A growing reliance on wind power in the Colorado energy market has had the unintended consequence of causing baseload coal-fired generators there to cycle up and down to accommodate the wind resource’s intermittent nature. This has led to an increase in emissions, a recent study has found.
Bentek Energy LLC, which conducted the study on behalf of the Independent Petroleum Association of Mountain States (IPAMS), recommends reducing the use of wind power until more gas-fired power generation can be developed to back up the resource. However, the American Wind Energy Association (AWEA) claims that gas interests are fretting over losing market share for gas-fired generation to wind.
Bentek looked at four years’ worth of Public Service Co. of Colorado (PSCO) hourly operational history and found that the cycling of coal plants has negated the emissions benefits of wind energy. The incidence of coal cycling has risen markedly with the introduction of 775 MW of wind capacity since 2007, Bentek said.
AWEA blasted the report.
“In fact, U.S. Department of Energy data show that adding wind energy to the grid leads to clear reductions in carbon dioxide and other emissions, as well as reductions in coal and natural gas use,” AWEA said. “Might this last data point have something to do with why a lobby group representing the natural gas industry is releasing a study to spread misinformation about wind energy?
“Specifically, DOE data tells us that renewable generation grew from providing 2.5% of Colorado’s electricity in 2007 to 6.1% of the state’s electricity in 2008, an increase in share of 3.6% that was almost perfectly matched by a 4.4% decrease in power plant CO2 [carbon dioxide] emissions from 2007 to 2008. After accounting for the fact that Colorado’s electricity production decreased slightly (by 0.8%) from 2007 to 2008, the fact that output from other sources of electricity supply was nearly unchanged, and that wind energy accounts for nearly all of Colorado’s renewable generation, it is conclusively clear that the increasing use of wind energy was directly responsible for this decline in emissions.”
An analysis last summer by Tudor, Pickering Holt & Co. Securities Inc. found that the rollout of wind power in Texas was reducing the demand for gas-fired generation. Power generation demand for gas in Texas is about 3.2 Bcf/d, the TPH analysts said at the time. If no incremental wind generation is added, this would grow to about 3.3 Bcf/d by 2013.
However, if, as TPH assumes, wind capacity grows to 18,500 MW by 2013 and wind power displaces gas-fired generation 75% of the time, the gas generation fleet capacity factor in the Electric Reliability Council of Texas (ERCOT) falls from 30% to 21%, and gas demand from power generators falls about 0.5 Bcf/d.
Coal-fired plants are intended for baseload generation, which causes them to run at stable rates. “Cycling causes coal-fired power plants to operate less efficiently and reduces the effectiveness of their environmental control equipment, which together drives up emissions,” Bentek said. “Paradoxically, using wind energy in such a way that it forces utilities to cycle their coal generation often results in greater SO2, NOX and CO2 [sulfur dioxide, nitrous oxide and carbon dioxide] emissions than would have occurred if less wind energy were generated and coal generation was not cycled.”
The culprit in Colorado — and also in Texas, Bentek said — is renewable portfolio standards (RPS) that dictate wind energy to be a “must-take” resource. “An analysis of the Electric Reliability Council of Texas (ERCOT), which also operates under a RPS mandate to utilize wind energy, validates the emissions findings for PSCO,” Bentek said. “The underlying problem is the same for both PSCO and ERCOT: wind generation frequently cannot be accommodated without forcing coal-fired units to cycle.
“Emission levels are increasing, not decreasing, at PSCO and ERCOT coal units because the units are being cycled to compensate for wind generation.”
The answer, Bentek said, is more gas-fired generation to backup the wind power. “Enacting [an] RPS that require[s] more than 5-10% of wind energy for electricity generation will significantly add to emissions unless more flexible natural gas generation is utilized.”
In the short term, wind power utilization should be limited to what can be offset by the cycling of existing gas-fired plants, Bentek said. Longer term, utilities operating under an RPS should add combined-cycle and combustion turbine gas-fueled plants to their generation mix, the firm said.
Last year Questar Corp. COO Charles Stanley, chair of the technology committee for America’s Natural Gas Alliance (ANGA), told NGI that he had heard anecdotal reports of coal-fired generators operating almost like gas-fired peakers to follow the peaks and valleys of wind generation output (see NGI, Sept. 7, 2009).
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