MATCHING VARIABLE ELECTRICITY SUPPLIES WITH VARIABLE DEMAND

It is sometimes suggested that renewable sources of electricity cannot provide more than about 20% of our electricity supplies because they are intermittent. But all sources of electricity are intermittent because they need to be taken out of service for scheduled maintenance and because, like any kind of equipment, they are liable to unscheduled breakdowns.

Not only are all sources of electricity variable but the demand for electricity is variable too—and there can be quite large changes from one minute to the next. The often-quoted example is how there can be a sharp peak in demand for electricity when there is a commercial break in a popular TV programme and many people go and put the kettle on to make a cup of tea.

There is a range of techniques available for matching constantly-varying supplies with constantly varying demands. When electricity supply systems are properly engineered, they should be able to accommodate sources of electricity that are 100% renewable.

Any or all of the following techniques may be used:

Large-scale 'HVDC' transmission grids. In an area like Europe, there are several potential benefits from building a 'Supergrid' of highly-efficient HVDC transmission lines to link existing HVAC transmission grids (see electricity transmission grids). One of the most important benefits is that this kind of large-scale grid can make it much easier to match variable supplies with variable demands. For example, the wind may stop blowing in any one spot but it almost never stops blowing everywhere across a wide area like Europe. If there is a peak in demand in any one area, it can almost always be met from spare capacity in one or more other areas.
Peaking power. One of the most useful attributes in any source of electricity is the ability to respond quickly to peaks in demand. Sources of electricity such as coal-fired power stations or nuclear power cannot respond quickly in that way and are really only suitable for 'base load'. Gas-fired power stations are much more responsive as are concentrating solar power plants that include provision for heat storage and backup supplies of heat when there is not enough sun. With those facilities, CSP plants can provide any combination of peaking power, intermediate power and base-load power.

An interesting possibility for the future is the use of Plug-in Hybrid Electric Vehicles (PHEVs) as a responsive source of power to help meet peaks in demand.

Storage of power. There are various methods for storing power but one of the cheapest and most effective is the storage of solar heat in melted salts or other substances, in conjunction with the generation of electricity using concentrating solar power (see generating electricity without the sun).
Methods for managing demand.
- Dynamic Demand. Some appliances, such as televisions or computers, need power at specific times. But other appliances, such as domestic refrigerators or large-scale commercial cold stores, are much more flexible in their requirements for electricity: a delay of an hour or two in switching them on may not matter much at all. The same is true of PHEVs mentioned above. If devices like that can detect when there is pressure on electricity supplies and delay their demands until supplies are more plentiful, that can be very helpful in matching variable supplies with variable demands. It turns out that a small drop in frequency in electricity supplies is a signal of pressure on the electricity supply system and that fridges and similar appliances can be equipped with devices that can detect that kind of fall in frequency and can ensure that electricity demands are delayed until more plentiful supplies are available. For more information, see Dynamic Demand.
- Interruptable service. Some large industrial customers may choose a lower-cost option of interruptable service, in which they pay a lower rate for their power in exchange for the right to have their service “interrupted”—temporarily cut off—in the event that demand is very high and power is needed somewhere else.
- Time-of-use billing is a related service in which consumers may payless for their power at a time when demand is lower, such as the middle of the night, or during a season when demand is lower.
The provision of spare generating capacity. All electricity supply systems have 'plant margin': the provision of generating capacity over and above what is strictly required to meet demand. This spare generating capacity can help make good any shortfalls in supplies.
Prediction. Weather forecasting can normally give a few hours notice of variations in output from wind farms. This gives time for alternative sources of power to be brought on stream or taken off stream. By contrast, when a conventional power plant fails, it normally does so without notice, giving power engineers little chance to provide alternatives.