WO2009115940A1

WO2009115940A1 - Light concentrating device

Info

Publication number: WO2009115940A1
Application number: PCT/IB2009/050996
Authority: WO
Inventors: Bernd Ackermann
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N.V.
Priority date: 2008-03-18
Filing date: 2009-03-10
Publication date: 2009-09-24
Also published as: TW200944713A

Abstract

The present invention relates to a light concentrating device comprising at least one translucent optical element (2) ortranslucentarray(1) ofoptical elements (2) designed and arranged to collect light (8) impinging on saidelement(s)(2) and to couple the collected light into an arrangement of lightguides (3) extending in a plane below said optical element(s) (2). A luminescent light concentrator (4) is arranged below said arrangement of lightguides (3) to collect and concentrate light penetrating throughsaid optical element (2) orarray (1) ofoptical elements (2) and said arrangement of lightguides (3). Thedevice allows collectingdaylightwith a high efficiency independent of the weather conditions, i.e. direct sunlight anddiffused skylight.

Description

Light concentrating device

FIELD OF THE INVENTION

The present invention relates to a light concentrating device comprising at least one optical element or an array of optical elements designed and arranged to collect light impinging on said element(s) and to couple the collected light into an arrangement of lightguides (e.g. optical fibers) extending in a plane below said optical element(s). Light concentrating devices of this type are in particular suitable for daylighting systems which can bring daylight deep into rooms and buildings. Fiber optic daylighting systems are especially interesting since they can be well integrated into the construction of buildings.

BACKGROUND OF THE INVENTION

It is well-known to concentrate sunlight by optical means in order to use it for solar power applications like heating, cooking, lighting, photovoltaic electricity generation, thermal electricity generation, horticulture, bioreactors etc. WO 03/091621 discloses a daylighting device in which a solar ray collecting device is arranged to collect sunlight and to direct said sunlight to a light transmitting device, which may be formed of an arrangement of lightguides. The solar ray collecting device comprises several convex and concave lenses which are arranged to be moveable in dependence of the angle of incidence of the solar light so as to focus the solar light into the light transmitting device. Instead of such a solar ray collecting device with the convex and concave lens arrangement it is also possible to collect direct sunlight by tracking the sun using an array of convex lenses mounted on a movable base plate, an array of mirrors (e.g. Cassegrain optics) mounted on a base plate in a similar way or an array of lenses or mirrors movable individually but synchronously to track the sun.

The devices collecting direct sun light by tracking the sun can reach a high collecting efficiency only in the case of direct sun light, but are not very efficient in the case of diffuse skylight. SUMMARY OF THE INVENTION

It is an object of the present invention to provide a light concentrating device, in particular for daylighting applications, which provides a high efficiency not only in the case of direct sunlight but also in the case of diffuse skylight.

The object is achieved with the light concentrating device according to claim 1. Advantageous embodiments of this device are subject matter of the dependent claims or are disclosed in the subsequent portions of the description. The proposed light concentrating device comprises at least one translucent optical element or translucent array of optical elements designed and arranged to collect light impinging on said optical element(s) and to couple the collected light into an arrangement of lightguides extending in a plane below said optical element(s). The proposed light concentrating device is characterized in that in addition to the above direct sun light collector a luminescent light concentrator is arranged below said arrangement of lightguides to collect and concentrate light penetrating through said optical element or array of optical elements and said arrangement of lightguides.

As already indicated above, devices collecting direct sunlight by tracking the sun collect virtually no diffuse skylight. Since in the proposed device a translucent optical element or a translucent array of optical elements is used to collect the direct sunlight, diffuse sky light not collected by said optical element(s) can penetrate through the optical element or array of optical elements and the arrangement of lightguides extending in a plane below said optical element(s) and can be collected by a further component, a luminescent light concentrator. By stacking the optical element or array of optical elements, which is preferably actuated to track the sun, on top of a luminescent light concentrator, the major part of diffuse sky light passing through the optical element or array of optical elements is collected by the luminescent light concentrator. This significantly improves the efficiency of the device under changing light or weather conditions. Luminescent light concentrators are known, for example, from A.A. Earp et al, "Optimization of a three-colour luminescent solar concentrator daylighting system", Solar Energy Materials & Solar Cells 84 (2004), 411 - 426. The description of the luminescent solar concentrator of this publication is incorporated herein by reference. Such luminescent concentrators are known to collect both direct sunlight and diffuse skylight, but are comparatively inefficient in collecting the direct sun light. With the claimed device combining direct sunlight collecting systems in a stack with such a luminescent solar concentrator, the device performs well both under clear sky (mainly direct sunlight) and overcast sky (mainly diffuse sunlight) conditions.

