Renewable energy forms an important component in future energy supplies for the electricity supply industry, the more so in the UK with increasing dependence upon imported gas and the retirement of coal and nuclear stations. Integrating renewable sources within the electricity supply system, however, requires special attention being paid to the specific variable power characteristics relevant to each respective source of energy and to the degree of penetration in meeting the system demand.
Inevitably, within the mix of renewable technologies in the future, a major role for wind generation is anticipated. The understanding of the nature and effects of the variability of wind power generation is therefore an important issue in ensuring both security of electrical power supply and security of electrical energy supply, two very different but linked problems.
The comprehensive studies referred to in this chapter examining the interaction of wind power with the UK grid system of supply produce some interesting results that may be summarized as follows:
• For the purposes of preserving security of power supply standards (defined in terms of loss of load probability, or LOLP) as wind capacity is installed in the system, the conventional planning margin capacity required is reduced by the capacity credit of wind generation.
• Detailed simulation and reliability analyses show that the capacity credit will never be more than the planning margin. This means that the total conventional plant capacity will never be less than the peak load irrespective of the amount of added wind capacity (a surprising result).
• As a consequence of the variable output, it is seen that wind power - and, by implication, the outputs of other renewable sources as well - can replace energy supplied from conventional sources, but not the need for most of their capacity. This will be a central problem for future studies and research.
• The immediate conclusion is that until new solutions emerge that will add substantially to the overall capacity credit of a more varied combination of variable energy sources, perhaps including very substantial energy storage capacities, much otherwise uneconomic conventional plant will need to be retained or replaced, either running on low or minimum output, or to be replaced by plant capable of frequent rapid start and ramping of output, such as (aero-derivative) OCGT generators (see also Chapter 6 in this volume).
These conclusions place an especially onerous security of supply requirement on market-driven investment and are perhaps not widely appreciated. They also represent a challenge to devising new methods for reducing the impact of the variability of renewable sources connected to the grid.
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