The paper presents a novel densely packed assembly for high concentrating photovoltaic applications, designed to fit 125x primary and 4x secondary reflective optics. This assembly can accommodate 144 multijunction cells and is one of the most populated modules presented so far. Based on the thermal simulation results, an aluminum-based insulated metal substrate has been used as baseplate; this technology is commonly exploited for Light Emitting Diode applications, due to its optimal thermal management. The original outline of the conductive copper layer has been developed to minimize Joule losses by reducing the number of interconnections among the cells in series. Oversized Schottky diodes have been employed for bypassing purposes. The whole design fits the IPC-2221 requirements. The plate has been manufactured using standard electronic processes and then characterized through an indoor test and the results are here presented and commented on. The assembly achieves a fill factor above 80% and an efficiency of 29.4% at 500x, less than 2% lower than that of a single cell commercial receiver. The novel design of the conductive pattern is conceived to decrease the power losses and the deployment of an insulated metal substrate represents an improvement towards the awaited cost-cutting for high concentrating photovoltaic technologies.
The basic idea behind the Concentrating Photovoltaics (CPV) is to reduce the cost of photovoltaic plants by replacing some of the expensive semiconductor material with a cheaper reflective or refractive material (such as a mirror or a lens) [
The IEEE defines a CPV receiver as “an assembly of one or more PV cells that accepts concentrated sunlight and incorporate the means for thermal and electric energy removal” [
Despite the fact that a number of HCPV modules have been presented in the literature [
This work is carried out as part of the BioCPV project [
Concentrator specifications.
Primary concentrator | Secondary concentrator | ||
---|---|---|---|
Geometric concentration ratio | 125x | Geometric concentration ratio | 4x |
Aperture area | 3 m |
Cell side aperture area | 10 mm |
Rim angle | 20° | Acceptance angle | 30° |
Focal length ( |
3.37 m | Length of CPC | 25 mm |
|
0.794 | Length of homogenizer | 10 mm |
Schematic of the system configuration and particular of the receiver.
The 125x primary concentrator is a parabolic dish with square opening and is made up of four sections to achieve an entry aperture area of 9 m2. The parabola has a focal length of 3.37 m and is truncated considering a rim angle of 20°. The 3 m × 3 m primary reflector focuses the light onto a 26.8 cm × 26.8 cm surface of the receiver. The secondary concentrator is made of 144 three-dimensional compound parabolic concentrators (CPCs) with a square 2 cm × 2 cm entrance aperture and a square 1 cm × 1 cm exit aperture. The CPCs are arranged in a 12 × 12 array and each CPC reflects the light on a single solar cell. A 10 mm length homogenizer is placed at the exit of each CPC in order to uniform the irradiation on the cells. In Figure
Cross-sectional view of the secondary optics.
Each plate is equipped with 144 cells and is rated at 2.6
A set of 3C40 cells, provided by AZUR SPACE [
Bypass diodes are essential in any PV applications to reduce the power losses from a series when at least one cell is shadowed and, at the same time, to prevent damages to the shaded cell itself [
In concentrating photovoltaic systems, it has been demonstrated that the installation of one bypass diode per cell maximizes the performances [
Comparison of the safety factors used by HCPV industries and in the developed system.
Company | Cell dimensions |
Max CR |
Max cell current |
Number of diodes per cell | Maximum forward current per diode |
Safety factor |
---|---|---|---|---|---|---|
AZUR SPACE [ |
|
1000 | 13.088 | 2 | 10 | 1.53 |
Emcore [ |
|
1000 | 4.4 | 1 | 10 | 2.23 |
Spectrolab [ |
|
500 | 6.95 | 1 | 12 | 1.73 |
Ergonsolair [ |
|
500 | 2 | 1 | 10 | 5.00 |
|
500 | 4.5 | 1 | 10 | 2.22 | |
|
500 | 7 | 1 | 12 | 1.71 | |
University of Exeter |
|
500 | 6.587 | 1 | 10 | 1.52 |
Taking into account the cell’s short-circuit current in the presented scheme (6.587 A), a 10 A Schottky diode grants an acceptable safety factor of 1.52. The peak repetitive reverse voltage is always higher than the cell voltage, even when a conservative safety factor is applied: the cell voltage is less than the 75% of the peak repetitive reverse voltage. Using two 10 A diodes would have enhanced the safety factor, but there is no space available to allocate them. The diodes applied in the systems are surface-mounted technologies, since they are cheaper than the discrete ones because they do not require any predrilled holes on the board [
The HCPV receivers are designed to maximize the extraction of electrical energy, to enhance the transfer of thermal energy, and to assure an adequate mechanical support. The choice of the geometry and the selection of the materials depend on many factors, such as the concentration and the cost, as well as the thermal management. In this case, the thermal behaviour of the substrate is taken into account: all the incoming energy that is not converted by the cells becomes heat and contribute to increasing the cell temperature. The PV cells are negatively affected by the increase in temperature, which cause drops in electrical efficiency and can lead to mechanical failures.
