The potential of pedal power

There have been some visitors here from an interesting article in a series on pedal power by Kris de Decker in Spain. It turns out that a significant amount of work on pedal-drive human-powered implements was done in the few decades between the invention of the bicycle and the widespread implementation of internal combustion and grid power, with another small bump during the 1970’s oil crises, and interesting demonstration projects here and there since.

We built our first pedal-powered apple grinder more or less on a lark, and we thought it was a good idea, but despite our expectations it was a real epiphany for me to see just how effective it was, and how pleasant it was to use. We went from operating a 3/4 horsepower electric garbage disposal right at the edge of its thermal limit, and cooling it (albeit in a grossly inefficient manner) with a 1 horsepower electric compressor, to a system powered by one pedaler that was much quieter, much faster, and produced nearly as fine a grind.

I think the real lesson here is less about pedal power than about the way that the availability of cheap energy and cheap goods leads to thoughtless waste, even among the well-intentioned. In cider year two, we had a pile of apples that would soon go soft, a brand new garbage disposal was 75 bucks, and the electricity to run it and the compressor cost a few pennies per hour – so we ended up using over 10x the energy we needed to do the job. The aesthetic attractiveness of pedal power mostly serves as a counter-incentive to cheap energy – ‘That would be so cool – let’s see if we can make this process efficient enough that we can power it with 100W…’ It took some engineering skill and some R&D time to figure out the right way to do it – over the next two years we fine-tuned the geometry of the grinding drum, added custom-machined post-crushers, etc. And the improvements we made are independent of the source of motive power – the same grinder assembly could easily be driven with a good quality electric motor with an efficiency of around 90%.

So once we started thinking that way, all sorts of possibilities came to mind. Right away we realized that the pressing half of the cider operation was another perfect case, where efficient application of well under 100W could do an impressive amount of practical work. Again, it took some engineering to select a suitable hydraulic pump, connect it appropriately to a bicycle drivetrain, and work out the picky details that only matter when you’re trying to be really efficient. For instance, we learned that a standard hydraulic check valve has a metal-to-metal seat, which leaks a tiny amount of oil when there’s significant back-pressure. We realized that we needed to get a check valve with a soft (o ring) seat, so that the press would hold pressure after the pedaler stopped pedaling. Normally this would not be an issue, since an engine-powered hydraulic system the engine is always running, so the pump is always working. Also, the pump would only make full pressure at a relatively high RPM, which was hard for pedalers to sustain for a long time, but with the check valve, a flywheel, and an accumulator we built out of an old dive tank, we could easily put in a short burst of vigorous pedaling and drive the system up to full pressure. The actual amount of mechanical work needed to press the cider is so modest that next year we’re going to demonstrate by hooking the press up to a junior-size bike, and let the growing crowd of kids press all 200 gallons.

Since then a whole list of promising practical applications have come to mind. High on the list is log splitting, where typically a large, loud single-cylinder gas engine runs at full-bore continuously, even though the full horsepower is only needed a tiny fraction of the time. I have a suspicion that our pedal-hydraulic power system could be hitched up almost unmodified to a log splitter with good effect, and there are probably other high force, low speed hydraulic applications as well. At the extreme end, it would be pretty sweet to see a mini excavator operating on the combined hydraulic power of five or six pedalers. Food preparation is another obvious area where a reasonable amount of work has been done, especially focused on the developing world. I’m interested to try a low-head pedal-powered irrigation pump, and possibly even a four-wheel, two station pedal-powered cultivation tractor, though the odds of any of these making it to the top of my engineering to-do-list are slim.

I guess the most obvious demonstration of the point I’m trying to make is those setups with a stationary bike where the pedaler attempts to make one or more incandescent light bulbs glow. Kris seems to go out of his way to point out that converting pedal power to electricity isn’t always the best approach; I think the discussion above should make it clear that his point is well taken here. But the futility of the pedal-light bulb demo isn’t really so much a demonstration of the limits of pedal power as it is of the lameness of incandescent bulbs – it would be interesting to do a demo with single pedaler, a well-engineered drive and generator, and an array of LEDs or compact fluorescents. I have a similar sense that a single pedaler could produce an impressive amount of audio, again given an efficient drive and a class D amplifier. And with the dramatic progress in low-power processors for mobile applications, and the availability of low-power electronic ink screens (developed by my former roommates, and used in e.g. the Kindle), pedaling could power an impressive amount of computation as well.


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