2.1 ELECTRICITY DEMAND

Electricity demand in an industrial society arises from a number of sources, including: Industry:

Commerce:

Public transport:

Domestic, homes:

It is clear from the above list why electricity demand fluctuates throughout every 24-hour period, as well as through the week and seasonally. It also varies from place to place and from country to country, depending on the mix of demand, the climate, and other factors. A daily load curve for an electricity system in a temperate climate is shown in Figure 4. From this it can be seen that there is a base load of about 60% of the maximum load for a weekday. This load curve is typical for developed countries.

The base-load demand for continuous, reliable supply of electricity on a large scale is the key factor in any system. The main investment of any electric utility is to meet that kind of demand.

As well as the daily and weekly variations in demand, there are gradual changes occurring in the pattern of electricity demand from year to year. In projecting demand patterns a decade or more into the future, planners must take note of such factors as:

Looking further ahead, there is major scope for the use of base-load electricity to charge the batteries for personal motor vehicles. In the last couple of years the popularity of hybrid cars, such as the Toyota Prius, and also Honda’s slightly different approach with hybrid diesel vehicles have put us within reach of practical electrical motoring for many people. This development has been enabled by the advent of much more efficient battery technology1, and with a further increase in battery capacity the possibility of using mainly energy derived from off-peak power, charging when parked overnight, is enhanced. This will mean that there is less reliance on the on-board internal combustion motor and more reliance on base-load power.

Some of these factors will affect total electricity consumption, while others will influence the relative importance of base-load demand. Production economics will require that as much of the electricity as possible is supplied from base-load generating plant. Government policies in many countries create scope for occasional input from any renewable generating capacity linked to the system.

2.2 ELECTRICITY SUPPLY

Because of the large fluctuations in demand over the course of the day, it is normal to have several types of power stations broadly categorized as base-load, intermediate-load or peak-load stations.

The base-load stations are usually steam-driven and run more or less continuously at near rated power output. Coal and nuclear power are the main energy sources used.

Intermediate-load and peak-load stations must be capable of being brought on line and shut down quickly once or twice daily. A variety of techniques are used for intermediate- and peak-load generation, including gas turbines, gas- and oil-fired steam boilers and hydroelectric generation.

Peak-load equipment tends to be characterized by low capital cost, and its relatively high fuel cost (unless hydro) is not a great problem.

Base-load plant is designed to minimize fuel cost, and the relatively high capital cost can be written off over the large amounts of electricity produced continuously.

Lowest overall power costs to the consumer are obtained when the peak-load increment is very small and a steady base load utilizes most of the available generating capacity fairly constantly. Any practical system has to allow for some of the plant being unserviceable or under maintenance for part of the time. Installed capacity should therefore be about 20% more than maximum load in a system, providing a reserve.

Base-load plants are likely to make up over half of a system’s total generating capacity, and produce more than 85% of the total electrical energy (cf. Figure 4). Almost one third of such a system’s capacity can broadly be classified as intermediate-load plant, supplying power throughout the working day and evening. The balance is peak-load in the strict sense, supplying short-term energy demand during high-load periods of the day or in emergencies, and with unit power cost being less critical.

The capital cost of peak-load equipment, such as gas turbines, is about half that of base-load coal-fired plant, and in addition it can be installed much more quickly. However, the fuel cost is relatively high compared with coal in a base-load station, per unit of power generated. Modern combined cycle gas turbine facilities, which have efficiencies substantially greater than that of coal-fired plants, reduce the difference.

Pumped water storage, using available base-load capacity overnight and on weekends, may be developed where topography permits, as an alternative to peak-load thermal power stations2. The capital cost may be low where there is existing hydroplant and such installations will have the effect of increasing the extent to which base-load equipment can contribute to total load through the week. However, there is an efficiency loss relative to inputs of around 35%.

In future the base-load contribution may be increased by using the surplus power in nonpeak periods to make hydrogen, either for peak generation or for transport fuel (see section 6.1). A further means of increasing the utilization of base-load plant is enabling it to follow the load to some extent, by varying the output.

As in other industries, there are economies of scale. Larger steam units result in reduced capital cost per kilowatt capacity, especially for base-load equipment. This means that location is sometimes determined as much by the supply of cooling water as by the fuel source. However, large power station units require a substantial electrical transmission grid and overall generating system to enable them to be operated effectively. Hence there are many situations where the economic virtues of small-scale gas-fired generating plants are put forward.

Goldisthal in Thuringia, Germany’s largest pumped water storage plant

2.3 FUELS FOR ELECTRICITY GENERATION TODAY

This book considers principally the question of electricity generation. In industrialized countries electricity generation takes about 40% of the primary energy supply. An increasing constraint on choosing the fuel for this is the carbon emissions involved.

In densely populated areas of the world, such as Japan and many parts of Europe and North America, coal supply is relatively remote from electricity demand. Also the high density of population and...