Curtailment

Even when renewable energy is available, the Hawaiian Electric utilities sometimes need to cut back on “as-available” sources during off-peak periods of low electricity use.

Generation and consumption of electricity must be in balance at all times to maintain reliability and power quality. As customer demand for power drops, for example overnight, generation must be turned down or off. If electricity generated exceeds demand, the electric system can overload and become unstable.

Combustion generators (whether using oil, diesel or biofuel) can be turned back, but only so far. Peak generators are turned off when the peak, usually 5 to 9 p.m. weekdays, is over. Cycling units are also turned down or off as demand drops in the late evening. Base-loaded units, usually the largest, steam turbine units, can only be turned down to a minimum generation level. If a steam generator is turned down too low, it must be turned off and restarted, a process that can take a long time.

Excessive on-and-off cycling can be a problem for combustion generators, leading to damage and faster deterioration. It is also inefficient, causing them to burn more fuel than necessary. This increases the cost of electricity.

Thus when the demand falls toward the minimum generation levels for firm-power generators, it becomes necessary to “curtail” non-firm renewable energy projects like wind energy, even though the wind is blowing and potentially creating electricity. If more than one non-firm renewable provider on the grid, newer projects on the system will usually be the first to be curtailed.

Curtailment can affect the cost of operating a renewable energy facility efficiently, raising prices or reducing the profits of the developer and owner. This makes it more difficult to attract outside developers to add more renewable energy to small, isolated grids like those in Hawaii.

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