DE2737847A1 - Solar energy electric or thermal energy converter - has stacked concentrators with fluorescence centres and solar cells each converting part of incident light - Google Patents

Abstract

Light is collected in a transparent layer (concentrator) whose index of refraction is greater than that of the surrounding medium, and which contains fluorescent centres. The light is then applied to a solar cell (or an absorber). More than one concentrator/solar cell combination is used. The concentrators are separated by a medium with a lower index of refraction than that of a concentrator. They are stacked, so that each concentrator converts a part of the incident spectrum into fluorescent light applies it to a solar cell or an absorber.

Description

Device with luminescence light concentrators for

Conversion of Solar Energy The invention relates to devices for converting light energy into electrical or thermal energy. She relates continued to rely on light concentrators, which are made up of thin layers of transparent solid or liquid substances with embedded fluorescence centers and which are in Connection with solar cells to convert solar energy into electrical energy to serve.

Background Art Recently, some patent applications and a magazine article about a new form of light concentrator for solar energy use emerged, which are listed here: / 1 / W.H. Weber and J. Lambe Applied Optics, Vol. 15, No. lo, October 1976/2 / Research Disclosure No. 29, Jan. 1975 pp. 20 and 21 as well as our following German patent applications / 3 / device for conversion from solar energy to electrical energy P 2629 641.3 - 13 / 4 / device for converting light energy into thermal energy P 2629 641.3 - 13/5 / device for converting solar energy into electrical energy P 2628 281.7 - 33 This Representations are all based on the following principle of light concentration: sunlight is captured in an inherently transparent layer, the fluorescence centers contains. The fluorescence centers absorb the radiation in a certain wavelength range, convert them into longer-wave radiation and re-emit them. Because the radiation is re-emitted in all directions, a very large part of this radiation remains due to total reflection in the slice and is transmitted in the slice plane. When the fluorescent dye is selected so that absorption and emission bands have as little overlap as possible, then this is the absorption length of the fluorescent light very large, i.e. large concentrator areas are possible. The concentrator can go out Plastic or glass are made in which the fluorescent molecules are dissolved, or also from a liquid solution contained between two transparent plates is. In Fig. 1 it is shown how a light beam L onto a fluorescent molecule of the Concentrator C hits, after the absorption is emitted with a wavelength shift and comes to the edge of the concentrator by total internal reflection. There he meets preferably with the interposition of an optical contact substance K (e.g.

highly viscous silicone oils) on a solar cell S.

In the publication by Weber and Lambe / 1 / only a one-layer arrangement is used, combined with a solar cell, in which the absorbed in the visible range Sunlight is converted into fluorescent light in the infrared and concentrated. Because this type of conversion of solar energy these authors call their arrangement "greenhouse luminescence collector", The arrangement described in / 2 / is also related to FIG Guiding the fluorescent light and the solar cell assembly around a one-light arrangement: It is true that there are different dyes in layers one on top of the other used, but the different layers are always there expressly optical interconnected so that not spectrally different fluorescence bands in optically completely separate layers and different spectrally matched solar cells as it happens in multi-layer systems, as it is done by us in the registrations / 3 /, / 4 / and / 5 / were proposed. In these registrations were the disadvantages of the single-layer systems explained in detail and advantages and several described new possibilities that deal with multi-layer systems - combined with Solar cells of different band gaps - result.

The basic structure of a multilayer fluorescence collector is shown in FIG. Fig. 2a shows a stack of three collectors C1-C3 with three solar cells in series. The absorption and emission spectra (the latter hatched) of the fluorescent molecules in the Collectors are sketched in Fig. 2b. Because each collector is fully transparent to the unabsorbed part of the spectrum is, in this way there is an almost complete separation of the solar spectrum. The solar cells have band gaps that correspond to the associated emission band of the collector are adapted.

In the present additional application a number of special, advantageous embodiments of light concentrators of the type as in ours cited applications.

Brief Description of the Drawings Fig. 1 Cross section through a Collector C. L is the incident light beam, K is an optical contact substance, Sa solar cell.

Fig. 2a) Cross section through a stack of collectors C1-C3, S1-S3 = adapted solar cells.

Fig. 2b) Emission and absorption spectra of the dye molecules, with which C1-C3 are doped. Issue takes place at) 3.

