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September 03, 2011

Stratosolar economic analysis

The StratoSolar PV solution represents an opportunity to make today’s PV technology cost effective without the massive subsidy needed to drive the technology to commercial viability in the 15 to 20 years historical trends would indicate will be necessary. It also makes PV an affordable alternative for locations like Germany and Japan where PV is unlikely to ever be viable without subsidy.

What the StratoSolar PV system does:
* Weather independent, photovoltaic solar power (PV)
* Locations up to latitude 60 produce market competitive electricity
* Electricity in utility scale systems from 10 MW to 1 GW in modular increments
* Cost competitive electricity without subsidy

Varying degrees of tracking are possible, and real systems will have results intermediate between flat plate and 2-axis tracking. At 20km sunlight can exceed 1.3kW/m2, which explains utilizations that exceed the theoretical 50% maximum achievable on the ground.


Stratosolar has a 23 page economic analysis of their system.

Nextbigfuture previously had an exclusive Sander Olson interview with Stratosolar president Edmund Kelly.

The OilDrum has an interview with Keith Henson (volunteer engineer with Stratosolar) about Stratosolar

A permanent high altitude platform could serve many additional purposes. Listed below are some examples of possible uses.
* Communications and observation platform
o Cell phone tower, data networks
o Radar for weather, commercial, military
o Science: astronomy, meteorology, earth science
o Laser communications network
o Tourism




The StratoSolar PV system has a reasonable operating cost mostly because the solar PV array (which dominates PV cost)

has a reasonable capital cost and a high utilization, with a resulting reasonable cost of electricity. The reasons for this are:
* The PV panels are exposed to 1.5 to 3.5X the solar energy of ground-based PV panels
* This means each square meter of PV panel gathers 1.5 to 3.5X the energy of ground-based PV panels
* The PV array uses no land. No land cost, or site development cost.
* The PV array support structure uses very little material due to light structural loads.
* All construction materials are standard, off the shelf, and low cost
* The PV panels are lower cost than ground-based PV panels due to reduced panel packaging cost
* The PV panels are higher efficiency than ground-based PV panels due to lower operating temperature and reduced reflection losses.

The extra capital costs incurred by the StratoSolar approach are the tether/HV cable, the winch, the gasbags and the hydrogen they contain. Adding everything up the capital cost of a StratoSolar plant in dollars per peak Watt ($/Wp) is the same as or lower than the same plant on the ground. (peak Watts is the standard way of defining the power output of PV panels) However the StratoSolar plant captures substantially more energy and generates substantially more kilo Watt hours (kWh) of electricity. Depending on geographic location the overall advantage in the cost of electricity generated in $/kWh over ground-based PV can exceed 3X.


Keith Henson Discussion at the Oildrum

After a year and a half of trying to find showstoppers, we feel confident that there are solutions to many of the problems identified. There is a lot of detailed engineering to do and there are problems we have identified but not solved—particularly how to construct the light pipe and the huge rotating aerodynamic shrouds. If anyone has ideas about how to construct objects of this size without wind destroying a partially completed section, please post your thoughts.

Hydrogen is expensive in terms of energy, about 50 MWh per ton. A tethered CSP plant would take a lot of hydrogen for buoyancy, around 5,000 tons. It also leaks hydrogen, around 1-3 percent per year. To fill one up would take 250,000 MWh or 250 GWh.

That's about 10.5 days for a one GW plant. The total energy investment for plastic, aluminum and steel wire, and a combined-cycle power plant is around 5 times as much as the hydrogen so the energy payback is about two months, about a tenth of the energy payback time for PV cells in a desert.

Wind: the biggest engineering problem

The optimization that led to a ~1 GW size is partly due to the volume of the buoyant support structures. The volume (and lift) goes up by the cube of the linear size while the area (and therefore wind resistance) goes up as the square of the size. Additionally, the light pipe losses go up as the reflectance raised to a power proportional to the length over the diameter.

Depending on the reflectance, the hollow light pipe optimizes at 30-50 meters. That size, and the resultant maximum wind drag for a cylinder, is just too high to be practical. A rotating aerodynamic shroud around the light pipe was required to reduce the wind drag into a manageable range (the shroud cuts wind drag by a factor of 10).

The shrouds turned out to be heavy, but large enough in volume that it was practical to offset their own weight and the weight of the light pipe and hold down cables with buoyancy cells. This prevents the exponential growth in cable size seen in space elevator designs when supported only from the top.

Even with aerodynamic shrouds, the models involve huge forces, 30-50 million Newtons of wind force, and four times that much excess buoyancy to keep the light pipe from bending over more than 15 degrees in a high wind. Made from inexpensive steel wire, the hold-down cables have a cross-section area similar to that of a medium-sized suspension bridge (upwards of 16 inches in diameter).

Every couple of years there is a high-wind event in the stratosphere that reaches 50 m/s (180 km/hr) for as much as a week. The 2-km diameter collector at the top simply can't take the stress of that high a wind, so provisions have to be made to fold it into an aerodynamic shape, and the power plant needs to burn gas or oil for that time if it is to be as reliable as a conventional power plant.
Can we start smaller? Not with the thermal type because the light pipe can't be made any smaller than a few tens of meters. If the pipe is a few meters, all the light is lost in the reflections.

Understanding that it isn't optimal, is it possible to put PV into the stratosphere? The power could come down at 20-30 kV in aluminum conductors inside the tethers.

There are enough advantages to consider this approach, particularly in Northern Europe and Japan. If the cost per square meter for cells supported in the stratosphere is close to the cost of a ground solar there is some, but not a not a lot of advantage, for putting PV solar collectors the in the stratosphere instead of a desert.It's a different story in places where there is persistent heavy cloud cover such as Germany and Japan. There the advantage over PV on the ground is a substantial multiple because of the high and predictable light levels.

The minimum size would be a lot smaller, in the few tens of MW.
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