WO2007073203A1

WO2007073203A1 - Solar cell module

Info

Publication number: WO2007073203A1
Application number: PCT/NO2006/000489
Authority: WO
Inventors: Erik Sauar; Ole M. Grimsrud; Erik Stensrud Marstein; Alf BJØRSETH
Original assignee: Renewable Energy Corporation Asa
Priority date: 2005-12-19
Filing date: 2006-12-19
Publication date: 2007-06-28

Abstract

The invention regards a solar cell module comprising a plurality of electrically interconnected solar cells having a front side and a back side and a transparent front plate overlying the front side of the solar cells, where the solar cells are arranged in a pattern where at least two cells are spaced from each other providing areas with no solar cells, and the transparent front plate comprises reflective areas overlapping with the areas with no solar cells. The reflective areas redirect light from areas with no cells onto the solar cells. The density of cells in the module, and hence the cost, may thus be reduced without significantly decreasing the efficiency of the module.

Description

Solar cell module

The invention regards a solar cell module.

Usually, solar cells are electrically connected, and combined into "modules", or solar panels. Solar panels have a sheet of glass on the front, and a resin encapsulation behind to keep the semiconductor wafers safe from the elements (rain, hail, etc) and give protection against corrosion. Solar cells are usually connected in series in modules, so that their voltages add.

In conventional flat-panel solar cell modules, the active elements, i.e. solar cells, account for the largest share of the costs due to expensive material and manufacturing process. For mono crystalline elements, which are produced from cylindrical ingots, this is a particular important issue, as the material is more expensive to produce. Additionally the round mono crystalline wafers traditionally are cut to have a square (or pseudo-square) shape in order to have a close packing of the active elements on the module, the cutting leading to loss of material and thus even higher costs.

It is thus desirable to reduce the density of active elements in each module, while still capturing mostly the same amount of light incident on the solar module. By achieving this, a solar cell module with reduced cost per watt can be provided.

US 5 994 641 describes a photovoltaic module comprising an array of electrically interconnected photovoltaic cells disposed in a planar and mutually spaced relationship between a light-transparent front cover member in sheet form, and a back sheet structure is provided with a light-reflecting means disposed between adjacent cells for reflecting light falling in the areas between cells back toward the cover member for further internal reflection onto the solar cells. The light will travel through the front cover member, into the air gap between the front cover and the back sheet structure towards the light-reflecting means on the back sheet structure. After the reflection by the light-reflecting means, the light will again travel through the air gap and then into the cover member with an angle greater than the critical angle to experience total reflection in the outer surface of the cover member. This method will lead to loss of light in all the transitions between the different media (front cover-air-air-front cover). This configuration also requires accurate alignment to ensure that the angle of the reflected light is greater than the critical angle of the front cover.

US 5 076 857 describes a photovoltaic cell where a glass cover plate comprises a plurality of v-grooves located over the front contact grid of the cell to avoid loss of light due to reflection from the front contact grid. The incident light travelling in a direction towards the contact grid on the cell is deflected by the v-grooves in such a way that the light rays will hit the active area of the cell rather than the contact grid. This is a method for improving the conversion efficiency of a cell, and does not address the problem of reducing the cell density in a solar module.

The object of the invention is to provide a solar cell module which is less costly to produce while still maintaining almost the same efficiency of the module. More specific it is an object of the invention to provide a solar cell module with a reduced density of active elements in each module, while still utilizing mostly the same amount of light incident on the solar module.

The object is achieved by means of the features in the patent claims.

The typical solar cell module according to the invention comprises a plurality of electrically interconnected solar cells having a front side and a back side and a transparent front plate overlying the front side of the solar cells, where the solar cells are arranged in a pattern where at least two cells are spaced from each other providing areas with no solar cells, and the transparent front plate comprises reflective areas overlapping with the areas with no solar cells. Note that the principle of the invention is independent of the current conventional column/row arrangement of solar cells in solar modules, and applies to any layout where the distance between the active elements in the solar module can be increased. This can be accomplished either by moving solar cells of standard size further apart or also by cutting standard solar cells so as to reduce their size and then separate them, in order to increase the portion of the module with no active elements.

According to one embodiment, the solar cells are arranged in rows and columns, where at least one of the rows or columns being spaced from each other.

By the term "solar cell" is in this document meant any photovoltaic device, such as mono crystalline cells, multi crystalline cells, ribbon silicon cells, thin film cells, etc. Such a solar cell may be produced from a single piece of material, for example by dividing one solar cell into several cells.

