A study of the Illinois electrical grid by the Argonne National Laboratory has found that while adding wind power will reduce carbon emission, there will be diminishing returns resulting from the need to keep ramping fossil fuel plants up and down to compensate for wind’s intermittency.

The study was undertaken in conjunction with the State of Illinois’ mandate to power the grid with 25 percent renewable resources by 2025. Currently renewables constitute only 2.2 percent of the grid. The blue block represents overall carbon emissions in millions of metric tons, scaled on the left side. The green ascending line represents the percentage of electricity supplied by wind, which is expected to be the main source of renewable electricity. The descending black line represents the total output of thermal power plants – mostly coal- and gas-fired, which will be required to maintain grid power. This line descends from the present to 2025 as the line for wind ascends, so that wind is taking its place.

The red band represents the extra carbon emissions that will come from firing coal and gas plants up and down as wind becomes more prominent on the grid. The band gradually doubles over the course of the 15 years. Argonne attributes this to the extra emissions that will come from running thermal plants at less than optimum power. When thermal plants are being brought up to full power they use more energy and produce more emissions because some heat is required simply to warm up the boiler. Argonne compares it to running a car engine in stop-and-go traffic. More emissions are produced since the engine is not always operating at optimum levels.

The line does produce some diminishing returns. But the overall effect is to reduce carbon emissions. In this model, Argonne predicts they will fall from 43 to 29 million metric tons. Very little of this is based on practical experience, however. The figure could be much more and it could be much less.