Solar Energy
SOLAR ENERGY
Energy from the sun is abundant and renewable. It is also the principal factor that has enabled and shaped life on our planet. The sun is directly or indirectly responsible for nearly all the energy on earth, except for radioactive decay heat from the earth's core, ocean tides associated with the gravitational attraction of Earth's moon, and the energy available from nuclear fission and fusion. Located approximately ninety-three million miles from Earth, the sun, an average-size star, belongs to the class of dwarf yellow stars whose members are more numerous than those of any other class. The energy radiated into outer space by the sun (solar radiation) is fueled by a fusion reaction in the sun's central core where the temperature is estimated to be about 10 million degrees. At this temperature the corresponding motion of matter is so violent that all atoms and molecules are reduced to fast-moving atomic nuclei and stripped electrons collectively known as a plasma. The nuclei collide frequently and energetically, producing fusion reactions of the type that occur in thermonuclear explosions.
While about two-thirds of the elements found on earth have been shown to be present in the sun, the most abundant element is hydrogen, constituting about 80 percent of the sun's mass (approximately two trillion trillion million kilograms). When hydrogen nuclei (i.e., protons) collide in the sun's core, they may fuse and create helium nuclei with four nucleons (two protons and two neutrons). Roughly 20 percent of the sun's mass is in the form of helium. Also created in each fusion reaction are two neutrinos, high-energy particles having no net electrical charge, which escape into outer space, and high-energy gamma radiation that interacts strongly with the sun's matter surrounding its core. As this radiation streams outward from the core, it collides with and transfers energy to nuclei and electrons, and heats the mass of the sun so that it achieves a surface temperature of several thousand degrees Celsius. The energy distribution of the radiation emitted by this surface is fairly close to that of a classical "black body" (i.e., a perfect emitter of radiation) at a temperature of 5,500°C, with much of the energy radiated in the visible portion of the electromagnetic spectrum. Energy is also emitted in the infrared, ultraviolet and x-ray portions of the spectrum (Figure 1).
The sun radiates energy uniformly in all directions, and at a distance of ninety-three million miles, Earth's disk intercepts only four parts in ten billion of the total energy radiated by the sun. Nevertheless, this very small fraction is what sustains life on Earth and, on an annual basis, is more than ten thousand times larger than all the energy currently used by Earth's human inhabitants. Total human energy use is less than 0.01 percent of the 1.5 billion billion kilowatt-hours (kWh) of energy per year the sun delivers to Earth. A kilowatt hour is one thousand watt hours and is the energy unit shown on your electric bill. A one-hundred-watt light bulb left on for ten hours will use one kWh of electric energy.
AVAILABILITY OF SOLAR ENERGY
While the amount of energy radiated by the sun does vary slightly due to sunspot activity, this variation is negligible compared to the energy released by the sun's basic radiative process. As a result, the amount of energy received at the outer boundary of Earth's atmosphere is called the Solar Constant because it varies so little. This number, averaged over Earth's orbit around the sun, is 1,367 watts per square meter (W/m2) on a surface perpendicular to the sun's rays. If, on average, Earth were closer to the sun than ninety-three million miles, this number would be larger; if it were farther from the sun, it would be smaller. In fact, Earth's orbit about the sun is not circular, but elliptical. As a result, the Solar Constant increases and decreases by about 3 percent from its average value at various times during the year. In the northern hemisphere the highest value is in the winter and the lowest is in the summer.
A non-negligible fraction of the solar radiation incident on the earth is lost by reflection from the top of the atmosphere and tops of clouds back into outer space. For the radiation penetrating the earth's atmosphere, some of the incident energy is lost due to scattering or absorption by air molecules, clouds, dust and aerosols. The radiation that reaches the earth's surface directly with negligible direction change and scattering in the atmosphere is called "direct" or "beam" radiation. The scattered radiation that reaches the earth's surface is called "diffuse" radiation. Some of the radiation may reach a receiver after reflection from the ground, and is called "albedo." The total radiation received by a surface can have all three components, which is called "global radiation."
