What the system does:
·         Weather independent, predictable 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

Key insights:
The idea exploits two environmental facts.  Firstly, the stratosphere is a permanent inversion layer in the earth’s atmosphere.  Inversion layers effectively isolate gas bodies.  The calm weather free stratosphere is isolated from the turbulent troposphere below.  There is no rain, hail, snow, or moisture in the stratosphere and wind force is much reduced and stable.  This means that buoyant platforms suspended in the stratosphere can be permanently stationed there without needing to be winched down in bad weather. It also means that PV panels in the stratosphere don’t suffer water based weather effects and can be simpler and cheaper to manufacture.

Secondly, light from the sun at 20km altitude is both strong and constant from dawn to dusk.  At 20km a platform is above over 90% of the atmosphere, so sunlight is not significantly scattered or absorbed and there are no clouds to interrupt power generation.  This means that on average PV panels produce multiples of the energy they can generate on the ground,  and just as important, the energy is highly predictable and not subject to interruption by clouds or storms.

The figure above helps illustrate where a StratoSolar PV system sits in the atmosphere and its relationship to various phenomena at mid latitudes. The PV platform floats in the stratosphere at the bottom of the ozone layer where the air temperature is about -55 degrees Celsius.  The cumulonimbus cloud illustrates the relative scale and altitude of a severe super-cell thunderstorm, the most violent weather event that affects the tethers in the troposphere. Some super-cell thunderstorms can punch through the tropopause and top out close to 20km, and a small percentage spawn tornadoes.  The jet stream, another strong wind phenomenon that affects the tethers is not shown explicitly, but when present it would be positioned just below the tropopause and be a few km thick.   Original artwork by Windows to the Universe staff (Randy Russell)
This image is derivative from content on Windows to the Universe® (http://windows2universe.org) © 2010, National Earth Science Teachers Association.  This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

Why it generates electricity at a reasonable cost:
For solar-power plants, almost the complete operating cost is the loan payment.  The StratoSolar PV system generates electricity at a reasonable market competitive cost mostly because the solar PV array (which dominates PV cost) has a similar capital cost to current ground based PV power plants but generates electricity with a higher utilization, resulting in a lower cost of electricity that is market competitive without subsidy at current PV cell cost.  The reasons for this are:

• The PV panels are exposed to 1.5 to 3.5X the solar energy (in kWh) of ground-based PV panels (depending on Latitude)
• 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 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 $/Wp) is the same as or lower than the same plant on the ground.  However the StratoSolar plant captures more energy and generates more 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.  See the PV cost section below for more detail on this topic.

While there is currently no economically viable electricity storage technology, the uninterrupted highly predictable PV power generation does not need spinning backup generation, and the power plants can be where they are needed rather than in deserts thousands of kilometers away.  These two benefits offer significant additional cost advantages to StratoSolar, but the benefits are far larger than economic.  As PV is attempting to be accepted as a viable utility scale electricity option these two problems are the major barriers to PV's acceptance as a viable technology for large-scale electricity generation.

As part of an integrated grid,  PV in the stratosphere could provide the bulk of electricity requirements at the lowest market cost, with nuclear, hydro and natural gas covering the rest.  Natural gas would be the natural choice to cover the shortfall during darkness.  Given that fuel costs dominate the cost of electricity generated  from natural gas, the cost penalty from the reduced power plant utilization factor would be small. Such an integrated electricity grid could have a carbon footprint with a fraction of the current US electricity generation carbon footprint and produce electricity with the lowest cost and the highest price stability.

This is a commercially competitive alternative energy solution.  By not covering huge land areas, it saves on an expensive, highly regulated, and uncertain resource that tends to delay construction and limit financing options.  It also allows great flexibility in location.  The competitive and highly profitable economics should lead to a business that is market financed and does not need government support or subsidy once demonstrated.  It is a bonus that this energy is carbon-free, and solves energy security issues.

A description of the system: 
A PV array attached to a rigid buoyant platform and permanently positioned in the stratosphere at an altitude of 20 km, gathers sunlight, converts it to electricity and transmits it down a tether/high voltage (HV) cable to the ground where it connects to the electricity distribution grid. 

The figure above shows an individual PV system in the center (a larger view is shown below).  The novel element of a StratoSolar power plant is a buoyant tethered platform supporting an array of PV panels floating in the stratosphere.  The strong and light tether incorporates a HV power cable that transfers electric power to the ground.  Excess buoyancy in the floating platform pre-tensions the tether and allows the platform to resist wind forces.

