Intermittency demystified

A new report by UKERC defines the cost of intermittency of wind power in the UK electricity system for penetration levels that can be foreseen in the next 20 years (expected to be up to 20% of electricity generation). Based on a review of 200 studies on the subject, and written/reviewed by leading academics on power engineering in Europe, the report provides a snapshot of current knowledge on the subject.

A complex subject such as the intermittency of wind power, and its impact on the electricity system remains a communication challenge. Therefore, it's good to be conscious of the context of the report, to which its findings can be applied:

• The study, while international in scope, draws its key findings for the UK electricity system (operating largely on thermal power stations, limited use of cogeneration, medium carbon intensity of electricity production, good quality wind resource available).
• The time horizon is 20 years, implying penetration levels of up to 20% of electricity generation by wind (note: a penetration level expressed as a pecentage of total system capacity will be much higher). No changes to the basic structure of the electricity system are foreseen, though local strengthening of the grid may be needed, and some of its operating principles adjusted. In particular, the report is based on the premise that a backbone of thermal power stations supports the electricity system, in which wind generators operate and reduce the need to burn fossil fuel, and therefore save emissions.
• The study addresses the cost of intermittency, not the cost of renewable energy sources.

Under these conditions, the report concludes a cost of intermittency of 0.5 - 0.8 p/kWh (0.8 - 1.2 cEuro/kWh), subdivided in 0.2 - 0.3 p/kWh balancing costs and 0.3 - 0.5 p/kWh for maintaining a higher system margin. Additional reserve requirements needed for system balancing amount to 5-10% of installed wind capacity.

Capacity credit is reported in 29 studies, with values of 15-20% of installed intermittent capacity at the 20% penetration level. All studies find a positive capacity credit, which declines as penetration level increases.

On emissions & fuel savings, the argument is sometimes made that intermittency reduces these savings, for example through the increased variability of thermal power operation to compensate for wind's variability. The study finds this effect to be negligible to small (up to 7% of emissions saved). Overall, when carefully read and understood, this study provides a substantial contribution to the debate. The cost of intermittency is significant, but affordable. When spread over all electricity users at the 20% penetration level, it adds 0.1 - 0.15 p/kWh to the cost of electricity.