Spinning Reserve

In order to ensure reliability, electricity grids need a certain amount of “spinning reserve”.  This is a source of generation that can be turned up quickly, to accomodate unexpected surges in demand, or loss of generation or transmission.

For fossil plants to be used as spinning reserve, they must be already warmed up, and that means they must burn some fuel just to keep them ready.  Waterpower provides Ontario with much of its quick start reserve, but we do use fossil units as well.

The rule that the grid operator uses is that we must always have an ability to replace the loss of the largest generating unit.  In Ontario, that is a unit of the Darlington nuclear station, or 900 MW.  So we always have to be able to replace that at a moment’s notice.  It can of course be replaced from multiple sources, including imports.  Because of our hydraulic and import abilities, we don’t need to keep fossil units running very much just for spinning reserve (but we do warm them up in advance of forecasted demand).
Nuclear can not be used for spinning reserve.  It is either on or it is off.  The same is true of co-generation or combined heat and power installations.

Interestingly, wind has been used in Denmark for spinning reserve.  The 200 MW offshore Horn’s Rev project can be throttled back by changing the pitch on the wind turbine blades.  And the offshore winds are very steady and predictable over the short run.  Wind turbines are easy to control, because they are modular.  It is easy to cut the output from a wind farm – just shut some turbines down.  And it is just as easy to turn them back on, and they will be producing power in less than a minute, when wind is available.  This can all be done under central control if needed, using the turbine’s SCADA system.

Of course, if you shut down a wind turbine, you lose the energy in the winds.  But if you shut down, or throttle back on a waterpower station, you can usually preserve the energy the storage pond.  So waterpower is always the preferred source of spinning reserve.

Reserve power is essential to maintaining a reliable electricity grid.  Wind’s flexibility to be shut down in small increments is a desirable characteristic, that could be used to maintain grid reliability.

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