  A rigid tetrahedral truss structure supports the PV array.  Buoyancy is from gasbags within the truss framework.  Models for the PV array power output are subject to simulation to a high degree of accuracy, with high confidence in the results.  While the buoyant structure is novel, there is no new science, and existing engineering design tools are sufficient.  The wind and buoyancy forces are well understood from an engineering perspective.  There are detailed meteorological models and historical data to provide an accurate statistical profile of the wind and buoyancy forces on the structure and tether.  The combination of accurate structural analysis and reliable meteorological data mean that structural viability can be determined to a high confidence level before construction.  Accurate models for sunlight and how it varies with location and altitude, daily and seasonally, provide an equally high confidence level for the power output.

The figure also shows two views of a large-scale system, the first view on the left with no wind and the second view on the right with a maximum wind load.  The large-scale system is a collection of 100 mechanically connected individual modular small-scale systems described above.  For clarity only some of the tethers are shown.  The benefits of connecting multiple smaller systems to make a larger system are reduced aerodynamic drag on the PV array and reduced impact on regulated airspace.  The array is directionally stable and can track the sun.  The reduced aerodynamic drag ensures that the structure can withstand the highest wind forces with a large safety margin and is safe to deploy on a permanent basis.  It also facilitates modular maintenance and repair, technology upgrades, and incremental overall system expansion.  Individual arrays can be winched down in a few hours when weather permits and can use adjacent tethers as guides to ensure safe control. 

Operationally there should be no need for people at 20km.  There is no need for large “hanger” structures on the ground, either for construction or maintenance.  During construction and maintenance the array structure is anchored at multiple points to the ground and effectively forms a roof over a protected space.  Maintenance on the ground only occurs during good weather and at night to avoid disruption in power output.

Another benefit of the modular approach is the system can grow and be financed incrementally, reducing the risk capital required to develop and demonstrate the system viability.

What are the benefits of 20km altitude?
·         More sunlight results in lower cost per kWh.  Under $0.10/kWh without subsidy with today's PV costs
·         Locations as far north as latitude 60 are practical and have the biggest relative cost advantage
·         No land needed for the PV array
·         Highly predictable power compared to ground based PV.  Weather never affects power.
·         No shading effects simplify the electrical design
·         The -50 degree Celsius cold environment at 20km altitude enhances PV efficiency
·         At 20km altitude the PV panels don’t have to handle rain, moisture, hail, snow or strong wind forces.  This reduces their packaging cost
·         The worst-case wind force on the PV array and the tether is low enough to allow for permanent tethering.  

Ground PV problems, none of which apply to StratoSolar

1. High cost of electricity needs subsidy but the high cost of subsidy is not politically sustainable
   
2. Intermittent output needs spinning backup generation, storage and/or a smart grid
   
3. Requires a long distance power grid which does not exist
   
4. Cheap natural gas is an easy alternative
   

Items 2 and 3 severely inhibit large scale deployment of ground based PV because they significantly increase the real cost of PV electricity.  Perhaps more significantly they are dependencies that need to be satisfied by different parties other than the PV plant developer. This potentially makes the development process a political nightmare.  So far the small scale of PV deployment relative to the electricity grid capacity has meant that these issues have mostly been ignored and the focus has been on item 1, the primary problem of high cost and the need for subsidy.   

The graph that follows takes a while to absorb, but is worth taking the time to do so.  It presents succinctly the true state of current and future PV electricity costs without the distortion of subsidies. It also clearly shows the benefit that StratoSolar brings to the table. For a more detailed analysis see this document. Link

The figure above shows a projection of the reduction in capital costs in $/Wp and resulting levelized cost of electricity(LCOE) in $/kWh with cumulative GWp installed capacity along a learning curve that has a forty plus year history.  LCOE is shown for several sunlight intensities that correspond to different geographic locations.  

A common way to refer to the variability in solar power input is to convert it to a utilization or capacity factor.  This is useful when comparing different power plant technologies.  Using this metric the lowest utilization is about 0.10, average for the US is about 0.15, desert is 0.20 to 0.25, and StratoSolar is about 0.30 to 0.35.

The horizontal band centered at $0.10/kWh represents electricity that is competitive in the marketplace without subsidy.  The width of the band represents the variability in electricity costs for different markets.  Europe and California tend to be at the high cost end of the band. This is an extremely significant barrier to cross.  Above the band is the world of government subsidy politics and low volume.  Below the band is the world of market economics and high volume.  

The vertical distance from this band to the different utilization lines represents the amount of subsidy needed.  As can be seen for current cumulative GWp the subsidy is very large.  This subsidy shows up in different ways.  In Europe the mechanism is mostly feed-in tariffs that result in higher costs of electricity to consumers.  In the US, the mechanism is mostly tax credits and accelerated depreciation ultimately paid by taxpayers. 

