Physical Testing and Analysis Physical testing investigates the physical properties and behavior of soils. The most fundamental level of physical testing in- cludes estimation of specific gravity, deter- mination of moisture content, sieve analy- sis, and hydrometer analysis. Specific gravity is defined as the ratio of “unit weight of soil solids only” to “unit weight of water.” For sandy soils the spe- cific gravity ranges from 2.63 to 2.67, while for clays it is between 2.67 and 2.90. The specific gravity of organic soils is less than 2.0. Moisture content is defined as the ra- tio of “the weight of water present in a soil sample” to “the weight of dry soil.” Mois- ture content is about 15 to 20 percent for coarse soils and80 to 100percent for clays. Organic soils may have moisture content in excess of 500 percent. In order todetermine the grain-sizedis- tribution of coarse soils, the soil sample is subjected to sieve analysis. This analysis is conducted by using a stack of sieves that have a decreasing sieve diameter from the top to the bottom. The sieves are usually made of woven wires. After the sieves are shaken for a specified period of time, the amount of sediment retained at each sieve is weighted. Based on these data, each par- ticle diameter is plotted in terms of the percentage of material that is finer than this particu- lar diameter. For the plot, a semilogarithmic paper is used. For the determination of particle size distribu- tion of fine soils, the hydrometer analysis is applied. The lower limit of the particle diameter that can be detected by this analysis is 0.001 millimeter. The hy- drometer measures the particle diameter indirectly. This analysis is based on the principle that the hy- drometer will be subject to higher buoyancy forces in a well-mixed water-sediment system. However, as the suspended solid particles settle, the density of the water-sediment mixture decreases, and the hydrome- ter tends tosink. The grain-size distributionis integral to studies involving sediment transport, protection against erosion, and soil contamination. Engineering Testing and Analysis Tests thatare most useful in geotechnicaland founda - tion engineering are those used to define the so- called Atterberg limits for clay soils: liquid limit, plas- tic limit, and shrinkage limit. The liquid limit is the moisture content that causes claysoils to behave as vis- cous liquids. Liquid limit is estimated by counting the number of “blows” required (using a Casagrande de- vice) to close a groove made in the soil sample. The plastic limit of moisture content is the point at which the soil sample will turn from a plastic state to a semisolid state. The plastic limit is determined by the ability of the sample to roll and form threads 3.18 mil- limeters in diameter without crumbling. When clay soils are losing moisture, their volume generally de- creases. The moisture content, expressed as a per- centage, at which the soil volume ceases to decrease is called the shrinkage limit. To determine the performance of soils under vari- ous loading conditions a number of soil tests are nec - essary. These tests include the Proctor compaction test, the unconfined compaction test, the shear test 1108 • Soil testing and analysis Global Resources A soil scientist inspects a field of canola plants, which will be used for cleansing soils rich in selenium. (United States Department of Agriculture) on sands, the consolidation test, and the triaxial tests in clay. The Proctor compaction test identifies the op- timum moisture content that would result in a maxi- mum dry unit weight of soil. This information is applied to the design of airports, highways, and struc- tural foundations. The unconfined compaction test determines the stress-strain relation of a soil speci- men and is useful in studies regarding retaining walls and landslides. The shear test in sands is used to estimate the abil- ity of sands to sustain shear loading. It is directly re- lated to itsangle of internal friction. Quantificationof the time-dependent settling of saturated clays sub- ject to increased loading is achieved by the consolida- tion test. Finally, the triaxial tests in clays define the general stress relationships for unconsolidated un- drained, consolidated drained, or consolidated un- drained specimens. Permeability tests are integral to hydrogeological studies. They define the flow of water through a soil sample under either a constant hydraulic head or a falling hydraulic head. Permeability tests are indica- tive not of soil porosity but of the connectivity among the pores and their ability to form conduits that allow the water to flow freely through the soil. Environmental Testing and Analysis Soils and sediment canbesubjected to contamination by a variety of pollutants. Fine sediments and organic soils, because of their large specific surface, show a particular affinity to adsorb or absorb chemicals in dissolved or particulate form. The most common chemical testing of soils involves estimation of the pH value; carbonate, chloride, sulfate, and organic con- tent; and total dissolved solids. The organic content can be defined easily through a loss-on-ignition test. The total dissolved solidscanbe estimated by evapora- tion. Panagiotis D. Scarlatos Further Reading Budhu, Muni. Soil Mechanics and Foundations. 