WO2012052542A1

WO2012052542A1 - Arrangement in a solar panel

Info

Publication number: WO2012052542A1
Application number: PCT/EP2011/068431
Authority: WO
Inventors: Tag Hammam
Original assignee: Tag Hammam
Priority date: 2010-10-21
Filing date: 2011-10-21
Publication date: 2012-04-26

Abstract

A junction rail designed to electrically connect the photodiodes in a solar module. The junction rail consists of two rows of conductive segments that are placed besides to each other and in addition the said segments are mutually electrically isolated. In an alternative embodiment of the junction rail, an anti-parallel connected diode / diodes or relay / relays, preferably MEMS relays and driver circuit is placed between the segments in order to bypass a shaded solar cell. The length of a segment is adapted to daisy chain two photodiodes, or two groups of parallel connected photodiodes. The two rows of segments are staggered corresponding to the spacing of two series-connected photo diodes, or two series-connected groups of parallel connected photodiodes. The affixed, preferably soldered, electrical conductors on the photodiode are connected at both ends to separate parallel junction rails. The junction rail design is so established that it also can mechanically relieve the solar module front glass plate at an applied external applied load, and in an alternative embodiment of the invention also facilitates heat dissipation from the solar cell.

Description

Arrangement in a solar panel

Technical Field

The present invention relates to an arrangement in a solar panel including a junction rail designed to electrically connect the photodiodes or solar cells in said panel.

Background of the invention

The predominant type of solar cells for terrestrial power generation is made up of a doped silicon wafer with electrical connections at both front and back of the wafer where the front corresponds to the photodiode anode or cathode. A typical solar cell is held within a square whose side is between 120 mm to 210 mm, and typically placed 40 to 80 solar cells in a module with a distance of 2 -10 mm between the two solar cells. Usually connected in series a large number of solar cells are so connected to obtain a higher electrical voltage from the module. A typical number of serially connected solar cells in a sequence is 10 pieces which form a row. Number of rows in a module can be, for example, 6 rows, joined in a junction box.

The electrical connection of a conventional solar cell usually consists of a large number of parallel silver lines on the front of the solar cell and an aluminum film with silver dots / silver lines on the back side. Serial connection between the solar cells is usually done by two or three tin plated copper conductive ribbons soldered perpendicularly to the silver lines on the front-side of the solar cell, where the ribbons continue on to the back-side of the next solar cell where the ribbons are soldered in place.

The electrical connection by ribbons on the solar cells in a module causes optical and electrical losses. The optical losses are mainly due to that the ribbons on the front of the solar cell shades the solar cell, while the electrical losses are mainly due to that the electrical conductive ribbons cannot be designed with desirable cross- section size, because the ribbons that are soldered onto the silicon wafer causes mechanical stress on the silicon wafer at cooling due to the nature of the difference in thermal expansion coefficient between copper and silicon. The strong contraction of a copper ribbon during cooling causes a bending moment on the silicon wafer. At best, the bending moment of the conductive ribbons on the front and the back- side of the wafer completely cancel each other out. However, if there is an imbalance in the bending moments the silicon wafer will bend, resulting in an increased risk of cracking of the silicon wafer, thus increasing the risk for failure.

When a single solar cell in a row of serially connected solar cells is shaded, it will block the current flow for all solar cells connected in the row. This can be prevented by bypassing the electrical current by connecting an anti-parallel connected diode / diodes to each solar cell, or a relay / relays with associated driver circuit. With today's technology, however, it requires extensive wiring, or the solar module surface needs to be made substantially larger. Therefore, as a rule all the series connected solar cells in two rows are connected to an anti-parallel connected diode, and thus all the solar cells in these two rows are bypassed when a single solar cell is shaded.

Soldering of the ribbons in solar cells is usually done in fully automatic machines. However, in cases where the soldering has to be redone on a solar cell, or when the silicon wafer breaks during the soldering process the solar cell need to be removed from the soldering machine and corrected manually. This is extremely difficult to automate due to that all solar cells in one row are connected in a long chain.

Furthermore, the fact is that In a row of serially connected solar cells the electrical current is determined by the solar cell that produces the least power. In the manufacture of solar cells a spread of performance is always obtained, and therefore series-connected solar cells are matched with respect to the current obtained under standardized metering conditions. Measuring the performance of the individual solar cells is made before soldering, however, the final measurement and classification step of the module is done when the assembling and

manufacturing is completed. If a solar cell in a row of serially connected solar cells produces less current than the other in the row, the entire row of solar cells will obtain a reduced performance. It would therefore be advantageous to measure the performance of individual solar cells after soldering, but this is not done today.

For solar cells for commercial power generation, it is essential that the cost per kWh from a Photovoltaic (PV) system is reduced to an absolute minimum. In this context, it is of utmost importance that the relationship between the cost of materials used for electrical connection and the electrical power generated from a solar module is optimized. One way to reduce the electrical losses is to increase the total cross-sectional area of the connecting conductors by increasing the number ribbons from two to three as proposed by Fujii Shuichi et.al at Kyocera Corporation [1], hereby incorporated in this description by this reference. A disadvantage of this method is that it increases the shading which increases the optical loss. There are alternative solutions that reduce the optical losses such as solar cells with all the contacts on the back-side [2], hereby incorporated in this description by this reference.

