One claim against solar and wind power is that they cause fossil fuel power plants to release significantly more emissions as they cycle electric production up and down to balance the load of electricity on the grid. A new study from the National Renewable Energy Laboratory (NREL) essentially calls that concern hogwash and dispels the myth — again. It’s the latest positive news for the wind and solar industries.
The new study, Phase 2 of the Western Wind and Solar Integration Study (WWSIS-2), finds that when emissions related to cycling of power plants for wind and solar power are compared to the emissions without renewable energy, wind and solar offer significant reductions in overall emissions. Carbon emissions induced by more frequent cycling of fossil fuel power plants are negligible at less than 0.2 percent. However, the study also finds that sulfur dioxide emissions reductions from wind and solar are 5 percent less than expected due to the cycling of fossil-fueled generators. Emissions of nitrogen oxides are reduced 2 percent more than expected, it states. So, there are some slight increases in emissions during the ramping up and down of fossil fuel plants.
More important are the overall reductions in emissions that would be caused by higher levels wind and solar electricity. The study finds that the high wind and solar scenarios (33 percent of electric generation) reduce carbon dioxide emissions by 29 percent to 34 percent across the Western Interconnection, which ranges from the Western tip of Texas to California and up into the Canadian provinces of British Columbia and Alberta.
While sulfur dioxide emissions were higher than expected because of cycling, the study still found that overall emissions were reduced by 14 percent to 24 percent in the high scenarios. Nitrogen oxides (NOx) are reduced more than expected by cycling — between 16 percent to 22 percent 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,” NREL says.
“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," says Debra Lew, NREL project manager for the study. "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 percent."
It should also be noted that cycling power plant production up and down is nothing new. "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," Lew explains. "Increased cycling to accommodate high levels of wind and solar generation increases operating costs by 2 percent to 5 percent 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."
High levels of wind and solar power — up to 33 percent in the study — 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, according to the study. That equates to added operations and maintenance costs between 47 cents to $1.28 per megawatt-hour (MWh) of generation for the power plants. As the costs of solar continue to come down, the costs-savings in terms of reduced fuel use will be greater as well. And NREL finds that solar could be cost-competitive with natural gas as soon as 2025 — without subsidies.
"From a system perspective, high proportions of wind and solar result in lower emissions and fuel costs for utility operators," Lew says. "The potential cycling impacts offset a small percentage of these reductions."
The newly released study is a follow-up to the first study, which was released in May 2010. The first study evaluated the feasibility of integrating high levels of wind and solar power into the western electricity grid, and raised questions about emissions and wear and tear costs. The 2010 study also found that 4 MWhs of renewables will replace 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,” the study says.
The research was supported by the Department of Energy’s Office of Energy Efficiency and Renewable Energy and the Office of Electricity Delivery and Energy Reliability. The study was undertaken by NREL, GE, Intertek-APTECH, and RePPAE. The study was reviewed by utilities companies, researchers, analysts, and other experts.
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