The basic concept of a luminescent light concentrator is to provide one or several layers of a translucent material doped with fluorescent or luminescent dyes which effectively absorb the impinging skylight and emit fluorescent or luminescent light in the visible wavelength region. These layers, preferably doped sheets of a glass or polymer material like PMMA (Polymethylmetacrylate) are coupled with their side faces to one or several lightguides in which the emitted light is transported. By using a stack of several sheets doped with dyes of different color, a high absorption of the impinging light and a nearly white emitted light, based on the combination or mixture of the colors of the different sheets, can be achieved. Skylight which is not absorbed in the upper layer is preferably absorbed in the intermediate or the lower layer by appropriate selection of the dyes, for example in the case of three layers with different dyes.

With the proposed light concentrating device a new type of solar concentrator is provided which can be applied in fiber optic daylighting. Fiber optic daylighting is especially attractive since it allows for seamless integration with artificial lighting and can be used in building renovation. The application of this light concentrator, however, is not limited to daylighting. Other applications may be for example in the fields of photovoltaics or bioreactors or may include fields like heating, cooking, photovoltaic electricity generation, thermal electricity generation, horticulture etc.

The direct sunlight collected by the optical element or array of optical elements and the remaining light collected by the luminescent light concentrator are both transported with the corresponding lightguides to the places in which the collected light is to be used. The lightguides are preferably formed of optical fibers or optical fiber bundles. In a preferred embodiment, the fibers are multimode fibers, in particular plastic optical fibers (POF) having a high numerical aperture. With the large diameter of such fibers, for example with a core diameter of nearly lmm, transmission is possible even if the ends of the fibers are slightly soiled or damaged or if the light axis is slightly off center.

The optical element or array of optical elements of the proposed device is preferably part of a tracking system which actively moves the optical elements for tracking the sun. Such tracking systems for optical elements are known in the art. The proposed device is not limited to any of such known systems as long as these arrays are sufficiently translucent to allow the penetration of diffused skylight to the underlying luminescent light concentrator. Examples of appropriate optical elements are conventional lenses or Fresnel lenses as known in the art. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described herein after.

BRIEF DESCRIPTION OF THE DRAWINGS

The following exemplary embodiments show examples of the proposed light concentrating device and components thereof with reference to the accompanying figures without limiting the scope of protection. The figures show:

Fig. 1 a schematic view of a device according to the present invention;

Fig. 2 a schematic view of a luminescent light concentrator which can be used in the proposed device; and

Fig. 3 a schematic view of optical elements which can be used in the array of optical elements of the proposed device.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 shows a schematic view of an embodiment of the proposed light concentrating device. This device comprises a two-dimensional array 1 of optical elements 2 which are in this example formed of lenses each mounted in a cone-formed holder. The holders can be actuated by appropriate driving units, not shown in this figure, in order to synchronously track the way of the sun. An appropriate tracking mechanism is for example shown in WO 2006/049560. With this tracking mechanism the direct sunlight is at any time collected very efficiently by the array 1 of optical elements 2. The lenses focus the sunlight to the entry aperture of optical fibers 3 which are appropriately bend to extend in a plane below the array 1 of optical elements 2. This arrangement of fibers 3 is schematically sketched in figure 1.

The light collected by the optical elements 2 is transported by the fibers 3 to the left side end of the fibers. By appropriately extending the length of the fibers 3 to the desired application, for example the room for artificial daylighting, the direct sun light is transported through the fibers 3 to this room. The holders of the lenses in this case are made of a transparent plastic material, so that impinging light which is not collected by the lenses penetrates through the array of lenses and through the arrangement of fibers 3 to the luminescent light concentrator 4 which is arranged below the arrangement of fibers 3 as indicated in figure 1. For this reason the tracking mechanism should also be constructed to allow the transmission of light as far as possible. Especially in case of diffuse skylight 14, i.e. in the case of an overcast sky, most of the light penetrates to the luminescent light concentrator 4, since only rays impinging parallel to the optical axis of the lenses of the array are focused on the entrance window of the fibers 3.

This luminescent light concentrator 4 is doped with fluorescent dyes which absorb a high portion of the impinging light. The light emitted by these fluorescent dyes is coupled into an optical lightguide 5, for example a further optical fiber or bundle of optical fibers, which is indicated on the left side of the luminescent light concentrator 4 in figure 1. The light emitted by the dyes can then in the same way as the direct sun light be transported to the desired application with the optical lightguide 5. The lenses in the above device may be convex lenses of a diameter of d =

3 cm and a focal length of f = 2.8 cm. The numerical aperture of the fiber of PMMA is 0.47 = V(ni² - n₂ ²), wherein ni is the refractive index of the core of the fiber and n₂ is the refractive index of the cladding of the fiber. The numerical aperture of a lens is sin (atan ( d / ( 2 * f ) ) ). If in simple cases, in which no graded index or similar fiber is used, both numerical apertures are identical, the diameter of the lens of a given focus length is ideally chosen maximum such that all collected light enters the fiber. The fibers 3 may be POF fibers having an inner core diameter of approximately lmm and formed to a bundle of fibers. Instead of individually moving each of the lenses or holders, the whole lens arrangement including the fibers 3 and luminescent concentrator 4 may be mounted on a mounting plate which is then moved by motors to track the sun.