However, all known alternatives will result in a more complex manufacturing process of the solar cell, which increases the manufacturing costs. It is well known to persons skilled in the art that the electrical power loss in a metallic conductor of constant cross-sectional area is P = 1 ² R. Since the generated current in a (parallel) conductor in a solar cell is proportional to the length of the conductor and as the resistive loss is also proportional to the length of the conductor, this implies that the electrical power dissipation in the above conductor is proportional to the cubic length of the conductor.

With the dominant technology today only one end of the ribbon conductor which is soldered onto the solar cell is connected to the next solar cell, but if the ribbon is connected at both ends to the next solar cell, this corresponds to the ribbon being replaced by two ribbons of half the length, thereby reducing the loss of the ribbon conductor by a quarter

( ½ ³ + y₂ ³ = /4 ). Michael Stietka et.al. [3], (hereby incorporated in this description by this reference) has pointed out that the electrical loss of the ribbons soldered onto the solar cell can be greatly reduced if they are connected at both ends, however, this involves a more complicated manufacturing process according to the above reference. Examples of such structures are described in reference [4] and reference [5], hereby incorporated in this description by these references.

The given reference [4] describes solar cells in which an amorphous silicon film has been deposited on a stainless steel foil which constitutes the electrical contact on the back side. The electrical contact on the front-side consists of fine screen- printed silver lines and perpendicular to these, a tin plated wire is soldered on. This wire is connected in both ends to the stainless steel foil, which is connected to the back-side of the next adjacent solar cell. This system is not suited for a solar cell made of a silicon wafer, because of the high mechanical stresses that occur when a foil is used as a contact as above. In the given references [5] the ribbons soldered onto the silicon wafer are connected to an adjacent contact device consisting of two films with an

intermediate electrically insulating film. The problems with that in reference [5] described solution is as follows: The design of the contact device implies that in order to obtain low electrical losses in the contact device the width of the foils in the contact device needs to be relatively large, resulting in a reduced density of solar cells in such a solar module. To increase the packing density for the above said contact device, require that the width of the foils is reduced which means that the thickness of the foils thus need to be increased accordingly to offset the electrical losses, this results in a significant increase in the total length of conductors from the solar cells to the above said contact device. The material of the said conductor which is soldered onto the solar cell is relatively costly, which is well known to persons familiar with the art. It is therefore important to minimize the length of the conductors which are attached to the silicon solar cell and therefore better solutions are needed than those offered by today's technology to satisfy the requirements of both the cost of production, electrical losses, and the packing density of a solar module. For example, assuming that the required length of conducting material per connection point for the above said contact device can be reduced by 5 mm for a solar cell with three conductors on each side which is a typical number for a conventional solar cell, will reduce the consumption of conducting material with 16 km per HW_peak, which represents a full circle around the equator of the earth at a solar cell production of 2.5 GW _peak. Conversely, for the solar cells described in reference [5] the number of conductors is assumed to be more numerous (several dozens of conductors) Soldering the conductor wires to the contact device in reference [5] is made during the lamination process, which excludes replacement of individual solar cells that breaks during soldering.

It is well known to persons skilled in the art that the efficiency of a silicon solar cell significantly decreases with increasing temperature. In a typical solar module design the glass plate rests on a supporting frame, it is also well known to persons skilled in the art that in such a solar module a hot air cushion is normally formed on the back-side of the solar module which causes restricted heat dissipation and thus an elevated temperature of the solar cells. An attempt to solve the above problem by providing the supporting frame of the solar module with ventilation holes has been presented in reference [6]. However, this solution is not optimal in terms of production cost and the intended heat dissipation effect.

US 20100170555 Al describes a solar cell that has at least one semiconductor layer arranged on metal support and is provided with a plurality of contact tracks arranged on the semiconductor layer. The contact tracks are connected to the solar cells by an adhesive bonding.

The use of adhesive bonding to connect the contact track to the solar cells has several drawbacks. One is that the solar cells expand and contract according to the temperature they experience. This can lead to the adhesive bonds being torn when the solar cells contract in cold weather. Further the expansion and the contraction of the solar cells exert stress on the contact track which again influences the conductive capability of the contact tracks.

Further the adhesive bond covers up a part of the solar cell which leads to a reduction in the effect of the solar cell.

Summary of the invention

It is an object of the present invention, to devise an arrangement in a solar panel that in at least partly overcomes the drawbacks mentioned above.

The present invention is a cost effectively, high-volume production, to join the conducting ribbons on the solar cell at both ends in order to reduce the electrical and optical losses in a solar module. A further object of the invention is as follows:

Provide opportunities to reduce material cost of a solar module.

Provide the possibility to reduce the mechanical stress that occurs on the solar cell after soldering of the conducting ribbons and thereby reduce the number of solar cells that break when soldering, or the possibility of using thinner silicon wafers, yet maintaining the same production yield.

