Ignoring the blatant ethical issues associated with this question, I’m genuinely curious from a scientific standpoint how efficient the human body is at generating energy. I’m a chemical engineering major and after learning about combustion engines and steam generation, there’s a great deal of inefficiency. After taking an intro to biochemistry course it seems like the human body is incredibly efficient at energy efficiency, using food as the fuel. I was also made curious by that one black mirror episode where people rode those standing bikes as their job, I think it was for power generation but I can’t really remember. Would it actually be a good substitute in terms of equivalent power and clean energy? Again, a horrible hypothetical given the history and current use of people in such dehumanizing ways, and if this really isn’t something to be discussed, I apologize.
In: Engineering
Abysmal.
Nearly all of the chemical energy you consume is “wasted” keeping you warm or making sure Rick Astley’s greatest hit is replaying in your biological supercomputer brain.
Your body expends something like 30% of its metabolic energy on the brain. Critical for human life of course, but useless for brute work.
An internal combustion engine will run around 30% efficiency with the rest being lost as heat.
A human would be somewhere below 5% most of the time, having wasted nearly all the energy input on nonsense like “brains” and “homeostasis”
Horrible. Remember that, before machines and tools, humans *were* the only power generation available to us, but the second we could offload that onto beasts of burden and machines, we did so, because they are so much better.
Afaik our muscles are about 30% efficient in terms of generating work from available energy. Given the incredible inefficiencies in producing human food (photosynthesis is about 2% efficient in converting solar energy into sugar), a humanpowered stationary bike will never outperform a solar panel.
Professional, Tour de France-type bicyclists generate about 400-500 watts of power per hour, which is equivalent to about 340-430 food calories. These bicyclists are going to vary in their calorie needs over that hour, but a good estimate is 1,000 calories per hour. That gives an efficiency of about 35-45%, which is not bad compared to a power plant that just burned the food they would have eaten and used it to drive a steam turbine. You might get even more efficiency if you directed the bicyclists to pedal at a more moderate pace.
However, there are a couple of big issues with using this as an actual means of power generation:
1. Scalability. Professional bicyclists are a rare breed, train a lot, need lots of recovery time, and frequently get injured (even aside from crashes). There’s no way this could be a work-a-day job that lots of people have.
2. Max scale. Half a kilowatt per hour is just not very much, even if its gotten with decent efficiency. The average US household uses 30 kilowatt hours of energy per day, so even if we somehow got *everyone* on a bike for a 16 hour shift, we’d produce less electricity than we currently use.
3. Economics. Food is expensive. Humans are even more expensive. Putting food into a power plant that also demands a wage is a ridiculous idea compared to just shoveling random biomass like wood or grass into a furnace. $1 per kilowatt hour is an extremely high price for electricity, but generating that electricity would cost you 2 hours of wages – well over $10 in any developed country (and less developed countries would have much lower electricity prices). That’s before you count any of the costs for buying/maintaining/storing the bikes and whatever you might have to contribute to the exceptional food and medical costs of your employees.
Horrible. Biological systems can be incredibly efficient if you’re looking at turning food into energy to keep them alive, but creating food isn’t very efficient in the first place and human bodies aren’t designed to externalize their energy usage in a manner that is mechanically efficient. You also have an “engine” that takes two decades to create and has all of the massive overhead of needing to support a complex neurological system that is intended to allow it to survive and reproduce, not just rotate a mechanical shaft all day long.
Or, to put it in practical terms: You could either put someone on a stationary bike to generate electricity to power a car, or you could just make them ride their bike instead of driving a car. Which sounds more efficient?
Awful.
To make the awfulness manifest, [have a look at this absolute unit attempting to make toast](https://youtu.be/S4O5voOCqAQ?si=Pim5OQ7W8hb2Sgom).
We’re very efficient at locomoting ourselves (again, A Bad Thing for those trying to lose weight via exercise), but this translates badly if trying to attach that power to anything external.
In that black mirror episode, half the point was just to occupy the population with something that seems useful, and half the point was that it’s a metaphor for some boring work that isn’t really that necessary, at least from worker pov. It was not about energy efficiency. It would likely be cheaper to literally burn the same amount of calories they feed the workers than make them into food and then electrical energy.
I guess I’m going to be a naysayer from the other comments, using[ this as my source.](https://phys.libretexts.org/Bookshelves/Conceptual_Physics/Body_Physics_-_Motion_to_Metabolism_(Davis)/10%3A_Powering_the_Body/10.09%3A_Efficiency_of_the_Human_Body#:~:text=Body%20Efficiency,-The%20efficiency%20of&text=Thermal%20energy%20generated%20during%20the,food%20energy%20into%20mechanical%20output)
Obviously we’re talking about “ideal” machines using math but I think we’re all agreeing that car-engine type systems are realistically only about 20% efficient, with the rest of the energy being wasted. On paper car engines could only reach a maximum of about 37% efficiency in an idealized universe of perfection.
The human body’s “chemical engine” runs at about 25% efficiency. Which means we’re actually *slightly better* than car engines and we’re much, much, much better than photosynthesis, which is roughly 2% efficient.
Humans have been said to eat 2000 kCal per day. With some math, this works out to about 97 Watts. Not a lot of power.
To make it worse, a human uses most of that power just keeping themselves warm, or thinking(humans are deeply invested in this activity, about 20% of total energy budget), or digesting food instead of doing useful work.
You can train a human. They can get lean and mean and eat 4000kCal in a day. You can get a lot more work out of them this way, and they do get more efficient, but the problem is muscle mass. Humans gain muscle mass as they do mechanical work(useful or otherwise), and that muscle mass has a baseline energy consumption. You can easily lose your efficiency gains if you let your human get too strong.
If you’re looking to bioreplace a truck engine, you should try a cow. They don’t waste as much energy thinking, and although their digestion is a serious high power system, it also runs on grass.
For commuter transit I’d say a horse. They’re fragile af, but smart, fast, and strong.
Shhhh…don’t give the Chinese any ideas. Next thing you know, there’ll be sprawling buildings with people strapped to stationary bikes generating electricity. Like an origin story for Conan.
And to answer your question, we are more efficient than machines, but converting that dietary intake into pedal action and into electricity will suffer a large efficiency loss.
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