Safety, performance and driving comforts are given high importance while developing modern day cars. All-Wheel Drive vehicles are exactly designed to fulfill such requirements. In modern times, human concern towards depleting fossil fuels and cognizance of ecological issues have led to new innovations in the field of Automotive engineering. One such outcome of the above process is the birth of electrical hybrid vehicles. The product under investigation is a combination of all wheel drive and hybrid system. A superior fuel economy can be achieved using hybrid system and optimized vehicle dynamic forces are accomplished by torque vectoring action which in turn provides All-Wheel Drive capabilities. Heat generation is inevitable whenever there is a conversion of energy from one form into another. In this master thesis investigation, a thermal simulation model for the product is built using 1D simulation tool AMESim and validation is done against the vehicle driving test data. AMESim tool was chosen for its proven track record related to vehicle thermal management. The vehicle CAN data are handled in MATLAB. In a nutshell, Simulation model accounts for heat generation sources, oil flow paths, power loss modeling and heat transfer phenomena. The final simulation model should be able to predict the transient temperature evolution in the rear drive when the speed and torque of motor is supplied as input. This simulation model can efficiently predict temperature patterns at various locations such as casing, motor inner parts as well as coolant at different places. Various driving cases were tried as input including harsh (high torque, low speed) ones. Simulation models like this helps Engineers in trying out new cooling strategies. Flow path optimization, flow rate, convection area, coolant pump controlling etc are the few variables worth mentioning in this regard.

Compact heat exchangers such as porous foam coldplates have great potential as a high heat flux cooling solution for electronics due to their large surface area to volume ratio and tortuous coolant path. The focus of this work was the development of unit cell modeling techniques for predicting the performance of coldplates with porous foam in the coolant path. Multiple computational fluid dynamics (CFD) models which predict porous foam coldplate pressure drop and heat transfer performance were constructed and compared to gain insight into how to best translate the foam microstructure into unit cell model geometry. Unit cell modeling in this study was realized by applying periodic boundary conditions to the coolant entrance and exit faces of a representative unit cell. A parametric study was also undertaken which evaluated dissimilar geometry translation recommendations from the literature. The use of an effective thermal conductivity for a representative orthogonal lattice of rectangular ligaments was compared to a porosity-matching technique of a similar lattice. Model accuracy was evaluated using experimental test data collected from a porous copper foam coldplate using deionized water as coolant. The compact heat exchanger testing facility which was designed and constructed for this investigation was shown to be capable of performing tests with coolant flow rates up to 300 mL/min and heat fluxes up to 290 W/cm2. The greatest technical challenge of the testing facility design proved to be the method of applying the heat flux across a 1 cm2 contact area. Based on the computational modeling results and experimental test data, porous foam modeling recommendations and porous foam coldplate design suggestions were generated.

It is well known that microprocessor performance can be improved by lowering the junction temperature. Two stage cascaded vapor compression refrigeration (VCR) is a mature, inexpensive, and reliable cooling technology that can offer chip temperatures down to ?? C. Recent studies have shown that for a power limited computer chip, there is a non-linear scaling effect that offers a 4.3X performance enhancement at ?? C. The heat transfer performance of a compact evaporator is often the bottleneck in sub-ambient heat removal. For this reason, the design of a deep sub-ambient compact evaporator is critical to the cooling system performance and has not been addressed in the literature. Four compact evaporator designs were investigated as feasible designs - a baseline case with no enhancement structures, micro channels, inline pin fin arrays, and alternating pin fin arrays. A parametric experimental investigation of four compact evaporator designs has been performed aiming at enhancing heat transfer. Each evaporator consists of oxygen free copper and has a footprint of 20 mm x 36 mm, with a total thickness of 3.1 mm. The micro channel evaporator contains 13 channels that are 400 um wide by 1.2 mm deep, and the pin fin evaporators contain approximately 80 pin fins that are 400 um wide by 1.2 mm tall with a pitch of 800 um. Two phase convective boiling of R508b refrigerant was investigated in each evaporator at flow rates of 50 - 70 g/min and saturation temperatures of ??to ??C. Pressure drop and local heat transfer measurements are reported and used to explain the performance of the various evaporator geometries. The results are compared to predictions from popular macro- and micro-channel heat transfer and pressure drop correlations. The challenges of implementing a two stage cascade VCR systems for microprocessor refrigeration are also discussed.

