An offshoot of the solar panels on the EV thread. While related somewhat it appears perhaps to deserve it’s own thread.
In the book ‘The Martian’ the character gets to run a electric vehicle which towed another one quite some distance over several ‘martian weeks’ on mars to make his destination. IIRC he had 28 solar panels plus a radiothermal generator which added a minor amount of power. Also the expected solar power is 1/4 that of earth - so on earth he could get the same about of power from about 7-8 panels. Additionally the gravity is less, but all travel is off road.
With all those factors and I’m sure many more was it reasonable for him to make those distances on mostly sunlight power?
In the related thread it was mentioned that . EV with a solar panel on earth (actually California) would be lucky to get 3 miles per day, but here we have effectively 7-8x as much energy coming in.
I’m doing a bit of re-reading and all the math is there for you to see starting with Sol Entry 197. It’s where he starts using “pirate-ninja” as the unit for kWh/Sol.
The situation is both better and worse than an earthbound EV. Not only does he need electrical energy to drive, he needs it to power his life support equipment, though he makes some changes to drastically reduce their power requirements (primarily by modifying the Oxygenator to use his RTG’s waste heat instead of an electric heater). However, he doesn’t just have solar panels on the roof of his vehicle. He has a stack of solar panels on the roof of his vehicle, and strapped to brackets on the sides. He drives at dusk/dawn and sets the array of panels out during the day.
His test run gets 57km from 18kWh, for an efficiency of .32kWh/km, which is twice the power used by the Model 3. Sounds about right - it’s big, clumsy, and off-road but he’s only driving 25kph. He straps 29 2m[sup]2[/sup] solar panels to his rover. Solar panel efficiency is discussed on Sol Entry 63, when he plans his run out to recover Pathfinder. I’m too lazy to check his math, but due to how the book was written I’d be surprised if it were off significantly.
There are so many variables. For example, also consider:
EVs on earth are driven at high speeds through thick air. At highway speeds, a significant fraction of the power is being used to overcome air resistance. A Mars rover wouldn’t be moving very fast, and the atmosphere is so thin that air resistance is negligible.
There is no stop-and-go traffic on Mars. That isn’t as big an issue with regenerative braking, but it still wastes some energy.
Commercial solar panels are typically around 15% efficiency. Higher efficiency panels are available, but for commercial applications, the higher output does not justify the higher cost. A Mars rover would probably use the highest efficiency panels available. The current Mars InSight lander has solar panels with 29.5% efficiency.
Also, sunlight on Mars is about 1/3 as bright as on the Sun, not 1/4.
At one point in the book he says the efficiency of the panels is 10.2%. At the time the book was written (2005-ish, I think), that was probably about what you could get for commercially available panels. But at the same time, the Dawn Mission was being built and that uses much higher efficiency triple-junction panels. You’d expect any NASA mission would get the best panels they could make.
It’s about half again further away, which would make it 1/4. However, Martian air is very thin compared to Earth’s, so there’s much less absorption by the atmosphere. So, 1/3 is probably about right. Unless there’s a dust storm, that is. And of course, he ran into one of those.