{"id":6671,"key":"Understanding_solar_concentrators","title":"Understanding solar concentrators","latest":{"id":1246268,"timestamp":"2026-05-06T14:03:25Z"},"content_model":"wikitext","license":{"url":"https://www.appropedia.org/Appropedia:Copyrights","title":"CC-BY-SA-4.0"},"source":"{{Open access}}\n\nTECHNICAL PAPER # 30\nUNDERSTANDING SOLAR CONCENTRATORS\nBy George M. Kaplan\n\nTechnical Reviewers
\nDr. Thomas E. Bowman
\nDr. Maurice Raiford
\nJesse Ribot
\n\nIllustrated By Rick Jali\n\nPublished By
\n\nVITA
\n1600 Wilson Boulevard, Suite 500
\nArlington, Virginia 22209 USA
\nTel: 703/276-1800 * Fax: 703/243-1865
\nInternet: pr-info@vita.org
\n\nUnderstanding Solar Concentrators
\nISBN: 0-86619-239-5
\n[C] 1985, Volunteers in Technical Assistance
\n\n== Preface ==\n\nThis paper is one of a series published by Volunteers in Technical Assistance to provide an introduction to specific state-of-the-art technologies of interest to people in developing countries. The papers are intended to be used as guidelines to help people choose technologies that are suitable to their situations. They are not intended to provide construction or implementation details. People are urged to contact VITA or a similar organization for further information and technical assistance if they find that a particular technology seems to meet their needs.\n\nThe papers in the series were written, reviewed, and illustrated almost entirely by VITA Volunteer technical experts on a purely voluntary basis. Some 500 volunteers were involved in the production of the first 100 titles issued, contributing approximately 5,000 hours of their time. VITA staff included Maria Giannuzzi as editor, Suzanne Brooks handling typesetting and layout, and Margaret Crouch as project manager.\n\nThe author of this paper, VITA Volunteer George M. Kaplan, is the president of KAPL Associates, a consulting firm specializing in program and project management, research and development, planning, evaluation, energy, and environment. The reviewers are also VITA volunteers. Dr. Thomas E. Bowman is Professor and Head of the Mechanical Engineering Department at the Florida Institute of Technology in Melbourne, Florida. Dr. Maurice Raiford is a solar energy consultant in Greensboro, North Carolina. Jesse Ribot is an energy analyst and consultant, and has assisted in the preparation of the VITA/USAID Djibouti National Energy Assessment.\n\nVITA is a private, nonprofit organization that supports people working on technical problems in developing countries. VITA offers information and assistance aimed at helping individuals and groups to select and implement technologies appropriate to their situations. VITA maintains an international Inquiry Service, a specialized documentation center, and a computerized roster of volunteer technical consultants; manages long-term field projects; and publishes a variety of technical manuals and papers.\n\n== I. Introduction ==\n\nAlthough solar energy research, development, and systems experiments\nwere conducted in the late 1800s and early 1900s, it was\nthe sharp increase in the price of oil in 1974 precipitated by\nthe Middle-Eastern oil embargo the previous year that escalated\nnational and international investment in solar energy. In the\nUnited States and other industrial countries, the technological\ntools and advancements produced during World War II, the post-war\nrebuilding and prosperity, the U.S. nuclear power and space\nprograms, and other technological achievements were applied to\nsolar energy research and development. The result was that research,\nwhich had been limited to backyard tinkerers and small\nspecialized companies, was spread to universities, national laboratories,\nand industry. The federal solar budget rose from less\nthan $1 million in early 1970s to over $1 billion in the early\n1980s; the budget is now about $200 million, with about $50\nmillion for solar thermal technology.\n\nSolar thermal technology is concerned principally with the utilization\nof solar energy by converting it to heat. In the concentrating\ntype of solar collector, solar energy is collected and\nconcentrated so that higher temperatures can be obtained; the\nlimit is the surface temperature of the sun. However, construction\nmaterials impose a lower, more practical limit for temperature\ncapability. Similarly, overall efficiency of energy collection,\nconcentration, and retention, as it relates to energy cost,\nimposes a practical limit on temperature capability.\n\nIf solar energy were very highly concentrated into a tiny volume,\nthe result would approach a miniature sun. If the same energy\nwere distributed along a thin line, the line would be cooler than\nthe miniature sun, but still hot. If distributed on a large\nsurface, the surface would be less hot than the line. There are\nsolar concentrators that focus sunlight into a point or a line.\nThere are also non-focusing concentrators. Each type has preferred\ntemperature-dependent applications.\n\nThe amount of energy per unit area that can be collected annually\nby a concentrator depends on the positioning of the concentrator\nrelative to the sun. Some types of collectors perform adequately\n(cost effectively) if left in a fixed position. These collectors\ngenerally have limited temperature capability, and provide little\nor no concentration of the incident sunlight. Most concentrators\nwould collect so little energy in a fixed position that they must\nbe provided with the capability to daily track the sun from\nmorning (east) to sunset (west) to be cost-effective. Some concentrators\ncan only be cost effective by tracking both the sun's\ndaily path and the sun's annual inclination (which causes the sun\nto appear to move in declination by 47 [degrees] over the year). Thus,\nconcentrators may be non-tracking, single-axis tracking (which\ntracks east to west), or two-axis tracking (which tracks both\neast to west and north to south). Two-axis tracking provides the\nmaximum solar energy collection but is not cost effective for\nmost applications or collector designs.