Fig. 3a) Cross section through a collector consisting of two layers C1, C2, where C1 and C2 are liquids held together by thin films will. LS are air gaps between C1 and C2, S1 and S2 are solar cells. TP: carrier plate, DP: cover plate, SP: side plate.

Fig. 3b) Cross section through a foil bag insert, consisting of three light chambers LS, two collector layers C1 and C2 and mirror layers Sp.

(The mirrors Sp can also be part of the container SP.) 4 absorption and emission spectra of a two-layer collector with special high efficiency. A 1 and 3 are lower layer emission bands, and r # 4 are emission bands the upper layer.

Fig. 5a) Square interlayer collector (top view) with absorption and emission properties according to Fig. 4. Sp are mirrors, S1 is a solar cell, adapted to wavelength 21 S2: solar cell, adapted to 3.IF1 is an interference filter, which has high transmittance and high reflectivity at 3. The following applies to 1F2 Corresponding.

Fig. 5b) Triangular two-layer collector Fig. 6a) Cross section through a two-layer collector with total re-emission in the upper layer C1, C2 = lower layer, Sp = mirror.

Fig. 6b) Another embodiment of the same principle. C1 = upper layer, C2 = lower layer with light scattering centers. F = fluorescent dye, Sp = mirror.

Fig. 7a) Cross section through the edge zone of a collector with increased Concentration. C is a transparent medium with a refractive index nl. KA is a Wedge approach with refractive index n2> nl. Sp is a mirror coating, S is a solar cell.

Fig. 7b) Cross section through collector with increased concentration. KA has convex interface.

Fig. 8 Perspective representation of a combination collector, wedge attachment, Solar cells.

5 = solar cells, Sp = mirror coating.

n Fig. 9a) Cross section through a collector with reduced self-absorption. FS is a thin layer with fluorescent molecules between transparent ones Plates whose refractive index curve is given in Fig. 9b).

9b) Course of the refractive index n in the z-direction.

Fig. Lo cross section through a collector concentrator with twofold Fluorescence conversion. C1 is a collector with refractive index n1 # 1.5, C2 is a layer with refractive index n # 2 2. S1, S2 are solar cells.

11 cross section through a three-layer collector C1-C3 with an interference filter IF over C3.

12 cross section through a collector C with built-in fluorescent molecules.

13 absorption and emission spectrum of an ideal fluorescent dye.

Fig. 3a shows in cross section a two-layer collector at which the collector layers are not made of solid plates as previously proposed stored fluorescent substances or self-supporting cuvettes, filled with fluorescent Solutions, consist of flat foil bags with one on top of the other Chambers that are alternately filled with gas (air) and fluorescent solutions.

Layers C1 and C2 filled with the mat fluorescent solutions the light-collecting and -guiding layers. The film thickness would between about 30 and 150 / µm. The foil bag insert shown in Fig. 3b, from the collector layers C1 and C2 and from the three air chambers LS, as well as consists of the mirror layers Sp, is easily between a lower support plate TP and a transparent cover plate with the help of side support plates pressed so that the confinement layers of the light guiding layers C1 and C2 be plane-parallel and have right-angled narrow sides. Set in Fig. 3a S1 and S2 represent solar cells that are attached to the light exit surfaces of the collectors C1 and C2 are pressed on. The air layers LS ensure that the total reflection in the collector layers C1 and C2 cannot be affected.

The transparent cover plate DP does not itself have to and cannot conduct light therefore consist of the cheapest glass or plastic. The foil bag insert can e.g. in one piece. are manufactured using foil welding technology. Compared to before proposed collectors from solid Plates or cuvettes sink the requirements for the transparency of the film material are very high, since the light now only needs to travel very short distances in the film material. This is particularly important in the near infrared, where highly transparent plastics are visible like plexiglass and others absorb relatively strongly. Liquids that are in the visible and near infrared are highly transparent, but can be made easier and cheaper produce. Other important advantages besides the extremely cheap production of the collectors are that high light concentration ratios can be achieved, since the layer thicknesses of the collector layers C1 and C2 are made very small can and that the plastic bags simply replaced when the dyes fade can be.

High efficiency can be achieved with the following two-layer collector, its absorption and emission bands associated with the two layers in FIG. 4 are sketched.

