Field of the Invention

The present invention relates to light collectors and to solar concentrators, especially building-integrated structures such as window blinds, using same.

Background to the Invention

Building-integrated solar concentrating structures that collect light and convert it into electricity via the photovoltaic effect are known. Typically, such structures are integrated into the glazed facade of the building structure. Amongst the properties of an ideal facade is an ability to allow the occupants to use daylight to see clearly within the building, while not being dazzled by glare. An ideal facade should also insulate the working environment within, rejecting external variations in temperature allowing efficient internal climate control. Ideally the unwanted incident energy should be collected and usefully employed, e.g. converted into electricity.

Criteria for assessing the performance of such structures may be said to include: the degree of glare (direct sunlight) elimination; the amount of scattered light available for internal lighting; the amount of energy saved by reducing the heating effect of the sun; and the amount of electricity generated. These multifunctional goals require the development of a system that effectively separates direct from scattered light, allowing the scattered to pass to the interior of the building while the direct is captured and used to generate electricity while controlling or using the heat by-product. Summary of the Invention

A first aspect of the invention provides a solar concentrator comprising at least one elongate lens associated with a respective elongate light collector, said at least one elongate lens being arranged to focus radiation onto a focal line, wherein said solar concentrator further includes means for causing the focal line of said at least one lens to be coincident with the respective light collector.

In preferred embodiments, said light collector comprises a substantially transparent elongate body having an optically active core running longitudinally of the body, and wherein said focal line of said at least one lens is arranged to be coincident with said optically active core. The preferred optically active core is arranged, in response to receiving incident radiation, to emit secondary radiation into said body, said secondary radiation being generated by the interaction of said incident radiation with said optically active core. Advantageously, the arrangement is such that the secondary radiation (or at least part of it) is guided along the light collector towards its ends. This may be achieved by, for example, reflection of the secondary radiation off the internal surfaces of the body, and/or by the provision of one or more reflective layers around all or part of the body (preferably located at one or more of the body's external surfaces). One or more transducers, for example photovoltaic cells, are preferably provided at one or both ends of the light collector in order to convert the secondary radiation into an alternative form, e.g. electrical power.

From another aspect, the invention provides a light collector comprising an elongate body having an optically active core. The preferred core is arranged to absorb primary radiation, especially sunlight, that is incident thereon during use, and to emit secondary radiation, generated by the interaction of said primary radiation with the optically active core, into the body. The body is adapted to trap at least a portion of said secondary radiation and, preferably, to direct said trapped secondary radiation longitudinally of the body. The body is typically formed from a transparent, or substantially transparent, material.

Preferably, at least one radiation transducer, typically in the form of one or more photovoltaic cells, is provided at at least one end of the body for receiving secondary radiation that is directed longitudinally of the body.

Typically, the arrangement is such that at least some of said trapped secondary radiation is trapped in the body by reflection off one or more internal surfaces of the body.

In preferred embodiments, one or more reflectors, especially dichroic reflectors, are provided on, or substantially at, one or more external surfaces of the body. The reflector(s) are adapted to reflect at least some of the secondary radiation emanating in use from the core back into the body, and preferably back into the core. The reflector(s) preferably take the form of a dichroic Fresnel mirror, especially a substantially flat dichroic Fresnel mirror.

Optionally, a reflector, especially a dichroic reflector, is provided within the body around substantially all, or part, of the core, preferably at the interface between the core and the rest of the body. The reflector, which may conveniently take the form of a reflective layer or coating, is adapted to reflect back into the core at least some of a proportion of the secondary radiation generated by the core that would otherwise be capable of escaping from the body. Typically, the reflector is adapted to reflect radiation that is, in use, incident thereon at an angle greater than a threshold angle. Most typically, the reflector is adapted to reflect radiation that is, in use, normally incident, or substantially normally incident thereon.

Preferably, the core is substantially coincident with, or substantially parallel with, the longitudinal axis of the body. The core typically extends substantially the whole length of the body, but preferably stops short of said radiation transducers when present.

The core typically comprises optically active particles (OAPs), for example luminescent particles (such as organic dyes, semiconductor quantum dots or phosphors (any of those materials or any combination of two or more of them)) and/or light scattering particles (such as metal nanoparticles and/or micrometer- size reflectors). The preferred arrangement is such that at least some of the incident primary radiation is absorbed and/or scattered by the OAPs and emitted from the core as secondary radiation. Preferably, the arrangement is such that the secondary radiation occurs in a relatively narrow frequency band when compared to the frequency band of the incident primary radiation. Advantageously, the core converts relatively concentrated primary radiation received by the core into secondary radiation in the body. Typically, the secondary radiation is relatively diluted, or less concentrated, in comparison with the primary radiation. In preferred embodiments, the primary radiation is directed onto the core by at least one lens. The secondary radiation may be said to be trapped in the body and, in use, propagates along the body with relatively low losses as a result of its dilution into a larger volume provided by the body.

From a second aspect, the invention provides a blind system comprising at least one elongate lens associated with a respective elongate light collector, and means for causing the focal line of the, or each, lens to be substantially coincident with the respective light collector.

Typically, the blind system comprises a plurality of spaced-apart, substantially parallel lenses, for example in the manner of a Venetian blind. Advantageously, the, or each, lens comprises a Fresnel lens.

