A recent analysis has generated some press interest for its conclusion that adding current levels of wind energy to the grid yields even greater reductions in emissions of harmful pollutants than expected, but that at levels of wind energy several times higher than are on the grid today, the incremental pollution savings of adding wind energy to the grid are somewhat smaller than they are at lower levels of wind. Unfortunately, this study’s findings have been misreported in the press, so we’d like to set the record straight:
– Much of the press coverage of this study is incorrectly reporting that the study finds that wind energy does not reduce pollution, or that the pollution savings are always smaller than expected. The study is explicitly clear that neither of those interpretations is correct. In reality, the report finds that wind’s emissions savings are currently larger than expected (12% carbon dioxide emissions savings with 10% of the electricity on the grid coming from wind, 21% carbon dioxide emissions savings at 20% wind), and only at wind levels many times higher than today’s do the incremental pollution savings of adding wind energy begin to decrease somewhat. However, for the reasons explained below, there is considerable doubt about the real-world validity of the second part of the report’s findings (that wind’s incremental emissions savings decline at very high levels of wind).
– The study acknowledges that its findings are a theoretical exercise based on the assumption that power plants in Illinois are operated in isolation from those in other states, and as a result the study’s conclusions have little to no bearing on how the actual utility system works, particularly at high levels of wind generation. As the study notes, “the analysis in this paper is limited to the state of Illinois… .” In reality, power plants in Illinois are operated as part of two massive interstate electric utility systems covering parts of 23 states and Manitoba, with the Midwest Independent System Operator (MISO) operating nearly all power plants in parts of 12 states stretching from Illinois to eastern Montana down to Missouri, and the PJM grid operator controlling Chicago-area power plants as well as power plants stretching east to New Jersey and down to Virginia. In contrast, the study assumed that grid operators only had the ability to control power plants in Illinois.
This causes the study to understate the pollution reduction benefits of large amounts of wind energy in several ways. Most importantly, the study assumed that at high levels of wind energy output in Illinois, grid operators would be forced to reduce the output of the state’s very large nuclear fleet, thereby resulting in no emissions savings at high levels of wind output. A chart in the study shows that it assumes that Illinois’s nuclear plants have their output reduced by over 10% as wind energy goes from providing 20% of the state’s electricity to 40% of its electricity. Because nuclear energy provides about half of Illinois’s electricity, this assumed reduction in nuclear output greatly erodes wind’s emissions savings; this explains why the study indicated wind’s emissions savings dropped off once wind energy reached the 20% level.
In the real world, those nuclear plants would likely never see their power output reduced, because that wind and nuclear power would be shipped out of state on the large power lines connecting Illinois to the other 23 states in MISO and PJM, allowing grid operators to reduce the output of more expensive fossil-fired power plants in those states. Grid operators use zero-fuel-cost wind energy to reduce the output of the highest fuel cost power plants first, which are almost always fossil-fired power plants. Low-fuel-cost nuclear power plants would only have their output reduced after the thousands of fossil-fired power plants across the entire 23-state region had their output reduced to zero, something that has never happened and never will for the foreseeable future. Instead, wind energy would allow grid operators to reduce the output of fossil-fired power plants in other states, thereby achieving the pollution reductions that are expected from adding wind energy. Moreover, had the study looked at nearly any state other than Illinois, it likely would have found that wind energy achieved or exceeded the expected emissions savings, as most states do not obtain half of their electricity from nuclear power.
By looking at Illinois in isolation, the study also overlooks another important aspect of how wind energy is reliably and efficiently integrated onto the power system today. There has always been a massive amount of variability and uncertainty on the electric utility system, as factories suddenly come on and offline, millions of people turn air conditioners and other appliances on and off, and conventional power plants unexpectedly break down. Most of these sources of variability cancel each other out, and grid operators have always used flexible power plants and exchanges with neighboring power systems to deal with the remaining variability. In the same way, most of the variability in wind energy output is canceled out by opposite changes in the output at other wind plants or by changes in electricity demand. By only looking at Illinois in isolation, the study ignores the fact that much of the variability in wind plant output would be canceled out by changes at wind plants in other states or by random changes in electricity demand in other states. In reality, grid operators in Illinois would also be able to exchange power with neighboring states to deal with that variability, and many times would be able to rely on power plants in other states that can change their output more efficiently than those in Illinois could. Finally, the study appears to have inaccurately modeled the variability of wind energy by assuming that all future wind plants in Illinois would have the same output characteristics as 15 small wind plants, even though dozens of studies have demonstrated that as more wind plants are added to the grid, their total output becomes much smoother as they are spread over a larger geographic area.
