Figure 10.27 Monthly Variation in Capacity Factor of a UK Wind Farm

Figure 10.27 Monthly Variation in Capacity Factor of a UK Wind Farm capacity. It may be seen that the wind-farm output is significantly greater during the winter months when, of course, the electrical load is highest. In the past, there has also been considerable study of the extent to which wind generation could be used to defer the construction of conventional generating plant. This question has been posed in terms of establishing the 'capacity credit' of the wind farm. Capacity credit is calculated by evaluating the capacity of conventional generation that need not be constructed while maintaining the same level of generation reliability (Milligan and Parsons, 1999). If these calculations are carried out carefully using conventional power system reliability techniques it is possible to establish long-

term capacity credits as being approximately equal to the measured capacity factors of the wind farms providing overall wind energy penetration into the power system is low. However, it should be emphasized that these are statistically based calculations and at particular times of peak demand (e.g., on clear cold winter days in Northern Europe) there may be very little wind generation.

The studies of the 1980s have now been largely superseded by the change to a market structure for generation and supply of electrical energy and developments in the conventional plant mix. There is no longer any central control over the construction of generating plants in the UK and the energy trading system is based on bilateral agreements with only the final balancing mechanism being under the control of the network operator. These arrangements are such as to reward predictability and controllability in generating plants and, over time, a market price should become visible to indicate the extent of the additional costs of wind-energy generation.

Conventional generators, as well as being a source of electrical energy (kWh), are also used to provide a range of so-called ancillary services including: (1) system frequency control, (2) reserve capacity, and (3) black start capability. These services are essential for the stable operation of the power system and if they are not provided by conventional thermal, usually steam, plant then alternative sources need to be found. In a deregulated environment, these services are purchased by the network operator. For example the National Grid Company in England and Wales estimates that 100 MW of additional reserve for a year can be obtained for £10 M (House of Lords, 1999). Using this figure, and assuming that for large penetrations of wind energy into the UK system it will be necessary to provide 15 percent of the wind output as additional reserve, a cost of 0.17p/kWh may immediately be calculated.

Synchronous generators also provide reactive power which is essential to support the voltage of a power system. It is difficult to transmit reactive power long distances in a power system and so local reactive sources are required to support the network voltage. Preliminary work from Denmark (Bruntt, Havsager and Knudsen, 1999) indicates that with the large offshore wind farms planned, which will use induction generators, there is a significant possibility of voltage collapse. This is now being addressed and possible remedial measures, including reactive compensators considered.

It is sometimes suggested that large penetrations of wind energy in a power system will lead to a requirement for energy storage. Although electro-chemical systems (flow batteries and fuel cells) are now becoming commercially available it will be very difficult for them to compete with conventional plant, particularly existing pumped storage, merely to provide generation ancillary services. It is likely the energy storage systems will have to use their distributed nature and location in the network to provide additional value, e.g., in terms of overcoming transmission constraints, as well as taking part in energy trading activities.

As wind power becomes a more significant component of the power system it will be necessary to integrate its operation more closely with that of the conventional plant. At present renewable generation is operated on a 'must run' basis with reactive power and voltage control being the responsibility of the network operator. This is entirely appropriate when wind energy is not significant compared with conventional generation but clearly alternative arrangements will be needed in the future.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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