Thermal effect of a thermoelectric generator on parallel microchannel heat sink

Thermoelectric generators (TEG) convert heat energy to electrical power by means of semiconductor charge carriers serving as working fluid. In this work, a TEG is applied to a parallel microchannel heat sink. The effect of the inlet plenum arrangement on the laminar flow distribution in the channels is considered at a wide range of the pressure drop along the heat sink. The particular focus of this study is geometrical effect of the TEG on the heat transfer characteristics in the micro-heat sink. The hydraulic diameter of the microchannels is 270 μm, and three heat fluxes are applied on the hot surface of the TEG. By considering the maximum temperature limitation for Bi₂Te₃ material and using the microchannel heat sink for cooling down the TEG system, an optimum pumping power is achieved. The results are in a good agreement with the previous experimental and theoretical studies.     

Numerical study on nonuniform segmented enhancement method for thermoelectric power generator

Thermoelectric power generators (TEGs) have received considerable attention in the vehicular waste heat recovery. Large temperature difference between hot-side end and cold-side end of TE modules should be maintained to keep large power generation. However, the scaling-up problem of inevitably temperature drop of exhaust gas along the flow direction of vehicles is introduced when multiple TE modules are assembled. In this work, a three-dimensional fluid–thermal–electric multiphysics model with equivalent properties of TE modules is established. The influencing mechanism of longitudinal vortex generator (LVG) pair number and distribution in the hot-side heat exchanger on the local power generation performance of the TEGs is examined. It is found that the extent of the local power generation enhancement gets smaller or even deteriorates at the downstream TE modules by uniformly increasing the LVG pair number along the flow direction. Hence, to make the heat flux efficiently utilized, a nonuniform segmented enhancement method is used for the hot-side heat exchanger. The results show that compared with the TEG with a uniform heat exchanger or a downstream-denser heat exchanger, the total Seebeck voltage generated by the TEG with an upstream-denser heat exchanger is 28% higher, and its output power is 64.4% higher at the matched load resistance. The TEG with the upstream-denser heat exchanger significantly improves the overall power generation performance without additional pumping power.     

Three dimensional numerical analyses and optimization of offset strip-fin microchannel heat sinks

This paper presents a numerical study on laminar forced convection of water in offset strip-fin microchannels network heat sinks for microelectronic cooling. A 3-dimensional mathematical model, consisting of N–S equations and energy conservation equation, with the conjugate heat transfer between the heat sink base and liquid coolant taken into consideration is solved numerically. The heat transfer and fluid flow characteristics in offset strip-fin microchannels heat sinks are analyzed and the heat transfer enhancement mechanism is discussed. Effects of geometric size of strip-fin on the heat sink performance are investigated. It is found that there is an optimal strip-fin size to minimize the pressure drop or pumping power on the constraint condition of maximum wall temperature, and this optimal size depends on the input heat flux and the maximum wall temperature. The results of this paper are helpful to the design and optimization of offset strip-fin microchannel heat sinks for microelectronic cooling.     

新能源汽车动力电池冷却系统热仿真及优化

动力电池的温度控制是新能源汽车发展中的一个难题,而电池冷却系统在动力电池的温度控制过程中起着相当重要的作用。利用Solidworks软件对电池包进行建模,利用ICEM CFD软件对电池包模型进行网格划分等前处理。利用Fluent软件并采用控制变量法分别对冷却管道截面宽度、冷却液质量流量和冷却液进口温度等3个对电池包散热性能影响较大的参数进行仿真计算和对比分析。根据仿真结果选择可优化电池包散热性能的参数,并在原方案基础上提出了一种新的冷却管道分布方案。经过仿真计算发现,该方案可有效降低电池在使用过程中的最高温度和温差,提高了电池冷却系统的散热性能。     

Performance effect on the TEG system for waste heat recovery in automobiles using ZnO and SiO2 nanofluid coolants

Enhancement in heat transfer of the cold side is vital to amplify the performance of a thermoelectric generator (TEG). With enriched thermophysical properties of nanofluids, significant improvement in heat transfer process can be obtained. The current study concerns the performance comparison of an automobile waste heat recovery system with EG‐water (EG‐W) mixture, ZnO, and SiO₂ nanofluid as coolants for the TEG system. The effects on performance parameters, that is, circuit voltage, conversion efficiency, and output power with exhaust inlet temperature, the total area of TEG, Reynolds number, and particle concentration of nanofluids for the TEG system have been investigated. A detailed performance analysis revealed an increase in voltage, power output, and conversion efficiency of the TEG system with SiO ₂ nanofluid, followed by ZnO and EG‐W coolants. The electric power and conversion efficiency for SiO ₂ nanofluid at an exhaust inlet temperature of 500K were enhanced by 11.80% and 11.39% respectively, in comparison with EG‐W coolants. Moreover, the model speculates that an optimal total area of TEGs exists for the maximum power output of the system. With SiO ₂ nanofluid as a coolant, the total area of TEGs can be diminished by up to 34% as compared with EG‐W, which brings significant convenience for the placement of TEGs and reduces the cost of the TEG system.     