The reflective areas are designed to reflect the incident light in such a way that the light hits one of the solar cells after reflection. This can be achieved by two or several reflections. In a preferred embodiment, the reflective areas are arranged on the surface of the transparent front plate facing the solar cells (back side). The light may then be reflected off the reflective areas and be redirected towards the front surface with an angle that is larger than the critical angle of the material of the transparent front plate in order to achieve total internal reflection. In this way, the reflected light will be reflected off the front surface of the transparent front plate and back into the transparent front plate. The light will then either hit a solar cell or hit another place on a reflective area to experience another reflection on the way to a solar cell. The extent of planarity of the front surface of the transparent front plate will influence the reflectivity properties of this surface. In order to achieve total internal reflection of a maximum amount of the reflected light, the front surface of the transparent front plate should be substantially planar in the area directly above the reflective areas. To account for incident light with angle of incidence different from zero, ie. angled incident light, the front surface of the transparent front plate should also be substantially planar in areas larger than the reflective areas. Experience has shown that light with incident angle larger than 30 degrees, contributes little to the current generated by the solar cells, and thus a planar area covering incident angles less than 30 degrees should be satisfactory. This enlargement (relative to the extent of the corresponding reflective areas) of the planar regions of the front surface of the transparent front plate would also allow (normally) incident light that enters the reflective area at a point close to the cell to be reflected back to the planar region of the front plate (at an angle of ~30° relative to the normal for grooves with vertex angle around 120°)

In order to fully benefit from spacing both between columns and rows of cells, the solar panels should preferably be adapted to track the movement of the sun. Without tracking the motion of the sun, a spacing only between the rows of cells or between the columns of cells would be most beneficial with respect to the effective utilization of the light incident on the entire front plate surface.

The reflective areas may be provided as grooves in the surface of the transparent front plate facing the solar cells. The grooves may be composed of plane surfaces (such as v-shaped grooves of the same height and angle, or other combinations of plane surfaces), by surfaces of other shapes, such as segments of spherical, elliptical, parabolic, or hyperbolic geometries, or more complex surfaces such as combination of surfaces with different shapes. One example can be combination of v-shape and curves. In the following we will, for simplicity, describe the case of v- shaped grooves, but the principles and methods described will apply also to other geometries. These grooves may be specified before production and manufactured directly into the glass using current production technology, without adding significant costs, or the grooves may be machined into the glass using standard techniques. The reflective areas may in some embodiments be separate elements connected/joined to the front plate, for example by gluing, welding, etc.

As described above, in one embodiment, the vertex angle of the v-shaped grooves is such that light incident on the grooves is reflected back into the transparent front plate with an angle larger than the critical angle of the material of the transparent front plate. An angle around 120° (for example in the range 110°- 130°) is optimal for glass with an optical index around 1.5. The v-shaped grooves may be coated with a reflective coating to enhance the reflection rate. The reflective coating may be a metal deposited onto the grooves, such as Al, Ag or Au. The reflection may alternatively be provided by other materials such as reflective polymers. Any degree of roughness on the surface of the grooves may lead to the angle of the reflected light varying over the reflective areas, this can lead to a non-negligible amount of loss of incident radiation. The surfaces of the grooves should thus be sufficiently plane in order for the light rays not to be reflected at an angle substantially different from the intended angle. The reflective areas are adapted to provide substantially specular (not diffuse) reflection of the incident light beam.

The roughness of the grooves must thus be sufficiently small so that only a small amount of the incident light encounters areas where the vertex angle of the grooves deviates by more than a few degrees from the desired angle. The tolerance for variation in angle depends on the desired angle, for example, when the desired angle is 120°, if the angle deviates by more than of the order 10° from this optimum, a non-negligible amount of the incident radiation is lost.

Depending on the roughness level of the grooves in the as-manufactured glass and on the requirements in each particular application, the surface of the grooves may thus need to be polished. This can be done using standard techniques (mechanical, chemical/etching, etc.). Alternatively, the roughness of the surface of the grooves may be reduced by controlled thermal heating, possibly in a mould, of the glass.

In one embodiment of the invention are all the v-shaped or otherwise shaped grooves are aligned in parallel, but they may also have several different orientations.

In another embodiment of the invention, the solar cell module comprises a transparent front plate comprising two layers with different refractive index. Preferably the layer with the higher refractive index will face the solar cells. By choosing the proportions between the refractive indexes, a focusing effect may be achieved which optimizes the characteristic angle of the reflective element while relieving the constraint on incident radiation covering a large range of incident angles. Alternatively the second, high index layer may be added separately between the transparent front plate and the solar cell. The reflective area will be provided in the high index layer. In one possible embodiment, the reflective areas are provided at the surface of the transparent front plate not facing the solar cells.

