Preface: From time to time, un-informed commenters on WUWT,
WattsUpWithThat.com, write that wind power integrated into a utility grid actually
increases overall emissions of CO2 and true pollutants such as SOx and NOx.
My engineering colleagues and I have long known that to be false. This 2013 government study by NREL shows
that claim to be false. From the
article, “the study finds that the high wind and solar scenarios reduce CO2
emissions by 29% --34% across the Western Interconnection. . .” because “adding
wind and solar to the grid greatly reduces the amount of fossil fuel – and associated
emissions – that would have been burned to provide power. . .” The “high wind and solar scenarios” have
one-fourth of the energy in the entire Western grid provided by wind and solar
sources. (emphasis added)
For
perspective, Iowa had approximately 25 percent, and California had 4.5 percent
of its in-state power generated from wind in 2012. (end preface)
NREL
Calculates Emissions and Costs of Power Plant Cycling Necessary for Increased
Wind and Solar in the West
September 24, 2013
New research from the Energy Department’s National Renewable
Energy Laboratory (NREL) quantifies the potential impacts of increasing wind
and solar power generation on the operators of fossil-fueled power plants in
the West. To accommodate higher amounts of wind and solar power on the electric
grid, utilities must ramp down and ramp up or stop and start conventional
generators more frequently to provide reliable power for their customers – a
practice called cycling.
The study finds that the carbon emissions induced by more frequent
cycling are negligible, less than 0.2%, compared with the carbon reductions achieved through the
wind and solar power generation evaluated in the study. Sulfur dioxide
emissions reductions from wind and solar are 5% less than expected because of
cycling of fossil-fueled generators. Emissions of nitrogen oxides are reduced
2% more than expected. The study also finds that high levels of wind and solar
power would reduce fossil fuel costs by approximately $7 billion per year
across the West, while incurring cycling costs of $35 million to $157 million
per year. For the average fossil-fueled plant, this results in an increase in
operations and maintenance costs of $0.47 to $1.28 per megawatt-hour (MWh) of
generation.
“Grid operators have always cycled power plants to accommodate
fluctuations in electricity demand as well as abrupt outages at conventional
power plants, and grid operators use the same tool to accommodate high levels
of wind and solar generation,” said Debra Lew, NREL project manager for the
study. “Increased cycling to accommodate high levels of wind and solar
generation increases operating costs by 2% to 5% for the average fossil-fueled
plant. However, our simulations show that from a system perspective, avoided
fuel costs are far greater than the increased cycling costs for fossil-fueled
plants.”
Phase 2
of the Western Wind and Solar Integration Study (WWSIS-2) is
a follow up to the WWSIS released in May 2010, which examined the viability,
benefits, and challenges of integrating high levels of wind and solar power
into the western electricity grid. WWSIS found it to be technically feasible if
certain operational changes could be made, but the first study raised questions
about the impact of cycling on wear-and-tear costs and emissions.
To calculate wear-and-tear costs and emissions impacts for the new
study, NREL designed five hypothetical scenarios to examine generating up to
33% wind and solar energy on the U.S. portion of the Western Interconnection
power system for the year 2020. This is equivalent to a quarter of the power in
the Western Interconnection (including Canada and Mexico) coming from wind and
solar energy on an annual basis. The study models cycling impacts representing
a range of wind and solar energy levels between none and 33%, and is not an
endorsement of any particular level.
The study assumes a future average natural gas price of
$4.60/MMBtu, significant cooperation between balancing authorities, and optimal
usage of transmission capacity (i.e., not reserving transmission for
contractual obligations). NREL modeled operations of the entire Western
Interconnection for that year in five-minute intervals to understand potential
impacts within every hour. With these assumptions, the study finds that the
high wind and solar scenarios reduce CO2 emissions by 29%–34% across the
Western Interconnection, with cycling having a negligible impact.
Cycling lessens the SO2 benefit by 2%–5%, so that SO2 emissions
are reduced by 14%–24% in the high scenarios. These impacts are modeled on an
overall Western Interconnection level, and changes on a regional basis could
vary. Further, the study does not examine cycling impacts on mercury and air
toxic control equipment now being retrofitted on coal units to comply with
recent EPA regulations.
Cycling actually improves the NOx benefit by 1%–2%, so that NOx emissions are reduced by 16%–22% in the high scenarios. This is because the average coal plant in the West has a lower NOx emissions rate at partial output than at full output.
"Adding wind and solar to the grid greatly reduces the amount
of fossil fuel — and associated emissions — that would have been burned to
provide power,” Lew said. “Our high wind and solar scenarios, in which
one-fourth of the energy in the entire western grid would come from these
sources, reduced the carbon footprint of the western grid by about one-third.
Cycling induces some inefficiencies, but the carbon emission reduction is
impacted by much less than 1%.”
WWSIS-2 does not consider other factors such as capital costs of
construction for wind, solar, fossil-fueled power plants, or transmission.
These costs are significant, but outside the scope of this study, which focuses
on operations.
“From a system perspective, high proportions of wind and solar
result in lower emissions and fuel costs for utility operators,” Lew said. “The
potential cycling impacts offset a small percentage of these reductions.”
According to the study, on average, 4 MWh of renewables displace 1
MWh of coal generation and 3 MWh of natural gas. The biggest potential cycling
impact is the significant increase in ramping of coal units. Other findings
include:
- Because of sunset and sunrise,
solar power creates the biggest ramping needs on the grid in this study.
However, because we know the path of the sun through the sky every day of
the year, system operators can predict these large ramping needs and plan
accordingly. Solar variability due to fast-moving clouds is much less
predictable, but it creates relatively smaller ramping needs.
- Errors in day-ahead wind forecasts
can make it challenging for operators to decide which power plants need to
be online the next day. However, because forecast accuracy increases four
hours ahead compared with 24 hours ahead, a four-hour-ahead decision on
whether to start up those power plants that can be ramped up relatively
quickly can help to mitigate these forecast errors.
- Despite the differences between
wind and solar in terms of grid operations, the study finds their impacts
on system-wide operational costs are remarkably similar.
WWSIS-2 was supported by the Energy Department’s Office of Energy
Efficiency and Renewable Energy, as well as its Office of Electricity Delivery
and Energy Reliability. The study was undertaken by NREL, GE, Intertek-APTECH,
and REPPAE, and underwent a rigorous technical review process that included
utilities, researchers, and analysts. The study can be downloaded at www.nrel.gov/wwsis.
NREL is the U.S. Department of Energy's primary national
laboratory for renewable energy and energy efficiency research and development.
NREL is operated for the Energy Department by The Alliance for Sustainable
Energy, LLC.
The news release is at http://www.nrel.gov/news/press/2013/3299.html
Roger E. Sowell, Esq.
Marina del Rey, California
Roger E. Sowell, Esq.
Marina del Rey, California
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