In this application, an active cooling is developed to dissipate the waste heat produced by the cell: the cell assembly is expected to be designed to optimize the heat transfer from the cell to the cooler. The most of the waste heat is removed from the cell by conduction [
The stationary pure conductive heat transfer equation is used to model the heat exchange between solids. The heat flux depends on the conductivity of the material
In the first approach, a single-cell geometry is reproduced in the software environment (Figure
Front view and cross section of the single-cell receiver developed in COMSOL.
The simulations are conducted to predict the steady-state thermal behaviour of the receiver. Three different substrates are considered: a printed circuit board (PCB), a direct bonded copper (DBC), and an insulated metal substrate (IMS). The thicknesses of the layers are established on the basis of the commercially available products or references and are reported in Table
Thicknesses and materials for the modelled substrates.
Layer | PCB | DBC | IMS |
---|---|---|---|
Interconnectors | 0.025 mm Ag | 0.025 mm Ag | 0.025 mm Ag |
Cell | 0.190 mm Ge | 0.190 mm Ge | 0.190 mm Ge |
Solder paste | 0.125 mm solder | 0.125 mm solder | 0.125 mm solder |
Conductive layer | 0.035 mm Cu | 0.30 mm Cu | 0.035 mm Cu |
Dielectric | 4.5 |
0.63 mm AlN | 4.5 |
Heat sink | 1.6 mm FR-4 | 0.30 mm Cu | 1.6 mm Al |
Reference | [ |
[ |
[ |
The COMSOL’s “Heat transfer” module, used in this simulation, requires three proprieties for each material: the thermal conductivity, the density, and the heat capacity at constant pressure. Wherever available, the COMSOL built-in materials are used, such as copper and aluminum. In other cases, the values are set according to external references (Table
Proprieties of materials (materials marked with
Materials | Thermal conductivity [W/Km] | Density [kg/m3] | Heat Capacity [J/kgK] |
---|---|---|---|
Aluminum nitride | 285 | 3260 | 740 |
Aluminum |
160 | 2700 | 900 |
Copper |
400 | 8700 | 385 |
FR-4 | 1.7 | 1850 | 600 |
Germanium | 60 | 5323 | 320 |
The solder pastes used in all the substrates and the marble resin, which acts as a dielectric in the considered PCBs and IMSs, are modelled as thin thermally resistive layers: for this function, COMSOL requires in input the thickness and the thermal conductivity only (Table
Conductivity and thickness of the thermally resistive layers.
Materials | Thermal conductivity [W/Km] | Thickness [mm] |
---|---|---|
Marble resin | 3.0 | 0.0045 |
Solder paste | 4.5 | 0.1250 |
No Joule heating is considered: as proved in [
According to Royne et al. [
The following simulations are conducted taking into account the Concentrator Standard Test Conditions (CSTCs) [
The minimum resistance per mass unit is then given as follows [
This second investigation confirmed that DBC and IMS behaved similarly in terms of heat removal, even in presence of a less performing cooler. The DBC achieved a maximum cell temperature of 75.6°C (Figure
Temperature distribution (a) and isothermal contours (b) in the PCB based assembly, in °C. Max cell’s temperature: 204.7°C.
Temperature distribution (a) and isothermal contours (b) in the DBC based assembly, in °C. Max cell’s temperature: 75.6°C.
