SkyLifters use sun and wind energy to fly.
The upper surface of the aerostat is the perfect place for solar collectors. SkyLifters can operate 100% solar powered or supplement with a combination of diesel/solar-electric.
Another major advantage of SkyLifters over their HTA cousins is that, as a routine flight practice and if conditions permit, SkyLifters will drift with the wind, effectively with thrust-off and using just a trickle of energy to power life-support, avionics and essential systems.
Overall, their cost per tonne-kilometre will be exceptionally economic because fuel is essentially only used for propulsion, not for lifting, and therefore efficiency is similar to road transport rather than current air transport.