The solar cells may be of an ordinary type, or can be adapted for this particular low density module type. Because the areas not covered by the solar cells will reflect light onto the solar cells, the shape of the solar cells may vary, e.g. be rounded, square, polygonal, etc. This means that the mono crystalline solar cells do not need to be cut to obtain a square/semi-square/rectangular shape for optimal packing, which again leads to less loss of expensive material and thus the option of making a less expensive module with high quality solar cells. Another possible modification relates to the electrical contact grid on the solar cell: As the distribution of light incident on the solar cell is modified in the current invention (due to the total incident light being the superposition of direct and reflected light), the design of the electrical contact grid may be re-designed/ re- optimized in order to minimize the resistive losses in the contact grid. As the solar cells will experience more incident light near the edges of the cells in the current invention, the cell will generate more current in the edge areas. As the resistive losses increase with the current, minimizing the resistive losses may be achieved by locating the bus bars in the contact grid in a position closer to the edges than in traditional solar cells. This can be accomplished by distributing the bus bars over the active area of the cell in such a way that each bus bar collects current generated from areas exposed to roughly the same amount of incident (direct + reflected) light, i.e. each bus bar collects approximately the same amount of current. Note that this could be done either by moving bus-bars in current conventional contact grids further apart, or by adding bus-bars so as to minimize the resistive losses due to the additional incident light.

The invention will now be described in more detail by means of examples accompanied by the figures.

Fig. Ia shows an illustration of the principle of the invention Fig. Ib shows one embodiment of the invention where the reflective areas are v- shaped grooves incorporated in the front plate.

Fig. 2a, 2b, 2c shows two possible configurations of the solar cells in a module according to the invention

Fig. 3 illustrates a part of one embodiment according to the invention Fig. 4 illustrates a part of another embodiment according to the invention

Fig. 5 illustrates the arrangement of the electrical conductors on a solar cell for use in a module according to the invention.

Fig. 6a and 6b illustrate two embodiments of the invention with different ratio between the width of the solar cells and the width of the reflective areas. Figure Ia illustrates the principle of the invention. The transparent front plate 10 overlies a plurality of solar cells 11 which are arranged spaced from each other, providing areas 13 with no solar cells. The solar cells 11 are electrically interconnected (not shown) and have a front side 14 and a back side 15. A reflective element 12 is arranged in or overlying the gap 13 between the solar cells. Light incident on the area 13 without any solar cell is reflected off the reflective element 12 and back into the transparent front plate 10, and reflected again off the interface between the front plate 10 and air by total internal reflection (TIR) towards a solar cell 11. Figure Ib illustrates one embodiment of the invention where the reflective areas 12 are v- shaped grooves 16 incorporated in the transparent front plate 10. The broken line 17 represents the plane formed by the side face of the transparent front plate which faces the solar cells, i.e. the back side of the front plate. The reflective areas 12 are overlying and overlapping with the areas 13 with no cells. This means that the reflective areas in their entirety lie above the plane 17, and thus the front side 14 of the solar cells. As seen in the figure, there is in this embodiment no optical interface between the transparent front plate and the reflective area as the reflective area is incorporated into the front plate and is not a separate element connected to the front plate. The reflective areas are incorporated/integrated in the front plate by directly forming the reflective pattern, here v-shaped grooves, in the front plate, for example by means of etching, engraving, carving, cutting or other suitable methods.

As in figure Ia, the light incident on the area 13 with no solar cells, i.e. the light rays being illustrated by arrow 18, will be reflected by the v-shaped grooves 16 of the reflective area 12 back into the transparent front plate 10. The light will then travel towards the interface between the front plate and air. The angle of the v- shaped grooves 16 is such that the incidence angle on the front plate/air interface is larger than the critical angle, i.e. the angle of incidence above which the total internal reflection occurs. The light will then be reflected off the front plate/air interface and towards the back side of the front plate. Depending on the width of the area between the solar cells, and thus the width of the reflective area, there may be one or several reflections of the light before the light hits the solar cell.

In figure 2a, 2b, and 2c, three possible configurations of solar cells in a module according to the invention are shown. In figure 2a are a number of multi crystalline solar cells 21 arranged on a module 20. The cells 21 are arranged in rows 23 and columns 22, and the columns 22 are spaced from each other. The reflective elements 24 are arranged overlapping the space between the columns.

Figure 2b shows a similar arrangement as in figure 2a, but the solar cells 21' are mono crystalline and thus rounded. Because the reflective elements may overlap all areas with no solar cells, there is no need for complete squaring of the solar cells with loss of expensive material as result.

Figure 2c shows a modification of the arrangement in figure 2b, where the mono crystalline cells are pseudo-squared. The optimal shape of mono-crystalline solar cells in a module using the current invention will depend on several factors, including the cost of module fabrication and system installation, as well as the fraction of light collected from the area between the cells.

