Electric utilities in the 13-state PJM Interconnection regional transmission territory have a balanced resource mix — coal, nuclear, gas and renewables — that is “well equipped” to support reliable operation of the regional grid, PJM has found in a new report, “PJM’s Evolving Resource Mix and System Reliability.”
But continued evolution of the resource mix — particularly the decommissioning of coal and nuclear plants and increasing reliance upon natural gas and renewables — could create reliability issues in the future.
PJM is in charge of maintaining the integrity of the electric grid within its territory, which includes all of Virginia. The study analyzed a spectrum of “portfolios” with different fuel mixes to see how they would affect a variety of electric reliability attributes such as voltage control, frequency response, and the ability to ramp production up and down as needed.
Of particular relevance to the ongoing energy debate in Virginia, PJM found that portfolios with 20% or greater of solar energy in the fuel mix would be “infeasible” because they would be unable to reliably meet night-time requirements. There don’t appear to be any upper bounds for natural gas, but excessive dependence upon gas could create vulnerabilities under a “polar vortex” scenario of sustained, bitterly cold temperatures.
In Virginia, Dominion Virginia Power has emphasized the importance of fuel source diversity, including coal and nuclear. Dominion’s plans for nuclear, which include extending the longevity of its Surry and North Anna nuclear units by an extra 20 years and possibly building a third nuclear unit at tremendous expense at North Anna, have proven particularly contentious. Solar constitutes a small percentage of Virginia’s fuel mix but is fast growing, and environmentalists are pushing for a much bigger role.
Across the PJM region, notes the study, the fuel mix has become more evenly balanced over time. In 2005, coal and nuclear generated 91% of the energy on the PJM system. But between 2010 and 2016, extensive coal capacity was retired and replaced mainly with gas and renewables. PJM’s installed capacity in 2016 consisted of 33% coal, 33% natural gas, 18% nuclear and 6% renewables and hydro. PJM has said in the past that the transmission grid was flexible enough that it could accommodate up to 30% renewables.
Each fuel source has advantages and disadvantages in helping electric utilities balance electricity supply and demand while sticking to tight parameters for frequency and voltage. Coal and nuclear are less responsive to changes in demand, taking far longer to ramp production up and down. Wind and solar are easy to turn off but, due to the variability of the wind and sun, cannot be turned on at will. Natural gas tends to be the most flexible, and PJM’s most reliable portfolios include large contributions from gas. Electric batteries also would provide considerable flexibility, but PJM does not foresee them being deployed on a large scale within the time-frame of the study.
States the study:
- Portfolios with the lowest unforced capacity shares of wind and solar tend to have the lowest composite reliability indices. (Note: “unforced capacity” refers to capacity in normal operating conditions as opposed to maximum “nameplate” capacity.)
- Composite reliability indices generally improve as capacity shares of nuclear, coal and natural gas increase.
- When coal and nuclear units are retired and replaced, portfolios with the highest composite reliability indices tend to be ones in which natural gas is the predominant replacement resource.
Bacon’s bottom line: PJM makes no judgment about the “best” fuel source mix, and it does not say that the most reliable fuel mixes are necessarily more desirable. If the goal is to increase renewables for reasons of reducing CO2 emissions, it is possible that some fuel mixes are reliable enough to accomplish both reliability and sustainability objectives.
Still, the PJM analysis suggests that high-renewable fuel mixes are “at risk for underperformance” and likely will need “additional technology requirements and/or new market rules” to ensure electric reliability.”