Each collector contains two fluorescent substances. Are accordingly in Fig. 4 in the upper layer two emission bands) 2 and # 44 shown by Marked hatching and in each case next to it unshaded the absorption bands. In the lower layer there are also two emission bands, namely> 1 and 23 and the associated 1 3 absorption bands. The absorption bands of both layers together completely cover the spectrum. The emission bands are where in the other layer has an absorption band. A particular advantage this Kind of assignment of the spectra of the two layers consists in the fact that the proportion the fluorescent light, which is otherwise lost because it is not totally reflected, can be reduced significantly. If you put one under the two collector layers Arranges mirror, as indicated in Fig. 4, so this type of loss can through Non-total reflection for the lower layer can even be printed to zero.

If you want in this arrangement, each with two fluorescent bands are guided in a collector layer, di-conversion of light into electrical The two fluorescent bands at the edge of the collectors have to be designed to optimize energy Separate and feed two different solar cells with an adapted band gap. For this purpose, it is proposed, according to FIG. 5, in which under a) a square and under b) a triangular collector layer C is shown in plan view, in case a) on the two adjacent edges on which the different solar cells 5 and S2 are located, in each case immediately in front of the solar cell an interference filter IF1 or IF2 made of dielectric Apply multiple layers (e.g. by vapor deposition), each with an interference filter is only permeable to a fluorescent band. This solution is recommended because the Interference filters are completely lossless because of the spectral separating power of the Filter according to FIG. 4 only low requirements are made (the Fluorescence bands are naturally very far separated from each other) and accordingly the transmission curves of the interference filters can be much wider than that Fluorescent bands be made. The condition that the interference filter show the necessary filter effect for a relatively large angle of incidence range. In Fig. 5 it is also sketched that the edges of the collector, which are not solar cells wear metallic mirror layers Sp.

The two-layer collector with mirror (Fig. 4) can become a very simple arrangement can be modified. Considering the above circumstance, that the lower collector in the arrangement with an underlying mirror according to FIG. 4 in principle is lossless, there is a simplification for some applications if one the radiation absorbed in the lower collector is re-emitted into the upper collector. Figures 6a and 6b show two ways of doing this. In Fig. 6a, C2 has one Structure that prevents light from propagating along the layer. This is how light becomes - e.g. light that is absorbed at 2 in C2 - re-emitted at 3 and absorbed in C1.

No light quanta from layer C2 are lost, but the photons only suffer a degradation of their energy. Another possibility is shown in Fig. 6b shown for the concentration of all radiation in C1. C2 consists of a light scattering Layer backed with a mirror Sp. Located on top of C2 a coating of fluorescent paint, which has very similar properties like the dyes normally dissolved in C2. The advantages of these two modifications are: All collected energy can be dissipated in C1 become (higher Light concentration), C2 may be made of lower quality material than that from C1 because the light paths are very short. This also applies to the properties of the Fluorescent substances in C2, in which the absorption and emission spectra overlap somewhat to be allowed to.

Fig. 7 shows in cross section collectors C, which have a refractive index nl own. There are wedge-shaped light bundling approaches on the narrow sides of the collectors KA made of transparent material with a refractive index n2 optically connected, where In the embodiment in the right half of the picture, the wedge attachment KA is not flat as in the picture on the left, but has a cylindrically curved surface with a lens effect.

The conical borders of KA are metallically mirrored. The solar cells S are attached to the narrower end of the wedge. The refractive index n2 is greater than nl, for example nl = 1.5 and n2 = 1.9. Through these wedge approaches light that runs to the right in collector C is focused on the reduced solar cell surface; the concentration of the radiation is increased. However, these are kind of an afterthought Bundling set narrow limits, as a result of the fact that if the wedge is tapered too much, the incident rays eventually reverse their direction of propagation without ever to reach the solar cell. The concentration ratio can be adjusted with such tools, which appear purely geometrically optically, are almost doubled.

A practical application can achieve the concentration just described Find wedge approaches with the following problem: In Fig. 8 is perspective by dashed lines a triangular collector plate, forming a right triangle, with mirrored narrow sides, marked with Sp, shown. The advantages this type of collector shape was explained in our previous registrations.

The luminance of the emanating from the hypotenuse of the triangular collector However, as one can easily see, fluorescent light is not constant, but rather decreases from the center of the hypotenuse to the corners of the triangle.

