EXTENDING PHOTOVOL TAlC OPERATION BEYOND 2000 SUNS USING A LIQUID METAL THERMAL INTERFACE WITH PASSIVE COOLING

Theodore G. van Kessel, Yves C. Martin, Robert L. Sandstrom, Supratik Guha, IBM T.J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, N.Y., 10598

ABSTRACT

Photovoltaic systems operating at solar concentration levels in excess of 500 suns require efficient cooling systems to manage the associated high incident power densities above 50 W/cm2. We present results for a liquid metal thermal interface with a 2-5 mm2CIW thermal resistance used in conjunction with a 1 cm 2 commercial triple junction concentrator cell to produce up to 75 W of electrical power. We show that the liquid metal interface can be used to extend the limits of passive cooling relative to polymer thermal interface materials for solar photovoltaic concentrator systems. We will discuss the operation of triple junction photovoltaic cells above the 2000 sun level employing the liquid metal thermal interface and passive cooling.

INTRODUCTION

Photovoltaic concentrator systems continue to evolve with efficiencies of 41 % and power levels of 2000 suns reported in the literature [1, 2, 3]. A photovoltaic concentrator cell operating at 2000 suns and 30% efficiency will generate -140 W/cm2 of waste heat that must be removed.

For reasons of compliance and rework ability, thermal greases and pastes containing conductive particles are often used to interface a heat producing chip to the heat sink. Commercial thermal greases with thermal resistance values in the range of 15 to 20 mm 2CIW are typical and will produce a corresponding interface ~ T of 21 C to 28 C at 140 W/cm2. In a this example case where the solar photovoltaic system must maintain the chip temperature at or below 85 C and tolerate ambient conditions of 35 C, the conventional thermal paste interface consumes up to 56% of the total 50 C thermal budget. This condition gets progressively worse as the power density rises. Additional structure including chip carriers and spreader plates further erode the thermal budget. If passive cooling is desired, the design of a thermal package with particular attention to the thermal interface materials is critical for CPV systems operating above at or above 2000 suns concentration. The liquid metal thermal interface (developed successfully for the high power microprocessors) provides a way to meet this requirement.

MATERIAL DESCRIPTION

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The liquid metal thermal interface (LMTI) is comprised of a gallium alloy (in our case, the gallium, indium, tin eutectic) with a melting point in the 10 C to 15 C range. Under normal operating conditions, the alloy is in liquid phase and is placed between the concentrator cell and the heat sink to form a thermal interface with a thermal resistance of 2-5 mm2CIW. Suitable chip coatings and containment materials have been developed for the microprocessor environment [4]. Compared to polymer thermal interface materials, liquid metal reduces the thermal interface resistance by as much as an order of magnitude, is mechanically compliant, reworkable, and has been developed and qualified for high volume usage successfully in the microprocessor cooling industry. In addition to a substantial benefit in interfacial thermal resistance, LMTI also offers the benefits of a lower application pressure (1 psi or less vs. 20 psi or more for paste) and mechanical flexibility.

TEST CONFIGURATION AND RESULTS

Figure 1 illustrates a test system comprising a 1 x 1 cm Spectrolab 3JT cell, directly attached using a liquid metal thermal interface to a Mikros water cooled heat sink. The test apparatus is mounted on a Celestron 3 axis telescope tracking system with a Fresnel lens capable of 2500 suns concentration. Use of the liquid cold plate enables simultaneous real time calorimetric thermal power and electric output power measurements to be made. Water cooling further enables the direct control of the experimental PV operating temperature via the inlet water temperature and flow.

The thermal improvement that the LMTI confers to the concentrator device operating temperature at high power is illustrated in figures 2 and 3 below. Figure 2 shows the relative difference between the chip temperature and the inlet water temperature at various flow rates for both a liquid metal thermal interface and a commercial thermal paste interface. In these experiments a copper absorber test chip instrumented with a thermocouple was directly attached to the Mikros water cooled heat sink. This configuration allows direct measurement and control of the heat exchanger and chip temperatures at a fixed applied power. The data shows a significant chip to heat sink inlet ~ T difference of -20 C between the commercial thermal grease and the liquid metal due to the difference in thermal resistance between the two materials.

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.... .-•••••• Fresnel concentrator •••••••••

.... . .... .......... L •• •• concentrator ~ ~cell

Itijt'I.~;f- I~~~i~~~tal >, :Y' S ,'> interface

cold plate

.. .. H20 connections

Fig. 1. Schematic of test arrangement comprising a 2500

Figure 3 illustrates I V measurements taken for a 1 x 1 cm Spectrolab 3JT photovoltaic cell at various operating points, using the setup described in Figure 1 at inlet water temperatures and flow rates derived from the flow calibration described above. As the data of Figure 3 shows, we are able to extract 68 W of electrical power at 2200 suns with the chip temperature maintained at 43 C. The highest value thus far obtained with our test system is 75 W of electric power. The liquid metal thermal interface has been shown to operate at at extreme power densities in excess of 750 W/cm 2 using similar liquid cooling methods and a high power thermal test chip [4).

