On Tuesday, May 14, 2013 10:35:52 AM UTC-6, wrote:

It strikes me the right answer for sailplanes is a front electric sustainer powered by a very small battery and fuel cell.

Gliders need light weight. Batteries are all sorts of things but they are not light weight.

It depends on the energy density of the chemistry (in Mega Joules/kg.)
Lead-acid: .17 mJ/kg.
Lithium Ion: 0.72-0.875 mJ/kg
lithium-Air: 9.0 mJ/kg.
Hydrogen-air fuel cell: 97 mJ/kg
IC engine w/fuel: Typically 9 mJ/kg

Lithium-air looks promising. Hydrogen-air fuel cells have proven to have long endurance in aircraft propulsion systems. Lithium batteries can deliver a huge surge of power for takeoff. A "range-extender" fuel cell/lithium battery hybrid looks interesting for electric vehicles but in a glider...?