DE29724095U1 - Photovoltaic arrangement with a device for concentrating solar radiation energy - Google Patents

Description

Jeck · Fleck · Herrmann *· ···..: &iacgr;*..&iacgr; . *..&iacgr; *··· rgr;«*««*» ^6S ·g

PATENT ATTORNEYS .!.*.·* * ,1&idigr;&&agr;*&phgr;*71 SO) H2 71&eegr;*&aacgr; 3 U 25'TdetaxW 7l 50)3 21 91 and 397 820 OSDN)

A 11800 - fle/poe Oct 29, 1997

HNE Elektronik GmbH & Co.
Satellite Reception Technology KG
Ferdinand-v.-Steinbeis-Ring 11

75447 Sternenfels

- 1

Photovoltaic arrangement with a device for concentrating solar radiation energy

The invention relates to a photovoltaic arrangement with a device for concentrating solar radiation energy.

Such a photovoltaic arrangement is disclosed as known in DE 41 08 503 C2. A spectrally selective mirror and also a holographic element are mentioned for splitting the solar radiation spectrum, although no further details are given on the structure and mode of operation of the holographic element.

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US Patent 4,433,199 also describes a photovoltaic arrangement in which a parabolic reflector, a lens system and optical fibers are mentioned to concentrate the radiation energy.

The costs of photovoltaic systems are largely determined by the cost of the energy-converting materials. Currently, the main materials used for solar cells are crystalline and amorphous silicon. With the help of solar concentrators for the radiant energy, silicon area can be saved for the same electrical output, thus making it possible to reduce the overall installation costs.

The effect of solar concentrators is based on the properties of light-collecting optical elements, such as lenses or curved mirrors, and has been known for a long time. Theoretically, the maximum concentration of sunlight is 46222 times the factor of 46222 with spherical concentration, and with cylindrical optics the maximum concentration factor is the square root of this value, namely 215. However, typical concentration factors that can be achieved are only around a few thousand for spherical concentrators and around 50 for cylindrical concentrators. To achieve such concentration values, concentrating lenses or mirrors and a tracking device are necessary. However, the costs for classic lenses, plastic Fresnel lenses or mirrors are so high that the savings in material costs for solar cells are offset by the costs for concentrating the radiation energy or light.

The invention is therefore based on the object of creating a photovoltaic arrangement with a device for concentrating sunlight,

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whose production costs are so low that they can be used as solar concentrators for
photovoltaic systems can be used economically.

This object is achieved with the features of claim 1. According to this, it is provided that at least a portion of the optical surface elements used for concentrating solar radiation is covered with diffractive structures. By means of the diffractive structures, not only is the concentration of the solar radiation energy achieved with relatively simple, inexpensive measures, but at the same time a spectral adaptation to the photovoltaic converter device can also be achieved.

Advantageous embodiments are the subject of the subclaims, as is apparent from the descriptions of the embodiments.

The invention is explained in more detail below using exemplary embodiments with reference to the drawings.

Fig. 1 shows a two-stage structure of a device for concentration

tration of solar radiation energy with diffractive structures,

Fig. 2 shows another embodiment of a device for

Concentration of solar radiation energy with diffractive structures,

Fig. 3 shows another embodiment of the device for con

concentration of solar radiation energy with diffractive structures,

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Fig. 4 a spectrally expanded image of the sun with diffractive structures,

Fig. 5 a diffractive structure in the form of a transmissive surface relief grating,

Fig. 6 shows a diagram of the diffraction efficiency of a diffractive

Structure in the form of a sawtooth structure with 600 nm design wavelength in the first diffraction order,

Fig. 7 a diffractive structure in the form of a cylindrical diffraction lens in plan view,

Fig. 8 a focusing with a diffractive structure in the form

a planar diffraction lens,

Fig. 9 an approximation of a diffractive structure in the form of a

Sawtooth grid through steps,

Fig. 10 a solar radiation spectrum on the earth’s surface,
Fig. 11 another solar radiation spectrum,

Fig. 12 a representation of the relative spectral conversion efficiency

of solar cells and

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Fig. 13 a thermally converted relative energy share in

approximation.

