The Sun glows because of the fusion of hydrogen atoms in its hot, gaseous core (see Figure 9.15, page 272). The energy radiating from the Sun is enormous: The amount that strikes Earth’s surface in just 2 hours exceeds global energy consumption from all sources in a year. The radiant energy the Sun emits, which we commonly call sunlight, looks orange and yellow, but it is actually a complex mixture of varying wavelengths of light (Figure 10.2).

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Approximately half of this radiant energy is visible to humans, with wavelengths between 400 and 700 nanometers (Figure 10.3). Infrared light, which we can perceive as heat, accounts for most of the light energy outside the visible range and is very important in Earth’s heat balance, as we shall see in detail in Chapter 14. The Sun also emits ultraviolet light, which consists of highly energetic, short wavelengths of light (> 400 nanometers). Most ultraviolet light is absorbed by ozone in the upper atmosphere before it reaches Earth’s surface (see Chapter 1, page 2).

One challenge with solar power, as with many sources of renewable energy, is that it provides only intermittent power—only during daytime hours on relatively clear days. Therefore, an electrical system must have additional storage capacity (e.g., a battery), and is currently only robust when combined with other forms of power generation, such as wind or hydroelectric power.

The uneven heating of Earth’s surface by sunlight produces a greater concentration of solar energy near the equator, between the Tropics of Cancer and Capricorn. In addition, the availability of solar energy at Earth’s surface is strongly affected by cloud cover. Consequently, arid and semiarid regions near the Tropics of Cancer and Capricorn receive some of the highest amounts of solar energy (Figure 10.4). However, the Sun provides useful energy supplies well into the temperate zones of North America, Europe, and Asia.

Historically, solar energy has been used to preserve foods through drying, to illuminate living and work spaces, and to heat living areas (Figure 10.5). More recently, solar energy has been used to generate electricity. Small solar energy systems can provide electricity for homes, businesses, and power needs in remote areas. Larger solar energy systems provide more electricity for contribution to the electric power grid.

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Because solar energy is a diffuse source of heat, one form of solar power uses mirrors to reflect and concentrate sunlight onto receivers (Figure 10.6a). One experimental power plant, Solar One, began operating in the Mojave Desert of Southern California in the early 1980s. Nearly 2,000 sun-tracking mirrors, known as heliostats, were arrayed in concentric circles around a central tower. Inside the tower, a reservoir of oil was heated to temperatures exceeding 600°C (1,000°F), providing constant heat to a boiler that drove a steam turbine. Solar One and its successor, Solar Two, demonstrated the reliability of solar power by producing enough electricity to power 7,500 homes; but they were shut down in 1999 because fossil fuels were relatively cheap and there were no economic incentives to support the plant in the long term. Since that early, experimental period, concentrating solar generation power plant designs have advanced considerably, and new plants are being built all over the world. In addition to towers, other solar concentrators use curved mirrors that focus energy on a pipe containing a liquid (Figure 10.6b) or use a mirrored dish that concentrates solar energy to run an engine that drives a generator.

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Solar energy can be used to generate electricity directly using photovoltaic cells (solar cells). In these systems, light induces the photoelectric effect, a flow of electrons in a semiconductor. A semiconductor is a material that conducts current, but only somewhat. The semiconductor has properties somewhere between that of an insulator (e.g., rubber), which blocks current, and a conductor (e.g., copper), which allows current to pass freely. Most semiconductors are crystalline materials, most commonly silicon. If a semiconductor absorbs enough light energy, an electron will move from its position within the semiconductor. The result is a flow of electrons, that is, an electrical current, as many electrons are stimulated.

Several industrial processes can be used to increase the efficiency of the photoelectric effect by producing solar cells made up of layers of semiconductors that differ in their relative number of electrons. The addition of phosphorus atoms produces an excess of electrons in a silicon-based semiconductor. These materials are used to manufacture n-layers (the “n” indicates “negative”). The insertion of boron atoms in a layer of silicon produces p-layers (the “p” indicates “positive”) that have fewer free electrons.

When an n-layer and a p-layer are joined, excess electrons in the n-layer near the junction instantaneously migrate from the n-layer to the p-layer, producing relatively positively charged and negatively charged regions along the junction between the two layers (Figure 10.7). When energized by a photon of light, the electrical field pushes the electrons on the p-layer side of the junction back through to the n-layer. This induces an electrical current in conductors attached to the surfaces of the n- and p-layers that can be tapped for applications ranging from lighting to powering an electric vehicle.