You need something which can gain Height without power... So obviously something which is light enough to use thermals...

The best bet would be to go electric, a glider type Airplane with an electric motor...

This way the Airplane has very good lift and you only drain the batteries every now and then, when the engine is off you don't use power. (except for control)...

The problem with using a glow motor is that it always requires fuel...

A good electric set-up could keep you in the sky for hours and hours...

Matt