2d ed. Hoboken, N.J.: Wiley, 2007. Das, BrajaM. Soil Mechanics Laboratory Manual. 5th ed. Austin, Tex.: Engineering Press, 1997. Day, Robert W. Soil Testing Manual: Procedures, Classifi- cation Data, and Sampling Practices. New York: McGraw-Hill, 2001. Head, K. H. Soil Technicians’ Handbook. New York: Halsted Press, 1989. Kézdi, Árpád. Soil Testing.Vol.2inHandbook of Soil Me - chanics. New York: Elsevier Scientific, 1974-1990. Liu, Cheng, and Jack B. Evett. Soil Properties: Testing, Measurement, and Evaluation. 6th ed. Upper Saddle River, N.J.: Prentice Hall, 2008. Mudroch, Alena, and José M. Azcue. Manual of Aquatic Sediment Sampling. Boca Raton, Fla.: Lewis, 1995. Web Site University of Massachusetts, Amherst, Department of Plant and Soil Sciences Soil and Plant Tissue Testing Laboratory: Results and Interpretation of Soil Tests http://www.umass.edu/plsoils/soiltest/interp1.htm See also: Aggregates; Bureau of Land Management, U.S.; Bureau of Reclamation, U.S.; Clays; Erosion and erosion control; Peat; Sand and gravel; Sedimentary processes, rocks, and mineral deposits; Soil; Soil man- agement. Solar cells. See Photovoltaic cells Solar chimneys Category: Energy resources Solar chimneys have long been used both to aid in nat- ural cooling of homes and for passive solar heating. In recent times, a device similar to the traditional solar chimney has been used together with a solar collector and wind turbine to create a means of generating elec- tricity from solar energy. Background There are three different applications for the device called a solar chimney. In all three cases the applica- tion is called a solar chimney. The use that has been around the longest is for enhanced ventilation and cooling of living space. A second use is a particular type of passive solar-heating system, also called a thermosyphon. The third application is for genera- tion of electricity from solar energy. The operation of a chimney in general is based on the fact that heated air will rise because of its decreased Global Resources Solar chimneys • 1109 density. A chimney is used to exhaust flue gases from combustion in a stove, furnace, or fireplace; the air in the chimney is heated by the combustion at the base of the chimney and thus rises, creating an updraft through the chimney. With a solar chimney, however, nothing burns at the base of the chimney; rather, the air in the chimney is heated by the Sun, causing it to rise up the chimney and create an updraft. For enhanced ventilation and natural cooling, the updraft caused by solar-heated air rising up the chim- ney is used to draw air through the house, increasing the ventilation rate and ideally drawing cool air into the house. The hot air is exhausted out the top of the chimney. Solar chimneys for natural cooling were in use hundreds of years ago in Rome and the Middle East. The device that is sometimescalleda solar chimney for passive solar heating is simply a rather standard air-heating solar collector mounted vertically on a house wall, facing the equator (facing south if in the Northern Hemisphere, and facing north if in the Southern Hemisphere). Just as in any solar chimney, there will be an upflow of solar-heated air. For this ap- plication, the heated air is sent into the living space to provide heating, rather than being sent out the top of the “chimney.” Cool air is drawn into the bottom of the vertical collector from the living space. In the late 1970’s, use of a large solar chimney, to- gether with an extensive solar collector at the base of the chimney and a wind turbine or turbines at the top to drive a generator, was proposed as another way to generate electricity from solar energy (in addition to photovoltaic cells and heating a fluid with solar en- ergy for use in generating electricity). The term “solar chimney” has come to be used for this latter system, as well as for the traditionaltypeused for enhanced cool- ing or for passiveheating.This concept has been dem- onstrated, and large facilities of this type are in the planning stages. Enhanced Ventilation The solar chimney for this application could be as simple as a traditional chimney painted black on the outside to increase the heating of the air in the chim- ney by the Sun shining on it. A more effective design would be to use glass for the side of the chimney fac- 1110 • Solar chimneys Global Resources Solar chimneys provide the potential for creating clean energy at minimal cost. (AP/Wide World Photos) ing the Sun, with a black