SOLAR COLLECTORS

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 focussing 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 focussed 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 focussing 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 US 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 US 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:

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

Figure 2 shows a diagrammatic cross-section at the line 2-2 in Figure 1;

Figure 3 shows an enlargement of the circled area in Figure 2;

Figure 4 shows a cross-section at the line 4-4 in Figure 1;

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

Figure 6 shows a diagrammatic cross-section at the line 6-6 in Figure 5;

Figure 7 shows a cross-section at the line 7-7 in Figure 5;

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

Figure 9 shows a diagrammatic cross-section at the line 9-9 in Figure 8;

Figure 10 shows an enlargement of the cross-sectional view seen in Figure 9;

Figure 11 shows a cross-section at the line 11-11 in Figure 8;

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

Figure 13 shows a diagrammatic cross-section at the line 13-13 in Figure 12;

Figure 14 shows a cross-section at the line 14-14 in Figure 12;

Figure 15 shows an enlargement of the cross-sectional view seen in Figure 13; and

Figure 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

Figures 1 to 4 illustrate a first embodiment of solar collector according to this invention. It includes a module 10 having a rectangular bounding frame 12 which supports at an assembly 14 of side-by-side, parallel generally wedge-shaped prisms 16 of, for instance, glass, acrylic or polystyrene as well as a linear refractor 18, typically in the form of a linear Fresnel lens. The linear refractor 18, which may also be of glass, acrylic or polystyrene, is in use exposed to solar radiation and may include an ultraviolet (UV) filter.

The individual prisms 16 are elongate both in a vertical sense and a horizontal sense. Referring in particular to Figure 3, each prism has major, planar side surfaces 20 and 21 which converge at an acute angle 22, in this case 3°, to one another towards a relatively narrow end 24 of the prism. The opposite end 26 of the prism is relatively wide and has mounted to it a series of PV cells 28 arranged side by side with one another in a direction into the plane of the paper in Figure 3. The cells are in turn mounted in contact with aluminium heat sinks 30 which remove excess heat from the cells. The numeral 32 indicates an axis about which the module 10 can be rotated.

In this embodiment, the narrow ends 24 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.

In Figure 1 , the prisms are arranged operationally with their narrow edges 24 extending N-S, and the linear refractor is designed to refract solar radiation incident thereon onto the major side surfaces 20 and 21 of the prisms.

The numerals 34 in Figure 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 the module 10, in a N-S direction. With the sun at equinox solar rays 34.1 incident upon the linear refractor are refracted towards the lower, i.e. wider end 26 of the illustrated prism 16. The angle at which the rays fall upon the major surface 20 of the prism 16 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 the surface 20 as exemplified by the numeral 34.2. In each case, rays which enter the prisms are totally internally reflected and are eventually incident upon the PV cells 28. To increase the acceptance angle of the prism, the surfaces 20 and 21 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 an angle 36 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 all rays 34 incident on the linear refractor over the given lateral dimension 38 of 72mm, corresponding to the solar aperture of the module 10, will reach the PV cells. These cells have a lateral dimension of the order of 5mm for the given prism dimensions. Thus the module 10 provides a concentration ratio of 72mm:5mm, i.e. approximately 14:1.

It is perceived that with this relatively high concentration ratio, the module 10 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. The module 10 is rotated at the appropriate angular speed about the axis 32 in order to track the sun during this relative movement. Thus the collector illustrated in Figures 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 the linear refractor 18. Thus all solar rays incident on the linear refractor 18 within the illustrated solar aperture, i.e. within the lateral dimension 38, will be concentrated onto the PV cells 28. The side by side spacing of the prisms 16 will accordingly be selected to ensure that all radiation incident on the refractor 18 is captured and concentrated.

Figures 5 to 7 illustrate a second embodiment of the invention. In these Figures, components corresponding to those in Figures 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 the narrow end 24 of each prism is spaced vertically below the linear refractor 18. In this case, the refractor 18 is formed with a gap 40 aligned with the central, vertical axis of the prism 16, and the gap is spanned by a reflector structure 42 composed of upstanding reflector panels 44 arranged at an acute angle to one another. In this case solar rays 34.7 which would not be refracted by the refractor 18 at an angle acceptable to the prism, i.e. the rays would otherwise be externally

reflected by the surfaces 20 and 21 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 the solar aperture 38 to a dimension of 144mm 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 the reflector structure 42.

