Capacity Factor and Capacity Credit [

When renewables displace significant amounts of conventional generation plant, an extra conventional plant capacity margin is required to maintain system supply reliability. It is important to recognize that an additional plant margin will only be required at times of low electricity demand and high input from variable sources. At other times an additional margin will not be required as there will be sufficient conventional plant available to meet demand irrespective of the contribution from variable sources.

The capacity factor of a generator, as mentioned in Section 2.4.3, is usually defined as the ratio of its yearly energy output to the output it would have produced if it operated continuously at its nameplate rating. Due to unavailability caused by maintenance schedules, breakdowns, etc., no plant achieves a capacity factor of unity. Base load thermal generators have a capacity factor of 85-90% when new, declining over the years until they are decommissioned. Wind turbines achieve capacity factors of 20-40% depending on the windi-ness of the site. In the UK the average capacity factor of onshore wind farms is ~30% with offshore schemes achieving higher values. Often, uninformed commentators quote the figure of 30% when they claim that wind turbines require back-up for 70% of the time. In fact wind turbines provide some power between cut-in and cut-out wind speeds for 80% of the time. The capacity factor does not determine back-up requirements, which must be assessed statistically.

Statistical analysis confirms that, for limited penetration, the capacity factor is a good guide of the probability that the generator in question will be available to contribute towards meeting demand [5]. On this basis variable sources are incapable of providing the same level of reliable or firm power during demand peaks as conventional generators, but they are still capable of providing a contribution to this. This firmness is known as the capacity credit and is a measure of the amount of load that can be provided by variable plant with no change in the LOLP. This can be illustrated by an example. A 1 GW dispersed installation of wind farms with a 30% load factor will provide the same yearly energy output as a 350 MW gas fuelled plant of 85% load factor. Both plant statistically have a capacity credit of 300 MW.

The calculations above assume that the availability of generated power is completely uncorrelated to the demand. Although this assumption is correct for the gas fuelled plant this is certainly not so in the case of the wind plant. As will be shown in the next section, due to aggregation of wind farms and the characteristics of the wind resource, there is a positive correlation between the availability of wind power and the demand.

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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