Efficiency and economic arguments:
According to a study written by Drs. Van Dyne and Raymer for the Tennessee Valley Authority, the average US farm consumes fuel at the rate of 82 litres per hectare (8.75 US gallons per acre) of land to produce one crop. However, average crops of rapeseed produce oil at an average rate of 1,029 L/ha (110 US gal/acre), and high-yield rapeseed fields produce about 1,356 L/ha (145 US gal/acre). The ratio of input to output in these cases is roughly 1:12.5 and 1:16.5. Photosynthesis is known to have an efficiency rate of about 16 % and if the entire mass of a crop is utilized for energy production, the overall efficiency of this chain is known to be about 1 %. This does not compare favorably to solar cells combined with an electric drive train. Biodiesel outcompetes solar cells in cost and ease of deployment. However, these statistics by themselves are not enough to show whether such a change makes economic sense.
Additional factors must be taken into account, such as: the fuel equivalent of the energy required for processing, the yield of fuel from raw oil, the return on cultivating food, and the relative cost of biodiesel versus petrodiesel. A 1998 joint study by the U.S. Department of Energy (DOE) and the U.S. Department of Agriculture (USDA) traced many of the various costs involved in the production of biodiesel and found that overall, it yields 3.2 units of fuel product energy for every unit of fossil fuel energy consumed. [7] That measure is referred to as the energy yield. A comparison to petroleum diesel, petroleum gasoline and bioethanol using the USDA numbers can be found at the Minnesota Department of Agriculture website[8] In the comparison petroleum diesel fuel is found to have a 0.843 energy yield, along with 0.805 for petroleum gasoline, and 1.34 for bioethanol. The 1998 study used soybean oil primarily as the base oil to calculate the energy yields. It is conceivable that higher oil yielding crops could increase the energy yield of biodiesel. The debate over the energy balance of biodiesel is ongoing, however.
Some nations and regions that have pondered transitioning fully to biofuels have found that doing so would require immense tracts of land if traditional crops are used. Considering only traditional plants and analyzing the amount of biodiesel that can be produced per unit area of cultivated land, some have concluded that it is likely that the United States, with one of the highest per capita energy demands of any country, does not have enough arable land to fuel all of the nation's vehicles. Other developed and developing nations may be in better situations, although many regions cannot afford to divert land away from food production. For third world countries, biodiesel sources that use marginal land could make more sense, e.g. honge nuts [9] grown along roads.
More recent studies using a species of algae that has oil contents of as high as 50% have concluded that as little as 28,000 km² or 0.3 % of the land area of the US could be utilized to produce enough biodiesel to replace all transportation fuel the country currently utilizes. Further encouragement comes from the fact that the land that could be most effective in growing the algae is desert land with high solar irradiation, but lower economic value for other uses and that the algae could utilize farm waste and excess CO2 from factories to help speed the growth of the algae. [10]
The direct source of the energy content of biodiesel is solar energy captured by plants during photosynthesis. The website biodiesel.co.uk[11]discusses the positive energy balance of biodiesel:
When straw was left in the field, biodiesel production was strongly energy positive, yielding 1 GJ biodiesel for every 0.561 GJ of energy input (a yield/cost ratio of 1.78).
When straw was burned as fuel and oilseed rapemeal was used as a fertilizer, the yield/cost ratio for biodiesel production was even better (3.71). In other words, for every unit of energy input to produce biodiesel, the output was 3.71 units (the difference of 2.71 units would be from solar energy).
Biodiesel is becoming of interest to companies interested in commercial scale production as well as the more usual home brew biodiesel user and the user of straight vegetable oil or waste vegetable oil in diesel engines. Homemade biodiesel processors are many and varied.