\n\nThe U.S. national solar energy research program has led the world\nboth in investment and breadth of program. Because the potential\nU.S. market is large, the U.S. national program was aimed at the\ndomestic market and was not intended specifically for export.\nThus, the U.S. experience is primarily applicable to the U.S. and\nmay not be relevant to other countries without modification.\n\nFor U.S. applications, for example, mirror-type concentrators are\nmore cost effective than lens-type concentrators for small, intermediate,\nand large systems for heat generation and use. Tracking\nsystems appear most effective for high-temperature applications.\nHowever, the effectiveness in the U.S. may be due to\nsophisticated technology, availability of skilled maintenance\npersonnel and spare parts, an excellent supporting infrastructure,\nrather than an inherent advantage of mirrors or tracking\nsystems. In a less industrialized environment, lens concentrators\nmay prove more appropriate.\n\nAlthough the terms \"collector\" and \"concentrator\" are used interchangeably\nin this paper, the terms are distinctive. A collector\nmay not concentrate solar radiation, while concentrators are\nconsidered collectors. No distinction will be made in this paper\nunless necessary.\n\n=== HISTORY OF SOLAR CONCENTRATORS ===\n\nThe concept of concentrating solar rays to heat a target area has\nbeen known for at least 4,000 years. In the clay tablet period\nof Mesopotamia, polished gold vessels were reputedly used to\nignite altar fires. Archimedes is said to have saved Syracuse\nfrom invasion by burning the Roman fleet with concentrated solar\nrays reflected from polished metal.\n\nExperiments to verify the story of Archimedes were performed in\nthe seventeenth century with polished metal plates. Glass lenses\nwere first used to smelt iron, copper, mercury, and other materials\nfrom their ores in the seventeenth century. The eighteenth\ncentury brought solar furnaces and solar ovens. Advancing tech-in\nthe nineteenth century produced steam engines and hot\nair engines operated with solar energy. Numerous solar engines\nand solar furnaces were constructed early in the twentieth century.\nExperimentation continued into the 1930s before languishing\nas inexpensive fossil fuels, particularly natural gas, became\nwidely available.\n\nThe U.S. solar energy program was initiated in 1970 as part of\nthe Research Applied to National Needs (RANN) program of the U.S.\nNational Science Foundation. This program expanded enormously as\na result of the oil embargo of 1974 and the price rise of oil and\nother fossil fuels. As the program goals changed from research\nand development and later to commercialization, program responsibility\nshifted to other federal agencies. The program is now\npart of the U.S. Department of Energy; the focus is again on\nlong-term high-cost, high-risk research and development unlikely\nto be undertaken by industry; responsibility for commercialization\nhas been shifted back to industry.\n\n=== NEEDS SERVED BY THE TECHNOLOGY ===\n\nSolar concentrators provide high energy density solar radiation\nto a target receiver, thus raising the temperature of the target.\nDepending on the degree of concentration, the optical properties\n(solar absorption and radiation) of the target surface, and the\ntarget's cooling rate, the following may occur:\n\n* the target will melt (high concentration);\n* the target will reach an equilibrium temperature with natural cooling (modest concentration); or\n* the target will reach an equilibrium temperature with a forced (circulating) coolant (intermediate concentration).\n\nThe first instance is that of a solar furnace. The second may be\nconsidered a solar cooker or solar oven. In the third instance,\nthe heated coolant is used directly as, for example, hot water or\nsteam in home or industrial applications, or indirectly, as a\nvapor (steam) to generate electricity. In the case of electricity\nproduction, common energy conversion devices provide an intermediate\nstep--shaft rotation--between the heated fluid and conversion\nto electricity.\n\nIf the target of the concentrated sunlight is a photovoltaic\ncell, or an array of cells, electricity will be produced directly.\nThe degree of solar concentration, cell conversion efficiency,\nthe design of the cell assembly, and the cell material will\ndetermine if natural circulation or forced circulation cooling is\nnecessary for efficient operation of the cell. Currently, the\ncost/unit area of a concentrator is less than the cost/unit cell\narea. As a result, concentrators are used to reduce cell area.\nShould the cell area become less expensive than the concentrator\narea, concentrators would not be utilized.\n\nThis paper deals principally with concentrators for thermal applications\nrather than for applications with photovoltaic cells.\nEmphasis is placed on applications in less developed countries.\n\n== II. OPERATING PRINCIPLES ==\n\n=== SUNLIGHT ===\n\nBefore discussing concentrators, a few words about the sun are in\norder. Beyond the earth's atmosphere the intensity of sunlight\nis about 1,350 watts per square meter (429 British thermal units\n[Btu] per hour per square foot). Passage through the atmosphere\ndepletes the intensity due to absorption by the various gases and\nvapors in the air and by scattering from these gases and vapors\nand from particles of dust and ice also in the air. Thus, sunlight\nreaching the earth is a mixture of direct (unscattered) and\ndiffuse (scattered) radiation. At sea level the intensity is\nreduced to approximately 1,000 watts/square meter (295 Btu/hour/\nsquare foot) on a bright clear day. The intensity is further reduced\non overcast days.\n\nMost concentrators utilize direct radiation only. These concentrators\nwork well on bright clear days, poorly on hazy days, and\nnot at all on drab gray days when the sunlight intensity is\nreduced and the light consists principally of diffuse radiation.\nAnother limiting factor is that the sun is not a point but has a\ndiameter equivalent to about one-half degree of arc. Concentrator\ndesign must consider this arc.