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Renewables
Why Renewables? The Challenges
Intermittency
Wind and solar electricity generation is intermittent, meaning that its output is governed not by the system's needs but by the natural fluctuations of the wind and the sun. This reduces the value of the wind and solar output. Geothermal and fossil-fired plants, in contrast, can be operated in response to system needs – to operate at full capacity when electricity needs are highest, and then turned down or off when needs are lower.
The penalty, or reduced value, that should be applied to wind and solar due to their intermittency is a contentious issue. At one extreme, some argue that wind and solar should receive no capacity value, that their only value is as a fuel saver: in other words, that wind and solar cannot be depended on at all as the wind could stop blowing or the clouds could roll in, and therefore that one always will need fossil-fuel plants as backup.
At the other end of the argument, some are of the view that a collection of wind turbines located at different locations, for example, can be viewed as a reliable power plant, as it is extremely unlikely that the wind will stop blowing everywhere all at once.
The truth, as usual, lies somewhere in between. It's clear, however, that a PV system or a wind turbine is not directly comparable to a natural gas plant with the same rated peak output, and that its output needs to be valued differently.
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