The IEA published a 2014 update to its 2010 PV technology report in September. It’s a comprehensive 55 page report with lots of historical data and forward projections to 2050 based on several scenarios. They upped their earlier 2010 estimate for PV generation for 2050 from 11% to 17% of electricity generation as part of scenarios where a combination of solar, wind, bio-fuels, hydro, nuclear etc become the bulk of world electricity generation by 2050. The scenarios have yearly PV installations rising from 36GW in 2013 to 200GW by 2025.

Their cost reduction projections are in line with the historical 20% learning rate and imply some growth in the amount of subsidies needed.  The overall cost of subsidy is a race between the yearly growth in projected installed capacity and the rate of reduction in costs. As such the report assumes policies that continue to provide subsidies as needed to maintain the growth they are projecting. Were the IEA’s projections of cumulative growth and cost reductions to be maintained, prices would eventually fall to where PV electricity was competitive in the late 2020s. When this occurs it would be realistic to expect that growth would increase, but the projections do not assume higher growth after prices fall to competitive levels. At high levels of PV market penetration, other constraints like storage and backup start to loom large.

PV optimists think the IEA is too conservative in its projections, largely because they foresee a more rapid drop in prices, based on projecting the recent rapid PV price drop forward, rather than the  slower historical 20% learning rate curve. The IEA’s PV price projections seem based on a more realistic assessment of the state of the business and the more likely to be correct.

Unfortunately, PV optimists and the IEA are both likely too optimistic. The IEA because it projects political willingness to provide more subsidies and PV optimists because they foresee  unrealistic price drops. StratoSolar could provide the price drop the PV optimists desire and enable the growth that the IEA projects. Unfortunately we will have to wait for both sets of optimists to be proven wrong before alternatives like StratoSolar get any serious attention.

By 
Edmund Kelly

StratoSolar's tether provides a support for fiber optic back-haul links and the platforms can carry very large payloads, both capabilities not possible with station keeping drones or airships. StratoSolar can also station keep at 20km in 50m/s winds.  With these unique benefits in mind its interesting to compare them with the current technical capabilities of drones and airships. 

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