Solar Energy Conversion at Night

This is a fun thought experiment; Why not have space based mirrors to reflect light down to earth at night to make solar do a little extra generating!

The cost of solar is coming down and the cost of lifting things into orbit is coming down, so why not. Plus gold is very reflective, light weight and extremely malleable [$1 Million in gold, weighs 60 lbs and can make (up to) 27,500 sqr meter mirror] Answers and Replies anorlunda Staff Emeritus All the numbers that you need to calculate the costs and the extra solar generation produced can be found on Wikipedia. Why don't you do the calculations, then report back here what you found. Last edited by a moderator: All the numbers that you need to calculate the costs and the extra solar generation produced can be found on Wikipedia. Why don't you do the calculations, then report back here what you found. It's hard to say, Little Help? There this NYTs article http://www.nytimes.com/1993/01/12/s...e-mirror-as-giant-night-light-for-earth.html? which calculates satellites in terms of multiple of moon light equivalents and I found this https://www.quora.com/Can-moon-ligh...nerate-the-electron-hole-pair-in-a-solar-cell showing how much a PV cell can generate from moonlight but the questions is more complex than that Solar isn't competitive away from the equator but solar mirrors work best when deployed away from the equator. So a mirror in space would be necessary if you want solar in a city like Anchorage Alaska at all. But sunlight in winter would have benefits other than electrical generation, less depression, reduced heating costs and general improved economic activity as people just do more in the winter. It's fun to dream/think big . . . Bystander Science Advisor Homework Helper Gold Member Fourteen hundred watts per square meter times what area? Fourteen hundred watts per square meter times what area? 1.$340 million to build and operate Znamya 3 http://motherboard.vice.com/read/the-man-who-turned-night-into-day
2. Which produced a light on the earth "35 miles to 55 miles across" http://www.nytimes.com/1993/01/12/s...e-mirror-as-giant-night-light-for-earth.html?
3. So with electrical costs about 11 cents a kilowatt hour (in Alaska https://www.aelp.com/Rates/ourrates.htm ) you'll get an additional $600 a year from a solar panel (365 days * 12 hours of night * 11.5 cents *1.4 (per bystander)) 4. Let's say the satellites have a ten year life and is funded by a high yield bond today at %5 so it will need a$44 million bond to payment to justify $340 million investment 5.$44 million / $600 is 70,000 square meter solar panels to justify cost It seems doable in Alaska Bystander Science Advisor Homework Helper Gold Member *1.4 (per bystander)) What area? Think it through; this is not full illumination. What area? Think it through; this is not full illumination. I got distracted by this "The Case for Space Solar Power" Kindle Edition https://www.amazon.com/dp/B00HNZ0Z96/?tag=pfamazon01-20 and this https://www.nasa.gov/pdf/716070main_Mankins_2011_PhI_SPS_Alpha.pdf Anyway 1400 seemed high, so full moon light produces 1.1 eV per sqr meter and Znamya 3 produces 50 moon lights so that's 55 eV so . . . you'll need 1.3 sqr kilometers to justify the cost. Much less doable since it would need to be three times larger than the largest solar park. On the other hand 1.3 sqr kilometers over an entire city is possible. Anyway I'm going to ask the the NASA scientist who wrote the book on space PV, Mankin, see what he says Last edited by a moderator: Drakkith Staff Emeritus Science Advisor Anyway 1400 seemed high, so full moon light produces 1.1 eV and Znamya 3 produces 50 moon lights so that's 55 eV eV? If that stands for electron volts, then that's not a measure of the output voltage of the solar panel. anorlunda Staff Emeritus Ignoring the problems with power density and economics calculations, what kind of orbit are you suggesting to keep the mirror aimed at the same spot on Earth? A very large very light mirror makes a good solar sail. How do you propose to keep the mirror's shape and to keep it from being blown away by the solar wind? Ignoring the problems with power density and economics calculations, what kind of orbit are you suggesting to keep the mirror aimed at the same spot on Earth? A very large very light mirror makes a good solar sail. How do you propose to keep the mirror's shape and to keep it from being blown away by the solar wind? I was thinking that too, It might be why Znamya_2.5 https://en.wikipedia.org/wiki/Znamya_(satellite)#Znamya_2.5 failed . Anyway I was thinking that some orbital PV could power a electromagnetic drive https://www.nasaspaceflight.com/2015/04/evaluating-nasas-futuristic-em-drive/ to constantly reposition the mirror . . . anorlunda Staff Emeritus https://en.wikipedia.org/wiki/Znamya_(satellite)#Znamya_2.5 said: The mirror deployed successfully, and, when illuminated, produced a 5 km wide bright spot, which traversed Europe from southern France to western Russia at a speed of 8 km/second You didn't read that sentence. The satellite did not keep the spot focused on one place. Anyway I was thinking that some orbital PV could power a electromagnetic drive https://www.nasaspaceflight.com/2015/04/evaluating-nasas-futuristic-em-drive/ to constantly reposition the mirror . . . Keep adding practicalities and 60 pounds soon becomes 60 tons. Also, that em drive (if it works at all) is far too weak to do that job. Your competition for orbiting solar is simply buying twice as many solar panels, instead of doubling their efficiency. With solar panel prices well below$1/watt (and prices dropping nearly 2%/month), that is very hard to beat.

