SOLAR ENERGY SYSTEM

Abstract: A solar energy system that includes a primary concentrator (12), a receiver (14) having a plurality of photovoltaic cells (26), and a prefilter (22) surrounding the receiver (14). The prefilter absorbs some of the radiation that is out of band with respect to the photovoltaic cells (26), and may include a conduit (28) for a cooling fluid. The cells on the receiver are positioned such that each cell receives the same solar energy flux (16). The receiver may include a phase change to protect the photovoltaic cells from excessive temperatures.

Description: 80LAR ENERGY SYSTEM Field of the Invention The present invention relates to systems for converting solar energy Into electrical energy, using photovoltaic oells. Background of the Invention Photovoltaic oells are often the most expensive components of solar energy systems. As a rβiult, one prior approaoh to building solar energy systems has been to use large quantities of low-eoit oelli, e.g., polyorystalline thin ilm cells, of the type designed to operate on unαonoentrated sunlight. However, this approaoh yields very low oonverslon efficiency, and has not proved economic. A second and generally more efficient approach to solar energy systems has been to use concentrators to concentrate the solar energy onto relatively high performance (and high ooβt) photovoltalo cells, such as OaAa/OaBb oells. Unfortunately, the use of concentrators presents a number of new problems. For example, virtually all known photovoltaic cells operate less efficiently as their temperature lnoreases. The use of concentrators tends to oausβ the cells to operate at a higher temperature, thereby deereaslng system βffiolβifcy. This creates a difficult tradeoff between oost and conversion efficiency. A further problem with concentrator designs la that the illumination field at the fooal point of a concentrator is usually very intense, and nonuniform. The intensity makes it difficult to take advantage of all of the Inoid β nt sunlight, because some of the area in the intense light field is required for Interconnecting conductors and gβomβtrlo cell mounting considerations. The result is poor utilisation of expensive photovoltalo oelli. Furthermore, the large β urr β nte generated by the intense light field require heavy oontaot rid lines on the cells, and for Interconnecting oonduetori. The nonunlformlty of the illumination results in different temperatures, currents, and voltages In ths Individual photovoltaic αells. in addition, the oelli in oonoentrator arrangement! are βubjeot to rapid thermal damage If coolant iβ loft, or if hot spots are formed by flaws in the concentrator. Tandem photovoltalo oelli have been developed to lnereaae converβion effiαlenoy In aolar energy ayatemi. In a tandem oelli aolar radiation that panes through a primary oell li incident on a eeoondary eell that typioally haβ a wavelength abiorbing region different from that of the primary cell. Thui the secondary cell can utilize some of the radiation not oonverted by the primary oell, thereby increasing the overall oonveraion ef iciency. However, in general, the secondary cell costs aa much aa the primary cell, but only produoes a fraction (e.g., 30 percent) of the amount of electricity produced by the primary cell. Thus the electrical energy produced per unit cost la often Quite low In tandem designs. Another prior attempt to Increase oonveraion efficlβnoy relates to the faot that In a aolar energy system, muoh of the inotdent solar energy that is not oonverted to eleotrioity appeara aa heat. Attempts have therefore been made to utilise aomβ of such heat in so-called oogβneratlon facilities that produoβ both electrical and thermal energy outputs. However, cogeneration systems present a undamental design conflict. The αonfliot is due to the act that the photovoltalo calls must be operated at as low a temperature as possible, in order to maximize their eleotrloal oonveraion effioiency. However, thermal energy must be delivered at as high a temperature as possible, in order to maximize the thermodynamio efficiently of the thermal energy aystem * One prior approaoh to solving the conflicting requirements set forth above has been distributed point focus systems, which oomprlse an array of email lenses, each of which has a photovoltalo oell at its foαal point. However, such systems typioally have relatively low concentration, and require more oell area than designs that use a single oentral receiver. Further, when tandem oells are used to boost βfflolβnoy, the rear oells in the tandem atao double the overall ooβt, while increasing the eleotrloal output by a smaller amount. Summary of the Invention The problems set forth above are substantially overoo e by the aolar energy system of the present Invention. In one preferred embodiment, the eyatem comprises photovoltalo means and prefiltering means. The photovoltalo means comprises an active area and a pair of eleetrodes, and absorbs electromagnetic radiation Ineldent upon the aotlve area within a response band of wavelengths. A portion of the absorbed eleotromagnetlo radiation is oonverted Into an eleotrloal potential dif erence between the eleotrodes. The prefiltering means oo prlses means for absorbing a substantial fraction of electromagnetic radiation incident upon the