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NREL: Utility ‘Cycling’ Causes Negligible CO2 Emission Increase

September 24, 2013 By Leon Walker

NREL

The carbon emissions induced by utilities ramping up or down generators to accommodate fluctuations in the amount of wind or solar power being fed to the grid are “negligible” when compared to the emissions avoided by the use of such renewable power, according to research from the Energy Department’s National Renewable Energy Laboratory.

According to phase two of the Western Wind and Solar Integration Study, wind- and solar-induced cycling will cause CO2 emissions reductions to fall by less than 0.2 percent. Sulfur dioxide emissions reductions from wind and solar are 5 percent less than expected because of cycling of fossil-fueled generators. Emissions of nitrogen oxides are reduced 2 percent more than expected, the study says.

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, or $35 million to $157 million per year. However, 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. 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. The study’s simulations show that from a system perspective, avoided fuel costs are far greater than the increased cycling costs for fossil-fueled plant.

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, the study says.

By contrast, 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.

In August, NREL published a report on the land use requirements of solar power plants based on actual land-use practices from existing solar facilities.

The report, Land-use Requirements for Solar Power Plants in the United States, shows results from data gathered from 72 percent of the solar power plants installed or under construction in the United States. Among the findings:

  • A large fixed tilt photovoltaic plant that generates 1 GWh per year requires, on average, 2.8 acres for the solar panels. This means that a solar power plant that provides all of the electricity for 1,000 homes would require 32 acres of land.
  • Small single-axis PV systems require on average 2.9 acres per annual GWh – or 3.8 acres when considering all unused area that falls inside the project boundary.
  • Concentrating solar power plants require on average 2.7 acres for solar collectors and other equipment per annual GWh; 3.5 acres for all land enclosed within the project boundary.


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