Publisher Summary

The effective use of existing, depletable fossil fuels and the utilization of renewable alternative energy sources demands economical and efficient energy storage with flexibility in operation and siting. The availability of suitable energy storage systems will have a favorable impact on some areas. This chapter highlights these areas. This broad range of possible applications for energy storage devices is unlikely to be satisfied by a single method. In addressing the demand for development of storage devices and systems with different technical and operational characteristics, two quite different groups of applications are considered. These are stationary applications and transportable applications. Electric power systems have a well established need for large-scale energy storage that is met in part by pumped hydroelectric storage. The full potential of utility energy storage can be attained only through development and implementation of more broadly applicable storage technologies. One of the clearest examples of the demand for transportable stored energy is the need for an alternative to the filled petrol tanks in the cars. While the main constraint for most stationary applications is low cost, there are, for the transportable applications, some additional technical constraints, such as power density and energy density, both with respect to volume and weight of the storage unit.

1. Utility load levelling – to improve load factors, reduce pollution in populated urban areas and to make better use of available plants and fuels

2. Storage for combined heat and power systems – to improve overall efficiency by offering optimum division between heat and power irrespective of load demands

3. Storage for electric vehicles – to replace petrol in the long term, reduce urban air pollution and improve utility plant factors

4. Utilisation of solar energy in its various manifestations – to relieve the burden on the finite fossil fuel resources and to improve the living environment

5. Storage for uninterruptable power supplies – to improve the reliability of supply for critical applications such as hospitals and computing facilities

6. Storage for remote location facilities such as telecommunication and meteorological stations

7. Storage for industrial mobile power units – to provide better working conditions, especially in confined areas such as warehouses, mines, etc.

Stationary applications

Electric power systems have a well established need for large-scale energy storage which is being met in part by pumped hydroelectric storage. The full potential of utility energy storage can be attained only through development and implementation of more broadly applicable storage technologies.

The variation of load throughout the day, week, and year imposes a demand for storage especially with the increase in the use of large coal or nuclear plants designed to operate at maximum efficiency on base load and a future increase in utilisation of variable energy sources such as solar, wind, ocean energy etc.

Figure 3.1 shows a typical weekly load curve of a utility with and without energy storage. As illustrated by the upper curve, intermediate and peaking power involves extensive generating capacity. The load variation shown here is typical of the US situation, but it applies to most other countries, where cheap off-peak electricity rates exist. In countries where this is not the case the daily variation tends to be larger. In any case it appears to be the fact all over the world that installed capacity is about double the yearly average load.

If large-scale energy storage were available as illustrated by the lower curve of Figure 3.1 then the relatively efficient and economical base load generation could be increased and the excess beyond off-peak demand (lower shaded areas) could be used to charge the storage system.

Discharge of the stored energy (upper shaded areas) during periods of peak power demand would then reduce or replace fuel-burning peaking plant capacity, thus conserving fuel resources. In addition the higher base-load level would replace part of the intermediate generation. Assuming that new base load plants use non oil-based fuel, there are further savings of both cost and of oil resources.

Use of energy storage to generate peaking power in this manner is termed ‘peak shaving’. Load levelling describes the more extensive use of storage to eliminate most or all conventional intermediate cycling equipment. Energy storage efficiencies in the example shown in Figure 3.1 were estimated at 75%, and if so the result may include an overall energy saving but not necessarily. The outcome depends on whether the higher efficiency of base load plants compared with peaking and intermediate equipment makes up for the storage inefficiency. In any case, a reduction in installed capacity is achieved.

The demand for storage in the case of combined heat and power production arises in periods throughout the day where relative demand for heat and power output cannot be met by the utility plant. At night the demand for electricity is usually very low, but the demand for domestic heating is high. A heat storage unit, e.g. a highly insulated hot water tank with few hours storage time capacity in this case, is essential.

Another major demand for stationary storage arises from the utilisation of renewable energy sources. These sources, which directly or indirectly relate to the solar radiation arriving on the surface of the earth, all vary with time. The variation of solar radiation itself is shown in Figure 3.2, from which the demand for long term heat storage is obvious. The demand for heating occurs in periods with a lack of solar energy, hence the storage time must be several months for solar panel heat storage. Also solar cell electricity requires storage especially because the present high cost of solar cells makes an optimum system design including a large battery attractive. The variation is less for indirect solar sources such as wind and ocean waves, and they fit much better to load demands throughout the year. Wave energy, which is essentially accumulated wind energy, fits the load demand quite well (see Figure 3.3) and wave energy installations might be connected to the utility grid without use of storage units. Both wind and wave installations impose a demand for storage in remote locations, where grid connections are not economical.

Transport applications

As already mentioned, one of the clearest examples of the demand for transportable stored energy is the need for an alternative to the filled petrol tanks in our cars. Whilst the main constraint for most stationary applications is low cost, there are, for the transportable applications, some additional technical constraints such as power density and energy density, both with respect to volume and weight of the storage unit.

In general, a good storage system for transport applications must meet, at least, the requirement referred to in Figure 3.4 and it must also be reasonably safe to handle and to operate. Most of the effort during recent years to bring about an alternative to petrol driven combustion-engine-vehicles has been concerned with the electric battery vehicle, for which the major demand is a battery with better energy density than that of the lead acid battery (see Chapter 5).

The development of the transport sector has enlarged the demand for alternative fuel and storage forms. Figure 3.5 shows some alternatives and it also shows that trends in the use of primary fuels and storage are...