NL1040088C2

NL1040088C2 - Concentrating solar panel with diffuse light conversion.

Info

Publication number: NL1040088C2
Application number: NL1040088A
Authority: NL
Inventors: Michiel Herman Mensink
Original assignee: Linesolar Ip B V
Priority date: 2013-03-11
Filing date: 2013-03-11
Publication date: 2014-09-15
Also published as: WO2014142650A1

Abstract

A solar panel comprises two different types of photoelectric cells and a focusing optical element that project incident radiation onto one type of photoelectric cells. Another type of photoelectric cells converts the indirect light rays. A reflective optical element may be added to the system to also concentrate the indirect light, and also reduce the amount of indirect light cells needed. Preferably, the cells for converting the concentrated direct light are highly efficient while the cells for converting indirect light can be less efficient and lower cost.

Description

CONCENTRATING SOLAR PANEL WITH DIFFUSE LIGHT CONVERSION

Description and claims

Solar panel

The invention relates to an innovative solar panel.

In solar panel technology, a focus has been on increasing solar energy conversion efficiency and on reducing production costs. There are two general categories of solar panels: non-concentration panels (also referred to as flat panels), in which almost the full panel area is covered by solar cells, and concentrating panels, in which focusing lenses or mirrors are used to focus sunlight on a small area, and thereby reduce the amount of expensive photovoltaic material. In a flat panel the production costs of solar cells is often 50%-80% of the entire solar panel production costs. Therefore a high concentration panel uses lenses to focus the sunlight onto a small spot and thereby achieve a factor 1000 concentration of the sunlight. In this concentration system the area that needs to be covered with expensive solar cells can be reduced by approximately a factor 1000, and thereby reduce costs. Most concentrating solar panels need to be mounted on a so-called tracker that positions the panel perpendicular to the sunlight, by rotating the panel slowly during the day so it faces East in the morning and West by sundown.

A common problem in concentrating panels is that only direct sunlight is converted to electrical power. The indirect (also referred to as diffuse) sunlight is only partially converted. In the example above, only approximately 1/1000th of the indirect light falling on a panel is converted, since indirect light falls on the panel from random directions and since only 1/1000th of the panel area is covered by solar cells. The amount of indirect light is not negligible. When clouds block the direct sunlight, all light is indirect. For this reason, concentrators are only economical in the Sunbelt - sunny regions throughout the world with a high number of sun hours. This includes parts of the North Africa, the Middle East, Australia, Southwest USA, Mexico and several other areas with high Direct Normal Irradiance (DNI). However even in a sunny and blue, cloudless sky, 10-25% of the light falling on a solar panel is indirect. This indirect light is sunlight that was reflected by particles and molecules in the air (which reflect more blue light than other colors of light and therefore the sky appears blue). In sparsely populated and mountainous areas the amount of indirect light is relatively low and mostly caused by scattering from molecules in the atmosphere. In populated or industrial areas the share of indirect light can be higher than 25%, due to smog, dust and fine sand particles. Concentrating panels there fore cannot convert a substantial portion of light that is converted by flat panels. Concentrators are thereby limited to areas with high DNI.

An object of the invention is to provide a solar concentrating panel that converts also a large proportion of the diffuse light.

This object may be achieved by a solar panel comprising a plurality of photoelectric cells, wherein a group of cells captures most of the direct light (the direct light solar cells) and wherein another group of cells captures most of the indirect light (the indirect light solar cells). The panel may include one or two types of optical concentrating elements, whereby one type of concentrating element focuses the direct sunlight with a medium to high ratio on the ‘direct light solar cells’. The indirect light will mostly fall on the 'indirect light solar cells. Another type of optical concentrating element may be used to concentrate the indirect light as well. Thereby, also the surface area covered by the indirect light solar cells can be substantially smaller than the surface area of the solar panel, thus also a smaller amount of indirect light solar cells need to be used.

Thereby a concentrating solar panel may be produced that allows conversion of a large proportion of the indirect light.

Another object of the invention is to provide a solar panel with high light conversion efficiency and low production costs.

This object may be achieved by a solar panel as described above, wherein the direct light solar cells consist of relatively expensive solar cells with high efficiency, and wherein the indirect light solar cells consist of relatively inexpensive solar cells with a relatively lower efficiency. Thereby a concentrator panel may be produced that is highly efficient, that has low production costs and that also allows for the conversion of most of the indirect light.