In preferred embodiments, said elongate light collector comprises a light collector embodying the first aspect of the invention. In such cases, the arrangement is such that the focal line of the, or each, lens is, in use, substantially coincident with the optically active core of the respective light collector. Preferably still, the arrangement is such that core of the light collector and the focal line of the respective lens are substantially the same length and substantially in register with one another.

The system typically includes a support structure for carrying the or each lens and means for operating the support structure to pivot or rotate the or each lens, typically about their longitudinal axis, and preferably also means for adjusting the distance between the or each lens at its respective light collector.

The support structure may comprise one or more sets of first and second substantially parallel support members, the lenses extending between and substantially perpendicular to the support members and being pivotably or rotatably coupled thereto.

The system may include any suitable electro -mechanical means for operating the support structure in the manner described. In typical embodiments, the system further includes a control system for monitoring the position of the Sun (or other radiation/light source) and providing appropriate control signals to the relevant electro -mechanical devices.

A third aspect of the invention provides a building structure, especially a window or facade structure, incorporating a blind system embodying the second aspect of the invention and/or a solar concentrator of the first aspect of the invention.

Further preferred features are recited in the dependent claims.

It will be seen that preferred embodiments of the invention provide a building integrated, or integratable, solar structure, in the preferred form of a window blind, that significantly improves the properties of glass, or other transparent, exterior building facades making them closer to the ideal. The preferred system harvests direct sunlight to suppress glare, reduce external heat load and generate electricity while most of the diffuse daylight is transmitted inside for daylight illumination.

The preferred system is designed particularly for use with highly glazed facades. This simple process description allows all of these desirables goals for a building facade to be achieved.

In preferred embodiments, notable features of the invention include one or more of the following:

1) The use of linear Fresnel lenses to concentrate direct sunlight in one dimension for mechanical simplicity.

2) The use of any suitable mechanism to move the lenses and track the sun so that the "lines of least confusion" generated by the lenses do not significantly change in width and do not move with respect to the building during the day and during the year.

3) The use of transparent elongate, or rod-shaped, light collectors designed to contain the spatially fixed "lines of least confusion" of the lenses and convert a large fraction of the incoming beam of concentrated sun-power into a flux of (red-shifted) trapped radiation guided towards the edges of the rods.

4) The use of solar cell(s) provided at the end(s) of the light collectors and optimised for the spectral properties of the trapped radiation which are substantially different from the incident solar spectrum.

In preferred embodiments, the system takes the form of Venetian blinds, but with the conventional fully opaque slats replaced by transparent lenses, especially linear Fresnel lenses. Reduction of glare may be achieved by the combined action of Fresnel lenses and light collectors with no need for additional obtrusive components. A plurality of systems or structures embodying the invention may be integrated with the glazed facade of a building.

Advantageously, opaque components such as solar cells and heat sinks are located at the edge of the structures in contrast to known structures. In prior solutions, solar cells are directly placed at the focal line of linear lenses or mirrors. This means that the photovoltaic function is essentially superimposed to the solar-blind function with no real added value being generated by the combination of the two functions.

Preferred embodiments of the invention provide the integration of at least three key functions, namely glare reduction/elimination, increased daylight transmission and high efficiency electricity generation, as well as providing a solution to the abatement of the solar factor in glass-clad buildings.

The process whereby the radiative power from the sun is first concentrated onto an optically active core and then diluted into the larger volume of the collector significantly reduces re-absorption (re-scattering) with respect to luminescent concentrators as described in the prior art.

Further advantageous aspects of the invention will become apparent to those ordinarily skilled in the art upon review of the following description of preferred embodiments and with reference to the accompanying drawings.

Brief Description of the Drawings

Embodiments of the invention are now described by way of example and with reference to the accompanying drawings in which like numerals are used to indicate like parts and in which: Figure 1 is a perspective view of part of a window blind structure embodying one aspect of the invention;

Figure 2 is a schematic side view of the structure of Figure 1 showing the incidence of direct sunlight;

Figure 3 is a perspective view of an assembly of a lens and a light collector suitable for use in the structure of Figure 1;

Figure 4 is a schematic side view of part of a light collector embodying another aspect of the invention and being suitable for use in the structure of Figure 1;

Figure 5 is a perspective view of the light collector of Figure 4;

Figure 6 is a schematic side view of part of an optically active core forming part of the light collector;

Figure 7 is an end view of part of the light collector including a Fresnel mirror;

Figure 8 is an alternative end view showing part of the light collector;

Figure 9 is a graph illustrating the performance of a typical light collector;

Figure 10 is a graph illustrating the absorption spectrum of lead sulphide (PbS) quantum dots;

Figure 11 is a schematic view of a linear system of energy concentration and collection;

Figure 12 is an illustration of the relationship between the Sun's azimuth and required distance between a lens and a collector; Figure 13 is a schematic side view of a lens control mechanism suitable for use in the structure of Figure 1 ; and

Figure 14 is a schematic side view of an alternative lens control mechanism suitable for use in the structure of Figure 1.

Detailed Description of the Drawings

Referring now in particular to Figure 1 of the drawings, there is shown, generally indicated as 10, part of a solar concentrating structure embodying one aspect of the invention, and in the preferred form of a blind. The blind 10 includes at least one, but typically a plurality of, lenses 12 each being associated with, or coupled to, a respective light collector 14. Each light collector 14 comprises an elongate body 16 and may be said to take the form of a rod. The preferred light collectors 14 embody another aspect of the invention and are described in further detail hereinafter with reference to Figures 3 to 5.