\n\n=== GENERIC TYPES AND USE ===\n\nAlthough the discussion that follows deals with concentrators as\nentities, concentrators are only a portion of an energy collection\nsystem. To be useful the concentrated rays must be directed\nto a target called a receiver, which converts the rays into\nanother form of energy, heat. The concentrator and receiver must\nbe matched for optimum performance. Frequently, the receiver is\nexpected to impart heat to a fluid in order that the heat be\nutilized or dissipated. When the main purpose of the concentrator\nis to obtain heat effectively, then the combination of concentrator\nand receiver must be carefully designed to reduce stray\nloss of energy from either the concentrator or receiver.\n\nThere are many ways to characterize concentrators. These include:\n\n* Means of concentration--reflection or refraction\n* Point, line, or non-focusing\n* Fixed or tracking concentrator\n* Fixed or tracking receiver\n\n=== Means of Concentration ===\n\nConcentration of light is achieved with mirrors (reflection) or\nwith transparent lens (refraction). Cameras and small telescopes\nuse lenses; large telescopes use mirrors. A mirror reflects\nincoming light so that the angle of the reflected ray is equal to\nthe angle of the incident ray (Figure 1). This relation also\n\n[[File:Solar 25p05a.gif|center|486px]]\n\nholds when the mirror is tilted (Figure 2). A single flat mirror\n\n[[File:Solar 25p05b.gif|center|486px]]\n\ndoes not concentrate but concentration can be obtained by superimposing\nthe reflections of many mirrors. Alternately, concentration\ncan be achieved by bending the mirror into a pre-determined\nshape and relying on the optical properties of the resulting\ncurved surface.\n\nThe lens relies on bending (refracting) incoming light so as to\nconverge to a common focus (Figure 3). As the size of the lens\n\n[[File:Solar 25p06a.gif|center|353px]]\n\nincreases, lens thickness also increases. A Fresnel lens (Figure 4)\n\n[[File:Solar 25p06b.gif|center|393px]]\n\nmaintains the optical characteristics of the standard lens by\nretaining the same curvature piecewise. This permits a significant\nreduction in the thickness and weight of the lens with only\na modest performance penalty.\n\nEach method of concentration has drawbacks. The mirror requires\na clean smooth reflecting surface: clean since dust particles\ncould scatter light away from the receiver or the light could be\npartly absorbed by a thin dirty film; smooth because contour\nerror can also result in missing the receiver. The reflecting\nmaterial may be placed on the surface of the mirror (first surface,\nFigure 5), or behind a transparent surface (second surface,\n\n[[File:Solar 25p07a.gif|center|393px]]\n\nFigure 6). Silver is the preferred reflector material with\n\n[[File:Solar 25p07b.gif|center|393px]]\n\naluminum second. Silver is very susceptible to degradation by\nmoisture and airborne contaminants. Available protective coatings\nhave not proven effective for silver in first surface application.\nAluminum is more durable but less reflective.\nSecond-surface mirrors have some energy loss due to absorption of\nlight by the transparent surface, usually glass or plastic, as\nthe light is incident and as it is reflected through the material.\nLow-iron glass is preferred over high-iron glass because\nof reduced absorption of light. If plastic is used, it must be\nstabilized against degradation by the ultraviolet light of the\nsun.\n\nBecause of the greater thickness of the lens, the degree of\nenergy absorption is higher than that of the second surface\nmirror. The Fresnel lens, which can be made much thinner than a\nstandard lens, has less energy loss due to energy absorption than\nthe standard lens.\n\nThe lens surface must also be clean and smooth for the same\nreasons as for the mirror. Fresnel lens performance is enhanced\nwhen the vertical portion has little or no slope error. Plastics\ncan be formed to produce Fresnel lens of higher quality and less\ncost than with glass. However, plastic lenses tend to deteriorate\nunder ultraviolet light and must be stabilized.\n\n=== Point, Line, or Non-Focusing ===\n\nOne criterion for selection of a specific concentrator is the\ndegree of concentration and hence temperature that is to be\nachieved. As a rule, concentrating energy onto a point produces\nhigh to very high temperature; and onto a line, moderate to high\ntemperature. Non-focusing concentrators produce low to moderate\ntemperature.\n\n[[File:Solar 25p08.gif|center|393px]]\n\nThe parabolic dish reflector (Figure 7) utilizes the\noptical properties of the parabolic curved surface to concentrate\ndirect light to the focal point. The dish geometry is\nfamiliar being used for automobile headlights, searchlights,\nradar, and to receive transmissions from broadcast satellites.\n\nStandard circular and Fresnel lenses are also point focus concentrators.\nThe Fresnel lens has been utilized in conjunction with\nphotovoltaic cells in several test installations in the United\nStates and abroad.\n\nThe overlapping images from many flat mirrors can be considered\nthe equivalent of point focusing. The focal shape is not a point\nbut rather the finite image of the sun further broadened by the\ncharacteristics of the reflector material and various errors in\nmanufacture and in the precision of image overlap. Figure 8\n\n[[File:Solar 25p09a.gif|center|393px]]\n\nillustrates the central receiver concept wherein heliostats (flat\nor slightly curved mirrors mounted on tracking devices) redirect\nthe sun's rays toward a receiver atop a tower. A 10-megawatt\nelectrical generating plant employing this principle has been\nsuccessfully operated in California since 1982.\n\nLine. The parabolic trough (Figure 9) is an example of line focus\n\n[[File:Solar 25p09b.gif|center|393px]]\n\noptics. The incident direct radiation is reflected from the\ntrough to the focal line the length of the trough. To maximize\nenergy collection the trough is designed to track the sun. The\ntrough may be oriented with the focal line running east-west,\nnorth-south, or north-south with simultaneous tilt toward the sun\n(polar mount).\n\nEach orientation has its own seasonal and yearly collection characteristics.\nNo one orientation is universally preferred (i.e.,\nis more cost-effective).