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Bad Math at TED

TED.com is a site that is known for very high quality content. Unfortunately on occasion they do get things wrong.

Rob Hopkins in his talk at TED Global 2009 claimed that 1 liter of oil “contains the energy equivalent of five weeks of human labor by 35 strong people” [1]. Now Rob has made a lot of good points and I look forward to watching his lecture when it becomes available, but I can’t let his claim about the energy of oil pass.

First we have to consider the functional usability of the energy. A Prius takes about 5 liters of petrol to drive 100Km and I believe that Toyota is going to improve this in the near future. Let’s assume for the sake of discussion that a hypothetical turbo-Diesel Prius based on the yet to be released new Toyota hybrid drive-train would take 3L of Diesel fuel per 100Km (Diesel engines are more efficient and Toyota is continuing to improve their technology). The Prius weighs about 1300Kg so let’s assume for the sake of discussion that 1L of Diesel fuel can move 1500Kg (vehicle plus driver and cargo) a distance of 33Km.

So the question becomes, how long would it take 35 strong people to move 1500Kg a distance of 33Km? 1500/35 gives a mass of 42Kg per person – any strong person can lift 42Kg with ease (it’s less than the mass of a good Trinitron monitor). 5 * 40 hour working weeks gives 200 hours of work, 33Km in 200 hours means an average of 165 meters per hour. I think that I could carry a 42Kg mass more than 165 meters per hour without excessive effort. If I was allowed to use some form of trolley then I could take it a lot further – I have moved monitors much faster than that while balanced on a wheeled chair!

It seems that the Bicycle Rickshaw [2] is one of the most efficient ways of moving passengers and cargo on roads. According to the reports I’ve heard a 100Kg passenger who comes from a first-world country (and can therefore pay well) will be welcomed as a rickshaw passenger. I think it’s reasonable to assume that a rickshaw driver can transport a passenger more than 33Km in one day. So if you had 35 strong rickshaw drivers working for a day they should be able to transport 3,500Kg of passengers and cargo for a distance that is greater than 33Km as opposed to a hypothetical future-technology Prius which can transport 1500Kg for 33Km while using a liter of Diesel fuel!

Now if we consider the fact that the 1500Kg that the Prius moves is comprised of 1300Kg of car and 200Kg of passengers and cargo we have 1 liter of oil in the Prius moving 200Kg a distance of 33Km vs 35 strong people working for a day and moving 3500Kg the same distance.

According to the Human Powered Transport Wikipedia page [3] someone who is “in shape” can produce 200W of cycling energy for more than an hour – that is 720KJ/hour. I wonder how many hours they could do that for in a day. It seems reasonable that a full 8 hour day of work would comprise at least the equivalent of 4 hours work, so that would be 2.88MJ per day or 72MJ for five 40 hour weeks. Therefore for 35 people it would be 2.52GJ of cycling energy over five 40 hour working weeks!

According to the Wikipedia page on Fuel Efficiency [4] one liter of Diesel fuel contains 38.6MJ of energy. If the energy in one liter of Diesel fuel was converted to motion with 100% efficiency then it might be equivalent to one strong person cycling for 13.4 days.

According to the Wikipedia page on Thermal Efficiency [5] the most thermally efficient engine is the Wärtsilä-Sulzer_RTA96-C [6] which can run at 51.7% efficiency which gives 163g of fuel used per KWh. So the RTA96-C could produce just over 22MJ of usable energy for 1 liter of fuel. That’s about equal to one person cycling for 7.6 days. Also note that the RTA96-C is an engine for a very large cargo ship, smaller engines are much less efficient.

There is no doubt that petro-chemicals are a concentrated source of fuel. I can carry a jerry-can which contains usable energy equivalent to more than 6 months of work by a laborer (according to my rough calculations). But there is no way I could carry enough food to keep someone alive and working for 6 months.

I look forward to watching Rob’s talk when it is available for download, I don’t think that getting one point spectacularly wrong reduces the value of his work. The Transition Towns [7] concept has many benefits to offer, even beyond Rob’s initial plans.