– The study also acknowledges that it uses very outdated and unreliable estimates for making assumptions about the efficiency of fossil-fired power plants at different output levels. The two main sources are rough estimates from documents from 1989 and 1993, which do not appear to be based on real-world data. As a result, these estimates ignore the fact that the current fleet of power plants is likely to be far more flexible and efficient than typical power plants were more than 20 years ago, particularly with a large influx of highly flexible natural gas-fired plants over the last 10-15 years. As the study itself notes, “We recognize that the limited data availability on start-up emissions adds uncertainty to the estimates outlined above,” and “We would like to emphasize that the accuracy of the results are limited by the assumptions used in the analysis.”
– Other analyses using more accurate assumptions and more reliable sources have found that wind’s emissions savings are as large or larger than expected. A recent analysis using real-world data derived from EPA emission monitors found that in an absolute worst case, wind energy achieves 98.3% of the expected carbon dioxide emissions savings, and 103.3% of the expected nitrogen oxide emissions savings. An ongoing phase of that analysis, due to be completed within the next several months, is likely to show that wind’s net emissions savings are even larger than expected. This result occurs because wind energy tends to disproportionately displace dirtier and less flexible coal generation instead of more flexible natural gas generation, so any slight decrease in power plant efficiency is more than offset by this additional emissions savings. This result was also found in the Argonne analysis, which noted that “…increasing wind generation leads to a shift in dispatch from coal toward natural gas,” though those emissions savings were masked by the larger impact of the incorrect assumption that wind energy would displace nuclear generation.
– Real-world data confirms that states that have added significant amounts of wind energy, such as Illinois, have seen fossil fuel use and emissions decline by as much as or more than expected. Department of Energy data for Colorado show that as wind energy jumped from providing 2.5% of the state’s electricity in 2007 to 6.1% of the state’s electricity in 2008, carbon dioxide emissions fell by 4.4%, nitrogen oxide and sulfur dioxide emissions fell by 6%, coal use fell by 3% (571,000 tons), and electric-sector natural gas use fell by 14%. DOE data for Texas show that as wind and other renewables’ share of Texas’s electric mix increased from 1.3% in 2005 to 4.4% in 2008, an increase in share of 3.1 percentage points. During that period, electric sector carbon dioxide emissions declined by 3.3%, even though electricity use actually increased by 2% during that time. Because of wind energy, the state of Texas was able to turn what would have been a carbon emissions increase into a decrease of 8,690,000 metric tons per year, equal to the emissions savings of taking around 1.5 million cars off the road. Similarly, thanks to the growth of wind energy in the state, Illinois saw a 0.5% decrease in CO2 emissions from 2006 to 2009, even though electricity use actually increased by 0.75% over that time period. In Minnesota, as wind energy grew from providing less than 4% of the state’s electricity in 2006 to almost 10% in 2009, electric sector carbon dioxide emissions fell by more than 10%, or 4 million metric tons per year.
As further evidence, four of the seven major independent grid operators in the U.S. have studied the emissions impact of adding wind energy to their power grids, and all four have found that adding wind energy drastically reduces emissions of carbon dioxide and other harmful pollutants. While the emissions savings depend somewhat on the existing share of coal-fired versus gas-fired generation in the region, as one would expect, it is impossible to dispute the findings of these four independent grid operators that adding wind energy to their grids has significantly reduced emissions. The results of these studies are summarized below.
Independent Grid Operators’ Calculations of Wind’s Emissions Savings
Finally, analysis of readily available DOE data puts to rest the idea that wind energy has a significant negative impact on the efficiency of fossil-fired power plants. The Department of Energy collects detailed data on the amount of fossil fuels consumed at power plants, as well as the amount of electricity produced by those power plants. By comparing how the efficiency of power plants has changed in states that have added significant amounts of wind energy against how it has changed in states that have not, one can test the hypothesis that wind energy is having a negative impact on the efficiency of fossil-fired power plants. The data clearly shows that there is no such relationship, and in fact states that use more wind energy have seen greater improvements in the efficiency of their fossil-fired power plants than states that use less wind energy. Specifically, coal plants in the 20 states that obtain the most electricity from wind saw their efficiency decline by only 1.00% between 2005 and 2010, versus 2.65% in the 30 other states. Increases in the efficiency at natural gas power plants were virtually identical in the top 20 wind states and the other states, at 1.89% and 2.03% improvements respectively. The conclusion that adding wind energy actually increases fossil plant efficiency makes intuitive sense, because as explained above, adding wind energy to the grid displaces the output of the most expensive, and therefore least efficient, fossil-fired power plants first.
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