The lenses 12 are elongate, or linear, line-focussing lenses, i.e. lenses of the type that focus incident light to a focal line. In the preferred embodiment, the lenses 12 comprise linear Fresnel lenses, or equivalent lenses. The advantages of using Fresnel lenses include that they have the desired characteristics of being substantially flat and lightweight. The arrangement is such that the focal line of each lens 12 is substantially coincident with the respective light collector 14 and, preferably, with the longitudinal axis of the respective collector 14 (since this is the preferred location of the core 20, described hereinafter).

The lenses 12 and collectors 14 are mounted on a framework or other support structure that is not shown in Figures 1 to 3 for reasons of clarity. Examples of suitable support structures are described hereinafter with reference to Figures 13 and 14. In the preferred embodiment, the solar concentrator 10 takes the form of a Venetian-type blind, each lens serving as a respective slat of the blind. To this end, the lenses 12 are substantially parallel with one another and the light collectors 14 are substantially parallel with one another and with the lenses 12.

Figure 2 illustrates the solar concentrator 10 in side view and shows incident radiation in the form of sunlight beams 18 concentrated by the lenses 12 onto the light collectors 14, the focal line of each linear lens 12 being made to coincide with the longitudinal axis of the corresponding collector 14. The relative position and/or orientation between each lens 12 and its respective collector 14 is adjustable by means of an appropriate tracking mechanism (not shown in Figures 1 and 2) to keep the focal line of each lens 12 incident on the respective collector 14 in the desired manner as the Sun moves during the course of a day. In the preferred embodiments, it is more convenient to adjust the position and/or orientation of the lenses 12 rather than the collectors 14. Suitable tracking mechanisms are identified hereinafter with reference to Figures 11 to 14.

The preferred collector 14 is substantially transparent along substantially its entire length and may be referred to as a transparent solar power collector (TSPC). The lenses 12 are substantially the same length as the respective collectors 14, although in preferred embodiments are slightly shorter than the collectors 14.

Under typical operating conditions and in preferred embodiments, the tracking system ensures that the direction of the direct sunlight is contained in the plane defined by the optic axis and the direction of the Fresnel grooves, and that the focal line of the lens is always superimposed to the longitudinal axis of the collector 14.

Referring now to Figures 3 to 5, the preferred collector 14 is described in more detail. The body 16 comprises a transparent, or substantially transparent, rod. The body 16 may be formed from any suitable material, for example glass, resins, acrylic or other plastics, that allow the longitudinal transmission of radiation, e.g. visible light, from one end of the body 16 to the other, preferably with a maximum attenuation of less than 50%.

The body 16 is substantially rectangular or square in transverse cross section, although in alternative embodiments it may take other cross sectional shapes, e.g. polygonal or circular.

The preferred collector 14 includes a core 20 running longitudinally of the body 16, preferably along the longitudinal axis of the body 16. It is preferred that the core 20 is shorter than the body 16 and is positioned such that it stops short of either end 22, 24 of the body 16. It is also preferred that the length of the core 20 is substantially the same as the length of the focal line (or line of least confusion) of the respective lens 12. The lenses 12 are arranged in use such that their focal line is substantially in register with the core 20 of the respective collector 14. The core 20 is preferably formed from a material having a refractive index that is the same, or substantially the same, as the rest of the body 16. The core 20 may be formed from any suitable material, for example glass, resins, acrylic or other plastics, that allow the longitudinal transmission of radiation, e.g. visible light, from one end of the core 20 to the other, preferably with a maximum attenuation of less than 50%. The core 20 and body 16 need not necessarily be formed from the same material. Preferably, a single continuous core 20 is provided although, in alternative embodiments, the core may be made up of a plurality of spaced apart core segments.

The core 20 is optically active in that it is not completely or substantially transparent to incident light or other radiation. The core 20 alters at least one property of the incident radiation to produce secondary radiation, the arrangement being such that the secondary radiation is directed towards the ends 22, 24 of the collector 14. The altered properties of the incident light may include one or more of the following: bandwidth, frequency (or wavelength), orientation, direction, speed, velocity or energy. For example, the core 20 may be adapted to absorb incident radiation 18 and to emit absorbed light in a different form and/or may be adapted to optically scatter, preferably elastically, incident radiation. In preferred embodiments, the core 20 absorbs incident radiation in a first frequency band and emits absorbed radiation in a second frequency band that is different to, typically narrower than, the first frequency band. Alternatively, or in addition, the optically active nature of the core may be such that the angular distribution of the emitted radiation is altered with respect to the incident radiation, for example this may be achieved by optical scattering or by causing the plane in which it an emitted beam of radiation propagates to be rotated with respect to the plane in which the corresponding incident beam(s) of radiation propagate.

