Biodiesel (FAME): Efficiency
Why soybean diesel pays back its energy but algae has not
What this tab covers How much of the energy available is actually captured or extracted, in per cent: theoretical limits, laboratory records, typical new equipment and in-service averages, each with exactly what is being measured.Transesterification converts oil into biodiesel almost one for one by mass, so the main efficiency question is the whole life cycle: soybean biodiesel returns about 5.5 units of fuel energy per unit of fossil energy. Algae-based fuels have so far returned less energy than they consume.
Efficiency by measure
| Measure | Range | Type | What is measured |
|---|---|---|---|
| Oil or fat to biodiesel (transesterification) | 100% by mass1 | Commercial | About 100 lb of oil plus 10 lb of methanol give about 100 lb of biodiesel and 10 lb of glycerin |
| US soybean biodiesel, 2006 farm data | 5.54 energy ratio2 | In service | Fossil energy ratio: energy in biodiesel per unit of fossil energy used in farming, crushing and conversion |
| US soybean biodiesel, 2002 farm data | 4.56 energy ratio2 | In service | Same measure with earlier data; the rise reflects better crop yields and crushing |
| Algal biocrude, research facility and models | 0.00092 to 0.35 energy ratio3 | Record | Second-order energy return on investment, from the experimental case (9.2 x 10^-4) to an optimistic literature-based model (0.35) |
Where the energy is lost
| Step | Loss |
|---|---|
| Glycerin co-product split off from the oil | about 10 lb per 100 lb of oil (by mass)1 |
| Algae: energy for mixing, harvesting, dewatering and extraction | Exceeds the energy in the fuel (energy ratio below 1)3 |
Vegetable oils and animal fats are already energy-dense liquids, but they are too thick to run well in modern diesel engines. Transesterification swaps the glycerol backbone of each fat molecule for methanol, giving thinner methyl esters (FAME, fatty acid methyl esters) that behave like diesel. Because the oil's mass is almost all kept, conversion losses are small; the energy cost lies mainly in growing the crop, crushing the seeds and supplying methanol.
Algae can produce far more oil per hectare in theory, but growing them at low concentration in water means huge volumes must be pumped, harvested and dried. A detailed study of a research facility found that the energy put in exceeded the energy recovered by a large margin, even under optimistic assumptions.
Sources
- Biodiesel Production and Distribution US Department of Energy, Alternative Fuels Data Center, 2025
- Energy life-cycle assessment of soybean biodiesel revisited (Pradhan et al., Transactions of the ASABE 54(3)) ASABE (abstract, University of Johannesburg repository), 2011
- The Energy Return on Investment for Algal Biocrude: Results for a Research Production Facility (Beal et al., BioEnergy Research) Springer (author copy, OurEnergyPolicy), 2011