# How efficient would it be for an outpost to use human-powered vehicles?

Considering how bicycles use human power very efficiently for transport, would it be reasonable for astronauts on an outpost on the Moon or Mars to use easily replaceable human-powered equipment, like a 3D-printed light-rover powered by pedals or levers, or something like that?

In other words, considering a self-sustaining outpost, are humans a more efficient machine to indirectly transform solar energy into motion than the combination of solar panels, chargers, batteries and motors? If not, how inefficient they are in comparison?

Update: There are many issues with astronauts wearing a space suit in a near vacuum and low gravity riding on something like a tricycle or a bike, but I'm asking only about energy efficiency.

• You mean like this? Yes ... Apr 18, 2015 at 1:38
• @TidalWave something like that, yes. Apr 18, 2015 at 1:56
• @DeerHunter There are patented solutions to the combined bicycle/radiation problem! Apr 18, 2015 at 6:47
• This comes down to a comparison of the efficiency of solar cells, batteries, rover propulsion versus the human food chain. For a proper comparison you'd have to calculate how much energy it costs to build a greenhouse, grow food (incl. mining or generating water), process the food etc. I'm not sure we know enough of the challenges involved in food production on the Moon/Mars yet to answer this question. Apr 20, 2015 at 8:24
• @Hobbes: Yes, and there are also other factors. For instance, humans must expend extra energy in exercise to maintain health, so perhaps Mars biking is a good way to do this. OTOH, if you collect solar energy, you can use it for lots of other things besides driving a vehicle. May 25, 2015 at 18:42

Human powered vehicles used within domed or cavern cities would seem to be extremely plausible. One way to look at it is a simple bicycle is less complex than a powered vehicle. The power source (aka a Human) is extremely complex and maintenance intensive, but given that we already have a functioning human and want to get that human to some other place, a bicycle or scooter is a fine way to do so at minimal cost. Of course, the city needs to be large enough to justify not simply walking. Humans always need some exercise, and so within a biodome using a human powered vehicle can contribute to some of the exercise requirements of a human. In that sense, it is free.

Outside of an artificial biosphere, in the hard vacuum of space, or under a thin atmosphere we run into more problems. All things being equal, a life support system would sustain a human on a powered vehicle much longer than it would a human-powered vehicle.

I had a little trouble finding exact numbers, but here is one study which shows how much an adult human breathes per minute:

A human simply sitting in a vehicle breathes 9-10L/minute, while doing somewhat strenuous exercise they will be breathing at least 30L/minute. Life support has to work at least 3 times harder - up to 6 times harder. What this means is that every minute spent riding a bike, would be a minute less work which could be done - or 2-5 minutes less work if the work mainly involves standing around. Or if travelling somewhere, the maximum range is halved or worse.

There is an obvious solution - upsize the life support system. This has an equally obvious problem - the human has to carry the life support system under their own power and thus work even harder.

I think we've established that humans become a lot harder to maintain once they are huffing and puffing, putting aside the oxygen problem, the question is, how does the efficiency compare with electric motors? Again it wasn't too easy to find clear answers to this, but one could look at this article: http://physics.ucsd.edu/do-the-math/2011/11/mpg-of-a-human/

That article and other sources assert a human is around 25% efficient at converting food energy into kinetic energy (i.e. on a bicycle). This compares unfavorably with an internal combustion engine, at around 40% efficient, and very unfavorably with an electric motor, at around 90% efficiency. In this case, the lost energy becomes heat. A human outputting 250 watts of power, also generates 750 watts of heat. An electric motor would generate about 30 watts of heat. Unfortunately for the human, all that waste heat is something life support has to get rid of, through radiators (heavy) and/or active cooling (energy intensive).

Now lets look at charging the battery. Solar panels are about 30% efficient at converting sunlight into electricity - it can be higher or lower, but 30% is a fair average assuming the environment is a near vacuum. Charging batteries can be extremely efficient - 80-90% for lithium batteries. So of the captured sunlight, 30% x 90% x 90% = 24% becomes mechanical energy driving the motor.

And lets look at charging the human. The wikipedia article on Photosynthetic efficiency is pretty good. To summarize, most crop plants are 0.25-0.5% efficient at creating food energy. That would give the human an abysmal 0.5% * 25% = 0.125% efficiency. Even with really efficient algae or something we'd still be struggling to break 5% efficiency.

It's looking really bad for the human, but to add insult to injury the solar panels can be just plonked outside in hard vacuum. The plants need a whole life support system of their own, water, atmosphere, temperature, fertilizers - all these things need to be regulated.

TL;DR A human powered vehicle is far less efficient than an electric vehicle: From sunlight to kinetic energy, only about 1% as efficient. Human powered vehicles might be justified within a biodome as humans need exercise anyway. But in a vacuum or hostile atmosphere the excessive life support requirements for a human doing strenuous exercise would make human powered vehicles much less practical than electric vehicles.

• Outstanding! That's the answer I was looking for. May 26, 2015 at 16:56
• The figures linked are per minute, not per kilometer. If doubling life support usage gets you to the destination in 1/4 the time as walking? Sure if you have a working electric vehicle, that's fine. But if your alternative is walking, a mild ride at 12km/h surely beats intense walk at 6km/h.
– SF.
Oct 24, 2016 at 6:41
• @SF that's a good point. A quick check indicates cycling uses about half the energy per km as walking. Still better to sit in an electric vehicle though, and if you have life support you have electricity. Oct 24, 2016 at 10:09
• The same sort of economics apply here on Earth, also--fossil-fuel powered transport often is greener than human powered transport! Oct 24, 2016 at 18:49