\n\nThe standard and Fresnel lenses can be fabricated in linear form\n(Figure 10) with the same cross section as the circular lens but\n\n[[File:Solar 25p10.gif|center|534px]]\n\nnow producing a focal line instead of a focal point. Plastic\nlinear Fresnel lenses of good quality can easily be produced by\nextrusion.\n\nThe hemispherical bowl (Figure 11) is another example of linear\n\n[[File:Solar 25p11a.gif|center|540px]]\n\nfocal optics. Unlike the trough or lens, two-axis tracking is\nmandatory. The hemispherical bowl is always fixed, and the receiver\ndoes the tracking. The focal line falls on the line connecting\nthe center of the sphere with the sun. The focal line is\nrestricted to the lower half of the radius by the optical properties\nof the bowl. Because some rays reach the focal line with\nonly one reflection and others require multiple reflections, the\nintensity is not uniform along the length of the focal line.\nFigure 12 shows a 65-foot (19.7-meter) diameter experimental bowl\n\n[[File:Solar 25p11b.gif|center|600px]]\n\nthat has operated successfully in Texas for many years. Annual\nenergy collection is lower than for other collector optics and\nthere appears to be no compensating advantages, except that it is\nmuch easier for a small receiver to track the sun's image than it\nis for a larger and much heavier concentrator.\n\nNon-Focusing. The hemispherical trough (Figure 13) and the flat\n\n[[File:Solar 25p12a.gif|center|486px]]\n\nplate collector with booster mirrors are examples of concentrators\nthat are non-focusing. Non-focusing concentrators do not\nfocus sunlight into a specific geometrical shape, but reflect\nsunlight onto a receiver, thus increasing the total amount of\nsunlight received. The category of non-focusing concentrators\nalso includes concentrators in which the focus is of poor quality.\nThe cylindrical collector (Figure 14), a variation of the\n\n[[File:Solar 25p12b.gif|center|437px]]\n\nhemispherical trough, is of interest because the entire cylinder\nmay be fabricated with inexpensive, inflatable plastic.\n\nA simple method of achieving a modest increase in concentration\non a large area is to use booster mirrors in conjunction with a\nflat plate collector (Figure 15). Before noon the mirrors face\n\n[[File:Solar 25p13a.gif|center|540px]]\n\neast; after noon they face west. The energy collection advantage\nof boosters for a flat plate collector is shown in Figure 16.\n\n[[File:Solar 25p13b.gif|center|437px]]\n\n=== Fixed or Tracking Concentrators ===\n\nMaximum energy collection on a daily or annual basis requires\ntracking of the sun (or the sun's reflected image) since concentrators,\nparticularly those capable of high concentration, utilize\nonly direct radiation. Thus a parabolic dish, when pointed\nat the sun, has reflected rays passing through the focus. As the\nsun moves, some of the reflected rays will miss the focus and, in\ntime, all will miss the focus. The dish must be moved to maintain\nthe reflected rays at the focus. The central receiver,\nparabolic dish, parabolic trough, standard lens, and Fresnel lens\nare examples of tracking concentrator systems.\n\nThe hemispherical bowl likewise must continuously track the sun.\nLarge bowls are too unwieldly to move. Thus, the receiver is\nmoved continuously instead. It tracks the focal line of the\nsphere (the reflected image of the sun) throughout the day.\n\nLike the hemispherical bowl, the Russell concentrator is fixed\nand the receiver must track the sun's image (Figure 17). This\n\n[[File:Solar 25p14.gif|center|486px]]\n\nconcentrator consists of long narrow mirrors whose centers all\nfall on the perimeter of a circle. The mirrors are oriented so\nthat all reflected images focus on a point on the same perimeter.\nAs the sun moves the focus moves along the perimeter.\nThe Winston collector is usually considered a non-tracking concentrator.\nIts energy collection can be increased by tracking. As\na trough-type collector (Figure 18), it consists of a parabolic\n\n[[File:Solar 25p15.gif|center|486px]]\n\nsurface whose axis is horizontal and whose focal point is close\nto the surface. The collector is frequently found as a paraboloid\nin shape but can also be in trough form. The collector accepts\nboth direct and diffuse radiation. The acceptance angle (angle\nof acceptance of sunlight) depends on the height of the parabola.\nThe shorter the height, the greater the acceptance angle and the\nperiod of daily operation, but the less the concentration and\nmaximum temperature capability. The collector has been utilized\nas a highly effective fixed collector, which reaches higher\ntemperature than a typical flat plate collector.\n\n=== Fixed or Tracking Receivers ===\n\nThe central receiver and parabolic trough have fixed receivers,\ndue to the optical characteristics of the systems. The parabolic\ndish receiver is usually positioned at the focus so as to move\nwith the dish as the dish tracks the sun. Neither the bowl nor\nthe Russell collector track the sun, hence their receivers must\ntrack the sun's image. The Winston collector, the cylindrical\ncollector, and the flat plate collector with booster mirrors are\nnormally utilized in fixed position and with fixed receivers. The\nflat plate is, of course, both the collector and the receiver.\n\n=== Other Fixed Concentrators ===\n\nThere are many ingenious concentrators that work quite well and\ncan be cost effective in some applications. The cusp collector\n(Figure 19), whose surface geometry is the locus of the position\n\n[[File:Solar 25p16a.gif|center|486px]]\n\nof the end of a string as it is unwrapped from a pipe can provide\na modest concentration suitable for hot water. A conical collector\n(Figure 20) can be substituted for the Winston paraboloid,\n\n[[File:Solar 25p16b.gif|center|540px]]\n\ngaining simplicity of manufacture with some performance penalty.\nSimilarly, flat reflectors can substitute for the parabolic sides\nof the Winston trough collector.\n\nTable 1 summarizes the characteristics and potential uses of the\nconcentrators described above.\n\nType of Concentrator: '''Parabolic Dish'''
