Common concerns and frequently asked questions
Why have permanently tethered platforms never been done before now?
Will financing and insurance be difficult to obtain?
How do you handle static electricity and lightning?
What is the buoyancy gas?
How do you handle hydrogen safety?
Will the tethers be a hazard to aviation?
Is station keeping an alternative to tethering?
Will icing on the tether be a problem?
How will you survive extreme weather events like thunderstorms and hurricanes?
Won't the structure be vulnerable during deployment?
How will you handle maintenance and repair?
Won't the large scale structures cast a large shadow?
What happens if a structure falls from the sky?
Are meteorites a problem?
How will you handle hydrogen leakage and air contamination?
Won’t PV on the ground be good enough, soon enough?
Question: Are 20 km long tethers practical or feasable?
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Question: Why have permanently tethered platforms never been done before now?
This is a question that is rarely asked directly, but is probably among the most important to address up front. Tethered communications platforms only need small payloads and a minimum size platform is more than adequate. Why the minimum platform is so large is the usual question that gets to the core issue of why permanent tethered platforms have never been implemented. The closest attempt to permanent buoyant tethered platforms implemented so far has been the air force radars suspended from aerostats at 4.5km altitude along the southern US border. These were developed in the 1980s before PV was a viable source of electricity so the radars they carry are powered from diesel generators. Sufficient fuel is carried aloft to sustain about two weeks of operation. Given that constraint, permanent uninterrupted operation was not achievable. However, even without the energy supply constraint, the simple physics of the environment of the troposphere also make permanent tethered platforms beyond what is achievable with current technology. Troposphere wind speed can exceed 100m/s and air density is around 1kg/m3. A tethered platform that can station keep under these conditions has to be a minimum of about 500 meters long and sustain wind pressure exceeding 2000 Pascals. There are additional environmental constraints including strong vertical wind shear in thunderstorms and rain, snow and ice. The combination of the size required and the forces a lightweight structure of this size can sustain make this impossible to achieve under steady state conditions, never mind the chaos of stormy conditions. The radar aerostats are about 80 meters long and can therefore operate in a wind of around 30 m/s before being blown over. This wind speed also produces forces within the range that the super pressure body with a pressurization of about 300 Pa can sustain mechanically. Higher winds or bad weather cause the platforms to be winched down. This highlights another aspect of current tethered aerostats. Even winched down they are exposed to damaging winds and accidental impact with the ground or other objects, wind born debris etc. If the weather changes suddenly they may not be winched down in time and suffer damage. All these operational conditions add up to current aerostats having a tough life and limited average lifetime. The Stratosphere at 20km has a maximum wind speed of 50 m/s and an air density of about .09 kg/m3. There is no water and there are no storms. This operational environment means a platform theoretical minimum length is around 150 meters and the maximum wind forces are around 150 Pascals, or more than an order of magnitude less than the troposphere. This along with the benign, stable, water free, weather free, environment mean that real tethered platforms of about 300 meters in length can permanently station keep at 20km altitude and their relatively fragile structures can mechanically sustain the benign wind forces. |
Question: Will financing and insurance be difficult to obtain?
As stated in the opening blog posting, the StratoSolar-PV alternative is the result of studying the concerns raised by the original CSP based design which was perceived to be too risky on several fronts.
· The risk of catastrophic loss from extreme weather events
· The complexity of developing many technologies at untested scales and new environments
· The complexity of needing many costly and risky elements to build a system
· The inability to demonstrate and develop a system on a small scale
The PV system attacks these concerns directly. The design reduces catastrophic risks, has many fewer technology development elements, has very few elements to build a system, and provides incremental engineering development and incremental system deployment starting from a much lower initial cost in order to reduce financial risk at each stage.
Catastrophic risks are reduced by reducing the wind loads on the tethers and the PV array platform to where the system can sustain winds beyond worst case known winds simultaneously at all altitudes. The tethers have a very low cross section, and the platform is horizontal with a low cross section and static with no moving parts.
The development can start with a small R&D engineering test platform and simple tether that will cost in the low millions of dollars. This gets across the psychological barrier of actually tethering something useful at 20km altitude. It also develops and tests all the platform structural and electrical elements.
The 10MW power platform will cost tens of millions of dollars. This expenditure will be incremental in nature.