prefiltering means within a prefiltering band. At least a portion of the prefiltering band lies outside of the response band of the photovoltalo means, but within, the solar band of the inoldent solar energy. The photovoltaic means and the prefiltering means are positioned such that the solar energy must pass through the prefiltering means to strike the active area of the photovoltalo means. Thus, some of the solar energy outside of the response band is absorbed by the prefiltering means, and is not Incident upon the photovoltaic means. By removing some of the out of band energy, the prefiltering means lets the photovoltaic means operate at a lower temperature, thereby Increasing the conversion efficiency. In a second preferred embodiment, the solar energy system of the present Invention oomprlses primary oonoentrator means and a reoeivβr. The concentrator means reoeives the Inoldent solar energy, and fooυses It toward a foαal spot. The reoeiver Is positioned substantially at the focal spot, and comprises a plurality of photovoltaic cells positioned suoh that each photovoltaic eell receives approximately the same solar energy flux (energy per unit area) from the oonoentrator means. In a preferred embodiment, the photovoltaic oells are arranged in a plurality of rings. The photovoltaic cells of each ring are at a constant radius from a oβntral axis common to all rings, and the active sur aces of the photovoltalo cells face outward, away from the central axis. In a further aspect, the present invention provides a photovoltaic module for converting solar energy Into eleotrloal energy. The module comprises a substrate having a relatively large first surface and a relatively small second surface. A first photovoltalo oell is mounted to the first surface, the first photovoltaic cell having a first response band. The module is positioned such that the aolar energy la inoldent onto the active area of the first photovoltaic oell. A second photovoltaic oell substantially smaller than the first photovoltaic oell la mounted to the second surface of the substrate. The substrate is formed suoh that a portion of the solar energy transmitted by the first photovoltalo oell Into the substrate is direoted onto the active area of the aecond photovoltaic cell. Becauae of the smaller area of the second photovoltalo oell, the eost per unit oell area is lower than in conventional tandem designs. In a further preferred aspect, the present invention provides a reoetvtr for t solar energy system that tnoludes a phase ohenge material. The phase change material Is seleoted suoh that It experiences e phase change (e.g., melting) at a temperature that is higher than the normal operating temperature range of the photovoltaic cells, but less than the temperature at which the photovoltalo cells will suffer thermal damage. The phase change material reduces the possibility that the photovoltalo cells will be damaged by ooolant loss. Brief Description of the Drawings FIGURE 1 is a diagram illustrating a aolar energy system including a concentrator. FIGURE 2 is a diagram illustrating a preferred embodiment of the receiver. FIGURE 3 is a graph Illustrating the prefiltering; FIGURE 4 is a cross-sβotlonβi view of the rβoβiverj FIGURE 5 is a plan view of a photovoltalo oell; FIGURE β is a perspective view showing the photovoltaic oells In a single ring} FIGURE 7 is a graph Illustrating a tβohnique for positioning the photovoltalo oells; FIGURE 8 la a partial oroββ-ββotlonal view ahowlng the mounting of a cell) FIGURE 9 is a schematic oroββ-seetlonal view of a two oell module) FIGURE 10 Is a schematic orosβ-section'ai view of a three oell module; FIGURE 11 is a. partial cross-sectional view showing the positioning of a multicell module in the receiver; and FIGURE 12 is a schematic cross-sectional view of a second preferred receiver design. Detailed Description of the Invention FIGURES 1-7 illustrate the construction and operation of a first preferred embodiment of the solar energy system of the present invention. Referring initially to FIGURE 1, the overall aystem oomprlses primary oonoentrator 12 and receiver 14. The oonoentrator has a shape of a three-dimensional paraboloid, with reoelver 14 positioned at the focus. The oonoentrator receives Inoldent solar energy, symbolized by rays 18, and refleots the solar energy βuoh that it is conαentrated onto reoelver 14. The receiver converts portions of the solar energy into eleotrloal energy and thermal energy. The thermal energy is In the form of a fluid heated above ambient temperature. A preferred 'embodiment of • reoelver 14 is illustrated in greater detail in FIGURE 2. The receiver oomprlaes a disk-shaped base 20, and hemiapherioal prefllter 22. One surface of base 20 (the lower surfaoe in FIGURE 2) comprises e serrated mounting surface 24 to which a plurality of photovoltalo oells 26 are mounted. The particular arrangement of the aerratlonβ and photovoltaic oells Is described in greater detail below. Prefllter 22 substantially surrounds the photovoltaic oells, such that the solar radiation from the concentrator (symbolized by rays 16) must pass through the prefllter before striking the cells. Prefllter 22 comprises a Jacket that Includes an internal passageway 28 through which a fluid oart flow. Passageway 28 communiαatea with paasagewey 30 formed in base 20. 