"We have a toilet on board, so we really try to have a sustainable life. I try to eat normally, I go to the toilet normally," Borschberg says. I asked if they were able to steal a lot of techniques from spaceflight, but not really: Solar Impulse has the benefit of gravity. That’s a game-changer for going to the bathroom.
https://www.theverge.com/2015/5/7/8566081/solar-impulse-2-airplane-pacific-ocean-record-flight