A number of suitable ways of adapting the core 20 to be optically active are described hereinafter by way of example, although it will be understood that any conventional means of rendering the core optically active may be used. The preferred core 20 may therefore be described as a longitudinal optically active core (LOAC). The main function of the core 20 is to convert the incident concentrated solar power (or other incident radiation) into a beam, or beams, of radiation trapped inside the body 16 and propagating towards the ends 22, 24 of the body 16. Typically, the absorbed light is converted into emitted beams 23 of radiation having different spectral (frequency) and/or angular distributions when compared to the incident radiation 18. The core 20 is substantially rectangular or square in transverse cross section, although in alternative embodiments it may take other cross sectional shapes, e.g. polygonal or circular. The cross sectional shape of the core 20 need not necessarily be the same as that of the body 16.

The drawings show the secondary radiation being emitted from a particular point of the core 20, although it will be understood that secondary radiation is emitted by all of the optically active core 20 when illuminated by the concentrated solar radiation 18. At least one photovoltaic, or solar, cell 28 is provided at at least one, and preferably both, ends 22, 24 of the body 16. The photovoltaic cells 28 are arranged to absorb the trapped radiation at the ends 22, 24 of the body 16 and convert it into electricity. In alternative embodiments (not illustrated) any other transducer(s) for converting solar energy, or other captured radiation, into another energy form may be provided as desired.

Optionally, the collector 14 includes or is associated with a heat pipe 30, preferably formed from any suitable transparent material. The heat pipe 30 may be either embedded in the core 20 (as illustrated) or may be otherwise in thermal contact with the core 20 to collect waste energy dissipated by the core 20 (as well as infrared radiation which cannot be converted into electricity) and to transfer it away from the collector 14. In the illustrated embodiment, the heat pipe 30 is arranged to direct heat energy to the ends 22, 24 of the body 16 whereupon it may be dumped to a heat sink (not shown) or directed to an external system (not shown), e.g. a heating system. The term "heat pipe" is intended to include, by way of example, a transparent pipe or conduit carrying running water, or other liquid or fluid, that would absorb substantially all of at least the residual infrared component of the incident solar radiation, thus preventing it from being transmitted through the collector 14.

During use, the incident sunlight 18 is concentrated by the lens 12 onto the core 20, whereupon in preferred embodiments it is elastically-scattered, or absorbed and subsequently re-emitted as secondary radiation beams 23 (only examples of each type of beam 18, 23 are shown for illustrative purposes). The transmitted, scattered, re-emitted or otherwise altered radiation 23 may be referred to as secondary radiation. As is described in more detail below, the light collector 14 is arranged to trap the secondary radiation 23 and guide as much of it as possible towards the solar cells 28. The preferred rectangular or square cross-sectional shape of both the collector 14 and the core 20 helps to maximise the collection, or trapping, of secondary radiation 23 within the body 16.

Referring in particular to Figures 4 and 5, during use a portion of secondary radiation 23 is trapped by total internal reflection inside the body 16 of the collector 14. The trapped secondary radiation 23 propagates along the body 16 towards the solar cells 28. The probability this radiation is re-absorbed, or re- scattered, by the core 20 depends on the ratio between the respective areas of the body 16 cross section and the core 20 cross section. The preferred arrangement whereby the core 20 stops short of the ends 22, 24 of the body 16 reduces the amount of re-absorption/re-scattering occurring near the ends 22, 24 of the body 16 and so allows a higher proportion of the radiation 23 to reach the cells 28.

The rest of the secondary radiation may be emitted within escape cones 32 and so would leak out of the collector 14 as indicated by arrows LR in Figure 4. By way of example, Figure 5 shows four escape cones 32 associated with a specific emission point on the core 20. The rectangular, or square, cross sectional shape of the body 16 causes the total solid angle spanned by the emission cones 32 to be significantly smaller than it would be in, say, a cylindrical body where the total escape solid angle is given by the total volume spanned by one of the cones as it completes a full rotation around the collector axis.

Secondary beams 23 which are emitted outside the escape cones 32 remain trapped within the body 16, the reflections at the flat surfaces of the body 16 only switching the beam 23 back and forth between an original direction and a complementary one that is always outside the escape cones 32. Moreover, the preferred rectangular or square cross sectional shape promotes the spread of the secondary radiation 23 so that it becomes substantially uniformly distributed across the section of the collector 14. If the section were circular, the trapped radiation would always bounce back towards the core after every reflection and therefore cancel any advantage associated with the dilution of the concentrated radiation in the core 20 to secondary radiation in the body outside of the core 20.

It will be seen from the foregoing that the core 20 converts the primary, or incident, radiation 18 into two types of secondary radiation, namely: radiation 23 trapped in the body 16 by total internal reflection; and scattered, transmitted and/or re-emitted radiation propagating from the core 20 towards the external surfaces of the body 16 within the escape cones 32.

In the preferred embodiment, the trapped beams 23 become substantially homogeneously spread over the whole volume of the body 16 and are eventually absorbed by the solar cells 28. As a result, the solar cells 28 are substantially uniformly illuminated irrespective of the alignment of the optical components. The effective dilution of the radiation in the body 16 outside of the core 20 reduces the probability of re-absorption/re-scattering by the core 20 a factor approximately equal to the ratio between the respective cross sectional areas of the body 16 and the core 20. This reduces the impact of non-radiative losses associated with every absorption-reemission and scattering processes.

By way of example, the total geometric concentration, C _g , of the incident beam 18 is the ratio between the area of the lenses 12 and twice the cross sectional area of the body 16. C _g is essentially the product of the primary concentration provided by the linear lens 12 (typically 30x to 5Ox), a subsequent dilution due to the emission of the secondary radiation from the core 20 to the whole volume of the collector 14 (typically small percentage value) and a further concentration stage due to the waveguide effect (typically 10Ox to 50Ox).

