Biofuel

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Can plants be used to fuel my car?

Big Ideas: 
  • Power from the sun’s radiation can be extracted by vegetation and processed to create a biofuel that can power cars.

What is biofuel?

Biofuel is fuel that is derived from organic matters such as sugar, starch, and plant oil. This kind of energy can be obtained from plants which are rich in these organic matters such as corn and soybean1,2.  Biofuel that is used in automobiles is in liquid form, very similar to gasoline.

Two main types of biofuel are biodiesel and ethanol. Chemically, ethanol is a type of alcohol – the commonly‐known drinking alcohol – and is the most common biofuel in the world. Biodiesel is chemically known as alkyl (most commonly methyl) esters, and is the most common biofuel in Europe. One notable advantage of biodiesel  fuels is that they can be used in diesel engines with little or no modification2.

How much energy can be harnessed from biofuel?

Biofuel comes from vegetation such as corn and soybean, which need to be planted on land. Thus, one important factor in determining the feasibility of using biofuel is the area of land needed to plant the vegetation. As an exercise, let’s calculate roughly how much land is needed to plant corn so that the energy harvested is able to power a car for a year.

As with most challenging exercises, the trick is to break the problem into smaller parts:

  1. How much energy does a car use in a year?
  2. Where does the energy of vegetation come from?
  3. How much energy can each square meter of land provide in a year?

How much energy does a car use in a year?

Many of us don’t normally think about precisely how much energy a car uses. If this smaller problem appears challenging, it’s time to break it down into even smaller parts. The question, “What information is needed to solve this problem?” should come to mind.

Let’s start off by calculating how much gasoline a car burns in a year. Surely, this would take us closer to finding the car’s energy consumption. The fuel efficiency of a typical small car is about 10 litres per 100 km traveled. We express this as

$  \dfrac{10 \textnormal{ L}}{100 \textnormal{ km}}  $

But what distance does a typical person drive their car each year? The distance from the Vancouver‐Burnaby boundary to UBC is about 20 km. Let’s assume that a high school teacher has to drive half of this distance, 10 km, every time he goes to work. To get back home from work, he has to drive another 10 km, for a total of 20 km every day.

Let’s proceed further to calculate how much distance the high school teacher drives in a year. There are about 52 weeks in a year. We will assume that the teacher will drive for 40 of the 52 weeks, and 5 days a week. This works out to be 40 x 5 = 200 days per year. Now we can calculate the amount of fuel used in a year:

$  \dfrac{10 \textnormal{ L}}{100 \textnormal{ km}} \times \dfrac{20 \textnormal{ km}}{1 \textnormal{ day}} \times \dfrac{200 \textnormal{days}}{1 \textnormal{ year}} = 400 \textnormal{ L}  $

The energy content of gasoline is about 32 MJ / L, so we have that the energy a car uses in a year is

$  400 \textnormal{ L} \times \dfrac{32 \textnormal{ MJ}}{1 \textnormal{ L}} = 128 \textnormal{ GJ}  $

That’s 128 gigajoules!

Where does the energy of vegetation come from?

Almost all of the energy that we use originates from the sun. This energy is called solar energy. Sometimes, we use solar energy directly, by using solar panels. Other times, solar energy is converted to other types of energy before we use it. In the case of biofuel, the sun’s energy is absorbed by the vegetation first, in a process called photosynthesis. After that, we harvest the vegetation, process the vegetation to create the biofuel that we can put in our automobiles, and finally use the energy by burning the biofuel.

The amount of energy we can gather from vegetation would depend on how much vegetation we grow. The more vegetation we grow, the more land area we need to use. This is an obvious statement: of course more land is required to grow more vegetation! However, there is another enlightening perspective of looking at the statement. Remember that the energy stored in vegetation came from the sun. If we grow vegetation in a larger area, what we are really doing is gathering sun’s energy from a larger area. This in turn means that we harvest more energy. This idea will be very useful in the remaining solution of our problem.

How much energy can each square meter of land provide in a year?