absorbing surface on the back wall of the chimney, so the air between the glass and the black absorber would be heated. There must be a vent into the living space at the bottom of the chimney, so that the air flow up the chimney will draw air from the living space into the bottom of the chim- ney to be heated. This will cause increased air flow or ventilation through the living space, making it more comfortable. Increased natural cooling can be ac- complished if the air drawn into the living space is cool. In some cases, the air into the house is drawn through an underground tunnel to cool it before it enters the house. Passive Solar Heating For a solar-chimney-type passive solar heater, the col- lector consists of insulation against the house and then a rectangular enclosure with a black absorber at the back, glazing in the front facing out, and an air- space between. The air between the glazing and the black absorber is heated by the solar radiation and rises up the airspace. Instead of venting the heated air out the top, as is done for enhanced ventilation, the heated air is directed into the living space through a vent that passesthroughthe building wallat the top of the solar chimney collector. Another ventthrough the building wall draws cool room air into the bottom of the collector. This type of device could also be used to increase summertime ventilation through the living space, by closing the vent into the living space at the top of the collector and opening the top of the collec- tor so that the hotair is exhausted out the top. For this use, the top of the collector should be near the top of the house, or else the heated air going out of the col- lector willheat the portion of the house above the col- lector. This type of passive solar heating system is also called a thermosiphon. Electricity Generation The type of solar chimney used to generate electricity is a combination of three established technologies: the greenhouse, the chimney, and the wind turbine. The chimney, which is a long tubular structure, is placed in the center of the circular greenhouse, and the wind turbine is mounted inside the chimney. The solar-to-electric energy conversion involves two inter- mediate stages. In the first stage, conversion of solar energy into thermal energy is accomplished in the greenhouse (also known as the collector). In the sec - ond stage, the chimney converts the generated ther - mal energy into kinetic energy and ultimately into electric energy by using a combination of a wind tur- bine and a generator. In its simplest form, the collector isa glass or plastic film cover stretched horizontally and raised above the ground. This covering serves as a trap for reradiation from the ground. The ground under the cover is heated and, in turn, heats the air flowing radially above it. The height of the collector cover gradually increases toward the centerof the collector, providing a smooth transition for the hot air flowing from the collector into the chimney. A flat collector of this kind can convert a significant amount of irradiated solar energy into heat. The soil surface under the collector cover is a convenient energy storage medium. During the day, a part of the incoming solar radiation is ab- sorbed by the ground; it is later released during the night. The mechanism is capable of providing a con- tinuous supply of power all year round. The chimney itself is the actual thermal engine. The upthrust of the air heated in the collector is pro- portional to the rise in air temperature in the collec- tor and the volume of the air flowing. The latter de- pends on the height of the chimney. Mechanical output in the form of rotational energy can be ex- tracted from the vertical air current flowing in the chimney by using a suitable turbine (or turbines). This mechanical energy can be converted into elec- tric energy by coupling the turbinestothegenerators. Solar chimneys do not necessarily need direct sun- light. They can exploit a component of the diffused radiation when the sky is clouded. The system also has an advantage over traditional systems that use wind to provide power in that it does not depend on the natu- ral occurrence ofwind, which always fluctuates. More- over, because the direction of air movement is fixed, the complicated tracking mechanism necessary for a horizontal-axis wind turbine is not needed. Solar chimneys are relatively reliable and simple to build, and they do not have adverse effects on the environ- ment. The necessary materials are readily available, and maintenance costs are minimal. Less than four years after the solar chimney was first proposed in the late 1970’s, construction of a pi- lot plant began in Manzanares, Spain. A 36-kilowatt pilot plant was built; it produced electricity for seven years, thus proving the efficiency and reliability of the technology. The chimney tower was 194.6 meters high, and the collector had