A macro-homogeneous, nonisothermal, two-phase, and steady state mathematical model is developed to investigate water and thermal management in polymer electrolyte membrane (PEM) fuel cells. An original two-phase energy balance approach is used to catch the thermal transport phenomena in cases when there is a signicant temperature dierence between the fuel cell temperature and the reactants inlet temperatures like during cold start-up. Model considers in depth electrode kinetics for both anode and cathode reactions. External and internal mass transfer resistances on fuel cell performance are accounted by means of a thin-film and agglomerate approach. Developed model accounts for all substantial transport phenomena including diffusion of multi-component gas mixtures in the porous media, electrochemical reactions in the catalytic regions, water and proton transport through the solid polymer electrolyte, transport of electrons within the solid matrix, heat transport in the gas and solid phases, phase change and transport of water through porous diffusion media and catalyst layers. In this study, it is truly shown how significant heat and water transport are to overall fuel cell performance. Model predictions are validated by comparison with experimental data, involving polarization curves, saturation and temperature gradients. For optimal electrode kinetics purposes, an alternative novel hollow core mesoporous shell (HCMS) carbon supported Pt and Pt-Pd electrocatalysts were synthesized by microwave irradiation. HCMS carbon spheres were produced by two different carbon precursors with the template replication of solid core mesoporous shell (SCMS) silica spheres. Compared to Pt/VX and ETEK electrocatalysts, HCMS carbon based Pt and Pt-Pd electrocatalysts showed promising cathode and anode electrodics performance in the fuel cell environment.

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High efficiency multi-junction (MJ) solar cells were packaged onto receiver systems. The efficiency change of concentrator cells under continuous high intensity illumination was done. Also, assessment of the receiver design on the overall performance of a Fresnel-type concentration system was investigated. We present on receiver designs including simulation results of their three-dimensional thermal operation and experimental results of tested packaged receivers to understand their efficiency in real world operation. Thermal measurements from solar simulators were obtained and used to calibrate the model in simulations. The best tested efficiency of 36.5% is obtained on a sample A receiver under 260 suns concentration by the XT-30 solar simulator and the corresponding cell operating temperature is ~30.5°C. The optimum copper thickness of a 5 cm by 5 cm simulated alumina receiver design was determined to be 6 mm and the corresponding cell temperature under 1000 suns concentration is ~36°C during operation.

The 2010 National Space Policy of the United State of America introduced by President Obama directed NASA to set far reaching exploration milestones that included a crewed mission to a Near Earth Asteroid by 2025 and a crewed mission to Martian orbit by the mid-2030s. The policy was directly influenced by the recommendations of the 2009 Review of United States Human Space Flight Plans Committee, which called for an evolutionary approach to human space exploration and emphasized the criticality of budgetary, programmatic, and program sustainability. One potential method of improving the sustainability of exploration architectures is the utilization of orbital propellant depots with commercial launch services. In any exploration architecture, upwards of seventy percent of the mass required in orbit is propellant. A propellant depot based architecture allows propellant to be delivered in small increments using existing commercial launch vehicles, but will require three to five times the number of launches as compared to the using the NASA planned 70 to 130 metric ton heavy lift launch system. Past studies have shown that the utilization of propellant depots in exploration architectures have the potential of providing the sustainability that the Review of United States Human Space Flight Plans Committee emphasized. However, there is a lack of comprehensive analysis to determine the feasibility of propellant depots within the framework of human space exploration. The objective of this research is to measure the feasibility of a propellant depot and commercial launch based exploration architecture by stochastic assessment of technical, reliability, and economic risks. A propellant depot thermal model was developed to analyze the effectiveness of various thermal management systems, determine their optimal configuration, quantify the uncertainties in the system models, and stochastically compute the performance feasibility of the propellant depot system. Probabilistic cost analysis captured the