US20090301469A1

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Publication number: US20090301469A1
Application number: US12/375,508
Authority: US
Inventors: Angus Muir Edington Scrimgeour
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-07-28
Filing date: 2007-07-27
Publication date: 2009-12-10
Also published as: WO2008012779A2; EP2052195A2; WO2008012779A3

Abstract

The invention concerns a solar collector (10) which has at least one radiation-transmitting prism (16) which is wedge shaped in cross section. The prism has major side surfaces (20, 21) converging at an acute angle to a relatively narrow, operatively upper end (24). The opposite, lower end (26) of the prism is wider. A refractor (18) is arranged over the prism to refract solar radiation incident thereon onto the major side surfaces of the prism, as the sun moves relative to the earth, at angles allowing such radiation to enter the prism and be internally reflected therein towards a target at or adjacent the relatively wide end of the prism. The configuration allows high levels of solar concentration to be achieved.

Description

BACKGROUND TO THE INVENTION

THIS invention relates to solar collectors.

There exist numerous devices designed to concentrate solar radiation for the purpose of generating electricity or heat. In the case of electricity generation, the function of the solar collector is to concentrate the radiation onto relatively small photovoltaic (PV) cells, while in the case of heat generation, the function of the collector is generally to concentrate the radiation onto a conduit or container conveying or storing a fluid, such as a liquid or gas, the temperature of which is to be elevated.

In the known devices it is recognised that for efficient collection and concentration of the solar energy it is necessary for the device to track the sun as the position of the sun relative to the earth changes during the year and/or as the position of the sun relative to the earth changes during the day. A single-axis system aligned N-S (north-south) should track the sun E-W (east-west) during the day, while a single axis system aligned E-W should track the sun N-S during the year.

Concentrator systems that employ focusing lenses for primary concentration require either biaxial tracking, i.e. both N-S and E-W, or a secondary tracking system that varies the position of the lens or target in order to ensure that the collected radiation is focused correctly on the target, i.e. PV cells or fluid conduit or container. The latter type of system, frequently referred to as a 1.5 times tracking system, typically moves the assembly of lenses, associated reflectors and/or target either individually or in arrays. The apparatus required to achieve such movement can however be expensive and complicated.

Where electricity is to be generated with the use of PV cells an added disadvantage of systems which employ a focusing lens is the fact that dirt particles on the lens create shadows which result in uneven distribution of radiation on the PV cells. Apart from the fact that this reduces the efficiency of the PV cells, it can also cause permanent damage to the cells. Dirt particles on the reflectors of a reflector-type concentrating system can also be problematical.

One example of a known solar collector, described in U.S. Pat. No. 4,282,862, uses an assembly of parallel wedges to reduce the angular dispersion of incident solar radiation. Radiation refracted by the wedges is then transported to the target by internal reflection in thin modules composed of wedge-shaped glass elements. A disadvantage of the system is however a relatively low concentration ratio of around 2:1. “Concentration ratio” refers to the ratio of the area of the solar aperture, i.e. the area on which the solar radiation is incident, to the area of the target onto which the radiation is concentrated. The low concentration ratio is indicative of a low level of efficiency. Another example, described in U.S. Pat. No. 4,344,417, makes use of a narrow, wedge-shaped collector to receive incident radiation and reflect it internally to the target area. The concentration ratio is however again relatively low, indicating a low level of efficiency.

Further examples of prior art collectors are described in JP 11305130 and JP 62266879. In the former case, the collector has wedge-shaped prisms and external reflectors arranged at a divergent angle with respect to one another in order to collect radiation over a larger solar aperture and to concentrate such radiation, by both internal reflection in the prisms and external reflection from the reflectors, onto a solar battery. In the latter case N-S aligned, connected wedge-shaped prisms are again used to concentrate incident radiation by internal reflection. The prism assembly is used in conjunction with a conventional solar panel.

It is an objective of the present invention to provide a novel and efficient solar collector.

SUMMARY OF THE INVENTION

According to the present invention there is provided a solar collector capable of single axis tracking and comprising:

at least one radiation-transmitting prism which is wedge shaped in cross section and which has major side surfaces converging at an acute angle to a relatively narrow, operatively upper end of the prism, the prism having an opposite, operatively lower, relatively wide end; and

a refractor arranged over the prism to refract solar radiation incident thereon onto the major side surfaces of the prism, as the sun moves relative to the earth, at angles allowing such radiation to enter the prism and be internally reflected therein towards a target at or adjacent the relatively wide end of the prism.

