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.

B.

Concentrating Collectors

For applications such as air conditioning, central power generation, and many industrial heat requirements, flat plate collectors cannot provide carrier fluids at high enough temperatures to be effective. They may be used as first-stage heat input devices; the temperature of the carrier fluid is then boosted by other conventional heating means. Alternatively, more complex and expensive concentrating collectors can be used. These devices reflect the Sun’s rays from a large area and focus it onto a small, blackened receiving area. The light intensity is concentrated to produce temperatures of several hundred or even several thousand degrees Celsius. The concentrators move to track the Sun using devices called heliostats.

Concentrators use curved mirrors with aluminum or silver reflecting surfaces that coat the front or back surfaces of glass or plastic. Researchers are developing cheap polymer films to replace the more expensive glass. One new technique uses a pliable membrane stretched across the front of a cylinder and another across the back with a partial vacuum between. The vacuum causes the membranes to form a spherical shape ideal for concentrating sunlight.

Concentrating solar energy is the least expensive way to generate large-scale electrical power from the Sun’s energy and therefore has the potential to make solar power available at a competitive rate. Consequently, government, industry, and utilities have formed partnerships to reduce the manufacturing costs of concentrators.

One important high-temperature application of concentrators is solar furnaces. The largest of these, located at Odeillo in the Pyrenees Mountains of France, uses 63 mirrors with a total area of approximately 2,835 sq m (about 30,515 sq ft) to produce temperatures as high as 3200°C (5800°F). Such furnaces are ideal for research requiring high temperatures and contaminant-free environments—for example, materials research to determine how substances will react when exposed to extremely high temperatures. Other methods of reaching such temperatures usually require chemical reactants that would also react with the substances to be studied, skewing the results.

Another type of concentrator called a central receiver, or 'power tower,' uses an array of sun-tracking reflectors mounted on computer-controlled heliostats to reflect and focus the Sun’s rays onto a water boiler mounted on a tower. The steam thus generated can be used in a conventional power-plant cycle to produce electricity. A U.S. demonstration in the Mohave Desert, Solar One, operated through most of the 1980s. During the early 1990s a second demonstration, called Solar Two, used molten salt heated in the boiler to 574°C (1065°F) to produce electricity. The hot salt was stored and later used to boil water into steam that drove a turbine to produce electricity.