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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.
I could see this working for either running cooling and such for refrigerated cargo or if they stick a battery in the trailer. In the latter case it would be possible to just charge it for free while the trailer sits in a lot somewhere. Then when the truck comes they plug in the battery and use the stored up power.