Green bicycles

I had an odd thought today. We all know that riding a bicycle is a relatively “green” form of transportation. Getting your body from point A to point B by bike clearly takes far less energy than, say, transporting yourself the same distance together with a 1.5 ton automobile.

But has anybody actually calculated how much it costs in energy to feed a bike rider? Humans are notoriously inefficient converters of solar energy into useful work, given how high we are on the food chain. If you are a vegetarian it might not be so bad. But once you start talking about eating a meal that contains, say, farm raised beef, the actual energy cost of bicycling those ten miles might not be so small after all.

Does anybody happen to know how to calculate this cost?

6 thoughts on “Green bicycles”

  1. eh, i think its still more efficient to bike.
    not everyone eats beef (especially likely not bike transportation people)

    once you cut out beef from the equation…i think it is a moot point.

    the biker would have to eat anyway, you see.

  2. (Below, when I discuss caloric expenditures, I am using “food calories”, that is the so so-called “kilogram calorie” or about 4.2 kilojoules. I am using only two significant digits throughout.)

    Well, of course calorie expenditure varies according to weight, speed, etc. But lets go with a 450 food calories/hour for an average speed of 15 mph. (Note that just walking at 3mph expends around 280 calories/hour, so bicycling is a far more caloric-efficient expenditure.) Simply being awake and sitting (e.g., driving in a car, or sitting down once one arrives) will expend around 100 calories/hours, so the marginal cost of cycling is 350 calories/hour; and 15 mph, that is around 23 calories/mile.

    Now the amount of joules per gallon of gas depends on temperature, but it can be approximated at 121 megajoules. Dividing by 4.184 kilojoules gives an approximate 29,000 food calories per gallon of gas. So, if you have a relatively efficient car that gets 25 miles to gallon, you are using approximately 1200 food calories per mile. In other words, bicycling is 52 times more efficient than driving under these assumptions.

    Now, you may claim that this does not reflect the true expenditure of energy — (e.g., the energy required to harvest, transport, and cook food; the energy required to refine and transport gasoline) but usually people compute “carbon footprints”. There are any number of spreadsheets you can find online. The problem is that calculations get difficult quickly. I’ve already made a huge number of assumptions in my calculations, and things get wild since the carbon footprint of food varies considerably (with consumption of beef being perhaps the highest, since cows produce considerable flatulence).

    By the way, I am not sure why you assume that the humans are less efficient consumers of solar energy than the organic material that decomposed to produce the oil in the first place.

  3. yes, but a couch potatoe needs less energy than somebody who bikes say 2h every day, right?

    I biked for one hour in the gym this morning, and the machine was saying something about 500 kcal that I “used” – if I remember correctly…

    I found http://www.foodcarbon.co.uk that lets you calculate how ones food choices affect the carbon footprint. unfortunately I couldn’t find any hint about how they calculate it..

  4. Here’s a paper that addresses the question. http://opim.wharton.upenn.edu/~ulrich/documents/ulrich-cycling-enviro-jul06.pdf
    He looks at efficiency of extracting energy from food and fossil fuels, and concludes that cycling is still more efficient.
    He also points out that longer lived individuals use up more energy during their lifetimes. I’m not sure you could really convince people to bike less using such an argument.

    The key paragraphs:
    “The average automobile in the U.S. achieves fuel economy of 11.6 km/kg (Davis and Diegel, 2004), which is equivalent to an energy consumption rate, at the vehicle’s fuel tank, of 3.8 MJ/km. For the OECD countries, this figure is 2.5 MJ/km (Schipper 2004). Extraction, refining, and transportation of automotive fuel requires approximately 20 percent of the energy in the crude oil (Brinkman et al. 2005), so the fossil fuel costs “at the well” are 1.25 times greater than these figures. I assume that an
    individual owns an automobile whether using a bicycle for transportation or not, and so the automotive energy consumption does not account for the energy required to manufacture the vehicle, which is typically about 10 percent of the energy consumed over the life of the vehicle.

    The average utility cyclist pedals at a speed of approximately 20 km/hr, which requires human power input on a conventional “roadster” style bicycle of 17 kJ/km (Wilson 2004) over and above resting energy requirements. The ratio of work done to food energy consumed for a cyclist at commuting power levels is approximately 22 percent (McDaniel et al. 2002), and so in dietary equilibrium, the cyclist consumes 77 kJ/km of fuel in the form of food to fuel this activity. The energy required to
    produce, process, and transform food is approximately 5.75 times greater than the energy content of the food itself (Coley 1998), and so the total energy cost of cycling is approximately 443 kJ/km. Of course this energy cost for a specific individual would vary based on diet, origin of food, bicycle technology, riding style, terrain, body mass, and metabolic efficiency. Despite the inefficiencies in the energy conversion processes to generate human power, the bicycle remains about 6-9 times more efficient per km traveled than the single-occupant automobile.”

  5. Intuitively the results on this are as I suspected – driving a car feels incredibly wasteful, whereas riding a bicycle does not.

    It’s good to have hard figures, and to know that sometimes our intuitions are reasonable after all. Thanks everyone!

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