The subsidy for ethanol expired on the last day of 2011 and I didn’t shed any tears. To make a gallon of ethanol, start with 23 pounds of corn with a potential energy of about 185,000 BTUs. Add 40,000 to 80,000 BTUS of fossil energy per gallon and presto you have a gallon of ethanol containing 77,000 BTUs. Therefore the efficiency of the corn-to-ethanol conversion, including the fossil energy, is around 30%. Then ethanol isn’t a very efficient motor fuel, so the conversion from heat to mechanical energy is around 15%. Really only about 5% of the energy in the corn is used to turn the wheels of the vehicle. No wonder ethanol requires subsides of $0.50 to $1.00 per gallon, including corn subsides! If you burn the corn in a pellet stove, you would harvest around 75% of the corn’s potential energy in space heat.
Pellets are a very efficient biofuel compared to ethanol. Pellets yield from 5 to 10 times the fossil energy required to harvest, process, and transport their biomass. And you can get 75% of the potential energy out of the pellets as space heat. If we were sophisticated as the Europeans who are making electricity and heat from biomass, we could get up to 85% of the potential energy out of pellets or wood chips. This process is called combined heat and power or cogeneration. It might be our best option for biomass conversion to a transportation fuel, by charging electric cars with electricity generated from biomass CHP.