uncertainty in the development cost of propellant depots and the fluctuation of commercial launch prices, and, along with the cost of launch failures, provided a metric for determining economic feasibility. Probabilistic reliability assessments using the launch schedule, launch reliability, and architecture requirements of each phase of the mission established launch success feasibility. Finally, an integrated stochastic optimization was performed to determine the feasibility of the exploration architecture. The final product of this research is an evaluation of propellant depots and commercial launch services as a practical method to achieving economic sustainability for human space exploration. A method for architecture feasibility assessment is demonstrated using stochastic system metrics and applied in the evaluation of technical, economic, and reliability feasibility of orbital propellant depots and commercial launch based exploration architectures. The results of the analysis showed the propellant depots based architectures to be technically feasible using current commercial launch vehicles, economically feasible for having a program budget less than $4 billion per year, and have launch reliability approaching the best single launch vehicle, Delta IV, with the use of redundant vehicles. These results serve to provide recommendations on the use of propellant depots in exploration architectures to the Moon, Near Earth Objects, Mars, and beyond.The 2010 National Space Policy of the United State of America introduced by President Obama directed NASA to set far reaching exploration milestones that included a crewed mission to a Near Earth Asteroid by 2025 and a crewed mission to Martian orbit by the mid-2030s. The policy was directly influenced by the recommendations of the 2009 Review of United States Human Space Flight Plans Committee, which called for an evolutionary approach to human space exploration and emphasized the criticality of budgetary, programmatic, and program sustainability. One potential method of improving the sustainability of exploration architectures is the utilization of orbital propellant depots with commercial launch services. In any exploration architecture, upwards of seventy percent of the mass required in orbit is propellant. A propellant depot based architecture allows propellant to be delivered in small increments using existing commercial launch vehicles, but will require three to five times the number of launches as compared to the using the NASA planned 70 to 130 metric ton heavy lift launch system. Past studies have shown that the utilization of propellant depots in exploration architectures have the potential of providing the sustainability that the Review of United States Human Space Flight Plans Committee emphasized. However, there is a lack of comprehensive analysis to determine the feasibility of propellant depots within the framework of human space exploration. The objective of this research is to measure the feasibility of a propellant depot and commercial launch based exploration architecture by stochastic assessment of technical, reliability, and economic risks. A propellant depot thermal model was developed to analyze the effectiveness of various thermal management systems, determine their optimal configuration, quantify the uncertainties in the system models, and stochastically compute the performance feasibility of the propellant depot system. Probabilistic cost analysis captured the uncertainty in the development cost of propellant depots and the fluctuation of commercial launch prices, and, along with the cost of launch failures, provided a metric for determining economic feasibility. Probabilistic reliability assessments using the launch schedule, launch reliability, and architecture requirements of each phase of the mission established launch success feasibility. Finally, an integrated stochastic optimization was performed to determine the feasibility of the exploration architecture. The final product of this research is an evaluation of propellant depots and commercial launch services as a practical method to achieving economic sustainability for human space exploration. A method for architecture feasibility assessment is demonstrated using stochastic system metrics and applied in the evaluation of technical, economic, and reliability feasibility of orbital propellant depots and commercial launch based exploration architectures. The results of the analysis showed the propellant depots based architectures to be technically feasible using current commercial launch vehicles, economically feasible for having a program budget less than $4 billion per year, and have launch reliability approaching the best single launch vehicle, Delta IV, with the use of redundant vehicles. These results serve to provide recommendations on the use of propellant depots in exploration architectures to the Moon, Near Earth Objects, Mars, and beyond.

