A short chat on biofuels

I'm going to assume we're interested in getting off fossil fuels, not just making a half-hearted 10% or 20% reduction in fossil fuel use.

Are biofuels part of the answer to sustainable energy?

When discussing this question (or indeed any question to do with energy), it's important to do the numbers.

There are three concerns about the numbers for biodiesel:

  1. Turning plants into biofuel requires many processing steps - spreading fertilizer, herbicide and pesticide; ploughing; harvesting; drying; transport; chemical transformation - all of which require investment of energy. How do those extra energy inputs compare to the final energy delivered? If the processing steps actually require you to put in roughly as much energy as the final energy delivered, then surely there would be little point in creating biofuels? I'll discuss the numbers below.
  2. To make biofuels requires land to grow plants on. Where would we grow the biofuels, how much land would be needed, and are we sure that no-one needs to do anything else with that land?
  3. Examples of the magic pixie-dust effect of biodiesel
    pump full of B5
    A pump full of B5
  4. The final concern is that biofuels are marketed like a magic pixie-dust, transforming whatever it touches to green. If you buy 'B20' biodiesel, for example, then it's actually just 20% biodiesel, and 80% normal diesel. So if pure biodiesel is doing any good at helping get us off fossil-fuels, B20 biodiesel is doing very little good, because someone who uses B20 is still using 80% normal diesel. Yet the marketers will paint the dispenser green and tell people that they've 'done their bit'. Amazingly, in the UK, even B5, which is just 5% biodiesel and 95% fossil fuel, is permitted to be marketed as 'biodiesel'. This is greenwash. Homeopathic greenwash.

Before we discuss the numbers, here are a few positive claims made about biodiesel. ([source])

  1. 'Biodiesel burns better in a diesel engine than conventional diesel', in that its emissions of unburned hydrocarbons, carbon monoxide, and particulate matter are lower, and emissions of sulfur oxides and sulfates are eliminated.
  2. Biodiesel leads to 50% less ozone-forming emissions than diesel.

We need to address the first two questions separately for each biofuel. The main biofuels in use today are biodiesel and bioethanol.


made from oilseed crops: soybeans, sunflower, canola, mustard, palm, coconut, safflower, hemp, jatropha; and used cooking oil from restaurants.

1. Energy costs of processing, expressed in terms of CO2

From a site supporting biodiesel: A 1998 biodiesel lifecycle study by the US Departments of Energy and Agriculture, concluded that biodiesel reduces net CO2 emissions by 78 percent compared to petroleum diesel.

However, this study has been criticised. 'Overly optimistic assessment in US DOE Report': 'These estimates were overly optimistic, and out of line with other analyses' For further reading see 'Biofuels for Oil Addicts '


According to a lifecycle analysis from MIT (Groode and Heywood): Ethanol made from corn -- consumes too much energy, inferior to gasoline, unless careful use made of co-products. Ethanol made from cellulose (eg from switchgrass or corn byproducts) -- not yet a commercial technology -- might be energetically better than break-even but economically unclear. Growing switchgrass is said to be promising because its life cycle is about 10 years, so transport costs are reduced.

Is there enough land (using current farming techniques)?

The quick answer:
  1. For the UK to grow its own transport fuel? Answer: No.
  2. For the USA to grow its own transport fuel? (The USA has a much lower population density!) Answer: No. [and a biodiesel advocate agrees with this]

In a bit more detail, let's look at the UK. Let's assume we take over 75% of the country and devote it entirely to biofuel production. The UK has an area of 4000m2 per person. So we have 3000m2 per person devoted to biofuel production. Now, bring along the sunlight. The average power of sunlight falling on Britain is 50W/m2. (That's 1000W/m2, the power of raw sunlight, multiplied by a daylight factor of (1/4), a sunniness factor of (1/3), and a latitude factor of 0.6.) [Correction: Actually 100 W/m2 is more accurate for the UK.] The best plants are roughly 1% efficient at turning sunlight into carbohydrates [Correction: actually 0.5% efficient is more accurate for energy crops in the UK], so the chemical power delivered by the 3000m2 biofuel plantation is 3000m2 x 0.5W/m2 = 1.5kW. Or to put it in the units your energy bills come in, 36 kWh per day. Don't forget, that's before we allow for the energy required in the processing steps. ../images/747/747FrontSideD.jpg How does this compare with the energy we use for transportation? The transport consumption of the UK averages out to 42 kWh per day per person. For example, taking just one intercontinental trip by plane per year requires an amount of energy equivalent to 24 kWh per day per person.

David MacKay
First written July 2007; updated Fri Dec 23 09:49:23 GMT 2011. See also Sustainable Energy - without the hot air (published December 2008).