WO2014037721A1

WO2014037721A1 - Concentrated photovoltaic (cpv) cell arrangement, module and method of fabrication

Info

Publication number: WO2014037721A1
Application number: PCT/GB2013/052320
Authority: WO
Inventors: Geoffrey Duggan
Original assignee: FULLSUN PHOTOVOLTAICS Ltd
Current assignee: FULLSUN PHOTOVOLTAICS Ltd
Priority date: 2012-09-05
Filing date: 2013-09-05
Publication date: 2014-03-13
Anticipated expiration: 2015-03-05
Also published as: GB201215834D0

Description

Concentrated Photovoltaic (CPV) cell arrangement, Module and

Method of Fabrication

The present invention relates to backplane architecture for a concentrated photovoltaic module. In particular, the architecture permits rapid assembly of a photovoltaic receiver package onto a backplane using high speed pick and place (PnP) tools.

The generation of electricity from clean, carbon-emission free sources at a cost which is competitive with traditional fossil fuel sources is needed to mitigate both the Earth's dwindling resources of fossil fuels and the threats posed by the effects of continued climate change.

Solar photovoltaic (PV) technology, the generation of electricity from sunlight, has the potential to be a major contributor to the solution of these problems. At the present time there are many variants of PV technology all vying to try and meet the target of being able to generate electricity in an economic and reliable manner. Such variants include technologies based on thin films such as CdTe and alloys of Copper Indium Gallium and Selenium (CIGS), thin film amorphous Silicon, thick films of crystalline and

multicrystalline silicon, organic materials like dyes and finally inorganic III- V, semiconductor materials.

PV cells made from III-V materials show the highest efficiency for the conversion of sunlight to electricity. By making so-called multijunction cells of different III-V materials grown on a Germanium substrate a world record cell conversion efficiency of >40% has been achieved. Concentrated PV (CPV) is a variation of solar PV technology which seeks to replace the large areas of semiconductor or thin-film materials by small areas of the more expensive, highly efficient III-V cells in combination with cheap plastic or glass Fresnel lenses or mirrors that concentrate the Sun's intensity several hundred times onto the cell . CPV modules or panels are mounted on a tracking system that keeps the focus of the Sun on the cell as it moves through the sky during the day.

A complete CPV system can be broken down into a number of sub-units:

• III-V PV cells, receivers, lenses or mirrors which are assembled into

modules - to concentrate the sun's rays and generate the direct current (DC)

• Dual axis tracker and control system - to accurately follow the sun and to maximize the energy harvested

• Inverters - to convert the DC to an alternating current (AC) and feed the current into the grid

CPV modules are constructed either using mirrors - a reflective system - or lenses - a refractive system. The main elements and technical challenges that need to be addressed in designing and constructing a CPV module are illustrated by reference to point focus reflective and refractive systems shown in Figure 1.

In the refractive system, light from the sun is focused using a Fresnel lens (5) onto a PV cell (4) that would usually be mounted onto a small printed circuit board (PCB) which acts as heatsink. In the example shown here the light first passes through a secondary optical element (SOE)(3) which acts to spread out the intensity of the focused light into a more uniform spot. The SOE also increases the acceptance angle of light that can be captured and focused onto the cell.

The reflective system shown above uses a primary concave mirror (1) and a convex secondary mirror (2), with the light from the sun being focused through an optical rod (3) onto the solar cell (4), again mounted onto a suitable heatsink. This optical rod performs the same function as the SOE in the refractive system . The PV cell together with a silicon bypass or protection diode, PCB heatsink and SOE form a, so called, receiver (Rx) unit. The Rx is the power generator of the CPV system so its design, performance and reliability are one of the keys to building an efficient and economic system . Most manufacturers of CPV systems today use a SOE in their Rx design.

The other key design consideration for the Rx is thermal management. Even though the PV cells are close to 40% efficient it still means that 60% of the incident radiation is converted to heat and has to be dissipated as efficiently as possible without raising the cell temperature excessively, since any rise in temperature is accompanied by a drop in conversion efficiency. Commonly used PCB substrates are aluminium based, as used in the high brightness LED industry, and are known as either IMS (insulated metal substrate) or MCPCB (metal core PCB). DBC (direct bonded copper) substrates, commonly used in power electronics applications, are also used by several CPV players.

A CPV module consists of an array of series connected Rx which have been mounted on the base of the module at the focus of either a parquet of Fresnel lenses or suitable reflective optics. Each module has to be mounted and accurately aligned on the dual axis sun tracker.