Depending on the application, diffractive optical or short-diffractive structures BS are more or less periodic arrangements of lines at intervals of 0.5 to 50 micrometers (//m). A line element is usually designed as a surface relief in the form of rectangular, sinusoidal or sawtooth-shaped grooves. Depending on the shape of the groove, different properties of the diffractive structure BS or the diffraction grating result.

If a diffractive structure BS in the form of an optical grating is illuminated at an angle α (Fig. 5), a splitting of the incident beam into different radiation with different propagation directions is observed. These propagation directions are called diffraction orders (m), whose deflection angles (e) result from the diffraction law.

= mX/p + sin(a)

m denotes the diffraction order, λ is the wavelength and ρ is the period width of the grating lines (grating period). Depending on the shape of the grooves, different amounts of light are obtained in the diffraction orders m. For the purpose of energy concentration, sawtooth-shaped diffraction gratings are particularly advantageous because, in the best case, they only have one diffraction order m. However, this only applies to light of one wavelength. The diffraction efficiency depends on the depth of the grooves and the wavelength λ. If the period of the diffracting structure BS is significantly larger than the wave-

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length &lgr;, the energy distribution in the diffraction orders m can be calculated with sufficient accuracy using the scalar diffraction theory (see e.g. Handbook of Optics, Vol. II, Chapter 8, Mac Graw Hill, 1995, ISBN 0-07-047974-7). The energy share of the m-th diffraction order for a sawtooth grating (blaze grating) is r _m in reflection and t _m in transmission

&eegr; Az ₂ (η-I)-Az

sin((m-—— )-π) sin((/w- )-π) ²

_r &aacgr; / _ &aacgr;.

where m denotes the diffraction order, η is the refractive index of a covering layer of a reflective grating (reflection grating) R or the refractive index of a carrier material of a transmission grating T and Δζ represents the geometric depth of the groove and λ the wavelength.

For the reflection grating R with a cover layer of refractive index 1.5, as found in reflective holographic foils, a groove depth of λ/3 in the 1st order is calculated. For an optimal diffraction efficiency in the 1st diffraction order at a wavelength of, for example, 600 nm, a depth of 200 nm is calculated, which can be achieved in foils using rotary embossing processes. At twice the depth, the diffraction efficiency in the 2nd order is again optimal, but then the deflection angle e doubles with the same grating period p, as can be seen from the grating equation. For twice the wavelength λ, an optimal diffraction efficiency in the 1st order with the same deflection angle e is also obtained.

J··, , I

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In Fig. 6, the diffraction efficiency of a sawtooth structure with optimal depth in the 1st diffraction order for a wavelength λ of 600 nm is plotted.

If one goes to very fine structures on the order of the wavelength λ or to very deep structures, then the scalar diffraction theory no longer applies with sufficient accuracy and one has to calculate the structures by rigorous numerical methods.

If the period of such a sawtooth grating is allowed to become smaller and smaller from the center outwards with a root characteristic, a cylindrical diffraction lens is obtained, as shown in Fig. 7. The step height is 150 - 300 nm when implemented as a reflective embossed foil for the above wavelength range λ, which is covered with a protective varnish.

If such structures are used on flat substrates, the maximum deflection angle at the edge of the diffraction lens under perpendicular illumination is sin(e) = mvl/p _min , where p _min is the smallest structure width to be produced.

Sawtooth gratings can be approximated in the form of steps using microlithography. With 4-step steps, a maximum diffraction efficiency of 80% is achieved, and with 8-step steps, an efficiency of 95% is achieved, so that with 4- or 8-step diffraction gratings, a sufficient approximation of the ideal structure for the application in question is already achieved.

An alternative option, but only for rotationally symmetrical, spherical lenses and mirrors, would be diamond turning. Cylindrical lenses can also be manufactured using so-called grating machines, in which grooves are drawn into a substrate using a diamond.

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The smallest width for a step section that can currently typically be realized using available microlithography systems is approximately 1 to 1.5 //m. This results in a minimum period of 4 to 6 μιη for the simplest approximation with 4 steps. From these values, a maximum deflection angle sin(e) at &lgr; = 0.8//m of 0.8//m/(4-6//m) « 0.15-0.2 or € « 8.5 - 11.5° can be derived for the 1st diffraction order. Sin(e) is also referred to as the numerical aperture. With diffractive lenses of this fine structure, a concentration of approx. 35 to 40 (215:5 or 6) can be achieved. If the second or an even higher diffraction order m is used, the deflection angle e is multiplied accordingly, which makes a correspondingly higher concentration possible. However, this requires a groove depth several times greater than that of the first diffraction order.

