I recently stumbled on the website of a company called Stratosolar which is proposing to deploy floating PV platforms 20km above the earth’s surface. The platforms would be 300 meters in length (along the direction of wind flow) and would be tethered to the ground by kevlar straps. Power cables which would conduct solar generated electricity to the ground would be attached to the kevlar straps. StratoSolar claims that the maximum wind speed of 50m/s at 20km altitude will allow stable, secure floating platform deployment with only small variation is horizontal position (According to their caclulation a length of 300 meters is required to achieve this stability.). Weights would be added to the tethers as a form of gravity energy storage. At 20Km of altitude each kg of weight would store 54Wh of gravitational potential energy (compare 38wh/kg for lead acid batteries). Excess electricity would be used to elevate the weights into the stratosphere, and the descending weights would be used to generate electricity during during period of low PV electricity production.
Twenty kilometers is above the cloud deck as well as being above a large portion of the atmosphere. These two effect lead to larger average incident radiation. StratoSolar claims that their designed deployments will produce three times as much electricity per unit area of deployment. All other things being equal this extra production would translate into one third lower cost of electricity production. However, it is far from clear that all other things are equal for stratospheric PV platform deployment. StratoSolar mention that the platforms could be filled with either helium or hydrogen. Helium would probably be used for inititial deployments, but StratoSolar admits bringing a high energy lifestyles to nine billion people (and this option seems to be the universal goal of climate change techno-fixers) would require the use of hydrogen as a buoyancy gas. There are very significant safety concerns with this use of such a highly flammable gas, but on their FAQ page StratoSolar claims that the engineering problems of hydrogen safety are solvable.
Siting PV panel above the clouds leads to high predictability of electricity production profiles which would lower the need for energy storage and which would make demand management schemes which try to match electricity use to the natural production profile easier to carry out. Furthermore this high predictability of solar electricity output can be achieved anywhere, including locations close to areas of high human population because the variable of cloud cover has been eliminated.
Since the power cables travel through all levels of the atmosphere below 20km the issue of lightning protection is very important. The design of the lighting protection system is discussed on the FAQ page
Whether or not the proposed energy storage in elevated weights is immune to weather is less clear. The weights will be moving through the troposphere which have potentially experience much higher winds than the height where the floating platforms will resides. The robustness of this energy storage scheme in the presence of violent wind is not clear.
Floating PV platforms will cast a shadow on the earth. The shadow will not be in fixed position during the day, and the path of the shadow will vary with the season. StratoSolar seems to to feel that landowners will not mind the relatively small amount of time that shadows from these platforms fall on their land. Whether or not such tolerance will apply in practice remains to be seen.
StratoSolar claims that their proposed solar energy production and storage scheme solves all of the problems associated with solar energy variability. This claim is not true since the seasonal variation of solar influx requires a scale of energy storage which larger than can be achieved by the proposed gravity energy storage scheme.
As far as I can tell from StratoSolar’s website they have not progressed beyond papers studies to real engineering on this PV deployment concept. While the ideas of stratospheric floating PV platforms is intriguing, I am not holding my breath waiting for a real world installation of one of these platforms.