Thermoelectric conversion of waste heat from IC engine‐driven vehicles: A review of its application,issues, and solutions

Thermoelectric generator (TEG) is a promising thermoelectric (TE) conversion technology to effectively recover and convert waste heat from vehicle exhaust into useful energy, ie, electricity. Exhaust TEG (ETEG) is a system that is incorporated into the exhaust manifold of a vehicle. Exhaust TEG comprises of a heat exchanger, TEG modules, heat sink, and power conditioning unit. The present work reviews different vehicular ETEGs based on engine type, engine‐rated power, type and number of TEG module, efficiency of ETEG and TEG, exhaust and coolant temperature, and power output of ETEG . In addition to these, the technical issues faced in these ETEGs are addressed under 2 categories, viz., primary (TEG with low ZT TE material and inefficient heat exchanger and heat sink) and secondary issues (low operating temperature TEG modules and installation position of ETEG). In addition to it, effects of vibration and thermal cycling of exhaust system on TEG modules that may arise in ETEG are also discussed. A review of preventive solutions to the issues is also presented. Finally, the economic aspects of an ETEG are also discussed. The review highlights the need of commercialization of TE materials with ZT > 2, high‐temperature operating range, and segmented TEG modules in large volumes so that their practice can be extended in vehicular applications. Heat exchanger modeling using computational fluid dynamics and interfacing with heat transfer theory is essential to maintain temperature uniformity across the TEG modules. Installation of ETEG in the exhaust pipe should be such that it does not affect the performance of the engine. It is also realized that sturdy TEG modules should be developed for long‐term operation to prevent degradation due to mechanical vibration and thermal cycling of the vehicle. Further, ETEG is economically beneficial in vehicles such as trucks owing to availability of high thermal energy in their exhaust stream.     

Evaluation of the power-generation capacity of wearable thermoelectric power generator

Employing thermoelectric generators (TEGs) to gather heat dissipating from the human body through the skin surface is a promising way to supply electronic power to wearable and pocket electronics. The uniqueness of this method lies in its direct utilization of the temperature difference between the environment and the human body, and complete elimination of power maintenance problems. However, most of the previous investigations on thermal energy harvesters are confined to the TEG and electronic system themselves because of the low quality of human energy. We evaluate the energy generation capacity of a wearable TEG subject to various conditions based on biological heat transfer theory. Through numerical simulation and corresponding parametric studies, we find that the temperature distribution in the thermopiles affects the criterion of the voltage output, suggesting that the temperature difference in a single point can be adopted as the criterion for uniform temperature distribution. However, the criterion has to be shifted to the sum of temperature difference on each thermocouple when the temperature distribution is inconsistent. In addition, the performance of the thermal energy harvester can be easily influenced by environmental conditions, as well as the physiological state and physical characteristics of the human body. To further validate the calculation results for the wearable TEG, a series of conceptual experiments are performed on a number of typical cases. The numerical simulation provides a good overview of the electricity generation capability of the TEG, which may prove useful in the design of future thermal energy harvesters.     

Evaluation of the power-generation capacity of wearable thermoelectric power generator

Employing thermoelectric generators (TEGs) to gather heat dissipating from the human body through the skin surface is a promising way to supply electronic power to wearable and pocket electronics. The uniqueness of this method lies in its direct utilization of the temperature difference between the environment and the human body, and complete elimination of power maintenance problems. However, most of the previous investigations on thermal energy harvesters are confined to the TEG and electronic system themselves because of the low quality of human energy. We evaluate the energy generation capacity of a wearable TEG subject to various conditions based on biological heat transfer theory. Through numerical simulation and corresponding parametric studies, we find that the temperature distribution in the thermopiles affects the criterion of the voltage output, suggesting that the temperature difference in a single point can be adopted as the criterion for uniform temperature distribution. However, the criterion has to be shifted to the sum of temperature difference on each thermocouple when the temperature distribution is inconsistent. In addition, the performance of the thermal energy harvester can be easily influenced by environmental conditions, as well as the physiological state and physical characteristics of the human body. To further validate the calculation results for the wearable TEG, a series of conceptual experiments are performed on a number of typical cases. The numerical simulation provides a good overview of the electricity generation capability of the TEG, which may prove useful in the design of future thermal energy harvesters.     

