Windeconomics: Records tumble in UK and US

WORLDWIDE: Two months ago we reported a UK record for wind generation, with wind supplying around 14% of total electricity demand one day in March.

Research… National Grid in the UK has studied reserve requirements (pic:Decc)
Research… National Grid in the UK has studied reserve requirements (pic:Decc)

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The strong winds that were responsible for the high output had reached Ireland the day before and a new record wind output of 1.54GW was established there on 21 March. With system demand at that time amounting to 4.26GW, wind's contribution reached 36%, also a record.

Another record was achieved in the US on 7 April, when Californian wind-power generation reached 4.19GW - 17% of system demand at the time. The California Public Utilities Commission reports that it is on target to meet the 25% by 2016 milestone as mandated by the renewables portfolio standard that aims for 33% by 2020. A recent report from the commission also calculates wind's procurement expenditure as $80/MWh. The only procurement expenditure that is cheaper than wind are biogas ($66.6/MWh), geothermal ($65/MWh) and small hydro ($58.9/MWh).

Still in the US, the Electric Reliability Council of Texas (Ercot), which manages about 80% of electricity in that state, recorded a peak wind output of 9,48GW on 9 February. Total wind capacity in its region is 10.41GW.

Managing supply and costs of wind

In the US region managed by the Midwest Independent System Operator (Miso) underscheduling of wind - when the wind contribution is underestimated - in the day-ahead market averaged 619MW in the quarter from December to February. Wind production during this period averaged 4GW.

Almost two thirds of the underscheduling was offset by "virtual supply" bids at wind sites. These bids are notional amounts of power and are bid to sell at the day-ahead price and buy at the real-time price. The bids appear to the market as a dispatchable supply resource. Virtual bids do not require any physical generation or load.

The UK's National Grid has also provided information about the accuracy of its wind power forecasting. This is on a four-hour basis and so is not directly comparable with the data quoted above from Miso. Over a six-month period last year, the mean absolute error varied between 2.3% and 9.3%. In its proposals for an incentive programme that forms part of the price-control regime, regulator Ofgem has proposed a summer-time target of 4.5% error in forecasting and a winter-time target of 6%.

A mechanism that is facilitating the assimilation of wind into networks is the ability to control it - or make it "dispatchable" in utility jargon. Miso estimates that nearly 40% of wind resources are now dispatchable in the region.

The costs of variability

Utility Ercot does not explicitly assign the costs of ancillary services, such as spinning reserve - a power supply that can adjust output to balance supply and demand - to wind energy, but the monthly costs of these services do not appear to be correlated with the level of wind generation. This suggests the impact of wind is small. In December 2012, for example, when wind production was high, ancillary service costs were low. The UK National Grid has provided typical estimates of the short-term operational reserve required for wind. In its absence, the requirement was about 3.8GW and the addition of 20GW of wind increases this by about 50%. Wind of 20GW corresponds to a penetration level of about 15%. Other utilities report similar requirements.

National Grid has also estimated the amount of additional short-term operating reserve (including spinning reserve and other types) required between April 2011 and September 2012 to cover for wind output being lower than forecast. Wind farms generated 23.7TWh of electricity in this period, but the energy provided by short-term reserve that was due to the wind output being lower than forecast was only 22GWh. The total required for the whole system was 246GWh.

This means there was a very small reduction in the quantity of emissions saved by wind energy, since short-term reserve often operates below full output and is less efficient. Low efficiency means higher emissions, and critics have argued that this means their carbon savings are lower than their calculated values. National Grid estimates that 10.9 million tonnes of CO2 was saved by wind energy during the period, with the abatement due to the extra short-term reserve being only 8,800 tonnes, or 0.08%.

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