A Space Solar Cell

[cwes99_03]

OK, thanks for the humor, i enjoyed immensely.

 

I was thinking while reading the parts about mercury that mercury is a perfectly good reflector, but that it doesn't send a concentrated beam, thus it doesn't reflect as bright as the sun does at noon-time.

 

Then your point about concentrating the beam that way would vaporize so much that it made me think more clearly. However, I'm not sure how you get 90% transmission of the energy by converting a full spectrum of sunlight into microwave or IR without the same problem.

 

BTW beaming a terrawatt microwave laser at the earth sounds like a super bad B scifi movie experiment that some errant US government agency might try out on a tuesday night scifi channel movie, but hey it would still be more interesting than annaconda. Wouldn't that type of experiment just melt down everything in the 10 mile radius and totally destroy all electronics in the same radius?

[Pyrotex]

...I'm not sure how you get 90% transmission of the energy by converting a full spectrum of sunlight into microwave or IR without the same problem....Wouldn't that type of experiment just melt down everything in the 10 mile radius and totally destroy all electronics in the same radius?

Good questions. You can't convert sunlight to microwaves with 90% efficiency. You convert sunlight to electricity << at the source >> with perhaps 25% efficiency (eg, using concave mirrors and Sterling Cycle liquid salt/water vapor turbine-generators at the focus). All your waste heat is discarded where it can do no harm, not at the end where it will set the remaining rain forests on fire.

 

Electricity to microwave (MW) is VERY efficient. Look up Klystron.

 

The safety of beaming MW to Earth is a factor of energy density: how many watts per square meter are you delivering? By having the receiving antennas spread over 10 sq miles, the average energy density is no more than sunlight. But MW can be converted back to electricity at 95% efficiency, sunlight only at 15-20%. MW of certain frequencies can hurt birds, trees, electronics. Avoid them as best as possible, and put recievers in relatively "safe" areas like the Gobi Dessert, the Sahara Dessert, the Australian Outback, and maybe Las Vegas, Nevada. :eek_big:

 

Beaming IR to the Earth is not good. :eek_big: You would have to convert IR to MW in some manner, or use the IR to produce electricity IN ORBIT where the waste heat wouldn't hurt anybody.

One way of using IR efficiently is to have an intense beam power a solar sail, for large spaceships traveling to the outer planets or to nearby stars.

[Pyrotex]

...I was speaking of a concentrated form ....

I just thought of something Kayra. I'm really sorry, but this may come as a disappointment. Brace yourself. Okay?

 

If we concentrate sunlight near Mercury and "beam" it back to Earth, what do we have? We have concentrated sunlight. What if we take ordinary sunlight at Earth's surface and pass it through a magnifying glass? We have concentrated sunlight. No difference. None whatsoever. :eek_big:

 

Or you could put a mile-wide mirror with integrated Fresnel lens in orbit and reflect/beam that down to the surface of Earth. We still have just concentrated sunlight.

 

No matter how much you concentrate light, the very best you can achieve is about 25% efficiency conversion to electricity. All the rest goes to heating up the surroundings. If the concentration is high enough, replace the word "heating" with "incinerating".

[Kayra]

I just thought of something Kayra. I'm really sorry, but this may come as a disappointment. Brace yourself. Okay?

 

If we concentrate sunlight near Mercury and "beam" it back to Earth, what do we have? We have concentrated sunlight. What if we take ordinary sunlight at Earth's surface and pass it through a magnifying glass? We have concentrated sunlight. No difference. None whatsoever. :hyper:

 

Or you could put a mile-wide mirror with integrated Fresnel lens in orbit and reflect/beam that down to the surface of Earth. We still have just concentrated sunlight.

 

No matter how much you concentrate light, the very best you can achieve is about 25% efficiency conversion to electricity. All the rest goes to heating up the surroundings. If the concentration is high enough, replace the word "heating" with "incinerating".

Consider me braced :eek_big:

 

All good points, except for the points :shrug:

 

I am sorry if I left the impression that all of that energy had to be going to one place, and used on one system of power generation. That was not my intention.

 

The ability to concentrate 10 times the sunlight was the point of going to mercury. A MUCH smaller structure is required to gather the same amount of sunlight if you do it at or just inside mercury orbit.

 

Conversion of sunlight to electricity (when not using solar cells) varies depending on the max temp. 40% is the current best we can do using Nuclear reactions, so it is reasonable to assume we could accomplish the same thing with High quality solar heat.

