Large-scale wind energy has its drawbacks – Harvard Gazette

In terms of energy production, the free lunch does not exist unfortunately.

As the world begins its large-scale transition to low-carbon energy sources, it is essential that the advantages and disadvantages of each type be well understood and that the environmental impacts of renewable energies, however small, be they in relation to coal and gas, are considered.

In two articles published today in the Environmental Research Letters and Joule journals, researchers at Harvard University have discovered that the transition to wind or solar power in the United States would require five to twenty times more land than expected, and the farms were built, would warm the average surface temperatures on the American continent by 0.24 degrees Celsius.

"The wind beats coal, regardless of the environmental action taken, but that does not mean its impacts are negligible," said David Keith, professor of applied physics at the Gordon McKay Chair of SEAS at Harvard School of Medicine. Engineering and Applied Sciences (SEAS). principal author of articles. "We need to move quickly away from fossil fuels to stop carbon emissions. In doing so, we must choose between different low-carbon technologies, all of which have social and environmental impacts. "

Keith is also a professor of public policy at Harvard Kennedy School.

To understand the impact of renewable technologies on the environment, one must begin by understanding how much land would be needed to meet the future energy demand of the United States. Even from today's energy demand, energy experts have long discussed the area and the associated power density.

In previous research, Keith and his co-authors modeled large-scale wind farms' production capacity and concluded that real-world wind energy production had been overestimated because they did not accurately capture interactions between turbines and the atmosphere.

In 2013, Keith explained how each wind turbine creates a "wind shadow" behind the air when the air has been slowed down by the blades of the wind turbine. Today's commercial-scale wind farms carefully manage space turbines to reduce the impact of these wind shadows, but with the hope that wind farms will continue to grow as demand grows. wind-generated electricity will increase, interactions and related weather effects can not be avoided.

What was missing from this previous research, however, were observations to support modeling. Then, a few months ago, the US Geological Survey released the locations of 57,636 wind turbines around the United States. Using this dataset, coupled with several other US government databases, Keith and his postdoctoral researcher Lee Miller were able to quantify the power density of 411 wind farms and 1,150 solar PV plants in operation in the United States in 2016.

"With respect to wind energy, we found that the average power density, that is the rate of energy production divided by the total area of ​​the wind turbine, was up to 100 times lower than the estimates of some energy experts, "said Miller, the first author of both papers. "Most of these estimates did not take into account the turbine-atmosphere interaction. For an isolated wind turbine, the interactions are not at all important, but once the wind farms have a depth of more than five to ten kilometers, these interactions have a major impact on the power density. "

Wind energy densities based on observation are also well below the significant estimates of the US Department of Energy and the Intergovernmental Panel on the Evolution of Energy. climate.

For solar energy, the average power density (measured in watts per square meter) is 10 times greater than wind energy, but also well below estimates by leading energy experts.

This research suggests that not only will wind farms need more land to meet the proposed renewable energy targets, but they will also become, on such a large scale, an active player in the climate system.

The next question, explored in the Joule review, was what impact such large scale wind farms would have on the climate system.

To estimate the impacts of wind energy, Keith and Miller established a baseline for the 2012-2014 US climate using a standard weather forecast model. Then they covered one-third of the continental United States with enough wind turbines to meet current US electricity demand. The researchers found that this scenario would warm the surface temperature of the continental United States by 0.24 degrees Celsius, with the largest changes occurring at night when surface temperatures increased to 1.5 degrees Celsius. This warming is the result of wind turbines that actively mix the atmosphere close to the ground and at altitude while escaping the movement of it.

This research supports more than 10 other studies that have observed warming near operational US wind farms. Miller and Keith compared their simulations to satellite observational studies in North Texas and found relatively constant temperature increases.

Miller and Keith quickly point out the disadvantage that the United States produces as much wind energy as it simulates in its scenario, but localized warming occurs in even smaller projections. The next question is to understand when the growing benefits of reducing emissions are about equal to the near-instantaneous impacts of wind energy.

Harvard researchers have found that the warming effect of wind turbines in the continental United States is actually more important than the effect of reducing emissions for the first century of operation. Indeed, the warming effect is primarily located in the wind farm, while greenhouse gas concentrations must be reduced globally before the benefits materialize.

Miller and Keith repeated the calculation for solar energy and found that its climate impacts were about 10 times less than those of wind energy.

"The direct impacts of wind energy on the climate are instantaneous, while the benefits of reducing emissions are slowly accumulating," Keith said. "If you are considering the next 10 years, wind energy has, in some ways, more impact on the climate than coal or gas. If you are considering the next thousand years, wind energy has a lot less impact on the climate than coal or gas.

"The work should not be considered as a fundamental criticism of wind energy," he said. "Some of the impacts of the wind on the climate will be beneficial – several global studies show that wind energy is cooling the polar regions. The work should rather be considered as a first step to become more serious in assessing these impacts for all renewable energies. Our hope is that our study, combined with recent direct observations, marks a turning point in which the climate impacts of wind energy begin to be seriously taken into account in strategic decisions regarding the decarbonization of the energy system.

This research was funded by the Fund for Innovative Research on Climate and Energy.