ABSTRACT
There is a great push for satisfying the steadily rising energy demand of modern society for renewable sources due to the depletion of fossil fuels and the increasing awareness of the overall CO2 emission effect on the climate. Nonetheless, beside the technological challenges associated to the energy conversion earlier discussed in this book, one of the greatest challenges is the intermittency of the resulting energy supply, especially the renewable ones, requiring the employment of efficient energy storage technologies. In this regard, there are to date only a few technologies that have gained practical relevance. The most important-also in terms of energy stored worldwide-is certainly pumped hydropower (PHP) with a worldwide contribution of around 99% (Dunn et al. 2011) because of its relatively high energy storage efficiency (ranging from about 70 to 85%) and the highest potential power ratings, typically from 100 MW up to 3000 MW (Chen et al. 2009). Apart from these highly advantageous characteristics, however, this technology heavily relies on suitable geographic conditions, providing
the opportunity to use two large water reservoirs at different heights (for instance, lifting 1 m3 of water by 1 m is equivalent to an energy storage of 3 Wh (Larcher and Tarascon 2015)) and the inclusion of a suitable dam system accompanied by relatively high investments (Chen et al. 2009). Similarly, the utilization of compressed air energy storage (CAES) devices, also capable of providing power ratings exceeding 100 MW, is dependent on certain geographic conditions, i.e., the presence of salt caverns, rock mines, or depleted gas fields to store the large amounts of compressed air. More importantly even, this technology can be combined basically only with gas turbine power plants, rendering it unsuitable for any other type of power plants, including those based on renewables (Chen et al. 2009). For these reasons, electrical and electrochemical energy storage-even though limited with respect to the achievable power ratings to several MW-has recently gained increasing interest, offering geographical independence, relatively lower installation cost, enhanced response times and energy storage efficiencies up to 95%, as well as the independence of the type of power plant. As a matter of fact, this rising interest is not least the result of the great success of electrochemical energy storage devices for comparably smaller applications such as portable electronics and (hybrid) electric vehicles, which would not have been possible or even conceivable without batteries. In fact, the targeted switch from combustion engine powered transportation to purely electric vehicles will ideally also contribute to the large-scale energy storage, if forward-looking approaches like the vehicleto-grid concept may turn into common reality one day ((Yang et al. 2011).
