Louis-Michel Collin

Thermal Resistance and Heat Spreading Characterization Platform for Concentrated Photovoltaic Cell Receivers

Concentrated photovoltaics (CPV) focus the sunlight on a cell area smaller than the aperture area, making the use of highly efficient multijunction solar cells cost-effective. However, the high heat flux generated under concentration can raise the cell temperature and reduce the benefits of higher concentration. Low thermal resistance cell packages (receivers) associated with effective heat sinking can alleviate this problem. This paper proposes a new experimental method and characterization platform to measure the thermal performance of a solar cell receiver in a specific cooling module. The platform injects a calibrated heat flux into a test receiver to measure its contribution to the thermal resistance, demonstrating an accuracy and reproducibility of ±0.15°C/W. A metric to evaluate the heat spreading capability of the receiver is defined and extracted from experimental measurements conducted with different thermal boundary conditions. Multiple receiver configurations and materials were characterized, demonstrating that the proposed test methodology and platform can capture their impact on the heat spreading capabilities. The results also highlight the importance of thermal interfaces and the benefits of spreading the heat in metallic layers before conducting it through the dielectric layers that form the receiver. The proposed metrics and characterization platform will therefore be beneficial for the design, experimental development, and selection of CPV receivers and cooling modules.

Distributed and self-adaptive microfluidic cell cooling for CPV dense array receivers

Temperature non uniformities of the CPV receivers lead to mismatch losses. In order to deal with this issue, a cooling device, formed by a matrix of microfluidic cells with individually variable coolant flow rate, has been developed. This device tailors the distribution of the heat extraction capacity over the CPV receiver to the local cooling needs in order to reduce the temperature non uniformities with respect to microchannel devices when submitted to uniform or non-uniform illumination profiles. At equal average temperature of the CPV receiver, power generation applying the matrix of microfluidic cells with individually variable coolant flow rate is 9.7% higher than the one with conventional microchannel technology.

Microchannel Design Study for 3D Microelectronics Cooling Using a Hybrid Analytical and Finite Element Method

For microelectronics cooling, microchannels are a potential solution to ensure reliability without sacrificing compactness, as they require relatively small space to remove high heat fluxes compared to air cooling. However, designing microchannels is a complex task where simulation models become a forefront tool to investigate and propose new solutions to increase the chip thermal performances with minimal impact on other aspects.This work evaluates numerically the impact of microchannel cooling in a standalone chip and a 3D assembly of two stacked chips with localized heat sources. To do so, a modeling approach was developed to combine finite element modeling of conduction in the chip using commercial software with analytical relations to capture the heat transfer and fluid flow in the microchannels. This approach leverages the multiphysics and post-processing capabilities of commercial software, but avoids the extensive discretization that would normally be required in microchannels with full finite element modeling. The study shows that increasing the flow rate is not as beneficial as increasing the number of channels (with constant total cross-section area). The effect of heat spreading was also found to be critical, favoring thicker dies. When switching to 3D chip configuration, the interdie underfill layer significantly increases the total thermal resistance and must be considered for thermal design. This effect can be significantly alleviated by increasing the interdie thermal conductivity through adding copper micropillars.Copyright

Thin micro-cold plate for hot-spot aware chip cooling

This work proposes a non-invasive and hot spot aware cooling approach by stacking a micro-cold plate at the backside of a chip. With trends such as 3D stacking and hotspot generation, microelectronics face major cooling challenges to ensure chip performance and reliability. It makes the liquid microchannel solutions more adapted than conventional air cooling for both space and heat removal. One approach is to concentrate the cooling in the vicinity of the heat sources to help to judiciously use the pumping power and contribute to keep the cooling solution thin and easily integrated. An experimental micro-cold plate has been fabricated through wafer level produced microchannels, capped with die-to-wafer pick-and-place operation. The microchannels from the cooling die are formed by Si DRIE and the die is capped with a Si wafer attached by SiNR adhesive on one side. An epoxy adhesive is then bonded to a thermal test chip with metallic lines as heaters and temperature sensors for a total stack thickness of 1.5 mm. It has then been characterized on a dedicated test bench. A cooling resistance of 3.5 °C/W is achieved with an electric power of only 1.2 W, showing a coefficient of performance of 5770 in respect of an hydraulic power of 2.6 mW and with a 609 W/cm2 heat flux. Finally, such micro-cold plate could be used as an “add-on solution” in applications where space and pumping power are limited, independently of the chip thickness or the possibility of etching its back surface.

