Fossil fuels, which we will need to replace with sustainable energy sources, developed over millions of years. As a result, fossil fuel energy is extraordinarily concentrated. Consequently, the amount of land area required to produce a given amount of energy will be less with fossil fuels and greater with renewables.

Vaclav Smil, of the University of Manitoba, has looked at the power density of coal mines and hydrocarbon fields and has concluded that their power densities are commonly 102 or 103 W/m

2, while the power density of biomass energy is well below 1 W/m2Here’s a list of power densities for various fuels.

Smil has also computed power densities for electrical generation plants based on different fuels.

In the previous article we talked about capacity factors. A plant’s capacity factor is the percentage of the power a plant produces over the course of a year divided by how much it theoretically can produce. When power densities are computed, capacity factor needs to be considered.

Using several different scenarios, he calculates that current coal-fired power plants have power densities that vary from 100 – 1000 W/m

2. America’s largest coal-fired electricity generating plant, Robert W. Scherer in Georgia has an installed capacity of about 3.5 GW, and produces its power using about 3500 acres of the earth’s surface. With a capacity factor of 75 percent, it has a power density of about 175 W/m².

We can use this information to compare various alternative energy sites around the world and can evaluate power densities for different technologies.

Olmedilla de Alarcón, the world’s largest solar photovoltaic facility in Spain, has an installed, or nameplate, capacity of 60 megawatts of peak power, but actually averages 9.7 megawatts. Its capacity factor is 16 percent. Solar plant capacity factors vary with available sunshine, of course, so Portugal’s Moura plant does better at 22 percent and Germany’s largest, Waldpolenz, is lower at 11 percent. The best power density of these three facilities is Olmedilla at 9 W/m

2.

How about concentrated solar? Planta Solar, located near Seville, is composed of plants PS10 and PS20. Their capacity factors are 25 percent and 28 percent and their overall power densities are 4 W/m² and 6 W/m² respectively.

Current wind power plants are averaging about 30 percent capacity factor and that number is expected to rise with larger and higher windmills. Unfortunately, larger windmills require more space in between them in order to minimize wake turbulence, so power densities of .5 to 2 percent are common. Right now, facilities worldwide generally use 30-70 acres per megawatt, so a wind farm capable of replacing the Scherer plant’s 2.45 GWs would need 30,000 to 70,000 acres. Compare this with the Scherer plant’s 3500 acres.

Similar calculations for wood-fired power plants show an even worse energy density of.06 W/m².

The difference in power densities translates to huge increases in land use devoted to energy.

Using this and similar information, the Nature Conservancy has looked at the effect of current and likely future U.S. energy policy and has concluded that at least 206,000 km

2, or nearly 80,000 square miles, will be impacted as we transition to more diffuse power sources.

80,000 square miles is nearly the land area of Kansas.

Want to save some electricity? Change your shower head. According to Greenpeace, a water-saving shower head can save you nearly 1500 kWh a year.