GB2058332A - Radiation Collector - Google Patents

Abstract

A radiation collector comprising an elongated receiver (4), spaced from one side of which is a lens (1), preferably Fresnel type, and spaced from the other side of which is a reflective surface (9) formed by silvering the back of transparent material (6). The longitudinal axes of lens (1), receiver (4) and reflective surface (9) are substantially parallel to one another and the longitudinal axis of the receiver (4) is on the plane of optical symmetry of the lens (1). The receiver (4) is positioned so that it is nearer the lens (1) than the focal plane of the lens (1). Substantially all the space between lens (1) and reflective surface (9) is occupied by said transparent material (6) which has to have a refractive index of at least 1.35 and being capable of withstanding temperatures of at least 75 DEG C. <IMAGE>

Description

SPECIFICATION Radiation Collector This invention relates to a radiation collector, especially one which is statically mounted.

A radiation collector has been described and claimed in the specification of our patent application 79.19252. Although an improvement over prior art radiation collectors it has one disadvantage. This is the relatively poor specular reflectivity of aluminium which is the most suitable material for the manufacture of the reflector.

We have now devised a radiation collector constructed of transparent material whereby it is possible to reduce greatly the dimensions of the collector elements. The use of transparent material also means that it is possible to appiy conventional silvering techniques to the underside of the reflector.

According to this invention a radiation collector comprises an elongated receiver, spaced from one side of which is a lens and spaced from the other side of which is a reflector having high specular reflectivity, the longitudinal axes of the lens, receiver and reflector being substantially parallel to one another. The receiver is positioned so that its longitudinal axis is substantially on the plane of optical symmetry of the lens. Also the receiver is positioned so that it is nearer the lens than the focal plane of the lens. Substantially all the space between the lens and the reflector and which surrounds the receiver is occupied by a material of high transparency having a refractive index of at least 1.35 and being capable of withstanding temperatures of at least 750C.It is also necessary for the distance between the receiver and the lens to be such that the product of said refractive index and the sine of the angle between the plane of optical symmetry of the lens and a tangent to the receiver intersecting said plane at the lens is substantially equal to the sine of the angle of acceptance of the system constituted by the lens and the reflector.

The receiver may be a conduit through which fluid can flow and the fluid, e.g., liquid, may be the sole means of extracting energy from the collector. Preferably the receiver is substantially circular in cross-section, but it could if desired be for example elliptical in cross-section. If desired its outer surface can be treated to increase its absorptivity and reduce its emissivity, e.g. coated with black chrome.

The lens is preferably of the Fresnel type so that its weight and thickness are minimum. The Fresnel lens may be made of for example, extruded radiation-transparent plastics material. If necessary for rigidity it may be adhered to one face of a relatively rigid optically flat radiationtransparent support plate such as glass or rigid plastics material.

Located on the other side of the receiver from the lens is a reflector. This has high specular reflectivity by which term we mean that the degree of dispersion of the reflected light is low and that little radiation is absorbed by the reflector.

Preferably the reflector is a silvered surface applied to the material of high transparency and with refractive index of at least 1.35. Less desirably the reflector is separate from said material of high transparency and it could be made of glass and highly silvered on one side so as to reflect substantially all the light.

Alternatively it can be made of a plastics material such as perspex (registered Trade Mark), also with one surface silvered.

The reflector is shaped and situated with respect to lens and receiver so that substantially all the rays impinging on the lens at or less than the angle of acceptance either directly impinge on the receiver or are reflected from the reflector onto the receiver. This means that the shape of the surface of the reflector is preferably at least partly defined by the locus of intersections of tangents drawn from the receiver with rays from the lens at a selected focal length thereof at the modified angle of acceptance of the collector, a line bisecting the angle defined between a tangent and a ray at each intersection being normal to the surface of the reflector. By modified angle of acceptance we mean the angle of refraction where the angle of incidence is the angle of acceptance.

In effect the reflector may be shaped as a pair of involutes joined together by a cusp which is substantially on the plane of optical symmetry of the lens, each end of the involute remote from the cusp merging smoothly with a surface which is very slightly concave towards the receiver.

The arrangement of the lens and receiver are such that the focal plane of the lens is further from the lens than the receiver. This is to avoid concentrating radiation along a very narrow band or line on the receiver so as to prevent damage due to excessive heating. For the same reason it is also preferable if the reflector is nearer the reeeiver than the focal plane of the lens.

Substantially all the space between the lens and the reflector and which surrounds the receiver is occupied by a material of high transparency. This material has to have a refractive index of at least 1.35 and be capable of withstanding temperatures of at least 75 or.

Since the concentration ratio, that is the ratio of the mean intensity of the radiation at the receiver to the intensity of the incident radiation for the collector is n/sin6 where 0 is the angle of acceptance and n is the refractive index, for a given angle of acceptance the concentration ratio can be increased quite considerably i.e. by a factor of n compared with cases where air occupies the space between the lens and the reflector. By reducing the scale of the radiation collector, for example so that the overall depth of lens, receiver and reflector is only about 13 mm, it is possible economically to fill the space between the lens and reflector with the transparent material of refractive index of at least 1.35. Thus, as explained above one can considerably increase the concentration ratio.Alternatively one could keep the concentration ratio at the hitherto common level of about 3:1 and increase the angle of acceptance. As another alternative one can increase both the concentration ratio and the angle of acceptance compared with systems where there is only air in the space between the lens and reflector.

In order to accommodate changes in dimensions of the receiver due to thermal expansion and contraction there should be a slight air gap surrounding the receiver. This will also mean a considerable reduction in temperature experiences by the material occupying the remainder of the space between lens and reflector due to the thermal insulating properties of air. However the air gap should not be too great because the benefit of the high refractive index of the surrounding material will be lost.