5 Base passageway 30 is in turn coupled to pump 86 by central passageway 32 that Is formed by tube 34, the tube extending outwardly from the center of the base. In operation, a heat transfer fluid suoh as water is pumped by pump 36 into tube 32, from which point the water spreads outward through a plurality of passageways 30 in the base. It will be understood that passageways 30 are 10 illustrated schematically in FIGURE 2, to simplify the illustration. The heat transfer fluid cools the base and the attached photovoltalo oells, as further deaαribed below, and is in turn heated by the baae to a first temperature above the inlet temperature. The onoe heated fluid then flows through passageway 28 in prefllter 22, and in so doing intercepts a portion of solar radiation 18 passing 15 through the prefllter. This raises the fluid temperature to a seoond temperature, above the first temperature. The twice heated fluid exiting from passageway 28 forms the thermal energy output of the system. This heated fluid may be used directly, for example aa hot water, or may be input to a heat exchanger for other known applications. 20 An Important function of the prefllter may be desαribed with reference to FIGURE 3. In this graph, curve 40 represents the spectrum of solar energy, i.e., the solar irradlanoe, at the earth's βurfaoe. The solar spectrum has an upper (ahort wavelength) cutoff at about .3 mlorons, and has a long tall extending Into the Infrared. Curve 42 represents the relative response of a typical gallium 25 araenlde photovoltaic oell. Response ourve 42 lies within response band 44 that haa a short wavelength limit 46 at about 0.3 mlorons, and a long wavelength limit 48 at about 0.9 miorons. A oomparlson of aolar spectrum 40 and response ourve 42 illustrates that gallium arsenide oells are a good choice for a solar energy system. However, other types of photovoltaic cells may also be used to 30 implement the present invention. Curve BO illustrates the percentage of absorbanoe of a 0.4 inoh thiok film of water, suoh is water flowing through prβ liter 22 of FIGURE 2. It oan ba aeen that ourve 50 lies within e prefiltering band 82 that has a short wavelength limit 84 very olose to the long wavelength limit 48 of response band 44. The 35 purpose of the prefiltering represented by ourve 50 is to remove as muoh as possible of the solar radiation that lies outside of response band 44, without letting auoh radiation reach the photovoltalo oells. In FIGURE 3, ourve 56 represents the result of multiplying the abaorbanoe represented by curve 50 by aolar spectrum 40. Thus the shaded region 58 represents the energy removed from the solar spectrum by a prefllter that αomprlaeβ a 0.4 inch water film. The prefllter enhances the performance of the receiver, by removing "out of band" infrared energy from the aolar spectrum. Because prefiltering band 52 lies almost entirely outaidβ response band 44, the removed energy represents energy that the photovoltaic oell can not oonvert to electricity. Overall, gallium arsenide photovoltaic cells are capable of oonverting about 20% of the energy of incident sunlight Into electricity. This leaves 8096 of the energy to appear as heat in the receiver. However, about 4096 of the Incident solar energy is out of the gallium arsenide response band. In the ideal oase, all of this energy is removed by pre iltering, leaving only 4096 of the incident solar energy to appear as heat in the reoelver. Thus up to a 5096 reduαtlon (8096 to 4096) of the heat generated in the receiver can be achieved. Such a reduction will significantly lower the oell operating temperature, thereby inoreasing the conversion effloienoy. Similar Improvements are possible with other types of photovoltaic cells, suoh as silicon oells. In general, the absorption of out-of-band energy In the prefllter may be enhanced by constructing the prefllter jacket from an Infrared absorbing glass such aa the Schott-Amβrlca KG-Sβrlβs glasses. The prefllter is especially advantageous for a cogenerative photovoltaic aystem. In particular, In addition to relieving the thermal load on the photovoltaic oells, the prefllter facilitates low temperature, effioient eleotrloal conversion, and simultaneously, enables a two-stage heat removal system eapable of delivering a heated fluid at approximately twloe the oell operating temperature (with respect to ambient). FIGURES 4-8 illustrate further details of a preferred embodiment of base 20 to which the photovoltalo oells are mounted. The disk-shaped baae is shown in cross section In FIGURE 4, and Is symmetric about central axis 60. The receiver comprises a series of mounting faoes 62(1) on whioh the photovoltaic oells 26 are mounted, the mounting faoes being oriented vertically In FIGURE 4. Each mounting faoe has the form of a cylindrical shell of height h that eneircles axis 60 at a radius R j . The different mounting faces all have different radii R-, but the same height h. The mounting faoes are Joined to one another by oonnectlng faces 64 that are oriented substantially parallel to the incoming aolar radiation 16 that has been refleoted towerdβ the reoelver by the primary oonoentrator. Thus the oonnectlng faoes do not blook any of the incoming radiation, and substantially all of suoh radiation falls on one of the mounting