WO2012125446A2

WO2012125446A2 - Solar electrical and thermal generators with curved reflectors

Info

Publication number: WO2012125446A2
Application number: PCT/US2012/028439
Authority: WO
Inventors: Aaron Bent; Bed Poudel; James Christopher Caylor; Stefan Abrecht
Original assignee: Gmz Energy Inc.
Priority date: 2011-03-11
Filing date: 2012-03-09
Publication date: 2012-09-20
Also published as: WO2012125446A3

Abstract

Various embodiments provide improved devices for solar electric and thermal energy conversion by including a reflector. The various embodiments may include solar electric conversion elements, solar thermal conversion elements, or both. The reflector may increase total energy output of a solar energy conversion device by concentrating solar radiation on a radiation absorber coupled with one or more conversion elements. Also, the reflector may prevent losses from gaps that would otherwise be present between solar energy conversion devices. In different embodiments, the reflector may be asymmetric or symmetric. Further, the reflector may be optimized in dimension and arrangement for a particular location. Optimization may be for year round performance and thereby avoid the need to shift the reflector or conversion device with the changing seasons.

Description

Solar Electrical and Thermal Generators with Curved Reflectors RELATED APPLICATIONS

[0001] This application claims the priority benefit of U.S. Application 61/451,877, filed on March 11, 2011, which is incorporated herein in its entirety.

BACKGROUND

[0002] Solar thermoelectric converters are known in the art. These converters rely upon the Seebeck effect to convert temperature differences into electricity. Solar energy may directly or indirectly heat a portion of the thermoelectric converter to create the necessary temperature difference. The efficiency of the energy conversion depends upon the temperature difference across the thermoelectric converter. Greater temperature differences allow for greater conversion efficiency.

[0003] Solar thermal converters are also known. Solar thermal converters convert solar energy into thermal energy or heat. Some countries have widespread use of roof-top hot- water systems based on solar thermal converters. In addition to functioning as hot water systems, solar thermal converters have been used to generate electrical energy by driving mechanical heat engines with steam generated from the solar thermal converter. In a solar thermal converter, one or more fluid conduits are provided in direct thermal contact with a solar radiation absorbing surface. The surface absorbs solar radiation and transfers heat to the conduits. The transferred heat raises the temperature of the fluid, such as oil, liquid salt or water flowing through the conduit. The heated fluid is then used in a power generator, such as a steam driven power generator to generate electricity. The term "fluid", as used herein includes both liquids and gases.

SUMMARY

[0004] Embodiments may include a device comprising an evacuated enclosure, a reflector configured to reflect solar radiation onto a radiation absorber, the radiation absorber being disposed in the enclosure and positioned to receive solar radiation from the reflector, the radiation absorber having a front surface and a back surface, the front surface being adapted for exposure to solar radiation so as to generate heat, and at least one solar conversion device disposed in the enclosure and thermally coupled to the absorber.

[0005] Further embodiments may include the at least one solar conversion device which comprises a thermoelectric converter and/or a solar thermal converter.

[0006] Further embodiments may include an energy conversion method, comprising receiving solar radiation at a solar absorber located in an evacuated enclosure, wherein at least some of the solar radiation is reflected from a reflector located outside of the evacuated enclosure, receiving heat from the solar absorber at an at least one solar conversion device, and generating electricity from the at least one solar conversion device.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate exemplary embodiments of the invention, and together with the general description given above and the detailed description given below, serve to explain the features of the invention.

[0008] FIG. 1 is a schematic side cross sectional view of a solar energy conversion module within an evacuated enclosure.

[0009] FIG. 2 is a schematic front cross sectional view of the solar energy conversion module within an evacuated enclosure in FIG. 1.

[0010] FIG. 3 is a schematic side cross sectional view of a solar energy conversion module within an evacuated enclosure including a fluid conduit.

[0011] FIG. 4 is a schematic front cross sectional view of solar energy conversion modules within evacuated enclosures under sunlight. [0012] FIG. 5 is a schematic front cross sectional view of solar energy conversion modules within evacuated enclosures under sunlight at an incidence angle.

[0013] FIG. 6 is a schematic front cross sectional view of tilted solar energy

conversion modules within evacuated enclosures under sunlight at an incidence angle.

[0014] FIG. 7 is a schematic front cross sectional view of tilted solar energy

conversion modules within evacuated enclosures under sunlight.

[0015] FIG. 8 is a schematic front cross sectional view of a solar energy conversion module within an evacuated enclosure coupled with a reflector.

[0016] FIG. 9 is schematic perspective view of a solar energy conversion module within an evacuated enclosure coupled with a reflector.

[0017] FIG. 10 is a schematic front cross sectional view of a solar energy conversion module within an evacuated enclosure coupled with a reflector and showing conversion elements.

[0018] FIG. 11 is a schematic front cross sectional view of a solar energy conversion module within an evacuated enclosure coupled with a reflector under low sun elevation light.

