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Question
I have an innovative new product concept that (hopefully) boils water rapidly using solar energy but I wanted to check the idea matches the theory first before I embark on a wild goose chase. The product is for social good.

Imagine 2 identical black-body discs made of a material with an emissivity of close to 1 and a high thermal conductivity. They are each 10cm in diameter and 1mm thick.

Each disc is angled towards the sun so that the angle of insolation is 90 degrees. The only difference is that one of the discs (disc A) has a magnifying lens (also 10cm in diameter) positioned in front and in parallel so that the suns rays are focused onto a small point. Disc A will effectively be 'in shadow' other than a high intensity heat/light spot in the centre.

Question 1: Will disc A heat up quicker than the other disc?
Question 2: Will disc A reach a higher temperature than the other disc?
Question 3: If the answer is yes to either of these questions (which I hope) then why as the energy coming from the sun is constant (I think something like on average 1050W per m2)?
Question 4: If yes, how might this process be exaggerated/maximised?

The product concept uses the discs to transfer heat to water that surrounds them. With this in mind I actually need the disc to radiate the 'acquired' solar heat back into the water through conduction as efficiently as possible. Other than maximising the surface area of the discs to the volume of the water is their any advice/suggestion/key knowledge you may be able to impart that would make this process the most efficient it could be?

Any information or advice is most appreciated.

Thank you in advance.

Answer
Questions 1 and 2 don't have the straightforward answers you're looking for.  It will not gain thermal energy faster than the other disc.  There will be faster thermal transfer from the hot spot locally, but the outer parts will actually heat up far slower.  They will also reach a lower temperature at equilibrium, when the heat flow in will equal the heat radiated.  Since much more radiated heat will come from the hot spot, there's less to radiate from the rest of the disc.  For question 3, the answer to neither of them is yes, so I wouldn't worry about it.  The Sun has a nice and relatively constant temperature (5700 Kelvin), so of course it radiates energy at a constant rate.  

The only way to enhance the heating of your disks is to use a large lens or reflector and collect more light.  But why use disks when you can use a lens or reflector to focus light directly on a ceramic panel with water channels or a water tank itself?  This is done all over the world every day, I'm not sure what you have in mind to improve the process that isn't already being done.  There's no reason to heat any discs.

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Dr. Stephen O. Nelson

Expertise

I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

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I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

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Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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