Figure 2 shows an example of a luminescent light concentrator 4 which may be used in the proposed device. This luminescent light concentrator 4 is formed of a stack of three colored luminescent solar concentrators 6 connected to clear flexible lightguides 7. Each colored luminescent solar concentrator consists of a clear PMMA matrix with dimensions of approx. l m x θ,15 m x 0,002 m and is doped with a colored fluorescent dye. For the purpose of light transport the solar concentrators are coupled with optically clear clue to the clear PMMA lightguides 7. With these lightguides 7 the light can be transported to underground rooms or windowless rooms in the center of a building. By appropriate selection of dyes, white light output can be achieved, given a good match with the color of daylight.

In the luminescent light concentrator shown, the upper sheet may contain a violet dye, the intermediate sheet may contain a green dye and the lower sheet may contain a pink dye. The dyes are selected to have a high quantum efficiency, to emit fluorescent light which in combination of all three colors is very close to white light and to overlap in their absorption bands such that most of the impinging light is absorbed by the three layers or sheets. Solar energy enters the stack where it may be absorbed and randomly re-emitted by the violet dye in the top sheet. If the emitted photons travels below the critical angle with respect to the top or side surface of the sheet it will leave the collector. Otherwise it will be totally internally reflected to the end of the collector and then passes through the lightguide 7 to the illuminating end of the lightguide 7. Photons that are not absorbed by the violet dye may be transmitted through to the next sheet, where they may be absorbed and re-emitted by the fluorescent green dye. Highly reflective mirrors are fixed to the back edge of each sheet to reflect photons that are originally directed away from the lightguides 7. Some emitted photons may also leave each sheet at its base and enter in the pink sheet at the bottom of the stack where they may be subsequently re-absorbed and re-emitted. A white or reflective (e.g. of aluminum) base plate is placed under the pink sheet to reflect any light reaching the base of the stack, thus increasing the absorption efficiency of the stack.

Figure 3 shows an example of optical elements which can be used in the array 1 of optical elements 2 of the proposed device. In this case the array of optical elements consists of two arrays of lenses 12, 13 arranged in two parallel planes one behind the other. The upper plane contains an array of convex lenses 12, the lower plane an array of concave lenses 13. The two lens arrays can be moved independent from one another in their corresponding planes allowing the tracking of the sun as can be seen from the three partial views showing different angles of the impinging direct sun light 8. The combination of the convergent lens 12 and the concave lens 13 focuses the impinging sunlight to the entrance aperture of the optical fiber 3 for further transport. Light which does not impinge under the correct angle does not enter the fibers 3 and passes to the luminescent light concentrator not shown in this figure.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. The different embodiments described above and in the claims can also be combined. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from the study of the drawings, the disclosure and the appended claims. For example, the construction of the luminescent light concentrator is not limited to the exemplary construction in the figures. The concentrator may have less or more sheets of dyes and for example may also have other dimensions. The array of optical elements at top of the device may also have any appropriate design able to collect direct sunlight and to be sufficiently translucent to allow diffused skylight which is not collected by the array to pass to the luminescent light concentrator. Furthermore, the distribution of the optical elements in the array and the arrangement of the lightguides is not critical and may have any appropriate form.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that measures are recited in mutually different dependent claims does not indicate that a combination of these measures can not be used to advantage. The reference signs in the claims should not be construed as limiting the scope of these claims. LIST OF REFERENCE SIGNS

1 array of optical elements

2 optical elements/lenses 3 optical fiber or fiber bundle

4 luminescent light concentrator

5 optical lightguide

6 luminescent solar concentrator (LSC)

7 flexible lightguide 8 direct sunlight

11 hole

12 convex lens

13 concave lens

14 diffuse skylight

Claims

CLAIMS:

1. A light concentrating device comprising at least one translucent optical element (2) or translucent array (1) of optical elements (2) designed and arranged to collect light (8) impinging on said element(s) (2) and to couple the collected light into an arrangement of lightguides (3) extending in a plane below said optical element(s) (2), wherein a luminescent light concentrator (4) is arranged below said arrangement of lightguides (3) to collect and concentrate light penetrating through said optical element (2) or array (1) of optical elements (2) and said arrangement of lightguides (3).

2. The device according to claim 1, wherein said arrangement of lightguides (3) is formed of optical fibers or optical fiber bundles.

3. The device according to claim 1, wherein said optical element (2) or array (1) of optical elements (2) is part of a tracking system which actively moves the optical element(s) (2) for tracking the sun.

4. The device according to claim 1, wherein said array (1) of optical elements is an array of lenses.

5. The device according to claim 1, wherein said luminescent light concentrator (4) is formed of one or several lightguides extending in a plane parallel to said plane of the arrangement of lightguides (3) and being at least partly doped with one or several luminescent materials.

6. The device according to claim 1, wherein said luminescent light concentrator (4) is formed of a stack of at least three luminescent solar concentrator sheets (6) coupled to clear flexible lightguides (7), each of said sheets being doped with a differently colored fluorescent or luminescent dye.

7. A daylighting system comprising one or several light concentrating devices according to any one of claims 1 to 6.