If these wind power peaks coincide with periods of low consumer demand, the wind power may have to be curtailed — unless the electricity network has connections with other utilities or some other means of utilising the power.
Data from western Denmark can help illustrate the problem. Last year, wind energy accounted for 28% of the area's electricity consumption. The total wind power capacity at the end of the year was 3,051MW and the minimum demand on the system was 918MW. At first sight, it might be difficult to assimilate wind power, but in practice this was not the case.
Low demand tends to occur during the summer months when winds are rarely at their strongest. Also it is unusual for all wind power plants to produce maximum output, partly because very high winds rarely cover the entire country, and also because there are inevitably some turbine outages. As a result, maximum output of a large quantity of wind in a region does generally not exceed around 80% of its capacity.
That was the case in 2010, as the maximum power delivered by western Denmark's wind turbines was 2,466MW, or 81% of the total capacity. Nevertheless, wind power production exceeded the total demand on the system for a total of 71 hours. Most of these periods were in the spring or autumn and generally only lasted a few hours (see chart, below).
Western Denmark has connections with eastern Denmark, Norway, Sweden and Germany and so assimilation of the wind energy was not a problem. Without these connections, it is unlikely that a system operator would have been happy to run the system with 100% wind.
A "cushion" of thermal plant would ensure that any rapid fall in the output of the wind farms would endanger the stability of the system. Ireland's system operator, Eirgrid, has suggested that the amount of wind-power generation should not exceed 75% of the total at any one time. Applying this criterion to western Denmark in 2010 would have meant that 2.3% of wind power would have been rejected. This is in line with an analysis by the Danish system operator a few years ago. That analysis suggested that, even with 50% wind, curtailments would only necessitate the rejection of about 7% of the total wind production.
These principles are fairly universal, but the severity of the curtailments may be worse if peak wind generation coincides more closely with periods of minimum consumer demand. Another factor that leads to curtailments is when wind is in competition with other generation sources. This has been the root cause of the problems encountered by the Bonneville Power Administration following excessive snowmelt in the US's Pacific Northwest, a story Windpower Monthly has covered extensively.
The cost for operators
Ideally, wind-power generators need to be paid for the "lost" energy, although the exact amount may be difficult to calculate. Without compensation developers will lose money - unless they can increase their prices. As wind is a capital-intensive generation technology, the prices they need to recover from their generation rises in proportion to the amount of lost energy. That means if 2% of the generation is likely to be curtailed, prices need to rise by about the same amount. A wind power developer expecting to receive EUR100/MWh for its electricity would need to receive EUR102/MWh if 2% of the output was likely to be curtailed.
Network operators compensate wind developers who are required to curtail their output in different ways - and in some cases not at all. A review by the American National Renewable Energy Laboratory in 2010 looked at various compensation schemes. Some operators, such as the Bonneville Power Administration and the New Zealand power authority, paid no compensation, whereas others, such as the Electric Reliability Council of Texas, paid partial compensation. In Texas, wind plants were set daily operating limits and compensation was paid if the system operator imposed tighter restrictions. Others, such as Southern California Edison, paid for the lost energy in full.
In Ireland, generators are paid at the prevailing market price for electricity. If this is lower than what they would otherwise be paid under the Irish support mechanism, they lose revenue.
In the UK and Germany, a market-based system requires generators to submit bids for the income they wish to receive if the system operator asks them to reduce their output. If the system operator needs to restrict generation, it will naturally select the bids with the lowest numerical value. So a generator with a bid of -EUR50/MWh will be selected ahead of one with a bid of -EUR100/MWh. The current limit on such bids in the UK is £99,999/MWh (EUR114,000/MWh) and this has caused some controversy recently, with some Scottish wind farms submitting bids as low as -£300/MWh.
Transmission is key
A recent report by the Irish system operator shows the dramatic effect of improved transmission connections with the UK. With 6GW of wind restricted to providing 75% of demand, constraints would mean that around 8% of wind-energy production would be lost. With 1GW of interconnections to the UK in place, that figure is halved.
The implicit assumption is that the UK network would be able to accept the additional power. As the UK is a much bigger network with 60GW of peak demand against Ireland's 5GW, this is likely to be the case. A similar situation applies in western Denmark, which can export surplus wind to Norway and Sweden, but may not be able to send power to Germany, as the country has a large capacity of wind power itself.
Improved transmission connections will generally enhance the efficiency of power networks, irrespective of the amount of wind. When Windpower Monthly explored the trade-off between the cost of new connections specifically for wind and the cost of curtailments in a recent article, it found that it was generally not worthwhile installing transmission lines with a rating greater than about 70% of the rated capacity of the wind plant.
Curtailments may be eased if wind turbines can be made more controllable and provide characteristics similar to those of thermal plant - increasing or decreasing output at the request of system operators. The manufacturers are already moving in this direction. This would ease the kind of restrictions suggested by Eirgrid. Its study shows that raising the amount of instantaneous wind power allowable from 75% to 80%, with a wind energy penetration level of around 40%, would reduce the curtailed energy approximately from 8% to 6%.