Today I read the news that Tesla is about to start production of electric semi trucks (aka tractor-trailers). Given the size of the trailer and its long, flat roof, I wonder if it would be feasible to fit them with solar arrays to help charge the battery? Could such a solar array generate enough power to reasonably increase the range of the truck?
Taking a single semitrailer as the load you have approximately 12.5m long by 2.5m wide of top surface, or 31.25m². With 20% efficient panels you might get as much as 6kW from the semitrailer, or 6kWh every 100km/60 miles on the highway (which is also where good solar access is most likely).
It's tricky to find energy consumption figures for heavy electric trucks, but BYD claim 124 miles from 435 kWh fully loaded, or about 2.2kWh/km (3.5kWh/mile) which more or less matches the 2kWh/km unsourced quote in wikipedia. So at best that truck would get an extra 12kWh or 6km/4 miles per battery charge.
In practice that's the best possible case, and no trucking company is going to run things that fine, or rely on getting solid sunshine for an entire trip. The real gain would likely be lower, not least because trucks operate 24/7 in most places and the solar peak only covers about 6-8 hours (less further from the equator, obviously) so the average benefit would be 1/3 or 1/4 the best benefit.
A second problem comes from the extra weight of the solar setup. Note that it will be on the trailer, so there will be extra weight for the link back to the tractor as well as the solar part of the charger. Mass-market solar panels alone weigh about 80kg/kW, but frameless ones only about 50kg/kWh, so we could expect the trailer to have direct-mounted panels and they would add 300kg to the trailer weight. Or to put it another way: solar panels would remove 300kg from the carrying capacity of the trailer. The extra air resistance should be minimal, and may be less important than the extra maintenance to keep the solar panels working, and the extra link/unlink time of the extra cabel between truck and trailer (not to mention the training time!).
I doubt anyone has, or will, do a study of exactly how this works out in practice, simply because the potential payoff is so low. It's much more effective to put the panels on the ground, somewhere they have good solar access, and connect them to the grid than on truck roofs.