There will typically be a plurality of the prisms assembled side by side with their narrow ends extending parallel to one another, and the collector is preferably configured for single axis tracking in a plane transverse to the narrow ends of the prisms. In an arrangement in which the narrow ends of the prisms extend N-S in use, the collector is movable to track the sun in an E-W plane during the course of a day, typically with means for rotating the collector about a N-S axis. Alternatively, in an arrangement in which the narrow ends of the prisms extend E-W in use, the collector is movable to track the sun in a N-S plane during the course of a year, typically with means for rotating the collector about an E-W axis.

The preferred refractor is a linear refractor, in particular a linear Fresnel lens.

The narrow ends of the prisms may be adjacent to or in contact with the refractor, or they may be spaced from the refractor. In the latter case the collector may include a reflector arrangement configured to reflect radiation incident thereon at angles appropriate for acceptance thereof by the prism for internal reflection therein, such as an arrangement including convergent reflectors which stand up from the refractor over the narrow ends of the prisms and are arranged to reflect solar radiation outwardly onto the refractor.

The collector may be located beneath a radiation transmitting cover, for instance in greenhouse or building heating application.

For improved concentration of the solar radiation, the collector may include a radiation transmitting secondary solar concentrator at the wider end of each prism. This may have side walls, typically planar or concave, which converge towards one another to a width less than that of the wider end of the prism. The secondary solar collector should be made of a material with a higher refractive index than the material of which the prism is made. Also, the prism and secondary solar concentrator should meet one another at a curved, typically an upwardly convex, interface.

The various embodiments of the invention described below can be used in an electricity generating mode in which case there will be a PV cell at the wider end of the prism or at the end of the secondary solar collector, or in a fluid heating mode in which case there will be a pipe conveying a fluid which is to be heated at the wider end of the prism or at the end of the secondary solar concentrator.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings.

In the drawings:

FIG. 1 shows a diagrammatic plan view of a solar collector according to one embodiment of the invention;

FIG. 2 shows a diagrammatic cross-section at the line2-2 inFIG. 1;

FIG. 3 shows an enlargement of the circled area inFIG. 2;

FIG. 4 shows a cross-section at the line4-4 inFIG. 1;

FIG. 5 shows a diagrammatic plan view of a solar collector according to a second embodiment of the invention;

FIG. 6 shows a diagrammatic cross-section at the line6-6 inFIG. 5;

FIG. 7 shows a cross-section at the line7-7 inFIG. 5;

FIG. 8 shows a diagrammatic plan view of a solar collector according to a third embodiment of the invention;

FIG. 9 shows a diagrammatic cross-section at the line9-9 inFIG. 8;

FIG. 10 shows an enlargement of the cross-sectional view seen inFIG. 9;

FIG. 11 shows a cross-section at the line11-11 inFIG. 8;

FIG. 12 shows a diagrammatic plan view of a solar collector according to a fourth embodiment of the invention;

FIG. 13 shows a diagrammatic cross-section at the line13-13 inFIG. 12;

FIG. 14 shows a cross-section at the line14-14 inFIG. 12;

FIG. 15 shows an enlargement of the cross-sectional view seen inFIG. 13; and

FIG. 16 illustrates a secondary solar concentrator which can be used in the embodiments illustrated in the earlier Figures.

In the Figures, the letters N, S, E and W refer respectively to north, south, east and west.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

FIGS. 1 to 4 illustrate a first embodiment of solar collector according to this invention. It includes amodule10 having arectangular bounding frame12 which supports at anassembly14 of side-by-side, parallel generally wedge-shapedprisms16 of, for instance, glass, acrylic or polystyrene as well as alinear refractor18, typically in the form of a linear Fresnel lens. Thelinear refractor18, which may also be of glass, acrylic or polystyrene, is in use exposed to solar radiation and may include an ultraviolet (UV) filter.

Theindividual prisms16 are elongate both in a vertical sense and a horizontal sense. Referring in particular toFIG. 3, each prism has major, planar side surfaces20 and21 which converge at anacute angle22, in thiscase 3°, to one another towards a relativelynarrow end24 of the prism. Theopposite end26 of the prism is relatively wide and has mounted to it a series ofPV cells28 arranged side by side with one another in a direction into the plane of the paper inFIG. 3. The cells are in turn mounted in contact withaluminium heat sinks30 which remove excess heat from the cells. The numeral32 indicates an axis about which themodule10 can be rotated.

In this embodiment, the narrow ends24 of the prisms are attached to the underside of the linear refractor. Although the narrow ends are shown as sharp edges, they may in practice be slightly truncated.

