### Author Topic: World's Largest Solar Plant Sets Itself on Fire  (Read 10157 times)

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#### CatalinaWOW

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##### Re: World's Largest Solar Plant Sets Itself on Fire
« Reply #25 on: June 08, 2016, 03:44:57 am »
There's always the reflection of the moon!

Actually, I'd almost be curious to see how bright you could get a spot using the moon and something like this.

I can't remember the exact proof at the moment  and I'm a bit busy to go looking for it but I'm pretty sure you can't heat a surface with a magnifying glass hotter than the surface of the thing radiating the energy. Something to do with thermodynamics and entropy.

https://what-if.xkcd.com/145/

Most of the arguments in this link are true, but the main point in the link is NOT applicable.  The vast majority of "Moonlight" is not light generated by the moon, it is light from the sun reflected from the moon.  So the limiting temperature based on thermodynamic arguments is the 6000 degree temperature of the surface of the sun.  If the moons surface were a flat reflector (or even better a slightly concave reflector), you could start fires by this "moonlight".  The reality is that the moons surface is a roughly lambertian reflector, and scatters the light reflected over an entire hemisphere, or a solid angle of pi steradians.   A magnifying mirror on the earth can only collect the portion of this light equal to the solid angle subtended by the lens divided by pi.  That solid angle is roughly the area of the lens divided by the square of the distance from the earth to the moon, so the attenuation is enormous.

#### djacobow

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##### Re: World's Largest Solar Plant Sets Itself on Fire
« Reply #26 on: June 08, 2016, 06:04:07 pm »

https://what-if.xkcd.com/145/

Most of the arguments in this link are true, but the main point in the link is NOT applicable.  The vast majority of "Moonlight" is not light generated by the moon, it is light from the sun reflected from the moon.  So the limiting temperature based on thermodynamic arguments is the 6000 degree temperature of the surface of the sun.  If the moons surface were a flat reflector (or even better a slightly concave reflector), you could start fires by this "moonlight".  The reality is that the moons surface is a roughly lambertian reflector, and scatters the light reflected over an entire hemisphere, or a solid angle of pi steradians.   A magnifying mirror on the earth can only collect the portion of this light equal to the solid angle subtended by the lens divided by pi.  That solid angle is roughly the area of the lens divided by the square of the distance from the earth to the moon, so the attenuation is enormous.

So just to clarify because I'm a bit thick: you are saying that it is theoretically possible to start a fire with moonlight, but that the efficiency of the system is so low that it is practically impossible?

Just for the record, the mirror array around a single tower at Ivanpah is about 2,600,000 m^2.

#### CatalinaWOW

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##### Re: World's Largest Solar Plant Sets Itself on Fire
« Reply #27 on: June 09, 2016, 04:12:31 am »

https://what-if.xkcd.com/145/

Most of the arguments in this link are true, but the main point in the link is NOT applicable.  The vast majority of "Moonlight" is not light generated by the moon, it is light from the sun reflected from the moon.  So the limiting temperature based on thermodynamic arguments is the 6000 degree temperature of the surface of the sun.  If the moons surface were a flat reflector (or even better a slightly concave reflector), you could start fires by this "moonlight".  The reality is that the moons surface is a roughly lambertian reflector, and scatters the light reflected over an entire hemisphere, or a solid angle of pi steradians.   A magnifying mirror on the earth can only collect the portion of this light equal to the solid angle subtended by the lens divided by pi.  That solid angle is roughly the area of the lens divided by the square of the distance from the earth to the moon, so the attenuation is enormous.

So just to clarify because I'm a bit thick: you are saying that it is theoretically possible to start a fire with moonlight, but that the efficiency of the system is so low that it is practically impossible?

Just for the record, the mirror array around a single tower at Ivanpah is about 2,600,000 m^2.

After further thought, I do believe it is very nearly impossible, if not flatly impossible.  I still reject the thermodynamic argument, which applies when objects are thermally radiating.  The simple counterexamples are the burning mirrors, solar energy plants, and solar furnaces which quite handily heat things to large fractions of solar temperature with no comparable rise in mirror temperature.  The temperature of the reflector is irrelevant.

The limiting factor involves another principle which is mentioned in the link.  A thought experiment will show the problem.  Imaging a large spherical mirror (half of a hemisphere and radius substantially larger than the moon).  In this case spherical describes the shape, the mirror is actually a segment of a sphere.   This mirror could, in principle return all of the energy reflected from a point on the moon.  With a suitable adjustment in shape it could deliver it to any other point between it and the moon, perhaps on the surface of the earth.  But that mirror cannot also reflect energy from other points on the moons surface to that same spot, so the power density delivered is the power density delivered at the moons surface by the sun, less losses from reflection.   The moon absorbs about 90% of sunlight, reflecting about 10%.  That 90% absorbed raises the temperature of the moon to only about 125 deg C, well below ignition temperature for most materials.  The temperature of the moons surface is limited by thermal conduction to the interior.  If you could better thermally isolate your moon reflection point, and choose a material with a very low ignition point, it is remotely conceivable that you could achieve ignition with this nine times lower energy flux.  But anything smaller than this impossibly big mirror with a carefully contrived ignition target will fail.

#### Artlav

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##### Re: World's Largest Solar Plant Sets Itself on Fire
« Reply #28 on: June 12, 2016, 09:49:54 pm »
Heh, in the Niven's classic story Ringworld there are sunflower plants with a mirror instead of seeds.
And, they reflectively aim sunlight towards any moving things.

Imagine flying over an indefinite field of them.

#### CatalinaWOW

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##### Re: World's Largest Solar Plant Sets Itself on Fire
« Reply #29 on: June 13, 2016, 12:19:47 am »
One of my favorite fiction stories (might even be called science fiction) involves a city which feels the home soccer team has been unfairly treated by a particular set of referees.  A silvered program with appropriate instructions is issued to the crowd and the problem is solved.  Technically feasible, practical details get in the way.  Flatness of a mirrored program.  Ability of each individual to aim his program and so on, but there is plenty of energy falling on a stadium.

#### bitslice

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##### Re: World's Largest Solar Plant Sets Itself on Fire
« Reply #30 on: June 13, 2016, 12:34:30 am »
Nobody bothers anymore counting how many animals have been killed by fossil fuel mining

There is definitely no dinosaurs left, I looked.

#### mtdoc

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##### Re: World's Largest Solar Plant Sets Itself on Fire
« Reply #31 on: June 13, 2016, 05:19:30 am »

So just to clarify because I'm a bit thick: you are saying that it is theoretically possible to start a fire with moonlight, but that the efficiency of the system is so low that it is practically impossible?

I've seen measurable wattage from a solar PV array during a full moon. So theoretically that power could be used to start a fire.

#### BillyD

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##### Re: World's Largest Solar Plant Sets Itself on Fire
« Reply #32 on: June 13, 2016, 07:06:52 am »
One of my favorite fiction stories (might even be called science fiction) involves a city which feels the home soccer team has been unfairly treated by a particular set of referees.  A silvered program with appropriate instructions is issued to the crowd and the problem is solved.  Technically feasible, practical details get in the way.  Flatness of a mirrored program.  Ability of each individual to aim his program and so on, but there is plenty of energy falling on a stadium.

Yes I can clearly remember reading that story too as a teenager! I think it was a book of short stories titled 'Science Stories For Boys' or somesuch, and the front cover had a picture of a guy operating an exoskeleton robot, good reading at the time!

Smf