Finance and Insurance costs depend on the risks and the rewards. Unlike nuclear power, liability insurance should be low. The primary insurance risk is the replacement of the partial or complete loss of a power plant. Understanding the probabilities of damaging or destructive events will only come with time and experience. The R&D process should provide a degree of confidence as it progresses over several years and the technology becomes familiar. A successful R&D program that results in a product that demonstrates competitive economics for solar power will be a powerful incentive to overcome what should by then be imagined risks. The first systems will be relatively small investments. If the market finds it too difficult to fund or insure the early deployment stage, it is reasonable to expect that government assistance perhaps in the form of loan guarantees will fill that gap. Governments currently seem happiest supporting alternative energy at the early deployment stage.
It is our expectation that when deployed in production the risks of loss will be lower than conventional power plants due to the mostly passive operation of the plants.
· The risk of catastrophic loss from extreme weather events
· The complexity of developing many technologies at untested scales and new environments
· The complexity of needing many costly and risky elements to build a system
· The inability to demonstrate and develop a system on a small scale
The PV system attacks these concerns directly. The design reduces catastrophic risks, has many fewer technology development elements, has very few elements to build a system, and provides incremental engineering development and incremental system deployment starting from a much lower initial cost in order to reduce financial risk at each stage.
Catastrophic risks are reduced by reducing the wind loads on the tethers and the PV array platform to where the system can sustain winds beyond worst case known winds simultaneously at all altitudes. The tethers have a very low cross section, and the platform is horizontal with a low cross section and static with no moving parts.
The development can start with a small R&D engineering test platform and simple tether that will cost in the low millions of dollars. This gets across the psychological barrier of actually tethering something useful at 20km altitude. It also develops and tests all the platform structural and electrical elements.
The 10MW power platform will cost tens of millions of dollars. This expenditure will be incremental in nature.
Finance and Insurance costs depend on the risks and the rewards. Unlike nuclear power, liability insurance should be low. The primary insurance risk is the replacement of the partial or complete loss of a power plant. Understanding the probabilities of damaging or destructive events will only come with time and experience. The R&D process should provide a degree of confidence as it progresses over several years and the technology becomes familiar. A successful R&D program that results in a product that demonstrates competitive economics for solar power will be a powerful incentive to overcome what should by then be imagined risks. The first systems will be relatively small investments. If the market finds it too difficult to fund or insure the early deployment stage, it is reasonable to expect that government assistance perhaps in the form of loan guarantees will fill that gap. Governments currently seem happiest supporting alternative energy at the early deployment stage.
It is our expectation that when deployed in production the risks of loss will be lower than conventional power plants due to the mostly passive operation of the plants.
Question: How do you handle static electricity and lightning?
The platforms will be grounded and this will have a small affect on the earth's fair weather electric field in the vicinity of the platform. The fair weather leakage current is normally on the order of one pico amp per square meter. The total leakage current from a 50 square kilometer platform will be about 100 micro amps. Normal grounding and shielding practice for electrical systems ensure this fair weather current is conducted to ground and static charge is not permitted to build up on the platform.
Lightning is handled with lightning conductor cables attached to tethers suspended from the platforms and electrically connected to ground, but not the platform. For large platforms these will form a Faraday cage around the perimeter of the platform. The usual surge protectors are also installed within the electrical system. For large systems, tethers, high voltage cables and lightning conductors are separate cables. Tethers are needed to provide mechanical support for conducting cables which are joined mechanically to the tethers.
For small systems, tethers and High Voltage cables and lightning shields are combined in one compound cable.
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. Current aerostat cables scaled up to about 10cm in diameter serve as the basis of one possible cable design.
For some detail see US patent 4842221 “Lightning 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/.
Using an aluminum 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 lightning 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 platform structure is well above thunderclouds so lightning will tend to strike suspended conductors.
Lightning is handled with lightning conductor cables attached to tethers suspended from the platforms and electrically connected to ground, but not the platform. For large platforms these will form a Faraday cage around the perimeter of the platform. The usual surge protectors are also installed within the electrical system. For large systems, tethers, high voltage cables and lightning conductors are separate cables. Tethers are needed to provide mechanical support for conducting cables which are joined mechanically to the tethers.
For small systems, tethers and High Voltage cables and lightning shields are combined in one compound cable.
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. Current aerostat cables scaled up to about 10cm in diameter serve as the basis of one possible cable design.