InFIG. 1, the prisms are arranged operationally with theirnarrow edges24 extending N-S, and the linear refractor is designed to refract solar radiation incident thereon onto the major side surfaces20 and21 of the prisms.

Thenumerals34 inFIG. 3 indicate solar rays, assumed to be parallel when incident upon the linear refractor. During the course of each year, the sun moves relative to the earth, and themodule10, in a N-S direction. With the sun at equinox solar rays34.1 incident upon the linear refractor are refracted towards the lower, i.e.wider end26 of the illustratedprism16. The angle at which the rays fall upon themajor surface20 of theprism16 is within the acceptance angle of the prism, i.e. the rays enter the prism and are not externally reflected at the prism/air interface. As the sun moves towards the solstice positions during the course of the year the rays impinge at successively higher positions on thesurface20 as exemplified by the numeral34.2. In each case, rays which enter the prisms are totally internally reflected and are eventually incident upon thePV cells28. To increase the acceptance angle of the prism, the

surfaces

20 and21 may be externally coated with a non-reflective coating.

For a prism having the given dimensions and a refractive index of 1.4, solar rays refracted at anangle36 will be within the acceptance angle of the prism and are accordingly received by the prism and internally reflected therein to be incident on the PV cells. Thus allrays34 incident on the linear refractor over the givenlateral dimension38 of 72 mm, corresponding to the solar aperture of themodule10, will reach the PV cells. These cells have a lateral dimension of the order of 5 mm for the given prism dimensions. Thus themodule10 provides a concentration ratio of 72 mm:5 mm, i.e. approximately 14:1.

It is perceived that with this relatively high concentration ratio, themodule10 will be able to function very efficiently as a solar collector.

During the course of the day, the sun moves relative to the earth from an easterly to a westerly position. Themodule10 is rotated at the appropriate angular speed about theaxis32 in order to track the sun during this relative movement. Thus the collector illustrated inFIGS. 1 to 4 requires single axis tracking only to take account of different solar angles during the course of the day. It is believed that it will be possible to achieve such single axis tracking in a simple, reliable and economical manner without the necessity for complicated arrangements to vary focal length as in 1.5 times tracking systems.

It will also be understood that for each latitude position of the sun, the solar rays are refracted parallel to one another on each side of the prism by thelinear refractor18. Thus all solar rays incident on thelinear refractor18 within the illustrated solar aperture, i.e. within thelateral dimension38, will be concentrated onto thePV cells28. The side by side spacing of theprisms16 will accordingly be selected to ensure that all radiation incident on therefractor18 is captured and concentrated.

FIGS. 5 to 7 illustrate a second embodiment of the invention. In these Figures, components corresponding to those inFIGS. 1 to 4 are designated with the same reference numerals.

A major difference between the second embodiment and the first embodiment is the fact that thenarrow end24 of each prism is spaced vertically below thelinear refractor18. In this case, therefractor18 is formed with a gap40 aligned with the central, vertical axis of theprism16, and the gap is spanned by areflector structure42 composed ofupstanding reflector panels44 arranged at an acute angle to one another. In this case solar rays34.7 which would not be refracted by therefractor18 at an angle acceptable to the prism, i.e. the rays would otherwise be externally reflected by the

surfaces

20 and21 of the prism, are reflected by the reflector panels to angles which result in acceptance by the prism. In this way it is possible to increase thesolar aperture38 to a dimension of 144 mm for the other, given dimensions. In this case, a concentration ratio in excess of 20:1, corresponding to high efficiency of the solar collector, can be obtained with relatively small additional cost attributable to the provision of thereflector structure42.

In the embodiment ofFIGS. 5 to 7 the prisms are, as inFIG. 1, aligned N-S and the facility is again provided for rotation about theN-S axis32 in order to track the sun during the course of the day.

FIGS. 8 to 11 illustrate a third embodiment. Once again, like components are designated by like numerals. In this case, theprisms16 are aligned E-W and the facility is provided for N-S tracking.

Thelinear refractors18 are arranged in a curved shape as shown diagrammatically InFIG. 9, within a light-transmittinggreenhouse dome58. Referring toFIG. 10, each prism is, as in the second embodiment, spaced some distance below arefractor18. In this case, the target for each prism is a PVsolar cell27 mounted on afluid pipe28 through which a fluid such as water is conveyed. The wider end of the prism is connected to thepipe28. The prism is connected to therefractor18 by light transmitting side panels50, possibly of acrylic, which also provide structural integrity.