[0019] FIG. 12 is a schematic front cross sectional view of a solar energy conversion module within an evacuated enclosure coupled with a reflector under high sun elevation light.

[0020] FIG. 13 is a schematic front cross sectional view of a solar energy conversion module within an evacuated enclosure coupled with a symmetric reflector.

[0021] FIG. 14 is a schematic front cross sectional view of a solar energy conversion module within an evacuated enclosure coupled with a symmetric reflector and showing conversion elements. [0022] FIG. 15 is a schematic front cross sectional view of a solar energy conversion module within an evacuated enclosure coupled with a symmetric reflector under sunlight.

[0023] FIG 16A and 16B are charts of power per enclosure for a system with a reflector and without a reflector, respectively, for varying incidence angles of sunlight.

[0024] FIG 17 A and 17B are charts of power per absorber area for a system with a reflector and without a reflector, respectively, for varying incidence angles of sunlight.

[0025] FIG 18A and 18B are charts of efficiency for a system with a reflector and without a reflector, respectively, for varying incidence angles of sunlight.

[0026] FIG. 19 is a diagram of a vertically oriented solar device rack.

[0027] FIG. 20 is a diagram of a solar device rack rotated to various orientations

[0028] FIG. 21 is a diagram of a solar device rack integrated into a building.

DETAILED DESCRIPTION

[0029] The various embodiments will be described in detail with reference to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. References made to particular examples and implementations are for illustrative purposes, and are not intended to limit the scope of the invention or the claims.

[0030] The various embodiments provide improved devices for solar electric and/or thermal energy conversion by including a reflector. The various embodiments may include solar electric conversion elements, solar thermal conversion elements, or both. The reflector may increase total energy output of a solar energy conversion device by concentrating solar radiation on a radiation absorber coupled with one or more conversion elements. Also, the reflector may prevent losses from gaps that would otherwise be present between solar energy conversion devices. In different

embodiments, the reflector may be asymmetric or symmetric. Further, the reflector may be optimized in dimension and arrangement for a particular location. Optimization may be for year round performance and thereby avoid the need to shift the reflector or conversion device with the changing seasons. A reflector may also capture more diffuse light than a solar energy conversion device's radiation absorber alone.

[0031] Multiple methods exist for generating electricity from solar energy. Various embodiments may include solar electric conversion elements, such as thermoelectric conversion elements. Thermoelectric conversion relies on the Seebeck effect to convert temperature differences into electricity. Thermoelectric converters operate more efficiently under greater temperature differences. To prevent unnecessary heat loss or heat transfer between components that may reduce the temperature difference of the thermoelectric converters, solar energy conversion modules may include one or more forms of insulation. One manner of insulating the various components is by placing the module in a vacuum, such as an evacuated enclosure.

[0032] Examples of solar energy conversion modules with an evacuated enclosure are illustrated in FIGS. 1 and 2. In some embodiments, the solar energy conversion module 100 includes a thermoelectric device 102 in an evacuated enclosure 104 (e.g., a glass tube) that extends along a longitudinal axis. As shown, the evacuated enclosure may be cylindrical, i.e., has a generally circular cross section, with a tapered end. However, in various embodiments, any elongated shape may be used. In some embodiments, the enclosure has a substantially oval cross section. In other

embodiments, the cross section may be square, rectangular, polygonal, irregular, etc. On or more of the ends of the enclosure may be a tapered end, a blunt end, a rounded end (e.g. including a hemispherical portion), etc.

[0033] Electrically conductive leads 114 and 116 are also depicted, which can provide appropriate electrical coupling within and/or between thermoelectric converters, and can be used to extract electrical energy generated by the converters 110.

[0034] In some embodiments, the thermoelectric device 102 is arranged in a substantially planar configuration. For example, in some embodiments, the separation between corresponding points on the top and bottom major surfaces of the device 102 deviates by less than 10% over the extent of the device. In some embodiments, the device has a curvature of less that 10% of the thickness of the device 102. The thermoelectric device 102 may include a top (hot side) thermal or heat absorber 106, a bottom (cold side) support structure 108, and thermoelectric converters 110 disposed there between (as shown, pairs of p-type and n-type legs of thermoelectric material). For some applications, e.g., those in which the solar conversion module 100 is used without a solar tracking system, a planar configuration is advantageous, as it may exhibit more uniform heating as the Sun moves across the sky during the day and over the course of the year.

[0035] The absorber 106 may be adapted for exposure to solar radiation, either directly or from a reflector. Although in this example the absorber is substantially flat, in other examples it can be curved. The solar radiation impinged on the absorber 106 can generate heat which can be transferred to the thermoelectric converters 110. More specifically, in this example the absorber 106 can be formed of a material that exhibits high absorption for solar radiation.

[0036] Thermoelectric converters 110 may be thermally coupled to the back of the absorber 106 to receive at least a portion of the generated heat. In this manner, one end of the converters is maintained at an elevated temperature. With the opposed end of the converters exposed to a lower temperature, the thermoelectric converters can generate electrical energy.