Temperature distribution (a) and isothermal contours (b) in the IMS based assembly, in °C. Max cell’s temperature: 73.8°C.
Similarly, a full scale simulation of the IMS-based receiver is conducted. A surface of 21 mm × 21 mm is left around of each cell. Each secondary concentrator requires a 20 mm × 20 mm entry aperture to achieve a 4x concentration on a 10 mm × 10 mm cell. Moreover, the thickness of the optics’ walls (1 mm) needs to be considered, leading the total area to 21 mm × 21 mm. In addition, a minimum of 2 mm tolerance is required on each side of the substrate and, on one of the sides, 8 mm is added to allocate the terminal tabs used for current extraction. Taking into account these values, an aluminum board sized 255.0 mm × 262.5 mm is designed.
The simulation predicts the thermal response of the assembly and proves the ability to remove the waste heat even in a densely packed configuration. The results are shown in Figures
Front view (a) and lateral view (b) of the temperature distribution of the full scaled board in °C. Max. cell’s temperature: 76.5°C. Min. cell’s temperature: 62.5°C.
Front view (a) and lateral view (b) of the isothermal contours in the assembly of the full scaled board in °C.
A maximum difference of temperature of 14°C is registered among the cells installed in the assembly: the minimum temperature, achieved by the cells on the edge, is due to the 1 cm room left on one side of the board to allocate the tabs for current extraction. Unfortunately, it is not possible to reduce that space and, on the other hand, adding the same room in the other edges will increase the temperature gradient, without any positive effect on the system’s performance.
In order to predict the behaviour of the system under a wider range of conditions, the system is then tested under the worst case conditions, when all the concentrated sunlight is converted into heat. In this case, the cell’s efficiency is considered to fall to 0%: the heat production then rises to 42.5 W/cm2 and a maximum cell’s temperature of 150°C can be allowed [
Front view (a) and lateral view (b) of the temperature distribution of the full scaled board in the worst case conditions in °C. Max. cell’s temperature: 115.6°C. Min. cell’s temperature: 91.9°C.
Front view (a) and lateral view (b) of the isothermal contours of the full scaled board in the worst case conditions in °C.
The design of the electrical circuit is developed to allocate all the components, to have a high efficiency, and to be easy to realize. In order to enhance the performances, the same copper plate is used to directly connect the negative pad of the cell with the positive one of the following cell. This way, the number of connections is reduced, limiting the contact resistances. To facilitate the manufacturing, the whole copper pattern is designed to be made of only few shapes, periodically repeated in the space to obtain the final drawing.
The pattern is realized taking into account the requirements and the restrictions of the optical geometries and the recommendations of the standards, which are listed in the next section. In the designing stage, AutoCAD is used to check the matching between the receiver’s geometry and the optics systems restrictions. A symbolic representation of the HCPV key components is shown in Figure
Key of the components schematics: (a) diode, (b) bare cell, and (c) cell with front interconnectors.
The design is drawn up according to the IPC-2221 Generic Standards on Printed Board Design [
Minimum width (in mm) of the 70
Current versus temperature increase | 3°C | 4°C | 5°C | 15°C | 25°C | 35°C | 45°C | 55°C | 65°C |
---|---|---|---|---|---|---|---|---|---|
6.5870 A | 4.26 | 3.58 | 3.12 | 1.60 | 1.18 | 0.96 | 0.82 | 0.73 | 0.66 |
3.2935 A | 1.64 | 1.37 | 1.20 | 0.62 | 0.45 | 0.37 | 0.32 | 0.28 | 0.25 |
Current distribution across the C2 copper shape. The dimensions are in mm.
Across the plate, adjacent copper shapes face various voltages while in operation. In conditions of open circuit at 500x, the negative pads of two consecutive cells face a difference up to 3.17 V. The ends of two consecutive rows meet a maximum difference of 76.08 V. The largest voltage difference is the one registered between the last pad of one series and the first one of the other one; there, the shapes face a maximum difference of 228.24 V. The standards strike out clearly the required electrical clearance between DC external coated conductors: 0.13 mm for any voltages lower than 100 V and 0.40 mm for voltages up to 300 V.