Allow for fully automatic production soldering of solar cells which include automated handling and replacement of solar cells that exhibit poor performance after soldering. Allow for significantly higher production rate at the soldering of solar cells compared to existing technologies.

Provide the possibility to mechanically reinforce the glass plate (figurl2), and thereby reduce the thickness of the glass. A thin sheet of glass reduces the optical transmission losses caused by the glass.

Provide the possibility of better heat dissipation through the effective venting away of the warm cushion of air that normally occur on the back-side of a solar module.

Provide the ability to easily connect an anti-parallel connected diode / diodes to the individual solar cells, or a relay / relays, preferably MEMS relays, with associated control circuit. This allows for a high degree of efficiency, obtained even if a single solar cell is shaded.

Secure the position of the solar cells during lamination, which means that the distance between the solar cells can be reduced, and thus the active portion of the surface of a solar module can be increased. Lamination means that the solar cells are enclosed between two polymer films. Usually solar cells are laminated between a glass plate and outer and thick multi-layer polymer film (called a backsheet).

The above aims and objectives have been achieved through the creation of an arrangement as disclosed in the appended claim 1. Further preferred embodiments are defined in the dependent claims. The invention consists of an arrangement in a solar panel that includes a multi-part junction rail that can be managed as a single component. Preferably, the aforementioned junction rail is placed on each side of the solar cells to be connected (Figure 1A). The junction rail is constructed of electrically conductive segments whose length is adapted to serially connect two solar cells, or two groups of parallel connected solar cells. Each junction rail is composed in such way that two rows of segments adjacent to each other are electrically isolated. The two rows of segments are staggered corresponding to the spacing of serially connected solar cells, or alternatively the spacing of a serially connected group of parallel connected solar cells. The junction rail has connection points for the fixed, preferably soldered, conductive ribbons from the photodiode / solar cell. Electrical contact between the junction rail and the affixed, preferably soldered, conductive ribbons from the solar cell is present only at the connection points. Electrical contact outside the connection points between the affixed, preferably soldered, conductive ribbons on the solar cell and the junction rail is prevented by isolation of the junction rail and / or the conductive ribbons on the solar cell.

An alternative to electrical insulation between the segments is to connect the segments besides each other by a bypass diode, whose function is to lead the current so that it bypasses the solar cell if it becomes shaded. Polarization of the bypass diode becomes the opposite of the solar cell, which is well known to persons familiar with the art and is described in, for example in reference [7], hereby incorporated in this description by this reference. An alternative to a bypass diode/diodes is to use a relay/relays (with associated control circuit), preferably MEMS relay/relays, which has the advantage that the forward voltage drop and leakage current is very low.

The solar cells to be connected can be of the conventional model that has contacts on both the front and back-side, alternatively, all the contacts are placed on one side, preferably the back-side of the solar cell. The attached, preferably soldered, electrical conductors on the solar cell are connected, preferably at both ends to each respective parallel junction rail. Series connected solar cells are connected to the cathode contacts at the first half of a segment and the second half of the same segment connects the anode contacts to the next solar cell in the series. The anode contacts of the solar cell are connected to the adjacent segment, which is also connected to the cathode contacts of the solar cell before it in the series. Thus, for a solar ceil with contacts to the silicon wafer on both the front-side and back-side of the silicon wafer, connecting the junction rail to the affixed conductive ribbons on the front-side of the solar cell with the conductive ribbons on the back- side of the next solar cell, this procedure is repeated until the required number of solar cells in a row has been linked.

Brief description of drawings

Figure 1A shows a schematic exploded view of three solar cells connected in series with two junction rails. Figure IB shows the front-side and back-side of the conductive ribbons before attachment to the solar cell. Figure 1C shows a section of a solar cell with affixed back-side and front-side conductive ribbons before connection with the junction rail.

Figure ID shows a segment before applying electrical insulation.

Figure IE shows cross section A-A in Figure 1A. Figure 2A shows an alternative design of the ends of the conductive ribbons.

Figure 2B shows a cross-section of a solar cell with attached front-side and backside conductive ribbons.

Figure 3A shows a further alternative design of the ends of the conductive ribbons that are bent sideways. Figure 3B shows a cross-section of a solar cell with affixed front-side and back-side conductive ribbons.

Figure 4A shows an alternative design in which the ends of the conductive ribbons are bent downward.

Figure 4B shows a section in which the curved ends of the conductive ribbons are connected to the junction rail.

Figure 4C shows the cross-section A - A in Figure 4B.

Figure 4D shows cross-section B - B in Figure 4B.

Figure 5 shows two junction rails besides to each other that have been merged into one unit. Figure 6A shows an alternative embodiment of the invention. Figure 6B shows the cross-section A - A in Figure 6A. Figure 6C shows cross-section B - B in Figure 6A. Figure 7 shows the segments that have been joined into a junction rail, where the segments have been provided with folds to absorb thermal movements that may occur in the solar module.

Figure 8 shows an alternative design of the segments. Figure 9 shows another alternative design of the segments.

Figure 10A shows three series-connected solar cells, where the junction rails have been provided with transverse support.

FigurlOB shows cross-section A - A in Figure 10A.