[0037] The thermoelectric converters themselves can be made from a variety of bulk materials and/or nanostructures. The converters preferably comprise plural sets of two converter elements— one p-type and one n-type semiconductor converter post or leg which are electrically connected to form a p-n junction. The thermoelectric converter materials can comprise, but are not limited to, one of: Bi₂Te₃, Bi₂Te₃__x Se_x (n-type)/Bi_x Se₂__x Te₃ (p-type), SiGe (e.g., Si₈oGe₂₀ ), PbTe, skutterudites, Zn₃Sb₄, AgPb_mSbTe_2+m, Bi₂Te₃ /Sb₂Te₃ quantum dot superlattices (QDSLs), PbTe/PbSeTe QDSLs, PbAgTe, and combinations thereof. The materials may comprise compacted nanoparticles or nanoparticles embedded in a bulk matrix material.

[0038] A heat conducting element 112 may extend between the support structure 108 and the evacuated enclosure which transfers heat away from the support structure to the enclosure, thereby helping to maintain the temperature differential between the hot and cold sides of the thermoelectric converters 110. For example, heat conducting element 112 may include any thermally conductive material, such as a metal (e.g., copper) or metal coated member, extending from the support structure 108 to the evacuated enclosure. The heat conducting element 112 may provide mechanical support for the thermoelectric device 102 within the enclosure 104, e.g., as shown in FIG. 2. In some embodiments, the heat conducting element 112 may be a solid member which substantially fills the lower half of the evacuated enclosure.

[0039] The heat conducting element 112 may include a curved portion which is conformal to a portion of the enclosure 104, e.g., as shown in FIG. 2. Conformal means that the element portion physically contacts and assumes the shape of the surface. In some embodiments the heat conducting element 112 may include a portion which is coated (e.g. metalized) directly on to the interior surface of the enclosure 104. Such a coating may be formed and/or patterned using any suitable technique to provide electrically and/or thermally isolated portions. One technique includes plating (e.g., electroplating) or depositing (e.g. using chemical vapor deposition techniques) a material layer, and then using lithographic and etching processes to pattern the material layer.

[0040] In other embodiments, heat conducting element 112 may contact the enclosure at one or more points or regions. For example, one or more "legs" may extend from the thermoelectric device to optional flat "foot" portions contacting the enclosure. The foot portions may include regions coated or metalized onto the enclosure. There may be of any number of legs and they may have any suitable shape (e.g. thin, thick, tapered, irregular, etc.). In some embodiments the legs may extend along the direction of the longitudinal axis, thereby forming fin-like members. [0041] Various alternative embodiments of this arrangement are discussed in pending application PCT/US 2010/036607 filed on May 28, 2010 to GMZ Energy, Inc. for "Thermoelectric System and Method of Operating Same," which is herein

incorporated by reference in its entirety.

[0042] Alternatively or in addition to the thermoelectric conversion elements, various embodiments may include solar thermal conversion elements, such as a solar fluid heating device or a solar thermal to electrical plant. A solar thermal to electrical conversion plant (which can be referred to simply as a "solar thermal plant") includes but is not limited to Rankine based and Stirling based plants, and includes trough, tower, and dish shaped plants, as will be described below. Such a system generates and/or co-generates solar electrical energy and/or solar thermal energy. Specifically, if the solar thermal conversion device is a solar fluid heating system, such as a solar hot water heating system, then the system can provide cogeneration of electricity using the solar thermoelectric device, and hot water for a facility, such as a building, using the solar hot water system.

[0043] In various embodiments that includes both the thermoelectric device and the solar fluid heating system, the fluid conduit may be physically separated and thermally decoupled from the solar radiation absorbing surface by the poorly thermally conducting thermoelectric material legs or posts. In this configuration, a proper temperature difference may be created across the thermoelectric legs or posts, and consequently, between the solar absorbing surface and the fluid conduits.

Alternate embodiments may include only the solar fluid heating device in which the fluid conduit is placed in thermal contact with the solar radiation absorbing surface for optimum transfer of the heat from the absorbing surface to the fluid.

[0044] The thermoelectric device generates electricity due to a temperature difference between its cold side and its hot side which is in thermal contact and optionally in physical contact with the absorbing surface. As used herein, the terms thermal contact or thermal integration between two surfaces means that heat is efficiently transferred between the surfaces either because the surfaces are in direct physical contact or are not in direct contact but are connected by a thermally conductive material, such as metal, etc.

[0045] If the fluid conduit of the solar thermal conversion device is also placed in thermal contact with the solar absorber (also referred to as a solar absorbing surface), then the fluid conduit will act as a heat sink. This will significantly reduce the temperature difference between the hot and cold sides of the thermoelectric device and would thus significantly decrease the efficiency of the thermoelectric device.