faces. However, to allow for imperfections in the primary concentrator, as well as for the finite angular size of the solar disk, a small amount of radiation will in fact strike the connecting faoes. The oonnecting faoes may therefore be coated with a reflecting meterial, 10 that any radiation that does strike the oonnecting faces is reflected onto the adjacent mounting faoes. A plurality of photovoltaic oells 26 are mounted on each mounting face, such that the completely oover the mounting faoe. As illustrated in FIGURE 5, in one preferred embodiment, each photovoltalo oell 26 oomprlses a rectangular body 70 having an active area 72 of height h that oovers most of one side of body 70, and a pair of eleotrodes 74 formed adjacent to the active area. The photovoltaic cell converts a portion of the solar energy incident on the active area into a potential differenβe between the eleotrodes. As Illustrated in FIGURE 6, the photovoltaic oells 26 that are mounted to a given mounting faoe 62(1) form a ring of radius R,, with aotive surfaces 72 facing outwardly away from oentral axis 60 of the receiver base. This design permits the use of a standard size photovoltaic oell. Aa will be described more fully below, cells 26 are positioned on mounting faoe 62(i) suoh that active areas 72 fully cover the mounting faoes, with the portion of the oells containing electrodes 74 being reoeived In slots in base 20. This arrangement maximizes the effeotlve aotive area of the reoelver. The relative positions of mounting faoes 62(1) are preferably selected such that an equal aolar energy flux falls on the photovoltaic oells on all the mounting faoes. A method of determining the appropriate mounting face positions is illustrated in FIGURE 7, for the oaae in whioh the prefllter does not bend the incoming rays from the oonoentrator (e.g., a hemispherical prefilter). In this diagram, referenoβ numeral 80 represents the primary concentrator, which is assumed to have a parabolic shape with ita center at the origin O of an X-Y coordinate system. For the purpose of this example, It will be assumed that in the illustrated X-Y plane, the shape of the primary oonoentrator is governed byi y ■ O.Oδx 2 CD The receiver 82 la centered at the fooal point of the parabola, at coordinates (0, 5). Reforenoe numeral 62(1) designates the mounting faoe or, eςulvalently, the active areas of the oells that will be affixed to the mounting faoe. Imagine that the βurfaoe of the primary oonoentrator is broken up into a aet of βlde-by-βlde annular areas, the boundaries between adjaoent annular areas being at X coordinates x . . . x,, X j + v . . . , with x : being the outermost edge of the primery oonoentrator. The oells on a given mounting surface will be arranged to intercept all of the solar radiation rβfleoted towards the reoelver by a given annular ring. Thus in FIGURE 7, the photovoltalo cells on mounting lurfaoe 62(1) will ' be positioned to interoept all aolar radiation refleoted from ring 86(1) that extends between oirole 88 at coordinates x j - j , and oirαle 80 at coordinates x l+1 , y 1+1 . Line 82 la constructed between reoelver 82. and oirole 88, and line 94 is construoted between receiver 82 and oirαle 80. The radius of the mounting aurfaαe 82(1) corresponding to ring 86(1) Is R j , the vertical position of the base of the mounting ring is at V } , and it is desired to find the set of radii R- and vertloal positions V j for all mounting rings, such that each mounting ring receives the same amount of aolar radiation per unit area. The area of the aolar radiation intercepted by ring 86(1) is A p - r(x j 2 - x i 2 ) (2) while the corresponding area of the photovoltalo oells that Interoept this radiation la Ap V * 2πR j h (3) It Is desired that the ratio of A p to A pv ahould be held constant at some suitable concentration factor C. Thus *ι 2 - *ι + ι 2 _ r ( ) 2 h which can be rearranged to produce \ " x 1+l (5) 2R^C A second equation for h can be found by determining the equations of lines 82 and 94ι aubetltutlng in the value x ■ R { to determine the Y ooordinatea of the top and bottom of mounting faoe 82(1), and ββttlng the difference between suoh Y coordinates equal to h. This produces (S-0.0Sx* l ) . | (5-O.OSx^ )R 1 h . - ' . : - ( β ) *H1 X 1 -9- Equations 5 and β can be solved iteratively, holding the height h constant, to determine the radii R* of the cylindrical bands on the receiver that will intercept, at. a constant concentration factor C, the energy reflected from a series of annular areas 88(1) on the primary concentrator. These radii can be inserted into the equations for the corresponding lines 84 to determine the vertical positions V- of the rings. A resulting receiver shape for a ooneentration factor of 500 is shown in FIGURE 4. The disposition of the oells determined In this manner minimizes the area of the photovoltaic cells required, and solves one of the most difficult problems encountered by photovoltaics in concentrator systems, i.e., nonuniform illumination of the cells. Further preferred aspects of the receiver design are illustrated in FIGURE 8. To provide maximum efficiency, the teohnlque for eleβtrioally lnteroonneoting the photovoltalo oells ehould not block any of the inoomlng radiation, or oause any of aotive areas of the oells to be otherwise unused. This