Environmental Engineering Reference
In-Depth Information
during which the coal plant reduces generation. 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, emis-
sion 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. Nearly 70% of the cycling instances identified for PSCO in 2008
occurred between 12:00 and 8:00 a.m. Similarly, 82% of coal cycling events at
ERCOT occurred at the same time of night.
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 genera-
tion. 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 fac-
tor. Most wind-driven cycling events appear to occur between 12:00 and
8:00 a.m.—during periods of lowest load. As a result, PSCO and ERCOT
utilities operate only their baseload facilities then. In the PSCO context, this
means the coal plants supplemented with some combined-cycle natural
gas and hydro are in operation. ERCOT's baseload includes nuclear, coal,
and combined-cycle plants. The extra emissions result because the RPS-
mandated must-take wind resources exceed the quantity of power generated
from combined cycle gas. PSCO's generation mix from 12:00 to 8:00 a.m. aver-
ages 62% coal, 20% combined cycle, and 18% hydro, wind, and purchases.
ERCOT's corresponding mix is 17% nuclear, 40% coal, 28% combined cycle,
6% combustion turbine, 9% wind, and 0% hydro. 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 sys-
tems to absorb wind without having to cycle their coal plants.
Regulatoryconlict
— The study results suggest that the RPS mandate is
in conflict with the Colorado State Implementation Plan for air emissions.
The RPS standard requires that more wind resources be utilized than can
be offset with lower-emission, natural gas generation equipment. That is the
case today when wind resources account for about 9% of PSCO's total sales.
Wind generation will increase in the coming years due to mandates to move
toward a 30% of total sales standard. 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.
National implications
— Congress is considering legislation that would
mandate a federal RPS. While our study paid only cursory attention to areas
beyond the ERCOT and PSCO territories, it is doubtful that a national RPS
can be imposed without creating the same emissions outcome found in the