In Fig. 7, the appearance of a cylindrical diffraction lens is shown by grayscale coding. The concentration of radiation energy with an ideal diffraction lens is shown in Fig. 8. In this configuration, the diffraction lens has no dispersing effect. Only the focal length is inversely proportional to the wavelength Λ, so that the focus is closer to the lens in the red than in the blue.

When sunlight is concentrated on a solar cell S, the problem of heating occurs because not all of the incident light energy is converted into electrical energy. The relative conversion efficiencies WE of amorphous and crystalline silicon are plotted in Fig. 12. In the green, where the solar radiation is strongest, the conversion efficiency WE of crystalline silicon becomes worse. The light that is not converted into electrical energy contributes to the heating of the solar cell S.

· · · · · · · ·· ···· •ϕ • ·· ·· ·· · · •
• • •
• • • I · · ··
· · · ·
··· · • ···· · ··· · •
• •
• · ·
· · ···· •
·· · * ·
·· · · ·· ·· · · ··· * ·

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As the temperature increases, the conversion efficiency WE decreases. For this reason, it is advantageous if different types of solar cells are only illuminated with the spectral components in which they have a high conversion efficiency WE. In Fig. 10 and Fig. 11, a solar spectrum is plotted on the earth's surface. Most of the energy is therefore radiated onto the earth in the green range, corresponding to the irradiance I.

For the further calculations, a global, 37° Air Mass 1.5 spectrum roughly sampled with a step size of 50 nm was used (see Fig. 11). For more precise calculations, better data would of course have to be used and, above all, the installation location would have to be taken into account. But the data are sufficient for a rough estimate of what is possible.

Solar cells S made of silicon are usually used, which have different spectral sensitivities depending on whether they are amorphous or crystalline (Fig. 12). The product of irradiation and spectral sensitivity is important for the conversion into electrical current. Crystalline silicon has a good optoelectronic conversion efficiency between 500 and 900 nm with a maximum at 900 nm, while amorphous silicon has a good conversion efficiency WE between 500 and 700 nm with a maximum at 600 nm. However, the proportion that is converted into thermal energy and thus contributes to the undesirable heating of the solar cell S differs. It is mainly the light in the visible range that contributes to the heating of crystalline silicon because the absorbed photons have a much higher energy than the band gap (at 1200 nm). The excess energy is converted thermally. The infrared portion above 1000 nm also contributes significantly to the absorption. From 1200 nm, silicon is transparent and the energy in the spectrum is no longer very high, so that this energy component is probably not very

• t

·

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contributes to heating. In Fig. 13, an estimated proportion of the thermal conversion is plotted against the wavelength λ . The estimate is based on the difference between the converted electrical energy and the total energy. The converted thermal energy against the wavelength λ is therefore low for crystalline silicon in the range between 700 and 1000 nm, while for amorphous silicon it is only low between 550 and 650 nm.

In order to prevent undesirable heating, only light between 700 and 1000 nm should reach the solar cell S in the case of crystalline silicon and only between 500 and 700 nm in the case of amorphous silicon.

These light components can be reflected back using interference filters. However, such interference filters are very expensive, which is why this method is unlikely to be an option for economic reasons.

On the structure of the diffraction gratings

Here, diffractive structures BS with different groove shapes and periods are used to direct light of different wavelengths &lgr; with different efficiencies in different directions.

The simplest variant is a cylindrical diffraction lens with a design wavelength of, for example, 600 nm, which is adapted for amorphous solar cells S (Fig. 7). With this design, the infrared component, which contributes most to heating, can be reduced by about half. However, the blue component can only be reduced by about 20%, since the 2nd diffraction order, which has its maximum at 300 nm, has a significant share in

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the solar cell S. In order to reduce this proportion even further, a two-stage design can be used, as shown in Fig. 1. In this case, a transmission grating T and a reflection grating R in the form of a reflective diffraction lens are used. The transmission grating T can be optimized for ultraviolet light of, for example, 200 nm. All UV light and also large blue components are diffracted and are focused by the diffraction lens R into a different focal line or focal point. The diffraction lens R is in turn designed for 600 nm, so that with this combination a strong reduction of the undesirable spectral component on the solar cell S can be achieved. The ultraviolet-blue focus can then be used in the event that there are solar cells S that are sensitive in this range. The green-red focus then illuminates a solar cell S that has a sensitivity curve like the amorphous silicon cell. In this design, only about half of the infrared light is focused on the solar cell S. The infrared portion can also be used in a 2-component film in conjunction with a concave mirror.

