A solar-powered high-altitude drone made by Titan Aerospace is intended to be able to remain in flight for about five years. When manufactured, it would have 3,000 solar panels producing about 7 kW of electricity and would be above the clouds, so it would be exposed to sunlight constantly during daylight hours. One is scheduled for completion next year.
The point of having such a plane would be to keep a payload in flight long-term, presumably in a manner similar to communications satellites. They could be used for surveillance applications such as environmental monitoring, fire monitoring and disaster response, among other things.