Markus Kayser has built a 3D printer that uses sun light to turn sand into 3D glass objects. Free energy making things from the free materials.
He calls the device a Solar Sinter. The process works by spreading thin layers of sand then focusing a beam of light with a magnifying lens on a point to melt the sand turning it into glass. The object is built up a layer at a time. The movement and focusing of the beam is powered with photovoltaic but the high energy melting is done with a magnifying lens. This allows the device to be very low cost and potentially long lasting
A rover equipped with a solar sinter could be placed in deserts on earth or another planet and let run making useful objects. Even though this device makes crude items similar 3D printing technology has been used to make high precision objects of many materials.
In addition to a 3D printer, which is an additive technique, he has made a solar cutter which is a subtractive technique.