6 comments to Bad Math at TED

  • Peje

    What happens if you add all the energy that is needed for pumping up, refining and transporting that liter of oil?

  • Rich

    Rob’s point is still valid. Niggling over the depth of the factoid doesn’t remove the point.

    We could go down to the definition of “human labour”, which may not mean a rickshaw-ride-by-a-westerner.. *shrug*

  • There already was a small car that needed 3l diesel fuel for 100km: http://en.wikipedia.org/wiki/Volkswagen_Lupo#Lupo_3L. I’m sure it can’t transport quite as much, but it definitely means that your estimate is by no means outlandish.

  • John Slee

    I didn’t RTFA :-) but are you factoring in the humans’ need for fuel?

    It’d be interesting, sort of, to discover how far individual humans with a carefully controlled diet could shift trolleys loaded up with a couple of 21″ CRTs in an 40-hour working week. Especially if they were wired up with a heart rate monitor, GPS and so on. I’d like to see bio-impedance analysis done each day too, to monitor changes in muscle/fat/water.

    I read somewhere that Peugeot were going to introduce a peoplemover-ish vehicle that has a turbo-diesel/electric hybrid powertrain. They apparently expect an average of ~3.9L/100km, which is plenty better than any Prius. I applaud Peugeot’s approach. It should be plenty more appealing than the Prius to many family users who would currently drive big 4WDs, Falcodores or other large cars, as it is a people mover rather than a small car.

    The average large car/4WD are actually semi-reasonable in consumption per person if you put 4+ people in them, but awful with one or two. The Peugeot on the other hand should be truly excellent with one or two people and in absolute terms still pretty good with a full load of kids and gear on the way to soccer. In per-passenger terms the fully loaded Peugeot should be _awesome_. I hope it succeeds.

  • etbe

    Peje: Unless the amount of energy that is needed to pump, refine, and transport the oil increases the overall volume by a factor of 50 or so it will still be way less energy than was stated on the TED side.

    Rich: I have yet to see Rob’s talk, but I expect it to be of very high quality overall. From what I’ve read his ideas seem to be good. Citing a factoid that is out by significantly more than an order of magnitude doesn’t help anyone though.

    http://en.wikipedia.org/wiki/Volkswagen_1-litre_car
    Johannes: Thanks for the link about the Lupo. The 1L/100Km concept car (scheduled for production in 2010) is also very interesting.

    John: The original post that I cited didn’t mention the need for human fuel. Obviously the amount of work that a human can do with 1Kg of food is quite limited, although I think that 1Kg of beef jerky could probably allow someone to do more usable work than could be done by a Diesel engine with 1Kg of fuel (which incidentally is slightly less than 1L).

    I agree that there is a significant untapped demand for a hybrid people-mover. 4WDs are totally unsuitable for families, there are too many safety issues (both driving at high speed and backing over young children). For transporting children hybrid vehicles which use batteries at low speed are definitely the way to go, a child’s face is a lot closer to the exhaust pipe than that of an adult…

    Making a 2000Kg people-mover that takes only 3.9L/100Km would be a real technical achievement. Are you sure you aren’t getting confused with the Peugeot concept car of a similar size to the Prius which was a hybrid turbo-Diesel and achieved about that efficiency?

    In regard to efficiency of various size vehicles, I believe that when you compare vehicles with all seats occupied people-movers tend to beat Falcons and Commodores right now in terms of fuel used per person. A Camry is really quite comfortable with 4 people and uses less fuel than a Falcon or Commodore.

  • John Slee

    Fairly sure it was a peoplemover, yes. It was mentioned in Wheels or Motor magazine in the last six months or so. When I’m in my Canberra home next I’ll try to remember to look through my archives and find it.

    Peugeot have been talking about/toying with diesel-electric hybrids for quite some years now. Their non-hybrid diesels area already remarkably efficient. A friend borrowed a 307 HDi wagon (edging toward a medium-sized car, IMHO) to move house from Canberra to Sydney and achieved an average over several round trips of (according to the trip computer) 4L/100km fully loaded, pretty good I think.

    For the sake of comparison, my BMW F650GS returns 3.6L/100km when carefully ridden.