Wind farms provide a variable supply of electricity because of the natural variability in wind speeds. However, wind speeds can be predicted days and hours ahead, meaning the output of a wind farm can be forecast in advance.
Modern wind turbines are designed to operate in a range of wind speeds. Typically a turbine will begin to generate electricity – or cut in – when wind speeds reach between 3 and 5 metres per second. It will stop generating – or cut out – when wind speeds exceed 25 metres.
The energy in wind increases with the wind speed (when wind speed doubles, its energy content increases 8 fold) so a turbine will generate more electricity as the wind speed increases. A turbine reaches its maximum output between 10 and 15 metres per second, and it will maintain maximum output until wind speeds reach about 25 metres per second (90 kilometres per hour). At this point the turbine will automatically shut down to prevent damage.
The wind speeds at which a turbine will cut in, cut out and reach maximum output varies for each model of wind turbine.
Sudden variations in generation caused by changes in wind speed are not as great as you might think. The control system in a wind turbine will automatically adjust the turbine to prevent surges in electricity output if wind speed increase suddenly. As turbines are arranged over a large area in a wind farm, often stretching several kilometres, a shift in wind speeds won’t affect all of the turbines at the same time. For example, generation will ramp up over the period of time it takes a gust to pass through a wind farm.
Typical power curve of a wind turbine (click to enlarge)
And when there is no wind?
New Zealand is a windy country - a wind turbine here will produce electricity for about 90% of the time.
On a quarterly and annual basis wind generation is much more consistent than hydro. Wind generation typically varies about 5 to 10% on an annual basis, where as hydro typically varies up to 20%. As energy sources are managed on an annual time frame (we talk about dry years, not dry days or minutes) it is this longer term consistency that is important.
Wind energy has a natural synergy with New Zealand’s existing hydro resources. One way of thinking about wind energy is that it is just another hydro inflow, but one that gets used before water flowing into a hydro lake because it can’t be stored.
If we use wind’s energy when it is available, then we can save the water stored in our hydro lakes. This stored water gives us more choices for generating electricity. We can:
- save the water to use another day – perhaps when low levels of wind generation or water inflows are expected
- use the water to meet fluctuations or daily peaks in demand
- use hydro instead of thermal generation to meet normal levels of demand.