StratoSolar
Keith Henson wrote: On Tue, Oct 12, 2010 at 11:00 AM, Dan Minette danmine...@att.net wrote: To: 'Killer Bs \(David Brin et al\) Discussion' We probably will never know if this StratoSolar method works. ... David Hobby hob...@newpaltz.edu wrote: I see bigger problems with losses in the light pipe. The plan seems to be to have a flexible tube lined with reflective material to guide the solar radiation down to steam turbines or whatever on the ground. Most of the light would have to reflect off the sides many times, losing at least a few percent of its intensity at each reflection. So nothing makes it to the ground, and the light pipe melts. There may be solutions to this too, but they're going to be tricky. How many reflections are you assuming light will make as it goes down the pipe, and how glancing are they? Keith-- Hi. Thanks for the details. I started thinking about the problem. It depends on the acceptance angle and the diameter of the light pipe. I'll give you that the spread for light come out of the whole array and into the pipe is 30 minutes, the same as the sun subtends in the sky. So that would be an average deviation of something like 10 minutes, or .003 radians. (Actually achieving that may be a headache, but I bet it could be done if it mattered. Although I believe that it doesn't matter that much, since even if light went into the pipe with only small angular errors, the average incidence angle would rapidly increase due to somewhat random reflections off the walls. See below.) This stuff: http://www.revelationlighting.co.uk/OLF%20Spec.pdf has a .99 reflectivity for angles less than 27 deg, That's pretty good reflectivity. Plastic tends to crinkle, though, so you'll need some sort of backing to help keep it flat. and almost all the loss comes from the points not being sharp. Lost me there. What points? At .999, which the optical guys say is not hard, and a 30 meter diameter light pipe, the loss is about 7%. One option is to fill the pipe with argon which reduces the Rayleigh scattering. Working backwards, you're assuming around ln(.93)/ln(.999) = 73 reflections? For a 30 km light pipe, that's around one reflection every 400 meters, for an average angle of 30/400 = .075 radians, or 4 degrees. It would take a thorough analysis, but I'm betting that successive reflections from the slightly crinkly walls of the light pipe would gradually increase the average incidence angle, pretty much like a random walk. O.K., I'll buy that, if you can get .999 reflectance at angles of a few degrees. There is 4 GW coming down the pipe. At 7% loss, 280 MW. The area of a 30 meter x 20 km pipe is 2 million square meters so the loss would be 140 W per square meter. In open air it is only going to get slightly warm. O.K., but what about localized losses? Suppose there's a sharp bend when the light pipe hits the jet stream, or something? If the pipe bends something like 45 degrees over 300 meters, then you'd have basically all the light hitting one side of the pipe over around 100 meters. And it would hit at a 10 or 15 degree angle, which probably decreases reflectivity to .995 or so? Then you've got .005 of 4GW hitting an area of around 100*30 square meters, giving .005*4GW/3000 = 7000 watts per meter. So that's as hot as grabbing a 60 watt incandescent bulb? It might still work, but things are getting tricky. For instance, after that one bend the average light ray is going to be hitting the sides of the pipe at 10 or 15 degree angles all the way down. (Unless you've got a mechanism to straighten out rays that are bouncing off the sides too much? I can't think of an easy one.) If you have a ray permanently at an angle of .2 radians, it hits every 150 meters, which would be around 100 times over 20 km. And if reflectivity is down to .995 at that angle, you're left with .995^100 = 60% of the light at the bottom. Another problem could be fluttering. If you have enough transient surface waves running over the light pipe, each one giving large random reflections to rays unlucky enough to hit it, you could rapidly have almost all of the rays bouncing off the walls at 20 or 30 degrees. That gives you more reflections per ray, each at larger angles with lower reflectance. Something like that could really cause big losses. It's an interesting problem. Thanks. ---David ___ http://box535.bluehost.com/mailman/listinfo/brin-l_mccmedia.com
Re: StratoSolar
