Some types of passive solar systems are called direct gain systems when they use the sun's heat without interference, and when the heat collection, storage and distribution all occur within the same space.
A direct gain system includes south-facing windows and a large mass placed within the space to receive the most direct sunlight in cold weather and the least direct sunlight in hot weather. In this type of system, sunlight passes through the windows, and its heat is trapped by the thermal mass in the room.
For winter, the mass absorbs solar heat during its exposure to direct sunlight and radiates that heat back into the space during the cooler night. During the summer, the reverse occurs. The thermal mass is prevented from receiving direct sunlight and absorbs the heat in the room, helping to keep the room's temperature cooler.
The most effective thermal mass is made from dense and heavy materials that will retain the heat even in the absence of direct sunlight. Examples include an internal wall or floor made of concrete, stone or masonry, especially if painted a flat, dark color; dark-colored cylinders, tanks or drums filled with water, or bins of rocks (see Indirect Gain).