Passive solar is any means to use the heat from the sun to provide for the heating or energy needs of a building without any means of mechanical assistance. The easiest method of passive solar involves the proper siting of a building to take advantage of the sun when it is at a low horizon or angle, but to provide shading when the sun is at its highest during the hottest part of the day. Another means of passive solar design is to build the floor of a building with some type of thermal mass such as concrete that absorbs the heat - and releases the heat at night when it is cooler.