On Oct 11, 2010, at 5:29 PM, Alberto Monteiro wrote: Keith Henson wrote: Since the 1970s, US politicians have given lip service to National Energy Self-sufficiency. The US has failed to achieve anything, largely because nobody had a good idea of how to make it work at the same or lower cost than importing oil. This method might not work either. However, it passes first-order physics and economics analysis and seems to deserve serious further study. You (USA) might be closer to self-sufficienty than you (Keith) think. Deepen the crisis (and reduce energy expendidure) and get a little more of shale gas, and you get there. Alberto Monteiro, minion of evil oil companies I still want to see someone work out a production scale process for seafloor methane-syngas-syncrude. Or even convert from flaring off natgas in the oilfield to field-scale syncrude production. If we have a finite amount of methane available, the least we can do is stop wasting it in production. Once you get to syncrude, you have perfectly reasonable refinery feedstock. Obviously it's a stopgap solution, but it would buy time to get off of a petroleum-based energy economy before the worst aspects of post-peak- oil economy start to kick in. (I would *really* like to see petroleum production start to migrate more toward plastics feedstock, and plastics in turn migrate away from disposable packaging -- the dreaded PETE water bottle included -- and more toward durable materials engineering. There's time yet to consider that. But that's later on in the plan. Along with recovering a lot of what's already been tossed into landfills .. which can be mined, if it comes down to it.) 'How do I print, Mr. Kahn?’ ‘How do I save?’ It’s Control-S! It’s ALWAYS Control-S!!” — Kahn Souphanousinphone ___ http://box535.bluehost.com/mailman/listinfo/brin-l_mccmedia.com
StratoSolar
StratoSolar This is off NDA so I can go into detail. For a few years, I was working on a way to reduce the cost of space-based solar power to the point it could displace fossil fuels. That's two cents or less per kWh, which is half the price of electric power from coal, and low enough that (off peak) it can be used to make synthetic hydrocarbon transport fuels for about a dollar a gallon. The rough economic analysis is based on a ten-year repayment of capital cost. Run 80,000 hours in ten years the return is $800 per kW per penny payment for a kWh. For power satellites, assuming 5kg/kW, $100 per kg lifted to GEO and about 1/3 of the cost going to transport, you get the required $1600/kW for 2 cents per kWh. With the help of Jordin Kare, Howard Davidson, Ron Clark, Spike Jones and others, by last January I had a proposal that looked like it would reach $100/kg cost to GEO. The general approach was discussed in an article in The Oil Drum about a year ago. It proposes huge lasers to get the average exhaust velocity up to the mission velocity. This gives a mass ratio to LEO of about 3and a throughput to GEO upwards of 100 t per hour. Late last year Howard became aware of a project an old friend of his, Ed Kelly, was working on. Ed is best known as a principal with Transmeta, a company that developed low-power processors some years ago. Howard introduced me to Ed. I have spent a lot of time going over Ed's spreadsheets and other details since last January. In the post-analysis, the reason ordinary ground solar power is so expensive is the huge amount of materials that are needed because solar energy is so dilute. (Wind has the same problem.) Ed's approach, which he named StratoSolar, was to reduce the mass from hundreds of kg per kW to a few tens of kg by moving the solar concentrator into the stratosphere as a large, lightweight, buoyant structure. This has significant advantages over being on the ground. There are no clouds at 20 km. The winds are light and steady and the low air density reduces the force on the structure. Because the primary concentrator can be pointed directly toward the sun, it gives close to full power whenever the sun is above the horizon. (Rough pointing--one to two degrees--can be done with combinations of thrusters, aerodynamic fins and reaction motors, fine pointing by stepper motors moving the mirror segments.) They work as far north as Stockholm. The concentrated sunlight gets to the ground via a hollow light pipe lined with highly reflective prismatic plastic. Preliminary optimization for kg/kW leads to a 30-meter diameter light pipe with less than 10% loss. A larger pipe has lower losses but uses more total material per kW. Because the mass is dependent on the pipe diameter and the power capacity on the area, StratoSolar plants optimize in large sizes, around 1 GW. That means the primary collector is a bit over 2 km in diameter and 100-200 meters thick. That gives plenty of room for gasbags to offset its weight. While the concentrator has neutral buoyancy, the light pipe has a lot of excess buoyancy. If you just think about it as a force diagram, the buoyancy needs to be 3-4 times the wind force to keep the angle the light pipe makes with the ground within 15-20 