spingears has mentioned something that I was going to bring up – you should probably blacken the container you are using, because the water itself, or a white or transparent container isn’t going to absorb a lot of sunlight. KLarge areas of the salt flats in the Utah desert are perpetually moist because the white salt keeps reflecting the sunlight back.
I’ve no doubt that you can boil water by concentrating enough sunlight on a container. There’s a huge solar mirror in France that can, I understand, melt metals at its focus. I’ll bet you don’t even have to blacken the container.
Okay so this is twenty years old, I still want to add a comment.
Speaking of fresnel lenses, there’s a series of YouTube videos by a guy who discovered that those older projection large screen televisions contain a large fresnel lens that can be salvaged out of it. On a sunny day these will melt rock.
An interesting and somewhat counterintuitive fact I learned from what-if is it’s impossible to focus light to get any hotter than the source of the light. Since the surface of the Sun is about 5000° C, you can’t focus sunlight to make something hotter than 5000° C, no matter how big a lens you use, or how many lenses you use, in any kind of clever arrangement.
That was difficult for me to wrap my head around, too, until I gave it some thought. Think of this analogy: you have a large, closed room, call it 50 degrees. Put one person in the room and they aren’t going to warm the room much. Add more and more people and the room is going to get warmer and warmer until the room temperature reaches 98 degrees. But if you double that number of people, the room won’t go up to 200 degrees. No matter how much you concentrate people, it will never get hotter than person temperature.
Non-imaging optics can actually produce a higher photon flux than at the surface of the sun (although this still can’t exceed the limit of the Second Law of Thermodynamics):
You are (theoretically) continuously dumping heat into an insulated room, but you are saying that the temperature will stop rising??
How about if, instead of people, you turn on an electric heater originally at room temperature, or a mass of plutonium? This is not an equilibrium situation.
I haven’t read the article yet, and I have no doubt that it is correct because Munroe’s stuff is pretty well researched. But I don’t subscribe to the “people in a room” analogy. Suppose this room has a cylinder filled with gas at the same pressure as the room. The cylinder has a piston. The gas reaches 98 degrees along with the rest of the room. If you use the cylinder to compress that gas down to a much smaller volume, that gas is going to get a lot hotter than 98 degrees. (This is how air conditioning works.)
The key word is Etendue. In particular it is conserved or in a lossy system can never increase.
You can’t focus light in a manner that exceeds the temperature of the surface of the source. If you could you could create a heat engine that breaks thermodynamics and become a perpetual motion machine.
The people in the room is actually an equilibrium situation, but the feedback mechanism is messy. If you put too many live bodies into the room, such that there’s more heat being generated than the room can shed at 37ºC, then the number of live bodies in the room will decrease.
The people in the room are converting chemical energy to heat. Like, starting with a room full of gasoline, air and matches all at 70 degrees. If, for some unknown reason, those things start actively converting chemical energy to heat, the room’s temperature will rise.
Alternately, let’s look at a room with a hot water radiator. Water enters the radiator at 212 degrees and makes the room warmer. Fill the room top to bottom with radiators, the room’s temperature will rise to 212 degrees, but will go no higher, no matter how many radiators you continue to add.
