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Clearly the longer term perspective involves the most appropriate means to measure these impacts. All of these plants are located near Denver and thus directly impact emissions levels along the Front Range. As total load decreased during the night, PSCO reduced generation at coal and gas units to allow wind to continue to generate. Beginning at p. In response, coal was ramped up from approximately 1, to 1, MW in 60 minutes beginning at a. Generation from all PSCO coal plants on September 28—29, , contrasts to generation a few days earlier September 22— Wind generation availability on September 28—29 resulted in a significant reduction in coal-fired generation.

As was done for the July 2 case study, the emission rates associated with generation from September 22—23 were applied to the September 28—29 event. The Pawnee, Comanche, and Cherokee coal units were cycled to balance the load. As with the July 2 event, the calculation method drives the results. PSCO Case Study Conclusions The case studies in this section conclude that cycling coal-fired facilities to compensate for intermittent must-take energy sources results in inefficient operation during the cycling event and for hours afterward.

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This inefficiency results in severe degradation of emission savings at best and net additional emissions in many cases. Variable generation sources such as stored energy and natural gas facilities are necessary on systems that utilize intermittent energy sources to fully realize emission savings.

Wind power is a musttake resource on both systems, but is curtailed more often at ERCOT because resources are much larger and can create reliability problems when the system is fully generating. Finally, both systems are dispatched by central operators that attempt to utilize as much wind generation as possible without disrupting reliability standards. More important than these similarities, however, are the distinctions.

ERCOT has far larger gas-fired generation capacity and requires publishing of detailed wind generation data that, when combined with CEMS data, enables precise definition of wind events, thus facilitating a better understanding of the emission implications of wind use. Since the wind blows at night when gas generation is relatively low as a percent of total generation, coal plants are cycled, producing more SO2, NOX, and CO2 than would have been the case had those coal plants not been cycled. In addition, hourly generation and emissions data are available through the CEMS system.

Due to the availability of the minute generation data, wind event details can be calculated more precisely. The frequency of cycling events in ERCOT is captured within this section along with a case study covering a 1-day period. Every day, as wind increases between p. On some days, such as November 9 and 10, coal generation drops significantly, but even on days of limited wind such as November 8, wind appears to push a small amount of coal generation offline. The solid bars indicate the number of wind-induced cycle events. The shaded portion represents cycling events not related to wind.

The categories capture the sizes of the events. For example, the first category — MW indicates that the number of times total coal-fired generation increased from to MW from hour to hour. This data indicates that most coal cycling in Texas is due to wind generation and that the number of wind-induced cycling instances is increasing rapidly.

The incremental growth in appears to have had a more profound impact on the incidence of cycling than did the larger growth in This suggests that the impact of wind is cumulative: the more wind that comes on the system, without corresponding additions of other generation forms, the more wind-induced coal cycling results.

Emission Impacts: J. Little wind generation was present on the morning of November 8. As a result, coal-fired generation produced power consistently throughout the morning until late evening. Coal units were cycled throughout that day to accommodate wind generation. One coal-fired plant was chosen to illustrate the impact of coal cycling. The J. Deeley plant was one of the plants that accommodated the wind on November 8—9. The graphic shows a sharp drop in generation, beginning about p.

SO2 initially followed suit and fell until generation began to rise about a.

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After that, SO2 rose with increased generation and did not flatten out when generation reached its peak around a. Deeley plant generation and emissions on November 8—9, NOX and CO2 both rose slightly as coal generation fell, but, as the generation came back online, emissions quickly returned to and held at their pre-event levels. Emission rates for SO2, CO2, and NOX rose significantly immediately after Deeley generation was cycled and decreased as generation was brought back online. SO2 rates did not return to their pre-event levels until late in the day.

Interestingly, when generation dropped around p. In comparison to November 8, emission rates on November 9 are significantly higher. If generation at Deeley remained constant instead of variable on November 9, the emission rates would have been similar to those of November 8. The stable day rates evidenced on November 8 before the wind event were used to calculate avoided emissions and then compared to the actual emissions from November 9. Cycling J.

Deeley to compensate for wind generation produced more SO2 and NOX emissions than would have been generated if the plant generated the same amount of power at a flat level. Due to cycling, J. Deeley plant generation and emission rates, November 8—9, Deeley plant generation, November 9, Operating these facilities irregularly or at nondesign levels leads to inefficient operation and higher emission levels.

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Identifying days when wind generation resulted in the cycling of coal units allowed for a precise understanding of the emission impacts. The gravity and frequency of these events increased as more wind generation was introduced to the system. This mirrors the results found on the PSCO system, supporting the theory that increased rates of cycling arise from the incremental integration of wind generation.

Furthermore, these wind-driven, coal-cycling events resulted in significantly more SO2 and NOX emissions than if wind generation had not been utilized. The same results were found on the PSCO system. When wind generation comes online, generation from coal and natural gas-fired plants is curtailed until the wind subsides, then nonwind generation is again ramped up to meet demand.

Cycling coal units in this manner drives their heat rates up and their operating efficiencies down, emitting more SO2, NOX, and CO2 than would have been emitted if the units had not been cycled. Two caveats must be understood when interpreting these results. First, we found no instances in which PSCO violated any of its air permits as a result of cycling coal. Furthermore, the study authors are not suggesting that PSCO violated its permits in extrapolating the case study results to estimate annual emissions.

The second caveat pertains to the data. Thus, it was possible to precisely identify wind events based on a sudden decline in coal generation coupled with a simultaneous increase in wind generation.


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Subsidiary conclusions based on our analysis include: Duration — Cycling coal-fired power plants has short- and long-term impacts. This study concludes that the impacts frequently have much longer durations. Many instances were found where cycling caused bag houses and other pollution controls to lose their calibration and take as long as 12 to 15 hours, sometimes as long as 24 hours, to settle back to pre-event emission rates. During these periods, emission rates normally exceeded what would be experienced if the plants ran at stable generation levels.

Timing — Wind-induced coal-plant cycling appears to be a night-time phenomenon.

Nonwind renewable implications — Coal-cycling issues do not appear to impact solar and other nonwind renewable energy forms. Solar energy is generated during daylight, thus coinciding with natural gas-fired generation. When solar energy peaks, the likelihood is much greater that natural gas-fired generation can be cycled to accommodate the energy. Generation mix — Composition of the generation stack is a critical factor.

In the PSCO context, this means the coal plants supplemented with some combined-cycle natural gas and hydro are in operation. The extra emissions result because the RPSmandated must-take wind resources exceed the quantity of power generated from combined cycle gas. Increasing the proportion of baseload generated by more flexible generation equipment such as natural gas-fired combined cycle plants and stored energy sources will enable systems to absorb wind without having to cycle their coal plants. The RPS standard requires that more wind resources be utilized than can be offset with lower-emission, natural gas generation equipment.

Without substantially more natural gas generation added to the PSCO system, the emission increases documented in this study will rise, further enlarging the degree to which Denver and the Front Range violate the State Implementation Plan limitations.