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Plants and AnimalsSolar Panels Reduce CO2 Emissions More Per Acre Than Trees — and...

Solar Panels Reduce CO2 Emissions More Per Acre Than Trees — and Much More Than Corn Ethanol

Solar Panels Reduce CO2 Emissions More Per Acre Than Trees — and Much More Than Corn Ethanol

Matthew Eisenson
|October 26, 2022

The story below was originally published on the Climate Law Blog from Columbia University’s Sabin Center for Climate Change Law. 

On September 21, the Latest York Times published an essay by Gabriel Popkin titled “Are There Higher Places to Put Large Solar Farms Than These Forests?” Popkin describes a recently approved 4,500-acre solar project in Virginia that may remove roughly 3,500 acres of forest and asks whether such projects could possibly be sited as an alternative on rooftops, parking lots, and other degraded land. This blog post provides some additional information and context to Popkin’s essay.

First, only a really small percentage of solar projects in the US are currently being sited on forested lands. While Popkin appropriately notes that roughly 50% of solar energy facilities, as measured by land area, are sited in deserts, the assertion that “greater than four-fifths of the remainder go on farmland, forestland or grasslands” requires additional context. Specifically, it is necessary to know that solar shouldn’t be being sited in equal quantities on these three sorts of land, as farmland hosts way more solar projects (33%) than either grassland (6%) or forests (4%). For comparison, nearly 3% of solar energy is currently sited in urban areas.

Second, while Popkin appropriately notes that forests, like solar farms, offer climate-change advantages, the essay doesn’t provide any information on the relative emissions advantages of forested land versus solar farms. To be clear, forests offer immense advantages that don’t have anything to do with carbon sequestration, including by serving as critical habitat for native wildlife, filtering drinking water, stopping erosion, and providing scenic and recreational advantages to thousands and thousands of individuals. Nevertheless, on the narrow but vital issue of carbon dioxide emissions, an acre of solar panels appears to offset more emissions every year than an acre planted with trees can sequester. In Virginia, where the first source of electricity is natural gas, the emissions intensity of electricity is 679 kilos of carbon dioxide per megawatt-hour (MWh), not including other greenhouse gases. In accordance with the Lawrence Berkeley National Laboratory, utility-scale solar energy produces between 394 and 447 MWh per acre per 12 months. Thus, an acre of solar panels producing zero-emissions electricity saves between 267,526 to 303,513 kilos, or 121 to 138 metric tons, of carbon dioxide per 12 months. By comparison, in keeping with the EPA, the typical acre of forest in the US sequesters 0.84 metric tons of carbon dioxide per 12 months. Thus, an acre of solar panels in Virginia reduces roughly 144 to 166 times more carbon dioxide per 12 months than an acre of forest.

What concerning the carbon that’s released when an acre of forest is removed? In accordance with the EPA, the typical acre of forest accommodates 81 metric tons of carbon, although the precise figure depends heavily on the species of trees within the forest. Roughly half of that quantity is sequestered within the soil. Even when all 81 metric tons of carbon, comprising 297 metric tons of carbon dioxide, were released upon conversion to a solar farm, those emissions can be offset inside 2-3 years of operation.

Third, Popkin suggests that siting solar projects on farmland may interfere with an “obvious a very important use: growing food.” Nevertheless, solar projects can coexist with and complement agriculture, including by improving pollinator habitat and allowing animals to graze between rows of panels. The Great Plains Institute has found, for instance, that “utility-scale solar could be compatible with other types of non-cultivated agriculture like pasture and grasslands.” As well as, recent research has shown that growing crops, comparable to tomatoes, in between rows of solar panels in hot, dry climates may increase yields by creating shade, which conserves water, increases humidity, and lowers temperatures. Likewise, the Michigan Department of Agriculture and Rural Development has determined that “the position of structures for industrial solar energy generation … is consistent with farming operations,” provided that certain measures are taken to keep up the integrity of agricultural land at the location, including by planting pollinator habitat and conservation cover. As well as, the income that farmers can earn by leasing out parts of their land for renewable energy projects allows them to maintain the remainder of their land in production while insulating them against low harvest years.

