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Merlynx
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« on: December 17, 2009, 10:00:59 AM » |
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Has anyone explored the efficiency of replacing solar cells with a thermopile?
I understand thermocouples have been developed which can approach the Carnot efficiency limit.
Carnot efficiency is highly dependent on the temperature of the cold sink.
A thin wing, made of 2 thermally isolated sheets connected by a thermopile, with the exterior surface matt black, would absorb the incident solar radiation (1000 to 1300 W/sq m) and re-radiate it from the other side.
At the temperature of background radiation approx. 3 degrees Kelvin, Carnot efficiency could easily reach 95%.
An alternate system would cover the exterior of the lifter in thermal panels, absorbing on the sunward side and radiating from the shadowed sides. There would be some loss of efficiency, but a bonus in weight efficiency by making it part of the main structure.
Just a thought
Merlynx
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Merlynx
Magical Engineer
Technical Metaphysicist
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Merlynx
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« Reply #2 on: December 20, 2009, 06:48:17 AM » |
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Good point on the stirling engines.
Still, any heat engine becomes very efficient if the background is 3 degrees Kelvin.
Could a solar-thermal system power the Lifter outside the atmosphere without needing a ground-based source?
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Merlynx
Magical Engineer
Technical Metaphysicist
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A_M_Swallow
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« Reply #3 on: December 20, 2009, 09:04:38 AM » |
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You are now asking questions about the mass of the machinery and size of the concentrating mirrors.
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Andrew Swallow
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Merlynx
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« Reply #4 on: December 21, 2009, 04:53:35 PM » |
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I suppose the important question is what kind of power is required to keep the lifter moving at 200 Kph (assuming we don't want to accelerate further)
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Merlynx
Magical Engineer
Technical Metaphysicist
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neil
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« Reply #5 on: December 26, 2009, 05:55:33 PM » |
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Hi Merlynx: That 95% must be theoretical, otherwise we would use your wings on GEO satellites which can cool to 100 degrees k on the shaded side and about 300 k on the sunlit side. My impression was that the the thermopile efficiency (about 10%) decreased only slightly as you went to large scale, but the sterling engines were not practical very small scale. A switch to stirling means the reliability is approaching the excellent reliability of the atomic radiation thermophile. Since making electricity transfers heat from the sunlit side to the shaded side, lots cooler than 100 degrees k would require lots of radiator fins even if the heating from Earth shine and moon shine was negligible. I wonder if the PV panels can also be thremopiles with very little mass increase?
The lifter can likely be powered at 200 Kph the last half of the trip to GEO and beyond, but air resistance in Earth's atmosphere likely makes even 20 Kph impossible with PV or thermopile wings. A dual propulsion system would reduce the pay load to zero. Neil
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Merlynx
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« Reply #6 on: December 27, 2009, 08:57:59 AM » |
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A dual power supply to the electric motors is exactly what I am thinking of, in order to remove the lasers entirely.
The lower 200 km of the ribbon (inside atmosphere) could be constructed as a power cable. (2 conducting sections seperated by an insulating section) Resistance to GEO is not important, the lifter could have contacts which bridge between the conducting sections, making the toatl circuit no more than 200 km long.
Outside Atmosphere power could be supplied by the solar-thermal system (which would be retracted inside atmosphere to minimize drag.
In the By the numbers thread I just estimated the power requirements to maintain 200 kph at 1 G, which would require 8000 sf of panels. (modified by the combined efficiency of the system)
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Merlynx
Magical Engineer
Technical Metaphysicist
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neil
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« Reply #7 on: December 31, 2009, 05:23:49 AM » |
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If I recall correctly the pinch rollers are projected at 33% efficiency, and the electric motor is optimised for lots of power at minimum mass. 30% may be optimistic for solar concentrated or thermocouples optimised for minimum mass. System efficiency 12%? so the 8000 square feet becomes 64,000 square feet, and needs to be kept facing the Sun as the climber turns with the ribbon twist, and folded while in Earth's atmosphere. Is 8000 square feet for a one ton or a 20 ton climber? Neil
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A_M_Swallow
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« Reply #8 on: January 03, 2010, 12:35:51 PM » |
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There is no particular need to climb through the atmosphere at 200 kmph, climb at 50 kmph and take 4 hours. That will save a lot of energy and air resistance. The same electric motors can be used by both power sources. The electrical pick-ups can descend or be discarded when above the air. Some forms of CNT can conduct electricity, so a mixture for the first 200 km is possible.
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Andrew Swallow
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Merlynx
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« Reply #9 on: July 12, 2010, 09:06:52 PM » |
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I have to apologise for the idea that started this thread.
I forgot to account for the physics of black-body radiation in space, and therefore overestimated the energy available from a solar-thermal system by at least a factor of 3.
<shrug>
Science is what happens when we understand why we were wrong.
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Merlynx
Magical Engineer
Technical Metaphysicist
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