\nType of Focus : Point
\nLens or Mirror : Mirror
\nSun's Concentration : > 1000
\nTracking : Yes
\nTracking Receiver : Yes
\nTemperature (C) : >2638
\nTemperature (F) : >3000
\nTypical Application : Electricity
\nComments : Small-scale appilcations
\n
\n\nType of Concentrator: '''Central Receiver'''
\nType of Focus : Point
\nLens or Mirror : Mirror
\nSun's Concentration : > 1000
\nTracking : Yes
\nTracking Receiver : No
\nTemperature (C) : >2638
\nTemperature (F) : >3000
\nTypical Application : Electricity
\nComments : Large-scale appilcations
\n
\n\nType of Concentrator: '''Lens (Round)'''
\nType of Focus : Point
\nLens or Mirror : Lens
\nSun's Concentration : > 1000
\nTracking : Yes
\nTracking Receiver : No
\nTemperature (C) : >2638
\nTemperature (F) : >3000
\nTypical Application : Electricity
\nComments : [[Photovoltaic cells]]
\n
\n\nType of Concentrator: '''Parabolic Trough'''
\nType of Focus : Line
\nLens or Mirror : Mirror
\nSun's Concentration : 100
\nTracking : Yes
\nTracking Receiver : No
\nTemperature (C) : 538
\nTemperature (F) : 1000
\nTypical Application : Electricity, Heat
\nComments : Small or Large Systems
\n
\n\nType of Concentrator: '''Fixed Mirror Moving Focus'''
\nType of Focus : Line
\nLens or Mirror : Mirror
\nSun's Concentration : 100
\nTracking : No
\nTracking Receiver : Yes
\nTemperature (C) : 538
\nTemperature (F) : 1000
\nTypical Application : Electricity, Heat
\nComments : Small or Large Systems. Not economic is U.S. experience
\n
\n\nType of Concentrator: '''Lens (linear)'''
\nType of Focus : Line
\nLens or Mirror : Mirror
\nSun's Concentration : 100
\nTracking : Yes
\nTracking Receiver : Yes
\nTemperature (C) : 538
\nTemperature (F) : 1000
\nTypical Application : Electricity, Heat
\nComments : Small or Large Systems. Little U.S. experience
\n
\n\nType of Concentrator: '''Sphere'''
\nType of Focus : Line
\nLens or Mirror : Mirror
\nSun's Concentration : 80
\nTracking : No
\nTracking Receiver : Yes
\nTemperature (C) : 538
\nTemperature (F) : 1000
\nTypical Application : Electricity
\nComments : Awkward in large scale
\n
\n\nType of Concentrator: '''Cylinder'''
\nType of Focus : Line
\nLens or Mirror : Mirror
\nSun's Concentration : 2
\nTracking : No
\nTracking Receiver : No
\nTemperature (C) : 121
\nTemperature (F) : 250
\nTypical Application : Heat
\nComments :
\n
\n\nType of Concentrator: '''Cusp'''
\nType of Focus : Line
\nLens or Mirror : Mirror
\nSun's Concentration : 1.5-2.5
\nTracking : No
\nTracking Receiver : No
\nTemperature (C) : 121
\nTemperature (F) : 250
\nTypical Application : Heat
\nComments :
\n
\n\nType of Concentrator: '''Winston'''
\nType of Focus : Line
\nLens or Mirror : Mirror
\nSun's Concentration : 3-6
\nTracking : No
\nTracking Receiver : No
\nTemperature (C) : 121
\nTemperature (F) : 250
\nTypical Application : Heat
\nComments : Concentration decreases as acceptance angle increases
\n
\n\nType of Concentrator: '''Flat plate w/ Booster'''
\nType of Focus : Area
\nLens or Mirror : Mirror
\nSun's Concentration : Between 1 and 2
\nTracking : No
\nTracking Receiver : No
\nTemperature (C) : 121
\nTemperature (F) : 250
\nTypical Application : Heat
\nComments :
\n
\n\n=== ANNUAL ENERGY COLLECTION EFFICIENCY ===\n\nCollectors that maintain their surfaces facing the sun (right\nangle for most collectors) have the highest annual collection\nefficiency. The parabolic dish and other two-axis tracking collectors\nare examples. The central receiver, although a two-axis\ntracking system, does not direct the heliostat reflectors to face\nthe sun but rather maintains an angle to the sun so that the\nimage is reflected to the receiver. As expected, its collection\nefficiency is lower than the dish. The parabolic trough is a\nsingle-axis tracking system; thus, the surface is only occasionally\nat a right angle to the sun and has a lower annual collection\nefficiency than the central receiver.\n\nFixed collectors with tracking receivers such as the bowl and\nRussell collector have even lower collection efficiency. The\nleast efficiency is exhibited by Winston and other fixed collectors\nand receivers.\n\nThe theoretical annual efficiency of the three principal concentrating\ncollectors utilized in the United States is 80 percent\nfor the dish, 60 percent for the central receiver, and 43 percent\nfor the parabolic trough on an annual basis. Collector efficiency\nis determined for the period extending from the beginning\nof tracking when the sun climbs to 15 degrees above the horizon\nuntil tracking stops when the sun declines below 15 degrees at\nthe end of the day. The efficiency depends on direct solar radiation\nand system optics.\n\nActual efficiency depends on mirror or lens surface accuracy,\nsurface dust and film, energy absorption by lens or mirror, the\nproperties of the reflecting, material, pointing accuracy, effects\nof temperature variations on these factors, weather--including\nclouds, dust and haze, and so on. The efficiency is further\nreduced by receiver performance and receiver subsystem design,\nincluding care given to reduction of heat loss by conduction,\nconvection, and radiation.\n\n== III. DESIGN VARIATIONS AND EXPERIENCE ==\n\n=== PARABOLIC SOLAR COLLECTORS ===\n\nA recent paper on the parabolic dish prepared by the Jet Propulsion\nLaboratory(*) describes nine designs sponsored by the U.S.\n\n(*) V.C. Truscello, \"Status of the Parabolic Dish Concentrator,\nProceedings of the Energy Research and Development Agency Conference\non Concentrating Solar Collectors, Georgia Institute of Technology,\nSeptember 26-28, 1977 (Washington, D. C.: U. S. Department\nof Energy, undated, circa 1982-1983).\n\nDepartment of Energy, eight privately-funded U.S. designs, and\n10 dishes developed by other countries. Although no two dishes\nare identical, they fall into four categories:\n\n1. Rigid reflector. The reflective surface is attached to a rigid curved structure. This is the standard (radar type) structure (Figure 21).\n\n[[File:Solar 25p20a.gif|center|437px]]\n\n2. Pressure-stabilized membrane. The reflective surface is attached to a flexible membrane, which takes the shape of a rigid, curved support structure by creation of a vacuum between the membrane and structure. The intent is to reduce cost by reducing weight of materials of construction (Figure 22).