Figure 11 shows a solar module in which two rows of series connected solar cells are series connected by means of the junction rails as shown in Figure 6.

Figure 12A shows a schematic diagram of a solar module from the front-side with the proposed junction rails depicted.

Figure 12B shows cross-section A - A in Figure 12A loaded with snow.

Figure 12C shows an alternative design of the ends of the junction rail for the cases when the junction rail partially relieves the glass.

Figure 12D shows an exploded view of a design in which the junction rails

(designed as shown in Figure 12C) rests against the bearing frame of the solar module.

Figure 13 shows the electrical and optical power loss caused by the conductive ribbons affixed on a solar cell.

Detailed description

The junction rail is set up to daisy chain a number of solar cells (Figure 1A), or daisy-chain a group of parallel connected solar cells. The junction rail is constructed of electrically conductive segments with a length adapted to series connect two solar cells, or two groups of parallel connected solar cells, and the junction rail is designed to stand upright in a solar module. Each junction rail is constructed of two rows of adjacent segments which are mutually electrically insulated. The two rows of segments are staggered corresponding to the spacing of series connected solar cells, or series connected groups of parallel connected solar cells. To daisy chain photodiodes, the anode contacts are connected 104 on a photodiode 101 to the first half of a segment 102 and the other half of the segment is connected to the cathode contacts 109, 110 of the next photo diode 107. The anode contacts 108 of the photodiode are connected to the adjacent segment 111 which is also connected to the cathode contacts 112 of the next photodiode 113 in the figure.

The conductive segments are made of an alloy where the main ingredient consists of copper or aluminum. On the segments there are connections points for the affixed, preferably soldered conductive ribbons from the solar cell 101, 102, 107, 108. The junction rail is insulated at the connection points in such a way that accidental contact between electrical conductive ribbons from the solar cell and the junction rail is prevented (Figure IE). The segments of a junction rail are mutually isolated and are bound together by a heat resistant electrically insulating glue or adhesive film 702. In cases where the electrical segments are made of aluminum, an electrical insulation in whole or in part can be achieved by anodization of the segments.

A segment may in its simplest form consist of a plate with the same thickness along its entire length. In an alternative version, the segments are thicker at the center and thinner at the ends in order to optimally balance the material consumption versus electrical loss (Figure 13). For persons skilled in the art, it is well known that for the inflow of current along an electrical conductor Kirchoff's first law applies I i + 1 2 = 1 3, which means that along the junction rail the current will increase for each point of inflow. For persons skilled in the art, it is also well known that power dissipation in the junction rail is proportional to current squared (P = 1 ² x R).

Thus, to minimize power dissipation in the junction rail it must be connected so that the inflow of electrical current from the conductive ribbons of a solar cell begins where the segment is at its thinnest, whereas the drain of electrical current from the segment to the conductive ribbons of the next solar cell has to start where the segment is at its thickest.

Bypass Diodes can be connected anti-parallel to the individual solar cells by placing flat bypass diodes between the segments 702. Alternatively, relays, preferably MEMS relays, along with a driver circuit can be used instead of bypass diodes. The conductive ribbons which are affixed to the solar cell, preferably soldered, are connected to the junction rail by for example soldering, ultrasonic welding, welding or using a conductive adhesive (Figure IE, 101, 102). The attachment points for the conductive ribbons on the back of the solar cell are preferably positioned on the upper part of the junction rail in order to minimize the length of the conductive ribbons. In cases where conductive ribbons from the back-side of the solar cell are connected to the underside of the junction rail a significantly longer total wiring length is obtained for the solar module (Figure 6B and 6C).

If there is a risk of accidental electrical contact between the front-side and the back-side conductive ribbons, or accidental contact between the conductive ribbon and the junction rail, it is important that there is insulation on the connecting points. The insulation can be arranged by the selective coating of an insulating layer on the junction rail or by selectively coating the conductive ribbons on the solar cell with an insulating layer. A series of solar cells with associated junction rails can be connected by means of electrical conductors with the next row of solar cells with associated junction rails. In an alternative design the junction rails are connected to a junction box in the solar module.

Furthermore, to reduce the loss of the light hitting the junction rails, the surface (edge) of the junction rail, which faces the glass on the solar module, should be dull and predominantly white. Thus, diffuse reflection can be obtained and thereby a part of the light hitting the junction rail through total reflection on the glass can reach the active surface of the solar cell. An example of a coating that can provide high reflectance is silicon rubber with white pigment or EVA (ethylene-vinyl acetate) with white pigment, and a white pigment which preferably can be used is titanium dioxide. In cases where the segments are made of aluminum or an aluminum alloy, a dull and a white surface can be obtained by anodization of the segments.

The connecting rails can in an alternative embodiment of the invention be designed in such way that they mechanically relieve the glass. Thereby, the thickness of the glass can be reduced, which means that the optical transmission loss caused by the glass is reduced. In typical commercial applications the glass plate is dimensioned to withstand snow loads of up to 5400 N / m ². With the above-described relief from the junction rails, it is possible to reduce the glass thickness by 25% resulting in a significant weight reduction of the solar module. In an alternative design of the junction rails which have been dimensioned to mechanically relieve the glass, the junction rails rest against the bearing frame of the solar module in such a way as to form a gap between the glass along with the laminated solar cells and the frame of the solar module. The gap allows air to pass between the glass and the frame whereby improved heat dissipation is obtained as the warm cushion of air that is normally formed on the underside of the module is ventilated away. The height of the column is within the range of 1 mm - 30 mm.

