The rush to find alternative energy sources to replace petroleum must be tempered with an understanding of sustainability. The notion held by some that biofuel is a viable replacement for petroleum is likely a result of misinformation. Biofuel, although able to supply energy comparable to petroleum based fuels, requires an enormous land area to grow feedstock and is energy expensive to produce. Relative to petroleum it produces a much lower energy output relative to the energy required to produce it.
For the United States alone, the amount of land needed to produce the required biofuel to match the current petroleum consumption, would be three times the size of Texas! On this scale, the resources required to plant the feedstock, irrigate, fertilize, harvest, and convert to biofuel is massive. It would undermine food production and have devastating consequences in general.
If you want to see how one arrives at the above figure, consider the amount of energy from petroleum used by the United States in 2007 (ref: http://www.eia.doe.gov/oiaf/aeo/supplement/pdf/suptab_10.pdf). It is 39.64 x 1015 Btu.
Assuming ethanol is the biofuel of choice, the energy density is 84,000 Btu per gallon (ref: http://bioenergy.ornl.gov/papers/misc/energy_conv.html).
An optimistic ethanol production is 1,000 gallons per acre of farmland (ref: http://www.washingtonpost.com/wp-dyn/content/article/2006/06/30/AR2006063001480.html).
The energy content that can be produced per acre is 84 x 106 Btu/acre.
Therefore, the total amount of land needed is 39.64×1015/84×106 = 4.7 x 108 acres = 1,900,000 km2, which is about three times the size of Texas.
The amount of quality farmland, the enormous amount of water needed for irrigation, the cost of fertilizer, as well as the use of large-scale fermentation equipment, are extremely resource depleting and very energy intensive. In fact, in terms of net energy output you are going to come up on the negative side or low on the positive side. In other words, it would take a lot of biofuel to make a lot of biofuel!
Not a single biofuel is the answer. Not a single feedstock has a high enough energy production per acre to keep the land and resource requirements to a reasonable level. This means that corn is not the answer. Sugarcane is not the answer. Switchgrass is not the answer. Jatropha is not the answer.
The photosynthetic efficiency of plants is just too low – much less than 1% – so the relatively tiny amount of incident solar energy stored in plant tissue means that you need a very large area to get an appreciable energy yield. As an estimate, the sunlight-to-energy conversion efficiency of plants is 30 times less than photovoltaic solar cells, and 60 times less than concentrated solar thermal power.
The only reasonable use of feedstock (biomass) is as a source of heat energy, obtained by burning directly. This way you aren’t using additional energy converting it into a biofuel, such as ethanol.
Certain biomass can be harvested on a large scale, without doing much different in terms of current land use. And certain plants such as switchgrass, are robust and can grow on marginal land, requiring little fertilizer. But as a national and global solution to energy demand, biomass or biofuel will not work.
Burning biomass as a source of energy is only viable for smaller niche applications, such as combined heating and power generation for greenhouses, small scale cooking and heating applications, or even small scale power generation. But this is on a much smaller energy scale than what is necessary to significantly offset worldwide petroleum use.
Long Term Energy Strategy
The long term solution to worldwide energy demands can be met by using all the renewable energy alternatives available. This involves a large scale electrification using renewable energy sources such as wind, hydro, geothermal, and solar power.
This would entail powering as many vehicles and machinery as possible, with electricity. In certain cases where battery capacity is insufficient, electrical power can be supplied directly from the grid, such as for rail transportation, or large machinery that is relatively stationary, such as mining and digging equipment, and other material handling equipment.
This electrification can also include to a large extent, home and residential heating using geothermal heat pumps which can reduce electric heating costs by as much as 70%.
Certain modes of transportation such as air and marine travel would have to continue using fossil fuels in the short-term because their very large energy requirements would not be met with current battery technology, now and likely well into the future. And for obvious reasons they cannot simply be “hooked up” to the electrical grid. So perhaps the only viable alternative is to use hydrogen power. It would take a massive infrastructure of hydrogen production to satisfy the worldwide energy needs in the aviation and marine industry, which account for about 10% of all petroleum use. Hydrogen is non-polluting and can be produced from water using electrolysis (electricity). See Alternative Fuel For Aviation And Marine Transportation.
Over the long term, the use of oil will likely dwindle to production of plastics and asphalt for roads, and certain other niche applications perhaps involving large machinery operating in areas where the only readily available fuel is petroleum based.