\n\n[[File:Solar 25p20b.gif|center|486px]]\n\n3. Fresnel lens or Fresnel mirror. The lens is built up from several narrow concentric parts; the mirror is a series of concentric reflective surfaces. The intent is to reduce cost by simplifying the compound curvature of the paraboloid (Figure 23).\n\n4. Secondary reflector. A second mirror, which may be hyperbolic(*) (cassegrain) or elliptic(**) (gregorian), reflects the rays from the parabolic reflector to a receiver behind the parabola. The intent is to eliminate the heavy receiver structural demands on the dish and also to provide easy access to the receiver for maintenance (Figure 24).\n\nThe rigid reflector has been the most popular since it resembles\ncurrent radar technology. The Shenandoah project, a U.S. Department\nof Energy demonstration project near Atlanta, Georgia, deployed\n114 7-meter-diameter dishes coated with a reflective film\nto produce 399 [degrees] C (750 [degrees] F) steam. The steam was used to generate\n400 kilowatts of electricity and process steam at 9.70 kilograms\nper square centimeter (138 pounds per square inch gauge [psig])\nfor an adjacent knitwear factory. After some initial problems,\nthe system is now operating satisfactorily. The project is a\njoint effort of the U.S. Department of Energy, the local power\ncompany, and the knit-wear factory. Its goal was to demonstrate\nthe viability of rigid-reflector collectors, not to be a commercial\nprototype.\n\n(*) A curve formed by the section of a cone cut\nby a plane that makes a greater angle with\nthe base than the side of the cone makes.\n\n(**) Oval-shaped.\n\n[[File:Solar 25p19.gif|center|393px]]\n\n=== CENTRAL RECEIVERS ===\n\nThe best U.S. example of a central receiver is Solar One, a joint\nproject of the U.S. Department of Energy and two Southern California\nutilities. This 10-megawatt electric pilot plant utilizes\n1,818 heliostats (or reflectors), each with 41.8 square meters\n(450 square feet) of second-surface glass mirrors. The heliostats\nsurround a tower on which the receiver is located. Most of the\nheliostats are located south of the tower. The plant has exceeded\nits specifications and is operating very successfully. The design\nwas based on a 100-megawatt plant and then reduced to 10 megawatts.\nAn optimized 10-megawatt plant would likely have a different\nheliostat field configuration.\n\nA 100-megawatt version (Solar 100) with similar technology is\nbeing considered by the utilities, assuming government investment\ncredits are provided. Without these financial incentives, the\nplant would not be economical in the United States due to falling\noil prices. However, such a plant may be economical in other\ncountries with high energy costs.\n\nHeliostats have evolved through a series of designs that reduced\nthe initial weight of over 97.6 kilograms/square meter (20\npounds/square foot) to about 39 kilograms/square meter (8 pounds/square\nfoot). Over 20 heliostat designs have been constructed and\ntested. The current preference is for a second-surface glass\nmirror on a glass backing. The U.S. Department of Energy's Solar\nEnergy Research Institute is developing a lightweight reflector\n(plastic/silver/plastic), which promises to drastically reduce\nthe cost of heliostats. When developed, the material may be of\ninterest for use in less-industrialized countries.\n\nHeliostat size is governed by rigidity and wind load requirements.\nDue to the present cost elements of heliostats (which are\ninfluenced by the fact that every heliostat needs its own tracking\nsystem), in the United States, system designs favor large\nheliostats. The distribution of cost elements may vary in other\ncountries. While only larger central receivers are likely to be\neconomical in the United States, some advanced developing countries\nmay be able to utilize the smaller Solar One technology\neconomically.\n\n=== LENSES ===\n\nCircular lenses, whether standard or Fresnel, tend to be limited\nin size, much like the parabolic dish. Size is also limited by\ncurrent fabrication capabilities. Small glass lenses for cameras\nand spotlights are available, as are larger plastic lenses. But\na 7-meter diameter lens (a size comparable to the Shenandoah\ndish) is certainly not widely available either in glass or plastic.\nIn large sizes, a glass lens would be very heavy; plastic,\nprobably in a Fresnel design, is likely to be the only practical\nlens, if available. Linear Fresnel lenses may offer the advantage\nof being fabricable in both small and large widths and lengths.\n\n=== PARABOLIC TROUGHS ===\n\nA significant number of parabolic troughs have been designed,\nbuilt, and tested, primarily with private funds. Many types are\navailable on the market. Troughs differ in their reflective\nmaterials, structural materials, receiver concepts, etc. The\nattainable temperature reaches about 540 [degrees] C (1000 [degrees] F). The designs\nvary with intended temperature application, since surface error,\ntracking error, and receiver losses assume considerable importance\nfor a high temperature design.\n\nTroughs have been utilized by many federal demonstration projects\nto provide process heat for industrial applications and to supply\nvapor for suitable small engines (e.g., irrigation pump devices).\nAll designs had initial problems, usually with materials and nonsolar\nhardware. After repair or modification, operation was reliable\nand successful. Many federally-funded projects tended to\nbe shut down when they ended and rarely restarted because of lack\nof sustained interest by the user. An excellent source of information\non private trough manufacturers is the Solar Energy Industries\nAssociation (SEIA) in Washington, D.C.\n\nTroughs may be attractive because of their relative simplicity.\nBecause their surface curvature is singular, not compound as for\ndishes, troughs are more easily fabricated. A second-surface\nreflective plastic with adhesive backing can be easily placed on\nthe curved substrate. A simple pipe or tube will serve adequately\nas the receiver although various simple techniques, such as a\nglass vacuum jacket around the receiver tube, will enhance performance.