The amount of solar radiation that reaches any point on the ground is extremely variable. As it passes through the atmosphere, 25 to 50 percent of the incident energy is lost due to reflection, scattering or absorption. Even on a cloud-free day about 30 percent is lost, and only 70 percent of 1,367 W/m2, or 960 W/m2, is available at the earth's surface. One must also take into account the earth's rotation and the resultant day-night (diurnal) cycle. If the sun shines 50 percent of the time (twelve hours per day, every day) on a one square meter surface, that surface receives no more than (960 W/m2) × (12 hours/day) × (365 days/year) = 4,200 kWh of solar energy per year. Since, on average, the sun actually shines less than twelve hours per day at any location, the maximum solar radiation a site can receive is closer to 2,600 kWh per square meter per year. To put this number into perspective, the average person on earth uses about 18,000 kWh of all forms of energy each year.
HISTORY
For centuries the idea of using the sun's heat and light has stimulated human imagination and inventiveness. The ancient Greeks, Romans, and Chinese used passive solar architectural techniques to heat, cool, and provide light to some of their buildings. For example, in 100 C.E., Pliny the Younger built a summer home in northern Italy that incorporated thin sheets of transparent mica as windows in one room. The room got warmer than the others and saved on short supplies of wood. To conserve firewood, the Romans heated their public baths by running the water over black tiles exposed to the sun. By the sixth century, sunrooms on private houses and public building were so common that the Justinian Code introduced "sun rights" to ensure access to the sun.
In more recent times, Joseph Priestly used concentrated sunlight to heat mercuric oxide and collected the resulting gas, thus discovering oxygen. In 1872, a solar distillation plant was built in Chile that provided six thousand gallons of fresh water from salt water daily for a mining operation. In 1878, at an exhibition in Paris, concentrated sunlight was used to create steam for a small steam engine that ran a printing press.
Many further solar energy developments and demonstrations took place in the first half of the twentieth century. However, only one solar technology survived the commercial competition with cheap fossil fuels. That exception was solar water heaters that were widely used in Japan, Israel, Australia and Florida, where electricity was expensive and before low-cost natural gas became available.
Also, passive solar, or climate responsive, buildings were in such demand by 1947 as a result of energy shortages during World War II, that the Libbey-Owens-Ford Glass Company published a book entitled Your Solar House, which profiled forty-nine solar architects. In the mid-1950s, architect Frank Bridgers designed the world's first commercial office building using passive design and solar water heating. This solar system has since been operating continuously, and is now in the U.S. National Historic Register as the world's first solar-heated office building.
It was the oil embargo of 1973 that refocused attention on use of solar energy, when the cost of fossil fuels (coal, oil and natural gas) increased dramatically and people began to appreciate that fossil fuels were a depletable resource. More recently, concerns about energy security (the dependence on other countries for one's energy supply), and the local and global environmental impacts of fossil fuel use, have led to the growing realization that we cannot project today's energy system, largely dependent on fossil fuels, into the long-term future. In the years following the oil embargo, much thought and effort has gone into rethinking and developing new energy options. As a result, a growing number of people in all parts of the world believe that renewable (i.e., non- depletable) energy in its various forms (all derived directly or indirectly from solar energy except for geothermal and tidal energy) will be the basis for a new sustainable energy system that will evolve in the twenty-first century.
USING SOLAR ENERGY
Solar energy can be used in many direct and indirect ways. It can be used directly to provide heat and electricity. Indirectly, non-uniform heating of Earth's surface by the sun results in the movement of large masses of air (i.e., wind). The energy associated with the wind's motion, its kinetic energy, can be tapped by special turbines designed for that purpose. Winds also create ocean waves, and techniques for tapping the kinetic energy in waves are under active development. Hydropower is another indirect form of solar energy, in that the sun's radiant energy heats water at Earth's surface, water evaporates, and it eventually returns to Earth as rainfall. This rainfall creates the river flows on which hydropower depends. In addition, sunlight is essential to photosynthesis, the process by which plants synthesize complex organic materials from carbon dioxide, water and inorganic salts. The solar energy used to drive this process is captured in the resulting biomass material and can be released by burning the biomass, gasifying it to produce hydrogen and other combustible gases, or converting it to liquid fuels for use in transportation and other applications.