The efficacy, reliability and versatility of the light emitting diode (LED) can outcompete most established light source technologies. However, they are particularly sensitive to high temperatures, which compromises their efficacy and reliability, undermining some of the technology s key benefits. Consequently, effective thermal management is essential to exploit the technology to its full potential. Thermal management is a well-established subject but its application in the relatively new LED lighting industry, with its specific constraints, is currently poorly defined. The question this thesis aims to answer is how can LED thermal management be achieved most effectively? This thesis starts with a review of the current state of the art, relevant thermal management technologies and market trends. This establishes current and future thermal management constraints in a commercial context. Methods to test and evaluate the thermal management performance of a luminaire system follow. The defined test methods, simulation benchmarks and operational constraints provide the foundation to develop effective thermal management strategies. Finally this work explores how the findings can be implemented in the development and comparison of multiple thermal management designs. These are optimised to assess the potential performance enhancement available when applied to a typical commercial system. The outcomes of this research showed that thermal management of LEDs can be expected to remain a key requirement but there are hints it is becoming less critical. The impacts of some common operating environments were studied, but appeared to have no significant effect on the thermal behaviour of a typical system. There are some active thermal management devices that warrant further attention, but passive systems are inherently well suited to LED luminaires and are readily adopted so were selected as the focus of this research. Using the techniques discussed in this thesis the performance of a commercially available component was evaluated. By optimising its geometry, a 5 % decrease in absolute thermal resistance or a 20 % increase in average heat transfer coefficient and 10 % reduction in heatsink mass can potentially be achieved . While greater lifecycle energy consumption savings were offered by minimising heatsink thermal resistance the most effective design was considered to be one optimised for maximum average heat transfer coefficient. Some more radical concepts were also considered. While these demonstrate the feasibility of passively manipulating fluid flow they had a detrimental impact on performance. Further analysis would be needed to conclusively dismiss these concepts but this work indicates there is very little potential in pursuing them further.

abstract: Semiconductor scaling technology has led to a sharp growth in transistor counts. This has resulted in an exponential increase on both power dissipation and heat flux (or power density) in modern microprocessors. These microprocessors are integrated as the major components in many modern embedded devices, which offer richer features and attain higher performance than ever before. Therefore, power and thermal management have become the significant design considerations for modern embedded devices. Dynamic voltage/frequency scaling (DVFS) and dynamic power management (DPM) are two well-known hardware capabilities offered by modern embedded processors. However, the power or thermal aware performance optimization is not fully explored for the mainstream embedded processors with discrete DVFS and DPM capabilities. Many key problems have not been answered yet. What is the maximum performance that an embedded processor can achieve under power or thermal constraint for a periodic application? Does there exist an efficient algorithm for the power or thermal management problems with guaranteed quality bound? These questions are hard to be answered because the discrete settings of DVFS and DPM enhance the complexity of many power and thermal management problems, which are generally NP-hard. The dissertation presents a comprehensive study on these NP-hard power and thermal management problems for embedded processors with discrete DVFS and DPM capabilities. In the domain of power management, the dissertation addresses the power minimization problem for real-time schedules, the energy-constrained make-span minimization problem on homogeneous and heterogeneous chip multiprocessors (CMP) architectures, and the battery aware energy management problem with nonlinear battery discharging model. In the domain of thermal management, the work addresses several thermal-constrained performance maximization problems for periodic embedded applications. All the addressed problems are proved to be NP-hard or strongly NP-hard in the study. Then the work focuses on the design of the off-line optimal or polynomial time approximation algorithms as solutions in the problem design space. Several addressed NP-hard problems are tackled by dynamic programming with optimal solutions and pseudo-polynomial run time complexity. Because the optimal algorithms are not efficient in worst case, the fully polynomial time approximation algorithms are provided as more efficient solutions. Some efficient heuristic algorithms are also presented as solutions to several addressed problems. The comprehensive study answers the key questions in order to fully explore the power and thermal management potentials on embedded processors with discrete DVFS and DPM capabilities. The provided solutions enable the theoretical analysis of the maximum performance for periodic embedded applications under power or thermal constraints. / Dissertation/Thesis / Ph.D. Computer Science 2012