The copper pattern is built to allocate two series of 72 cells each: each series is expected to produce 6.440 A at about 208 V at the maximum power point. A 4.5
In the designing stage AutoCAD is used in order to check the matching between the receiver’s geometry and the optics systems restrictions. The main challenge is to fit all the components in the available space. A 4x secondary system is placed above the plate: this means that the surface available to allocate the cell, the diode, the interconnectors, and the conductive layers, inclusive of the clearances, is four times larger than the cell’s active area (10 mm × 10 mm). The edges of the optics structure have a thickness of a 1 mm; the available surface rises then up to 21 mm × 21 mm and it is marked by the red square in Figure
Components distribution across the 21 mm × 21 mm surface, delimited by the red square.
The geometry, shown in Figure
A four-cell V3.1 pattern. In (a), cells and diodes are reported with dimensions in mm. In (b) the copper shapes employed for the pattern are highlighted.
The four shapes of the V3.0: C1 recurs twice, C2 recurs 132 times, C3 recurs twice, and C4 recurs 10 times in the 144-cell design. Dimensions are in mm.
The large, 144-cell conductive layer is composed of 146 copper elements, opportunely etched on the substrate, as shown in Figure
Complete assembly: the dimensions of the aluminum board are shown.
In any electronic application, interconnectors represent one of the weakest points, because of the high electrical contact resistance and the fragility of the bonding. The original design of the copper pattern designed in this work has only one set of interconnections between adjacent cells: one shape of copper is used both as landing surface for the interconnectors coming from the negative pole of the cell and as mounting pad for the positive pole of the adjacent cell. This approach allows lowering the electrical resistance of the circuit and, thus, the electrical losses.
A second feature of the design is the scalability: it can be easily adapted to allocate a different number of cells. Opportunely combining the copper shapes, it is possible to create less or more populated arrays of aligned cells. In the present work, the design has been used to produce several single-cell receivers, a 16-cell receiver [
An investigation about the effects of the thermal expansion is sorted out, in order to prevent problems due to the high temperatures involved. The plate is designed and manufactured at a room temperature of 25°C. In case failures, the system can face temperature up to 150°C. The maximum thermal expansion is then calculated between these two extreme temperatures: a maximum difference of temperature (
Copper has a coefficient of thermal expansion (
Wire bonding is used to interconnect the front of the cell with the conductive layer. It is a standard process in electronics and is considered to be extremely reliable after the introduction of automatic wire bonding, low temperature bonding processes, and effective pad cleaning methods [
The approach suggested by Shah [
The electrical resistance depends on the electrical resistivity of the material (
In the fabrication of the cell receiver, 70 wires per cell are bonded, applying then a safety factor of about 1.4. This factor can be judged as low, but it is safe enough if the already conservative approach used for sizing is considered. Anyway, the surplus of wires is installed to overtake two issues that can occur during manufacturing and in operation: wire bond nonsticks and nonuniform current generation. Some contamination on the dies from the soldering process causes some wire bond nonsticks, like those shown in Figure
A completely bonded cell’s tab (a) and a cell’s tab with two missing wires (b).
The plate is designed to work at a peak power of 2.678
Power losses breakdown in the conductive pattern.
Component code | Geometry | Power losses per piece [W] | Number of repetition in 144-cell design | Total power losses [W] |
---|---|---|---|---|
C1 |
|
0.013 | 2 | 0.026 |
|
||||
C2 |
|
0.077 | 132 | 10.202 |
|
||||
C3 |
|
0.014 | 10 | 0.140 |
|
||||
C4 |
|
0.067 | 2 | 0.134 |
70 aluminum wires are installed on each cell, to transfer a current of 6.440 A at the maximum power point. Taking into account an electrical resistivity of
The 262.5 mm × 255.0 mm IMS plate is developed to accommodate 144 cells and to fit the 4x secondary concentrators as described earlier (Figure
The cell assembly populated with cells (CC), interconnectors (IC), diodes (D), and terminal tabs.