To absorb thermal motions the junction rail should be fitted with one or more folds in the space between the solar cells 703. Preferably transverse support is also placed in the space between the solar cells in order to ensure that the junction rails are standing upright. The transverse support will then rest against the glass (Figure 10B, 101) and the surface facing the glass should preferably be dull and white to obtain diffuse reflection. Preferably the transverse support is surrounded by an electrically insulating layer, and where the transverse support is made of aluminum or an aluminum alloy the insulating layer is preferably achieved by anodizing. The transverse support is preferably double-sided (Fig. 10A), however, single-sided transverse support is preferably used for the junction rails adjacent to the frame of the solar module. In the designs of solar module in which the junction rail mechanically relieves the glass, the transverse support contributes to obtain mechanical stability and thus prevents folding or buckling of the junction rail.

The junction rail can also be used to connect the conductive ribbons on a solar cell in which all contacts are located on the back-side of the solar cell.

The invention provides the following benefits and opportunities:

The above-described junction rail is a multi-part component that can be rationally managed in automatic production lines for soldering. This enables in high-volume production a cost-effective method to connect the conductive ribbons on the solar cell at both ends in order to reduce the electrical and optical losses in a solar module.

Provides opportunity to reduce the mechanical stress on the solar cell arising after soldering of the conductive ribbons, and thereby reduce the number of solar cells that break when soldering, or alternatively the possibility is given to use thinner wafers while maintaining the production yield. With the proposed invention the opportunity is given to first solder the conductive ribbons on the solar cell and thereafter connect the conductive ribbons to the junction rail. It is therefore perfectly feasible that in a fully automated production verify performance of the individual solar cells after the soldering of the conductive ribbons on the solar cells. If a solar cell is not working satisfactorily, it can be replaced automatically before the solar cells are connected to the junction rail. This reduces the performance loss that occurs if a cell in a series-connected string of cells is not functioning satisfactorily after soldering.

One advantage of the invention is that the conductive ribbons that will be soldered onto the silicon wafer can easily be heated resistively to the required soldering temperature. This allows for better temperature control of the soldering process as the temperature of the individual conductive ribbons can thereby be controlled. Furthermore, significantly shorter soldering time is expected compared to infrared light heating or soldering iron which is commonly used today. The junction rail provides the possibility to reduce the total material cost of a solar module.

The junction rail can be designed to mechanically reinforce the glass plate, and thereby the thickness of the glass can be reduced. A thinner plate of glass reduces the optical transmission losses caused by the glass, and reduces the weight of solar module.

The junction rail secure the position of the solar cells during lamination, which means that the distance between the solar cells can be reduced, and thus, the active portion of the surface of a solar module is increased. Lamination means that the solar cells are enclosed between two polymer films. Usually solar cells are laminated between a glass plate and an outer thick multi-layer polymer film (called a backsheet).

For a solar cell with the described junction rail it is easy to use different geometries for the conductive ribbons on the front-side and back-side of the solar cell. This allows that the cross-section area of the conductive ribbons to be increased to up to twice the area by using a wider but thinner conductive ribbons on the back. A prerequisite is that the geometry of the conductive ribbons on the front and the back is cut so that the bending moment arising from the cooling after soldering is equal on both sides. The invention makes it possible to maintain a high degree of automation, replacing the normal 1.5 - 2.5 mm wide and 0.1 -0.2 mm thick tin plated copper conductive ribbons that are soldered onto the silicon wafer with thin wires. Furthermore, the number of ribbon 100 pieces) on each side of the solar cell. With the described junction rail it becomes easy to connect a diode / diodes anti- parallel to each solar cell, or relay / relays, preferably MEMS relays and associated driver circuit, whose function is to bypass the solar cells if they become shaded. In those cases, bypass diodes are used, preferably flat and thin bypass diodes

(thickness approximately 0.12 mm) are placed between the segments (Figure 7, position 2). This may be relevant for solar modules in which parts of the module is shaded by for example chimneys.

Example 1.

The example below shows the benefits of diverting the current at both ends of the conductive ribbons on a solar cell compared to the dominant technology to drain current in only one end of the conductive ribbon. The basis for the calculations and also the values of a standard solar cell is taken from reference [3] . The starting point is a multi-crystalline solar cell with a size of 155 x 155 mm, with a rating of 35mA/cm ² and 0.5 V.

In general, for a given conductor on the front-side of a solar cell the following relation hold :

Increasing the conductor width causes an increase in the optical losses.

Increasing the conductor width while maintaining the height/thickness results in decreased electrical losses in the conductor as a result of the increased cross- sectional area. Increasing the conductor height/thickness while maintaining the cross-sectional area (that is, that the width is reduced accordingly) causes an increase In the bending moment on the silicon wafer.

