Money? Markets? Autoregressive heteroskedasticity? Ask me about it.
Corn ethanol is all the rage, but after reading a post citing a pre-combustion efficiency of 0.13% (yes that is zero point thirteen), a post is in order to set things straight.
Aldo V da Rosa's textbook Fundamentals of Renewable Energy Processes states:
The 135 kg of sucrose found in 1 ton of b&c are transformed into 70 liters of ethanol with a combustion energy of 1.7 GJ. The practical sucrose-ethanol efficiency is, therefore, 76% (compare with theoretical 97%). One hectare of sugar cane yields 4600 liters of ethanol per year (without any additional energy input because bagasse produced exceeds the amount needed to distill the final product). This however does not include the energy used in tilling, transportation, and so on. Thus, the solar energy-to-ethanol conversion efficiency is 0.13%.
(For reference, one hectare is 2.47105 acres).
4600 L of ethanol is 78783 moles, which provides 110831 MJ of energy (taking the heat of combustion = -1406.8 KJ/mol). This is equivalent to 0.0035 MW over a year. With average internal combustion engine efficiency of 20%, this becomes 0.0007 MW in effective energy, slightly below the energy required to power a home. Remember this is without transportation and delivery costs.
In contrast, a recent solar stirling engine project was a 4500 acre array constructed in southern california that produced 500 MW, or 0.274 MW per hectare. Taking a (really) conservative electrical energy efficiency of 80% (below legal requirements for cars), we get 0.219 MW, or enough to power about 220 homes.
Granted, I would suspect the solar array to have much higher starting capital cost, but a 300x increase in efficiency is nothing to sneeze at. So much for corn ethanol as the "fuel of the future"…
