MANAGING VARIABILITY BY DAVID MILBORROW, 24 JUNE 2009

Meeting the European Union's Directive that 15% of UK final energy consumption should come from renewable sources by 2020 demands a significant increase in the proportion of electricity that comes

from renewable sources. Modelling for the Government suggests that this level may be about to 32%1 and suggests that wind energy will contribute a major share -- about 22% of electricity production in
the UK by 2020.

Numerous misconceptions surround the issues associated with the integration of the variable sources of renewable energy into electricity networks. "What happens when the wind stops blowing", is a
common question -- the implication being that the lights may go out. "Every megawatt of wind energy needs a megawatt of backup plant", is a slightly more sophisticated -- but still incorrect --
assertion.

So where does the truth lie? In a nutshell: nothing will happen when the wind stops blowing, simply because it never stops blowing, suddenly, over the whole of the British Isles. The variations from
distributed wind are generally less than the demand fluctuations regularly encountered on electricity networks. To cope with these, every network has reserves scheduled at all times and what matters is
the additional reserve needed to cope with the variability of wind. That is only a few percent of the rated power of wind plant -- not 100%.

There are also concerns that a system with a high proportion of variable renewables would risk power cuts at times of peak demand. The ability of wind energy to contribute to these peak demands needs to
be examined. This introduces the concept of ‘capacity credit’ -- how much thermal plant can be retired with the introduction of wind energy?

National Grid, in common with other electricity utilities, is on record affirming that any limits to the penetration of wind energy are likely to be economic rather than technical. As the proportion of wind
energy rises above about 20-25%, it may be necessary to constrain wind output at times. The aim of this paper is to clarify the issues in more detail, drawing upon the analysis that has been carried out
during the past 30 years (one recent review identified over 150 references) and also to review ways in which the impacts of variability may be mitigated.

To read the full report, please click here.