The dilution of the radiation allows a significant overall light collection efficiency (ratio between number of incident photons and number of photons reaching the solar cells per unit time) to be obtained over a typical length required in, say, a

Venetian blind-type structure, or other embodiment. In order to reduce the amount of secondary radiation that escapes from the collector 14, it is preferred to provide the collector 14 with one or more reflecting and/or filtering elements. In particular, one or more reflecting and/or filtering elements, especially a dichroic element, may be provided on, or at, at least one of the external surfaces of the body 16 and/or at the interface between the core 20 and the body 16.

With reference to Figure 6, a dichroic mirror or reflector in the form of a dichroic coating or layer 34 may be provided on or at the boundary surface between the core 20 and the body 16. The layer 34 may partly or wholly cover the boundary surface. The dichroic layer 34 is designed to reflect radiation emanating from the core 20 and having certain physical characteristics while allowing other radiation emanating from the core 20 to pass. In particular, the layer 34 is arranged to reflect radiation that would otherwise be within an escape cone 32 and to allow other radiation to pass. To this end, the design of the layer 34 is such that its spectral, or frequency, response depends on the angle of incidence of radiation thereon. In particular, radiation Rl incident thereon at a relatively steep angle, e.g. substantially normal incidence, is reflected back into the core 20, while radiation R2 incident thereon at a more shallow angle is transmitted through the layer 34 and is able to leave the core 20.

The provision of the dichroic layer 34 is particularly useful in cases where the frequency of the secondary radiation is narrow-band (compared to the broadband incident solar radiation). This occurs when the core 20 contains (high-efficiency) luminescent centres, for example quantum dots or organic dyes (described in more detail hereinafter). The boundary between the core 20 and the rest of the body 16 is provided with a dichroic coating or layer adapted to reflect only the secondary (narrow-band) radiation at a relatively steep, e.g. normal, or substantially normal, incidence, while being transparent to radiation having any other incidence angle.

The layer 34 traps the radiation Rl in the core 20 until it is re-absorbed or re- scattered (see point Pl in Figure 6) and partly re-emitted at an angle outside the escape cones 32.

With reference to Figure 7, a dichroic reflector/filter in the form of a dichroic mirror 36 may be provided on, or at, at least one of the external surfaces of the body 16. Preferably, the mirror 36 comprises a Fresnel dichroic mirror. The dichroic mirror 36 is designed to reflect radiation emanating from the core 20 and having certain physical characteristics while allowing other radiation emanating from the core 20 to pass. In particular, the mirror 36 is arranged to reflect radiation that would otherwise be within an escape cone 32 and to allow other radiation to pass. To this end, the design of the mirror 36 is such that radiation R3 incident thereon at a relatively steep angle, e.g. substantially normal incidence, is reflected back to the core 20, while radiation incident thereon at a more shallow angle is transmitted through the mirror 36 and is able to leave the collector 14. In the illustrated embodiment, secondary radiation emitted within the escape cones 32 is let through the outer surface of the body 14 whereupon the dichroic Fresnel mirror 36 reflects the leakage radiation back to the core 20. This solution is particularly effective only if the core 20 is sufficiently small with respect to the body 16 that the escaping radiation hits the mirror 36 at normal, or substantially normal, incidence.

Figure 8 shows a specific example of a dichroic mirror 36A and a Fresnel dichroic mirror 36B mounted on the external surface of a collector 14 which is, by way of example, approximately 2cm x 2cm in cross sectional area (Figure 8 only shows approximately a quarter of the collector in end view, the axes representing distance in centimetres). It is assumed that the secondary radiation is emitted from the core at point 0, 0. It will be seen that the mirrors 36A, 36B reflect rays that are incident substantially normally thereon. Either the dichroic mirror 36 or dichroic layer 34 allows a significant part of the non-trapped radiation to be recycled in the collector 14 by re- absorption/scattering.

When the secondary radiation is either red-shifted (i.e. is of decreased frequency when compared to the incident solar radiation 18), or has a much narrower spectral distribution with respect to the incident solar radiation 18, the mirror 36 or the layer 34 can be provided on the surfaces or the body 16 and/or core, respectively, that are directly exposed to the primary beam 18, since they would only prevent a relatively small proportion of the (broadband) incident radiation 18 from reaching the core 20. This is always the case when the secondary beams 23 are dominated by the narrow-band emission of luminescent dyes or quantum dots. In preferred embodiments, therefore, dichroic mirrors 36 and/or dichroic layers 34 are applied either to all of the relevant boundary surfaces of the body 16 and/or core 20, or to all of said surfaces except those upon which the primary radiation 18 is incident.

Figure 9 illustrates, by way of example, the performance of a Im long collector 14 having 500 ^χ geometric concentration and secondary beam dilution = 0.04, with an optimised core 20 based on PbS (lead sulphide) quantum dots having 85% radiative quantum efficiency. Curve Cl shows the concentrated direct-sunlight spectrum (i.e. of the primary beam 18). Curve C2 shows the narrow-band radiation collected at the ends 22, 24 of the collector. Curve C3 shows the primary beam radiation transmitted by the core 18. It will be seen that the secondary radiation C2 is emitted in a relatively narrow frequency band when compared to the primary radiation Cl. Figure 10 shows the absorption spectrum of PbS quantum dots used in the example of Figure 9.