In principle, energy can also be extracted from the kinetic energy of large currents of water circulating beneath the ocean's surface. These currents are also driven by the sun's energy, much as are wind currents, but in this case it is a slower-moving, much more dense fluid. Research into practical use of this energy resource is in its very earliest stages, but is looking increasingly promising.
Finally, there are renewable sources of energy that are not direct or indirect forms of solar energy. These include geothermal energy in the form of hot water or steam derived from reservoirs below the surface of Earth (hydrothermal energy), hot dry rock, and extremely hot liquid rock (magma). These resources derive their energy content from the large amount of heat generated by radioactive decay of elements in Earth's core. Energy can also be tapped from tidal flows resulting from Earth's gravitational interaction with the moon.
SOLAR HEATING AND COOLING
Solar heating and cooling systems are classified either as passive or active. Passive systems make coordinated use of traditional building elements such as insulation, south-facing glass, and massive floors and walls to naturally provide for the heating, cooling and lighting needs of the occupants. They do not require pumps to circulate liquids through pipes, or fans to circulate air through ducts. Careful design is the key. Active solar heating and cooling systems circulate liquids or air through pipes or other channels to move the necessary heat to where it can be used, and utilize other components to collect, store and control the energy from the solar energy source. Systems that combine both passive and active features are called hybrid systems. Solar heating systems are used primarily for hot water heating, interior space heating, and industrial and agricultural process heat applications. Solar cooling systems use solar heat to energize heat-driven refrigeration systems or to recycle dessicant-cooling systems. They are most often combined with solar hot water and space heating systems to provide year-round use of the solar energy system.
PASSIVE SYSTEMS
There are several types of passive system designs: direct gain, thermal storage, solar greenhouse, roof pond, and convective loop. The simplest is the direct gain design (Figure 2), which in the northern hemisphere is an expanse of south-facing glass (usually double glazed). The energy in the sunlight entering directly through the windows is absorbed, converted to heat, and stored in the thermal mass of the floors and walls. A thermal storage wall system consists of a massive sunlight-absorbing wall behind south-facing double glazing. The wall may contain water or masonry to store the energy during the day and release it during the night. Another type of passive system is the solar greenhouse (or sunspace), which combines direct gain in the greenhouse and a thermal storage wall between the greenhouse and the rest of the house. Solar energy provides the heat for the greenhouse and a good share of the energy for heating the living space in the house.
A fourth type of passive solar heating and cooling system is the roof pond, in which containers of water are used to collect and store the sun's energy. Movable insulating panels are used to control the gain and loss of heat. In winter, the insulation is opened during the day to allow collection of the sun's heat; at night the insulation is closed to minimize loss of heat that is to be released to the house. In summer, the insulation is closed during the day to block the sun, and opened to the sky at night to provide radiative cooling.
The fifth type of passive system is the natural convective loop, in which the collector is placed below the living space and the hot air that is created rises to provide heat where it is needed. This same principle is used in passive solar hot water heating systems known as thermosiphons. The storage tank is placed above the collector. Water is heated in the collector, becomes less dense, and rises (convects) into the storage tank. Colder water in the storage tank is displaced and moves down to the collector where it is heated to continue the cycle.
More than one million residences and twenty thousand commercial buildings across the United States now employ passive solar design.
ACTIVE SYSTEMS
An active solar heating and cooling system consists of a solar energy collector (flat plate or concentrating), a storage component to supply heat when the sun is not shining, a heat distribution system, controls, and a back-up energy source to supply heat when the sun is not shining and the storage system is depleted.
The simplest type of active solar collector, the flat-plate collector, is a plate with a black surface under glass that absorbs visible solar radiation, heats up, and reradiates in the infrared portion of the electromagnetic spectrum. Because the glass is transparent to visible but not to infrared radiation, the plate gets increasingly hotter until thermal losses equal the solar heat gain. This is the same process that occurs in a car on a sunny day, and in the atmosphere, where gases such as carbon dioxide and methane play the role of the glass. It is called the greenhouse effect. Careful design can limit these losses and produce moderately high temperatures at the plate—as much as 160°F (71°C). The plate transfers its heat to circulating air or water, which can then be used for space heating or for producing hot water for use in homes, businesses or swimming pools.
