# Solar Energy Conversion at Night

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1. Dec 15, 2016

### John d Marano

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] 2. Dec 15, 2016 ### anorlunda 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: Dec 16, 2016 3. Dec 15, 2016 ### John d Marano 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 . . . 4. Dec 15, 2016 ### Bystander Fourteen hundred watts per square meter times what area? 5. Dec 15, 2016 ### John d Marano 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 6. Dec 15, 2016 ### Bystander What area? Think it through; this is not full illumination. 7. Dec 15, 2016 ### John d Marano I got distracted by this "The Case for Space Solar Power" Kindle Edition https://www.amazon.com/The-Case-Space-Solar-Power-ebook/dp/B00HNZ0Z96 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: May 8, 2017 8. Dec 15, 2016 ### Drakkith ### Staff: Mentor eV? If that stands for electron volts, then that's not a measure of the output voltage of the solar panel. 9. Dec 16, 2016 ### anorlunda 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? 10. Dec 16, 2016 ### John d Marano 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 . . . 11. Dec 16, 2016 ### anorlunda 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.

12. Dec 16, 2016

### John d Marano

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 . . .

13. Dec 16, 2016

### Staff: 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.

14. Dec 16, 2016

### John d Marano

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

Last edited: Dec 16, 2016
15. Dec 16, 2016

### Staff: 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.

16. Dec 17, 2016

### mheslep

17. Dec 17, 2016

### John d Marano

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/The-Case-Space-Solar-Power-ebook/dp/B00HNZ0Z96 and think about it . . .

Last edited by a moderator: May 8, 2017
18. Dec 17, 2016

### Staff: Mentor

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.

19. Dec 17, 2016

### John d Marano

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 . . .

20. Dec 17, 2016

### mheslep

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.