may be aohieved by mounting the photovoltalo oells 26 on eaoh mounting face 62 such that active surfaces 72 receive the Incoming radiation 16, with electrodes 74 positioned in a reoess 100 formed in reoelver body 20. A pair of wires 104 are connected to the eleotrodes, and lead the electrical voltage signal out of the reoelver body Into a suitable utilization eirouit. FIGURE 8 Illustrates two additional preferred aspects of the present invention. A slot 110 Is formed in reoelver body 88, such that the inner end of slot 110 Is positioned behind mounting faoe 62. A plug 112 is then positioned in slot 110 In the position illustrated. Plug 112 may be formed of the aame material (e.g., copper) as base 20, and may be held in plaαe by a high temperature silver solder or the like. The portion of the elot above plug 112 Is then filled with a phase change material 114, for example, an lndlum-tln alloy. The portion of alot 110 below plug 112 forma a passageway 118 for a cooling fluid, suoh as the cooling passageway 30 sohematloally ahown in FIGURE 2. Phaβe ohange material 114 provides overheating protection for the reoelver. The phase change material la aeleoted such that It expβrlenoes a phase transition at a temperature that Is above the normal operating temperature range of the photovoltaic oells, but below the thermal damage threshold for the photovoltaic materials. Thus, when the temperature rises above the normal operating range to the phese transition temperature, the phase of the phase change material will change, thereby absorbing a quantity of heat, while maintaining the reoelver at a aubβtantlally constant temperature, until the phase ehenge Is complete. In general, the most convenient phaβe ohange material will -10- be those having melting points in the required temperature range. For galli arsenide solar cells, a typical maximum operating temperature is 85°C, whil typical thermal damage threshold temperature Is 350°C. Thus, a preferred ph change material would be a material such as an Indium-tin alloy, having a melti point in this range. The location of passageways 116 just behind the photovoltaic cells minimizes the distance that the heat must be conducted. If It is assumed that t heat conduction path through copper base 20 from oell 26 to cooli passageway 116 Is 0.1 inohes with a thermooonductlvity of 3.85 watts/cm, a that the heat transfer coefficient of the cooling fluid Is 6000 BTU/hr-ft 2 °F f water flowing at 3 ft/sec, the maximum temperature difference between the ce and the fluid will be 3.8°C. The main considerations for the hydraulic design of the cooling aystem a that the volume of flow be sufficient to carry away the excess heat with appropriate rise in temperature; that the flow velocity be adequate to produce film heat transfer coefficient sufficiently low to maintain the desired c operating temperature; and that the strength of the receiver will be adequate contain the combination of pressure produced by the fluid pump and t hydrostatic pressure produoed by. the rise in temperature of the fluid. T temperature rise in the fluid above an assumed input temperature of 30°C may calculated by assuming no eleotrloal load (the worst case). The result of t calculation is a thermal heat load of 23,350 watts, a desired temperature rlaβ 120°C for an exit temperature of 150°C, and a flow rate of .718 gallons p minute. The flow velocity is determined by the coolant ohannβl size and the flo volume. However, the velocity must also meet the constraint imposed by the fil heat transfer coefficient requirement. As deaoribed above* a flow velocity of ft/sec meets the heat transfer requirements. If the oron section o passageways 116 is 0.5 om, then the required height of the passageway may b determined to be about 1 cm. The hydrostatic pressure generated by water a 1S0°C la about 26 pel. The pumping pressure drop at a flow rate of 0.718 gallon per minute will be much less than the hydrostatic pressure. Copper walls 0.1 inc thick can easily withstand such pressures. Aa previous^ mentioned, tandem solar oells have been used to increase th efficiency of the solar-to-eieαtrioal oonveraion process. In a tandem oell design, primary cell Intercepts the incident solar energy, and converts a fraction of it t elβotrlαity. Some of the βoler energy not converted to electricity produces hea In the primary cell. However, a seoond unconverted portion Is transmitte -11- thrσugh the primary oell, and strikes a secondary oell that has a response band different from that of the primary cell. The secondary cell is used to generate electricity from those portions of the solar spectrum to which the primary cell is not ' responsive. A basic problem with tandem cell designs is that they require essentially double the cell area, but the eeoondary cell typioally only produces up to about 3096 of the eleotrloal energy produced by the primary cell. Thus the cost is approximately doubled, while only producing a 3096 increase in output. FIGURES 9-11 illustrate preferred embodiments of photovoltaic cell modules that overcome this problem. Referring initially to FIGURE 9, a first preferred cell module comprises a wedge-shaped substrate 120 having a pair of relatively large surfaces 122 and 124, and a pair of relatively small surfaces 126 and 128. Larger surfaces 122 and 124 are