ABSTRACT
A convenient reference case for discussing energy storage options is oil, a substance formed by millions of years of geological processing of biomass (Sørensen, 1984). Currently, world economy is based on the transient usage of this unique and ‚nite resource, available for a century or two at nearly zero extraction cost and offering very high energy storage density and convenient conversions to and from the store that may be a simple metal or composite container. An example of ‚lling this type of store is given in Figure 7.1. The crude oil has been modestly re‚ned to diesel oil, capable of furnishing a fuel-to-wheel ef‚ciency of 27% when used in a common-rail internal combustion engine based on the diesel cycle to accomplish the chemical-to-mechanical work conversion step when extracting fuel from the store (Sørensen, 2005). Few alternative energy storage systems are able to compete with the ‚lled oil tank in terms of performance. Below I give an overview of the current status of different storage devices followed by a more detailed discussion of some key options, including not only those involving storage facilities but also other ways of handling intermittent energy production, for example, by making use of energy exchange and trade arrangements.