One embodiment of a solar panel according to the invention uses a one-direction optical concentrator for the direct light, meaning it focuses the direct light in one direction (similar to the focusing effect of a cylinder lens). The light concentration factor in this implementation may range from 5 to 70, however is preferably 20 to 50. A specific implementation uses a glass plate that incorporates a multitude of plastic cylindrical lenses. Narrow line-shaped or strip-shaped PV-cells are positioned in the focus of these lenses and convert the direct sunlight. Each cylinder-shaped cross section of the lens element may have a width of 40 mm while each PV cell may have a width of just 1 mm. Thereby only 1 /40th of surface area of the panel needs to be covered with solar cells for converting the direct light, compared to the area needed in a flat panel. Thanks to this fractional amount needed, it is economical to use cells with a higher efficiency that are somewhat more expensive solar cells. The panel may have for instance 25 parallel rows of lenses and corresponding rows of narrow PV cells, to obtain a panel width of approximately 1000mm. The length of the panel may be approximately 1600mm, similar to many other solar panels. In between two rows of narrow PV cells, the panel incorporates another group of PV cells for converting the indirect light. These cells can be much wider and may consist of a type of cells that is less expensive. Also the cell-type may be better suited towards converting indirect light. Indirect light has a somewhat different energy distribution across the spectrum compared to direct light. Further, the light intensity of indirect light is substantially lower (factor 3-20) compared to direct light. The cells for indirect light may have lower costs compared to the cells for converting the direct light.

Another embodiment of the solar panel according to the invention includes an additional optical concentrating element for concentrating the indirect light. This element may be implemented with a reflector. The reflector may concentrate the indirect light by a factor 2-6, and conversely implies that less than 1/2-1/6th of the module area needs to be covered with indirect light solar cells. This concentrator may have a profile that concentrates the direct light in 1 direction. Alternatively, it may be shaped such that it concentrates the light in 2 directions.

A multitude of choices and options are available to implement the focusing of direct light in one direction. The topside of the transparent protective covering plate may contain the lens profiles, however the ridges between the profiles may trap dust and dirt. Preferably, a tempered solar glass plate is used with a flat topside and rows of lens-profiles on the inside. The lenses may consist of a plastic material such as silicone or PMMA. Directly below these lenses is a medium with lower refractive index such as air, another gas, a cooling liquid or even a transparent solid with a lower refractive index compared to the top plate. The lens profiles may be cylindrical, however a preferable option is to use an a-cylindrical lens profile. In this case, the surface of the lens is not equivalent to a part of any cylinder. Rather, the profile may be closer to a hyperbola or other shape of a higher order formula. Such a-cylindrical profiles can reduce optical aberrations including spherical aberration, coma and astigmatism. Another option is to implement the (cylindrical or a-cylindrical) lens profile in segments, as a so-called Fresnel profile. In this case each lens-profile is divided in several sub-lenses. The advantage of such a Fresnel arrangement is that the thickness of a lens is greatly reduced, and therefore production costs may be reduced. Another option to focus direct light in one direction is to use a transparent material in which the refractive index varies - a gradient index material. And yet another option to focus light in one direction is to use a diffractive optical element such as a hologram instead of the refractive optical elements described in various implementations above. Yet another option for any implementation of the optical elements described above, is to create a non-symmetrical profile. In that case the narrow solar cell is not positioned below the center of the optical element.

A multitude of choices and options are also available to implement concentration of the indirect light. Preferably a reflector is positioned inside the lower index medium, between the solar cell and the transparent covering plate. The reflector may consist of a metal or plastic profile that is covered with a reflective coating or a reflective foil. Alternatively the reflector may be based on the principle of total internal reflection. In this case the reflector consist of a high refractive index material through which the light travels. The reflector profile may have various shapes including straight, spherical or higher order. Preferably the shape is hyperbolical so as to reflect most of the incoming light on to the solar cells. The profile may be asymmetrical, so that light is not concentrated in the center of the optical aperture but towards one side. The profile may be divided along the profile into segments that are slightly different from each other, and may contain features in the direction perpendicular to the profile. The reflector may even concentrate the indirect light in 2 directions, in order to increase the concentration factor and further reduce the amount of indirect light solar cells.

In yet another embodiment of the solar panel according to the invention, the direct light is concentrated in two directions. The concentration factor can be 50-1500. In this implementation the direct light solar cells are not strip-shaped and narrow, but rather they have a similar width and length. The concentrating optical element focuses the light in two directions, but otherwise the same implementations apply as discussed above for the one direction concentrator. In this embodiment the indirect light is concentrated in two directions by a reflector. The area covered by the indirect cells has a similar width and length, however contains a gap or hole on the position where the direct light is focused and where the direct light cell is positioned. The reflector focuses the light in two directions, but otherwise the same implementations apply as discussed above for the one directional concentrator.

