Dealing with wind farms' variable power output

Power output from an individual wind turbine or wind farm varies over time, depending on the weather conditions. The fact that wind output varies is not itself a problem, provided that good information is available in advance to predict how much power wind farms will be producing at any given time. Predictability, by means of accurate forecasting, is an essential tool to the successful integration of wind power into the electric power system.

Extensive evaluation and modelling is carried out before a wind farm is built. That provides a great deal of information about what these wind plants will deliver on a seasonal or monthly basis. This is reinforced once wind farms are in operation.

In countries with 'priority access' for renewable energy producers, such as Germany and Spain, TSOs manage the grid as a whole to ensure that the system can always accept the maximum output from the wind plants in windy periods, yet maintain the power supply to power consumers during less windy periods. TSOs can now be supplied not only with seasonal/monthly expectations, but a combination of good meteorological forecasting and sophisticated software enables them also to have reliable hour-by-hour forecasts of available wind power. These are of particular value for their complex task.

Woolnorth wind farm, Tasmania, Australia

© Wind Power Works

While there can be variations between forecast and actual, this is of course also the case on the demand side. Any power system is influenced by a large number of planned and unplanned factors, but they have been designed to cope effectively with these variations through their confguration, control systems and interconnection.

On the demand side, changes in weather makes people switch on or off their heating, cooling and lighting, and millions of consumers expect instant power for hair dryers, washing machines and TVs – sometimes all at the same time such as during a popular TV programme.

On the supply side, also, there are variations, and not only from renewable energy sources. When a large power station suddenly shuts down, whether by accident or for maintenance, this causes the immediate loss of many hundreds of megawatts of capacity. With wind power, which is produced by hundreds or thousands of individual wind turbines rather than a few large power stations, such sudden drops should never occur, and significant variations should be forecast and planned for. this makes it easier for system operators to predict and manage the changes in supply.

Grid operators in a number of european countries, including Spain, Denmark and Portugal, have now introduced central control centres that can monitor and manage efficiently the entire national feet of wind turbines. Thanks to a combination of technology, forecasting and TSO expertise, in some parts of Europe wind power is routinely providing 30% of the electricity supply, and at times this goes far higher. In western Denmark, and in parts of Germany, wind has at times provided more than 100% of demand, meaning that 'spare' electricity generated from wind has been available to export to neighbouring grid systems.

In addition, large, interconnected grids lessen the overall impact if the wind stops blowing in one particular place. the idea of new electric 'superhighways', such as the 'supergrid' proposed to tap into the vast offshore wind resource in northern Europe and link these countries' electricity grids with the entire continent, would dramatically help to smooth out the variable output of the individual wind turbines.

A frequent misunderstanding concerning wind power relates to the amount of 'back up' generation capacity required to balance the variability of wind power in a system. the additional balancing costs associated with large-scale wind integration tend to amount to less than 10% of wind power generation costs, depending on the power system flexibility, the accuracy of short-term forecasting and functioning of the individual power market. The effect of this on the consumer power price is close to zero.1

1 European Wind Energy Association: Wind Energy – The Facts, 2009