In article ,
Eric Greenwell wrote:

small fuel cell.

I've wondered if a gas-electric hybrid would be practical. A gas motor
powerful enough to supply half the energy of a 2000' climb at 500 fpm
would be about 20 hp, a fair sized motor. So again, maybe the solution
is a big enough Li battery to do the whole 2000', then use 4 hp motor to
recharge the battery over the next 30 minutes or so.

Apparently there was a gas-electric hybrid glider at the AERO
Friedrichshafen show, or at least a poster about one. SoaringCafe has
pictures from the show. One pic is of a diagram of an Albastar 18m
glider with internal gasoline engine driving a generator linked to a FES.