It will be understood that all subject matter disclosed with reference to the method according to the invention, is also applicable to the solar panel according to the invention, and vice versa.

Further advantages, features and effects of the invention will become apparent from the appended drawings, showing non-limiting embodiments of the invention, in which:

Figures 1A - 1C depict schematic views of a one-dimensional (1D) concentrator solar panel without the use of a reflective optical element according to embodiments of the invention;

Figures 2 depicts a schematic view of a one-dimensional concentrator solar panel that includes a reflective optical element to concentrate indirect light according to embodiments of the invention; - Figures 3A - 3C depict schematic views of a two dimensional (2D) solar panel (in which case the light is focused in two directions) without the use of a reflective optical element according to embodiments of the invention;

Figures 4A - 4C depict schematic views of a two dimensional (2D) solar panel that includes a reflective optical element to concentrate indirect light depicts a cross-sectional view with reflective optical elements that may be used according to embodiments of the invention;

Figure 5A - 5D depict schematic cross-sectional views of the focusing optical elements that may be used according to embodiments of the invention; A solar panel will be described that may be arranged for concentration of sunlight as well as conversion of indirect light.

The Solar Panel (schematically depicted in Figures 1A, 1B and 1C) is a one-directional concentration solar panel. It comprises of a hardened, ultraclear and AR coated glass plate 20, with thickness 3.2mm, length 1600mm and width 995 mm. The glass plate incorporates 24 parallel rows integrated PMMA lenses 25 having a width of 40mm and a length of approximately 1580mm. The lens profile has a radius of 14.7mm and a conic constant of -2.2. These lenses focus the direct light rays 10 on narrow and long strips of crystalline silicon PV cells 50, with each cell having a width of approximately 1.4mm and a length of 156mm. Ten such strip-shaped cells are placed in each row to cover the length of the panel. These silicon cells are optimally designed to receive and convert concentrated light with high efficiency. The indirect light rays 11 are incoming to the solar panel from random directions, and will mostly hit the wide strips of lower cost multi-crystalline silicon solar cells 60 for converting indirect light at reasonable efficiency and low costs. These solar cell strips have a width of approximately 38mm and a length of 156mm. Each solar cell in the panel is electrically connected to conductive tracks made from copper, which are integrated with the back plate 70. Groups of cells are connected in parallel, while some groups of cells are connected in series. The back plate 70 consists of materials such as glass fibers and epoxies and is structured similarly to printed circuit boards used for electronic devices. The panel contains an air gap 30 that is filled with dried air. A spacer and sealing 80 have a heigth of approximately 50mm and a width of approximately 10mm, and run around the perimeter of the panel to ensure that the PV strips 50 are positioned at the correct focal distance from the lenses 25. The sealing ensures that the air gap inside the panel is closed off from dust, water and celldamaging gases (e.g., sulphorous oxide).

In another embodiment, a Solar Panel that is similar to the one described above also incorporates reflective optical elements, as schematically depicted in Figure 2. These reflectors are created from ABS plastic and have a wall thickness of 2mm. The reflectors also enhance mechanical properties of the solar panel, and are attached to the bottom plate and the top plate. Two types of reflectors are used in the panel: a one side reflective element 40 along the edges of the panel and two sided reflective elements 42 in between the rows of solar cells. The base 43 of the reflective element 42 has a width of 26mm; the base 44 of reflective element 41 has a width of 13mm. The reflector surfaces 41 have a curved shape, preferably parabolic and are covered with a reflective coating that reflects and concentrates the indirect light by a factor 3 onto the solar cells. Thanks to this concentration by a factor 3 the width of indirect light cells 60 do not need to have a width of 38 mm. However these strips now have a width of just 6 mm and two such strips are positioned along the sides of the direct light cells 50. Therefore the cost of the solar panel may be reduced.