\nSingle-axis tracking is less complex than two-axis\ntracking.\n\n== IV. SPECIAL TOPICS ==\n\n=== RECEIVERS ===\n\nThe concentrated sunlight must be converted to a useful form of\nenergy, usually heat. If desired, heat can be converted to electricity\nby means of an engine and generator. The receiver should\nbe designed to minimize heat loss. Heat loss occurs through\nradiation to a cooler object; through convection currents created\nby heating air in contact with the hot receiver surface; and\nthrough conduction from the hot parts of the receiver to colder\nparts and to attached structural members and insulation.\nHeat retention by the receiver is enhanced by covering the receiver\nwith a selective coating which will absorb virtually all\nthe concentrated radiation but will reradiate comparatively\nlittle energy. Furthermore, since the total energy radiated\ndepends directly on the radiating area, the receiver surface area\nshould be minimized. Convection can be reduced by preventing the\nbuild-up of air currents that remove air heated by the receiver\nand provide the receiver with colder air for continued heat loss.\nA transparent window (glass or plastic depending on temperature)\ncan reduce air currents.\n\nThe window introduces other heat loss and heat gain effects.\nSome energy will be reflected from the front surface and rear\nsurface of the window and never reach the receiver. Additional\nenergy will be absorbed by the window and not reach the receiver.\nThe inner surface of the window may be coated with a heat mirror\nsuch as tin oxide, which reduces the radiation loss by reflecting\nradiated energy back to the receiver. Etching of the outer surface\nof a glass window reduces the reflection from the surface.\n\nInsulation serves to reduce convection and radiation losses from\nparts of the receiver outside the path of the incoming radiation.\nConduction loss is reduced by decreasing the cross-section of\nstructures in direct contact with the receiver, and using poor\nheat conductors for these structures where possible. Creating a\nvacuum between the window and the receiver will further reduce\nconvection and conduction losses.\n\nFigure 25 shows the reflectivity of several mirror systems. Note\n\n[[File:Solar 25p24a.gif|center|540px]]\n\nnot only the differences in reflectivity but also that for some\nmaterials the reflected energy falls within a small solid angle*\n(Figure 26). These materials allow a small target area for\n\n[[File:Solar 25p24b.gif|center|486px]]\n\nreceipt of the reflected rays. If a larger solid angle is required\nto enclose the reflection, then a compromise between\ntarget size and loss of reflected rays must be made. Energy which\nis not reflected is converted to heat at the reflecting surface.\nThis may require positive cooling efforts to ease or eliminate\nthermal stress.\n\n=== COST ===\n\nConcentrator cost represents only one portion of the cost of a\nsystem. The cost of the quantity of heat delivered at the required\ntemperature is the preferred method of determining cost.\nFor a given system, the cost per million kilowatt-hours, or kWh\n(per million Btu) usually decreases as the total number of kWh\n(Btu) delivered increases, i.e., as system size increases. Similarly,\nthe cost per million kWh (per million Btu) is likely to be\nless at lower temperatures than at higher temperatures. In general,\nthe higher the concentration and complexity, the higher the\ncost.\n\n(*) If you have an angle, one side of which is vertical and the\nother side not vertical, and that side is rotated around the vertical\n(maintaining the same angle), the angle created is called\nthe solid angle.\n\nCost is frequently represented by purchase price but not always.\nSellers may reduce selling price to penetrate a market, to expand\nmarket share, to anticipate future manufacturing economies and\ncost reductions, and to limit or exclude potential competition.\nSellers with a monopoly or a preferred position may sell at\nhigher than reasonable rates. Sellers faced with unknown or\nindeterminate risks and liabilities for the product will try to\ntransfer the risk to the purchaser through higher prices or other\nmeans.\n\nIn the United States, many solar energy systems are cost effective\nonly because of federal and state tax policies to aid the\nsolar energy industry. These systems cost two to five times more\nthan competing energy systems. However, energy costs in many\nless-developed countries are several times greater than in the\nUnited States, and therefore solar systems may be cost effective\nin those countries.\n\nIn the United States, the cost of a solar thermal electric system\nutilizing relatively new technology and incorporating research\nand development costs would range from $10 to about $30 per watt.\nThe central receiver experiment in California (Solar One) cost\nabout $15 per watt; a proposed 100-megawatt plant incorporating\nthe lessons of Solar One and the economies of a ten-fold increase\nin size is anticipated to cost about $4 per watt. Heliostats were\nabout one-third of the total cost of Solar One, and are expected\nto be about one-half the cost of the large plant. (A coal-fired\nelectric plant costs about $1.00-$1.40 per watt of installed\ncapacity.)\n\nStudies of dish technologies indicate costs ranging to $50 per\nwatt for the system, with dish costs of one-third to one-half of\nthe system cost. Dish technology is well behind heliostat experience.\nParabolic troughs appear to cost about $538 per square\nmeter ($50 per square foot) at present with possible reduction to\nabout $270 per square meter ($25 per square foot) with a larger\nmarket. Again, these costs reflect only one-third to one-half the\nsystem cost.\n\nOf possible interest to developing countries is the class of\ncollectors using transparent plastic in cylindrical form with the\nreflector film partially located in the lower arc and a \"black\"\ntube located at the focus. This type of collector appears to\noffer low cost. Some versions using an evacuated glass tube with\nan inner blackened copper tube in \"once through\" (straight tube)\nor bayonet style are commercially available in the United States\n(Figures 27, 28, and 29).