Figure 3 illustrates how the reflective areas 30 may be incorporated in the transparent front plate 31. The reflective 30 areas are provided as v-shaped grooves 32 stretching in parallel along the length of the area without solar cells. Preferably a metal or other high reflective coating may be deposited on the v-shaped grooves to enhance the reflection ratio of the reflective areas.

Figure 4 illustrates an embodiment where the transparent front plate 40 comprises two layers 41, 42. The layer 42 facing the solar cells has higher refractive index (e.g., n~2) than the layer 41 facing the incident light (e.g., n~1.5). The reflective area 43 has the same shape as in figure 3 and is provided in the high indexed layer 42. This leads to a focusing effect on the light and makes the system more insensitive for the field of view of the module. Figure 5 illustrates, in one embodiment of the invention, how the spacing between busbars can be re-optimized in order to capture the full benefit of the current invention: The distance between the edge of the cell and the busbar is denoted x, and the distance between the busbar and the center line of the cell is denoted α-x (in conventional modules, α=l to minimize the total resistive losses). The reflective areas between the columns of cells have a width of 2-z, and an amount γ % of the light entering these reflective areas are redirected towards the active cell area. The new distance between the busbars is recalculated in order to minimize the resistive losses in the contact grid. To illustrate the method, we assume as a first approximation that the redirected light is uniformly distributed between the edge and the busbar on the active cell, α is obtained from α-x = x + γ-z. As a concrete example, if the cell width is 150mm (i.e. 2-χ-[l+α] = 150mm), z=10mm and γ=90%, we obtain α = 1.27.

Figures 6a and 6b illustrate two embodiments of the invention with different ratio between the width of the solar cells and the width of the reflective areas. In figure 6a the solar cells 60 and the reflective areas 61 (and the spacing between the cells) have the same width d. This gives a concentration ratio of 2, i.e. the solar cells receives twice as much light as they would without the reflective areas arranged in the spacing between the cells. The light distribution on the solar cells will in this case be uniform as all parts of the cells will receive the same amount of reflected light. In this example the cell width is in the area of ~10-15 mm and the ratio between cell width and glass thickness is around 4.

In figure 6b the solar cells 60 still have width d, and the reflective areas 61 ' have half of this width ¹Ad. In this case the concentration ratio will be 1.5, and the light distribution on the solar cells will be non-uniform as only the edge sections of the solar cells will receive reflected light. In this case the arrangement of bus bars as described in connection with figure 5 will be advantageous as the current will be more equal distributed in the conductors/bus bars. With optimal number of bus bars, and by carefully choosing the distribution of the bus bars over the cell area, it can be achieved that each bus bar collects the same amount of current while minimizing the resistive losses. Other ratios between the width of the solar cells and the reflective areas may of course be chosen according to the desired concentration ratio.

Claims

1. Solar cell module comprising a plurality of electrically interconnected solar cells having a front side and a back side and a transparent front plate overlying the front side of the solar cells, the solar cells being arranged in a pattern where at least two cells are spaced from each other providing areas with no solar cells, characterised in that reflective areas are incorporated in the transparent front plate, overlying and overlapping with the areas with no solar cells.

2. Solar cell module according to claim 1, characterised in that the surface roughness of the reflective areas is small enough to give a non-negligible angle variation of the reflected light.

3. Solar cell module according to claim 1, characterised in that the reflective areas are provided as grooves in the surface of the transparent front plate facing the solar cells.

4. Solar cell module according to claim 3, characterised in that the grooves are composed of plane surfaces.

5. Solar cell module according to claim 3, characterised in that the grooves are v-shaped.

6. Solar cell module according to claim 3, characterised in that the grooves are composed of a combination of surfaces with different shapes.

7. Solar cell module according to one of the claims 3-6, characterised in that the angle of the grooves are such that light incident on the grooves is reflected back into the transparent front plate with an angle larger than the critical angle.

8. Solar cell module according to claim 5, characterised in that the vertex angle of the grooves are in the range 110°- 130°

9. Solar cell module according to one of the claims 3-8, characterised in that the grooves are coated with a reflecting coating such as Al, Ag, Au, reflective polymers, etc.

10. Solar cell module according to one of the claims 3-9, characterised in that all the grooves are aligned.

11. Solar cell module according to one of the claims 3-9, characterised in that the v-shaped grooves have several different orientations.

12. Solar cell module according to one of the claims 1-11, characterised in that the transparent front plate comprises two layers with different refractive index, the layer with the higher refractive index facing the solar cells.

13. Solar cell module according to claim 1, characterised in that the reflective areas are provided at the surface of the transparent front plate not facing the solar cells.

14. Solar cell for use in a module according to one of the preceding claims, characterised in that the cell comprises a number of bus bars which are distributed over the cell area in such a way that each bus bar collects the same amount of current while minimizing the resistive losses.