not parallel, but are oriented at a small angle (e.g., 10°) with respect to one another. Λ primary photovoltaic oell 130 is formed on surface 122, and a eeoondary photovoltaic cell 132 is formed on surface 126. The primary and secondary cells are preferably designed so that they have different response bands. By way of example, aubatrate 120 may oompriae gallium araenlde, primary cell ISO may comprise a gallium arsenide photovoltaic cell, and eeoondary oell 132 may comprise a silicon, GaSb, Ge, or GalnAs photovoltalo oell. The oell module shown In FIGURE 9 is positioned such that solar radiation 140 la incident upon primary oell 130. A large portion of the incident solar radiation that is out-of-band with respect to the primary cell is transmitted through the primary oell into substrate 120, Because of the angle between surfaces 122 and 124, this tranamltted radiation 142 is reflected back and forth between surfaces 122 and 124, toward surface 126. At surface 126, the radiation traveling through the aubatrate strikes secondary oell 132, and a portion of such radiation is oonverted into electricity. Substrate 120 in effect acts as a concentrator, since the radiation transmitted by the relatively large area primary cell is eonαentrated onto a muoh amaller eeoondary cell. To insure that the radiation tranamltted by the primary cell does not escape through surface 124, it is desirable to fabrioate substrate 120 from a relatively high refraotive index material, suoh as gallium araenlde, and/or to plaαe a reflective ooating on surface 124. Generally, in module shown in FIGURE 9, the area of secondary cell 132 may be 10-150 times smaller than the area of the primary cell. A second preferred embodiment of a multloell module Is Illustrated In FIGURE 10. This embodiment comprises substrate 140 having relatively large -12- aurfaces 142 and 144 and relatively small surfαoeβ 146 and 148. A first primar cell 150 and a second primary cell 152 are formed on surface 142, with cell 15 lying between cell 150 and surface 142. A smaller secondary cell 154 Is formed o surface 146. The three oells preferably all have different response bands. Fo example, substrate 140 may comprise silicon, primary cell 150 may compris gallium arsenide, primary cell 152 may comprise silicon, and secondary cell 15 may comprise GaSb, Gβ, or GaRnAs. In such a design, the first primary cel (GaAs) will transmit energy longer than about .9 microns, and the second primar cell (SI) will absorb and convert energy in the 0.9-1.1 micron band, and transmit energy longer than about 1.1 micron. Secondary cell 154 will have a response band at least partially in the range longer than 1.1 mlorons. For the modules of FIGURES 9 and 10, the uae of a high Index of refraction substrate will make the module very tolerant to angular variations in the incident radiation. For example, for a gallium arsenide substrate, the incident radiation can vary up to 60 degrees from normal, and still be trapped in the substrate. Conical prisms can also be used. Instead of wedges, if higher concentration factors are desired. FIGURE 11 illustrates how cell modules such as those shown in FIGURES 9 and 10 may be Integrated into a receiver of the type illustrated In FIGURE 4. The receiver body 160 is shaped aa in FIGURE 4, except that an additional out-back or reoess is formed in each mounting aυrface, the reoβss being shaped so as to receive oell module 162 auoh that the primary oell 164 of the cell module oocuples the tame position as photovoltaic oell 26 in the FIGURE 4 design. The overall receiver operates in a manner similar to that described above in connection with FIGURES 1-8. A further preferred embodiment for the receiver is schematically illustrated in FIGURE 12. Aa In the embodiment of FIGURE 2, the reoelver oompriaes a disk- shaped base 170 and prefllter 172. The photovoltaic cells are mounted to base 170, in a manner generally similar to that ahown in FIGURE 4. However, in this design, prefllter 172 comprises a solid ahell 174 having a nonspherical shape. Base 170 is mounted in an enclosure 176 having a transparent upper surface 178. A mounting block 180 underlying enclosure 176 includes passageway 182 through which a cooling fluid Is pumped. Pβssageway 182 communicates with the volume 190 between ahell 174 and enolosure 176, via oondult 184. Thus, as in the deaign of FIGURE 2, the ooollng fluid first cools base 170, end then passes through the prefllter, where it dlrβotly absorbs some of the out-of-band incoming radiation. However, In the design In FIGURE 12, the prefllter also functions as a lens, to ■ 13- focus and therefore further concentrate inoomlng solar radiation 192. In this case, the focusing provided will be a function of the index of refraction of the α.oollng fluid. Therefore in this design, it is generally desirable to use a high index fluid, suoh as a slllαon-based oil or mineral oil, to increase the focusing. Using the prefllter to act as a seαondary oonαentratlng lens can relieve requirements of the expensive primary concentrator, thereby lowering aystem costs. While the preferred embodiments of the invention have been illustrated and described, variations will be apparent to those skilled in the art. Accordingly, the scope of the Invention Is to be determined by reference to the following claims.

