Osvaldo Arenas

Determining passive cooling limits in CPV using an analytical thermal model

We propose an original thermal analytical model aiming to predict the practical limits of passive cooling systems for high concentration photovoltaic modules. The analytical model is described and validated by comparison with a commercial 3D finite element model. The limiting performances of flat plate cooling systems in natural convection are then derived and discussed.

Electrothermal Mapping of AlGaN/GaN HEMTs Using Microresistance Thermometer Detectors

Self-heating effects in AlGaN/GaN high-electron mobility transistors (HEMTs) can notably reduce electron mobility and produce reliability concerns. Electrothermal characterization and appropriate thermal management are required to address this situation. This letter presents the measurement of channel temperature (T_ch) of GaN HEMTs in multiple bias conditions with a good accuracy. The measurements are executed using the integrated microresistance thermometer detector (μRTD) technique in AlGaN/GaN HEMTs on SiC and sapphire substrates. The integrated Ti/Pt μRTD sensor with linear resistance-temperature characteristic is used to obtain an I_ds-V_ds-T_ch map for each device. Thermal resistances are compared for similar operation conditions, obtaining RTH = 34.7 °C · W^-1 for the HEMT on SiC and RTH = 157.2 °C · W^-1 for the HEMT on sapphire.

Creating screen‐printed passive components for microwave applications

Purpose – The purpose of this paper is to share valuable information about low‐cost microwave circuit research with academic and industrial communities that work, or want to work, in this field.Design/methodology/approach – Screen‐printing technology has been chosen as the fabrication method because of simplicity and low costs. Different materials and printing parameters were tested in four generations of microstrip lines. After obtaining a satisfactory fabrication method, passive microwave components were printed, assembled, characterized and modeled.Findings – Results demonstrated that the proposed low‐cost method allows fabricating low loss microstrip lines (15.63×10−3 dB/mm at 10 GHz), filters, inductors, and capacitors that work well up to 12 GHz.Research limitations/implications – Model accuracy of inductors and capacitors can be improved. The use of more precise calibration and de‐embedding techniques is necessary. More components can be fabricated and modeled to increase the flexibility and applic...

Integration of Micro Resistance Thermometer Detectors in AlGaN/GaN Devices

Temperature measurements in AlGaN/GaN high electron mobility transistors are required for proper device design, modeling and achieving appropriate reliability. These measurements usually require sophisticated equipment and extensive calibration. This study evaluates the feasibility of temperature measurements by integration of a Pt resistance thermal detector (RTD) in an “un-gated” transistor and evaluating their electrical interactions. The integrated RTD presents the advantage of being independent of the device. Micro RTD showed a linear response in the calibration interval (0 to 206 °C). Measured temperature values using the micro RTD are in agreement with 3D finite element simulations at multiple bias conditions in the “un-gated” transistor. Measurements show no noticeable electrical perturbation between the device and RTD under simultaneous operation.

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.

Thermal test and simulation of alumina receiver with high efficiency multi-junction solar cell for concentrator systems

The thermal performances of multi-junction solar cells, mounted on receivers, are studied to determine the change in device efficiency with respect to sunlight concentration under continuous illumination. Experimental characterization of the device was performed by measuring the solar cell current-voltage curve using both flash and continuous-illumination solar simulators. We are able to extract the change in efficiency and open circuit voltage with respect to the change in concentration from experiments with respect to the application of thermal paste between the receiver and the heat exchange. We show the efficiency linearly decrease at a rate of -0.0094%/°C (no paste) and -0.0043%/°C (paste). We used the calibrated numerical model to determine the solar cell temperature and incorporate the corresponding efficiency when scaled up to 2000 sun concentrations under continuous illumination.

Characterization of Dynamic Self-Heating in GaN HEMTs Using Gate Resistance Measurement

This letter reports on a new method for the characterization of transistors transient self-heating based on gate end-to-end resistance measurement. An alternative power signal is injected to the device output (between drain and source) at constant gate-to-source voltage. The dependence of gate resistance with temperature is used to extract the thermal impedance of the device in frequency domain via electrical measurement. This new method is validated on common-gate AlGaN/GaN high-electron-mobility transistors on Si substrate under different experimental conditions, which demonstrates its potential to provide complete dynamic self-heating models for power transistors.

A 1000x utility-scale parabolic frame tracker for multidisciplinary CPV research

A dual-dish concentrating solar research system is introduced in which multiple low-cost single-axis-focusing mirrors have their foci overlapped into a single intense compound focus. A CPV receiver for such a focus is also introduced, with cooled secondary mirrors and a Dense Receiver Array (DRA) with shingled cell rows to eliminate inter-row gaps. CTE-matched micro-channel cold plates are used for low-resistance cooling and fin tube radiators provide ample heat-rejection surface. The ratio of the DRA’s cell area to focusing mirrors’ area allows reaching a concentration factor of 1000x. A cost breakdown is presented and discussed and areas that still need significant improvement to be able to compete with flat panel costs are identified, along with research works in progress in those areas.

Assessment And Comparison Of Concentrator Cell Carrier Efficiencies Under Very High Fluxes

The present work aimed at assessing and comparing the thermal performances of two different types of cell carriers exposed to natural sunlight beams concentrated up to 1,500–4,500 suns. Metallic and hybrid metal‐ceramic carriers of various dimensions, or bonded to cells of different sizes, were considered. Temperature profiles inside the carriers exposed to concentrated beams were measured using temperature sensors placed at two different locations. 3D heat transfer simulations of a carrier bonded either to the real Ge‐based solar cell or to the dummy cell instrumented for our temperature measurements showed that the measured temperatures differed by less than a couple of degrees from the real solar cell surface temperatures within a large range of concentration. Experimental results and thermal simulations confirmed the need to select a high‐conductivity carrier combined with a very efficient active device for cooling the solar cells under very high concentration. In addition, the key role played by ther...

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.