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MATCHING SUPPLIES OF ELECTRICITY TO VARIABLE DEMANDS FOR ELECTRICITY

It is sometimes suggested that variability in renewable sources of electricity puts a limit on their use. 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. With all sources of power, load factors are normally well short of 100%.

The variability of sources such as wind power is much less of an issue than is sometimes suggested, as described in Managing Variability (PDF, 402 KB, a report by independent consultant David Milborrow commissioned by Greenpeace, WWF, RSPB, Friends of the Earth, July 2009).

Not only are all sources of electricity intermittent, and many of them are 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 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.

A demonstration of the way that renewables can provide a comprehensive and robust source of power is the "Combined Power Plant" which links and controls 36 wind, solar, biomass and hydropower installations spread throughout Germany. It is just as reliable and powerful as a conventional large-scale power station.

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. Large-scale storage facilities, such as pumped-storage systems in Norway and the Alps, may be widely shared. Submarine HVDC transmission lines that have been laid between Norway and Denmark and between Norway and the Netherlands enable both pairs of countries to benefit in this way.
  • Complementary sources of power. In load-balancing via the grid, it is helpful if different kinds of generators have complementary characteristics. For example, there is a good fit between solar power—which is strongest in the summer—and wind power—which is strongest in the winter (see, for example, Seasonal optimal mix of wind and solar power in a future, highly renewable Europe, Dominik Heide and others, Renewable Energy 35, 2483-2489, 2010).
  • Power on demand. 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'. Non-renewable sources that provide power on demand are gas-fired power stations and stand-by generators (See Emergency power systems to be worth £1.5bn by 2020, The Telegraph, 2011-11-09). Renewable sources of power that can provide power on demand include:
  • An interesting possibility which is likely to become increasingly important in the next few years is the use of Plug-in Hybrid Electric Vehicles (PHEVs) or electric vehicles as a responsive source of power to help meet peaks in demand (see vehicle-to-grid technology).
  • 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). Another possibility is to use surplus electricity to create hydrogen by electrolysis of water. The stored hydrogen may be used to generate electricity when required. Yet another possibility is 'liquid air'.
  • 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. Large cold stores, for example, may take advantage of relatively cheap surplus power when it is available and delay drawing current—perhaps for several hours—when electricity prices are high. The same is true of PHEVs or electric vehicles 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.
    • Phase-change materials. The use of phase-change materials in fridges, buildings and elsewhere can provide flexibility about when power is needed and, in some cases, reduce overall power consumption. The phase-change material can absorb or release heat without changing temperature. See:
    • Ice in air conditioners. A related idea is the basis for a new device, now commercially available, which integrates an ice-making function into air-conditioners. Ice is made at times when electricity supply is plentiful; air conditioning is accomplished when the weather is hot, mainly using the cold stored in the ice.
    • Storing surplus power from wind turbines. When there is excess power from wind turbines, it is possible to store it as heat in district heating schemes and draw on it as required (see District heating from wind: Kirkwall).
    • Interruptible 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. Electricity supply systems normally 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. The most useful kinds of capacity are those that can be switched on or off easily, depending on demand.
  • 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.

In connection with the problem of balancing electricity supply and demand, it is relevant to point out that nuclear power is an embarrassment:

  • Like any kind of equipment, a nuclear power station can fail. But, since most nuclear power stations are relatively large sources of power, their failure is more disruptive than when smaller units fail.
  • Nuclear power is an inflexible source of power: it cannot be turned up quickly to meet peaks in demand and it cannot be turned off easily when supplies exceed demand.

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Last updated: 2014-02-03 (ISO 8601)