Solar Furnace

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At an intensity of 1366 watts/m2, the lunar surface is idea for the use of solar furnace technology. By concentrating the sunlight on to a small area you can raise the temperature well past that necessary to melt iron, regolith, incubate sealed containers to make silane or drive water off lunar soil.

Except for fresnel lenses which are great for solar welding, the design involves using mirrors coated with aluminum to reflect the sunlight to a small point. The back of the mirror can be of any rigid material and aluminum is a common element on the moon with a silver like reflective properties. Due to the lack of wind and lower gravity of the moon, less than a tenth of materials are required to make similar size furnace compared to earth.

See Solar Thermal for details of how to use a solar furnace to generate energy.

The ideal location of a solar furnace is at the lunar poles. This is because a vertical orientation allows for easier access to where the point of concentration is and the sunlight is virtually continuous for half the year if not all of it. Closer to the equator and you have the problem of a more horizontal axis and that means that your focal spot is above you in a hard to reach location thus limiting its efficiency.

Concentration methods

Parabolic mirror

This design focus all of the energy on a small point that blurs due to the perceived width of the sun. Reflective designs do not have the separation of light frequencies that refractive methods have and are thus simpler to focus. The only limit on the diameter is how big your mounting platform can handle in terms of weight. Using materials slightly thicker than foil is fully workable. The advantage of this design is that it has both a single focus point and single point of angle adjustment.

Fresnel lens

This involves a transparent sheet with grooves that act like miniature lenses. If you use this to focus light on a highly reflective solar tube, you can transfer this heat to a specific location several meters away. For the needs of welding, this would be very useful. A fresnel lens of a meter in diameter would be enough to raise the focus temperature well over that needed to fuse iron. Any bigger than this and you are likely to have problems.

Mirror arrays

This can come in two forms. The first is an array of flat mirrors scattered over the surface each with their own targeting control. This is currently being used in the desert to generate power on earth.

The second is long mirrors that do not concentrate the sunlight to a point. This is desirable for packing because to get high density you need a regular shape. This would be efficient for really large scale solar furnaces where the target point is a strip where things heat up. This is the environment you need for melting things or keeping them at a high temperature for a long time.