A novel balloon-based solar-plus-storage idea hatched by a research team in Tokyo is getting a lukewarm reception from experts consulted by GTM.

The idea involves capturing energy with solar panels placed on balloons floating more than 3.7 miles above the earth. The energy would then be used or stored in the balloon, in the form of hydrogen. 

“This technology is definitely in the realm of looking great in theory, but implementation will require overcoming major commercialization hurdles,” said Ravi Manghani, a senior analyst for energy storage at GTM Research.

​“Even if we assume this product passes the hurdle of technology feasibility, it will surely be hard to finance such projects in the very near term,” he said.

The solar balloons idea was dreamed up by Jean-François Guillemoles, director of research for the French National Center for Scientific Research at NextPV, a joint lab between CNRS and the University of Tokyo.

“The main problem with photovoltaic energy is that sunlight can be obscured by clouds, which makes electrical production intermittent and uncertain,” explained Guillemoles in a CNRS blog posting. “But above the cloud cover, the sun shines all day, every day.”

Guillemoles has calculated that a high-altitude tracking collector could pick up five times the solar power of a flat-plate PV system at ground level in a temperate latitude.

A diagram of the concept shows a balloon’s PV coating feeding power to the grid during the day, and at the same time powering a fuel cell to produce hydrogen that is stored in the balloon, keeping the system aloft.

“A moored high-altitude balloon of reasonable size could store about 10 days' equivalent of its own solar electricity production, which is more than enough to meet energy needs overnight, until production resumes in the morning,” claimed Guillemoles.

He said the balloons could be “moored or mobile.”

Tethered balloons are already in use, he told GTM, and the PV plants would be grounded in case of storms. "Places with frequent storms would not be suitable. [The balloons] should be signaled properly according to regulations. Signaling lamps would not consume much."

NextPV is currently studying how a mobile balloon would deliver energy back to earth and how the return on investment would compare to a traditional solar system, although “the point is not only cost; it is ease of installation and low footprint,” Guillemoles said.

One potential challenge for the concept is that “hydrogen as a carrier gas for balloons is not allowed in many places,” he admitted.

Manghani highlighted at least three other potential stumbling blocks. The first, he said, is that hydrogen fuel cells have been tough to commercialize. "There are certain operational limitations that require frequent operations and maintenance stoppages.”

This is an economic challenge for fuel-cell systems on land, he said, and “servicing these systems at high altitude can be an order of magnitude tougher.”

A second problem is that “adding storage will add to the weight of the device, making it costlier to maintain afloat.”

One way to avoid this might be to position the fuel cell at the base of the tether, although that would presumably complicate the design by requiring a hydrogen pipeline to run from the balloon to the earth's surface.

Finally, said Manghani: “The schematic shows energy harvesting and storage devices, but to connect to existing grids, there will be need to power conditioning as well as monitoring and controls. Again, adding those can increase costs.”

Utility-scale solar systems typically have balance-of-system components that aggregate and reduce the need for individual pieces of equipment, Manghani said. “Adding those components for individual balloon systems will drive up the costs as well.”

Even if all these issues can be overcome, the economic case for balloon-based solar might be extremely marginal outside cloudy climates, according to Oliver Soper, founding director of the renewable energy technical consultancy OST Energy.

Unlike wind, where power is a function of the cube of the air speed, PV output has a direct relationship with solar intensity, he said.

Thus, while it might make sense to harvest stronger winds using airborne turbines, the potential returns for high-altitude solar are likely more modest.

Soper noted that existing PV technology can already achieve yields of three times the northern European average in sunny locations such as Chile or South Africa.

“You would have to look at the round-trip efficiency of converting solar to hydrogen and back to electricity on the ground,” he said.

If the losses are greater than the extra energy you can generate at height, “then this concept is not going to work.”