Furthermore, any discussion of a purported conflict between agriculture and energy production must also acknowledge that greater than one-third of all corn grown in the US is used not for food—and even to feed livestock—but for energy. In total, greater than 30 million acres of farmland, covering an area roughly the scale of Louisiana, are currently used to grow corn for ethanol. All of that land could possibly be redeployed to solar energy production without affecting food production.

Importantly, converting the land currently used for growing corn ethanol to solar energy would greatly increase the quantity of energy produced on that land. Indeed, an evaluation from PV Magazine recently found that converting the land currently used for corn ethanol to solar energy could meet all the nation’s electricity needs. Likewise, a UK-based evaluation from Carbon Transient found that “a hectare of solar panels delivers between 48 and 112 times more driving distance, when used to charge an electrical vehicle, than that land could deliver if used to grow biofuels for cars.”

Based by myself calculations (below), an acre of solar panels produces roughly 40 times more energy than an acre dedicated to growing corn for ethanol—and that is without bearing in mind the undeniable fact that electric vehicles use energy more efficiently than gas-powered cars:

  • As noted above, solar energy produces between 394 and 447 megawatt hours (MWh) per acre per 12 months.
  • In accordance with the University of Nebraska-Lincoln, one acre of corn produces roughly 462 gallons of ethanol.[2]
  • With a heat content of 76,300 BTU per gallon of ethanol, 462 gallons of ethanol accommodates 35,250,600 BTU.
  • Applying a regular conversion factor of 3,412,000 BTU per MWh, one acre of corn produces a quantity of ethanol similar to 10.3 MWh.
  • Thus, an acre of solar panels produces roughly 38 to 43 times more energy per acre than corn ethanol, even assuming a comparatively high output per acre of corn.

Fourth, as Popkin appropriately acknowledges, rooftops and parking lots are “generally dearer to develop than forest or farmland.” Nevertheless, Popkin doesn’t explain how much dearer it’s to construct solar on rooftops or parking lots. In accordance with the National Renewable Energy Laboratory, the average cost per watt of putting in rooftop solar projects is roughly 1.75-3 times as expensive as utility-scale solar. The typical cost per watt of a utility-scale solar system is $0.89, in comparison with $1.56 for a industrial rooftop project and $2.65 for a residential rooftop project.

Comparison of installation costs of residential solar, commercial rooftop solar, and utility-scale solar.

Comparison of installation costs of residential solar, industrial rooftop solar, and utility-scale solar. Source: National Renewable Energy Laboratory

Constructing solar canopies over parking lots also appears to be dearer than utility-scale solar. The industry publication PV Magazine has used $3 per watt as a back-of-the-envelope figure, while Energy Sage has estimated, based on data from its solar energy marketplace, that the typical installation cost is $3.31 per watt. To offer one real-world example, the 12.3-megawatt solar cover under construction at JFK International Airport will cost $56 million, or $4.55 per watt. While the development costs of solar canopies could also be offset in some cases by charging a premium for the shaded parking spots underneath, it is going to be tougher to recoup such costs in places where parking is free. And these are only the installation costs; it is usually dearer to keep up small, widely dispersed units than one large system.

Comparison of operations and maintenance costs of residential, commercial, and utility-scale solar.

Comparison of operations and maintenance costs of residential, industrial, and utility-scale solar. Source: National Renewable Energy Laboratory 

Ultimately, achieving net-zero carbon dioxide emissions by the early 2050s to limit warming to 1.5 degrees Celsius would require siting an unprecedented variety of renewable energy facilities in a really short time. At the moment, siting solar projects on forested land stays relatively rare; within the rare instances when solar is sited on forested land, those projects appear to offset more emissions on a per-acre basis than trees can sequester; the 30 million acres of farmland which might be currently getting used to provide corn ethanol could produce far more energy as solar farms without affecting food production; and utility-scale solar projects remain significantly cheaper to put in and maintain than rooftop and car parking zone solar projects.

Matthew Eisenson works on the Renewable Energy Legal Defense Initiative on the Sabin Center for Climate Change Law


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