\n\nThe hemispherical bowl has been tested in Crosbyton, Texas, by\nthe U.S. Department of Energy. The unit, 20 meters in diameter,\nproduced high temperatures and high pressure steam suitable for\nmodern steam turbines. The compound curvature is difficult to\nbuild, as is the two-axis tracking required of the receiver.\nHowever, a tracking receiver is simpler than a tracking concentrator.\nThe concentrator may be more acceptable in smaller size\nand lower concentration (temperature). The reduction in concentration\nwill decrease temperature, which increases the number of\nmaterials that can be used for the receiver, and may ease fabrication\nof the sphere.\n\nTo compare solar thermal technologies, costs should be reduced to\ncommon bases such as cost per watt electric or per kWh (Btu). The\nbase should distinguish between average and peak capacity; the\namount of storage incorporated; temperature, if heat is the\ndesired end product; and the yearly energy delivered. Other\ntechnologies have their own bases; photovoltaics use cost per\npeak watt, and installed cost per annual kilowatt-hour produced.\nElectricity from wind energy, as well as from other solar electric\ntechnologies, may have different value to the user depending\non the time of generation. These considerations should be included\nin any evaluation methodology for selection of cost-effective\nsystems.\n\n== V. COMPARING THE ALTERNATIVES ==\n\nSimple flat plate collectors are the most widely used and most\ncost-effective solar collectors. Their primary use is for domestic\nand commercial (e.g., hospitals, restaurants, etc.) hot water\napplications; however they may also be used in preheat systems\nfor higher temperature applications. They can achieve a temperature\nof about 38 [degrees] C (100 [degrees] F) above the ambient by capturing sunlight,\nconverting sunlight to heat, and carefully minimizing\nunwanted heat loss from the collector.\n\nFlat plate (usually non-tracking) collectors are the simplest to\nfabricate. Simple, unsophisticated, functioning collectors can\neasily be built with simple tools. Care must be taken to enhance\nsolar collection and prevent thermal losses. Careful use of local\nmaterials to the maximum extent possible can reduce cost. While\nselective absorbers enhance performance and yield higher temperature,\nalmost any \"black\" surface will perform adequately. Some\nsimple, low-cost flat plate collectors may be better than concentrators\nfor temperatures below 93 [degrees] C (200 [degrees] F), particularly in\nless-industrialized countries. Expectations of better performance\nfor flat plate (non-concentrating) collectors over concentrating\ncollectors, for the same temperature application, have not been\nverified in practice. The expectations were based on utilization\nof both direct and diffuse radiation by flat plate collectors and\nuse of only direct radiation by concentrators.\n\n== BIBLIOGRAPHY/SUGGESTED READING LIST ==\n\nReports and Conference Proceedings\n\nDougherty, D.A. Line-Pocus Receiver Heat Losses. SERI/TR-632-868. Golden, Colorado: Solar Energy Research Institute, July 1982.\n\nMurphy, L.M. Technical and Cost Potential for Lightweight, Stretched-Membrane Heliostat Technology. SERI/TP-253-2070. Golden, Colorado: Solar Energy Research Institute, January 1984.\n\nScholten, W.B. A Comparison of Energy Delivery Capabilities of Solar Collectors. McLean, Virginia: Science Applications, Inc., 1983.\n\nSolar Energy Research Institute. Solar Thermal Technology Annual Evaluation Report, Fiscal Year 1983. Golden, Colorado: Solar Energy Research Institute, August 1984.\n\nTruscello, V.C. \"Status of the Parabolic Dish Concentrator.\" Proceedings of the Energy Research and Development Agency Conference on Concentrating Solar Collectors. Georgia Institute of Technology, September 26-28, 1977. Washington, D.C.: U.S. Department of Energy, undated (circa 1982-1983).\n\nU.S. Department of Energy. Solar Parabolic Dish Annual Technology Evaluation Report, Fiscal Year 1982. DOE/JPL1060-63. Washington, D.C.: U.S. Department of Energy, September 15, 1983.\n\nU.S. Department of Energy/Sandia Laboratories. Proceedings of the Line-Focus solar Thermal Energy Technology Development Conference, A Seminar for Industry (September 9-11, 1980). Washington, D.C.: U.S. Department of Energy, September 1980.\n\n== Books ==\n\nDuffie, J.A., and Beckman, W.A. Solar Engineering of Thermal Processes. New York, New York: John Wiley and Sons, 1980.\n\nKreith, F., and Kreider, J.F. Principles of Solar Engineering. Washington, D. C.: Hemisphere Publishing Corp., 1978.\n\nLunde, P.J. Solar Thermal Engineering. New York, New York: John Wiley and Sons, 1980.\n\nMeinel, A.B., and Meinel, M.P. Applied Solar Energy. Reading, Massachusetts: Addison-Wesley Publishing Co., 1976.\n\n== SOURCES OF INFORMATION ==\n\nGovernment Printing Office
\nWashington, D.C. 20402 USA
\n\nJet Propulsion Laboratory
\n4800 Oak Grove Drive
\nPasadena, California 91103 USA
\n\nNational Technical Information Service
\n5285 Port Royal Road
\nSpringfield, Virginia 22161 USA
\n\nSolar Energy Industries Association
\n1717 Massachusses Avenue N.W.
\nWashington, D.C. 20036 USA
\n\nSolar Energy Research Institute
\n1617 Cole Boulevard
\nGolden, Colorado 80401 USA
\n\nU.S. Department of Energy
\nOffice of Thermal Systems
\n1000 Independence Avenue, S.W.
\nWashington, D.C. 20585 USA
\n\n== See also ==\n\n* [http://ieeexplore.ieee.org/iel4/5796/16147/00747949.pdf?arnumber=747949 Component Efficiencies From The Operation Of The Crosbyton Solar Bowl]
\n\nEnergy Conversion Engineering Conference, 1990. IECEC-90. Proceedings of the 25th Intersociety
\nPublication Date: 12-17 Aug 1990
\nVolume: 5, On page(s): 185-195
\nISBN: 0-8169-0490-1]\n\n{{Page data\n| license = CC-BY-SA-3.0\n| affiliation = VITA\n| description = Appropedia shares how solar concentrators work and why they matter. Learn about designs that capture more sunlight with simple tools.\n}}\n\n[[Category:Solar]]\n[[Category:Design]]\n[[Category:Books]]"}