Higher temperatures—up to 350°F (177°C)—can be achieved in evacuated tube collectors that encase both the cylindrical absorber surface and the tubes carrying the circulating fluid in a larger tube containing a vacuum which serves as highly efficient insulation. Still another high-temperature system is the parabolic-trough collector that uses curved reflecting surfaces to focus sunlight on a receiver tube placed along the focal line of the curved surface. Such concentrating systems use only direct sunlight, and can achieve temperatures as high as 750°F (400°C), but do require tracking of the sun.
Water heating accounts for approximately 25 percent of the total energy used in a typical single-family home. An estimated 1.5 million residential and commercial solar water-heating systems have been installed in the U.S. In Tokyo—there are nearly that many buildings with solar water heating. Large numbers of solar water heaters have also been installed in Greece, Israel, and other countries. More than 300,000 swimming pools are heated by solar energy in the United States.
SOLAR THERMAL POWER SYSTEMS
A solar thermal power system, increasingly referred to as concentrating solar power, to differentiate it from solar heating of residential/commercial air and water, tracks the sun and concentrates direct sunlight to create heat that is transferred to water to create steam, which is then used to generate electricity. There are three types of solar thermal power systems: troughs that concentrate sunlight along the focal axes of parabolic collectors (the most mature form of solar thermal power technology), power towers with a central receiver surrounded by a field of concentrating mirrors, and dish-engine systems that use radar-type reflecting dishes to focus sunlight on a heat-driven engine placed at the dish's focal point. Solar ponds are also a form of solar thermal power technology that does not require tracking.
Trough Systems
Trough systems currently account for more than 90 percent of the world's solar electric capacity. They use parabolic reflectors in long trough configurations to focus and concentrate sunlight (up to one hundred times) on oil-filled glass tubes placed along the trough's focal line. The dark oil absorbs the solar radiation, gets hot (up to 750°F (400°C)), and transfers its heat to water, creating steam that is fed into a conventional turbine-generator. Troughs are modular and can be grouped together to create large amounts of heat or power. Troughs operating in the Mojave Desert in the United States have been feeding up to 354 MW of electrical energy reliably into the Southern California power grid since the early 1990s, usually with minimal maintenance. While not currently cost competitive with fossil fuel-powered sources of electricity (trough electricity costs are about 12 cents per kWh), construction of the trough systems was encouraged by generous power purchase agreements for renewable electricity issued by the State of California, and Federal tax incentives.
Power Towers
Power towers, also known as central receivers, have a large field of mirrors, called heliostats, surrounding a fixed receiver mounted on a tall tower. Each of the heliostats independently tracks the sun and focuses sunlight on the receiver where it heats a fluid (water or air) to a very high temperature (1,200°F (650°C)). The fluid is then allowed to expand through a turbine connected to an electric generator. Another version heats a nitrate salt to 1,050°F (565°C), which provides a means for storage of thermal energy for use even when the sun is not shining. The heat stored in this salt, which becomes molten at these elevated temperatures, is then transferred to water to produce steam that drives a turbine-generator. Ten megawatt electrical power towers have been built and tested in the United States, one using water as the material to be heated in the receiver, and one using nitrate salt. A smaller unit that heats air has also been operated at the Platforma Solar test site in Almeria, Spain. While there are no commercial power tower units operating today, technical feasibility has been established and, in commercial production, electricity production costs are projected to be well below ten cents per kWh. At such costs, power towers will begin to compete with more traditional fossil fuel-powered generating systems. It is believed that power towers will be practical in sizes up to 200 MW (electrical).
Dish-Engine Systems
A dish-engine system uses a dish-shaped parabolic reflector, or a collection of smaller mirrors in the shape of a dish, to focus the sun's rays onto a receiver mounted above the dish at its focal point. Two-axis tracking is used to maximize solar energy capture, and most often a Stirling engine (a sealed engine that can be driven by any source of external heat) is mounted on the receiver. Such systems are not yet commercial, but have been tested in sizes ranging from 5 to 50 kW (electrical). Conversion efficiencies of up to 29 percent have been achieved. Dish-engine systems are also modular, and can be used individually or in large assemblies.