a radius of 122 meters. S. A. Sherif, updated by Harlan H. Bengtson Global Resources Solar chimneys • 1111 Further Reading Afonso, Clito, and Oliveira, Armando. “Solar Chim- neys: Simulation and Experiment.” Energy and Buildings 32, no. 1 (2000): 71-79. Dai, Y. J., H. B. Huang, and R. Z. Wang. “Case Study of Solar Chimney Power Plants in Northwestern Re- gions of China.” Renewable Energy 28, no. 8 (2003): 1295-1304. Dai, Y. J., K. Sumathy, R. Z. Wang, and Y. G. Li. “En- hancement of Natural Ventilation in a Solar House with a Solar Chimney and a Solid Adsorption Cooling Cavity.” Solar Energy 74, no. 1 (2003): 65-75. El-Haroun, A. A. “The Effect of Wind Speed at the Top of the Tower on the Performance and Energy Generated from Thermosyphon Solar Turbine.” International Journal of Solar Energy 22, no. 1 (2002): 9-18. Pretorius, J. P., and D. G. Kröger. “Critical Evaluation of Solar Chimney Power Plant Performance.” Solar Energy 80, no. 5 (2006): 535-544. Schlaich, Jörg, Rudolf Bergermann, Wolfgang Schiel, and Gerhard Weinrebe. “Design of Commercial Solar Updraft Tower Systems—Utilization of Solar Induced Convective Flows for Power Generation.” Journal of Solar Energy Engineering 127, no. 1 (2005): 117-124. See also: Electrical power; Photovoltaic cells; Solar energy; Wind energy. Solar energy Category: Energy resources Present patterns of nonrenewable energy usage cannot be sustained; diminishing supplies, increased prices, and concerns over global warming have made the move to sustainable sources of energy inevitable. Solar is the only renewable energy resource plentiful enough to serve as the foundation of a new global energy econ- omy. Not only is it abundant and clean, but also it has become both economically competitive and politically viable. Background Solar energy has provided continuously almost all of Earth’s energy, which humans have exploited, di - rectly or indirectly, since prehistoric times. The steady evolution of solar technology, however, has been in - terrupted sporadically by the discovery of plentiful and inexpensive nonrenewable fuels, such as coal, oil, and uranium. Successive civilizations have shortsight- edly overexploited each new resource by treating it as an inexhaustible supply governed only by price and availability. Designing buildings to optimize the use of the Sun during various seasons began in ancient Greece. Dur- ing the fourth century b.c.e., a severe shortage of wood for fuel necessitated that homes be designed to take advantage of the abundant sunlight during the moderately cool winters while taking advantage of shade during the summer. Because glass or other transparent materials for doors and windows did not yet exist, houses had to be designed to collect as much sunlight as possible during the short winter days. This was achieved by designing houses with (in the North- ern Hemisphere) south-facing covered porches that were closed to the north. During the winter the low- angled sunlight streamed through the porch and warmed the interior rooms. During summer, rooms were shaded from the high Sun by the porch roof. The ancient Romans used wood at such a prodi- gious rate that by the first century b.c.e., timber had to be imported from more than 1,000 kilometers away. The highcost of imported wood led theRomans to copy Grecian solar architecture, but they added clear glass window coverings to keep solar energy trapped within a house. They also expanded the uses of solar heating to include greenhouses and public baths. Solar design became so entrenched in the Roman state that guarantees of “sun rights” were in- corporated into Roman law. After the fall of Rome, however, solar architecture wasforgotten until the Re- naissance. During the sixteenth century there was a revival of horticulture and greenhouses were used to growexotic fruits and vegetables in northern Europe. By the eighteenthcentury, new glass-manufacturing techniques allowed the production of large windows for greenhouses. Experimenters also developed “hot boxes,” glass-enclosed devices for achieving tempera- tures up to 88° Celsius. During the nineteenth century, the greenhouse evolved, at least for wealthy people, into a place for the ostentatious display of exotic plants. While the Industrial Revolution was fueled by coal, some far- sighted individuals saw the vast energy of the Sun as an untapped source of power. Focusing collectors, con - sisting of concentrating mirrors, developed during 1112 • Solar energy Global Resources the seventeenth century, were being used by the latter half of the nineteenth century to focus sunlight to produce steam for operating small steam engines. In the early twentieth century, the perfection of large solar steam engines for pumping water and for irrigation occurred. Concurrently, active systems were invented and marketed to heat water for domestic use, and passive systems were rediscovered as an en- ergy-efficient