Solar Energy

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Earth's surface receives energy from processes in Earth's interior and from the Sun. Heat from the interior comes from radioactive elements in the mantle and core, tidal kneading by the Moon and Sun, and residual heat from the earth's formation. This interior heat is radiated through the surface at a global rate of 3 × 10¹³ watts (W)—about .07 W per square yard (.06 W/m²). The Sun, in contrast, provides 1.73 × 10¹⁷ W, 5,700 times more power than Earth radiates from within and about 30,000 times more than is released by all human activity. Clouds, air, land, and sea absorb 69% of the energy arriving from the Sun and reflect the rest back into space. The ocean, which covers about 70% of the earth's surface, does about 70% of the absorbing of solar energy.

Between its absorption as heat and its final return to space as infrared radiation, solar energy takes many forms, including kinetic energy in flowing air and water or latent heat in evaporated water. Solar energy keeps the oceans and atmosphere from freezing and drives all winds and currents. A small fraction of Earth's solar energy income is intercepted by green plants, providing the flow of food energy that sustains most

earthly life. Only a few organisms, including thermophilic bacteria infiltrating the crust and organisms specialized to live in the vicinity of hydothermal deep-sea vents, derive their energy from Earth's interior rather than from the Sun.

Regional variations in solar input contribute to weather patterns and seasonal changes. On average Earth's surface is more nearly at a right angle to the Sun's rays near the equator, so the tropics absorb more solar energy than the higher latitudes. This creates an energy imbalance between the equator and the poles, an imbalance that the circulation of the atmosphere and oceans redress by transporting energy away from the equator. During each half of the year the daylight side of each hemisphere is tilted at a steeper angle to the sun than during the other half, and so intercepts less solar energy; this results in seasonal climatic changes.

Solar energy is also of technological importance. Utilization of the Sun as an energy source has been routine on spacecraft for decades and is becoming more frequent on the ground. Electromagnetic radiation from the Sun, unlike the major conventional power sources, produces no smokestack emissions, greenhouse gases, or radioactive wastes; and its production cannot be manipulated for profit or political leverage. On the down side, sunlight is a diffuse or spread-out energy source compared to any fuel and is directly available only during the day. Yet, even at high latitudes in Europe and North America, where most of the world's energy is consumed, the ground receives from the Sun a long-term average of 83.6 W per square yard (100 W/m²). This average is inclusive of "dark" hours. Both indirect and direct harvesting of this energy income is possible. Indirect solar schemes, including wind power, wood heat, and the burning of alcohol, methane, or hydrogen, run on energy derived at second hand from sunlight. Direct schemes use sunlight as such to heat buildings or water, generate electricity, or supply high-temperature process heat to industrial systems.

Because conventional electricity generation is expensive and polluting, much effort has been devoted to solar electricity generation. Electricity can be generated from sunlight either thermally or photovoltaically. Thermal methods focus the Sun's rays on looped pipes through which molten salt, hot air, or steam flows. This hot fluid is then used either at first or second hand to run generators, much as heat from coal or nuclear fuel is used in conventional power plants. Photovoltaic electrical generation depends on flat, specially designed transistors (solar cells) that convert incident light to electricity. At 83.6 W/yard² (100 W/m²) average solar input, 38 square yards (32 m²) of 33% efficient solar cells—a square 18 feet (5.5 m) on a side—could supply 800 kilowatt-hours of electricity per

month, the approximate usage of the average U.S. household. An efficiency of 32.3% has been demonstrated in the laboratory, but most commercial photovoltaic cells are only about 10% efficient. Unlike the unused heat from a ton of coal or uranium, however, the sunlight not converted to electricity by a solar cell entails neither monetary cost nor pollution, and so cannot be viewed as waste.

Despite its obvious advantages, photovoltaic electricity generation has long been limited to specialized off-grid applications by the high cost of solar cells. However, cell prices have fallen steadily, and several large-scale photovoltaic electricity projects are now under way in the U.S. and elsewhere.

Atmospheric Circulation; Coronal Ejections and Magnetic Storms; Energy Transformations; Global Warming; Insolation and Total Solar Irradiation; Meteorology; Ocean Circulation and Currents; Seasonal Winds; Solar Illumination: Seasonal and Diurnal Patterns; Solar Sunspot Cycles; Sun; Ultraviolet Rays and Radiation

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