You didn't read that sentence. The satellite did not keep the spot focused on one place.

Keep adding practicalities and 60 pounds soon becomes 60 tons. Also, that em drive (if it works at all) is far too weak to do that job.

Your competition for orbiting solar is simply buying twice as many solar panels, instead of doubling their efficiency. With solar panel prices well below \$1/watt (and prices dropping nearly 2%/month), that is very hard to beat.

Znamya_2.5 failed http://news.bbc.co.uk/2/hi/science/nature/273220.stm

"The total force exerted on an 800 by 800 meter solar sail, for example, is about 5 newtons (1.1 lbf) at Earth's distance from the Sun,[2] making it a low-thrust propulsion system, similar to spacecraft propelled by electric engines" https://en.wikipedia.org/wiki/Solar_sail so perhaps an electric engine can offset the motion caused by sunlight

A better competition for solar mirrors would be concentrating sunlight. Solar concentration causes a lot of heat but in a place like Alaska the extra heat can be used to heat a home.

I was just thinking (in a fun way) that solar at night might make a solar PV productive enough to help partially offset the cost of a space mirror . . .

jim mcnamara
Mentor
All 'coolness' and practicality of completing this idea aside -

Consider photoperiodism or 'diurnal rhythms' in both plants and animals and us humans
https://en.wikipedia.org/wiki/Photoperiodism

FYI: There is a fair percentage of living things that require night - low light levels and subsequent light periods. In a large urban complex these species do not do well.
Example: Greenhouses in those no night areas areas use large blackout curtains to get required dark periods in order to get flowers to bloom. Poinsettias come to mind this time of year.

We humans respond to photoperiod, too. But we have essentially removed darkness in urban areas. Why do you suppose lots of people have darkened bedrooms with heavy curtains? When you live in a remote temperate area, people there do not usually have massive bedroom curtains. Why do shift (think graveyard) workers tend to have all kinds of extra added health issues?
Example: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2904525/

If we start adding large patches of night lighting we could end up losing plants or insects or animals in a large area where they are in fact integral to what we require in terms of food, esthetics, and shelter. At best we would drive up the cost of producing certain agricultural products.

HyperTechno and berkeman
All 'coolness' and practicality of completing this idea aside -

Consider photoperiodism or 'diurnal rhythms' in both plants and animals and us humans
https://en.wikipedia.org/wiki/Photoperiodism

FYI: There is a fair percentage of living things that require night - low light levels and subsequent light periods. In a large urban complex these species do not do well.

Humans do not do well in polar regions https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3793275/ a little light in the winter might help

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jim mcnamara
Mentor
That is a personal opinion. Do you know anything about subarctic flora and fauna? I do. Please cite some relevant paper on the subject.

See Fraas 2015

http://www.mtu.edu/ece/department/faculty/full-time/zekavat/SSP2015_fraas.pdf

First major hurdle for any space solar power idea,is that it must have an overall advantage over deploying similar technology on the ground and avoiding the launch costs.