[0046] In contrast, if the fluid conduit is placed in thermal contact with the cold side of the thermoelectric device, then the fluid conduit will act as a heat sink and increase the temperature difference between the hot and cold sides of the thermoelectric device and thus improve the efficiency of the thermoelectric device. Since the thermoelectric converters (e.g., semiconductor legs or posts) of the thermoelectric device are poor thermal converters, the fluid conduit is not in thermal contact (i.e., not thermally integrated) with the solar absorber surface. Thus, the fluid conduit does not act as a heat sink for the solar absorber surface and does not interfere with the operation of the thermoelectric device.

[0047] Furthermore, the cold side of the thermoelectric device is still sufficiently warm (i.e., is above room temperature) to heat the fluid, such as water, salt, or oil, inside the fluid conduit to a desired temperature. For example, for a hot water heating system, the cold side of the thermoelectric device may be maintained at a temperature of about 50°C to about 150°C, such as for example less than 100°C, preferably 50°C to 70°C, which is sufficiently high to heat water to about 40°C to about 150°C for home, commercial or industrial use. Thus, the water heated by the cold side of the

thermoelectric device may be provided from the fluid conduit into the facility as hot water for various uses, such as hot water for showers or sinks, hot water or steam for use in radiators for room heating, etc. Alternatively, if the fluid, such as oil or salt is sufficiently heated, then it may be used in a thermal power plant to generate electricity. For example, the oil or salt may be heated above its boiling point.

Alternatively, the oil or salt may be heated below its boiling point, but to a sufficiently high temperature so that it is used to heat water into steam, which is feed into steam turbine to generate electricity.

[0048] FIG. 3 illustrates a solar energy conversion module 300 with both solar electric and solar thermal conversion elements. Similar to FIG. 1, module 300 may include a thermoelectric device 102 to generate electricity from a temperature difference.

Module 300 may also include a fluid conduit 304. As discussed above, the fluid conduit 304 may be attached to the cold side of thermoelectric device 102. The fluid conduit 304 may thereby act as a heat sink to increase the temperature difference across the thermoelectric device and improve electricity generation.

[0049] Fluid conduit 304 may be connected to module 302. Module 302 may be any device operative to use the heat transferred by the fluid medium of conduit 304, such as a heat exchanger or generator.

[0050] FIG. 4 illustrates a cross sectional view of an example arrangement of enclosures 104 each with a solar energy device 402 inside. Devices 402 are illustrated as lines for simplicity but may include a thermal absorber 106 as well as

thermoelectric devices 102, solar thermal devices, such as a fluid conduit 304, or both. In the arrangement of FIG. 4, the solar energy devices 402 are horizontal such that most of the solar radiation with an incidence angle of zero degrees, as illustrated by the vertical lines, may be collected by the absorber of devices 402. However, FIG. 5 illustrates that the same configuration may only absorb a fraction of solar radiation with an incidence angle of negative sixty degrees. This angle is measured with zero degrees as perpendicular to the ground. Such an incidence angle may be the result of a low sun elevation, which may occur depending on the time of day, season, and location (e.g., in winter in the Northern Hemisphere). Lack of absorption during low sun elevation may be a serious problem because low sun elevations occur in winter which may be when more thermal energy is desired.

[0051] One possible solution to increase absorption at low elevation sunlight is to tilt the solar energy devices 402 as illustrated in FIG. 6. However, the enclosures 104 may need to be spaced further apart to avoid shading of neighboring devices 402. FIG. 7 illustrates the problem with this arrangement though. For high sun elevations, increased spacing leads to increased losses of radiation in the gaps between the enclosures 104.

[0052] The problems of this trade off may be avoided by optically coupling reflectors with the absorbers in the enclosures 104. FIG. 8 illustrates a device 800 with a reflector 802 and an enclosure 104 including a tilted solar energy device 402. FIG. 9 illustrates device 800 from another perspective and shows how the reflector 802 may run lengthwise with enclosure 104 and absorber 106. FIG. 9 also illustrates optional support(s) 902 for device 800. The support(s) may comprise a rack, a roof, a stand, or any other suitable support that supports the enclosure horizontally (i.e., parallel to ground) or tilted at an angle of 1-90 with respect to the horizontal. The reflector 802 may be concave and angled to reflect incident solar radiation on the solar device 402 (e.g., on the absorber of the solar device). One non-limiting example of a reflector is a compound parabolic collector (CPC). Other reflector types, such as flat mirrors, may also be used. Dimensions and positioning of the reflector 802 and solar device 402 may be optimized depending on the anticipated range of incident sunlight geometry during the year. Angle X 806 may be defined as a maximum incidence angle of sunlight, e.g., the angle of usable solar radiation for a reflector. This angle will depend on the anticipated range of incident solar radiation throughout a year. Thus, the device 800 is preferably stationary (i.e., does not move or rotate) as a function of time during the year, but is still able to maximize usable solar radiation throughout the year. For any given location, the incidence angle of sunlight may vary through this range across the seasons and each day. North and South are labeled in FIG. 8 under the assumption that the Sun rises in the East and sets in the West and that the lengths of the modules are aligned in an East- West orientation. Under these assumptions, the cross section and the incidence angle would be aligned with a North-South direction.