degrees of vertical. The materials required—aluminum, plastic, steel wire, and hydrogen (for buoyancy)—are all inexpensive and do not need to be processed to tighter specifications than the norm for commercial products. The sunlight is absorbed and converted to heat at the bottom. The heat is used to run an ordinary, 45%-60%-efficient, one or two stage power plant. About half the heat during the day is used to heat a solid heat thermal storage medium. This will provide enough stored heat to run the plant overnight. Graphite is a good choice, but any high temperature solid would work. Cowper blast furnace stoves (regenerators, dating from 1837) produce air as hot as 1400 deg C, just about the limit for turbine inlet temperature. While stoves for this application are big (typically 70,000 cubic meters), they are dead simple and should cost well under $100 million for a GW plant. That cost adds 1/8 of a cent per kWh to the cost of power. This is less than 1/10th the cost of any other proposed storage mechanism. Our rough estimate for the cost is around $1.2 B per GW, or $1200 per kW. Using the above ten-year payback, the cost to generate power should be around 1.5 cents per kWh. It will take building a few to learn how to manufacture them and get accurate cost numbers. However, if this is close, it will solve the long-term energy problems and get the human race off fossil fuels by simply under pricing them. Like any other large project, there are a million details. We have given thought to such topics as ozone, lightning, hydrogen fires, thunderstorms, icing, interaction with aircraft, high wind loads, aerodynamic shrouds, UV damage, turbine throttling, maintenance access
Re: StratoSolar
Keith Henson wrote: Since the 1970s, US politicians have given lip service to National Energy Self-sufficiency. The US has failed to achieve anything, largely because nobody had a good idea of how to make it work at the same or lower cost than importing oil. This method might not work either. However, it passes first-order physics and economics analysis and seems to deserve serious further study. You (USA) might be closer to self-sufficienty than you (Keith) think. Deepen the crisis (and reduce energy expendidure) and get a little more of shale gas, and you get there. Alberto Monteiro, minion of evil oil companies ___ http://box535.bluehost.com/mailman/listinfo/brin-l_mccmedia.com
RE: StratoSolar
Just a quick point. Run 80,000 hours in ten years the return is $800 per kW per penny payment for a kWh. For power satellites, assuming 5kg/kW, $100 per kg lifted to GEO and about 1/3 of the cost going to transport, you get the required $1600/kW for 2 cents per kWh. Well, that seems really low, so I looked up present costs. At http://crowlspace.com/?page_id=50 there is a talk promoting space based solar. It was honest enough to admit: The launch cost from Earth to low earth orbit is the greatest impediment to this project. It is currently about $5,000 per pound to low earth orbit, and it has been about that cost for a long time. Geosynchronous orbit would raise the cost to 10,000/pound. Given the fact that, as mentioned in the talk, lift costs have been fairly constant, where does the factor of 200 improvement come from? How do you know it will happen when it hasn't? Dan M. ___ http://box535.bluehost.com/mailman/listinfo/brin-l_mccmedia.com
Re: StratoSolar
Keith Henson wrote: StratoSolar This is off NDA so I can go into detail. ... Ed's approach, which he named StratoSolar, was to reduce the mass from hundreds of kg per kW to a few tens of kg by moving the solar concentrator into the stratosphere as a large, lightweight, buoyant structure. ... The concentrated sunlight gets to the ground via a hollow light pipe lined with highly reflective prismatic plastic. Preliminary optimization for kg/kW leads to a 30-meter diameter light pipe with less than 10% loss. A larger pipe has lower losses but uses more total material per kW. Keith-- Hi. StratoSolar is interesting. I looked at the website when you mentioned it a month ago. At the time, this was my main objection: I see bigger problems with losses in the light pipe. The plan seems to be to have a flexible tube lined with reflective material to guide the solar radiation down to steam turbines or whatever on the ground. Most of the light would have to reflect off the sides many times, losing at least a few percent of its intensity at each reflection. So nothing makes it to the ground, and the light pipe melts. There may be solutions to this too, but they're going to be tricky. How many reflections are you assuming light will make as it goes down the pipe, and how glancing are they? ---David ___ http://box535.bluehost.com/mailman/listinfo/brin-l_mccmedia.com
