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
The human body does not generate energy. It consumes 2000 kilocalories a day just to exist, and it hasn’t done any work yet. You’d be better off building a turbine that runs off burning wheat and using that to do work.
ELI5 – Growing food to feed people to make energy as heat or muscle movement to do work is very inefficient. You get more energy out of burning wood or fuel.
Pet Peeve: This is one of the reason those Matrix movies are so dumb. It’s thermodynamically idiotic.
Human muscles are in the neighborhood of 23% efficiency: put in 100 joules of chemical energy, get 23 joules of mechanical work and 87 joules of heat. This is actually really close to the peak efficiency of modern internal combustion engines.
This is surprising when viewed in the context of transportation energy efficiency: bikes are like 30 times more efficient in terms of energy per distance than cars.
It turns out that most of the difference isn’t about engine efficiency, but mass and aerodynamics. A car and driver weighs far more than a bicycle and rider. On the same grocery run, that mass makes a huge difference accelerating from a stop. On a longer trip, cars tend to move a lot faster than bikes and have more frontal area, so the energy lost to drag is enormous.
There are exotic human powered vehicles, such as fully enclosed aerodynamic recumbent bicycles, that can achieve high speeds under human power on a flat course. They don’t climb well though, as they’re relatively heavy, as well as expensive.
On the other end, an electric car’s motor is likely pushing 85% efficiency, and the charging process is similar. Grid transmission losses aren’t that bad: 5% or so. So an EV is probably only around 10 times worse than a bike: it still has the mass and frontal area issues, but much better energy efficiency.
The human body is incredibly efficient at converting food into energy for its own use, but for outputting/ transferring raw power, we are extremely inefficient.
Take a look at some of the attempts at human-powered flight. A lot of engineering had to go into the designs and they still are not an effective mode of transportation.
A small, solar powered, electric motor could theoretically sustain that machine in flight at a steady pace for a much longer amount of time with far less input requirements.
To supplement the extra energy needed to create machine work from humans, the caloric intake would need to increase, which would mean an increase in need for food supplies, which would likely offset emissions from wasted electrical energy in the form of heat: and to be clear here, I don’t have numbers to support this, just a hypothetical statement. But then one could argue that the processes to make the solar panels and wires and motor are also environmentally harmful. It’s a tall order to ask that anyone be able to solve this mathematically taking into all possible factors.
lets just say matrix have a horrible plot line with the “battery” idea.
yes there were others, but thats the one that bugs me the most.
A fun fact along this vein that I like is that a bike powered by a human with a normal diet emits more CO2 than an electric bike charged from the normal grid.
Well, imagine a gym where people are on treadmills generating power for the city. It’s like turning calories into kilowatts—the original green energy source with a side of cardio.
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