[0053] Angle X 806 will depend on the location of the device 800. The Sun would rise each day and reach a certain peak elevation around midday. This elevation would vary with the seasons, being lower in winter and higher in the summer in the Northern Hemisphere and vice versa in the Southern Hemisphere. The actual elevation would depend on the location of the device 800. For example, if the device 800 is located in San Diego, U.S.A., or somewhere of similar latitude of about 32° N, the elevation would be highest in summer at an angle of roughly eighty-one degrees. This angle may be measured with zero degrees being parallel to the ground facing south as opposed to the previously mentioned angles measured with zero degrees being perpendicular to the ground. The midday elevation may be lowest in the winter at an angle of about thirty-two degrees. At higher latitudes, such as in Boston, U.S.A., the peak elevations would be lower. Conversely, near the tropics, the elevations would be higher, such as about a hundred degrees. The same would be true in the Southern Hemisphere with lower elevations further from the Equator.

[0054] Based on this range of sun elevations and corresponding incidence angles, the dimensions of the device 800 may be set. The solar energy device 402 may be tilted an angle Y 812 to absorb the low incidence angle radiation of a location. In other words, angle Y 812 is the tilt angle of a flat absorber 106 of device 402 around the longitudinal axis of the enclosure with zero degrees being horizontal or parallel to the ground. In some embodiments, the enclosure 104 may not be level with the ground, for example if the enclosure is angled by a support structure 902 (e.g., tilted rack) and/or on a slanted roof. In these embodiments, zero degrees may be the plane representing horizontal if the enclosure was parallel to the ground.

[0055] To absorb the high incidence radiation, the acceptance angle Z 808, which may be defined as the range of reflected incident light angles, may be set by adjusting the distance 804 between the reflector 802 and the edge of the solar energy device 402 furthest from the reflector. Ray 810 illustrates light with the highest incidence angle that will strike the solar energy device 402 after being reflected, and therefore represents the limit of acceptance angle Z 808.

[0056] In some embodiments, the acceptance angle Z 808 is high enough to reflect solar radiation including the maximum incidence angle X 806. Thus, for the tropics example, angle X 806 would be just over one hundred degrees and angle Z 808 may be set to collect and reflect light the solar energy device 402 would not absorb. If device 402 is set at a tilt angle Y 812 of forty- five degrees to accept low incidence radiation, the spacing between enclosures needed to avoid shading by neighboring devices would create gap losses. Reflectors may prevent these losses. The tilted devices 402 collect the low incidence radiation, so the reflectors may collect and reflect the high incidence radiation to devices 402. In some embodiments, this may include radiation with incidence angles above the complimentary angle of the tilt angle Y 812 up to the maximum incidence angle X 806.

[0057] Therefore, continuing the tropics example, a tilt angle Y 812 of forty- five degrees and a maximum incidence angle X 806 of one hundred degrees means an acceptance angle Z 808 of about fifty-five degrees may be desired to cover the full range of incident radiation.

[0058] In other embodiments, the angles will not follow this relationship and may vary independently or in other relationships.

[0059] The reflector 802 may vary in length and angle, but will preferably be long enough to leave little gap between enclosures and angled according to acceptance angle Z 808.

[0060] Depending on the limits of the conditions at the location of the device 800, the angles may be shifted to optimize concentration. For example, a lower acceptance angle Z 808 may allow a higher solar radiation concentration. However, lower acceptance angles may only be preferred at higher latitude locations with lower maximum incidence angle X 806, such as northern Europe or North America.

Further, the tilt angle Y 812 may be increased yielding a higher optical concentration. This may also lead to a higher acceptance angle Z 808.

[0061] In various embodiments, the angles may be shifted for other effects. For example, a high acceptance angle may better for diffuse light, e.g. sunlight reflected from snow or clouds as opposed to direct sunlight. A high acceptance angle may provide a larger surface to collect or concentrate the diffuse light. [0062] Particular ranges of angles may be used in certain preferred embodiments. In some embodiments, angle Y 812 may range from 15 to 85 degrees. At relatively high latitudes where the maximum angle X 806 reaches below 75, such as about 71.5 degrees, angle Y 812 may range between 30 and 75 degrees and angle Z 808 may range between 40 and 60 degrees. For example, angle Y 812 may be 45 degrees and angle Z 808 may be 45 degrees. At lower latitudes, where the maximum angle X 806 may reach above 75, such as about 80 degrees, angle Y 812 may range between 50 and 85 degrees and angle Z 808 may range between 45 and 65 degrees. For example, angle Y 812 may be 60 degrees and angle Z 808 may be 50 degrees.

[0063] FIG. 10 illustrates the positioning of the absorber 106, thermoelectric converters 110, and fluid conduit 304 of a device 800 with respect to a reflector 802. The cold side of the thermoelectric device 102 may be thermally coupled with the fluid conduit 304 as discussed above. Solar radiation may impinge the absorber 106 directly and/or via reflection by reflector 802. This radiation may heat the absorber, thereby allowing the thermoelectric converters 110 to generate electricity. The heat may also pass to fluid conduit 304 to be transported elsewhere by a fluid medium. The enclosure 104 and device 102 may be the same as illustrated in FIGS. 1-3.