For a given height of the conductive ribbons on the solar cell there is an optimal total width of the conductive ribbons on a solar cell in which the combined electrical and optical loss reaches an optimal minimum. For a standard solar cell with a height of the conductive ribbons of 0.16 mm an optimal minimum loss is obtained at a total width of 5 mm (Figure 13). The combined electrical and optical loss is then equal to 254 mW.

With a similar solar cell as defined in the above example, but where the conductive ribbons are connected to the previously described junction rail (Figure 1A) an optimal minimum loss is obtained at a total width of 2.5 mm (Figure 13). The combined electrical and optical loss is then equal to 127 mW. Thus, the example shows that the electrical and optical loss can be reduced corresponding to 254-127 = 127 mW when the two junction rails according to Figure 1A is used. To this should be added the electrical loss in the junction rails. Assuming that the segments of the junction rails are made of aluminum, and has a height of 12 mm and a thickness of 0.5 mm, the electric power loss caused by the two junction rails will amount to 10 mW per solar cell.

Thus a solar cell with the above-described junction rails provides a reduction of the electrical and optical losses equivalent to 127-10 = 117 mW, and in addition the amount of the required silver paste is reduced with about 20 %, and the

mechanical stress from the soldered conductive ribbons on the silicon wafer is reduced by 50 %.

While the figures all show junctions rails composed of conductive segments that are rectangular in cross section, the rectangular shape being regarded as most practical for most applications, the segments can have any shape in cross section that is found to be beneficial for some specific purpose, such as circular or semi-circular.

Reference numerals used on the drawings

Figure 1 A shows a schematic exploded view of the three solar cells connected in series with two junction rails.

101 : first solar cell, front side shown.

102: conductive segment of upper junction rail.

103 : conductive segment of lower junction rail.

104: first back-side conductors affixed to the first solar cell. 105: first front-side conductors affixed to the first solar cell. 106: second conductor affixed to the first solar cell. 107: second solar cell, front-side shown.

108: first back-side mounted conductor on the second solar cell. 109: first front-side conductor affixed to the second solar cell.

110: second front-side conductor affixed to the second solar cell.

Il l : The inner part of the upper junction rail.

112: first front-side conductor affixed to the third solar cell.

113 : third solar cell, front-side shown. Figure IB shows a perspective view and a cross sectional view of the front-side and back-side conductive ribbons before attachment to the solar cell. The width of the conductors is between 25 microns and 3 millimeters and the height / thickness is between 25 microns and 250 microns.

104: back-side conductor 105 : front-side conductor

Figure 1C shows a section of a solar cell with affixed back-side and front-side conductive ribbons before connection with the junction rail.

101 : solar cell.

104: back-side conductor. 105 : front-side conductor. Figure ID shows a segment before the addition of electrical insulation. The width / thickness of a segment is in the range between 50 microns and 3 millimeters and the height of a segment is in the range between 1 millimeter and 60 millimeters.

114: connection point for back-side conductor on solar cell. 115: connection point for the front-side conductor on the solar cell.

116: indentation for the front conductor on the solar cell.

117: indentation for the back-side conductor of the solar cell.

Figure IE shows cross section AA of Figure 1A.

101 : solar cell. 104: back-side conductor on the solar cell.

105: front-side conductor on the solar cell.

114: connection point for the back-side conductor.

115 : connection point for the front-side conductor.

118: electrical insulation of the coupling rail. 119: electrical conductor on the junction rail.

Figure 2A shows an alternative design of the ends of the conductors. For thin silicon wafers, 100 microns or less, the total cross-sectional area of conductors can be increased without increasing the optical losses. This is done by increasing the width of the back-side conductor while making it thinner, under the condition that the bending moments from the front-side and back-side conductor cancel each other out. Example: for a solar cell with a thickness of 100 microns and in which both front-side and back-side conductors have a width of 1 mm and height 0.15 mm, the back-side conductor can be replaced by a conductor of the width of 3 mm and a height of 0.0725 mm and thereby maintaining the balancing of bending moments. This measure reduces the total electrical losses in the conductors in the example above by 18% due to the resulting larger total cross-sectional area (0.3 mm ² compared to 0.3675 mm ²).

201 : front-side conductor.

202: back-side conductor. Figure 2B shows a section of a solar cell with affixed front-side conductor and backside conductor.

101 : solar cell.

201 : front-side contact.

202: back-side contact. Figure 3A shows a further alternative design of the ends of the conductors which are bent sideways. This design is suitable for use in thin conductors.

301 : front-side conductor.

302: back-side conductor.

Figure 3B shows a section of a solar cell with attached front-side conductor and back-side conductor.

101 : solar cell.

301 : front-side conductor.

302: back-side conductor.

Figure 4A shows an alternative design in which the ends of the conductors are bent downward. The original form of he conductors is shown in Figure IB.

101 : solar cell.

401 : front-side conductor. 402: back-side conductor.

Figure 4B shows a section in which the curved ends of the conductors are connected to the junction rail.

101 : solar cell. 102: the junction rail.

401 : front-side conductor.

402: back-side conductor.

403: connection point for back-side conductors.