Power-tower and dish-engine systems have higher concentration ratios than troughs, and therefore the potential to achieve higher efficiencies and lower costs for converting heat to electricity. All three systems can be hybridized (i.e., backed up by a fossil fuel system for supplying the heat), an important feature that allows the system to produce power whenever it is needed and not just when the sun is shining (dispatchability).
Solar Ponds
A solar pond does not concentrate solar radiation, but collects solar energy in the pond's water by absorbing both the direct and diffuse components of sunlight. Solar ponds contain salt in high concentrations near the bottom, with decreasing concentrations closer to the surface. This variation in concentration, known as a salt-density gradient, suppresses the natural tendency of hot water to rise, thus allowing the heated water to remain in the bottom layers of the pond while the surface layers stay relatively cool. Temperature differences between the bottom and top layers are sufficient to drive an organic Rankine-cycle engine that uses a volatile organic substance as the working fluid instead of steam. Temperatures of 90°C are routinely achieved in the pond bottom, and solar ponds are sufficiently large to provide some degree of energy storage.
The largest solar pond in the United States is a tenth of an acre experimental facility in El Paso, Texas, which has been operating reliably since 1986. The pond runs a 70 kW (electrical) turbine-generator and a 5,000 gallon per day desalination unit, while also providing process heat to an adjacent food processing plant. The potential of solar ponds to provide fresh water, process heat and electricity, especially for island communities and coastal desert regions, appears promising, but has not been fully investigated.
PHOTOVOLTAICS
Photovoltaics (photo for light, voltaic for electricity), often abbreviated as PV, is the direct conversion of sunlight to electricity. Considered by many to be the most promising of the renewable electric technologies in the long term, it is an attractive alternative to conventional sources of electricity for many reasons: it is silent and non-polluting, requires no special training to operate, is modular and versatile, has no moving parts, and is highly reliable and largely maintenance-free. It can be installed almost anywhere, uses both direct and diffuse radiation, and can be incorporated into a variety of building products (roofing tiles and shingles, facades, overhangs, awnings, windows, atriums).
The photoelectric effect (the creation of an electrical current when light shines on a photosensitive material connected in an electrical circuit) was first observed in 1839 by the French scientist Edward Becquerel. More than one hundred years went by before researchers in the United States Bell Laboratories developed the first modern PV cell in 1954. Four years later, PV was used to power a satellite in space and has provided reliable electric power for space exploration ever since.
The 1960s brought the first terrestrial applications for PV. At that time the technology was very expensive, with PV collectors, called modules, costing upwards of $1,000 per peak watt. The term "peak watts" is used to characterize the power output of a PV module when incident solar radiation is at its peak. Nevertheless, PV was a preferred choice in remote locations where no other form of power production was feasible. Over the past three decades, steady advances in technology and manufacturing have brought the price of modules down more than 200-fold, to $4 per peak watt. Further reductions in module cost to $1 to $2 per peak watt are expected within the next decade.
A photovoltaic cell (often called a solar cell) consists of layers of semiconductor materials with different electronic properties. In most of today's solar cells the semiconductor is silicon, an abundant element in the earth's crust. By doping (i.e., chemically introducing impurity elements) most of the silicon with boron to give it a positive or p-type electrical character, and doping a thin layer on the front of the cell with phosphorus to give it a negative or n-type character, a transition region between the two types of doped silicon is formed that contains an electric field. This transition region is called a junction.Light consists of energy packets called photons. When light is incident on the cell, some of the photons are absorbed in the region of the junction and energy is transferred to the semiconductor, freeing electrons in the silicon. If the photons have enough energy, the electrons will be able to overcome the opposing electric field at the junction and move freely through the silicon and into an external circuit. As these electrons stimulate current flow in the external circuit, their energy can be converted into useful work (Figure 3).
Photovoltaic systems for specific applications are produced by connecting individual modules in series and parallel to provide the desired voltage and current (Figure 4). Each module is constructed of individual solar cells also connected in series and parallel. Modules are typically available in ratings from a few peak watts to 250 peak watts.