It varies by a maximum of about a factor of 2. In FIG. 8 is still with solid lines a wedge approach is shown, in which, in contrast to Fig. 7 now the wedge angle varies along the hypotenuse so that the concentration ratio from the center of the hypotenuse to the corners of the triangle so increases that the Irradiance of the solar cells attached to the narrow end of the wedge attachment S is spatially constant. This is the prerequisite for the fact that all solar cells S1 to 5 generate the same electrical voltage.

n If the individual solar cells S1 to 5 now have the same n area on, as indicated in Fig. 8, all solar cells S1 to 5 generate the same photocurrent, can therefore as n electrically identical current sources either in parallel, in series or be switched combined in such a way that because of the matched internal resistances of the solar cells, the useful electrical power reaches its maximum value.

It has been stated several times that the self-absorption of fluorescent light in the collector, caused by the overlap of the absorption and emission spectrum a fluorescent substance, one extraordinary for the efficiency of the collector important Role play. An exemplary embodiment of a collector will now be explained in FIG with which, even if there is a non-negligible overlap the spectra of a dye, high efficiency can be achieved. The goal of this Method is to run the distances in the fluorescent medium as much as possible to reduce. This can be achieved according to FIG. 9a by adding a thin fluorescent layer FS (a liquid or solid dye solution) between two transparent plates is embedded, the one from the middle layer (Z = 0) outwards in the Z-direction show decreasing refractive index curve. Such a refractive index curve n (Z) is sketched in Fig. 9b. Outside the sandwich arrangement one has the refractive index n = 1 of the surrounding air. As shown in Fig. 9a by the curved lines, the fluorescent light can leave the layer FS over a short distance. It will be because of the refractive index gradient finally guided the rays in parallel planes, They later no longer pass through the middle class FS, in which they self-absorption would experience. This Breohungeindeirprofil is obtained naob in principle known Dosage Hubs.

Another method of covering up self-absorption is in Fig. Lo shown. Collector plates C1 with a thicker cross-section are shown, with the lower refractive index, for example, n = 1.5, which has an optical Contacting layer of a second, thinner collector plate C2 connected in series is. The plane of the plate of C2 is rotated by 900 in relation to the plane of the plate of C1. C2 has a higher refractive index, for example n2 = 2.

C1 contains a first fluorescent dye, if possible one absorbs a large part of the solar radiation, C2 contains a second fluorescent dye, which absorbs the fluorescent light from C1, converts it into fluorescent light and concentrates it gives off to solar cells S1 and S2 at the edge of C2. The layer thickness of C1 is relative thick, e.g., 1 cm, so that the dye concentration can be made small. In C1 has to travel long light paths, but because of the low dye concentration the self-absorption losses remain small. The layer thickness of C2 is small, e.g. 1 mm, so that a high concentration of dye is required. Since the light paths in C2 are very small, self-sorption also falls there - despite the high concentration does not matter. The high refractive index of C2 is necessary so that also in C2 Fluorescent light is captured and guided by total reflection. It is still add that it is minimized with this two-stage fluorescence collector the light paths is also beneficial, both C1 and C2 from lined up Assemble triangular collectors.

In Fig. 11 is another alternative for overcoming the self-absorption losses shown. In a multi-stage collector, e.g. with spectra according to Fig. 2b, a large part of the light quanta reabsorbed in the first stage from the collector scattered out. The light scattered in this way then lies on the absorption band the next stage and is captured again. The same process takes place in the following stages, so that finally a large part of the energy in the last, e.g. in the third Level is collected. To now this energy to gain fully, one can cover the last stage with an interference filter IF. which is transparent to most of the absorption wavelength at this stage but is reflected in the overlap area of the spectra. So the reabsorbed Light reflects until it finally goes in one direction and in one wavelength range which is carried out in C3.

Another possibility, the self-absorption losses in fluorescence collectors to avoid is indicated in FIG. Here is a collector plate again in cross section C shown on which sunlight falls from above.

The fluorescent molecules in C, indicated by cigar-shaped structures are, by known methods, with their molecular axes parallel to each other aligned.

For the molecules drawn on the left, a double arrow indicates the direction of polarization of the electron transfer of the fluorescent molecule upon light absorption. Are the Fluorescent molecules are selected in such a way that the electron transition takes place when light is emitted takes place, approximately by 900 compared to which the absorption is rotated, like drawn in Fig. 12 for the molecules in the right half, the fluorescent light not again - or only slightly - in other identical fluorescent molecules are absorbed because of the different polarization of the transitions, independently whether the emission and absorption bands of the fluorescent molecule overlap or not. The result is a strong reduction in self-absorption.

