The lowest estimates of additional reserve requirements come from the Nordic countries and the UK where generation schedules are set for just one or four hours in advance. The short time scales help reduce errors in forecasting wind power production so that less reserve capacity is needed, keeping costs down. With wind power supplying 10% of electricity on a power system with short "gate closure" times, the required increase in reserve capacity is between 1% and 5% of the wind capacity. With 20% wind power, the increases rises to between 4% and 7% of the wind capacity.
On power systems with much longer scheduling requirements, forecasting the contribution from wind energy is less accurate, which pushes up the need for reserve power. Estimates for Germany, based on day-ahead scheduling, suggest between 8% and 9% of the wind capacity is needed as reserve with a 15% wind penetration. Data from Minnesota, however, also based on day-ahead uncertainty, suggest the additional reserve is only about 5% of the wind capacity.
With 20% wind energy, the study finds that additional balancing costs are mostly between EUR 1/MWh and EUR 4/MWh. As the authors remark, this is 10% or less of the wholesale value of wind energy. The study also looks at the cost of grid reinforcements when adding wind power and reports that this roughly ranges from EUR 50/kW to EUR 100/kW, depending on the quantity of wind energy to be connected.
A centralised and well connected power system is essential for integrating large amounts of a variable power supply because fluctuations in demand and supply balance one another, the study points out. "Transmission is the key to aggregation benefits, electricity markets and larger balancing areas," it states. Large balancing areas help reduce the impact of wind's variable supply and forecast errors. They also help in pooling more cost effective balancing resources. But it is essential for high penetrations of wind power that electricity is able to flow freely within large geographic areas.
The study also looks at how much conventional wind plant can be replaced by the addition of wind power capacity. Data from Ireland, Norway, Minnesota and Britain suggest that wind power can displace conventional plant equivalent to about 35% of its installed capacity with modest amounts of wind on a system. By the time the wind capacity reaches 50% of the peak load, however, that percentage has fallen to around 15%. In areas of the world with a relatively poor wind resource, such as Germany, the capacity credit of wind power is thereby significantly lower.
The study's findings so far are presented in an interim report. Twelve organisations are involved from Europe and the United States, including power system operators, electricity generators and research centres. The aim of the study, which is co-ordinated by Finland's VTT Technical Research Centre, is to develop guidelines on recommended methodologies when estimating the system impacts and the costs of wind power integration. The final report is due at the end of 2008 and will include information from further national studies and a more detailed examination of what happens at high levels of wind penetration.