The Dimensions of the Challenge

The purpose of this page is to compare the different non-fossil energy sources that have been proposed with respect to their capabilities. Where appropriate, we will compare the land areas required for each with the land area available. This comparison applies only to the United States.

Electricity

First, consider the amount of electricity the US uses, a total of just over 4 billion MWH/year.[43]

Wind

What really limits wind power is the small amount of storage available; hydroelectric dams can treat a small part of their capacity as short-term storage for wind power. But for the purpose of this calculation, we shall pretend that the limitation doesn't apply.

Currently, typical wind-turbines on wind farms are sized at 1.5 MW, with a rotor-tip height of 450 feet and a rotor diameter of 231 feet.[53][54]. Allowing a generous load factor of 0.35 [17], each turbine yields 4602 MWH/year, so 869,000 turbines would be required. The minimum turbine spacing recommended is five times the rotor diameter [55], so each 1.5 MW turbine requires (5 X 231 ft)^2 = 1,334,025 square feet, or 0.048 square miles, or 30 acres. To reduce the interference between them, clustered turbines require 60 acres each, or 0.096 sq miles.

To provide all the electricity the US uses would require more than 83,440 square miles. That would be a strip of land 80 miles wide running from the Montana/Canada border to the Arizona/Mexico border.

Solar

Solar energy has the same storage limitations as wind power, but we still shall pretend that the limitation doesn't apply.

For the US, an average insolation would be around 5.5 KWH/m^2/day[45], or 2 MWH/m^2/year. Allowing a generous 20% efficiency[46], the output would be 0.4 MWH/m^2/year. To provide all the electricity the US uses would require 10 billion square meters or 3861 square miles of solar panels. That would be a panel 1-1/2 miles wide running from San Diego to Boston.

Nuclear

Nuclear plants are operating at about 90% load factors.[47] However, new ones will run somewhat lower, so an average load factor of 80% will be assumed. For 1000 MW power plants, 571 would be required to provide all the electricity the US uses, compared with 104 that currently are in operation.

Up to now, this discussion has ignored the difference between peak power demands and gross power generation. In the case of solar and wind power, it didn't matter because neither can reliably supply energy at any time, let alone meet peak demands. Nuclear power is available at all times, though, so peak demand can be met. The current US electric capacity is about 890,000 MW[50], so about a thousand 1000-MW power plants would be required, or a smaller number of larger plants.

Motor Fuels

The US uses about 140 billion gallons of gasoline per year.[39] Since ethanol has only 70% of the energy content of gasoline[41], at 439 gallons per acre[40], the US would have to plant 456 million acres, or 713,000 square miles in corn to displace gasoline with ethanol. That is about one-fourth of the area of the 48 contiguous US states.

The US consumes 63 billion gallons of diesel fuel per year.[39] The land area required to grow enough soybeans to displace the petrodiesel with biodiesel, at 63 gallons per acre[40], would be one billion acres or 1,563,000 square miles, about half of the area of the 48 contiguous US states.

These calculated land areas seem too high to be correct, but they are in line with calculations done by others. For example, [42] finds that, if all vehicles were diesel-powered, the land area required would be 58% of the US including Alaska. Another calculation [51] shows that if all the corn and soybean crops in the US were converted into biofuels they would replace just 12 percent of the gasoline used and just 6 percent of the diesel fuel.

Of the land in all of the US, only 18% or 650,000 square miles is arable[48], and almost all of that is being cultivated for food and fiber. Also, the calculations shown above assume that all the land would have the same yields as the prime farmland currently under cultivation and that there would be sufficient water for irrigation. Neither of those conditions is true, of course, so plainly there isn't enough land.

Clearly, biofuels won't provide much liquid fuel. There is a possibility that these land areas can be reduced by two thirds if hydrogen is injected into the biomass during processing. For example, 0.77 gallons of biodiesel can be produced by adding 1 kg of hydrogen[42], which requires 39.3 KWH of energy to produce from water. The biodiesel equivalent of US diesel consumption is 70 billion gallons per year; to produce enough hydrogen would require 2.75 trillion KWH per year. The fact remains, though, that biofuels can only be part of the solution.

The world's best hope for vehicle power is hydrogen. Fuel cells are highly efficient at delivering energy from hydrogen, and could be affordable if they were manufactured in volume.

As a rough estimate, let's say the amount of hydrogen needed would be the energy-equivalent of 100 billion gallons of diesel fuel per year, chosen mainly because it's a round number about half of the total of gasoline and diesel fuel: 50 billion gallons probably is too little and 150 billion gallons probably is more than necessary. The heat value of diesel fuel is about 38 KWH/gallon[49], so our energy equivalent is 3.8 billion MWH/year. For our rough purposes, this is the same as our current electrical usage.

Unfortunately, the process for converting water to hydrogen at normal temperatures is less than 30% efficient. So, the electricity required would be more than three times our current electrical usage. To generate that much electricity with solar panels would require a panel 5 miles wide running from San Diego to Boston. To generate the electricity with wind turbines would require a strip of land 260 miles wide running from the northern Montana border to the southern Arizona border with 2,870,000 turbines, all rated at 1.5 MW.

It is possible to produce hydrogen efficiently in a thermochemical process, using nuclear-generated heat. The nominal efficiency is over 45%.[26] But the heat left over from the conversion can be used to generate electricity, so the hydrogen production is nearly 100% efficient. The nuclear plants can produce electricity and hydrogen at the same time. More power plants aren't required because the additional heat will be available during off-peak hours.

Currently, hydrogen storage is the weak link. It's practical only for local transportation, but intense research is underway.

Conclusion

Barring some startling new energy development, what all this shows is that solar panels, wind turbines, and biofuels won't provide major parts of the world's energy. If global warming is to be avoided, the only two technologies that can provide sufficient energy are nuclear and hydrogen.