In the case of the embodiment shown in Fig. 2, the incident radiation ES is concentrated by a parabolic mirror SP, which is covered with a diffraction film BF, which in itself has no focusing properties. A transmissive film TF acts essentially on the UV-blue component, and the reflective diffraction film BF on the surface of the parabola acts essentially on the green-red component. The infrared component is only slightly diffracted and is collected in a focus by the parabolic film BF. A polycrystalline silicon cell can then be attached to the infrared focus, which is then no longer heated by the blue-green component of the sunlight.

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Further variations can be achieved by imparting a focusing effect to the grating foils BF, TF in addition to a deflecting effect. Such foils can be obtained by using only the outer parts of a diffraction lens (off-axis lens).

If, for example, the transmissive film TF according to Fig. 2 is designed as an off-axis lens (Fig. 3), the diffracted light is focused on the parabolic mirror SP so that only parts of the diffracted light are illuminated. The surface areas can then be covered with films with different diffracting structures BS. Different colored coatings for the films are also conceivable, which absorb different spectral components so that the effect of selective concentration can be further enhanced.

Further design possibilities arise from the dispersive property of diffraction gratings, which direct light of different colors into different angles. White light is therefore split into an angular range that depends on the grating period ρ. The image A of the sun's disk is created slightly shifted under each color (Fig. 4). Depending on whether the splitting is large or small, the colors are well separated or only visible at the edges. Mathematically speaking, the image of the sun is folded with its spectrum.

Depending on the size of the solar cell S and the amount of splitting, a greater or lesser proportion of the different spectral components of the sunlight are imaged onto the solar cell S.

The size and position of the solar cell S can influence the spectral component that hits the solar cell S.

·

·

t:

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The diffractive structures BS described can be produced relatively easily and inexpensively using microlithography. The particular advantage of the diffractive structures produced in this way is their flatness and mass reproducibility by injection molding or embossing processes.

Claims (12)

1. Photovoltaic arrangement with a device for concentrating solar radiation energy, characterized in that at least a part of optical surface elements used for concentrating solar radiation (ES) is covered with diffractive structures (BS). 2. Photovoltaic arrangement according to claim 1, characterized in that the diffractive structures (BS) are designed for transmission and/or for reflection. 3. Photovoltaic arrangement according to claim 1 or 2, characterized in that the diffracting structures (BS) are designed as sawtooth-shaped grooves. 4. Photovoltaic arrangement according to one of the preceding claims, characterized in that the diffraction efficiency of the diffracting structures (BS) is different for different wavelengths (λ). 5. Photovoltaic arrangement according to one of the preceding claims, characterized in that the diffractive structures (BS) have concentrating properties. 6. Photovoltaic arrangement according to one of the preceding claims, characterized in that the diffractive structures (BS) are designed as films (BF, TF) which are applied to transparent carrier materials in the case of transmission. 7. Photovoltaic arrangement according to one of the preceding claims, characterized in that, in conjunction with concentrating surface elements, solar radiation (ES) of different wavelengths (λ) is concentrated on energy converters (S) of different conversion efficiencies (WE). 8. Photovoltaic arrangement according to one of the preceding claims, characterized in that the diffractive structures (BS) are designed as diffraction lenses or collecting mirrors. 9. Photovoltaic arrangement according to one of the preceding claims, characterized in that a collecting concave mirror is covered with a diffractive structure (BS). 10. Photovoltaic arrangement according to one of the preceding claims, characterized in that at least two partial surfaces of the concentrating system have diffraction structures (BS). 11. Photovoltaic arrangement according to one of the preceding claims, characterized in that the transmitting carrier materials or the transparent lacquers covering the diffractive structures (BS) are colored in such a way that different parts of the solar spectrum are absorbed. 12. Photovoltaic arrangement according to one of the preceding claims, characterized in that solar radiation (ES) of different wavelength ranges is concentrated at different positions.