For some detail see US patent 4842221 “Lightning 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/.
Using an aluminum 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 lightning 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 platform structure is well above thunderclouds so lightning will tend to strike suspended conductors.
Question: What is the buoyancy gas?
Buoyant stratospheric solar power platforms need substantial amounts of buoyancy gas. The logical choice is helium. Based on our reference PV platform design the estimated helium required is from 2tonnes/MWp to 5tonnes/MWp. A reference 20MWp modular platform would need about 100tonnes of helium. A 1GWp plant would need between 2000tonnes and 5000tonnes of helium. The table below shows 2011 USGS statistics for world helium annual production and estimated reserves. With helium supply numbers like this, 20GWp small-scale plants would not stress the available helium resource, and there are sufficient reserves to expand yearly production to meet such demand. However larger utility scale GWp plants would stress current production and require a significant growth in annual production. Any substantial deployment of utility scale stratospheric PV plants would severely stress the available resource, and ultimately limit deployment to considerably less than 1000GWp.
This means that hydrogen is necessary for large-scale deployment. Hydrogen is effectively a limitless resource already produced in substantially higher volume than Helium (>50Mt/year). Only half the mass of hydrogen is needed compared to helium (1tonne/MWp to 2.5tonnes/MWp), and Hydrogen is considerably cheaper (<$6/kg compared to >$15/kg). Hydrogen is also considerably easier to contain in gasbags.
Hydrogen’s one Achilles heel is flammability. A separate FAQ answers concerns about Hydrogen safety. A reasonable strategy is to use helium to simplify initial development and deployment and transition to hydrogen as volume grows and engineering of hydrogen-based systems demonstrates market acceptable safety levels. Helium could always satisfy the lower volume and stronger safety requirements of military platforms.
U.S. Geological Survey, Mineral Commodity Summaries, January 2011 Helium statistics.
Production (t) Resource (t)
United States (extracted from natural gas) 13,752 3,679,160
United States (from Cliffside Field) 8,573
Algeria 3,215 1,464,520
Canada - 357,200
China - 196,460
Poland 464 -
Qatar - 1,803,860
Russia 982 1,214,480
Other countries - -
World total (rounded) 26,790 9,287,200
Buoyant stratospheric solar power platforms need substantial amounts of buoyancy gas. The logical choice is helium. Based on our reference PV platform design the estimated helium required is from 2tonnes/MWp to 5tonnes/MWp. A reference 20MWp modular platform would need about 100tonnes of helium. A 1GWp plant would need between 2000tonnes and 5000tonnes of helium. The table below shows 2011 USGS statistics for world helium annual production and estimated reserves. With helium supply numbers like this, 20GWp small-scale plants would not stress the available helium resource, and there are sufficient reserves to expand yearly production to meet such demand. However larger utility scale GWp plants would stress current production and require a significant growth in annual production. Any substantial deployment of utility scale stratospheric PV plants would severely stress the available resource, and ultimately limit deployment to considerably less than 1000GWp.
This means that hydrogen is necessary for large-scale deployment. Hydrogen is effectively a limitless resource already produced in substantially higher volume than Helium (>50Mt/year). Only half the mass of hydrogen is needed compared to helium (1tonne/MWp to 2.5tonnes/MWp), and Hydrogen is considerably cheaper (<$6/kg compared to >$15/kg). Hydrogen is also considerably easier to contain in gasbags.
Hydrogen’s one Achilles heel is flammability. A separate FAQ answers concerns about Hydrogen safety. A reasonable strategy is to use helium to simplify initial development and deployment and transition to hydrogen as volume grows and engineering of hydrogen-based systems demonstrates market acceptable safety levels. Helium could always satisfy the lower volume and stronger safety requirements of military platforms.
U.S. Geological Survey, Mineral Commodity Summaries, January 2011 Helium statistics.
Production (t) Resource (t)
United States (extracted from natural gas) 13,752 3,679,160
United States (from Cliffside Field) 8,573
Algeria 3,215 1,464,520
Canada - 357,200
China - 196,460
Poland 464 -
Qatar - 1,803,860
Russia 982 1,214,480
Other countries - -
World total (rounded) 26,790 9,287,200
Question: How do you handle 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, lightning 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 fire 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 usually a secondary consequence and not the primary cause of destruction or loss of life. The Akron, the Macon and the Shenandoah all used Helium as the buoyancy gas, and all were damaged or destroyed in stormy weather.
