VARIABILITY IN ELECTRICITY SUPPLY AND DEMAND

Opponents of wind farms often argue that wind energy is a poor source of power "because the wind does not blow all the time". Likewise, it is clear that PV solar panels do not produce electricity at night.

These things are true but what is normally overlooked in this kind argument is that all sources of electricity are intermittent. Coal-fired or nuclear power stations need routine maintenance and, like any other kind of equipment, they do suffer from unscheduled breakdowns. This means that they are often out of action and may have a 'load factor' as low as 50%, meaning that they produce only 50% of their theoretical maximum output. Wind farms produce some electricity for about 80% of the time and they normally have a load factor of about 30%.

Another point to bear in mind is that the demand for electricity fluctuates wildly, often from minute to minute. The often-quoted example is how a commercial break in a popular TV programme can cause a large spike in demand when people throughout the country go to make a cup of tea and put the kettle on.

So, with or without renewable forms of energy, intermittency is an unavoidable feature of electricity supply and power engineers have devised several ways to cope with it:

• Conventional power stations are sometimes kept on 'spinning reserve', turning over at a low level so that their output can be ramped up quickly when other supplies drop or demand surges. This is a rather wasteful practice because energy is lost while a power station is on stand-by but spinning and, with fossil fuels, it releases CO₂ into the atmosphere.
• More generally, the electricity supply system can be designed to include a mixture of different sources of electricity. Those that cannot easily be switched on and off provide 'base load', those that can be ramped up or shut down fairly slowly provide 'intermediate load', while those that can be switched on and off fairly quickly provide 'dispatchable peaking power'.
• Of these, the last are most useful for ironing out peaks and troughs in supply and demand. There are several options here:
  • Gas fired conventional power stations can be switched on and off relatively quickly.
  • Pumped storage power stations, such as the Dinorwig power station in North Wales, can store energy and release it quickly on to the grid when required.
  • Tidal lagoons can be designed to provide much the same peak load capacity as pumped storage power stations.
  • Concentrating solar power plants that store solar heat in melted salt or other medium can also provide dispatchable peaking power.
  • Dynamic demand: fridges and other devices that do not require power at specific times can be designed to draw power when there is power to spare and avoid drawing power when demand outstrips supply. 'Plug-in Hybrid Electric Vehicles' (PHEVs) can actually feed electricity into the grid if there is a shortfall from other sources.
• Different sources of electricity may complement each other. Solar power is greatest in the summer, whereas wind power and wave power are greatest in the winter. By combining sources of power, shortfalls from one source can be compensated by surpluses from other sources.
• Perhaps the most effective way to cope with variations in supply and demand is to connect different sources of electricity together over a wide area using an electricity transmission grid. Although the wind does not blow all the time in any one spot, it is very rare for it to stop blowing everywhere across a wide area like Europe or the USA. With modern, highly-efficient 'HVDC' transmission lines, losses from transmission are very low. They are more than offset by gains in efficiency in the system because surplus electricity in one area can be transmitted to any area where there may be a shortage. Without that facility, surplus electricity in any area is simply wasted.

By using several different sources of renewable energy and connecting them with a wide-area HVDC transmission grid, it seems likely that the electricity supply system can be designed to cope with variations in supply and demand and largely eliminate the wastefulness of 'spinning reserve'.