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
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.
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