Dynamic Demand (electric Power) - The Need For Spinning Reserve

The Need For Spinning Reserve

The power utilities are able to predict to a reasonable accuracy (generally to within one or two percent) the demand pattern throughout any particular day. This means that the free market in electricity is able to schedule just enough base load in advance. Any remaining imbalance would then be due either to inaccuracies in the prediction, or unscheduled changes in supply (such as a power station fault) and/or demand. Such imbalances are removed by requesting generators to operate in so called frequency response mode (also called frequency control mode), altering their output continuously to keep the frequency near the required value.

The grid frequency is a system-wide indicator of overall power imbalance. For example, it will drop if there is too much demand because generators will start to slow down slightly. A generator in frequency-response mode will, under nominal conditions, run at reduced output in order to maintain a buffer of spare capacity. It will then continually alter its output on a second-to-second basis to the needs of the grid with droop speed control.

This spinning reserve is a significant expense to the power utilities as often fuel must be burned or potential power sales lost to maintain it. The kind of generation used for fast response is usually fossil fuel powered which produces emissions of between 0.48 and 1.3 tonnes of CO₂ equivalent for every megawatt hour (MWh) generated. Thus a significant environmental burden, in the form of in increased greenhouse gas emissions, is associated with this imbalance.