Experiments and simulations on low-temperature waste heat harvesting system by thermoelectric power generators

In this case study, a system to recover waste heat comprised 24 thermoelectric generators (TEG) to convert heat from the exhaust pipe of an automobile to electrical energy has been constructed. Simulations and experiments for the thermoelectric module in this system are undertaken to assess the feasibility of these applications. A slopping block is designed on the basis of simulation results to uniform the interior thermal field that improves the performance of TEG modules. Besides simulations, the system is designed and assembled. Measurements followed the connection of the system to the middle of an exhaust pipe. Open circuit voltage and maximum power output of the system are characterized as a function of temperature difference. Through these simulations and experiments, the power generated with a commercial TEG module is presented. Overview this case study and our previous work, the results establish the fundamental development of low-temperature waste heat thermoelectric generator system that enhances the TEG efficiency for vehicles.     

Thermal‐hydraulic design features of a micronuclear reactor power source applied for multipurpose

Microheat pipe cooled reactor power source (HRP) designed for space or underwater vehicles meets the future demands, such as safer structure, longer operating time, and fewer mechanical moving parts. In this paper, potassium heat pipe cooled reactor power source system which generates 50 kWe electricity is proposed. The reactor core using uranium nitride fuel is cooled by 37 potassium high‐temperature heat pipes. The shields are designed as tungsten and water, and reactor reactivity is controlled by control drums. The thermoelectric generator (TEG) consists of thermoelectric conversion units and seawater cooler. The thermoelectric conversion units convert thermal energy to electric energy through the high‐performance thermoelectric material. A code applied for designing and analyzing the reactor power system is developed. It consists of multichannel reactor core model, heat pipe model using thermal resistance network, thermoelectric conversion, and thermal conductivity model. Then, the sensitivity analysis is performed on two key parameters including the length of the heat pipe condensation section and the cold junction temperature of the TE cell. Meanwhile, the steady‐state calculations are conducted. Results show that the maximum fuel temperature is 938 K located in the center of reactor core and the outlet temperature of coolant reaches 316 K. Both of them are within the limitation. It is concluded that the preliminary design of HPR design is reasonable and reliable. The designed residual heat removal system has sufficient safety margin to release the decay heat of the reactor. This research provides valuable analysis for the application of micronuclear power source.     

Optimized thermal coupling of micro thermoelectric generators for improved output performance

There is a significant push to increase the output power of thermoelectric generators (TEGs) in order to make them more competitive energy harvesters. The thermal coupling of TEGs has a major impact on the effective temperature gradient across the generator and therefore the power output achieved. The application of micro fluidic heat transfer systems (μHTS) can significantly reduce the thermal contact resistance and thus enhance the TEG's performance. This paper reports on the characterization and optimization of a μTEG integrated with a two layer μHTS. The main advantage of the presented system is the combination of very low heat transfer resistances with small pumping powers in a compact volume. The influence of the most relevant system parameters, i.e. microchannel width, applied flow rate and the μTEG thickness on the system's net output performance are investigated. The dimensions of the μHTS/μTEG system can be optimized for specific temperature application ranges, and the maximum net power can be tracked by adjusting the heat transfer resistance during operation. A system net output power of 126 mW/cm² was achieved with a module ZT of 0.1 at a fluid flow rate of 0.07 l/min and an applied temperature difference of 95K.It was concluded that for systems with good thermal coupling, the thermoelectric material optimization should focus more on the power factor than on the figure of merit ZT itself, since the influence of the thermal resistance of the TE material is negligible.     

Numerical study of the inlet/outlet arrangement effect on microchannel heat sink performance

In this study, fluid flow and heat transfer in microchannel heat sinks are numerically investigated. The three-dimensional governing equations for both fluid flow and heat transfer are solved using the finite-volume scheme. The computational domain is taken as the entire heat sink including the inlet/outlet ports, inlet/outlet plenums, and microchannels. The particular focus of this study is the inlet/outlet arrangement effects on the fluid flow and heat transfer inside the heat sinks.The microchannel heat sinks with various inlet/outlet arrangements are investigated in this study. All of the geometric dimensions of these heat sinks are the same except the inlet/outlet locations. Because of the difference in inlet/outlet arrangements, the resultant flow fields and temperature distributions inside these heat sinks are also different under a given pressure drop across the heat sink. Using the averaged velocities and fluid temperatures in each channel to quantify the fluid flow and temperature maldistributions, it is found that better uniformities in velocity and temperature can be found in the heat sinks having coolant supply and collection vertically via inlet/outlet ports opened on the heat sink cover plate. Using the thermal resistance, overall heat transfer coefficient and pressure drop coefficient to quantify the heat sink performance, it is also found these heat sinks have better performance among the heat sinks studied. Based on the results from this study, it is suggested that better heat sink performance can be achieved when the coolant is supplied and collected vertically.     

