Static electricity and lightening, hydrogen safety, station keeping and FAA approval

Thanks for the comments.  These questions and answers should eventually form the basis for a FAQ section   

Static Electricity and Lightning:

Regarding static electricity and lightening I would refer you to US patent 4842221 “Lightening hardened tether cable and an aerostat tethered to a mooring system therewith” (1989).  This discusses the design of tethers associated with the high altitude radar aerostats that have been used by the air force since the seventies. These have exceeded altitudes of 10km.  See also the TCOM web site http://www.tcomlp.com/.  Basically the cable outer protective polymer layer is slightly conductive to bleed charge to a grounded co-axial shield which also serves as the conductor for lightning strikes.  These cables scaled up to about 10cm in diameter serve as the basis of one possible StratoSolar cable design.

  Using an aluminum alloy metal strut based rigid truss for the buoyant structure is in part motivated by having a grounded conductive frame to simplify solutions for static electricity and lightening protection.  Similarly the use of metallic film coated plastics for gasbags as well as providing low leakage gas containment also provides conveniently grounded surfaces to avoid static buildup.  The PV structure is well above thunderclouds, but lightening up strokes occur, and a rare form of up strokes called “Blue jets” are a recent area of investigation.  The outer surfaces of the PV array structure will incorporate lightening arrestors, much like current high altitude aerostats.

Hydrogen safety:

Given the need for hydrogen as the buoyancy gas, a great deal of engineering is devoted to alleviating concerns about fire.  This topic could fill several books, so I can only touch on it briefly.  A fire requires hydrogen gas leakage, confinement of a hydrogen-air mixture, and an ignition source.  Prevention focuses on avoiding these three conditions.  Ventilation, inert gas boundary bags, and the static electricity, lightening protection and electrical distribution system safety systems provide a first layer of defense.  Also all materials used are non-flammable.  Hydrogen dissipates rapidly so ensuring it can do so starves any fire.  Active measures include instrumentation to detect hydrogen and fire, emergency hydrogen venting systems and active suppression systems using inert gas.  Hydrogen is a widely used material with a large body of safe engineering practice and hydrogen economy advocates have discussed its inherent safety attributes.  For example see http://www.rmi.org/rmi/Library%2FE03-05_TwentyHydrogenMyths .  The Hindenburg is usually cited as the classic example of the dangers of hydrogen, but even to this day controversy surrounds the cause of the fire, and in rigid airships as a whole, fire was not the dominant cause of destruction or loss of life. 

Station Keeping:

Station keeping is difficult.  High altitude station keeping airships powered by PV arrays and batteries have been investigated for a decade or more, and teeter on the edge of practicality.  See the HAA stratospheric winds paper reference in the bibliography section on the StratoSolar web site.  Basically, the highest occasional stratospheric winds that come from excursions of the polar vortex can get to 40m/s.  Countering this wind requires a very large motor thrust.  It also needs to work at night when power would have to come from batteries that weigh a lot, cost a lot, and don’t have a very long life.  At very large scale (several kilometers in diameter), a thin aerodynamic disk collector might be able to station keep.  The cost of motors and batteries, the power loss needed for thrust, the over 50% loss due to microwave conversion at both ends and atmospheric attenuation in between, and the cost of the rectenna array on the ground would all add up to make it way too expensive compared to a simple tethered array.

FAA:

The FAA would clearly have concerns.  Compared to various proposals to harness wind power from the jet stream using enormous numbers of windmills, the StratoSolar impact on airspace would be minimal.  StratoSolar relies on a few large systems, probably placed in groups away from air traffic corridors.  It would have little impact on commercial aviation which already deals with a complex air traffic control system.  The PV array structures are well above the cruising altitude of aircraft, so the danger is from the tethers.  It is possible to conceive of safety systems mounted on the PV arrays that actively track possible aircraft impact on tethers and automatically sever the tether to avoid impact.  This could be done in a controlled way at connectors spaced periodically along the tethers.  The redundancy provided by many tethers would make this possible.  California might need 30 systems overall, probably in two groups, one near LA, and one near the bay area to satisfy all its daylight electricity needs.  If commercially viable electrical energy storage becomes viable, additional StratoSolar systems could satisfy more of our energy needs boosting the number of systems.  On the other hand, improved PV efficiencies could reduce the number of systems.