MULTI RECEIVER CONCENTRATOR PHOTOVOLTAIC TESTING AT EXTREME CONCENTRATIONS

Theodore van Kessel1, Ayman Abduljabar2, Abdulaziz Alyahya2, Badr Alyousef2, Alhassan Badahdah2, Hussam Khonkar2, Peter Kirchner1, Yves Martin1, Dennis Manzer2, Naim Moumen1, Aparna Prabhakar1,

Thomas Picunko1, Robert Sandstrom1, Yaseen Al-Saaedi2, Brent Wacaser1 and Supratik Guha1 1IBM T.J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, N.Y., 10598

2King Abdulaziz City for Science and Technology (KACST) P.O. Box 6086 Riyadh 11442 Saudi Arabia

ABSTRACT

Practical multi receiver concentrator photovoltaic systems operating at high solar concentration levels up to 2000 suns experience large radiation, thermal and electrical loads in addition to large power density transients under routine operation. These systems require efficient cooling to manage the associated incident power densities between 100 to 200 W/cm2. Photovoltaic cells and thermal interface materials experience considerable stress under these load conditions. Their assembly is sensitive to contamination and process optimization. Efficient optical coupling of light at high concentration requires precise component alignment and tracking. We will discuss high power testing of single and multi receiver, high concentration systems comprising commercial triple junction cells, Fresnel optics, electric actuators, and cooled through a metal thermal interface using active and passive cooling methods.

INTRODUCTION The material presented here reflects an effort on our part to explore the operation and reliability of photovoltaic concentrators in multi receiver configurations in the 900 to 2000 sun concentration range. We have constructed multi receiver systems to explore optics, tracking systems, passive cooling and inverter coupling. We are particularly interested in the reliability of the cells and the quality of power generated in high concentration multi receiver configurations. We have employed Fresnel optics in conjunction with light cup and light pipe secondary optics configurations to achieve concentration in multiple prototype embodiments. At an operating incident power density of 100 W/cm2, the cooling system must have overall performance on the order of 0.7 C/W or less to maintain the chip temperature at or below 85 C under 35 C ambient conditions. Practical multi cell systems benefit from passive cooling arrangements. We have constructed multi cell concentrator systems comprising passive cooling systems. High power semiconductor cooling packages are reliability tested by a variety of methods including power cycling, temperature and humidity, thermal aging, mechanical shock, vibration, and low temperature shock. Power cycling and temperature and humidity are particularly revealing tests for metal thermal interface failure. Full

qualification for 30 year operation may require 50000 to 100000 full power cycles. We have tested multiple packages to >10000 power cycles with good results. Optical components are tested with similar methods but with added emphasis on accelerated ultraviolet testing of adhesives and lenses at high flux.

MATERIAL DESCRIPTION

For our testing purposes, we have chosen the Spectrolab CDO-100 cell. The cell has a 1 cm2 active area with a nominal efficiency of 36% at 500 suns. We attach the cell to a carrier using a low melt PbSn solder. The carrier is subsequently interfaced to a passive cold plate using a liquid metal thermal interface comprising GaInSn. Effort is made to create and maintain a thin bond line for both the low melt solder and liquid metal materials in order to achieve the best thermal performance. Proprietary coatings are employed to assure compatibility of all the above materials. Cell packaging comprises an aluminum nitride ceramic. Secondary optics, cell package, thermal interface and electrical contacts are assembled into field re-workable receiver. Fresnel optics from multiple sources have been employed in combination with secondary optics comprising reflective light cups and refractive light pipes to achieve concentrations of interest. The receivers are further packaged with the primary lens and heat sink into re-workable modules and mounted on a tracking system actuated with electric motors. Testing includes both laboratory component testing and live assembly testing with up to 10 receivers on a single tracker. Significant effort has been spent on efficient power coupling of multiple cells to maximum power tracking inverters. For ease of testing we have chosen to two commercial micro inverter types that operate in a range from 200 to 350 W and are tied to the grid.

TEST CONFIGURATIONS AND OBSERVATIONS Figure 1 illustrates the optical configuration. We employ a large Fresnel lens, mechanical mounts and tracking system to image the sun on to the concentrator cell. Fresnel lenses were custom made using multiple vendors for our testing. The cell mounting and stack arrangement is illustrated in figure 2 and shows the relative positions of the thermal interface and cell isolation materials.

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Figure 3 illustrates the PV cell attached to the aluminum nitride package.

Figure 1 Illustration of the optics, cell and heat spreader configuration

Figure 2 Illustration of the package thermal stack

Figure 3 Illustration of the assembled package A progression of testers has been employed to evaluate the on sun performance of these receivers. Figure 4 shows our first tester comprising a large Fresnel lens and a liquid cooled cell heat sink assembly. This tester was built to drive cells to extreme concentrations over 200 W/cm2 and is presently used for package and

thermal interface cycle testing. Figure 5 illustrates a 4 cell, embodiment that operates at an optical concentration of 900X used to evaluate coplanar placement of cells.

Figure 4 Illustration of high power single cell tester

Figure 5 Illustration of a 4 cell per module tester Our most current tester is a 10 cell embodiment shown in Figure 6 below. Cell DC output power of 35 W has been demonstrated thus far on this tester. This tester comprises field replaceable modules that incorporate the Fresnel lens, secondary optic, receiver and heat sink into a single assembly. This facilitates the rapid interchange of different configurations for testing. These modules are mounted on

low melt alloy

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PV cell

cold plate

Fresnel concentrator

Solar illumination

concentrator cell cold

plate

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a tracking system as shown in the figure that is capable of tracking in both azimuth – elevation and elevation azimuth modes using different motor mounts. Both light cup and light pipe embodiments have been tested. Finally the individual modules are electrically coupled to maximum power point tracking inverters. We have explored two micro inverter types with input power ranging from 200 to 350 W.

