In passive solar heating, the greenhouse effect is used to heat buildings without the need for pumps or fans to distribute the collected heat. Cer­tain design features can be put to use in a passive solar heating system to warm buildings in winter and help them remain cool in summer. For example, large south-facing windows (or, in the Southern Hemisphere, north-facing windows) receive more total sunlight during the day than windows facing other directions. The sunlight en­tering through the windows provides heat that is then stored in floors, walls, or containers of water. This stored heat can be transmitted throughout the building naturally by convection (the circulation that occurs because warm air rises and cool air

sinks}. Buildings with passive solar heating systems must be well insulated so that accumulated heat does not escape. Passive solar heating is most effective in sunny climates, in cloudy regions it must be augmented by some other form of energy.

    In active solar heating, a series of collection devices mounted on a roof or in a field is used to gather solar energy. The most com­mon collection device is a flat solar panel or plate of black metal (which absorbs the sun's energy), enclosed in an insulated box. The absorbed heat is transferred to liquid or air inside the panel, which is then pumped to a building or a storage tank. Active solar heating is especially effective for water. Be­cause approximately 8 percent of the energy con­sumed in the United States goes toward heating water, active solar heating has the potential to sup­ply a significant amount of the nation's energy demand.

The use of solar energy for space heating will undoubtedly become more important as other en­ergy supplies dwindle. Furthermore, when dimin­ishing supplies of fossil fuels force their prices higher, solar heating, now usually costlier than more conventional forms of space heating, will become more competitive. Solar air conditioning is not yet commercially feasible.

Solar Thermal Electric Generation

Electricity can be produced by solar thermal elec­tric generation, in which trough-shaped mirrors (guided by computers), tracking the sun for opti­mum efficiency, center sunlight on oil-filled pipes and heat the oil to 390°C (735°F) (Figure 12-7). The hot oil is circulated to a water storage system

and used to change water to steam, which turns a turbine to generate electricity. Alternatively, the heat can be used in industrial processes or for desalinization (removal of salt from water) and water purification.

    Solar thermal energy systems are efficient at trapping the sun's energy; currently, up to 22 percent of the energy that hits the collector is con­verted to electricity. With improved engineering, manufacturing, and construction methods, solar thermal energy is becoming cost-competitive with fossil fuels. One problem with this form of energy is that a backup system must be available to generate electricity when solar power is not operating—at night and during cloudy days. LUZ International Corporation, the Los Angeles-based company that has a virtual monopoly on solar thermal systems in the United States, uses natural gas to augment solar thermal heat. LUZ has installed several solar ther­mal systems in California and is planning larger ones in Nevada and Brazil.

    Solar Power Towers: An Alternative Solar Thermal Technique The solar power tower, also called central receiver energy generation, is a tall build­ing or tower surrounded by numerous mirrors. The mirrors move to follow the sun, focusing solar radiation on a central receiver at the top of the tower; there a liquid is heated to produce steam, which is used to generate electricity. Some of the heat may be stored (as hot liquid) to be used

For electricity generation during the night, when solar energy is unavailable.

    Solar power towers are being tested in pilot plants in New Mexico and California, and Europe­ans are planning the Phoebus project, a massive central receiver to be built in Jordan. Whether we will be able to use solar power towers to generate increased amounts of electricity in the future is uncertain. The construction and maintenance of this type of facility are prohibitively expensive and require large tracts of land. Also, using current technology, it is very costly to store electrical en­ergy for use at night.

Photovoltaic Solar Cells

It is possible to convert sunlight directly into elec­tricity by using photovoltaic (PV) solar cells. Photovoltaic cells are wafers or thin films of crystalline silicon that are treated with cer­tain metals in such a way that they generate elec­tricity (that is, a flow of electrons) when solar en­ergy is absorbed. Photovoltaic solar cells are arranged on large panels that are set up to absorb sunlight.

    Our current photovoltaic solar cell technology, which is also used to power satellites, watches, and calculators, has several limitations that prevent the cells' widespread use to generate electricity. Photo­voltaic solar cells are not very efficient at convert­ing solar energy to electricity, they are extremely expensive to produce; and the number of solar pan­els needed for large-scale use requires a great deal of land. On the positive side, PVs generate electricity with no pollution and minimal maintenance. They can be used on any scale, from small, portable mod­ules to multi-megawatt power plants. Also, the cost of producing electricity from photovoltaic... de­clined more rapidly from 1970 to 1990 than had been predicted. Additional technological progress may eventually make photovoltaic economically competitive with conventional energy sources. For example, the production of a new type of solar cell from a thin film of a semiconductor material prom­ises to decrease costs. New ways to grow silicon crystals are also being investigated.