Commercially available PV systems most often include modules made from single-crystal or polycrystalline silicon or from thin layers of amorphous (non-crystalline) silicon. The thin-film modules use considerably less semiconductor material but have lower efficiencies for converting sunlight to direct-current electricity. Cells and modules made from other thin-film PV materials such as copper-indiumdiselenide and cadmium telluride are under active development and are beginning to enter the market. Most experts consider thin-film technology to be the future of the PV industry because of the reduced material requirements, the reduced energy required to manufacture thin film devices, and the ability to manufacture thin films on a mass-production basis.
Amorphous silicon modules experience a conversion efficiency loss of about 10 percent when initially exposed to sunlight, but then stabilize at the reduced figure. The mechanism for this reduction is being actively investigated, but is still not well understood. Individual modules made with other PV materials do not exhibit such loss of conversion efficiency, but combinations of modules in arrays do exhibit systematic reductions in power output over their lifetimes. Estimated at about 1 percent per year on average, based on data to date, these reductions are most likely associated with deteriorating electrical connections and non-module electrical components.
Complete PV systems require other components in addition to the PV modules. This "balance-of-system" generally includes a support structure for the modules to orient them properly to the sun, an inverter to convert direct-current electricity to alternating-current electricity, a storage system for electrical energy (usually batteries), an electronic charge regulator to prevent damage to the batteries by protecting against overcharging by the PV array and excessive discharge by the electrical load, and related wiring and safety features (Figure 5).
The output of a solar cell can be increased significantly by concentrating the incident sunlight onto a small area. Commercially available concentrator systems use low-cost lenses, mirrors or troughs in conjunction with tracking systems to focus sunlight on a small, high efficiency but expensive solar cell. One-axis trackers follow the sun during the day; two-axis trackers track daily and yearly variations in the sun's position. The higher cost of the cell and the associated tracking equipment, and the inability to use diffuse radiation, are offset by the higher conversion efficiencies achieved.
The cost of PV electricity is largely determined by four factors: cost of the PV modules, the efficiency of converting sunlight to electricity, the balance-of-system costs, and the lifetime of the systems. There are many combinations of these factors that provide about the same cost of electricity. Modules are now available that are designed to last at least thirty years.
Another scheme that has been proposed for use of photovoltaics is the solar satellite power system. Such a system would place large individual PV arrays (up to ten megawatts peak power) in synchronous orbit around the earth, and beam the collected and converted solar energy through the atmosphere in the form of microwaves to large receiving antennas on earth. The microwave generators would be powered by the PV electricity. The microwave energy collected at the surface would then be converted back to electricity for broad distribution. While of some interest, there are major issues associated with cost, environmental impact, and energy security.
PHOTOVOLTAIC APPLICATIONS
By its very nature PV can be used wherever electricity is needed and sunlight is available. For many years PV systems have been the preferred power sources for buildings or other facilities in remote areas not served by the utility grid. As the cost of PV systems continues to decline, there is growing consensus that distributed PV systems on buildings that are grid-connected may be the first application to reach widespread commercialization. Typical off-grid applications for PV today include communications (microwave repeaters, emergency call boxes), cathodic protection (pipelines, bridges), lighting (billboards, highway signs, security lighting, parking lots), monitoring equipment (meteorological information, water quality, pipeline systems), warning signals (navigational beacons, railroad signs, power plant stacks), and remote loads (village power, parks and campgrounds, water pumping and irrigation, vacation cabins). On-grid applications include support for utility transmission and distribution (T&D) systems, where the benefits derive from avoiding or deferring T&D system investments, and from improving the quality of electrical service.
More than 200,000 homes in the U.S. currently use some type of PV technology, and more than 10,000 U.S. homes are powered entirely by PV. A large number of homes in Japan, where consumer electricity rates are three times higher than in the U.S., have roof-integrated PV systems. Half a million PV systems have been installed in developing countries by the World Bank. More than 10,000 have been installed in Sri Lanka, 60,000 in Indonesia, 150,000 in Kenya, 85,000 in Zimbabwe, 40,000 in Mexico, and 100,000 in China. Global use of PV is expected to grow rapidly in the coming decades.
