ABSTRACT
Energy is useful only if available when and where it is wanted. Carrying energy to where it is wanted is called distribution or transmission; keeping it available until when it is wanted is called storage. Within natural ecology, biomass is an energy store for animals and parasites, with seeds becoming a form of distribution. Within society and technology, energy storage, local distribution and long-distance transmission are not new concepts. Fossil and nuclear fuels are effectively energy stores, whose energy density is large, and high-voltage cables allow transmission of electricity. However, as renewable supplies increase, there is a need to develop other storage methods, including secondary fuels, and to sustain and improve distribution and transmission, especially for electricity. As discussed in Chapter 1, renewable energy supplies have different requirements for storage and distribution than do fossil and nuclear energy supplies. Usually the low intensity and widespread location of most renewable sources favour decentralised end-use, and the variable time dependence favours integration of several supplies with storage in a common system. Nevertheless, some renewable sources are of relatively large scale, e.g. large hydroelectric, geothermal and offshore windfarms, and so suitable for relatively intensive use (e.g. aluminium smelting and high-voltage power transmission). Since the use of renewable energy supplies constitutes a diversion of a
continuing natural flow of energy, there are problems in matching supply and demand in the time domain, i.e. in matching the rate at which energy is used. This varies with time on scales of months (e.g. house heating in temperate climates), days (e.g. artificial lighting) and even seconds (e.g. starting motors). In contrast to fossil fuels and nuclear power, the primary input power of renewable energy sources is outside our control. As discussed more fully in Chapter 1, we have the choice of either matching the load to the availability of renewable energy supply or storing the energy for future use. Energy can be stored in many forms, i.e. chemical, heat, electric, potential or kinetic energy. Moreover by linking supplies and consumption
in a grid (e.g. hot water, gas pipe, vehicle transportation and networked electricity), the controlled system has access to forms of virtual storage, e.g. as pressurised gas in pipes, export and import of electricity. The extent of this virtual storage varies as the grid-intensive parameters are adjusted (e.g. voltage, temperature, pressure or speed). Incorporation of small and intermediate scale renewables sources in a widespread grid system is called embedded generation, especially for electricity. Table 16.1 and Figure 16.1 summarise the performance of various
storage mechanisms. ‘Performance’ can be measured in units such as MJ$−1MJm−3 and MJkg−1. Of these, the first unit (cost effectiveness) is usually the deciding factor for commerce, but is the hardest to estimate, see Chapter 17; note that ‘cost’ here is wholesale cost before taxes and that taxation, especially of transport fuels, varies greatly between countries. The second unit is important when space is at a premium, e.g. in buildings of fixed size. The third unit is considered when weight is vital, e.g. in aircraft. In this chapter we show how these performance figures are estimated.
