Choosing Between Dependable and Intermittent Clean Sources of Energy
On June 2 Exelon announced the early retirement of two nuclear power plants (Quad Cities and Clinton) in Illinois. On June 21 Pacific Gas and Electric announced a decision to close the Diablo Canyon nuclear plant in California. Those decisions have been explained in part on cost. The plants are aging, maintenance cost has been increasing, and low natural gas prices have reduced the cost of competing electricity. When a nuclear plant's contract comes up for renewal, politicians who think natural gas prices may stay low forever refuse to renew the nuclear plant's contract at a higher price.
But the most serious cost, a warning, a canary in the mine, is the result of flawed clean energy policy: Wind is given precedence. As wind penetration increases, a conflict develops between too much wind and baseload nuclear power plants. The conflict is explained in the attached figure. 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 percent of the annual average system load. At 30 percent wind, the residual curve drops below zero for 16 hours at different times in December 2015. This means that even if all other generators on the grid are shut down there is still too much wind and some wind turbines need to be cycled down.
On the PJM system, baseload nuclear comprises 33 GW of capacity (horizontal dash line in the figure). Wind conflicts with nuclear when the residual load curve drops below the 33 GW nuclear level, this would happen for 256 hours using December 2015 loads. During that time the utility has too much clean power and must choose to shut down either wind or nuclear. Most Renewable Portfolio Standard (RPS) legislation mandates the utility to buy wind and cycle down nuclear. Since cycling down nuclear is very expensive, existing RPS standards can drive baseload nuclear power off the grid.
When a nuclear plant is down it takes days to restart. 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.
Decommissioning well-functioning nuclear power plants will be expensive and will increase system CO2 emissions. One solution to this dilemma is to change the flawed RPS to a clean energy standard that includes nuclear power and let the utility shut down wind generation. The marketplace can also be changed so that when 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.
This piece was also supported by Dr. Harry Winsor, nuclear physicist and former DARPA program manager; Adjunct Prof. John Rudesill, chemical engineering at the University of Maryland BC; and Paul Acchione a Professional Engineer (Mechanical) and a Canadian based nuclear power consultant.