Solar Energy

I

Introduction

Solar Energy, radiation produced by nuclear fusion reactions deep in the Sun’s core (see Nuclear Energy). The Sun provides almost all the heat and light Earth receives and therefore sustains every living being.

Solar energy travels to Earth through space in discrete packets of energy called photons (see Electromagnetic Radiation). On the side of Earth facing the Sun, a square kilometer at the outer edge of our atmosphere receives 1,400 megawatts of solar power every minute, which is about the capacity of the largest electric-generating plant in Nevada. Only half of that amount, however, reaches Earth’s surface. The atmosphere and clouds absorb or scatter the other half of the incoming sunlight. The amount of light that reaches any particular point on the ground depends on the time of day, the day of the year, the amount of cloud cover, and the latitude at that point. The solar intensity varies with the time of day, peaking at solar noon and declining to a minimum at sunset. The total radiation power (1.4 kilowatts per square meter, called the solar constant) varies only slightly, about 0.2 percent every 30 years. Any substantial change would alter or end life on Earth.

II

Indirect Collection of Solar Energy

People can make indirect use of solar energy that has been naturally collected. Earth's atmosphere, oceans, and plant life, for example, collect solar energy that people later extract to power technology.

The Sun's energy, acting on the oceans and atmosphere, produces winds that for centuries have turned windmills and driven sailing ships (see Wind Energy). Modern windmills are strong, light, weather-resistant, aerodynamically designed machines that produce electricity when attached to generators.

---

---

Approximately 30 percent of the solar power reaching Earth is consumed by the continuous circulation of water, a system called the water cycle or hydrologic cycle. The Sun’s heat evaporates water from the oceans. Winds transport some of the water vapor from the oceans over the land where it falls as rain. Rainwater seeps into the ground or collects into streams or lakes and eventually returns to the ocean. Thus, radiant energy from the Sun is transformed to potential energy of water in streams and rivers. People can tap the power stored in the water cycle by directing these flowing waters through modern turbines. Power produced in this way is called hydroelectric power. See Waterpower; Dam.

The oceans also collect and store solar energy. A significant fraction of the Sun’s radiation reflects or scatters from the water’s surface. The remaining fraction enters the water and rapidly diminishes with depth as the energy is absorbed and converted to heat or chemical energy. This absorption creates differences in temperature between layers of water in the ocean called temperature gradients. In some locations, these differences approach 20°C (36°F) over a depth of a few hundred meters. These large masses of water existing at different temperatures create a potential for generating power. Energy flows from the high-temperature water to the low-temperature water (see Thermodynamics). The flow can be harnessed, to turn a turbine to produce electricity for example. Such systems, called ocean thermal energy conversion (OTEC) systems, require enormous heat exchangers and other hardware in the ocean to produce electricity in the megawatt range. Almost all of the major United States OTEC experiments in recent years have taken place in Hawaii.

Plants, through photosynthesis, convert solar energy to chemical energy, which fuels plant growth. People, in turn, use this stored solar energy through fuels such as wood, alcohol, and methane that are extracted from the plant life (biomass). Fossil fuels such as oil and coal are derived from geologically ancient plant life. People also eat and digest plants, or animals fed on plants, to obtain energy for their bodies.

III

Direct Collection of Solar Energy

People have devised two main types of artificial collectors to directly capture and utilize solar energy: flat plate collectors and concentrating collectors. Both require large surface areas exposed to the Sun since so little of the Sun’s energy reaches Earth’s surface. Even in areas of the United States that receive a lot of sunshine, a collector surface as big as a two-car garage floor is needed to gather the energy that one person typically uses during a single day.

A

Flat Plate Collectors

Flat plate collectors are typically flat, thin boxes with a transparent cover that are mounted on rooftops facing the Sun. The Sun heats a blackened metal plate inside the box, called an absorber plate, that in turn heats fluid (air or water) running through tubes within the collector. The energy transferred to the carrier fluid, divided by the total solar energy that falls on the collector, is called the collector efficiency. Flat plate collectors are typically capable of heating carrier fluids up to 82°C (180°F). Their efficiency in making use of the available energy varies between 40 and 80 percent, depending on the type of collector.

These collectors are used for water and space heating. Homes employ collectors fixed in place on roofs. In the Northern Hemisphere, they are oriented to face true south (± 20°); in the Southern Hemisphere, they are oriented to face north. For year-round applications such as providing hot water, they are tilted relative to the horizontal at an angle equal to the latitude ± 15°.

In addition to the flat plate collectors, typical hot-water and space heating systems include circulating pumps, temperature sensors, automatic controllers to activate the circulating pump, and a storage device. Either air or a liquid (water or a water-antifreeze mixture) can be used as the fluid in the solar heating system. A rock bed or a well-insulated water storage tank typically serves as an energy storage medium.