In the foregoing description, the primary (or incident) radiation comprises sunlight. It will be understood that other sources of primary radiation may be used. In general, the invention may be used with radiation of any wavelength that is capable of being absorbed by, or otherwise received by, the core and converted into secondary radiation, which may include visible and non- visible light.

It will be apparent from the foregoing that all of the opaque components of the collector 14, e.g. solar cells and heat sinks, are confined to the ends 22, 24 of the body 16 thus making the body 16 substantially transparent. Also, all of the electrical components are physically separated from the optical components and so can be hidden from the field of view.

In preferred embodiments, core 20 is substantially parallel or coincident with the longitudinal principal axis of the collector 14 and its transverse dimensions are chosen to be substantially equal to, or slightly larger than, the width of the line of least confusion (also known as the focal line) generated by the associated lens 12 when properly aligned to the incident beam of direct sunlight.

The core 20 typically contains optically active particles (OAPs) whose function is to scatter, or otherwise alter, the incident beam of concentrated sunlight, or other incident radiation. There are three preferred options:

A). Luminescent particles such as organic dyes, semiconductor quantum dots or phosphors (any of those materials or any combination of two or more of them), advantageously engineered to absorb a significant proportion of the concentrated direct sunlight, or other incident radiation, and to re-emit secondary radiation over a narrow wavelength band compared to the bandwidth of the incident radiation. This narrow band is typically centred near the absorption edge of the OAPs.

The luminescent particles or centres are advantageously selected to provide the best possible combination of the following properties: i) high absorption cross section over a standard AM1.5d solar spectrum ii) high quantum efficiency of radiative de-excitation iii) radiation emitted in a spectral range as close as possible to the absorption edge of a commercially- viable single-junction solar cell for concentrators but all inside the wider spectral range where the cell quantum efficiency is close to its maximum; and/or

B) Light scattering particles such as metal nanoparticles or micrometer-size reflectors, advantageously arranged to diffuse the largest possible fraction of concentrated light at large angles so that it becomes trapped inside the collector 14 by internal reflection. In the case of metal nanoparticles, a combination of different materials, sizes and shapes may be chosen in order to obtain a superposition of different plasmonic resonance peaks covering the widest possible portion of the solar spectrum. The nanoparticles should advantageously have a minimum size to minimise light absorption and maximise the light scattering efficiency, and/or.

C) Either a mixture of luminescent particles and metal nanoparticles, or specially designed nanomolecules made of at least one luminescent particle and one metal nanoparticle. In both cases the optical properties of the particles or centres are advantageously tailored in such a way that the large dipole fields generated by the metal nanoparticle at the plasmonic resonance is exploited to enhance the light absorption coefficient of the luminescent particle particularly near its absorption edge.

The optimum transverse optical thickness of the core 20 is the result of a trade-off between maximising the absorption/scattering of the primary beam 18 and minimising the re-absorption/scattering of the propagating secondary beam 23. As a result, the primary beam 18 is substantially attenuated as it crosses the core 20 but a residual beam is transmitted towards the external surface of the body 16 at the opposite side to the incidence area. This beam may then be intercepted by the mirror 36. By way of example, the core 20 may take the form of either: a portion of the body 16 where the OAPs are directly dispersed in the material from which the body 16 is made; or a longitudinal cavity formed inside the body 16 and filled with OAPs in solution. In the latter case solvents having substantially the same refractive index as the surrounding body material are preferably used to optimise the continuity of the refractive index at the interface between the core 20 and the rest of the body 16. This promotes the escape of any emitted radiation from the core 20 and its dilution into the whole volume of the body 16.

The preferred rectangular or square cross sectional shape of the body helps to ensure that no significant fraction of the secondary beam 23 is lost through the escape cones 32 as a result of multiple reflections at the external surface of the body 16, and to cause the secondary radiation 23 to be spread uniformly through the volume of the body 16. Consequently, the spatial homogeneity of the radiation flux at the two ends 22, 24 of the collector 14 where the photovoltaic cells 28 are mounted is maximised, and the rate of re-absorption/scattering of the secondary beam 23 by the core 20 is minimised.

The dichroic mirrors/reflectors 34, 36 may for example comprise either multilayer dielectric reflectors or heavily doped semiconductor oxides such as ITO or ZnO. The preferred dichroic layer 34 is arranged to trap as much narrow-band radiation emitted inside the escape cones 32 of the collector 14 as possible so that it is reabsorbed, and to transmit the secondary beam 23 so that it is efficiently spread over the whole volume of the collector 14. The preferred dichroic mirror(s) 36 are fitted outside the collector 14 so that there is an air gap between the mirror 36 and the external surface of the collector 14. The mirror 36 is typically designed for the wide-band radiation transmitted by the core 20 to be recycled back to the core 20 and eventually re-irradiated in the opposite direction to the primary beam 18. Due to the preferred rectangular section of the external surface, the dichroic mirror 36 is substantially elliptical in shape and so can be replaced by a flat Fresnel mirror as illustrated in Figures 7 and 8. The mirror(s) 36 may cover substantially the whole surface of the body 16 (apart from the ends 22, 24), in which case its reflectivity should be negligible at wavelengths below the narrow emission band of the secondary beam 23 in order to maximise the transmission of the primary beam 18 to the core 20.