Question: Will the tethers be a hazard to aviation and birds?
This is a short answer. There is a more detailed discussion here.
StratoSolar relies on a few large systems, placed away from air traffic corridors, each system in its own small restricted airspace. The small total area of these restricted air-spaces will have little impact on commercial aviation which already deals with a complex air traffic control system where aircraft follow well defined routes to avoid each other and existing restricted air-spaces.
The FAA clearly has concerns which we are addressing. Compared to various proposals to harness wind power from the jet stream using enormous numbers of windmills, the StratoSolar impact on airspace is minimal.
The PV array structures are well above the cruising altitude of aircraft and also above regulated airspace. The danger to aircraft and birds is from the tethers which are in regulated airspace above 18,000 feet. Tethers will carry lights, radar reflectors and transponders which will alert planes and Air Traffic Control (ATC) to their presence. The systems will also be visible as ground hazards in the GPWS/TAWS system carried by commercial aircraft. VFR pilots would be breaking the rules by entering a tether restricted airspace. If they are flying without instruments they will be dependent on visual aids like lights and warning systems like Obstacle Collision Avoidance System (OCAS). http://www.ocasinc.com/
StratoSolar PV platforms will carry OCAS radars to monitor airspace and warn aircraft if they are on a collision course with tethers. Given their 500km visibility horizon from the platforms, these radars will probably become part of the official air traffic control system. It is also possible to actively track possible aircraft impact on tethers and automatically move the tethers to avoid impact. The redundancy provided by many tethers make this possible.
StratoSolar relies on a few large systems, placed away from air traffic corridors, each system in its own small restricted airspace. The small total area of these restricted air-spaces will have little impact on commercial aviation which already deals with a complex air traffic control system where aircraft follow well defined routes to avoid each other and existing restricted air-spaces.
The FAA clearly has concerns which we are addressing. Compared to various proposals to harness wind power from the jet stream using enormous numbers of windmills, the StratoSolar impact on airspace is minimal.
The PV array structures are well above the cruising altitude of aircraft and also above regulated airspace. The danger to aircraft and birds is from the tethers which are in regulated airspace above 18,000 feet. Tethers will carry lights, radar reflectors and transponders which will alert planes and Air Traffic Control (ATC) to their presence. The systems will also be visible as ground hazards in the GPWS/TAWS system carried by commercial aircraft. VFR pilots would be breaking the rules by entering a tether restricted airspace. If they are flying without instruments they will be dependent on visual aids like lights and warning systems like Obstacle Collision Avoidance System (OCAS). http://www.ocasinc.com/
StratoSolar PV platforms will carry OCAS radars to monitor airspace and warn aircraft if they are on a collision course with tethers. Given their 500km visibility horizon from the platforms, these radars will probably become part of the official air traffic control system. It is also possible to actively track possible aircraft impact on tethers and automatically move the tethers to avoid impact. The redundancy provided by many tethers make this possible.
Question: Is station keeping an alternative to tethering?
Station keeping is difficult. High altitude station keeping airships (HAA) powered by electricity from PV arrays and batteries have been investigated for a decade or more, and teeter on the edge of practicality. Despite large investment none have yet succeeded. See the HAA stratospheric winds paper reference in the bibliography section on the StratoSolar web site. PV solar power is very reduced at northern latitudes in winter, which restricts operation to lower latitudes. The highest occasional stratospheric winds that come from excursions of the polar vortex in winter can get to 55m/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. For power generation systems, the cost of motors and batteries, the power loss needed for thrust, the over 50% loss in power transmission 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 with a high voltage cable transmitting power to the ground.
Station keeping for drone aircraft for wireless "satellite" communications face the same constraints as HAA. These have been investigated since the Pathfinder in the 1980's, which achieved solar powered stratospheric flight, but not station keeping continuous flight. Gradually over the years the electric motor technology, the PV technology and the air frame technology have improved considerably to where there are several proposals, both military and commercial that hope to achieve sustained station keeping at low Latitudes, where solar energy in winter is not reduced significantly and maximum wind-speeds are less than 35 m/s because the excursions of the polar vortex don't extend below latitude 30.