Remember that the energy originates from the sun. Clearly, we need to know how much power the sun provides. Solar irradiance measurements show that the sun provides about 1400 W/m2 of power3.  In other words, the amount of energy contained in the solar radiation that reaches a square meter of land is 1400 J (recall that a watt is a joule per second).

It would be hasty to assume that plants absorb all that power! In fact, plants absorb only a small portion of sunlight, and the rest of the sunlight is reflected – this reflection of light is why we can see plants. For many plants, including corn, the efficiency of absorbing sunlight is approximately 0.5%4. Only 0.5% of sun’s energy ends up in the vegetation we harvest!

Putting these numbers together, we get the amount of energy we can obtain from vegetation per square meter, in a year:

$  \dfrac{1400 \textnormal{ W}}{ 1 \textnormal{ m}^2} \times 0.5 \% \times \dfrac{86400 \textnormal{ s}}{1 \textnormal{ day}} \times \dfrac{365.25 \textnormal{ days}}{1 \textnormal{ year}} \times 1 \textnormal{ year} = 0.2209 \textnormal{ GJ/m}^2  $

or about 0.22 GJ / m2.

Remarks:

  • The above calculation assumes there is sunlight every second of the year. This of course, is not a very good assumption since the sun is only up during the day, and not during night. Based on this observation, the above number should be reduced by a factor of approximately 2 to reflect the fact that the sun is in the sky only about half the time.
  • Another faulty assumption is that we can grow corn all year long. However, it is more realistic to assume that corn can only be grown in the summer. This means that the above number should be reduced by a factor of 4.
  • The figure 1400 W/m2 assumes that we are at the equator, with the sun beaming directly down.  More realistically, we can assume that corn is grown in the United States, with a latitude of about 45 degrees. This means that the solar irradiance is reduced by a factor of cos(45o) or $ \sqrt{2} $.

Thus, a more realistic amount of energy we can obtain from vegetation per square metre is

$ 0.22 \textnormal{ GJ/m}^2 \div 2 \div 4 \div \sqrt{2} = 0.0194 \textnormal{ GJ/m}^2 $

How much energy can be harnessed from biofuel?

Now, we know that a car uses 128 GJ a year, and vegetation provides about 0.2 GJ/m2. We can finally come back to the question we were trying to answer:

$ 128 \textnormal{ GJ} \div 0.0194 \textnormal{ GJ/m}^2 = 6582.5 \textnormal{ m}^2 $

It takes about 6500 m2 of land to grow vegetation to produce biofuel that can power a single car for a year. 6500 m2 is equivalent to 3.5 Olympic ice hockey rinks or approximately 1 CFL football field without the end zones.

For more information see5.

  • 1. Thomas, George.Overview of Storage Development. San Ramon, CA : Hydrogen Program Review, 2000.
  • 2. a. b. UN‐Energy. Sustainable Bioenergy: A Framework for Decision Makers. 2009.
  • 3. Composite TSI Time Series. Active Cavity Radiometer Irradiance Monitor. (Online) http://acrim.com/TSI%20Monitoring.htm. [20 January 2010].
  • 4. Andrews, J. and Jelley, N. Basic biochemistry from an energy viewpoint. In: Energy Science - Principles, Technologies and Impacts. New York: Oxford University Press, 2007, Chapter 7.
  • 5. Wikipedia contributors. Biofuel. Wikipedia, the Free Encyclopedia. (Online) http://en.wikipedia.org/w/index.php?title=Biofuel&oldi. [8 January 2010].

Comments

My partner and I presented

My partner and I presented your article to our Physics 12 class as part of our unit on energy. We really enjoyed reading your comparison of the energy produced by biofuels. With the current push for more sustainable fuels, it is important to look at the feasibility of each option before supporting and subsiding them. We generally agree with your estimates. However, we noticed that the energy needed to grow and ferment the crops was not accounted for. This would cause a significant increase in the amount of land needed to power a car. Additionally, the energy used to power the farm equipment, produce the fertilizer, and run the distillation process would likely be harnessed from fossil fuels or coal. Thank you for providing us with such interesting reading!

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