One of the most significant challenges for wind power is its intermittency. The wind does not blow at a constant speed, and so wind turbines cannot generate electricity consistently or cover predicted peaks in demand.
Since wind is intermittent, it can not be assumed that it will be operating at its full capacity at times of peak demand. Only 10% of UK wind generating capacity can be assumed to be available to meet peak demand for electricity. The DECC estimates that the average load factor of UK onshore wind farms is about 28% – that is, they generated around 28% of their theoretical maximum electrical output and that the average load factor of our offshore wind farms is about 39%.
When there is no wind or wind speeds are low, wind turbines generate no electricity. At high wind speeds, wind turbines are at risk of getting damaged, and so are designed to brake automatically at wind speeds above a certain level.
The intermittent output of wind turbines would pose a reliability challenge if wind power were the main source of UK electricity. It must therefore be one component of a diverse energy mix – where other energy sources compensate for wind's intermittency.
Our energy mix must include flexible energy sources, whose output can be ramped up at short notice to cover peaks in demand for electricity. But this need for flexibility can affect the overall affordability of the electricity supply.
Siting UK wind farms across more diverse locations could also help to address wind's intermittency. The wind speed at any one location cannot be relied upon to stay constant. But if wind turbines were installed in more diverse locations, the likelihood that at least some will be turning at any one time would increase, and the average total output across all could become more consistent and predictable.
The Government is committed to increasing the proportion of UK energy generated from renewable sources to 15% by 2020. Much of this new generating capacity is expected to come from wind farms.
Another proposed strategy is to combine UK and European renewables into a ‘supergrid’ – electricity would be distributed from areas with low demand but high output to areas with low output and high demand.
Large-scale electricity storage is not yet economically viable in the UK, but could present another solution to wind's intermittency in the future. Most sites suitable for the construction of pumped hydroelectricity storage stations, which require two reservoirs close together but at different heights, are already in use as hydroelectric power stations.
Another development is compressed air energy storage (CAES): energy generated at peak times is used to pump air into an airtight chamber at high pressure. When demand is especially high, the compressed air can be released from the chamber and used to drive generators.