Today's CPV technology aims to concentrate the sun anywhere between 300x and lOOOx. The geometric concentration factor is defined as the ratio between the area of the collection (Fresnel) lens and the illuminated area of the PV cell . Generally, these systems are designed around the use of PV cells that are, for example, 10mm x 10mm or 5.5mm x 5.5mm . This implies that for example, a 625x system has to have lenses that are larger than the cell by 25x in each planar direction. With such large lenses it is difficult to focus the sun's rays efficiently without the focal length of the lens being long; typically the z-dimension would be 1.5x to 3x the x-y dimension of the lens. All of the different CPV system developers and suppliers have their own design of module but almost all have the same basic feature; a deep sided metal box with Rx mounted in a series connected matrix on a metal or glass baseplate with a lens parquet (array of lenses) aligned to the positions of the Rx below. Because the module has deep sides this has implications for the amount of material than needs to be used with attendant implications for the cost of construction and the design of the tracker that needs to hold the modules rigidly and aligned, especially in windy conditions where the deep design offers more wind resistance.

This invention seeks to address the shortcomings of the current CPV module designs by:

a. Simplifying the receiver design so that the receiver (Rx) can be realized from a PV cell and optionally a secondary optical element (SOE) that form part of a package that can be surface mounted onto a common printed circuit board (PCB) backplane; b. Utilizing small PV cells means that Fresnel lenses with a short focal length can be used to focus light onto individual Rx; thus ensuring that the CPV module has a "low profile" which minimizes the use of materials and provides a beneficial profile for wind resistance;

c. Utilizing the PCB as the baseplate of the CPV module onto which interconnecting circuitry can be patterned directly, thus removing the need for subsequent interconnection soldering or welding that would add cost and potentially reduce the reliability of the module; It also removes the need for any additional external heatsinking

d. Utilizing the PCB nature of the baseplate/backplane to allow additional functionality of the module allowing

i. On-board monitoring and diagnostics of module

performance,

ii. In-module conversion of the direct-current (DC)

generated by the PV cell to alternating-current (AC) through the provision of micro-inverting electronics

mounted directly onto the PCB.

According to a first aspect of the present invention, there is provided a Concentrated Photovoltaic (CPV) cell arrangement comprising an array of photovoltaic cells each connected to a patterned conductive backplane electrically connecting the array of photovoltaic cells together, the patterned conductive backplane comprising a heat sink layer, insulating layer, a conductive layer and a resist layer such that the photovoltaic cell is connected to the conductive layer through exposed regions in the resist layer.

Preferably, a photovoltaic cell is mounted on a conductive base and wherein the conductive base is connected to the conductive layer of the patterned conductive backplane.

Preferably, the conductive layer is a patterned metal interconnect layer.

Preferably, the conductive base of a photovoltaic cell is soldered or epoxy bonded to exposed regions on the patterned metal interconnect layer. More preferably, the resist layer is deposited over the conductive layer. In this way, the resist is exposed to define pads for soldering the conductive base of a photovoltaic cell to exposed regions on the patterned metal

interconnect layer.

The arrangement of the patterned conductive backplane is preferred as having the heat sink layer affixed to the back of the insulating layer. To enable efficient thermal management the heat sink layer may feature additional metallic fins. A further preferred arrangement is that the insulating layer comprises a dielectric over the heat sink layer and under the pattered metal interconnect layer. A further preferred arrangement includes a secondary optical element mounted over the photovoltaic cell, the secondary optical element being a moulded lens containing an optically transparent lens material disposed over the photovoltaic cell. A preferred lens is an aspheric lens with optionally the optically transparent lens material being silicone.

Alternatively the lens may be glass.

In a refractive system, the array of photovoltaic cells are arranged to receive light via a primary optical element, eg., a Fresnel lens.

According to a second aspect of the present invention, a method of forming a photovoltaic cell arrangement comprising picking a photovoltaic cell and placing the photovoltaic cell on a patterned conductive backplane electrically connecting an array of photovoltaic cells together, the patterned conductive backplane comprising a heat sink layer, insulating layer, a conductive layer and a resist layer such that the photovoltaic cell is connected to the conductive layer through exposed regions in the resist layer.

Preferably, the method includes soldering or epoxy bonding the photovoltaic cell to the exposed regions in the resist layer and melting the solder or epoxy to ensure electrical contact with the conductive layer.

Embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings of which :

Figure 1 represents schematic illustrations of point-focus reflective and refractive CPV optical systems;

Figure 2a is cross section of an overmoulded photovoltaic leadless chip package, also known as a receiver (Rx);

Figure 2b illustrates the way in which the PV cell is connected to the Cu leadframe portion of the Rx; Figure 3 is a schematic diagram of how the Rx are soldered to the IMS baseplate;

Figure 4 is a schematic diagram showing the functionality of the Fresnel lens parquet and its relation to the Rx arranged on the IMS baseplate;

Figures 5a-c are schematic diagrams in cross section of the CPV module showing the lens support and details showing the way in which one embodiment of the invention seals the module against water and dust ingress;

Figures 6a-b are schematic diagrams of a circuit layout that connects the PV cell and reverse biased bypass diode silicon diode in parallel on the IMS backplane and an example of how an array of Rx and bypass diodes could be arranged in parallel strings on the IMS backplane. The solder resist layer has been omitted for clarity;

Figure 7 is a schematic diagram of Rx and bypass diodes together with an IC chip that is capable of performing local maximum power point tracking; and

Figure 8 is a schematic diagram of Rx and bypass diodes together with an IC chip that is capable of performing local DC to AC conversion. Again, the solder resist layer has been omitted in both these diagrams.