Alternatively, the mirror(s) 36 may cover only the portions of external surface of the body 36 that are never illuminated directly by the primary beam 18, in which case the high reflectivity spectral range of the mirror 36 can be extended to include a significant fraction of the primary-beam radiation that is transmitted by the core (see for example curve C3 in Figure 9).

The optimum optical thickness of the core 20, i.e. the optimum density of OAPs, is advantageously chosen to maximise the radiation collection efficiency at the two ends 22, 24 of the collector 14. The optimum value is a trade-off between the maximisation of primary beam 18 scattering by the core 20 (which would require a relatively high density of OAPs) and the transmission of the trapped secondary beam 23 (which is increased by decreasing the density of OAPs). The secondary beam 23 is diluted and homogeneously spread over a relatively large volume compared to the volume of the core 20 (which is typically between 10 and 30 times smaller). The combination of the concentration of the primary beam 18

(typically 30 ^χ to 50 ^χ ) and the dilution of secondary beam 23 allows the efficient collection of secondary radiation 23, especially from collectors 14 whose length is of the order of approximately 1 metre or more.

In alternative embodiments (not illustrated), the core 20 may be replaced by an optical fibre co-doped with rare-earth atoms (for example Erbium, Ytterbium, Thulium, Holmium etc.) and efficient broadband absorbers (e.g. semiconductor nanoparticles/quantum dots). Alternatively still, core 20 and the cells 28 may be replaced by a longitudinal Graetzel cell (sometimes referred to as a dye-sensitzed solar cell). One implementation (not illustrated) of a Graetzel cell has the inner face of the core comprising an electrically conductive but optically transparent anode contact and suitable nanoparticles coated with a photosensitive dye, with the void of the core filled with an electrolyte and a central cathode running substantially the length of the core 20. Photo voltage and photocurrent is created between the two contacts.

Referring now to Figures 11 to 14, there is described suitable examples of support structures and control mechanisms suitable for use in the apparatus 10. With reference first to Figure 11 , the typical design constraints to be considered when designing a control system for a linear system of energy concentration and collection, including the apparatus 10, are outlined.

The angle of the Sun within the sky changes in both elevation (the angle up from the horizon) and azimuth (the angle along the horizon relative 'straight ahead'). To maintain the focus of direct radiation DR from the Sun on the light collector LC requires that two constraints b and c are met and for optimal energy transfer it is desirable that constraint a is also met, where with reference to Figure 11 : a - is the lens is orthogonal to the collector. b - is the angle of rotation of lens about the collector equals the sun's elevation c - is the distance of the lens from the collector (centre to centre) is related to the azimuth of the Sun as described below.

As the magnitude of Sun's angle of azimuth increases, the distance of separation required between lens LS and light collector LC will decrease. Where a _a is the sun's angle of azimuth, and/is the focal distance of the lens, the distance c will be: c=fcos (a _a ).

In some circumstances, it may be advantageous not to enforce one or more of the constraints. In particular, relaxing constraint a facilitates privacy or allows a greater exterior view. It can also control the ratio of absorbed direct to scattered sunlight. This could be useful at times when there is insufficient diffuse light DL to illuminate the interior.

A further constraint on the system involves safety. It is important to ensure that neither people nor objects come into contact with potentially hazardous concentrated light. It is therefore desirable that the apparatus 10 has an inner transparent panel (e.g. of glass or plastic) prohibiting any object from coming within a distance of 2/ from the lenses, where/is the focal distance of the lens By extending the planar cross-section of Figure 11, it may be understood that the lens angle and the distance to the light collector must be controlled.

In Figure 12, a is the Sun's angle of azimuth,/is the focal distance of the lens LS, and d is the distance between the lens and collector LC. It may be seen that the Sun's angle of azimuth relates d to fin the vector triangle illustrated as 'plan view'.

The function of the lens control mechanism is to simultaneously orientate a plurality of lenses with respect to their respective light collectors. In preferred embodiments, the light collectors are fixed relative to the window frame or other building structure to which they are mounted. This arrangement is advantageous if, for example, the light collector carries a coolant, or other liquid, e.g. via the heat pipe 30.

Referring now to Figure 13, a first example of a lens control mechanism is shown, generally indicated as 50. The mechanism 50 includes a support structure 52 adapted to carry a plurality of lenses LS, for example the lenses 12. The lenses LS are pivotable or rotatably mounted on the support structure 52 so that they are pivotable, or rotatable about a respective parallel axis, which in the cases of the lenses 12 is the respective longitudinal axis. In the illustrated embodiment, the structure 52 comprises at least one set of first and second parallel support members 54, 56 between which the lenses LS are mounted. In use, the support members 54, 56 are vertically disposed. The arrangement is such that the longitudinal axis of each lens LS runs substantially perpendicular with the support members 54, 56. Two or more sets of support members 54, 56 are typically provided, spaced apart along the length of the lenses LS. The lenses LS are coupled to each support member 54, 56 at, or adjacent, each of the longitudinal edges of the lenses LS by any suitable rotatable or pivotable link 58. The support members 54, 56 may take any suitable form, for example rigid, or semi-rigid members formed from any suitable material, e.g. metal or plastics, or flexible members such as cable, chains or the like.