Inversores de sistemas fotovoltaicos são equipamentos de eletrônica de potência que fornecem energia elétrica em corrente alternada (CA) a partir de uma fonte de energia elétrica em corrente contínua (CC), no caso, os módulos fotovoltaicos. Estes inversores quando em operação aumentam sua temperatura. Este aumento de temperatura é indesejável, porém é inerente ao seu funcionamento. Equipamentos eletrônicos possuem um limite seguro de temperatura de operação, acima do qual podem ocorrer instabilidades de operação, redução da vida útil ou até mesmo falha drástica. O conhecimento da eficiência de conversão elétrica e das perdas responsáveis pelo aquecimento é importante para o adequado dimensionamento de inversores quando aplicados em sistemas fotovoltaicos conectados à rede, assim como para o desenvolvimento do inversor como produto. Para proteção, para aumento da vida útil, para maior confiabilidade, para maior estabilidade e para maior segurança de operação de componentes, inversores possuem rotinas em seus algoritmos de controle com estratégias automatizadas dedicadas ao gerenciamento térmico. Estas rotinas de proteção e gerenciamento térmico, sempre quando acionadas, tendem a reduzir a capacidade de conversão de potência do inversor, seja pelo acionamento de um ventilador auxiliar, seja pelo deslocamento do ponto de operação em máxima potência. Fabricantes de inversores tratam deste assunto pelo termo “temperature derating” (em inglês) Esta tese apresenta um estudo relacionado a influência da temperatura sobre o desempenho de inversores fotovoltaicos conectados à rede. Avalia tipos de estratégias de gerenciamento térmico e proteção de uso corrente em inversores comerciais. Apresenta resultados de ensaios experimentais para determinação de parâmetros térmicos característicos dos inversores. Descreve um modelo preditivo da temperatura de operação em regime transiente. A partir da determinação experimental de parâmetros térmicos, o modelo preditivo de temperatura de operação foi implementado no software de simulação dinâmica para dimensionamento e avaliação de sistemas fotovoltaicos FVCONECT, desenvolvido no LABSOL/UFRGS, estando o mesmo apto para simular a operação e estimar perdas anuais de desempenho energético decorrentes das rotinas de gerenciamento térmico, dos seus efeitos e das limitações impostas durante a operação de inversores fotovoltaicos conectados à rede. Um dos resultados da simulação é a evolução da temperatura do inversor, permitindo avaliar a frequência e amplitude dos ciclos térmicos ao qual o inversor é submetido e, consequentemente, determinar uma estimativa para durabilidade do inversor. / Photovoltaic inverters are electronic power devices that provide electrical energy in alternating current (AC) from a source of electrical energy in direct current (DC) - a photovoltaic generator, in this case. Inverters increase their temperature when in operation. This rise in temperature is not desirable, but inherent to its operation. Any electronic equipment has a safe operating temperature limit. When this limit is surpassed, operating instability, life reduction or even drastic failure may occur. The knowledge of the electrical conversion efficiency and the losses responsible for the heating is important for the proper sizing of grid-tie inverters in photovoltaic systems, as well as for the development of the inverter as a product. In order to increase the useful life of the device and its components, for greater reliability, safety, stability and security of operation, inverters have routines in their algorithms of control with automated strategies dedicated to the thermal management. These protection and thermal management routines, whenever activated, tend to reduce the power conversion capacity of the inverter, either by the activation of an auxiliary fan or by the displacement of the operating point at maximum power. Inverter manufacturers address this issue by the term "temperature derating". This thesis presents a methodology to evaluate the influence of the performance of different strategies to avoid excessive temperature of the inverter components on its performance and durability It is also made an evaluation of different thermal management strategies and protection used in commercial inverters. Results of experimental tests for determination of thermal parameters characteristic of the inverters are presented. A predictive model of transient operating inverter temperature is also described. From the experimental determination of thermal parameters, the predictive model of operating temperature was implemented to the FVCONECT, a dynamic simulation software for sizing and evaluation of photovoltaic systems developed in LABSOL / UFRGS. With this modification, the software was able to simulate the operation and estimate losses of energy due to the thermal management routines, their effects and the limitations imposed during the operation of grid-tie inverters. One of the results of the simulation is the evolution of the inverter temperature, allowing to evaluate the frequency and amplitude of thermal cycles to which the inverter is subjected and, as a consequence, an estimate of durability of the inverter.