The constraining technology is energy storage for night time flight. Commercial attempts like Titan Aerospace and Ascenta rely on batteries and improving battery technology for energy storage. Neither has yet demonstrated station keeping, and Titan has stated publicly that it is betting on better battery technology. DARPA has funded Boeing to build a large payload station keeping drone called Solar Eagle. This project has been scaled back to focus solely on the energy storage solution which relies on hydrogen energy storage using an electrolyzer and fuel cell arrangement. This technology has clearly yet to demonstrate operational viability.
To summarize, station keeping for power generation platforms will probably never be economically viable, and station keeping for communications and observation has yet to demonstrate operational viability and even then will only be able to operate at low latitudes.
Station keeping for drone aircraft for wireless "satellite" communications face the same constraints as HAA. These have been investigated since the Pathfinder in the 1980's, which achieved solar powered stratospheric flight, but not station keeping continuous flight. Gradually over the years the electric motor technology, the PV technology and the air frame technology have improved considerably to where there are several proposals, both military and commercial that hope to achieve sustained station keeping at low Latitudes, where solar energy in winter is not reduced significantly and maximum wind-speeds are less than 35 m/s because the excursions of the polar vortex don't extend below latitude 30.
The constraining technology is energy storage for night time flight. Commercial attempts like Titan Aerospace and Ascenta rely on batteries and improving battery technology for energy storage. Neither has yet demonstrated station keeping, and Titan has stated publicly that it is betting on better battery technology. DARPA has funded Boeing to build a large payload station keeping drone called Solar Eagle. This project has been scaled back to focus solely on the energy storage solution which relies on hydrogen energy storage using an electrolyzer and fuel cell arrangement. This technology has clearly yet to demonstrate operational viability.
To summarize, station keeping for power generation platforms will probably never be economically viable, and station keeping for communications and observation has yet to demonstrate operational viability and even then will only be able to operate at low latitudes.
Question: Will icing on the tether be a problem?
Icing may be a problem for short segments of tethers below 10km altitude. Icing occurs in the troposphere where tethered aerostats have been deployed for many decades. Icing has not been a documented problem for these aerostats or tethers. Should it become an issue there are several engineering solutions to help prevent it.
Question: How will you survive extreme weather events like hurricanes?
For more on this topic see here. Extreme weather events occur in the troposphere usually well below 12.5km altitude, but large hurricanes can reach to 15 km altitude. The PV platforms are safely above the troposphere at an altitude of 20 km (65,000 feet) where there is no violent stormy weather and maximum wind forces are less than one tenth of the maximum wind forces in the troposphere. Only the tethers and high voltage cables are exposed to extreme weather events like thunderstorms and hurricanes. The tethers are very durable and have a small cross section area exposed to wind. The forces on the tether from extreme winds add a manageable tension in the tethers and small resulting platform deflections. The biggest source for potential damage to tethers is from wind carried debris striking the bottom 1km during tornadoes or hurricanes. The lowest 1km of tether will be protected from debris with a reinforced Kevlar shroud. There are always many redundant tethers, so even if one is damaged the rest can carry the load.
Question: Won’t the structure be vulnerable during deployment?
During construction, before deployment, system elements are folded flat on the ground covered by tarps, protected from weather. It is important to monitor weather and wind at all altitudes below 20 km before deployment, much like deploying a large oil production platform today. Particular attention has to be paid to the wandering jet stream, especially when deploying small platforms and the initial platform elements of large platforms. However the window of exposure is very small. Systems can be deployed from ground to 20 km altitude in less than two hours. Modern technology can monitor and predict winds and weather with sufficient accuracy that a two-hour window of benign weather for deployment can be guaranteed with a very high degree of confidence.
Question: How will you handle maintenance and repair?
The systems are designed with a high degree of redundancy. Failure of individual components will result in small losses of power output. It is envisaged that system elements will be winched down perhaps once every several years to repair or replace the accumulated failed components and perhaps replenish the small hydrogen loss. As with deployment, the window of exposure to bad weather is very small and system elements will be brought down only when risks are minimal.
Question: Won’t the large-scale structures cast a large shadow?
While the structures are large they are small compared to clouds and are much higher than most clouds. The shadow footprint on the ground is small and transient as the earth rotates. See the animations on the web site to get a visual appreciation of the scale.
Question: What happens if a structure falls from the sky?