Claims: 2/01437 -14- The embodlmentβ of the Invention In which an βxolusive property or privilege Is claimed are defined as follows: 1. A solar energy system for converting solar energy Into electrical energy, the solar energy comprising electromagnetic radiation having wavelengths within a aolar band, the system comprising: photovoltaic means comprising an active area, a pair of electrodes, and means for absorbing electromagnetic radiation Incident upon the active area within a response band of wavelengths and for converting a portion of the electromagnetic radiation so absorbed into an electrical potential difference between the electrodes, the response band including a portion of the solar band; and prefiltering means comprising means for absorbing a substantial fraction of electromagnetic radiation incident upon the prefiltering means within a prefiltering band of wavelengths, at least a portion of the prefiltering band lying outside of the response band and within the solar band, the photovoltaic means and the prefiltering means being positioned such that at least a portion of the solar energy must pass through the prefiltering means to strike the active area of the photovoltaic means, whereby some of the solar energy outside the response band is absorbed by the prefiltering means and is not incident upon the photovoltaic means. 2. The aystem of Claim 1, wherein the prefiltering means comprises means forming a conduit and means for pumping a fluid through the conduit, the conduit being positioned such that said portion of the solar energy must pass through the conduit and through the fluid contained therein to strike the active area of the photovoltaic means. 3. The system of Claim 2, further comprising a base having a passageway formed therein, the photovoltaic means being mounted to the base, the passageway communicating with the conduit such that said fluid can be pumped serially through the passageway and then through the conduit. 4. The system of Claim 2, wherein the prefiltering means Is shaped and formed so as to focus the solar energy onto the photovoltaic means. -15- 5. The system of Claim 1, wherein the response band extends between a short wavelength limit and a long wavelength limit, and wherein the prefiltering means absorbs a substantial fraction of the electromagnetic radiation that lies within the solar band and that has a wavelength longer than the long wavelength limit. 6. The system of Claim 1, wherein the prefiltering means has an approximately hemispherical shape, and wherein the photovoltaic means is positioned approximately at the center of said hemispherical shape. 7. The system of Claim 1, wherein the prefiltering means Is shaped and ormed so as to focus the solar energy onto the photovoltaic means. 8. The solar energy system of Claim 1, further comprising primary concentrator means for receiving the solar energy and for focusing the solar energy towards the prefiltering means, the prefiltering means being positioned such that at least a portion of the solar energy focused by the primary concentrator means towards the prefiltering means must pass through the prefiltering means to strike the active area of the photovoltaic means. 9. The system of Claim 8, wherein the prefiltering means comprises means forming a conduit and means for pumping a fluid through the conduit, the conduit being positioned such that said portion of the solar energy focused by the primary concentrator means must pass through the conduit and through the fluid contained therein to strike the active area of the photovoltaic means. 10. The system of Claim 9, further comprising a base having a passageway formed therein, the photovoltaic means being mounted to the bate, the passageway communicating with the conduit such that said fluid can be pumped serially through the passageway and then through the conduit. 11. The system of Claim 9, wherein the prefiltering means is shaped and formed so as to further focus the solar energy focused towards the prefiltering means by the primary concentrator means. 12. Λ solar energy system for converting solar energy Into electrical energy, the system comprising! primary concentrator means for receiving incident solar energy and for focusing the solar energy towards a focal spot; and a receiver positioned substantially at the focal spot, the reoeiver comprising a plurality of photovoltaic cells, each photovoltaic cell comprising an active area, a pair of electrodes, and means for absorbing electromagnetic radiation incident upon the active area and fo -converting a portion of the electromagnetic radiation so absorbed into an electrical potential difference between the electrodes; the photovoltaic cells being positioned such that each photovoltaic cell receives approximately the same solar energy flux from the primary concentrator means. 