Air Emissions Due To Wind And Solar Power

Renewables portfolio standards (RPS) encourage large-scale deployment of wind and solar electric power. Their power output varies rapidly, even when several sites are added together. In many locations, natural gas generators are the lowest cost resource available to compensate for this variability, and must ramp up and down quickly to keep the grid stable, affecting their emissions of NO x and CO2. We model a wind or solar photovoltaic plus gas system using measured 1-min time-resolved emissions and heat rate data from two types of natural gas generators, and power data from four wind plants and one solar plant. Over a wide range of renewable penetration, we find CO2 emissions achieve ∼80% of the emissions reductions expected if the power fluctuations caused no additional emissions. Using steam injection, gas generators achieve only 30−50% of expected NO x emissions reductions, and with dry control NO x emissions increase substantially. We quantify the interaction between state RPSs and NO x constraints, finding that states with substantial RPSs could see significant upward pressure on NO x permit prices, if the gas turbines we modeled are representative of the plants used to mitigate wind and solar power variability.