DOE: cycling costs negligible against the benefits of renewables

New research from the U.S. Department of Energy’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 variable wind and solar power, utilities must ramp up and down, or stop and start, conventional generators more frequently to provide reliable power for their customers, NREL noted in a Sept 24 statement.

The study finds that the carbon emissions induced by more frequent cycling are negligible (<0.2%) compared with the carbon reductions achieved through the wind and solar power generation. SO2 emissions reductions due to wind and solar are 5% less than expected because of cycling of fossil-fueled generators. NOx emissions 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 $7bn per year across the West, while incurring cycling costs of $35m to $157m 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 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 concentrations of wind and solar into the western egrid. That first WWSIS study found it to be technically feasible if certain operational changes could be made, but it raised questions about the impact of cycling on wear-and-tear costs and emissions.

Study builds hypothetical scenario with 33% renewables

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 on an annual basis. The study models cycling impacts representing a range of wind and solar concentrations between none and 33%, and is not an endorsement of any particular level, NREL added.

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.

“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.

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.

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.

About Barry Cassell 20414 Articles
Barry Cassell is Chief Analyst for GenerationHub covering coal and emission controls issues, projects and policy. He has covered the coal and power generation industry for more than 24 years, beginning in November 2011 at GenerationHub and prior to that as editor of SNL Energy’s Coal Report. He was formerly with Coal Outlook for 15 years as the publication’s editor and contributing writer, and prior to that he was editor of Coal & Synfuels Technology and associate editor of The Energy Report. He has a bachelor’s degree from Central Michigan University.