[0064] FIGS. 11 and 12 illustrate a device 800 in different seasons. In FIG. 11, the Sun is at a low elevation, such as in winter in the Northern Hemisphere. The sunlight has a low incidence angle and is illustrated striking the absorber of solar energy device 402 directly. In contrast, in the summer, the Sun is at a high elevation in FIG. 12 and the sunlight has a high incidence angle. FIG. 12 illustrates how some sunlight strikes the device 402 directly. However, the device 402 is tilted, and therefore would not convert sunlight in the gaps between the enclosures 104. This problem is avoided by the reflectors 802 located in the gaps collecting and reflecting the sunlight to device 402 that would otherwise be lost.

[0065] FIGS. 8-12 illustrate devices with an asymmetric CPC reflector positioned to reflect sunlight to one side and a tilted device 402 positioned to collect reflected sunlight from a CPC reflector located only on one side of the device 402. Various alternative embodiments may include a device 1300 with a symmetric CPC reflector 1302 as illustrated in FIG. 13. A symmetric reflector 1302 may collect and reflect solar radiation from both sides of a tubular enclosure 104. The solar radiation may be reflected on to either side of solar energy device 402, which may be positioned vertically (e.g., perpendicular to the ground or horizontal) and contain absorbers positioned on both the front and back sides of device 402 to received the reflected radiation from both sides. In further embodiments, solar energy device 402 may be tilted -10 to 10 degrees from vertical.

[0066] FIG. 14 illustrates an example embodiment of a device 1300 with a symmetric reflector 1302 and with solar energy conversion elements. Solar energy device 402 may include absorbers 106 A, 106B on both front and back sides as illustrated in FIG. 14. Solar energy device 402 may also include thermoelectric converters 110 and fluid conduit 304 between absorbers 106 A, 106B.

[0067] FIG. 15 illustrates a device 1300 with impinging solar radiation. As shown, the symmetric reflector 1302 may be shaped to reflect and concentrate the solar radiation onto both sides of the solar energy device 402 inside an enclosure 104. Unlike asymmetric reflectors that are positioned on only one side of the enclosure, symmetric reflectors 1302 may be joined with neighboring reflectors. Symmetric reflectors may even be manufactured as a single curved sheet of many reflectors.

[0068] A symmetric reflector device 1300 may be extended to provide a higher acceptance angle and collect and reflect a greater amount of diffuse light than an asymmetric reflector. Symmetric reflectors may also allow the devices 1300 to be mounted east-west or north-south or on tilted or flat roofs. However, asymmetric reflectors may provide some gains in efficiency due to relatively lower absorber surface area.

[0069] Test results comparing systems with and without reflectors have demonstrated the increased power and efficiency achieved with reflectors. FIGS. 16A and 16B compare power (in Watts) per tubular enclosure over different angles of incident sunlight. Each tube is 70mm in outer diameter, 2mm thick, and 1.613m in length and contains a solar absorber coupled with a solar conversion device. FIG. 16A shows power output per tube for a standard configuration without reflectors and with horizontal absorbers similar to the system in FIG. 4. FIG. 16B shows the substantially higher power output per tube in a system with a tilted absorber and an asymmetric reflector, similar to the configurations in FIGS. 11 and 12. FIGS. 16A and 16B each include a model (i.e., computer simulation) line and three separate tests (lines T1-T3) to insure consistency. The model results are higher than the actual test results, but actual results may come closer to the model as the test systems are optimized.

[0070] FIGS. 17A and 17B illustrate power per absorber area for the same

configurations as FIGS. 16A and 16B respectively. Absorber area was calculated based on an absorber width of 62mm. FIGS. 17A and 17B include a plot of power in units of W/m² for three tests and a model varying over different angles of incident sunlight. Power output per area with a reflector, as seen in FIG. 17B, is higher than without a reflector, as seen in FIG. 17 A, for all angles of incident sunlight.

[0071] FIGS. 18A and 18B illustrate system efficiency over different angles of incident sunlight, where system efficiency is defined as the amount of power produced per available area. Available area is defined as the area between the cross sectional centers of neighboring enclosures. Thus, available area for the configuration in FIG. 4 (without a reflector) would be smaller than for the configuration in FIG. 12 (with a reflector) because the enclosures are closer together without intervening reflectors. The enclosures in FIG. 12 are spread further apart with reflectors in between, and therefore the configuration of FIG. 12 has a higher available area. Despite a higher area, the system with a reflector, as shown in FIG. 18B, has a greater efficiency than the system without a reflector, as shown in FIG. 18 A, for most angles of incident sunlight.

[0072] Further embodiments may include various systems or arrangements of solar energy conversion modules, such as arrangements of a plurality of devices 800 with asymmetrical reflectors or devices 1300 with symmetrical reflectors. These arrangements may include one or more solar energy conversion modules in racks or other structures, such as support structure 902.