Thepipe28 is rotatable about its own E-W aligned axis in order to track the sun during the course of the year. Rotation of the pipe is accompanied by rotation of theprism16 andrefractor18. One or more counterweights (not shown) may also be provided to assist the rotational movement. The numerals34.7 indicate solar rays refracted by therefractor18 at mid-day for different latitude angles of the sun while the numerals34.8 indicate solar rays refracted by the refractor at times early and late in the day, for example 08h00 and 16h00, again for different latitude angles of the sun. In the mid-day position, the rays are refracted to thewider end26 of the prism while in the early morning and late afternoon positions, the rays are refracted to the narrow end of the prism. In each case, internal reflection within the prism transports the radiation to the target area. These solar positions are also indicated inFIG. 11.

With the dimensions given inFIGS. 8 to 11, concentration ratios of the order of 28:1 can be obtained.

In another embodiment similar to that ofFIGS. 8 to 11, pipe(s)28 could be arranged to move in an arc, in a N-S plane as inFIG. 9, as opposed to rotating.

The embodiments ofFIGS. 1 to 4 and5 to7 are described above in electricity generating applications. It will however be understood that such embodiments could also be used for water or other fluid heating duties, in which case the PV cells would be replaced by fluid transfer pipes, fluid containers or the like. It will also be understood that in this application there would be a requirement for fluid pipe connections able to take account of rotational movements about theaxis32. The embodiment ofFIGS. 8 to 11 is considered particularly appropriate for fluid heating, particularly in situations where thepipe28 rotates about its axis and accordingly does not change position.

In other embodiments of the invention, not illustrated, PV cells could be embedded during moulding in the wider ends of theprisms16.

FIG. 16 illustrates a modification in which a secondary solar radiation concentrator is indicated by thereference numeral70. Theconcentrator70, which extends for the full length of theprism16, is a solid or liquid body made of a material having a higher refractive index than the material of which the prism is made. In one example, the prism is made of an acrylic, such as PMMA (Polymethyl methacrylate) having a refractive index of less than 1.5 and thesecondary concentrator70 of polystyrol or glass having a refractive index of more than 1.5.

The secondary concentrator is placed at the wider, lower end of theprism16 and is intimately connected to the prism at an upwardly convex interface defined by aconvex surface72 of the secondary concentrator and aconcave surface74 of the prism. Thesecondary concentrator70 has planar side surfaces76 and78 and a planar lower surface80 to which, in this example, a heat transmitting coupler81 is intimately attached. The coupler81 is in intimate contact with thepipe28.

The numeral82 indicates a solar ray which enters theprism16 through theside surface20, is refracted at the prism/air interface and travels through the lower part of the prism to the convex interface between the prism and thesecondary concentrator70. At this interface the ray is refracted into the secondary concentrator and is thereafter reflected internally for eventual impingement on the coupler81. It will be understood that other solar rays that have been internally reflected in the prism will likewise be refracted into thesecondary concentrator70 for subsequent passage directly or through internal reflection onto the coupler.

To ensure that rays which enter thesecondary concentrator70 are reflected onto the coupler81, inwardly facing mirrors84 (only one shown) may be placed against the

surfaces

76 and78 or these surfaces may themselves be mirrored.

Instead of side surfaces76 and78 which are planar, the side surfaces of the secondary concentrator may be concave as indicated diagrammatically by the numeral86, or convex.

The convex interface defined by the

surfaces

72 and74 is preferred to a planar, horizontal interface because it will tend to refract radiation in the appropriate direction for subsequent reflection onto the coupler81. A secondary concentrator having a convex interface as illustrated may be referred to as a secondary convex concentrator (SCC).

As exemplified above it is preferred that the refractive index of the SCC be greater than that of theprism16 in order to ensure that solar rays are appropriately refracted.

It will be understood that the SCC seen inFIG. 16 will increase the concentration ratio further, implying high levels of solar concentration efficiency. It will furthermore be understood that the SCC could equally well be used to achieve highly efficient concentration of solar radiation onto a PV cell in place of the coupler81 andpipe28 in electricity generating applications.

SCCs such as that described above can be used in conjunction with theprisms16 in any of the embodiments seen in the drawings.

FIGS. 12 to 15 illustrate another embodiment of the invention in whichSCCs70 are used. In this embodiment, the numeral60 indicates an enclosure mounted for example in a fixed position on a building (not shown). The enclosure60 has a light-transmitting roof panel62, possibly made of glass or a fluoropolymer.Solar collection units63, each including alinear refractor18,prism16 andSCC70, are supported by bearings64 fixed in spaced metal frames66. In each case a heat pipe is rotatable in the associated bearing and thelinear refractor18 is supported by support arms or radiation-transmittingsheets67. Each unit includes a counterweight68 connected to the associated heat pipe by anarm69. The counterweight may, for instance, be provided by a weight or by a length of heavy rod or pipe extending parallel to the associatedprism16.

