Titan (Google) and Ascenta (Facebook) drones have serious technical hurdles to overcome. Their first problem is they are only trying to station keep at a max wind speed of 30m/s at 20km altitude. This is insufficient even for India/Pakistan/Bangladesh where the wind speed at 20km can exceed 35m/s. A more realistic max that would cover operations above latitude 30 where it is possible to encounter the wandering polar vortex is 50m/s. Power required increases with velocity cubed, so thats a far off dream.
Their second problem is energy storage. They need to run on batteries for a min of 14 hours during darkness at latitude 30. Each 1KW needs 14kWh of battery storage. LiPo is 200Wh/kg so each KW of power needs 70Kg of storage. The drone engines need about 5KW at 30m/s. Thats 350kg, right at or over the edge of the total mass budget.
Both these problems put the drones in the "not ready for prime time" category. They will work in light winds but won't be able to station keep reliably until better energy storage comes along. Titan's CEO said as much before they were bought by Google.
Darpa pulled development for Boeing's Solar Eagle drone after a couple of years and is now only focusing on developing fuel cells/electrolyzers for energy storage using high pressure hydrogen gas. That reinforces the reality of the energy storage problem.
The Various High altitude airships, including Lockheed's ill fated HALE-D were only trying to meet an inadequate 20m/s spec, but could not even get close to that. Getting sufficient power from the area available for PV panels, and adequate energy storage within the mass budget constrain their operational envelope.
So the contrast is between drones and airships that are real but don't meet station keeping requirements and StratoSolar that can theoretically meet all station keeping requirements, provides unique capabilities but lacks credibility without a practical demonstration.
By Edmund Kelly