Platforms will be located over lightly populated land, possibly agricultural land, forest, hills or inshore ocean. This land will most likely be owned and managed by the platform owners. The platform structures are very lightweight for their size. For a structure to fall it must lose most of its buoyancy gas, either through fire or structural collapse, or a mixture of both. The massively redundant tethers ensure that a platform will not become adrift in the sky. Almost all scenarios would result in debris falling on the lightly populated location directly under the platform with, heavy objects like DC converters falling close to the tether anchor. For almost all disaster scenarios there would be ample time to issue warnings to evacuate or go to protected shelters. This combination of low-density material falling in a lightly populated and controlled area would result in very little collateral damage.
Question: How will you handle hydrogen leakage and air contamination?
The rate of hydrogen or helium loss from gasbags is very low, less than 0.1% a year. This is due to a combination of the low permeation rate of metalized polyester (Mylar), the low surface to volume ratio and the low temperature and pressure environment. This loss can be replaced easily as platform elements or gravity storage weights are brought up and down. The amount of oxygen entering gas bags from air infiltration is very low and can be handled in a variety of ways including the use of passive oxygen scavenger materials and active filtering.
Question: Will meteorites be a problem?
Almost all meteorites are small and burn up in the mesosphere which is between 50km to 80km altitude. The Stratosphere is below the mesosphere and extends from 12km to 50km. StratoSolar is at about 20km in the low Stratosphere. Very rarely (once every 100 years or so) a larger meteor like the recent Chelyabinsk meteor makes it to the stratosphere before burning up, but even then the probability of a strike on a StratoSolar platform is very very remote.
Almost all meteorites are small and burn up in the mesosphere which is between 50km to 80km altitude. The Stratosphere is below the mesosphere and extends from 12km to 50km. StratoSolar is at about 20km in the low Stratosphere. Very rarely (once every 100 years or so) a larger meteor like the recent Chelyabinsk meteor makes it to the stratosphere before burning up, but even then the probability of a strike on a StratoSolar platform is very very remote.
Question: Won’t PV on the ground be good enough, soon enough?
The PV panel rapid price drop since 2010 has some optimists projecting this short term price reduction forward in time and anticipating overall system costs of $1/W within a few years. However the reason for the price drop has been an extreme manufacturing overcapacity with manufacturers shipping panels in many cases at a loss. This situation is not sustainable, and there are signs that supply is coming into balance with demand and polysilicon and PV panel prices are firming up. The realistic assessment is that PV prices may actually rise a bit from current depressed levels and will stay close to current prices for several years before resuming the historical 20% PV panel price reduction. This puts the $1/W system price out well past 2020.
The $1/W price is not nirvana. At that price, PV electricity generation with low cost financing will be competitive without subsidy in some sunny locations. This does not cover storage, backup or transmission costs. Also in less sunny locations the cost of PV generation will be higher and will not be competitive without subsidy. This is not to say that the PV business will decline. It will be a profitable business with a reasonable growth rate that will satisfy the industry participants, but the growth in installed PV capacity will only be a small fraction of what is necessary to have an impact on reducing CO2 emissions.
StratoSolar is not just a one off price reduction. All the reasons that PV panels will continue their historic price reduction still apply to StratoSolar. This means that when PV prices fall to a point where ground PV generation is competitive in sunny locations, StratoSolar can generate electricity at about $0.02/kWh, a low enough cost to manufacture synthetic fuels that are cost competitive with fossil fuels.
StratoSolar offers a low risk shot at an opportunity to make today’s PV the solution it aspires to be but unfortunately, is very far from achieving.
The PV panel rapid price drop since 2010 has some optimists projecting this short term price reduction forward in time and anticipating overall system costs of $1/W within a few years. However the reason for the price drop has been an extreme manufacturing overcapacity with manufacturers shipping panels in many cases at a loss. This situation is not sustainable, and there are signs that supply is coming into balance with demand and polysilicon and PV panel prices are firming up. The realistic assessment is that PV prices may actually rise a bit from current depressed levels and will stay close to current prices for several years before resuming the historical 20% PV panel price reduction. This puts the $1/W system price out well past 2020.
The $1/W price is not nirvana. At that price, PV electricity generation with low cost financing will be competitive without subsidy in some sunny locations. This does not cover storage, backup or transmission costs. Also in less sunny locations the cost of PV generation will be higher and will not be competitive without subsidy. This is not to say that the PV business will decline. It will be a profitable business with a reasonable growth rate that will satisfy the industry participants, but the growth in installed PV capacity will only be a small fraction of what is necessary to have an impact on reducing CO2 emissions.
