The duck curve (solar in California)

The charts below illustrate what’s become known as the ‘duck curve’.

It’s a phenomenon first described in California, but the principles of which apply in other power systems with substantial solar capacity.

The top chart shows how, on a sunny day, generation from solar rises rapidly in the morning, is substantial during the middle of the day and then falls rapidly again in the late afternoon/early evening.

The middle chart shows the impact on the market for electricity from conventional thermal power plants (the purple line). As the sun rises, demand is eaten into by solar, shrinking the market available for those thermal plants (as well as requiring rapid ramp rates up and down on the evening and morning respectively).

The bottom chart shows how this shrinking daytime market for thermal power plants impacts prices: on this particular day they dropped right down to zero for a substantial time. Of course, if exposed to market prices rather than selling at a price fixed by a PPA (power purchase agreement), solar projects would suffer too.

This certainly presents a problem for future projects: after all, why would a utility sign a PPA when they can get power for free when the sun is shining? It's one reason why storage is an increasing feature of solar projects in this part of the world, allowing output to be time-shifted and sold when it has higher value.

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