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# Calculating Solar Energy's Land-Use Footprint

New statistics from the National Renewable Energy Laboratory (NREL) reveal exactly how much land is needed to site a solar plant of various sizes and technologies, based on actual plants and projects and not models or projections. The takeway: your mileage may vary.

NREL's previous estimates and calculations of solar energy's land-use requirements, published several years ago, suggested that it could meet the U.S.' total electric demand (circa 2005 levels) with a footprint of about 0.6 percent of the nation's total land area, or somewhere around 14-15 million acres.

Now, though, NREL has pooled data from more than two-thirds (72 percent) of solar photovoltaic (PV) and concentrated solar thermal (CSP) power plants already installed or being built across the country, as of 3Q12 data from SEIA: 2.1 GW (AC) of generation capacity and 4.6 GW (AC) under construction. Not surprisingly, they determined the required footprint varies widely depending what solar energy technology is applied, weighing between and how one calculates the "direct impact" (physical infrastructure development) vs. "total" area impacted including the surrounding land.

Not surprisingly, they found a wide range of total land-use requirements depending on the type of solar technology and systems deployed at a site. Overall, generation-weighted solar power plants require on average a total of 3.5 acres/GWh/year, ranging from 3 acres/GWh/year (CSP towers) to 5.5 acres/GWh/year (small 2-axis flat-panel PV). Direct land impacts on a generation-weighted basis 2.9 acres/GWh/year. On a capacity-weighted basis, total land requirements average out to 8.9 acres/MWac, and 7.3 acres/MWac for direct land use. Redefining its calculations, NREL determines that a large fixed-tilt solar PV plant requires 2.8 acres per GWh/year of generation. Put another way, a PV plant spanning 32 acres could power 1,000 households.

Summary of land-use requirements for PV and CSP projects in the U.S. Credit: NREL

A few studies in recent years have attempted to compare multiple energy generation sources solely by land use requirements:

• Broad examinations of land-use requirements across various energy generation sources were performed several years ago, most notably by Columbia U.'s Vasilis Fthenakis et al (summed up in their Figure 1.6), and by Clinton Andrews et al. for the Lincoln Institute of Land Use (specifically Figure 5.1). One of the takeaways from the Fthenakis study: on a life-cycle electricity-output basis, direct and indirect land impacts for utility-scale PV in the U.S. Southwest are less than the average U.S. power plant using surface-mined coal.
• The Geothermal Energy Association looks at land use requirements over a 30-year period for a mix of energy generation sources, finding that geothermal rates the best.

Such comparisons are tricky, though, with baseline definitions hard to normalize. Energy options are site-dependent and restricted to certain locations or resource areas; other factors impacting land-use requirements include the specific technologies deployed, from solar cell types to tracking systems to inclusion of energy storage. Just including the physical footprint of actual energy generation, a spread-out utility-scale solar plant is comparably land-intensive vs. a standalone coal plant, if one removes the mining aspect from the equation. Nuclear plants have the smallest land-requirement footprint for their energy output. But what about the lands impacted from a power plant's operation (i.e. pollution, or resulting waste)? How should we add the impact from transportation requirements, roads and rails? And perhaps a bigger land-use consideration is, what is required for transmission & interconnection to get the energy to those who use it? Not to mention that land use is only one of many factors when determining siting for any power plant.

In the end, NREL's new numbers about solar land-use aren't meant to say that it is better, or worse, than other energy generation sources; it's really just to help advance the discussion with more reliable data. "All these land use numbers are being thrown around, but there has been nothing concrete," stated Sean Ong, lead author of the new NREL report. "Now people will actually have numbers to cite when they conduct analyses and publish reports."

Read more solar energy news here.

Lead image: Footprint made up of stones on a sandy background, via Shutterstock

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