The synthetic liquid fuel and synthetic gas bands show the ranges where the reduction in LCOE enables the manufacture of synthetic fuels using conventional manufacturing technologies that have already been proven at scale.

This chart clearly illustrates several points:

1. The same plant with the same capital cost produces electricity with highly variable cost depending on location.  E.g. at the 2010, $3.50/Wp   capital cost, northern Europe generates electricity for about $0.60/kWh, and StratoSolar generates electricity for $0.12/kWh.  StratoSolar has the best location (which can be over northern Europe) and lowest cost.
   
2. The $3.50/Wp capital cost is approximately the 2010 cost.  At historical rates of improvement, the $1.50/W cost may occur by 2020 at best.  Even in the best desert locations, the resulting ground based PV electricity will still cost $0.12/kWh which will not be competitive without subsidy in 2020.
3. The amount of subsidy required over the next ten years to maintain the historical PV capacity growth rate will become economically difficult to sustain if the capacity increases at the historical exponential rate.
4. StratoSolar will produce electricity without subsidy with current $3.00/Wp PV technology and will benefit equally from the PV $/Wp cost improvement path, producing increasingly competitive lower cost electricity.
   
5. StratoSolar can do this for northern climes for which PV is not an economically viable option.

Utility scale PV in the desert needs huge additional investment in electricity distribution and backup generation that is not factored into the normal PV $/Wp estimates for construction cost and also has environmental and political problems.  This means the "desert" trend line in the chart that appears closest to economic viability, is likely too optimistic.

This chart helps illustrate how far currently ground PV is from commercial viability and hints at the enormous cost of subsidy it will take globally to sustain the historical rate of improvement necessary.  StratoSolar represents a low cost way to leverage the historical investment in PV technology into commercial viability at locations where PV is unlikely ever to be commercially viable and with today’s PV capital cost in $/Wp, which will apply even if the historical rate of $/Wp improvement slows.

The unsustainable high cost of ground PV subsidy:
PV generated electricity is not economically competitive with electricity generated from burning fossil fuels.  Though it is not broadly understood, the existence of the industry depends almost completely on government subsidies. The goal of the subsidies is to bring PV costs down to where they will be competitive.  PV costs have come down remarkably and with a consistent learning rate. However the costs are still too high and the volumes to drive the cost down imply subsidies of a magnitude that cannot be sustained.

The historical rate of PV plant cost reduction has been approximately 20% for each doubling in capacity manufactured and installed. link The PV cost volume graph below shows a projection of this trend forward at current rates until 2027.  The future will not unfold as predictably as this graph would imply, but it does give a general sense of the magnitude of things.  This rate of improvement from the current cost base will produce a growing and unsustainable subsidy burden as the GWp capacity rises exponentially while the cost of electricity does not fall below $0.10/kWh until around 2025. 

The subsidy cost projection graph below illustrates the growth and magnitude of the implied subsidy which amounts to a total of about $1000B.  If the political will to provide the subsidies that sustain the capacity growth diminishes, then the improvement in the $/Wp capital costs will slow and the unsubsidized market viability of PV will be delayed beyond 2025.  

Because StratoSolar can produce market competitive electricity with today's PV technology it can quickly reduce or eliminate the cost of subsidy and thereby ensure the growth in volume of GWp capacity that will maintain or even increase the rate of cost improvement in PV technology. Getting to economic viability sooner with StratoSolar means the cost of the subsidy is greatly reduced, or given the unlikelihood of sustaining the subsidy, StratoSolar can ensure that historical growth in PV volume will continue.

  Renewables source:  http://www.ethree.com/public_projects/renewable_energy_costing_tool.html. 
  CCGT source:  http://www.cpuc.ca.gov/PUC/energy/Procurement/LTPP/LTPP2010/2010+LTPP+Tools+and+Spreadsheets.htm 

The figure above illustrates the actual 2010 cost of subsidy in California for various energy sources. Busbar shows the $/MWh cost to utilities.  MACRS is accelerated depreciation, and tax credit is effectively a cash rebate.  This does not reflect the benefit of loan guarantees or free federal land.  Even with all these subsidies, as the busbar price in the figure above shows, the California utilities are paying more than market prices for PV electricity.  The historical rate of PV plant cost reduction has been approximately 20% for each doubling in capacity manufactured and installed.  This rate of improvement from the current cost base as explained above will produce an unsustainable subsidy burden. 

The CCGT cost in the figure above is for combined cycle gas turbine, the dominant power plant type in California.  Gas prices have fallen and are predicted to stay low, so the CCGT electricity cost will fall, making PV even less competitive.