[0073] Further embodiments may include vertical arrangements of solar energy conversion modules, such as that shown in FIG. 19. FIG. 19 illustrates a solar device rack 1900 for vertically mounting several devices 800. One or more support structures 1902 may support one or more evacuated enclosures 104 each with a solar energy device 402 inside on the rack 1900. Additional support structures 1904 may support a reflector 802 beside each of the evacuated enclosures 104. Alternatively, the same support structure may be used to support both the enclosure 104 and its associated reflector 802. Reflector 802 may be asymmetric, such as a compound parabolic collector, or symmetric. Preferably, the rack extends in a non-zero angle with respect to a horizontal direction, and each enclosure 104 and reflector 802 pair attached to the rack 900 is attached above rather than to the side of the adjacent enclosure / reflector pair. Each enclosure 104 and associated reflector 802 of each pair are elongated in the horizontal direction, with the reflector 802 located above the enclosure 104.

[0074] Dimensions and positioning of the reflector 802 and solar device 402 may be optimized depending on the anticipated range of incident sunlight geometry during the year. For example, they may be positioned relative to each other such that most sunlight directly strikes the solar energy device 402 while some remaining sunlight is reflected from the reflector 802 onto the solar energy device 402.

[0075] A solar device rack 1900 arranged vertically may be used in areas with sunlight having a low incidence angle, such as areas farther from the Equator. Further embodiments include a rack that moves to various angles from vertical to horizontal based on the seasons. FIG. 20 illustrates solar device rack orientations 1900a, 1900b, and 1900c at various angles. Unlike some conventional solar tracking technology that shifts across two axes over the course of the day to follow the sun, a solar device rack 1900 may rotate on a single axis over the course of the seasons. For example, a solar device rack may be in a horizontal orientation 1900a in seasons with sunlight having a high incidence angle, such as summer. As the season shifts to autumn, the solar device rack 1900 may rotate up into a tilted orientation 1900b, such as 30-60 degrees with respect to horizontal. This rotation may be in various increments, such as a gradual rotation to track the gradual shift in incidence angle of sunlight over the seasons. In seasons with low incidence sunlight, such as winter, the solar device rack 1900 may rotate into a vertical orientation 1900c. The solar device rack 1900 may rotate back down to a tilted orientation 1900b for spring and then repeat this cycle.

[0076] Solar device racks 1900 may be integrated into buildings. For example, FIG. 21 illustrates vertical solar device racks 1900 integrated into the side of a building 2104. A shade or awning 2102 may be used to prevent reflected light from hitting pedestrians below. In alternate embodiments, the shade 2102 may be part of the solar device rack 1900. In various embodiments, solar device racks 1900 may include lights 2106. Although shown in FIG. 21 as outside of the rack 1900, these lights 2106 may be incorporated into one or more racks 1900 or inside one or more evacuated enclosures 104. The lights 2106 may be powered by the solar energy devices 402. The lights 2106 may be used for various purposes, such as decoration or advertising.

[0077] The preceding description of the disclosed aspects is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects without departing from the scope of the invention. Thus, the present invention is not intended to be limited to the aspects shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

What is claimed is:

1. A device comprising:

an evacuated enclosure;

a reflector configured to reflect solar radiation onto a radiation absorber, the radiation absorber being disposed in the enclosure and positioned to receive solar radiation from the reflector, the radiation absorber having a front surface and a back surface, the front surface being adapted for exposure to solar radiation so as to generate heat; and

at least one solar conversion device disposed in the enclosure and thermally coupled to the absorber.

2. The device of claim 1, wherein the at least one solar conversion device comprises a thermoelectric converter, the thermoelectric converter having a high- temperature end thermally coupled to the absorber to receive at least a portion of the generated heat and a low-temperature end, such that a temperature differential is achieved across the at least one thermoelectric converter.

3. The device of claim 2, further comprising:

a support structure disposed in the enclosure coupled to the low-temperature end of the thermoelectric converter, wherein the support structure removes heat from the low-temperature end of the thermoelectric converter.

4. The device of claim 3, further comprising:

a heat conducting element extending between the support structure and the evacuated enclosure and adapted to transfer heat from the support structure to the enclosure.

5. The device of claim 1, wherein the at least one solar conversion device comprises a solar thermal converter disposed in the enclosure and comprising a fluid conduit.

6. The device of claim 5, further comprising:

at least one thermoelectric converter disposed in the enclosure, the

thermoelectric converter having a high-temperature end thermally coupled to the absorber to receive at least a portion of the generated heat and a low-temperature end, such that a temperature differential is achieved across the at least one thermoelectric converter.

7. The device of claim 6, wherein the fluid conduit is coupled to the low-temperature end of the thermoelectric converter, wherein the fluid conduit removes heat from the low-temperature end of the thermoelectric converter.

8. The device of claim 7, wherein the reflector is a compound parabolic reflector which is located outside the enclosure.