StratoSolar is not just a one off price reduction. All the reasons that PV panels will continue their historic price reduction still apply to StratoSolar. This means that when PV prices fall to a point where ground PV generation is competitive in sunny locations, StratoSolar can generate electricity at about $0.02/kWh, a low enough cost to manufacture synthetic fuels that are cost competitive with fossil fuels.
StratoSolar offers a low risk shot at an opportunity to make today’s PV the solution it aspires to be but unfortunately, is very far from achieving.
Question: Are 20 km long tethers practical?
Tethers have to carry their own weight, the net buoyancy of the tethered platform and the wind forces that act on the tether and the platform. A common naïve assumption is that the weight of the tether is the biggest problem and solutions like tapering the tether from wide at the top to narrower will be necessary. With modern materials this is not an issue.
A theoretical material property is breaking length, the length of constant cross section material where the suspended weight matches the material breaking strength. This table shows breaking length for a variety of materials. As can be seen the breaking length for Kevlar is 256 km and for UHMWPE is 378 km both of which vastly exceed 20 km. These are the two candidate materials, though as the table shows there are others, like PBO that may also be suitable candidates.
The major force carried by the tether is the net buoyancy of the platform, followed by the maximum wind force. Its own weight is less than 10% for UHMWPE.
Another concern is material creep under the constant tension. Kevlar has very low creep, and UHMWPE has low creep at the low operational temperatures encountered over most of the tether length.
To calibrate an understanding of tether weight, a 1cm square cable simplifies the math. It’s cross sectional area is 1 cm2. That is (1/10,000)m2. The tether length is 20 km or 20,000 m. Multiplying 1/10,000 by 20,000, gives a volume of two cubic meters (2m3). For UHMWPE with a density of 970 kg/m3, that’s 1,940 kg. For Kevlar with density of 1480 kg/m3, that’s 2,960 kg.
So for each square centimeter of cross sectional area, the 20 km tether weighs nearly 2 tonnes for UHMWPE and nearly 3 tonnes for Kevlar.
The 50% de-rated breaking strength of UHMWPE material is about 1.8E+9 Pa. The strength of one square cm, (1E-4m2 ) is then 1.8E+5N. That’s 180,000 N or about eighteen tonnes. So a 1 cm2 cross section cable supporting 18 tonnes of load weighs 2 tonnes and can therefore carry 16 tonnes.
Tethers have to carry their own weight, the net buoyancy of the tethered platform and the wind forces that act on the tether and the platform. A common naïve assumption is that the weight of the tether is the biggest problem and solutions like tapering the tether from wide at the top to narrower will be necessary. With modern materials this is not an issue.
A theoretical material property is breaking length, the length of constant cross section material where the suspended weight matches the material breaking strength. This table shows breaking length for a variety of materials. As can be seen the breaking length for Kevlar is 256 km and for UHMWPE is 378 km both of which vastly exceed 20 km. These are the two candidate materials, though as the table shows there are others, like PBO that may also be suitable candidates.
The major force carried by the tether is the net buoyancy of the platform, followed by the maximum wind force. Its own weight is less than 10% for UHMWPE.
Another concern is material creep under the constant tension. Kevlar has very low creep, and UHMWPE has low creep at the low operational temperatures encountered over most of the tether length.
To calibrate an understanding of tether weight, a 1cm square cable simplifies the math. It’s cross sectional area is 1 cm2. That is (1/10,000)m2. The tether length is 20 km or 20,000 m. Multiplying 1/10,000 by 20,000, gives a volume of two cubic meters (2m3). For UHMWPE with a density of 970 kg/m3, that’s 1,940 kg. For Kevlar with density of 1480 kg/m3, that’s 2,960 kg.
So for each square centimeter of cross sectional area, the 20 km tether weighs nearly 2 tonnes for UHMWPE and nearly 3 tonnes for Kevlar.
The 50% de-rated breaking strength of UHMWPE material is about 1.8E+9 Pa. The strength of one square cm, (1E-4m2 ) is then 1.8E+5N. That’s 180,000 N or about eighteen tonnes. So a 1 cm2 cross section cable supporting 18 tonnes of load weighs 2 tonnes and can therefore carry 16 tonnes.