As wind is added to an electric power system with fossil fuel plants, the utility throttles back fossil fuel plants reducing emissions whenever the wind blows. Eventually the system has hours with too much wind. This generally occurs at low load, the middle of the night, with high wind. At that time the utility has no choice but to shut down (curtail) wind turbines.

The adjacent figure illustrates the conflict. The solid black curve is the total system load for PJM (the electricity system operator for PA, NJ, MD, VA & DE). The day/night system load variation for December’s 31 days is apparent. The residual load (dashed line) is the power that must be produced by all the non-wind generators in the PJM system. It is the result of subtracting hourly wind power from the hourly system load. For this illustration, real wind data was scaled up to be 30% of the annual average system load. At 30% wind, the residual curve drops below zero (black horizontal line) for 16 hours at different times in December 2015. This means that even if all other generators on the grid were dispatchable (turn on-off on command), wind turbines would need to be shut down for 16 hours during December.

On the PJM system, base load nuclear comprises 33 GW of capacity (horizontal green dash line in the figure). The dispatch conflict between wind and nuclear occurs when the residual load curve drops below the 33 GW nuclear level (red residual curve). This would happen for 256 hours using December 2015 loads. During that time there is too much clean power and the utility must choose whether to shut down either wind or nuclear. Renewable Portfolio Standard (RPS) legislation mandates the utility to buy wind and cycle down nuclear.

Cycling down nuclear is expensive; with existing technology Xenon poisoning prevents restart for about 3 days. Before restart completes, wind is likely to have abated and the system would require fossil fuel backup. Thus, emissions would be higher than they would be if the utility shut down wind and kept the nuclear plant running. So the low cost, low emission solution is to keep the nuclear plant on line and shut down wind. Ontario Canada reached this conclusion in 2011, in 2016 over 50% of wind production was either curtailed or exported at deep discounts.

One solution to this dilemma is to establish a supplementary Interruptable Load Marketplace so that when clean energy supply exceeds demand, surplus electricity can be sold at reduced rates to loads that can tolerate intermittent supply. This would encourage new variable load industries like using cheap electricity to electrolyze water and produce hydrogen.