Are biofuels reasonable?
Just reading something by Obama, set me to wondering whether it was possible to meet energy requirements from ethanol from plants. So I did some calculations (and some statistics gathering thanks to Google). The numbers refer to the USA:
The power from sunlight (the solar constant) is about 1.4 kW/m2
This is if the sun is overhead – away from the equator maybe we get half this – 0.7 kW/m2
Changing this to Joules of energy per year, you get about 2 X 1010
The growing season might be half a year, so that is 1010 Joules per year – on a square meter.
Suppose plants convert sunlight to fuel with an efficiency of 10%. This gives us 109 Joules per year on a square meter.
So how much land is needed?
Total energy consumption of the US in ‘liquid form’ = petroleum, diesel etc is 40 quadrillion BTU, which converts to 4 X 1019 Joules per year.
So we need 4 X 1010 square meters of land. How much have we got?
The US has about 940 million acres of agricultural land, which is 4 X 1012 square meters.
So only 1% of agricultural land could meet energy needs. If the conversion efficiency is down to 1%, that’s still only 10% of the land.
So they are reasonable.
To get a better grasp, how much land does the US currently have under/or available to the plough (not that the US is without wilderness) ?
If you do the same calculation for UK how do the numbers stack up ? Here we have higher population density and a whole lot less land available (‘cos so much of it is already under concrete).