However, solar energy is site-specific, and detailed observation of a potential site is necessary to evaluate the daily and seasonal variability of its solar energy potential.2 Solar energy may be used by passive or active solar systems. Passive solar energy systems do not use mechanical pumps or other active technologies to move air or water. Instead, they typically use architectural designs that enhance the absorption of solar energy. Since the rise of civilization, many societies have used passive solar energy. Islamic architects, for example, have traditionally used passive solar energy in hot climates to cool buildings. The total amount of solar energy reaching Earth’s surface is tremendous. For example, on a global scale, ten weeks of solar energy is roughly equal to the energy Thousands of buildings in the United States not just in the sunny Southwest but in other parts of the country, such as New England—now use passive solar systems. Passive solar energy promotes cooling in hot weather and retaining heat in cold weather.
Methods include (1) overhangs on buildings to block summer (high-angle) sunlight but allow winter (low-angle) sunlight to penetrate and warm rooms; (2) building a wall that absorbs sunlight during the day and radiates heat that warms the room at night; (3) planting deciduous trees on the sunny side of a building. In summer, these shade and cool the building; in the winter, with the leaves gone, they let the sunlight in.
Passive solar energy also provides natural lighting to buildings through windows and skylights. Modern window glass can have a special glazing that transmits visible light, blocks infrared, and provides insulation. Active solar energy systems require mechanical power, such as electric pumps, to circulate air, water, or other fluids from solar collectors to a location where the heat is stored and then pumped to where the energy is used. Solar collectors to provide space heating or hot water are usually flat, glass-covered plates over a black background where a heat-absorbing fluid (water or some other liquid) is circulated through tubes