way to helpheathomes in a varietyofcli- mates. The discovery of convenient and inexpensive natural gas, however, virtually eliminated the solar in- dustry. By the end of World War II, not one of the doz- ens of active solar energy manufacturers remained. During the 1970’s and continuing through 1985, the interest in solar energy for residential and com- mercial purposes grew. This growth was caused by lim- ited supplies and steeply rising costs of oil, coal, and electricity. Additional incentives for installing active solar-heating systems were provided for homeowners by federal tax credits for alternate energy devices. However, by the mid-1980’s, the price of gasoline was dropping, electricity rates had stabilized, and the tax credit program expired without renewal. The effects on the solar-energy market were immediate and dev- astating. On the positive side, the decade of high growth had created an enhanced public awareness of solar energy. Because fossil fuels remained relatively inexpen- sive during the 1990’s, and because of the high initial cost of solar-energy systems, solar did not significantly impact world energy consumption. However, during the first decadeof the twenty-first century, the priceof fossil fuels began to escalate again because of the de- pletion of oil reserves. The U.S. Office of Technology Assessment projected that at the 2009 rate of use all known oil reserves would be depleted by 2037. As fossil-fuel prices continue to rise, solar heating has in- creased in economic viability. In addition, the cost of electricity from photovoltaic (PV)cells decreased dra- matically after the1990’s. Futuretechnological break- throughs and economies of scale will undoubtedly provide additional economic advantages to solar PV energy. Nature of Solar Energy Every day, theEarth receives ten thousand timesmore energy from the Sun than humans derive from all other alternative energies and nonrenewable fuels combined. Above Earth’s atmosphere, 170 billion megawatts of power are available, but the energy’s intensity is diluted to 430 British thermal units per hour per square foot (Btu/hr/ft 2 ); this is attenuated to between 100 and 200 Btu/hr/ft 2 at Earth’s surface, thus requiring large collector areas to capture signifi- cant amounts ofusable energy. Nevertheless,industri- alized societies have come to realize that past patterns of energy consumption cannot be sustained. A viable energy future requires not only that solar energy be harnessed but also that technologically advanced so- cieties modify their lifestyles to live in closer harmony with nature before an energy crisis of global propor- tions decimates humanity. Utilizing solar energy as a major source of energy for the world has several advantages. First, solar en- ergy is virtually inexhaustible and is constant (at least above the Earth’s atmosphere). Second, it is clean; the only direct environmental impact may be aes- thetic—some active collection systems are conspicu- ously ugly. However, more attractive large-scale de- vices could be designed, and smaller units could be integrated into residential structures so as to be less obvious. Finally, the collected energy is “free” after the initial cost of purchase and installation. On the other hand, there are several disadvantages associated with solar energy. First, being diffuse, the Sun’s energy must be collected and concentrated. Second, it is intermittent. It is only available during daylight hours, and even then it may be obscured by cloud cover. Hence it must be stored, and storage is neither convenient nor efficient. Finally, active col- lection devices are constructed of relatively expen- sive nonrenewable resources such as aluminum and copper. Types of Solar Energy Systems All solar heating systems share two common elements: a device for collecting energy from the Sun to provide electricity, heat, or air-conditioning and a facility for storing the energy when it is not needed. PV systems convert the Sun’s radiant energy directly into electric- ity, while thermal solar units provide heat for interior spaces or hot water for domestic uses. Concentrating collectors are used to create steam that can power air- conditioning units or generate electricity. Thermal solar energy is captured in active or in passive systems. Active systems require electricity to power pumps or fans, while passive systems convert sunlight directly into interior space heating. Active systems may be subdivided into those that use flat- panel stationary collectors and systems that focus in - Global Resources Solar energy • 1113 coming solar rays in order to achieve temperatures high enough to create steam. Active solar heating sys- tems transfer a Sun-heated medium (air or water) from an exterior south-facing collector (north-facing if located in the Southern Hemisphere) to the point of use or to a storage facility. For air systems the stor- age facility is a