The tremendous difficulty in storing electricity in any large quantity has shaped the architecture of power systems as they stand today. Various options exist for the large-scale storage of energy to ease operation and affect overall economies. However, energy storage of any kind is expensive and incurs significant power losses. Care must be taken in its economic evaluation.
The options available are as follows: pumped storage, compressed air, heat, hydrogen gas, secondary batteries, flywheels and superconducting coils.
Very rapid changes in load may occur (for example 1300MW/min at the end of some programmes on British TV) or the outage of lines or generators. An instantaneous loss of 1320 MW of generation (two 660 MW generating units) is considered when planning the operation of the Great Britain system. Hence a considerable amount of conventional steam plant must operate partially loaded to respond to these events. This is very expensive because there is a fixed heat loss for a steam turbogenerator regardless of output, and the efficiency of a thermal generating unit is reduced at part load. Therefore a significant amount of energy storage capable of instantaneous use would be an effective method of meeting such loadings, and by far the most important method to date is that of pumped storage.
Figure 1.11 Tidal stream energy (Figure adapted from Marine Current Turbines)
A pumped storage scheme consists of an upper and a lower reservoir and turbine- generators which can be used as both turbines and pumps. The upper reservoir typically has sufficient storage for 4-6 hours of full-load generation.
The sequence of operation is as follows. During times of peak load on the power system the turbines are driven by water from the upper reservoir and the electrical machines generate in the normal manner. During the night, when only base load stations are in operation and electricity is being produced at its cheapest, the water in the lower reservoir is pumped back into the higher one ready for the next day's peak load. At these times of low network load, each generator changes to synchronous motor action and, being supplied from the general power network, drives its turbine which now acts as a pump.
Typical operating efficiencies attained are:
- • Motor and generator 96%
- • Pump and turbine 77%
- • Pipeline and tunnel 97%
- • Transmission 95%
giving an overall efficiency of 68%. A further advantage is that the synchronous machines can be easily used as synchronous compensators to control reactive power if required.
A large pumped hydro scheme in Britain uses six 330 MVA pump-turbine (Francis-type reversible) generator-motor units generating at 18 kV. The flow of water and hence power output is controlled by guide vanes associated with the turbine. The maximum pumping power is 1830 MW. The machines are 92.5% efficient as turbines and 91.7% efficient as pumps giving an exceptionally high round trip efficient of 85%. The operating speed of the 12-pole electrical machines is 500 r.p.m. Such a plant can be used to provide fine frequency control for the whole British system. The machines will be expected to start and stop about 40 times a day as well as
Figure 1.12 Wave power generation (Figure adapted from Pelamis)
Figure 1.13 Storage using compressed air in conjunction with a gas turbine generator
provide frequency response in the event of a sudden load pick up or tripping of other generators.