Some renewable energy, such as electricity generated by wind turbines, is produced in an unpredictable and intermittent cycle, when the wind blows. This energy supply does not always match the energy demands, and therefore, very often wind turbines are turned down (this is called wind curtailment). In times when there is a peak in electricity demand, grid operators will ramp up fossil fuel power plants to ensure the electricity supply, but these cause substantial emissions. The ability to store underground large amounts of hydrogen generated by rapid response electrolysers, provides a means for generating peak power without causing emissions. See Grid balancing.