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I am thinking of building a prototype for a solar generator using a fresnel lens to concentrate sun rays to a relatively small area, for the purpose of harnessing the thermal energy.
The problem with this is that the temperature at the concentrated point, temperatures as high as 3000F are possible.
What (preferably inexpensive) material can be placed at the focal point of the lens, to transfer the heat away to do some useful work?
Crucial to this is how you intend to exploit this energy as this will be very significant in how quickly and uniformly heat is extracted from the target.
3000F is outside the working range of most readily available engineering materials and if you want something which can withstand that sort of temperature outright you are in the realms of things like engineering ceramics, carbon composites and tungsten.
Having said that as long as you are removing heat quickly enough you should be able to keep the temperature in a sensible range indeed many heat engines have components which are exposed to flame temperatures in excess of their melting points. A gasoline engine will break pretty quickly if the cooling system fails.
So a logical approach is to use a heat engine with a working fluid with good heat transfer properties here steam power is an obvious choice and solar steam engines do indeed exist (note that this is an illustration rather than a reccomendation of how to do it in detail).
Obviously any sort of boiler or steam power is fraught with hazards and not something which should be attempted without taking detailed and specialist engineering advice. There may also be leagal requirements for inspection and certification of boilers whcih apply.
As mentioned in the video answer simple balck steel pipe should be adequate as long as you have enought coolant flow and has the advanatge that there are existing standards for steam pipe and the associated fittings and it is raltiavely inexpensive and straightforward to work with.
This approach has the advantage that you are using the working fluid to directly take heat from the target of the lense with minimal intermediate steps between the heat source and the engine.
Also steam power is a mature and well developed technology so if you are more interested in eveluating the potential output of the lense you could just run it as an open system and measure the energy trasnfer to the water, avoiding the hazrds of a pressurised steam system.
It's not that easy. You're combining several questions (unintentionally, I guess). The first question is how to make sure you absorb as many photons as possible. This is typically achieved both by using materials which are "black" in the wavelengths of interest and by making the surface as rough as possible to increase the chances of absorbing photons which reflect on first impact.
Next, you want a material with high thermal conductivity so that you can pull the heat (energy) into a sink. If you don't, then much of the energy will be re-radiated in accordance with black-body radiation theory.
If you're just building a "proof of concept" setup, a thin layer of carbon dust in good contact with a thin-wall metal pipe might do. The thin layer enhances heat transfer to the pipe; pump water or ethylene glycol (automotive radiator fluid) thru the pipe to transfer the heat to something else (perhaps a Stirling engine.