OTHER DIRECT APPLICATIONS OF SOLAR ENERGY
In addition to the applications already discussed, solar energy can also be used for solar cooking (use of a reflecting surface to focus sunlight on pots used for food preparation), distillation (use of solar heat to evaporate and desalinate seawater), purification (use of concentrated ultraviolet radiation to kill organisms in contaminated water), and agriculture (use of solar heat to dry crops and grains, space and water heating, heating of commercial greenhouses, and use of PV electricity to power water pumping and other remote facilities). Many other applications will also become feasible as PV costs continue to decline.
Production of PV modules is still relatively small, but has been growing at a steady and significant rate. Global production in 1998 reached 150 megawatts peak. As costs come down and new manufacturing facilities are placed in operation, this number will grow rapidly.
SOLAR ENERGY'S POTENTIAL AND PROBLEMS
The amount of solar energy available on earth is many times greater than all the energy collectively used by people around the world. When one takes into account all the various forms in which solar energy is directly or indirectly available, along with energy available from geothermal and nuclear sources, it becomes clear that there is no global shortage of energy. The only shortage is that of low-cost energy. In principle there is no reason that solar energy in its various forms could not supply all the world's energy. To give but one example, all the electricity used by the United States (a little more than 3 trillion kWh) could be generated by commercially available PV modules covering half of a 100 mile by 100 mile area in the Nevada desert.
Greater use of solar and other forms of renewable energy is increasingly seen as the long-term response to growing demand for electricity and transportation fuels in developed and developing countries. For example, electricity derived from renewable resources can be used to electrolyze water and create hydrogen, which can then be used in fuel cells to provide electricity or power electric vehicles. As a result of the issues raised by the oil embargo of 1973, and increasing awareness of the local and global environmental damage associated with use of fossil fuels, many people now believe that the world must undergo a transition to a clean and sustainable energy system during the twenty-first century. There is great hope that renewable energy will be the basis for that new, sustainable system.
This is not to suggest that widespread use of solar energy is not facing major barriers. For many applications, today's high cost of solar energy systems is an important limitation. Nevertheless, significant progress has been and will continue to be made in improving system performance and reducing associated energy costs. A particular challenge to solar energy research and development efforts is that incident solar energy is not highly concentrated and is intermittent. The former necessitates use of concentrator systems, or large collection areas that can have impacts on ecosystems due to land use and disturbances during the construction stage of large-scale power plants. Centralized, multi-megawatt power facilities can also result in significant visual impacts.
The fact that solar energy is an intermittent energy resource means that energy storage systems (e.g., batteries, ultracapacitors, flywheels, and even hydrogen) will be required if solar energy is to be utilized widely. In addition, a variety of toxic chemicals are used in the manufacture of PV cells; however, studies of the risks associated with their manufacture and disposal indicate little threat to surroundings and the environment.
CONCLUSIONS
Solar energy offers a clean, sustainable alternative to continued use of fossil fuels. In its various forms it is already providing useful amounts of energy on a global basis, and will provide steadily increasing amounts in the twenty-first century, especially as developing countries require more energy to improve their economies.
In February 1979 the Carter Administration released a 30-federal agency study entitled Domestic Policy Review of Solar Energy. This study found that, with increasing oil prices and "comprehensive and aggressive initiatives at the Federal, State and local levels," renewable energy sources "could provide about 20 percent of the nation's energy by the year 2000." This did not occur, as oil prices actually dropped in the 1980s, Federal funding for renewables was reduced, and, for a period of time, energy issues largely disappeared from public view. Today, renewable energy sources supply about 12 percent of U.S. electricity and 8 percent of total U.S. energy consumption.
Continuing research and development efforts by the public and private sectors have significantly reduced the costs of solar energy, and further significant reductions are expected. Together with growing public concern about global climate change, it is anticipated that the 21st century will see widespread deployment of solar energy and other renewable energy systems, in both developing and developed countries.
Allan R. Hoffman
See also: Solar Energy, Historical Evolution of the Use of.
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