bed of rocks, while water systems store heat in tanks of water. Active concentrating systems focus sunlight by one of two possible means: power towers consisting of mirror arrays, which reflect sun- light from a large area into a small central region, or troughs of parabolic mirrors, which focus sunlight to a central axis. A solar furnace is a specialized type of focusing col- lector that uses large arrays of parabolic mirrors to concentrate sunlight to a focal point where extremely high temperatures are achieved. The world’s largest solar furnace was constructed in 1970 in the French Pyrenees, where sunny weather occurs three hundred days annually. An array of sixty-three flat, movable mirrors on a hillside reflects sunlight into a huge curved mirror with an area of 1,800 square meters, which then focuses the light into a 0.09-square-meter spot. This delivers 1,000 kilowatts of power and cre- ates a temperature that can exceed 2,980° Celsius. Furnaces of this type are primarily used for materials research. Passive solar-heating systems use no external power; they collect light from the Sun and transform it into heat, which warms a building by natural convection. The collectors are glass windows on the south wall, and the room’sinteriorair mass stores the heat. There are three main types of passive systems: direct gain, in- direct gain, and attached gain. Direct gain systems re- quire large expanses of south-facing glass to admit sunlight into an interior space where ample mass has been incorporated to avoid overheating. Indirect gain systems require a massive walltobe positioned di- rectly behind the south-facing glass. The third system, attached gain, consists of a greenhouse attached to the exterior south wall, but accessible to the interior through insulated doors. The doors can be opened to heat the house when the greenhouse is warm and closed when it is cold. A fourth system, the thermo- siphon, uses flat-plate collectors for domestic hot water production, but because no electricity is used, it is technically passive. A water storage tank islocated at a higher elevation than the top of the collector. Water in the collector,heated by the Sun,rises by convection and enters the storage tank, creating a siphon effect that keeps the fluid circulating. PV cells use the photoelectric effect to transform solar radiation directly into electric current. The cells are made of semiconducting materials that act like in- sulators until impinging sunlight puts electrons in the conducting state, effectively making each cell a small battery. When large arrays of such cells are con- nected, sufficient power can be generated to power individual residences or produce electricity for a cen - tralized power plant. 1114 • Solar energy Global Resources Comparison of Two Types of Solar Energy Collectors Solar Photovoltaic Collector Solar Thermal Collector Converts solar energy directly into electricity for immediate use. Collects heat from solar energy for conversion into electricity. Electricity can be converted into heat for thermal use. Heat is used directly. Solar radiation of only a very small range of energy can be utilized. Radiation of a wide range of energy can be used. Requires additional storage devices that are costly and inefficient. Some have built-in storage devices that are relatively inexpensive and efficient. Ideal for micropower and small appliances. Unsuitable for micropower and small appliances. Active Solar Heating Systems These types of systems, first developed in the early de- cades of the twentieth century, were largely forgotten (after some considerable initial interest) when inex- pensive fossil fuels became widely available. The oil embargo of 1973 renewed interest in these systems, and government tax credits helped homeowners de- fray the initial high cost of purchasing and installing active systems. All active solar systems have the following compo- nents: a collector, a pump or fan to circulate the heat transfer fluid, and a means of storing excess energy. Active systems have two basic uses. Either they are used to heat the interior of a building (or at least to preheat the circulating fluid), or they are used for do- mestic hot water (DHW). The circulating fluid for DHW systems is always water, and the storage unit is a water tank, typicallyhaving a 50-gallon capacity.Space heating units may useeithercirculatingair or circulat- ing water, but air systems are more common. Solar flat-plate collectors consist of an enclosed rectangular box containing a metal plate with a flat black surface coveredby nonreflecting tempered glass. Tubes to conduct water across the collector are sol- dered to the plate for water systems, while in air sys- tems channels direct the airstream. The entire box must be watertight and well insulated. When sunlight strikes the black surface, light is changed into heat, which is transferred to the fluid moving across