In yet another embodiment a concentrating Solar Panel uses optical elements that focus the direct light in two directions, and achieves a concentration ratio of 1000 for the direct light. The solar panel is schematically depicted in Figures 3A, 3B and 3C. The solar panel comprises of a solar glass plate 20 have a width and length of 1000mm, with 100 integrated segmented lenses 25, whereby each lens has a length and width of 100mm and consists of a clear silicone that focus the direct light rays 10 on a small square multi-junction cell 51 with a length and width 3.3 mm. These cells are optimized to convert highly concentrated light with a high efficiency. The indirect light rays 11 have random directions and will mostly hit the surface area besides the multi-junction cell. This area is covered with low cost CIGS based thin film solar cells 61, in order to convert the indirect light at reasonable efficiency and low costs. All solar cells are electrically connected to copper tracks that are bound to the back plate 71, which consists of materials such as glass fibers and epoxies, and structures similar to printed circuit boards used for electronic devices. Other materials and structures may also be used as a back plate. The panel contains an air gap 30 that is filled with dried air or with another gas. A spacer and sealing 81 around the perimeter of the panel ensure that the multi-junction cells 51 are positioned at the correct focal distance from the lenses 25. The spacer and sealing 81 have a height of approximately 120 mm and ensure that the air gap inside the panel is closed off from dust, water and cell-damaging gases (e.g., sulphorous oxide).

In yet another embodiment a Solar Panel uses optical elements that focus the direct light in two directions with a concentration ratio of 1000 for the direct light, and also incorporates reflective optical elements (schematically depicted in Figures 4A, 4B and 4C) to concentrate indirect light. The solar panel is similar to the panel described in the previous paragraph however also incorporate reflective elements 45. Each multi-junction cell 51 is surrounded on all four sides by reflective elements 45. The reflectors have a base 47 with a width of 25 mm. The reflector surfaces 46 are curved and covered with a reflective coating to concentrate the indirect light by a factor 4. The surface area that needs to be covered with indirect light cells is approximately 75% smaller compared to the solar panel described in the previous paragraph.

Figure 5A - 5D depict schematic cross-sectional views of different types of focusing optical elements that may be used according to different embodiments of the invention. Figure 5A depicts a thick lens profile 26, wherein each lens surface has a continuous shape. The lens profile may be described by a cross section of a cone, or by a higher order formula. Figure 5B depicts a thinner implementation of a lens; in this case the profile 27 is discontinuous and consists of multiple segments, each with a different shape. In this case the focal point of the lens is not straight below the center of the profile, however is positioned towards one side. The profile is not symmetric. Please note that in any of the possible embodiments described above it is possible to create an optimal design by using a non-symmetrical element for focusing the direct light and a non-symmetrical element for reflecting the indirect light. Figure 5C depicts a diffractive optical element, in which case a thin diffractive layer consisting 28 causes the focusing effect. Figure 5D depicts an optical element that uses a material 29 with a so-called gradient refractive index. A gradual change in the refractive index within the material causes a focusing effect.

Claims

A solar panel consisting of at least two types of photoelectric cells and a plurality of focusing optical elements that project incident light directly from the sun onto one type of photoelectric cells, while another type of photoelectric cells indirectly collect light that enters the panel from other directions, and then partially convert this light into electrical power

The solar panel of claim 1, wherein the focusing optical elements focus the light rays in only 1 direction, directly below the center of each optical element

The solar panel of claim 2, wherein additional reflective optical elements reflect the indirect light rays and concentrate on a smaller surface

The solar panel of claims 2 and 3, wherein the focusing optical elements do not project the light rays in the center below each optical element, but rather from the center

The solar panel of claims 2, 3 and 4, wherein the focusing optical elements focus the light rays through refraction, and have a continuous or just discontinuous profile

The solar panel of claims 2, 3 and 4, wherein the focusing optical elements focus the light rays by means of diffraction or by means of a material with an internally varying refractive index

The solar panel of claims 5 and 6, wherein the reflective surface of the reflective optical element has a constant profile in 1 straight direction and which contains discontinuities

The solar panel of claim 1, wherein the focusing optical elements focus the light rays in two directions directly below the center of each optical element

The solar panel of claim 8, wherein additional reflective optical elements reflect the indirect light rays and concentrate on a smaller surface

The solar panel of claims 8 and 9, wherein the focusing optical elements do not project the light rays in the center below each optical element, but rather from the center

The solar panel of claims 8, 9 and 10, wherein the focusing optical elements focus the light rays through refraction, and have a continuous or just discontinuous profile

The solar panel of claims 8, 9 and 10, wherein the focusing optical elements focus the light rays by means of diffraction or by means of a material with an internally varying refractive index

The solar panel of claims 11 and 12, wherein the reflective surface of the reflective optical element has a constant profile in 1 straight direction

The solar panel of claims 11 and 12, wherein the reflective surface of the reflective optical element varies in two directions and may include discontinuities