Solar Furnace

Did you check out the website for the Spanish solar power station with multiple heliostats? www.psa.es/webeng

.

Did you see this: apparently old back projection TVs had a Fresnel lens as part of the screen, and, you can pick them up for almost nothing.

http://www.instructables.com/id/Giant-Fresnel-Lens-Deathray-An-Experiment-in-Opti/

Watch out for his warning to ensure you ware suitable protective (dark) glasses.

Spanish plant is electricity producing plant while Fresnel Lens are used for medium temperature industrial processes. My interest is in high temperature/high electricity consumption industrial processes such as smelting, high temperature heating/preheating in minerals and metals processing industries. Efficiency loss in transformation of heat generated by solar power into electricity is massive, in photovoltaic cells it is 15%, in concentrated solar plants it is 20%. Direct utilisation of thermal power will be much more efficient and cheaper then transformation into electricity and heating afterwards.

The Spanish plant melts sodium to transport the heat to where they make the steam for running the turbine.

You said you wanted to melt or at least preheat metal. What is the difference between heating sodium and metal placed at the focal point?

Forget Fresnel lenses. Even with the best optical grade lens material some residual heat wil be picked up. For what you want to do this residue wil cause problems as the lens itself heats up

The mirrors must also be very efficient reflectors to prevent soaking up enough heat to distort or damage the mirror and supporting surface. But at least you can put cooling systems on the back of the mirrors.

Liquid sodium salt warmed up to 300C has viscosity of the water and it can easily pass thrugh thin tubes made of stainless steel. This tubing represents solar collector where solar energy is passed onto the working medium/salt. To heat metal you would need to pass it through similar exchange process. Solar furnaces are used to preform such task, however their design i currently limited/non-existent. Direct exposure of metal to strong solar flux may blow the surface off while deeper parts would be left cold. As you know light/photons do not penetrate surface of material. Therefore some exchange fluid or furnace design must accommodate such energy exposure.