How to replace fossil fuels with renewables is the source of much discussion and political strife, particularly when it comes to the NIMBY (Not In My Back Yard) dynamic.  How much landscape will need to be consumed to satisfy our appetite for energy with solar, wind, biomass, and hydro?  How many forests ground up for biomass? How much cropland devoured for ethanol?  How many eyes will have to feast on solar farms and how many mountain tops will we festoon with twirling turbine blades?

Maybe this scary menu can be more easily digested with a bit of quantitative analysis. The space requirements for these options can be calculated accurately enough to give us an idea of the kinds of decisions involved in shifting from our fast food energy diet to the more healthful  renewables.

The USA consumes about 28 trillion kilowatt-hours (kWh) of energy per year. That includes all the uses (housing, electricity, transportation, commerce, manufacturing, imported energy, etc.). About 81% of current energy use in the U.S. is from fossil fuels, including oil, natural gas, and coal.

All our energy except nuclear, tidal, and geothermal ultimately comes from the sun (fossil fuels come from decayed living things formed  by photosynthesis and compressed for 300 million years). But ironically the more direct and immediate manifestations of the sun’s energy – solar, hydro, wind, biomass- take a lot more space than the seemingly indirect fossils do.

While fossil fuels are now beginning to scar the landscape profoundly (mountaintop removal for coal, tar sands destruction of wilderness, fracking throughout public lands), historically one of their chief merits has been their concentrated form. To obtain the same amount of energy from any of the renewables takes more space, but how much more varies greatly among the renewables.

Space requirements of renewables can be determined by analyzing how much energy they can provide per unit of land area per unit of time. As energy, area, and time can be measured in many different units, this can be confusing, but here we will use the kilowatt-hour for energy, the square foot for area, and the year as a time unit, or kWh/sqft/yr.

Since it’s quite tricky to calculate energy per unit of area over time for hydropower due to extreme variation in the geography involved, we will stick to biomass, wind and solar here. The following figures apply:

•  Biomass – wood or corn ethanol 0.1 kWh/sqft/yr

•  Biomass – switchgrass   0.4 kWh/sqft/yr

•  Wind turbines (onshore)   2-3 kWh/sqft/yr

•  Wind turbine (offshore)  3-5 kWh/sqft/yr

•  Solar Photovoltaics 15-20 kWh/sqft/yr

As you can see, a solar panel farm can generate around 200 times the energy of a Maine forest of equal area. In producing electricity from biomass, we lose about two thirds of its energy, making it 600 times more demanding than solar, space-wise.

What are the implications? This shows, for one,  that If we tried to run our country entirely on high yield biomass, it would require 68 trillion square feet or about 2.4 million square miles. That’s about 65% of the entire land area of the country!

The calculations above indicate that the area required to generate all our current power from photovoltaics would be around 60,000 square miles, but due to the efficiencies of electricity as a power source (another, future article), this could actually be reduced to 30,000 sq. miles (Maine’s area is about 35,000 sq. miles). The National Renewable Energy Laboratory did a study that found that there is enough appropriate rooftop area in the US to provide about 10% of that area (nrel.gov).  About half of that is on smaller buildings, typically homes. The rest is on larger rooftops such as apartment buildings and commercial and industrial buildings.

Again based on the numbers above, if we provided all of our energy from wind it would require over 200,000 square miles, almost the size of Texas. However, since the land between turbines can be used for crops, grazing or tree growth the physical footprint of the turbines is only about 5% of that, or about 10,000 square miles.  Then there is the offshore wind potential that takes up no actual land, but is more expensive to install and maintain.

So there we have some rough outlines of the size of the challenge.  But space is not the only consideration in designing our energy-edible landscape.  The “intermittency” problem demands a mix of different sources and storage methods to level off the energy supply. Increasing the efficiency of our energy use through better-insulated homes, heat pumps, electric cars, less food waste, etc. could be used to reduce the area required to satisfy our energy appetite. Look at it as a whole smorgasbord of options.

Thus, good news and bad news for energy-hungry Americans: A quantitative approach to the space demands of renewable energy sources shows that it is complicated.  We as a society may have to make some compromises; that is, SOMEone’s backyard will have to be made available!

But the numbers also show that with a thoughtful process for developing an appropriate mix of renewables – based on analysis of power per area – and a little attention to moderation in our diet, it appears our landscape could accommodate our appetites and perhaps sustain our children’s children too.  Let’s get cooking.

Paul Stancioff, PhD., is a professor of Physics at the University of Maine Farmington who studies energy economics on the side.  He can be reached at pauls@maine.edu.  Cynthia Stancioff, MA, Public Administration, is an amateur naturalist and wordsmith.

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