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Sweden is testing a semi-truck trailer covered in 100 square meters of solar panels
(www.popsci.com)
This is a most excellent place for technology news and articles.
Solar is getting more efficient by the week. It'll always be a marginal gain but it's still a gain. An applications engineer somewhere is likely doing the cost benefit analysis to determine the cost per panel to km of drivable energy produced to determine the x number of year return on the panels. If you assume the lifespan of a commercial long haul truck is about 20 years it could add up to a decent amount of energy savings and the panels would still retain some salvage value after that lifespan.
I love your optimism, bit that's not how any of this works.
It will never be more efficient to put panels on the vehicle. Any vehicle ever. Dirt, trees, buildings, bridges, tunnels, etc. All block light. And panels available today, are around 23% efficiency. And they only get worse over time, estimated a 90% lots at 20 years.
Could they make better panels someday, sure. Would it still make more sense to put those panels on top of buildings or an open areas where they can get lots of sun. Yes.
The issue is that while it may be a trivial gain, that same photovoltaic material would generate more energy in a fixed installation. Also, as an installation on a truck, the weight of the system contributes to the energy needed to move the truck, somewhat negating the benefit.
So sure, have your electric truck. The trickle charging of any onboard solar system wouldn't even be noticeable though, and it's better to have the panels on grid helping drive the charging infrastructure. I saw someone guesstimate a theoretical peak of 25kw. My car charges at home at half of that, and even for my comparatively tiny car, that takes a long time to restore range, compared to it driving down the road. The truck might be able to get an extra 5 miles of range per hour of peak sunlight with 25kw system under realistic conditions, and that same material might be able to extract 40-50% more energy over time in a fixed installation.