Hot spot aware microchannel cooling add-on for microelectronic chips in mobile devices

This work proposes an experimental microchannel solution to cool microelectronic chips with hot spots, using a non-intrusive technique. In microelectronics, approaches such as die thinning induces acute stress on cooling because it increases the hotspot phenomena and reduces chip bulk thickness aimed for microchannels. In mobile devices, the heat must be removed using limited pumping power and cooling space. Microchannels etched in the backside of the chip, usually considered as an efficient cooling solution, are impracticable on highly thinned chips. This work experimentally investigates the cooling performance of a non-invasive and hot spot aware microchannel die that is in direct fluidic contact with the backside of the chip. It also proposes a confinement-wise metric. A thermal resistance of 2.8 °C/W is achieved at heat flux of 1185 W/cm2 per heat source, for a total dissipated power of 20 W and a maximum allowed temperature rise of 55 °C. Such performance is obtained with only 19.2 kPa of pressure drop and 9.4 ml/min of flow rate, making a hydraulic power of only 3 mW, representing a coefficient of performance of 6500. Therefore, backside cooling appears as a compact and low consumption solution for highly confined heat on chips for mobile applications.

Self-adaptive microvalve array for energy efficient fluidic cooling in microelectronic systems

In the present work, the performance of temperature-regulated microvalves is investigated analytically for energy efficient fluidic cooling of microelectronic systems. The objectives are to decrease the overall mass flow rate of coolant (hence decreasing the pumping power) as well as to improve the temperature uniformity across the chip surface with hot spots. For this purpose, temperature-regulated microvalves are used to manage the coolant mass flow rate distribution throughout the chip based on the local chip temperature. The aim of this study is to find the optimum temperature response function of the microvalves to have more energy efficient cooling. Linear, quadratic and exponential temperature response behaviors are considered for the microvalves. Results show that for the linear microvalves, the mass flow rate and the temperature non-uniformity across the chip decrease by 50% and 29% respectively by using active self-adaptive microvalves, compared to the reference condition without any microvalve. These enhancement values are respectively 45% and 55% when using exponential instead of linear microvalves. This study shows that the concept of self-adaptive microvalve arrays for distributed chip cooling can have a significant impact on power and performance, opening a new approach for microfluidic cooling compared to traditional fixed microchannels.

High Performance Concentrated Photovoltaic Module Development Using Temperature Sensors

Resistance Temperature Detectors were used to investigate the thermal performance of three different cell carrier structures. A carrier configuration with 0.78 mm thick C110 grade copper and 54 μm thick dielectric epoxy was chosen to fabricate five identical CPV modules with 1 cm2 GaInP/GaAs/Ge solar cells. Modules were tested up to ∼864 Suns in steady state illumination conditions presenting a temperature increase factor of 0.0376 °C/Sun.

Advances in Cell Carriers for CPV Applications

High concentration solar applications require appropriate thermal management to prevent cell efficiency losses due to overheating. Cell carrier thermal characteristics play a role in the temperature of the solar cell. Thermal analysis of a conventional, ceramic‐based, cell carrier shows that it is suitable for high concentration applications but is not optimal for heat transfer. Calibrated 3D finite element simulations at 280 suns show that a metallic carrier produced an increase in cell temperature that is 24.4 K less than for the ceramic carrier. An enhanced metallic carrier was introduced and fabricated. Electric breakdown tests showed it can withstand a voltage of at least 1000 V. Results of temperature profiling on the metallic carrier’s surface showed a smaller ΔT, indicating an improved heat distribution.