the heated surface. Water systems typically require pro- pylene glycol in the circulating fluid to prevent freez- ing. A heatexchanger transfers the heatto the storage tank; the hot water created is used for DHW or pre- heated water for hydronic heating systems. A differentialthermostat, whichcompares the tem- perature of the collector to the storage tank tempera- ture, is employed so the collector fluid flows only when the flat-plate collector is warmer than the stor- age water. Whenever the collector is at least 10° Cel- sius warmer than the storage tank, the pump is acti- vated until the temperatures equalize. When air is used as the working fluid, excess heat is stored either in smooth rocks or in a phase-change material. Rock storageconsists of a largebin of 1-inch- diameter rocks, filling a 280-cubic-foot volume and weighing 6.3 metric tons.When the collector iswarmer than the house and heat is desired, a fan pumps the air directly from the collector into the house. When the residence reaches the desired temperature, the air is directed through the rock bin, transferring the heat into the rocks. At night, air from the house can be circulated through the rocks to reclaim the stored heat. Phase-change materials store heat by changing from a solid to a liquid at a temperature somewhere be- tween 27° and 32° Celsius. The most common sub- Global Resources Solar energy • 1115 Sunlight Collector Pump Heat exchanger Domestic hot water out Cold water in Hot water storage tank Active Solar Domestic Hot Water System stance used in solar applications is Glauber’s salt (so - dium sulfate decahydrate). Heat from the Sun is absorbed by the Glauber’s salt as it melts. In the eve- ning, when the temperature drops below the melting point, the salt resolidifies and releases its stored heat to the room. Because there are several unsolved prob- lems with phase-change storage systems, most air- heating systems use rock bin storage. Air systems have several advantages over space- heating water systems. They are less expensive, the fluid is not subject to freezing, and leaks are not cata- strophic. Air, however, is not as efficient a heat-transfer medium as water, the storage facility is considerably larger and heavier, fans require more electricity to op- erate than pumps, and air ducts require much more space thanwater lines. Also, air blowing directly into a room from the collectors may have a temperature of only 27° Celsius. Although this will heat the room, it feels cold and drafty because of the higher human body temperature. An auxiliary heater is required for most applica- tions of solar energy in the United States. Comparing DHW andspace-heating requirements, DHW has one major advantage: It can be used all year. During the summer months when the Sunishighin the sky, DHW systems are most efficient and can often provide most of a family’s hot water needs. Typically, for a family of four, two or three solar collectors of 1.2 meters by 2.4 meters apiece can provide at least one-half of the an- nual hot water needs. In order to heat the interior space of an average home in middle or northern lati- tudes, however, twenty or thirty collectors are re- quired. Thus, using solar collectors for DHW costs more than most homeowners can afford, while active space-heating systems are often too expensive for the average homeowner to consider. Solar thermal power plants concentratesunlightin order to produce high-temperature steam that drives a turbine to produce electricity in the conventional manner. Two types of collector technologies are in use: parabolic troughs and central receivers. Trough systems use parabolic reflectors to concentrate sun- light onto an oil-filled tube positioned along the focal line. The heated oil is piped to a central location, where it produces the steam required to drive the tur- bine. A system of thistypelocatedin Southern Califor- nia supplies 350 megawatts of power on clear days, with a conversion efficiency of 25 percent and at a price competitive with electricity produced by fossil- fuel power plants. Central receivers, or “power towers,” use a large array of movable Sun-tracking mirrors to reflect sun- light to a central location on top of a tower. At this point, the concentrated sunlight produces tempera- tures ranging from 538° to 1,480° Celsius, which va- porizes a working fluid that drives the turbine. A 10- megawatt pilot plant in Southern California, later modified to produce 200 megawatts of power, pro- vides electricity at a cost comparable to that of elec- tricity from conventional power plants. After the suc- cessful conclusion of these demonstration projects, many commercial plants in the 30- to 50-megawatt range were designed for the southwestern United States, Spain, Italy, Egypt, and Morocco. By the end of 2006, fifteen large thermal solar generating stations were operational in the United States alone, ten in California and