Figure 6 Illustration of 10 cell tester Tracking and control of the systems shown above has been demonstrated using both custom and off the shelf sun sensing electronics. Both have been shown capable of maintaining pointing accuracy well within a 0.1 degree envelope. Power produced with these systems is dissipated by controlled resistive loads for test purposes when producing IV curves for individual cells or groups of cells. Alternately we directly couple power to the grid using a micro inverter / MPPT tracker. Modules have been constructed configurations that include lenses, light pipes, light cups and other material from different vendors. These modules are tested to identify the best components and vendors. We will describe some of the issues we have had with assembly and testing below. Soldering the cell to the substrate package was initially performed by clamping the cell in a manually operated reflow fixture that protected the top surface of the cell. This methodology has been replaced with an automated process for cell placement and reflow. Initial results were mixed with some assemblies showing solder voiding. Figure 7 illustrates this condition. While it

is not uncommon to have voiding in solder joints, maintaining the size and number to small values is critical to achieving both mechanical and thermal performance. Remedies include dispense of the optimum amount of solder paste, correct reflow oven temperature profile, solder particle size and flux balance. Once these parameters are optimized, voiding is minimized or eliminated.

Figure 7 Ultra sound image of cell showing solder voiding.

Figure 8 Ultra sound image of cell showing reduced solder voiding.

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Testing experience indicates that small voids on the order of half the substrate thickness do not pose a problem thermally or mechanically. Never the less, void free is the correct criterion. A related issue is the migration of flux and reflow related contaminants to the top surface of the cell. In our manual cell reflow process the cell is protected from vapor and particulate re deposition from reflow process. Automation of this process exposes the top surface to process contaminants. These contaminants interfere with the bonding of light pipe secondary optics or in the case of light cup assemblies expose the cell to both mechanical cracking and electrical shorting under intense solar loads. We have implemented a multi step solvent clean to insure that the cell and package are thoroughly contaminant free. Proper bonding of the light cup to the substrate requires that a precise amount of adhesive be dispensed to achieve the correct bond line. It is further critical that the light pipe be mechanically placed precisely relative to the cell and maintained in the correct orientation until the adhesive cure is complete. Early assemblies had issues in both these areas. Once corrected reliable bonding resulted.

Figure 9 Illustration of a light pipe bonded to a cell. Live testing of concentrator assemblies required that our cell assemblies couple efficiently to the micro inverters used to grid tie the system. The micro inverters that we have examined are designed to accept DC power from flat panel photovoltaics with higher voltage and lower current than the typical values produced by triple junction cells operating at high concentration. Individual cell assemblies produce on the order of 12 to 15 amps each depending on configuration. The conventional serial connection of cells does not produce convenient matching of voltage, current and power to the micro inverters. We further observe that in a serial arrangement of cells, the performance can be

limited by weaker cells in the string.

Figure 10 Illustration of a light pipe incorrectly bonded to a cell. A variety of factors affect the individual cell performance. These include cell to cell differences in manufacture, individual alignment, illumination uniformity, and temperature. Collectively these impact the overall system efficiency by several percent.

Figure 11 Schematic of serially connected photovoltaic cells To remedy this, we have developed a collection of matching networks using DC to DC converter circuits and methods borrowed from server power distribution systems. These networks match the current and voltage produced by a given number of cells to the input requirements of the micro inverter. The matching network further enables

solar cells Maximum power

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individual cells to operate closer to their optimum power point. The matching network employs both serial and parallel strategies to achieve overall power conversion efficiency of approximately 95%. An illustration of the method is shown schematically in Figure 12. We note that some embodiments of the matching network can be optimized to reduce the total amount of copper used in the interconnect wiring of individual cells.

Figure 12 Illustration of cells connected to an inverter using a matching network

CONCLUSION We have constructed high concentration photovoltaic embodiments with one to 10 cells per system over a power density range from approximately 75 W/cm2 to 200 W/cm2 using both active and passive cooling systems in stand alone and grid tied embodiments. Given our testing experience to date, we conclude it is both feasible and reasonable to expect effective operation at high concentration.

REFERENCES [1] Pending patent, “Method and apparatus for deploying a liquid metal thermal interface for chip cooling”, USPTO Application No. 20070238282. [2] US Patent US7440281, “Thermal interface apparatus”. [3] R. Sherif, et al.,”The Path to 1 GW of Concentrator Photovoltaics Using Multijunction Solar Cells”, Thirty-first IEEE Photovoltaic Specialists Conference, 2005, pp. 17-22. [4] Y. Martin, et al., "Liquid Metal Thermal Interface for High Volume Production", Proceedings of the Fortieth Annual IMAPS Conference, 2007, pp. 1-3.

[5] Ralf Leutz, et. al., “Optical tests for reliability and efficiency of photovoltaic concentrators”, Proc. of SPIE, 7048, 704809, 2008, pp. 2-5. [6] González JR et al., “Reliability analysis of temperature step-stress tests on III–V high concentrator solar cells”, Microelectron Reliability. (2009), doi:10.1016/j.microrel.2009.04.001, pp. 2-3. [7] Jeffrey M. Gordon et. al., ”Toward ultrahigh-flux photovoltaic concentration”, Applied Physics Letters, 84, Number 18, 3 May 2004, pp.3642-3643. [8] Omer Korech, “High-Flux Characterization Of Ultrasmall Multijunction Concentrator Solar Cells”, Applied Physics Letters, 91, 064101, 2007, pp. 1-3.

solar cells

maximum power point tracker AC inverter

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AC power connection to grid

matching network

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