In the embodiment of Figures 5 to 7 the prisms are, as in Figure 1 , aligned N-S and the facility is again provided for rotation about the N-S axis 32 in order to track the sun during the course of the day.

Figures 8 to 11 illustrate a third embodiment. Once again, like components are designated by like numerals. In this case, the prisms 16 are aligned E- W and the facility is provided for N-S tracking.

The linear refractors 18 are arranged in a curved shape as shown diagrammatically In Figure 9, within a light-transmitting greenhouse dome 58. Referring to Figure 10, each prism is, as in the second embodiment, spaced some distance below a refractor 18. In this case, the target for each prism is a PV solar cell 27 mounted on a fluid pipe 28 through which a fluid such as water is conveyed. The wider end of the prism is connected to the pipe 28. The prism is connected to the refractor 18 by light transmitting side panels 50, possibly of acrylic, which also provide structural integrity.

The pipe 28 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 the prism 16 and refractor 18. One or more counterweights (not shown) may also be provided to assist the rotational movement. The numerals 34.7 indicate solar rays refracted by the refractor 18 at mid-day for different latitude angles of the sun while the numerals 34.8 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 the wider end 26 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 in Figure 11.

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

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

The embodiments of Figures 1 to 4 and 5 to 7 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 the axis 32. The embodiment of Figures 8 to 11 is considered particularly appropriate for fluid heating, particularly in situations where the pipe 28 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 the prisms 16.

Figure 16 illustrates a modification in which a secondary solar radiation concentrator is indicated by the reference numeral 70. The concentrator 70, which extends for the full length of the prism 16, 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 the secondary concentrator 70 of polystyrol or glass having a refractive index of more than 1.5.

The secondary concentrator is placed at the wider, lower end of the prism 16 and is intimately connected to the prism at an upwardly convex interface defined by a convex surface 72 of the secondary concentrator and a concave surface 74 of the prism. The secondary concentrator 70 has planar side surfaces 76 and 78 and a planar lower surface 80 to which, in this example, a heat transmitting coupler 81 is intimately attached. The coupler 81 is in intimate contact with the pipe 28.

The numeral 82 indicates a solar ray which enters the prism 16 through the side surface 20, is refracted at the prism/air interface and travels through the lower part of the prism to the convex interface between the prism and the secondary concentrator 70. At this interface the ray is refracted into the secondary concentrator and is thereafter reflected internally for eventual impingement on the coupler 81. It will be understood that other solar rays that have been internally reflected in the prism will likewise be refracted into the secondary concentrator 70 for subsequent passage directly or through internal reflection onto the coupler.

To ensure that rays which enter the secondary concentrator 70 are reflected onto the coupler 81 , inwardly facing mirrors 84 (only one shown) may be placed against the surfaces 76 and 78 or these surfaces may themselves be mirrored.

Instead of side surfaces 76 and 78 which are planar, the side surfaces of the secondary concentrator may be concave as indicated diagrammatically by the numeral 86, or convex.

The convex interface defined by the surfaces 72 and 74 is preferred to a planar, horizontal interface because it will tend to refract radiation in the appropriate direction for subsequent reflection onto the coupler 81. 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 the prism 16 in order to ensure that solar rays are appropriately refracted.

It will be understood that the SCC seen in Figure 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 coupler 81 and pipe 28 in electricity generating applications.

SCCs such as that described above can be used in conjunction with the prisms 16 in any of the embodiments seen in the drawings.

Figures 12 to 15 illustrate another embodiment of the invention in which SCCs 70 are used. In this embodiment, the numeral 60 indicates an enclosure mounted for example in a fixed position on a building (not shown). The enclosure 60 has a light-transmitting roof panel 62, possibly made of glass or a fluoropolymer. Solar collection units 63, each including a linear refractor 18, prism 16 and SCC 70, are supported by bearings 64 fixed in spaced metal frames 66. In each case a heat pipe is rotatable in the associated bearing and the linear refractor 18 is supported by support arms or radiation-transmitting sheets 67. Each unit includes a counterweight 68 connected to the associated heat pipe by an arm 69. The counterweight may, for instance, be provided by a weight or by a length of heavy rod or pipe extending parallel to the associated prism 16.

Like the embodiment of Figures 8 to 11 , the embodiment of Figures 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 the units 63 are shown as independent of one another it will be understood that they would in practice be linked and would move synchronously.

As in Figure 10 the numeral 34.7 in Figure 15 designates solar rays refracted by a refractor 18 at mid-day for different latitude angles of the sun while the numeral 34.8 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 focussing 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.