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Comment by brnt | Hacker News Reader

I love lowtechmag, but it's almost a rule they are economical with details too, and often to hide a (pretty obvious) compromise that not everyone would be willing to make.

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On top of that humans are very bad at converting fuel into work.

"In a rapidly changing world where inexpensive and reliable energy going forward is no longer a given"

In such a world a steam engine will give you more useful output for the same input - you could literally burn your food in the steam engine and get more useful work vs eating the food.

This kind of thing only makes sense in a world that has not invented steam engines, or at the minimum using an animal that can eat grass that you can't. (And even then burning the grass will give you more work.)

According to [1], human ‘fuel’ efficiency on a bike is about 20%. That is way ahead of any simple piston powered steam engine (should be below 10%, citation needed) that you would install in a workshop.

[1] https://physics.stackexchange.com/questions/46788/how-effici...

Steam engines are closer to 40% efficient, not 10%.

And they can burn the entire fuel, unlike a person who can only eat the choice parts and must waste large portions. (For example corn - the person can only eat the kernels, the steam engine can productively use the husk and the cob.)

Also, your 20% figure is for energy absorbed after digestion (they check via oxygen consumption), but digestion itself is wasteful - lots of usable fuel is not absorbed when someone eats, and is emitted as waste.

It's really not even close.

It's a problem also for comparing bicycles vs cars - even though they are heavier, cars are surprisingly efficient compared to the entire production process of delivering food to humans (lots of energy is used in the farm).

If I remember my figures correctly 2 people in a car is more efficient than 1 omnivore on a bicycle. And a single person in a car is more efficient (less CO2 emitted) than a carnivore on a bicycle.

(The car comparison depends a lot on what the person eats.)

Steam turbines can easily reach 40% efficiency. Simple (single expansion) steam piston engines are much less efficient, usually well below 10%.

Sources or calculations please and please elaborate on the system boundaries. Thx in advance.

The extremely obvious compromise is that you need to have a high-PSI air tank on hand as well as a donor bike. The donor bike can probably be gotten for very cheap or free, but the tank...

I looked at the prices of 80-gal 200 PSI rated tanks online, and the conclusion is...not great.

It's going to run you roughly $1000, for which you can realistically buy a whole set of cordless battery powered tools AND an outlet-powered air compressor on top of that.

The point is that this solution works without access to electricity and is resilient to the loss of industrial supply chains

How are you going to make an air tank that can handle 200 PSI without industrial supply chains?

There are a bunch that are already made. I think this setup is really impractical, but once you have it, you aren't reliant on the supply chain to feed the power plant, but you are reliant on it for finding a source of food to power the human pedaling.

> I looked at the prices of 80-gal 200 PSI rated tanks online, and the conclusion is...not great.

Or you can get a bunch of empty propane tanks for a few bucks and call it a day…

For the tank:

> There are many used air compressors for sale that have faulty motors or pumps, and are therefore quite inexpensive

And the pneumatic parts/equipment:

> An additional benefit of using compressed air for energy is the low cost of air tools. The current trend is pushing toward more battery powered equipment, so people are selling off their “outdated” pneumatic tools.

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