    One of the main benefits of photovoltaic de­vices for utility companies is that they can be pur­chased in small modular units that become opera­tional in a short amount of time. A utility company can purchase photovoltaic elements to increase its generating capacity in small increments, rather than committing a billion dollars (or more) and a decade (or more) of construction for a massive con­ventional power plant. Used in this supplementary way, the photovoltaic units can provide the addi­tional energy needed, for example, to power air conditioners and irrigation pumps on hot, sunny days.

    Although there has been steady improvement in the design and materials of photovoltaic solar cells, until their efficiency is improved it is unlikely that they can be used to generate electricity on a large scale, because we simply don't have enough space. At current efficiencies, for example, several, thousand acres of solar panels would be required to

absorb enough solar energy to produce the electric­ity generated by a single conventional power plant. The use of photovoltaic solar cells for smaller generating requirements is promising, how­ever. In remote areas that are not served by any electrical power plant (such as rural areas of developing countries), it is more economical to utilize photovoltaic cells for electricity than to extend power lines. Photovoltaic is the energy choice to pump water, refrigerate vaccines, grind grain, charge batteries, and supply rural homes with lighting. Thousands of people in developing coun­tries of Asia, Latin America, and Africa have in-stalled photovoltaic solar cells on the roofs of their homes. A PV panel the size of two pizza boxes can supply a rural household with enough electricity for five lights, a radio, and a television.

    Several pilot programs in California are suc­cessfully generating electricity using photovoltaic. Although efficiency is improving and costs are coming down, the total electricity produced by photovoltaic worldwide is about one-tenth of that produced by a single large nuclear power plant. Energy experts project that photovoltaic solar cells will not become a significant source of energy until well into the 21st century.

Solar Hydrogen: Solar electricity generated by photovoltaic can be used to split water into the gases oxygen and hydrogen. Hydrogen is a clean fuel (it produces water and heat when it is burned) and produces no sulfur oxides, no carbon monox­ide, no hydrocarbon particulates, and no CO₂ emis­sions. It does produce some nitrogen oxides, but the amounts of these pollutants are fairly easy to con­trol. Hydrogen has the potential to provide energy for transportation (in the form of hydrogen-powered electric automobiles) as well as for heating buildings and producing electricity.

    It may seem wasteful to use electricity gener­ated from solar energy to make hydrogen, which can then be used to generate more electricity. However, electricity that is generated by existing photovoltaic cells cannot be stored long-term; it must be used immediately. Hydrogen offers a con­venient way to store solar energy as chemical en­ergy. It can be transported by pipeline, possibly less expensively than electricity can be transported by wire.

    Production of hydrogen from solar electricity currently has an efficiency of 8 percent, which means that only 8 percent of the solar energy ab­sorbed by the photovoltaic cells is actually con­verted into the chemical energy of hydrogen fuel. Scientists are working to improve the efficiency, which will decrease costs and make solar hydrogen fuel more attractive. A pilot plant that makes solar hydrogen fuel is being tested in Saudi Arabia under the joint sponsorship of Germany and Saudi Ara­bia.

Solar Ponds

Because water absorbs solar energy, it is possible to build a pond of water specifically to collect solar energy. Solar ponds are generally dug 1 to several meters deep and are frequently lined with black

plastic. Because a large percentage of solar radiation penetrates to the bottom of the pond, the tem­perature near the bottom may be as high as 100^DC; the water near the surface remains at air tempera­ture.

Under normal conditions, warm water rises to the top of a body of water because it is less dense than cool water. However, the water at the bottom of a solar pond is made denser than the surface water by the addition of salt. Therefore, minimal mixing occurs between the warm, dense bottom layer and the cooler, less dense surface layer.

One of the problems associated with solar ponds is the large amount of land needed. Also, there is potential for environmental contamination, if the brackish water leaked into the surroundings, it would harm plants and other wildlife.