I(A)

35.0

2200 Suns, , 30.0

25.0 Tc = 43C, p. max = SSW \

\ 1500 Suns,

~ sun Fresnel concentrator, Spectrolab 3JT cell interfaced 20.0

with liquid metal to a Mikros copper mini channel water cooled cold plate. 15.0

Tc = 26C, p. max = 52W

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ED

!i)

~ 040 ..... .... 0 -_3)

8 I-<l2)

10

0 0 !i) 100 1!i) 2Xl 2D ~ :m

flow rate (cc/min)

Fig. 2. Temperature difference between the chip and cooling water (at inlet) as a function of water flow rate for the liquid metal and a thermal grease interface. Test configuration comprises a Mikros water cold plate and a 1 cm2 absorber chip at 100 W of applied power.

With improved cooling capability, it is possible to operate the PV device at significantly higher power levels while retaining the benefits of higher photovoltaic efficiencies at low temperatures. Data taken by this method was used to establish chip temperature vs. inlet temperature and flow calibration at different power levels.

978-1-4244-1641-7/08/$25.00 ©2008 IEEE

\ ,

-' 1500Suns,~ , ,

10.0

5.0 Tc = 59C, p. max = 49W

0.0 :,1

o 1 V (volts) 2 3

Fig. 3. High concentration I-V curves using a liquid metal thermal interface, Mikros water cooler and a Fresnel concentrator lens. Note that temperature effect on electrical efficiency is small: for ~ Tc = +33C, Pe decreases by -10%

In commercial operation, a passive cooling arrangement is considered by some to be a reliability advantage. In practice a vapor chamber or heat pipe type heat exchanger is used at the power densities considered here to efficiently spread the heat load. The reduced thermal interface resistance the liquid metal thermal interface may be used to extend the limits of passive cooling relative to polymeric thermal interface materials and thermal pastes in particular. This is illustrated in Figure 4. In this case a passive cooling estimate was created using experimental data taken from a vapor chamber heat sink (see Figure 5) that was constructed, optimized and tested in the laboratory (in still air) to perform at 0.275 CIW and combined with the thermal performance data shown in figures 2 and 3. The vapor chamber heat sink was fabricated out of copper using a nucleation film and attached to approximately 3250 cc of finned air convection heat exchange volume. The vapor chamber was partially filled with de ionized water and evacuated.

Figure 4 shows that thermal paste limits operation to 150 W/cm2 or solar concentrations of 1500 suns for devices that are rated for operation up to 85 C with a 35 C ambient temperature. The liquid metal thermal interface extends this limit to 2400 suns. Similar results can be obtained with metal solder, however solder interfaces suffer the same drawbacks that they do in microprocessor cooling technology in that they are not compatible with direct die attach vapor chamber based cooling solutions (due to reflow temperatures) and are not easily field serviceable. Using the above data, we project that standard paste based thermal interfaces will be less suitable when used with passive cooling beyond solar concentration levels of 1500 suns.

85

V / /

/

thermal Paste I / /

ShinEtsu 7783 ~J I

75

/ /

/ /i liquid Metal

V/~ " J /

35 V o 40 80 120 160 200 240

power (W)

Fig. 4. Estimate of maximum cell temperature as a function of incident solar power for a passively cooled system comprising a 1 cm by 1 cm Spectrolab 3JT cell directly attached to a custom vapor chamber heat sink using liquid metal and paste thermal interface methods

978-1-4244-1641-7/08/$25.00 ©2008 IEEE

90

80

70

§: 60 Q) ::; 50

iii Cii 40 Co E 30

JB 20

10

temperature (C) vs. power (watts)

411

/ /'"

,./ ./

/ V

o 50 100 150 200 250 300

power (watts)

Fig. 5. Measured chip temperature versus power performance of a custom vapor chamber heat sink

CONCLUSION

Liquid metal efficiently couples the thermal load from the photovoltaic concentrator cell to the heat sink and offers a practical opportunity to extend the use of passive cooling in high power solar concentration photovoltaic systems to 2400 suns.

REFERENCES

[1] R. McConnell, M. Symko-Davies, "Multijunction Photovoltaic Technologies for High Performance Concentrators", Fortieth IEEE World Conference on Photovoltaic Energy Conversion, 2006, pp. 1-4.

[2] C. Algora, et al.,"A GaAs Solar Cell with an Efficiency of 26.2% at 1000 Suns and 25% at 2000 Suns", IEEE Transactions on Electron Devices, Vol. 48, No.5, 2001, pp.840-843.

[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 aI., "Liquid Metal Thermal Interface for High Volume Production", Proceedings of the Fortieth AnnuallMAPS Conference, 2007, pp. 1-3

[5] R. Winston, "Planar Concentrators Near the Etendue Limit", Optics Letters, Vol. 30, No.19, 2005, pp. 2617-2619.