Besides access to light 24/7 I was thinking the same thing. However the closer you get to the sun the more intense the rays 1/5 into the sun from Earth the rays are 25 times more intense.

So if you have mirrors closer to the sun to direct the light towards a satellite in Earth orbit you could get some real power going. I'm going to read this book https://www.amazon.com/dp/B00HNZ0Z96/?tag=pfamazon01-20 and think about it . . .

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Drakkith
Staff Emeritus
Besides access to light 24/7 I was thinking the same thing. However the closer you get to the sun the more intense the rays 1/5 into the sun from Earth the rays are 25 times more intense.

So if you have mirrors closer to the sun to direct the light towards a satellite in Earth orbit you could get some real power going.

You can't focus the light over that distance. By the time it reaches the satellite in Earth orbit it will have spread out and you'll have lost any benefit to putting a mirror closer to the Sun.

You can't focus the light over that distance. By the time it reaches the satellite in Earth orbit it will have spread out and you'll have lost any benefit to putting a mirror closer to the Sun.

I was thinking of putting a solar sail facing the sun (constructed of a flimsy material so that undesirable parts of the spectrum can pass through) and have the desirable light concentrate on a concave mirror (hopefully it won't heat up too much because it's reflecting only part of the spectrum). So that the force exerted on the sail will be counteracted [partially] by the force on the mirror. And so the light will pass though a hole in the center of the sail and the focal point of the mirror will be one point half way between satellites.

Since putting a satellite 1/5 closer to the sun would be the equivalent of putting 25 in orbit around the Earth it could still make sense to to have booster satellites in a series until you reach Earth orbit but I have a feeling boosting satellites wont work . . .

mheslep
Gold Member
None the above closer-to-sun ideas are workable. You can't do 24 hrs either, as distance from something like L1 makes the spot on earth the size of, I dunno, Alaska, and with much less power per area than received directly from the sun. Again, see the Fraas paper above for best coverage available from a polar orbit.

Drakkith
Staff Emeritus
I was thinking of putting a solar sail facing the sun (constructed of a flimsy material so that undesirable parts of the spectrum can pass through) and have the desirable light concentrate on a concave mirror (hopefully it won't heat up too much because it's reflecting only part of the spectrum). So that the force exerted on the sail will be counteracted [partially] by the force on the mirror. And so the light will pass though a hole in the center of the sail and the focal point of the mirror will be one point half way between satellites.

This is not possible in the near future:
• We do not have the technology to construct a mirror with a radius of curvature of 40 million miles (around what you'd need it to be for your satellite near the Sun). This curvature is so shallow that we don't have a way of measuring the curvature across small areas, which means that we can't manufacture it correctly.
• The light cannot be focused over such a large distance unless your mirror is many kilometers across thanks to diffraction. It is the same reason a laser beam expands as it travels through space.
• Even if you could focus the light over that distance, the size of the image of the Sun is proportional to the focal length of the optical system. With a mirror with a focal length of 20 million kilometers, the image of the Sun would be 1.4 million kilometers across. That's how large your receiving system would need to be to capture all of the light from the mirror. That's about 100x the diameter of the Earth itself.

This is not possible in the near future:
• We do not have the technology to construct a mirror with a radius of curvature of 40 million miles (around what you'd need it to be for your satellite near the Sun). This curvature is so shallow that we don't have a way of measuring the curvature across small areas, which means that we can't manufacture it correctly.
• The light cannot be focused over such a large distance unless your mirror is many kilometers across thanks to diffraction. It is the same reason a laser beam expands as it travels through space.
• Even if you could focus the light over that distance, the size of the image of the Sun is proportional to the focal length of the optical system. With a mirror with a focal length of 20 million kilometers, the image of the Sun would be 1.4 million kilometers across. That's how large your receiving system would need to be to capture all of the light from the mirror. That's about 100x the diameter of the Earth itself.
You're right, I was thinking of converting the light to electricity and then transmitting the energy on a different point of the spectrum with a better divergence (than regular light) and then use a series of booster satellites to pass along the energy but that doesn't add up.

It makes more sense to put those satellites in orbit around Earth. I'm still curious though I'm going to read that book about space solar PV . . . This was fun to think about though

Drakkith
Staff Emeritus