The plate is tested in a WACOM
A short-circuit current of 11.6 mA is measured and open circuit voltages of 181 V and 180 V are recorded for the two cell’s series on the assembly. The discrepancy in the voltage outputs might be due to the hand-placement of the components and to the solder paste contamination found during the wire bonding. The fill factor ranges between 80.3% and 80.9%. The high values of fill factor prove a low series resistance in the board, whereas the shape of the
The measured outputs, shown in Table
Electrical outputs per cell of the two series of the produced cell assembly, compared with those of the commercial 3C40A assembly, under AM1.5, 1000 W/m2, at 28°C.
Assembly | Number of cells |
|
|
|
|
|
F.F. |
---|---|---|---|---|---|---|---|
Series A | 72 | 11.57 | 2.50 | 23.58 | 10.31 | 2.29 | 0.814 |
Series B | 72 | 11.59 | 2.51 | 23.38 | 10.32 | 2.26 | 0.801 |
3C40A | 1 | 12.11 | 2.58 | 25.64 | 11.00 | 2.33 | 0.820 |
Comparison of the
For a better prediction, the measured values are refined to simulate a full scale characterization. In the following numerical investigation, the values of the series A are considered: an average open circuit voltage of 2.50 V per cell is generated. The equations reported in [
A minimum short-circuit current of 5.77 A and an average open circuit voltage per cell of 3.08 V are predicted at 500x, under CSTCs (Table
Refined electrical outputs of the two series of the produced cell assembly, compared with those of the commercial 3C40A assembly for a concentration of 500x under AM1.5, 1000 W/m2, at 25°C.
|
|
|
---|---|---|
Series A | 5.77 | 3.08 |
Series B | 5.78 | 3.09 |
3C40A | 6.04 | 3.16 |
In a similar way, the maximum power point values can be predicted. At 500x, each cell is expected to work at a maximum power point power of 14.7 W, achieving, under 1000 W/m2 DNI, an efficiency of 29.4%. The commercial assembly instead reaches an efficiency of 31.9%. The difference between the two efficiencies might be due to the dimensions of the tested boards.
The cell’s datasheet reports a peak efficiency of 37.2% at 500x, under standard test conditions. The characterization has been conducted at one sun, instead of at full 500x scale: the cells are designed to work at high concentrations, so they are expected to differently behave at one sun [
In a real full-scale scenario, a combination of optical, mismatch, and Ohmic losses, along with the impacts of the temperature and the spectra, can occur and negatively affect the performances of the system [
A new, densely packed assembly for 500x HCPV applications has been developed on an insulated metal substrate. For the first time, the design of a large, densely packed HCPV receiver has been detailed in a scientific paper. This assembly represents a novelty for the unique low-resistance design of the conductive layers. The application of IMS can represent a step ahead towards the awaited cost-cutting for HCPV. The receiver is designed to accommodate 144 cells and to work at a power output of 2.6
Area
Cross-sectional area of the wire (m2)
Specific heat capacity (J/K)
Diameter of the wire (m)
Focal length of the primary concentrator (m)
Heat transfer coefficient W/(m2K)
Current (A)
Thermal conductivity (W/mK)
Boltzmann constant (J/K)
Maximum geometric dimension (mm)
Length of the wire (m)
Ideality factor
Number of cells
Number of wires per cell
Conduction heat flux (W/m2)
Elementary charge
Heat generated by the Joule losses (W)
Heat removed through conduction (W)
Volumetric heat source (W/m3)
Electrical resistance (Ω)
Thermal resistance per unit of surface (m2K/W)
Series resistance (Ω)
Temperature (°C)
Thickness of the layer (m)
Voltage
Volume of the cell (m3)
Concentrating Ratio.
Coefficient of thermal expansion (ppm/°C)
Deformation due to thermal expansion (mm)
Difference between the maximum operating temperature and the reference temperature of the copper layer (°C)
Difference of temperature between the two ends of the wire (°C)
Electrical resistivity (Ωm)
Density (kg/m3).
Ambient
Downside surface of the layer
External environment
Surface
Thin thermally resistive layer
Upside surface of the layer
Wire.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was financially supported by EPSRC-DST funded BioCPV project (EP/J000345/1). The authors would like to thank Cubik Innovation Ltd. and Custom Interconnect Ltd. for the assistance during the fabrication.