New Operating Plant Paradigm
Aside from being an interesting statistic, the reduction in demand for electricity is producing some unintended consequences for the operators of our electric grids.
Coal-fired plants, which produce about 50% of the annual kilowatt-hours consumed by the U.S., may soon need to operate in a cycling mode instead of steady-state base-load. This change in operating mode is especially likely at night, when demand is lowest and when wind power production is highest. Because wind power has zero fuel cost, all of the electricity produced by wind will be taken onto the grid (except in transmission-constrained areas), and the balance of the electical demand will be produced by "must run" nuclear plants, hydro plants, and gas and coal generation units.
Historically, simple-cycle and combined-cycle gas turbines have been ramped down or taken off the grid at night to handle demand reduction. But with so much new wind generation added during the past few years (and more coming), coal-fired plants may soon be cycled on a daily basis as well.
The big challenge on the Texas ERCOT grid used to be planning for the summer peak demand and having sufficient operating spinning reserves. Not any more. According to Trip Doggett, ERCOT’s CEO, the big challenge today is to manage the minimum load at night with so much wind power on the grid. The ERCOT peak load is 64 GW, and the minimum load is 22.4 GW. Doggett notes that "by 2010, we could have almost 10 GW of wind power operating during the minimum load periods." He expects coal-fired plants to be turned down or turned off completely in some cases. "On average, there are 51 hours per year when the down-regulation of non-wind generation will be especially tough."
It is difficult to know exactly how cycling will affect the maintenance costs and reliability of coal-fired units, as there is very little historical data. Since 2000, much has been learned about cycling with high-efficiency gas turbine combined-cycle plants that were designed to run at baseload and later assigned to cycling duty. Heat exchanger welds began showing fatigue and required more frequent repair. Certain models of gas turbines handled the cycling better than others did, but all gas turbines, even the aeroderivative units, had to be reevaluated to understand the effect of thermal cycling.
The utility industry needs to gear up for an accelerated study on coal plant operation and maintenance with cycling. No matter what your opinions might be relating to climate change, cap and trade, and renewable energy standards, it appears certain that wind power will become a larger fraction of the U.S. generation mix during the next few years.
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