404: connection point for front-side conductors. Figure 4C shows the cross-section A-A in Figure 4B.

101 : solar cell.

105: front-side conductor on the solar cell.

104: back-side conductor on the solar cell.

118: electrical insulation of the junction rail. 119: the electrical conductor of the junction rail.

403: connection point for back-side conductor.

404: connection point for front-side conductor.

Figure 4D shows cross-section B-B in Figure 4B.

101 : solar cell. 105: front-side conductor on the solar cell. 104: back-side conductor on the solar cell.

118: electrical insulation of the junction rail.

119: the electrical conductor of the junction rail.

403 : connection point for the back-side conductive ribbons. 404: connection point for the front-side conductive ribbons.

Figure 5 shows two junction rails besides to each other that have been merged into one unit. The figure shows the connection as shown in Figure 4 with the downward bent conductive ribbons. The advantage of this design is that the risk of accidental short circuit between two adjacent junction rails is reduced. 101 : solar cell.

501 : front-side conductive ribbon.

504: connection point for back-side conductive ribbon.

503: connection point for front-side conductive ribbon.

502: back-side conductive ribbon. 505: inner conductive segment of the junction rail.

506: outer conductive segments of the junction rail.

Figure 6A shows a design where the front conductive ribbons from the solar cells are attached on the upper edge of the junction rail, and where the back-side conductive ribbons are attached to the lower edge of the junction rail. One difference compared to the previously described designs of the conductive ribbons way of connection to the junction rail is that the required total length of the conductors is considerably longer when many conductive ribbons on the solar cell are connected, resulting in higher material consumption of conductive materials.

101 : solar cell. 111 : the junction rail.

501 : front conductive ribbon.

Figure 6B shows the cross-section A - A of Figure 6A.

101 : solar cell. 501 : front-side conductive ribbon.

505: inner segments of the junction rail.

506: outer segments of the junction rail.

601 : connection point of the back-side conductive ribbon.

602: connection point of the front-side conductive ribbon. Figure 6C shows cross-section B - B in Figure 6A.

101 : solar cell.

501 : front-side conductive ribbon. 502: inner segment of the junction rail. 505: contact point of the back-side conductive ribbon. 601 : outer segments of the junction rail.

602: contact point of the front-side conductive ribbon.

Figure 7 shows segments that have been merged to one junction rail, where the segments have been provided with folds to absorb thermal movements that may occur in the solar module. 701 : a segment.

702: joint between two electrically isolated segments. 703: a fold in the segment intended to absorb thermal motion in the solar module. Figure 8 shows an alternative design of the segments. 701 : a segment.

702: joint between two electrically isolated segments. 703: a fold in the segment intended to absorb thermal motion in the solar module.

Figure 9 shows another alternative design of the segments, where the segment's thickness decreases in the direction towards the ends, this in order to minimize material consumption in relation to the electrical losses in the segments, and provides the ability to increase the packing density of solar cells in a module. 701 : a segment.

702: joint between two electrically isolated segments.

703 : a fold in the segment intended to absorb thermal motion in the solar module.

Figure 10A shows three series-connected solar cell, where the junction rails have been provided with transverse support in order to ensure that the junction rails are standing upright on the edge in the solar module. In addition, in the designs of solar module in which the junction rail mechanically relieves the glass, the transverse support contributes to the mechanical stability and thus prevents folding or buckling of the junction rail.

101 : the front-side of the first solar cell in the figure. 102: upper junction rail in the figure.

103: lower junction rail in the figure.

104: first back-side conductive ribbon affixed to the first solar cell in the figure. 105 : first front-side conductive ribbon affixed to the first solar cell in the figure. 106: second front-side conductive ribbon affixed to the first solar cell in the figure. 107 : the front-side of the second solar cell in the figure.

108: first back-side conductive ribbon affixed to the second solar cell in the figure.

109: first front-side conductive ribbon affixed to the second solar cell in the figure. 110: the second front-side conductive ribbon affixed to the second solar cell in the figure.

1001 : transverse support. 1002: transverse support.

FigurlOB shows cross section A - A in Figure 10A. 101 : solar cell.

104: back-side conductive ribbon affixed to the solar.

105: front-side conductive ribbon affixed to the solar cell.

1001 : transverse support.

1003 : glass 1004: EVA (ethylene-vinyl acetate) film.

1005: the junction rail.

Figure 11 shows a solar module in which two rows of series connected solar cells are series connected by means of the junction rails as shown in Figure 6. The two above rows have been linked in the rows endpoints. 101 : solar cell.

1101 : electrical interconnection of the two adjacent junction rails. 1102: electrical interconnection of the two outer junction rails in the figure.

1103: electrical interconnection of the two junction rails that surround a row of solar cells.

1104: external electrical connection. Figure 12A shows a schematic diagram of a solar module from the front of the proposed junction rails depicted.

101 : solar cell.

1201 : aluminum frame.

1202: the junction rail. Figure 12B shows cross section A - A in Figure 12A loaded with snow. The junction rails partially relieve the glass.

101 : solar cell.

1003: glass.

1201 : aluminum frame. 1202: junction rail.