9. The device of claim 8, wherein the radiation absorber comprises a radiation absorber plate which is tilted about a longitudinal axis of the enclosure toward the reflector such that a major surface of the radiation absorber plate is positioned to receive solar radiation from the reflector.

10. The device of claim 9, wherein the radiation absorber is tilted about the

longitudinal axis of the enclosure in a range of 15 to 85 degrees from horizontal when the enclosure is level.

11. The device of claim 9, wherein the radiation absorber is tilted about the

longitudinal axis of the enclosure in a range of 30 to 75 degrees from horizontal when the enclosure is level, and wherein the reflector is angled to reflect solar radiation with an incidence angle in a range of 40 to 60 degrees from horizontal from South or North.

12. The device of claim 9, wherein the radiation absorber is tilted about the

longitudinal axis of the enclosure in a range of 50 to 85 degrees from horizontal when the enclosure is level, and wherein the reflector is angled to reflect solar radiation with an incidence angle in a range of 45 to 65 degrees from horizontal from South or North.

13. The device of claim 8, wherein:

the radiation absorber comprises two sides;

the at least one solar conversion device is disposed between the two sides of the radiation absorber; and

the two sides being positioned to receive solar radiation from different portions of the reflector.

14. The device of claim 13, wherein the compound parabolic reflector is symmetrical about the enclosure and reflects solar radiation to the two sides.

15. A device comprising:

an evacuated enclosure;

a symmetrical reflector configured to reflect solar radiation onto a radiation absorber, the radiation absorber being disposed in the enclosure and positioned to receive solar radiation from the reflector, the radiation absorber comprising two sides being adapted for exposure to solar radiation so as to generate heat, the two sides being positioned to receive solar radiation from different portions of the reflector; and at least one solar conversion device disposed between the two sides of the radiation absorber and thermally coupled to the absorber.

16. The device of claim 15, wherein the radiation absorber is tilted about the longitudinal axis of the enclosure in a range of -10 to 10 degrees from vertical when the enclosure is level.

17. An energy conversion method, comprising: receiving solar radiation at a solar absorber located in an evacuated enclosure, wherein at least some of the solar radiation is reflected from a reflector located outside of the evacuated enclosure;

receiving heat from the solar absorber at an at least one solar conversion device; and

generating electricity from the at least one solar conversion device.

18. The method of claim 17, wherein the at least one solar conversion device comprises a thermoelectric converter, the thermoelectric converter having a high- temperature end thermally coupled to the absorber which receives at least a portion of the heat, and a low-temperature end, such that a temperature differential is achieved across the at least one thermoelectric converter.

19. The method of claim 18, further comprising providing heat from a cold side of the thermoelectric converter to a fluid conduit.

20. The method of claim 17, wherein the at least one solar conversion device comprises a solar thermal converter disposed in the enclosure and comprising a fluid conduit

21. The method of claim 17, wherein the reflector is a compound parabolic reflector.

22. The method of claim 21, wherein the radiation absorber comprises a radiation absorber plate which is tilted about a longitudinal axis of the enclosure toward the reflector such that a major surface of the radiation absorber plate receives the reflected solar radiation from the reflector.

23. The method of claim 22, wherein the radiation absorber is tilted about the longitudinal axis of the enclosure in a range of 15 to 85 degrees from horizontal when the enclosure is level.

24. The method of claim 22, wherein the radiation absorber is tilted about the longitudinal axis of the enclosure in a range of 30 to 75 degrees from horizontal when the enclosure is level, and wherein the reflector is angled to reflect solar radiation with an incidence angle in a range of 40 to 60 degrees from horizontal from South or North.

25. The method of claim 22, wherein the radiation absorber is tilted about the longitudinal axis of the enclosure in a range of 50 to 85 degrees from horizontal when the enclosure is level, and wherein the reflector is angled to reflect solar radiation with an incidence angle in a range of 45 to 65 degrees from horizontal from South or North.

26. The method of claim 21, wherein:

the radiation absorber comprises two sides;

the at least one solar conversion device is disposed between the two sides of the radiation absorber;

the two sides being positioned to receive solar radiation from different portions of the reflector; and

the compound parabolic reflector is symmetrical about the enclosure and reflects solar radiation to the two sides.

27. A system, comprising:

a rack supporting a plurality of devices, each device comprising:

an evacuated enclosure;

at least one solar conversion device disposed in the enclosure and thermally coupled to the absorber; wherein the rack extends in a non-zero angle with respect to a horizontal direction, and each enclosure and reflector pair attached to the rack is attached above of an adjacent enclosure / reflector pair.

28. The system of claim 27, wherein the rack is vertically oriented.

29. The system of claim 27, wherein each enclosure and reflector pair are elongated in the horizontal direction, with the reflector located above the enclosure.

30. The system of claim 27, wherein the rack is incrementally rotated between a horizontal orientation and a vertical orientation.

31. The system of claim 27, wherein the rack is integrated into a building.

32. The system of claim 31, further comprising a shade or awning located beneath the evacuated enclosures of the plurality of devices.