The sun is the ultimate provider of green energy. Solar power is constant and renewable. It's not going to run out anytime soon. However, you don't need unsightly solar arrays to enjoy the benefits of solar energy. Passive solar energy is simple, has little to no moving parts and requires minimal upkeep. Passive solar-energy systems are designed to use natural principals of heat transfer instead of machines such as furnaces and air conditioners [source: Passive Solar Resources].
Passive solar technology uses the building's walls, windows and floors to collect, store and release the sun's energy. However, passive solar homes still need mechanical equipment, such as a forced-air system or radiant flooring to keep the temperatures cool in the summer and warm in the winter [source: Arizona Solar Center].
The easiest passive solar design systems to install are called "direct gain." In direct gain systems, sunlight passes through windows where the light is converted into thermal energy. The walls and floors directly absorb and store the heat energy. As long as the room temperature is high, the interior of the homes will hold on to the heat. When the temperature drops at night, the stored heat radiates through the living space. Builders can also install plastic or metal water pipes inside a wall. When the sun hits the walls the water in the pipes heats up. That water can be pumped throughout the house as a source of heat [source: Arizona Solar Center].