Fri, 15/04/2011 - 12:08 — dan

*Much too long for a comment, responding to this at transition.net:*

On the energy in petrol: the quote above we're checking is "a 40 litre fill-up at a petrol station is the equivalent of about four years of human manual work". So the question is: how much human work does 40 litres of petrol equate to? I get it to be 41 days, or about 1 day per litre. It also turns out, at current UK fuel prices, direct people-power would be a hundred times more expensive than liquid fuel! So -

Let's look at diesel, so we can compare humans to a diesel generator. I'm only dealing with the work that can be done directly here, not embodied energy or calorific value. Bang goes the theory's human powered station did this brilliantly. Here's a useful image from that page, listing number of cyclists and the wattage required. Note: we all output about 100 watts even at rest: that chart works out as cyclists' average 120 watts *useable* output, so they're being required to do at least 150-200 watts - but as I say, it's the work done we're interested in.

(Note also: they could have set it up with twice the number of cyclists to give them an easy time, but keeping it around 120 watts each made sure people had to really work at peak loads. Much better telly!)

The basic fact from that: you can get about 120 watts (0.12 kw) of electricity from a cyclist, if you're working them. How does that compare to a diesel generator? Wikipedia and this conversion chart work out about the same value: about 3 kw hours per litre. 40 litres is thus 120 kwh. Our cyclist at 0.12 kw would output 120 kwh in a nice round 1000 hours, or 41 days with no breaks. At which point they'd probably want a little lie-down.

That's handy: it means 1 litre of diesel is equivalent to 1 day's cycling, if we're getting that cyclist to generate electricity. (Note: the diesel chart range varies between 2.93 and 3.66 kwh/litre, depending on size of generator, for 1/2-load, by my calculations. Same kinds of load changes would likely apply to cycle-power, but let's not worry about that right now...)

Now, average UK household daily electricity use is about 9 or 10 kwh I believe. That would be, what, 3 or 4 litres of diesel. At current prices that's £4.20 - £5.60. Minimum wage is £5.93 at the moment. 10 kwh is one cyclist going for just over 83 hours. Presuming you have a team of them in your basement, and they're not paid for lallygagging, that'd be £492 a day. (I guess they'd have to buy their own energy drinks from that...)

So that's a small comfort: diesel prices would have to increase a hundred-fold before human domestic power stations become a viable option! (obviously all ceteris paribus: plenty of other things would go awry a looong time before oil prices got anywhere near that cost, but it's a fun comparison. For a given definition of 'fun'.)

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