Device for utilizing solar energy

DEVICE FOR UTILIZING SOLAR ENERGY

HELD OF THE INVENTION:

The present invention relates to means for concentrating solar energy onto a solar cell and converts part of the solar spectrum, and especially the part in the ultraviolet and blue region, to longer wave-lengths, which are better utilized by solar cells. The invention further relates to means for converting part of the solar radiation to longer wavelength and for dissipating undesired heat, while the concentrated solar radiation is concentrated onto a solar cell. Other and further features of the invention will become apparent hereinafter.

BACKGROUND OF THE INVENTION:

Silicon semiconductors play a key role in most types of solar energy conversion due to their electro-optical properties and the technology of production of such cells Is highly developed. It is one of the serious drawbacks of such cells that they are quite expensive and thus their widespread use for the conversion of solar energy to electrical energy Is not yet economical. One of the ways to overcome this drawback Is to concentrate the Incoming flux of solar radiation onto a solar cell, Increasing conversion efficiency and substantially reducing the surface area of the solar cells required per unit area of solar flux. Various suggestions have been made how to focus solar energy on solar cells, but conventional means concentrate the entire solar radiation, including heat radiation. Solar cells generally have a .maximum of sensitivity about 800 nm, and if heat energy and radiation In other parts of the sp-ctrwn 1s concentrated on such solar cells, the overall efficiency Is quite low.

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In British patent No. 1,174,380 there are described and claimed . uranyl activated phosphors» These emit green light when exposed to ultraviolet radiation, X-rays,cathode rays and Ion bombardment* The barium containing phosphorus compounds of the British patent are not transparent, and cannot be used for the purposees of the present Invention which relates to certain novel transpatent glasses.

British patent No. 1,178,803 relates to transparent uniformly coalesced glasses of silica oortalning at least one oxide Ingredient per million atoms ox' silicon, said oxide being other than a rare earth oxide. A multitude of luminescent activator elements Is mentioned, and amongst these also uranium.Typical glasses of the said British patent contain tungsten or tungsten in combination with another cation. There is no mention in this patent of uranyl ions and of their use in glasses.

י 14 Recently Weber et al, App. Phys« 15, (1976) 2299-2300 suggested to use luminescent solar collectors and proposed to use neodymium doped laser glass or rhodamino-6-G as luminescent material.

Their calculations show the relation between trapping of radiation and the refractive Index of the collector.

SUMMARY OF THE INVENTION:

According to the present invention there 1s provided a device adapted to convert part of solar radiation which 1s generally not utilized efficiently by conventional solar cells to radiation which 1s utilized to a larger degree by such cells, and which concentrates solar radiation on solar cells, thus diminishing the area of such cells required for a given area of flux.

The device according to the present Invention comprises a suitably doped medium which absorbs strongly in the UV and blue regions of the solar spectrum and which converts this radiation to longer wavelengths, the geometrical arrangement of the device being such that the solar energy 1s focused on a solar cell, resulting 1n a highly Increased eff1c1|n^y of solar energy conversion to electrical energy. According to a preferred embodiment of the Invention uranyl-doped glass 1s used as medium for the oconversion of the UV and blue part of the spectrum to higher wavelengths. The energy obtained from the uranyl-doped glass can be further converted to a still better utilized wavelength, namely 1n the 800 nm region, by adding a rare earth to the doped glasses.

The device according to the present Invention 1s 1n a geometrical form which results In the focusing of the solar energy onto a small-size solar cell. The form of choice 1s a planar collector, provided with reflecting means on Its lower and lateral sides, one of the lateral sides being attached to a solar cell. The most simple shape 1s that of a rectangular slab, as this 1s least expensive to manufacture. It Is within the ambit of the present Invention to use other suitable geometrical shapes which result 1n the desired focusing of the radiation on a solar .

dill. The uranyl ion may be Incorporated Into various types of glass: for example there may be mentioned glasses such as Ba0״P₂0₅; SrO’P₂O₅; Ca0״P₂0_g; silicate glasses of the type Ca0*Sl0₂^Na₂0.

The slab of glass can be of any suitable dimension; thicknesses of from about 1 mm to about 10 nm were tested and found to be suitable.

The ratio of thickness to lateral dimensions can vary within wide limits: for example a range of from 1:10 to about 1:100 of thickness to lateral dimension was tested and good results were obtained.

The absorption of uranyl ion is quite high, and it is about orders of magnitude higher than that of neodymium, and thus it is possible to attain a complete absorption of solar radiation with

4*Φ I small concentrations of U0₂ . The uranyl ion absorbs in the part of the spectrum to which the silicon cells are not sensitive and thus energy which 1s otherwise lost 1s utilized by the device of the invention. Uranyl fluoresces with a high quantum efficiency at ambient temperature at longer wavelengths at which the sensitivity of the sillcon cell is higher that of natural solar radiation.

The geometrical configuration results in an efficient utilization of the desired part of the radiation, while at the same time unwanted heat is dissipated by the comparatively large area of the collector. Non-radi©active uranyl is abundant: it is a byproduct 235 of separation of U and quite Inexpensive.

When uranyl doped glass is doped by the addition of Eu the wavelength of the emitted radiation is about 612 nm and this further increases the utilization of radiation by solar silicon cells.

The absorption spectrum of uranyl ion in phosphate glass has been studied by Lieblich-SofCr et al.* Inorg. Chlm, Acta (1978) and the spectrum is presented as Fig. 1, Exci ation of uranyl in these bands results In a strong visible fluorescence (Figure 2) which has a quantum efficiency of about 50 % at ambient temperature and a lifetime of about 300 ps. According to the geometrical configurations used the emitted radiation is collected at one of the edges of the device since most of the light is emitted at angles more grazing to the surface wthan the Initial angle of total internal reflection. The trapped light propagates by successive reflections to the narrow edges, provided with reflecting means, until it reaches the one edge where the solar cell 1s located, where it is allowed to escape and reach tills cell. The effect is similar to that of fiber-optic waveguides.

BRIEF DESCRIPTION OF THE DRAWINGS!

Figure 1 is an Absorption Spectrum of UOg** In phosphate glass; Figure 2 is an emission Spectrum of UOg** In phosphate glass; Figure 3 is a schematic perspective view, not according to scale, of a device according to the Invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT:

The invention is Illustrated with reference to the enclosed schematlcal drawing, which Is to be construed In a non-1Imitative manner.

As shown 1n Figure 3, a device II according to the present Invention comprises a rectangular slab 12 of suitable doped glass, such as uranyl doped glass, which Is provided at its lower surface with a reflecting layer 13 and at 3 of Its lateral sides with reflecting layers 14, 15 and 16. The fourth edge 17 1s open and attached to a solar cell 18, of the silicon type. The ratio of glass-surface area on which solar radiation impinges to the area of the solar cell 18 was In this specific case about 18:1. The device wS?Irradiated by a high-pressure mercury lamp (100 w, distance: 10 cm) and a considerable increase of current and voltage was obtained. In Table I there is presented the ratio of short-circuit current of the cell obtained by the concentrated light via the collector to that of the current obtained from the cell connected to a blank undoped glass of Identical geometry: I/I blank; while the ratio of current obtained by the cell connected to the collector against the cell exposed to direct Illumination 1s given as I/I cell. The decrease at higher concentrations of uranyl 10n Is due to the fluorescence concentration quenching of uranyl. When the ratio 1s Increased above the 18:1 ratio used, a further Increase can be obtained. The device 1s advantageously used in conjunction with the type of silicon cells developed recently which are capable of utilizing a high flux density at a high efficiency.