The present invention will now be described by reference to the drawings which describe preferred embodiments of the invention.

The preferred CPV module is made from the following component parts

1. A receiver having :

a. A PV cell - preferably a high efficiency, III-V multi-junction cell that has an optical aperture of ~ 1.5mm x 1.5mm; b. A leadless chip package in which the PV cell is die attached to a metallic leadframe by a suitable electrically conductive solder or epoxy;

c. An aspheric silicone lens, a secondary optical element (SOE) that is capable of focusing light from an upper primary optical element (POE), usually a Fresnel lens, onto the optically active area of the PV cell. The SOE is preferably produced by overmoulding the silicone lens onto the PV cell and leadframe or chip carrier which results in a simplified packaging process that simultaneously protects the PV cell and that is small enough to be assembled onto the CPV module's backplane using high speed pick and place (PnP) tools readily available from the optoelectronics and in particular the high brightness LED industry.

2. A metal cored printed circuit board (MCPCB) that forms the baseplate of the CPV module and which can be patterned with electronic circuitry that allows series and parallel connections of receivers and bypass, protection diodes.

a. Receivers are placed into position on the PCB and subsequently re-flow soldered to allow electrical connections and final positioning.

3. A matrix of Fresnel lenses, often referred to as a parquet, that is positioned at a predetermined vertical position relative to the PCB backplane

4. A metallic "spacer" or module sidewall that holds the Fresnel lens parquet and PCB backplane at the appropriate distance apart from one another and when held in place with an appropriate sealant and adhesive material, most likely a silicone, ensures that the module is sealed against moisture and dust particle ingress.

Figure 2a illustrates a cross section through a leadless chip package 200 into which has been attached a multi-junction photovoltaic (PV) cell (4). Following the curing of the solder that attaches the PV cell to the metallic leadframe (8), the PV cell (4) has wire bonds (7) attached to its upper surface to complete the electrical circuit and the open package is then placed into an appropriate mould, the inner surface of which has been highly polished in the shape of an aspheric lens (6) to focus incident light onto the optical aperture of the PV cell. The mould is then filled with an optically clear silicone, for example, which once cured completely

encapsulates and protects the PV cell .

Figure 2b shows how the PV cell is mounted and bonded to the two sections of the Cu pads that are an integral part of the package. The package is made rigid by the simultaneous moulding of sides and base that separates the two Cu connector pads. In one embodiment the entire package is made from a material able to withstand high temperatures whilst also being resistant to damage by intense levels of UV, candidate materials would be, but not limited to, a silicone. In another embodiment the sides of the package would be first moulded in a material able to withstand high temperatures and intense levels of UV, such as a liquid crystal polymer (LCP) or even a metal, Finally, in this version, the transparent silicone lens would be overmoulded, simultaneously filling the open cavity, encasing the PV cell and forming the lens to focus the light from the primary optical element - the Fresnel lens or similar.

Figure 3 shows a cross section of two moulded packages that have been soldered 9 to pads 11 that form part of the electronic circuitry that has been patterned onto a MCPCB 10 or IMS. The Rx packages 200 are picked and placed accurately and at high speed onto exposed features on the PCB (10) that are defined by openings in the solder resist layer 12. The whole PCB would then be placed into a standard re-flow oven to melt the solder to ensure electrical contact with the circuitry on the PCB and hold the PV cells accurately in place.

The specific embodiment described above with reference to Figure 3 illustrates two moulded packages soldered 9 to pads 11 following the curing of the solder that attaches the PV cell to a metallic leadframe (8) as described with reference to Figure 2. In Figure 2, the PV cell (4) has wire bonds (7) attached to its upper surface to complete the electrical circuit and the open package is then placed into an appropriate mould, the inner surface of which has been highly polished in the shape of an aspheric lens (6) to focus incident light onto the optical aperture of the PV cell .

In an alternative embodiment, the PV cell 4 can be mounted directly upon pads 11 without the base of a PV cell being first soldered to a metallic leadframe 8. In such an embodiment, the PV cell is soldered to one pad 11 and is wire bonded from its upper surface to an adjacent pad to complete the electrical circuit.

