A solar furnace is any device that creates heat by concentrating solar radiation through the use of reflectors. A small one may be used to cook food without consuming firewood, whereas a larger one can generate electricity by heating a gas. There have been attempts to introduce the technology to developing countries to minimize firewood-related deforestation, but only with limited success.
The principle of operation of a solar furnace is quite simple. Take two mirrors, point them at a focus, and the intensity of sunlight in that focus will increase by around threefold. This is because, instead of only getting light from the sun, the focus gets light from the sun and two mirrors. Make that ten mirrors, and you start to get levels of heat that are useful for applications like cooking food.
At least a few groups of hobbyists have made a solar furnace that generates over a thousand degrees of heat, capable of reducing most organic material to ash. In principle, the main limitation of how much solar energy can be focused on one spot is a function of how accurately the mirrors can be aimed at the focus. The solar furnace is one of those technologies that our ancestors would have found extremely useful, if only they had the means to manufacture highly reflective mirrors, which they didn’t. Ancient mirrors were merely polished silver or copper, whereas modern mirrors consist of a thin layer of aluminum deposited on glass.
A dish-Stirling system is a solar furnace that uses parabolic mirrors to focus sunlight on rock salt, which melts and can be used to heat water and produce steam to drive a turbine. These are more efficient than photovoltaic solar cells. It is said that the Greek inventor Archimedes used a solar furnace to set enemy ships on fire during a war, but this is unlikely, as he didn’t possess the technology to focus mirrors that precisely on a target more than a few feet away.