Suitable transparent materials having a refractive index of at least 1.35 include acrylic resin, silicone rubber, polycarbonates and methylpentene polymer.

Preferably the refractive index is at least 1.58.

Also it is desirable that the material shall be capable of withstanding temperatures of at least 1 000C and more preferably at least 1 500C.

As stated above it is preferable if the face of the material remote from the lens is silvered so as to form the reflector. This face of the material will of course have to be correctly shaped so that the reflecting surface is as previously described.

If desired a glass or transparent plastics cover plate may be used to reduce convection losses and to protect the lens from dirt and U.V.

radiation.

It is preferred that the underside of the reflector be backed by a layer of supporting thermal insulating material, for example foamed plastics insulating material. This layer can if desired by covered by an outer skin, for example an aluminium sheet.

The combination described above comprises a single lens, a single receiver and a single reflector.

In preferred embodiments the radiation collector comprises a plurality of these combinations arranged side by side with the longitudinal axes of the lenses, receivers and reflectors being substantially parallel. Usually each combination of lens, receiver and reflector will be contiguous. In one particular embodiment the transparent material of refractive index of at least 1.35 can be formed in one piece to allow a series of receivers to be housed in a series of substantially parallel recesses formed in the materiai. The underside of this material can also be shaped so as to conform to the configuration of a plurality of parallel reflectors, one for each receiver.

When the radiation collector is mounted for use the plane of symmetry of the or each lens of the collector should extend substantially in an East-West direction. Also the collector should preferably be tilted so as to receive radiation at the smallest angles of incidence during the part of the year when the collector is to be used to supply energy.

The invention is now described with reference to the accompanying drawing which is a crosssectional elevation of one particular embodiment of the radiation collector.

The lens 1 is separated by a small air gap 2 from a toughened glass cover plate 3. Two tubular receivers shown at 4 are seated in recesses 5 formed in slabs of transparent methyl pentene polymer 6. Each receiver tube 4 is connected at each end to a header tube (not shown) and this enables there to be a slight air gap between the tube 4 and the surrounding methyl pentene polymer slab. The top portion of each tube 4 is also covered by a slab 7 of methyl pentene polymer, there also being a slight air gap between slab 7 and receiver tube 4. Each slab 7 forms part of the lens assembly 1, the lens assembly 1 being stuck to slab 6 (shown at 8) by a transparent adhesive.

Slab 6 is cut and shaped so that the underside conforms to the shape of reflectors. These reflectors are formed by silvering the underside of slab 6 at 9.

Beneath slab 6 is a slab or slabs of foamed thermal insulation 10. This insulation which is polyurethane foam is in turn supported by an aluminium plate 11.

With the arrangement shown it is possible to use receivers of 3.75 mm diameter, with an aperture of 37.5 mm, the refractive index of methyl pentene polymer being 1.465. The concentration ratio is approximately 3:1 and the actual depth of the lens, receiver and reflector is about 13 mm. With a 2.5 mm thick cover plate, an air gap of about 3 mm and thermal insulation about 5.5 mm deep below each reflector, the overall thickness of the collector is about 24 mm.

Not only are such collectors very thin compared with previously known collectors but their weight is substantially less than that for the previously known collectors which is in the region of 1 5 to 20 kg/m2. Another advantage of the collectors of this invention is the possibility of automatic manufacture with a consequent reduction in costs.

Claims (13)

Claims

1. A radiation collector comprising an elongated receiver, spaced from one side of which is a lens and spaced from the other side of which is a reflector having high specular reflectivity, the longitudinal axes of the lens, receiver and reflector being substantially parallel to one another, the longitudinal axis of the receiver being on the plane of optical symmetry of the lens, the receiver being positioned so that it is nearer the lens than the focal plane of the lens, substantially all the space between the lens and the reflector and which surrounds the receiver being occupied by a material of high transparency having a refractive index of at least 1.35 and being capable of withstanding temperatures of at least 750C and the distance between the receiver and the lens being such that the product of said refractive index and the sine of the angle between the plane of optical symmetry of the lens and a tangent to the receiver intersecting said plane at the lens is substantially equal to the sine of the angle of acceptance of the system constituted by the lens and the reflector.

2. A collector according to claim 1 wherein the outer surface of the receiver is coated with a selective surface having a high absorptivity and a low emissivity.

3. A collector according to either of claims 1 and 2 wherein the lens is of the Fresnel type.

4. A collector according to any one of the preceding claims wherein the shape of the surface of the reflector is at least partly defined by the locus of intersections of tangents drawn from the receiver with rays from the lens at a selected focal length thereof at the modified angle of acceptance (as hereinbefore defined), a line bisecting the angle defined between a tangent and a ray at each intersection being normal to the surface of the reflector.

5. A collector according to any one of the preceding claims wherein the reflector is nearer to the receiver than the focal plane of the lens.

6. A collector according to any one of the preceding claims wherein said material of high transparency has a refractive index of at least 1.58.

7. A collector according to any one of the preceding claims wherein said material of high transparency is capable of withstanding temperatures of at least 1 000C.

8. A collector according to claim 7 wherein said temperature is at least 1 500 C.

9. A collector according to any one of the preceding claims wherein the reflector comprises the silvered surface formed by silvering the face of said material of high transparency remote from the lens.

10. A collector according to any one of the preceding claims which includes a glass or transparent cover plate.

11. A collector according to any one of the preceding claims wherein the underside of the reflector is supported by a layer of thermal insulating material.

12. A plurality of collectors according to any one of the preceding claims arranged side by side with the longitudinal axes of the lenses, receivers and reflectors being substantially parallel.

13. A collector according to claim 1 substantially as hereinbefore described with reference to the drawing.