal, monthly or even annual cycles. Similarly, the demand can vary diurnally or annually. We need a way to buffer power, that is, store energy when excess is produced and then release it when production levels do not meet the requirements. Only then can we rely on, say, wind or solar power as our primary sources of energy . There are other reasons why it is necessary to store large amounts of energy. Depending on how storage is distributed, it may also help the network withstand peaks in demand. Storing energy allows transmission and distribution to operate at full capacity, decreasing the demand for newer or upgraded lines. Storing energy for shorter periods may be useful for smoothing out small peaks and sags in voltage . There is clearly a need for energy storage, specifically energy storage in a larger scale than before. Traditional energy storage methods, such as the electrochemical cell, are not necessarily applicable to larger-scale systems, and their efficiency may be suboptimal. Meanwhile, a number of new and promising methods are in development. Some of these are based on old concepts applied to modern energy storage, others are completely new ideas. Some are more mature than others, but most can be, and are, improved . This report aims to introduce some of the new methods available or under development for large-scale storage of energy. While the definition of ``large scale'' may not always be a clear one, the focus is on solutions which could be applied in the same scale as the energy production methods. An introduction to the different methods follows, along with discussion of some of the physics behind the technologies, some advantages and disadvantages, and their feasibility. Comparison between the different methods, where applicable, is also covered . It can be difficult to define what ``large scale'' actually means when talking about energy storage methods. A power plant certainly requires a large-scale energy storage system whereas a portable CD player does not, but is a car an example of a large-scale application? In some cases a particular technology may be suitable for large-scale storage even if it is currently used on a smaller scale. For the purposes of this report, the scale at which the technology is used at the moment is secondary as long as there is potential for future large-scale use. Also, the basic properties of the different energy storage methods vary largely . While one method of storage may be ideal to smooth out annual fluctuations, another one may only be suitable to satisfy very short peak power requirements.
Fuel cells, like all electrochemical cells, convert stored chemical energy directly into electrical...