Figure 4 shows us in cross section how the array of receivers are positioned relative to a parquet 14 of Fresnel lenses whose centres are positioned such that parallel light from the Sun's rays are focussed to a position either at the apex of the silicone lens or just inside its body. The lens parquet could be made from PMMA for example or of a thin layer of silicone that is laminated to a suitable glass - a so-called Silicone-on-glass (SOG) lens.

Figure 5a is a cross section of the assembled module where the lens parquet 14 is held in place by a support structure 16. The support structure would be typically aluminium; it could however be any material that is rigid enough to support the lens and will not deteriorate over more than twenty years exposure of extremes of temperature a prolonged exposure to UV. It should also be able to withstand high concentrations of sunlight should the tracker malfunction and an "off axis" beam be directed toward the support structure.

Figures 5b and 5c show detail of how the support structure is attached to the base of the module and the lens parquet. In this example a thin, continuous bead of an appropriate silicone adhesive (15) is sandwiched between the parts to be joined and then cured once in place. We have shown an example using a bead of silicone sealant but other materials such as 3M VHB tape (or equivalent) might also be used.

Figure 6a shows an example of how the Rx 200 and silicon bypass diode 19 can be connected in parallel on the IMS baseplate with the Rx 200 and bypass diodes reflow soldered to pads 18, 20 opened up on the IMS, while Figure 6b shows one example of the way in which the Rx 200 and silicon bypass diodes are arranged on the IMS baseplate. In this particular embodiment we show two strings of Rx 200 that are connected in parallel on the IMS baseplate with each of the individual Rx 200 being sited below the centre of a Fresnel lens of the primary optical element.

The Rx 200 are arranged in parallel strings so as to optimise the voltage and current being delivered from each individual module to either a central inverter or a micro-inverter that is sited within the module.

Figure 7 shows an example of how the Rx 200 can be connected on the PCB together with an integrated circuit or set of microchips (21) that can locally optimise the maximum power point tracking (MPPT) of each individual module on a distributed array which will significantly increase the overall power generated compared to an array with just a central inverter.

Figure 8 shows an example of how the Rx 200 can be easily connected to an integrated circuit or chip set (22) that will allow each individual module to convert the local DC to AC without the need of a central, expensive and potentially unreliable inverter.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto.

Claims

1. A Concentrated Photovoltaic (CPV) cell arrangement comprising an array of photovoltaic cells each mounted on a patterned conductive backplane electrically connecting the array of photovoltaic cells together, the patterned conductive backplane comprising a heat sink layer, insulating layer, a conductive layer and a resist layer such that the photovoltaic cell is connected to the conductive layer through exposed regions in the resist layer.

2. A CPV cell as claimed in claim 1, wherein a photovoltaic cell is mounted on a conductive base and wherein the conductive base is connected to the conductive layer of the patterned conductive backplane.

3. A CPV cell arrangement as claimed in claim 1 or 2, wherein the conductive layer is a patterned metal interconnect layer.

4. A CPV cell arrangement as claimed in claim 2, wherein the conductive base of a photovoltaic cell is soldered to exposed regions on the patterned metal interconnect layer.

5. A CPV cell arrangement as claimed in any preceding claim, wherein the resist layer is deposited over the conductive layer.

6. A CPV cell arrangement as claimed in any preceding claim, wherein the heat sink layer is affixed to the back of the insulating layer.

7. A CPV cell arrangement as claimed in claim 6, wherein the heat sink layer incorporates metallic fins.

8. A CPV cell arrangement as claimed in any preceding claim, wherein the insulating layer comprises a dielectric.

9. A CPV cell arrangement as claimed in any preceding claim, wherein a secondary optical element is mounted upon the photovoltaic cell, the secondary optical element being a moulded lens containing an optically transparent lens material .

10. A CPV cell arrangement as claimed in claim 9, wherein the secondary optical element is an aspheric lens.

11. A CPV cell arrangement as claimed in claim 9 or 10, wherein the optically transparent lens material is silicone.

12. A CPV module comprising a CPV cell arrangement as claimed in any preceding claim, wherein the array of photovoltaic cells are arranged to receive light via a primary optical element, eg., a Fresnel lens.

13. A method of forming a photovoltaic cell arrangement comprising picking a photovoltaic cell and placing the the photovoltaic cell on a patterned conductive backplane electrically connecting an array of photovoltaic cells together, the patterned conductive backplane comprising a heat sink layer, insulating layer, a conductive layer and a resist layer such that the photovoltaic cell is connected to the conductive layer through exposed regions in the resist layer.

14. A method as claimed in claim 13, including soldering the photovoltaic cell to the exposed regions in the resist layer and melting the solder to ensure electrical contact with the conductive layer.

15. A concentrated photovoltaic receiver substantially as hereinbefore described and/or with reference to Figures 2 to 8 of the accompanying drawings.

16. A method of forming a photovoltaic receiver substantially as hereinbefore described and/or with reference to Figures 2 to 8 of the accompanying drawings.