The potential impacts of increasing wind and solar power generation on fossil-fueled power plants in the West


NREL conducts research to assess potential impacts of increasing wind and solar power generation on the operators of fossil-fueled power plants in the West. 

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.

According to the study, carbon emissions induced by more frequent cycling are negligible at 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 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.”

According to the study, on average, 4MWh of renewables displace 1MWh of coal generation and 3MWh 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.

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 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.

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.

The study can be downloaded at