1203: roof or other building structure.

1204: snow.

Figure 12C shows an alternative design of the ends of the junction rail for the cases where the junction rail partially relieves the glass. 1205 : schematic sketch of a junction rail with the alternative design of the ends.

Figure 12D shows an exploded view of an embodiment in which the junction rails rest against the solar module's bearing frame in such a way as to form a gap between the glass along with the laminated solar cells and the frame of the solar module. The gap allows air to pass between the glass and the frame by which an improved heat dissipation is obtained by letting the warm cushion of air that is normally formed on the underside of the module be ventilated away. The arrows in the figure show the airflow around and through the module.

101 : solar cell.

1003: glass.

1203 : roof or other building structure.

1204: bearing frame of the solar module. 1205: junction rail in 12C resting on the bearing frame of the solar in such a way as to form a gap between the glass and the frame.

Figure 13. The diagram shows the electrical and optical power loss caused by the affixed conductive ribbons on a solar cell as a function of the total width of the conductive ribbons of a standard solar cell with both a conventional connection of the soldered conductive ribbons and also a standard solar cell with two junction rails, arrows indicate the minimum (optimal) value of the combined electrical and optical power loss for each graph respectively, 254 mW and 127 mW, obtained at a total summarized width of the conductive ribbon of 5 mm and 2.5 mm respectively.

The invention is not limited to the embodiments described in the figures but can be varied freely within the scope of the subsequent patent claims.

Foregoing publications:

1. US2007295381 (Al), PCT/JP2005/006548

Solar Cell Module and Photovoltaic Power Generator Using This

2. US7468485 (Bl) Back Side Contact Solar Cell with Doped Polysilicon Regions 3. Michael Stietka och Johann Summhammer. "Wire Cell: a More Efficient Silicon

Solar Cell and module" Presented at 23rd European Photovoltaic Solar Energy Conference, Valencia, Spain, 1-5 Sep. 2008.

4. US 005651837A. Solar Cell Module and Manufacturing Method Thereof 5. WO/2004/021455, PCT/CA2003/001278. Electrode for Photovoltaic Cells, Photovoltaic Cells, Photovoltaic Cell and Photovoltaic Module

6. United States Patent 7774998, Ventilated photovoltaic module frame

7. http : //www.microsemi.com/micnotes/304.pdf

Claims

1. An arrangement in a solar module, the solar module including a number of wafer based photodiodes (101, 107, 113), each photodiode having anode and cathode conductors (105, 106; 109, 110) attached thereon,

characterized in one or more first conductive segments (102) organized in a first row,

one or more second conductive segments (103) organized in a second row, wherein said first and second segments are shaped as elongate bars,

the first and second segment(s) being placed besides each other,

the first and second segments being mutually staggered with electrical insulating material (118) joining the segments (102, 103) into a single junction rail, the anode conductors being connected with the first conductive segment(s) and the cathode conductors are connected with the second conductive segment(s).

2. An arrangement according to claim 1, wherein the photodiodes are covered by a sun facing transparent plate (1003), the plate (1003) resting on said first and second segments (102, 103).

3. An arrangement according to claim 1, wherein the segments (102, 103) are mutually electrically insulated or two segments that are overlapped are connected with an anti-parallel connected diode/diodes or relay/relays (702), preferably MEMS relays and associated driver circuit.

4. An arrangement according to claim 1, wherein the segments are (102, 103) made of an alloy, which consists wholly or partly of copper or aluminum.

5. An arrangement according to claim 1, wherein the segments (102, 103) are made of aluminum or an aluminum alloy in which the electrical isolation of the segment has been fully or partially achieved by anodizing the segment.

6. An arrangement according to claims 1 - 2, wherein the segments are

rectangular in cross section and shaped as flat strips standing upright on its edge.

7. An arrangement according to claim 1, wherein electrical connections between the attached conductors on the photodiodes and the segments of the junction rail are performed by soldering, welding, ultrasonic welding or gluing.

8. An arrangement according to claim 1, wherein, when series connecting of photodiodes/solar cells, each segment has a length that is adapted to be able to daisy chain two photodiodes, alternatively when series connecting groups of parallel connected photodiodes/solar cells, each segment has a length that is adapted to daisy chain two groups of parallel connected photodiodes/solar cells.

9. An arrangement according to claim 1-8, wherein surfaces which faces the front side glass on the solar module, are dull and white.

10. An arrangement according to claim 1, wherein the segments include one or more folds (703).

11. An arrangement according to claim 1, wherein said rail includes one or more transverse support(s) (1001, 1002) to ensure that the junction rail is standing upright on its edge on the solar module.

12. An arrangement according to claim 11, wherein the surface of said transverse support (1001, 1002) facing the transparent plate on the solar module is dull and white.

13. An arrangement according to claim 1, wherein each segment consists of a metal strip (701) of the same thickness all along its full length.

14. An arrangement according to claim 1, wherein each segment consists of a shaped plate that is thinner towards the segment ends.

15. An arrangement according to claim 2, wherein ends of the junction rail rest against a frame (1201) that surrounds the solar module forming a gap between the plate (1003) and frame (1201).