The mechanism 50 further includes means for effecting a pivotal or rotational movement of the lenses LS. In the illustrated embodiment, this comprises rotational drive means 60, e.g. a rotational drive motor, typically a servo motor, coupled to the support structure 52 by means of a linkage mechanism 62. The linkage mechanism 62 may comprise a first link 64, which is typically rigid or semi-rigid, extending between, and pivotably coupled to each, support member 54, 56, for example in the same manner as the lenses LS, and a second link 66 coupled between the drive means 60 and the first link 64. The preferred arrangement is such that the drive means 60 imparts, in use, rotational movement to the second link 66 about an axis that is substantially parallel with the rotational axes of the lenses LS (as indicated by arrow d2 in Figure 13). This causes a corresponding raising or lowering of the first link 64 which has the effect of rotating or pivoting the lenses LS.

The mechanism 50 further includes means for effecting relative displacement of the lenses LS towards and away from the collectors LC. In the illustrated embodiment, this is achieved by providing link 66 as a longitudinally extendible link, and by providing means for effecting extension and retraction of the link 66. By way of example, link 66 may take the form of a linear actuator. The mechanism 50 has two degrees of freedom annotated as dl and d2 in Figure 13. The drive means 60 controls the parameter b, which in correct operation tracks the Sun's altitude. The driven linear axis dl controls the distance between lens and collector (parameter c). As described above, under correct operation this is related to the Sun's azimuth by the relationship given in equation 1.

In order to control the operation of the drive means 60 and linear actuator 66 (or equivalent mechanisms), it is preferred to provide a control system (not shown) for monitoring the movement of the Sun and providing appropriate control signals to the relevant electro-mechanical devices. Alternatively, the drive means and linear actuator, or equivalent mechanisms, could be operated manually by a human operator.

Solar tracking systems are well known and are not described in detail herein. A number of methodologies may be employed. For large grid tied applications, open loop control systems based on geographical location and an accurate determination of time may be used. An alternative approach is to use a closed loop control strategy with a sensing head pointing towards the Sun. Both approaches are embodied by commercially available units, for example as provided by Array Technologies Inc, of Albuquerque, New Mexico under the Trade Mark

WATTSUN (http : //www . w atlsun . com/) . Each strategy has know weaknesses and recent attempts have been made to produce a hybrid tracker. For concentrator systems, the accuracy of the tracking should be typically less than 0.1 of a degree, with the consequence that purveyor of such systems often produce their own trackers. For the embodiment described herein where only a single axis tracks the

Sun, this axis can use a sensor head based tracker. The control of the other axis may use an open loop approach. This arrangement may employ a tracking controller with an "Equation of Time" based hybrid tracker. The time based tracker is a well known and accurate method of determining the position of the Sun based on geographical location and time, but to maintain a high degree of accuracy required it should be complimented with information from a sensor head tracking the Sun's actual position in the single tracked axis. The information from both approaches along with information about the power output may be integrated to produce an accurate and compensated tracking system.

Note that in the mechanism 50, constraint a cannot be broken as it is mechanically fixed, thus the lens and respective collector remain perpendicular to each other.

Referring now to Figure 14, a second example of a lens control mechanism is shown, generally indicated as 150. The mechanism 150 includes a support structure 152 adapted to carry a plurality of lenses LS, for example the lenses 12. The structure 150 may be substantially similar to the structure 50 and so similar descriptions apply.

The mechanism 150 includes means for effecting relative displacement of the lenses LS towards and away from the collectors LC. In the illustrated embodiment, this is achieved by means of a linear drive mechanism 170 comprising, for example a linear bearing mechanism 172.

The mechanism 150 further includes means for effecting a pivotal or rotational movement of the lenses LS. In the illustrated embodiment, this comprises a respective pulley 174, 176 for each support member 154, 156. In this case, the support members 154, 156, or at least the part of them that is coupled to the respective pulley 174, 176, comprises a flexible linkage, e.g. a cable. The mechanism 150 also includes means for rotating each pulley 174, 176, preferably independently of one another. The rotating means may comprise any suitable rotating drive means (not shown). The pulleys 174, 176 are coupled to the drive mechanism 170 by any suitable means, e.g. frame 178.

In use, rotation of the pulleys 174, 176 causes a corresponding raising or lowering of the respective support member 154, 156 which in turn causes rotational movement of the lenses LS. As for the mechanism 50, the operation of the mechanism 150 may be controlled to track the Sun's movement by any suitable means (not shown).

The mechanism 150 of Figure 14 has three controlled degrees of freedom dl, d2 and d3. Two of these d2, d3 are effected by a respective pulleys 174, 176, the third being controlled by the linear actuating mechanism 170. Constraints a, b and c may be satisfied for any given azimuth and elevation of the Sun by selecting an appropriate set of values for dl, d2, and d3.

An advantage of the system 150 over the system 50 is greater flexibility - particularly in that constrain a may be broken. In the physical implementation of the mechanism 150 the control linkages could be any combination of rigid, semirigid or flexible (e.g. cable). It is envisaged, however, that the control linkages would be mostly be flexible cables.

The invention is not limited to the embodiments described herein which may be modified or varied without departing from the scope of the invention