five in Arizona. Passive Solar Space Heating This type of heating is achieved entirely by natural means—heat circulates by natural convectionwithout pumps or fans. A well-designed passive system should include these elements: south-facing insulated (dou- ble-paned) windows to collect the winter sunlight, interior thermal mass to prevent overheating, night insulation to cover the windows, overhangs above the windows to keep out the summer sunlight, and suffi- cient insulation to minimize heat loss. Passive systems may be categorized as one of three types, depending on the relative locations of the win- dows and thermal mass. These are termed direct-gain, indirect-gain, and attached-gain (or greenhouse) sys- tems. In direct-gain systems large south-facing win- dows admit sunlight, which falls on the thermal mass, usually brick, tile, or concrete. The mass, which may be incorporated into a floor, a wall, or even an earth- filled planter, must be sized to the total area of south- facing glass—the greater the area, the greater the mass required to prevent overheating and to store heat efficiently for evening. Without sufficient ther- mal mass, indoor temperatures can exceed 32° Cel- sius during the day while plummeting to uncomfort- ably low temperatures at night. Indirect-gain systems have a massive wall, con- structed of brick or barrels of water, located close to the south-facing glass. The outside-facing surface of the mass is painted black, which transforms sunlight into heat. The heat is released through vents into the interior living space by natural convection and is ab - sorbed by the thermal mass. During the night the 1116 • Solar energy Global Resources vents are closed, preventing heat loss by convection, while the mass radiates heat into the interior space. Attached-gain, or greenhouse, systems are added to an exterior south-facing wall of a house. The struc- ture is usually completely glass, while the mass is con- crete and/or soil for growing plants. During a sunny day, heat from the greenhouse is admitted into the residence, while at night the openings are sealed. When properly designed, an attached greenhouse can be used to provide food as well as heat during the winter season. When carefully designed and appropriately inte- grated into a residence during construction, passive solar systems can save 50 percent on heating expenses for a 5 percent increase in construction costs. During the 1990’s, approximately 7 percent of new homes built in the United States incorporated passive solar heating features; this percentage increased during the first decade of the twenty-first century. Photovoltaic Systems Although passive and active solar technologies are well understood, the direct conversion of sunlight into electricity by means of photocells is still being de - veloped. Once too expensive for typical homeowners, new technologies are reducing the cost so that it is be- coming a cost-effective and viable energy alternative, particularly inremoteareas where the costof running conventional power lines would be prohibitive. Solar PV cells installed on individual residences are particularly beneficial for providing intermediate load demand because they provide electricity during the sunniest and hottest part of the day, when the de- mand for air-conditioning is maximum. Such systems are also cost-effective for utility companies because if less electricity needs to be provided, the cost of up- grading transmission lines and associated equipment is reduced. When individual PV systems are tied to the grid, excess energy produced during the day can be fed back into the grid, while nighttime requirements are available from the grid. Home systems connected to the grid eliminate the need for large banks of stor- age batteries, which are expensive and potentially dangerous. Cost-effective grid-connected systems are becoming common in Japan and Germany. As the world market for PV systems increases from less than 1 percent of new generating systems to hun- dreds of times this level by the mid-twenty-first cen - tury, these systems will also become significantly less expensive. When PV systems are installed during Global Resources Solar energy • 1117 As of 2009, this commercial solar tower in Sanlucar la Mayor, Spain, was the most powerful on the planet. (AP/Wide World Photos) . terms of the percentage of material that is finer than this particu- lar diameter. For the plot, a semilogarithmic paper is used. For the determination of particle size distribu- tion of fine. However, during the first decadeof the twenty-first century, the priceof fossil fuels began to escalate again because of the de- pletion of oil reserves. The U.S. Office of Technology Assessment projected. a varietyofcli- mates. The discovery of convenient and inexpensive natural gas, however, virtually eliminated the solar in- dustry. By the end of World War II, not one of the doz- ens of active