Like the embodiment ofFIGS. 8 to 11, the embodiment ofFIGS. 12 to 15 employs single axis tracking, for each unit about an E-W axis, to track the sun as it moves relative to the earth during the course of the year. Although theunits63 are shown as independent of one another it will be understood that they would in practice be linked and would move synchronously.

As inFIG. 10 the numeral34.7 inFIG. 15 designates solar rays refracted by arefractor18 at mid-day for different latitude angles of the sun while the numeral34.8 designates solar rays refracted by the refractor at times early and late in the day, for example 08h00 and 16h00, again for different latitude angles of the sun.

Apart from the simplicity of the single axis tracking systems which are employed in the embodiments described above, and the high concentration ratios and favourable efficiency which can be obtained with such embodiments, an important advantage of these embodiments, compared to known system using focusing lenses, is the fact that internal reflection by the prisms ensures that radiation is evenly distributed across the receiving surfaces of the PV cells in electricity generating applications, and that shadows attributable to dirt particles on the lenses do not occur.

Claims

1. A solar collector comprising:

at least one radiation-transmitting prisms which is wedge-shaped in cross-section and which has major side surfaces converging at an acute angle to a relatively narrow end of the prism, having an opposite, relatively wide end; and

2. A solar collector according toclaim 1 comprising a plurality of the prisms assembled side by side with their narrow ends extending parallel to one another.

3. A solar collector according toclaim 2 wherein the collector is configured for single axis tracking in a plane transverse to the narrow ends of the prisms.

4. A solar collector according toclaim 3 wherein the narrow ends of the prisms extend N-S in use and the collector is movable to track the sun in an E-W plane during the course of a day.

5. A collector according toclaim 4 comprising means for rotating the collector about a N-S axis.

6. A solar collector according toclaim 3 wherein the narrow ends of the prisms extend E-W in use and the collector is movable to track the sun in a N-S plane during the course of a year.

7. A solar collector according toclaim 6 comprising means for rotating the collector about an E-W axis.

8. A solar collector according toclaim 1 wherein the refractor is a linear refractor.

9. A solar collector according toclaim 8 wherein the refractor is a linear Fresnel lens.

10. A solar collector according toclaim 1 wherein the narrow ends of the prisms are adjacent to or in contact with the refractor.

11. A solar collection according toclaim 1 wherein the narrow ends of the prism's are spaced from the linear refractor.

12. A solar collector according toclaim 11 wherein the collector includes a reflector arrangement to reflect radiation incident thereon at angles appropriate for acceptance thereof by the prism for internal reflection therein.

13. A solar collector according toclaim 12 wherein the reflector arrangement comprises convergent reflectors which stand up from the refractor over the narrow ends of the prisms and are arranged to reflect solar radiation outwardly onto the refractor.

14. A solar collector according toclaim 2 wherein the collector is located beneath a radiation transmitting cover.

15. A solar collector according toclaim 2 wherein the prisms are spaced from the refractor and are connected to the refractor by radiation-transmitting side panels.

16. A solar collector according toclaim 1 comprising a radiation transmitting secondary solar concentrator at the wider end of each prism.

17. A solar collector according toclaim 16 wherein the secondary solar concentrator has side walls which converge towards one another to a width less than that of the wider end of the prism.

18. A solar collector according toclaim 17 wherein the side walls are planar or concave.

19. A solar collector according toclaim 16 wherein the secondary solar collector is made of a material with a higher refractive index than the material of which the prism is made.

20. A solar collector according toclaim 17 wherein the prism and secondary solar concentrator meet one another at a curved interface.

21. A solar collector according toclaim 20 wherein a convex surface of the secondary solar concentrator mates with a concave surface of the prism at the interface.

22. A solar collector according toclaim 1 comprising a PV cell at the wider end of each prism.

23. A solar collector according toclaim 16 comprising a PV cell at an end of the secondary solar collector remote from the prism.

24. A solar collector according toclaim 1 comprising a pipe conveying a fluid which is to be heated at the wider end of the prism.

25. A solar collector according toclaim 16 comprising a pipe conveying a fluid which is to be heated at an end of the secondary solar concentrator remote from the prism.

26. A solar collector according toclaim 1 wherein each prism is made of glass, acrylic or polystyrene.

27. A solar collector according toclaim 16 comprising a pipe conveying a fluid which is to be heated at an end of the secondary solar concentrator remote from the prism.