 

The "Concentrated Sunlight" that we beam back to earth will, as you mentioned, naturally defocus. I have no idea what the energy density per square meter would be by the time it reached earth, but if it was to high it could be intentionally defocus-ed before it was beamed.

 

This would leave you with a rather large "Patch" of space where the energy density of the sun was 100 times (or whatever the collector material could withstand) higher then from the sun naturally, and falling off in a gradient. Anything requiring heat would place a second collector near the center and focus the sunlight onto multiple points on earth. With multiple collectors in orbit you could arrange to focus as much heat energy at any point on earth, as required. Perhaps even enough to attempt fusion. By making energy of this magnitude available, a purpose and a method will be found.

 

In the outer parts of the "patch" you could put your solar cells. They would be placed in such an area that they received as much sunlight as they could stand without damaging them. Alternately (or both) you could put those sterling engines you mentioned. Generate your electricity, convert it, and beam it back to the earth.

 

Reflect some to the various currently operating solar farms, and sell them the ability to generate electricity 24/7/365 (weather permitting).

 

The net result: With one collector near the sun, all earth orbit devices would either operate at much higher efficiencies or be much smaller in size, while generating far more power. It would still be a matter of doing a cost/benefit analysis though :hyper:

 

There is a lot to be said for "concentrated sunlight" :eek_big:

[Pyrotex]

Consider me braced :eek_big:...The ability to concentrate 10 times the sunlight was the point of going to mercury. A MUCH smaller structure is required to gather the same amount of sunlight if you do it at or just inside mercury orbit....:shrug:

Kayra, Kayra, Kayra... :hyper:

Okay, so a 1 sq km structure near Mercury gets 10 times as much sunlight as an identical 1 sq km structure near Earth.

So build a 10 sq km structure on Earth. Same thing. For 0.00037% the cost.

 

Chop the 10 sq km structure into two million 10x10 m structures (50% loss in atmosphere). It's all the same thing. And they'll all fit comfortably into an arid corner of Nevada.

 

It all boils down to just the number of photons per second. It doesn't matter where you get them. Mercury photons aren't any more intense or "special" than Earth photons. It's just numbers. I'm sorry. I'm really, really sorry. No kidding.

 

:eek_big: Never mind. I'm joking. You're idea is just fine. Go ahead and build your energy structure near Mercury. {sigh} But I want gargoyles at the corners of all the struts.

[Kayra]

LOL

 

Thanks for the chuckle.

 

Seriously though, would it be that much cheaper (or even equivalent cost) to build vast structures on earth that are subject to weather and day/night cycle? I had thought that was the entire reason for looking to space based Solar collection in the first place.

 

If that is the case, then I gues there is no point :eek_big:

[Pyrotex]

LOL Thanks for the chuckle.

Seriously though, would it be that much cheaper...I had thought that was the entire reason for looking to space based Solar collection in the first place....shrug:

Well, don't give up yet.

There was a reason to build space based Solar. I'm kinda cheating here, because I got to read the original proposal to NASA, NASA's cost analysis, and the independant analysis of that. Here is the reason:

 

1. Converting heat to electricity is inefficient. This usually yields greenhouse gases, too. Hydroelectric is limited.

2. Converting light at the ground to electricity is inefficient and plagued by sand, wind, night, storm, rain, bird droppings.

3. We can convert humongo amounts of light into electricity in Space, where none of those things pose a problem.

4. Electricity/MW/electricity transport to the Earth is feasible, and if done correctly, produces no greenhouse gases, very little heat, minimum danger to life.

5. Solar collection in Space can be scaled up as big as we want.

 

NASA's estimate for electricity cost at the ground was 8 to 12 times the $/kw*hr as we produce already. Independent analysis lowered this factor down to 1 to 3. The actual cost is somewhere in between, and may be going down slowly as new technologies develop. If we ever get a space elevator built and polycarbon (nanotube) composites down to a reasonable price, then Space/Solar power may actually become cheaper than terrestrial production.

 

The location for the powersats was assumed to be at geosync. Known technology can beam MW that far with little loss. But distances more than 3...5...10 times that become iffy. You could do it from L1, but it would require multiple recievers on Earth. You could do it from the Moon's surface, but you lose 55-75% right off due to the Moon's day/night cycle.

[Kayra]

Ahh, that is starting to make sense.

 

Just how much sunlight can a solar cell be designed to work with efficiently i wonder.

 

Take a long look at graphene next chance you get.

I would bet my mercury solar collector that this simple material will put CNT's to shame in our near future. The stuff is showing some pretty amazing electrical properties as well.

(what the hell is a massless electron? and how can this material conduct without holes?)