In addition, the proportion of the light that is not totally reflected would also be greatly reduced, since a dipole does not radiate in the direction of its axis, as also in Fig. 12 indicated on the right outside.

The fluorescent dyes used in the collectors must be specific Satisfy conditions. The loss of energy due to fluorescence (Stokes loss) is supposed to be kept as small as possible. This condition is met by a possible narrow emission band that is as close as possible to the long-wave absorption edge. In Fig. 13 it is shown that the energy gap E of a semiconductor solar cell is at most at which the energy corresponding to the wavelength h (E) may lie, i.e. the photoelectric Conversion efficiency is determined by the parameters mentioned earlier.

The same goes for the thermal conversion efficiency, the is determined by a a. An example of 2 fluorescent dyes that meet these conditions meet are the rare earth chelates.

L e r s e i t e

Claims (17)

Device for converting light energy into electrical energy (or thermal energy), in which the light in a transparent layer, whose refractive index is larger than that of the surrounding medium and contains the fluorescence centers (hereinafter Collector or concentrator called), caught on and a solar cell (or a Absorber), characterized in that more than one combination Concentrator / solar cell (or absorber) with the interposition of a medium smaller refractive index than that of the concentrators are stacked on top of each other, each concentrator part of the incident spectrum in fluorescent light converts and supplies a solar cell (or an absorber). 2. Apparatus according to claim 1, characterized in that the concentrators consist of alternating layers of liquid and air, which are covered by plastic films are separated from each other, and that also the concentrators from above by a dimensionally stable cover plate and limited from below by a carrier plate. 3. Device according to claim 1, of two layers and a rear Consisting of mirrors, characterized in that each layer has two spectral ranges absorbed and emitted in two areas and that - with the exception of the long and short wave Boundaries - one layer in each case absorbs in the area in which the other emitted. 4. Apparatus according to claim 3, characterized in that the radiation on two side edges of each concentrator it is taken that each of these side edges is provided with an interference filter for each of the two of the Concentrator supplied wavelengths is transparent and that the other range reflected. 5. Apparatus according to claim 3, characterized in that the lower Concentrator has such a shape that light propagation in a straight line in it is impossible. 6. Apparatus according to claim 3, characterized in that the lower Concentrator consists of a transparent layer that contains light scattering centers and that further covers the surface of this concentrator with fluorescent paint is. 7. Apparatus according to claim 1-6, characterized in that the the light-supplying, non-mirrored side edge of each concentrator with a wedge-shaped Approach is provided in such a way that the cross section of the light beam is reduced and that the approach consists of a substance that has a higher refractive index owns than the concentrator. 8. Apparatus according to claim 1-7, characterized in that the The boundary surface between the concentrator and the wedge base is convex from the concentrator seen trained. 9. Apparatus according to claim 1-8, characterized in that the free surfaces of the wedge are mirrored. lo. Device according to claim 1 - 9, characterized in that the Has notches approach in the direction perpendicular to the concentrator plane. 11. The device according to claim 1-8, characterized in that the Wedge angle of the approach along the concentrator edge varied so that at the exit surface the same light intensity is achieved everywhere. 12. The apparatus according to claim 11, characterized in that on the Exit surface of the wedge approach solar cells are attached, which are divided in this way are that the individual cells have the same area. 13. The device according to claim 1, characterized in that the concentrator consists of a thin layer of fluorescent material divided into two thicker ones transparent plates is embedded and that the refractive index of the system of decreases inside out. 14. The device according to claim 1, characterized in that one first stage of fluorescence conversion is carried out in a relatively thick concentrator and that the light thus obtained is in a second, perpendicular to the first Concentrator is guided, the second concentrator having a higher refractive index than the first one has. 15. The device according to claim 1, characterized in that in one Multi-layer concentrator converts the lowest layer and the longest wavelength that this bottom layer carries an interference filter on its upper side, which reflects the entire emission band of this layer and all shorter wavelengths lets happen. 16. The device according to claim 1, characterized in that the concentrators Contain fluorescent molecules that are aligned parallel to each other and that Furthermore, these molecules are such that the direction of polarization of the The electron transition causing absorption is approximately perpendicular to the direction of polarization of the electron transition causing the emission. 17. The device according to claim 1, characterized in that the long-wave End of the emission band of the fluorescent dye less than 4000 cm from long-wave end of the absorption band is removed.