13. The system of Claim 12, wherein the photovoltaic cells are arranged in a plurality of rings, each ring comprising a plurality of photovoltaic cells positioned side-by-side with respect to one another, the photovoltaic oelli of each ring 1 lying at a constant radius R | from a central axis common to all rings, the active surface of each photovoltaic cell facing away rom the central axis. 14. The system of Claim 13, wherein all photovoltaic cells and all rings have the same height as measured along said central axis. 15. The system of Claim 14, wherein the photovoltaic ceils in each ring are positioned to receive the solar energy focused by a different annular area on the primary concentrator, said annular areas being contiguous to one another. 16. The system of Claim 12, wherein all photovoltaic ceils have a common response band within which the photovoltaic cells can convert electromagnetic radiation into said potential difference, wherein said solar energy comprises electromagnetic radiation having wavelength within a solar band, and wherein the system further comprises prefiltering means comprising means for absorbing a substantial fraction of electromagnetic radiation incident upon the prefiltering means within a prefiltering band of wavelengths, at least a portion of the prefiltering band lying outside of the response band and within the solar band, the receiver and the prefiltering means being positioned such that at least a portion of the solar _energy must pass through the prefiltering means to strike the active area of the photovoltaic cells, whereby some of the solar energy outside the response band is absorbed by the prefllterlnp means and it not incident upon the photovoltaic cells. 17. A p otovolluic module for converting soiar energy into eieetrieβl energy, the solar energy comprising electromagnetic radiation having wavelengths within a solβr band, the module comprising: a substrate having first and second surfaces, the first surface being substantially larger than the second surface; ; a first photovoltaic cell mounted to the first surface, the first photovoltaic cell comprising a first active area, a pair of first electrodes, and first means for absorbing, electromagnetic radiation incident upon the first active area within a first response band of wavelengths and for converting a portion of the electromagnetic radiation so absorbed Into an electrical potential difference between the first electrodes, the response band Including a portion of the solar band, the module being positlonable such that the solar energy is incident upon the first active area, whereby a portion of tht-solar energy outside the first response oand is transmitted through the first photovoltaic cell Into the substrate; a second photovoltaic cell mounted to the second surface, the second photovoltaic cell comprising a second active area, a pair of second electrodes, and second electrode means for absorbing electromagnetic radiation incident upon the second active area within a second response band of wavelengths different from the first response band and for converting a portion of the electromagnetic radiation so absorbed Into an electrical potential difference between the second electrodes, the second response band including a portion of the solar band, the active area of the second photovoltaic cell being substantially smaller than the active area of the first photovoltaic cell; and the substrate being formed and shaped such that a portion of the solar energy transmitted by the first ohotovoltaic cell Into the substrate is directed onto the active area of the second photovoltaic cell. 18. The photovoltaic module of Claim 17, wherein the first photovoltaic cell comprises an inner photovoltaic cell mounted to the first surface and an outer photovoltaic cell mounted to the inner photovoltaic cell, the outer and inner photovoltaic cells having different response bands. 19. The photovoltaic module of Claim 17, wherein the substrate comprises a wedge-shaped body having a pair of opposite, nonparallel, relatively long surfaces and e pair of opposite relatively short surfaces, the first surface comprising one of the relatively long surfaces, and the second surface comprising one of the relatively short surface*!. 20. A solar energy system for converting solar energy into electrical energy, the system comprising: primary concentrator means for receiving incident solar energy and for focusing the solar energy towards a focal spot! and a receiver positioned substantially at the focal spot, the receiver comprising a base and a plurality of photovoltaic cells mounted to the base, the receiver being shaped so as to form one or more enclosures containing a phase change material having a phase change temperature that is higher than a maximum operating temperature of the photovoltaic cells and smaller than a thermal damage threshold for the photovoltaic cells.

Inventors: HORNE WILLIAM E, HORNE, WILLIAM, E.
Applicants: UNITED SOLAR TECHNOLOGIES INC, UNITED SOLAR TECHNOLOGIES, INC.
Application year: 1992
Main classification: H01L31
IPC codes: F24J2, G02B17, H01L31