3D model of the AT solar power tower 2. The solar power tower is placed in a pit in the soil so as to take advantage of the increased insulation provided by the soil. A reflective surface is added (ie thin metal surface) to redirect any sunlight falling into the pit towards the pyramid. Additional sunlight can be provided to the pyramid by means of heliostats (placed on buildings). The inside of the pyramid is filled with oil, and a (Alpha-type) Stirling motor was added (with 4 pistons to smoothen operation). Cooling of the lower pistons is done by means of water. The pyramid is also made directional, so it must be oriented towards the south (in areas in the northern hemisphere) or towards the north (in areas in the southern hemisphere). See the 'Purchasing_the_models' link at appropedia's AT_CAD_Team for additional information on the model