Performance Assessment Comparison of Variable Fin Density Microchannels With Offset Configurations

Current technologies for the cooling of integrated circuits (IC) chips employ single-phase liquid flow through microchannel heat sinks. The surface temperature in these devices increases along the flow direction, leading to low heat transfer coefficients toward the channel exit. Enhancement of the heat transfer coefficient while mitigating the temperature nonuniformity has been possible with the utilization of variable fin density microchannels. A microchannel cooling layer suitable for three-dimensional (3D) stacking of IC chips is studied in this paper. The effect of the variation of geometrical parameters and operating conditions on the overall performance of the cooling layer is reported. The overall performance of eight different configurations presenting offset strip fins with variable fin density has been evaluated and compared to two cases with smooth rectangular microchannels, one case for each channel height. The cooling layer, with a length of 8 mm and a width of 7.92 mm, dissipates a total thermal power of 200 W by means of pumping coolant through 12 flow channels. Results from this investigation demonstrate the capability of the proposed configurations to decrease the surface temperature nonuniformities and to maintain the surface temperature below the allowable limits for IC chips while achieving low pressure drops. A parameter for the overall performance assessment, named the overall performance index, has been proposed; by computing and properly comparing the values for this parameter, the best performing configuration is identified.     

Thermal analysis of high‐power lithium‐ion battery packs using flow network approach

High‐power applications of lithium‐ion batteries require efficient thermal management systems. In this work, a lumped capacitance heat transfer model is developed in conjunction with a flow network approach to study performance of a commercial‐size lithium‐ion battery pack, under various design and operating conditions of a thermal management system. In order to assess the battery thermal management system, capabilities of air, silicone oil, and water are examined as three potential coolant fluids. Different flow configurations are considered, and temperature dispersions, cell‐averaged voltage distributions, and parasitic losses due to the fan/pump power demand are calculated. It is found that application of a coolant with an appropriate viscosity and heat capacity, such as water, in conjunction with a flow configuration with more than one inlet will result in uniform temperature and voltage distributions in the battery pack while keeping the power requirement at low, acceptable levels. Simulation results are presented and compared with literature for model validation and to show the superior capability of the proposed battery pack design methodology. Copyright © 2014 John Wiley & Sons, Ltd.     

A 500 W low-temperature thermoelectric generator: Design and experimental study

Most of the current thermal power-generation technologies must first convert thermal energy to mechanical work before producing electricity. In this study, a direct heat to electricity (DHE) technology using the thermoelectric effect, without the need to change through mechanical energy, was applied to harvest low-enthalpy thermal work. Such a power generation system has been designed and built using thermoelectric generator (TEG) modules. Experiments have been conducted to measure the output power at different conditions: different inlet temperature and temperature differences between hot and cold sides. TEG modules manufactured with different materials have also been tested. The power generator assembled with 96 TEG modules had an installed power of 500 W at a temperature difference of around 200 °C. An output power of over 160 W has been generated with a temperature difference of 80 °C. The power generated by the thermoelectric system is almost directly proportional to the temperature difference between the hot and the cold sides. The cost of the DHE power generator is lower than that of photovoltaics (PV) in terms of equivalent energy generated.     

Experimental investigations of flow boiling heat transfer and pressure drop in straight and expanding microchannels – A comparative study

Flow boiling experiments were conducted in straight and expanding microchannels with similar dimensions and operating conditions. Deionized water was used as the coolant. The test vehicles were made from copper with a footprint area of 25 mm × 25 mm. Microchannels having nominal width of 300 μm and a nominal aspect ratio of 4 were formed by wire cut Electro Discharge Machining process. The measured surface roughness (Ra) was about 2.0 μm. To facilitate easier comparison with the straight microchannels and also to simplify the method of fabrication, the expanding channels were formed with the removal of fins at selected location from the straight microchannel design, instead of using a diverging channel. Tests were performed on both the microchannels over a range of mass fluxes, heat fluxes and an inlet temperature of 90 °C. It was observed that the two-phase pressure drop across the expanding microchannel heat sink was significantly lower as compared to its straight counterpart. The pressure drop and wall temperature fluctuations were seen reduced in the expanding microchannel heat sink. It was also noted that the expanding microchannel heat sink had a better heat transfer performance than the straight microchannel heat sink, under similar operating conditions. This phenomenon in expanding microchannel heat sink, which was observed in spite of it having a lower convective heat transfer area, is explained based on its improved flow boiling stability that reduces the pressure drop oscillations, temperature oscillations and hence partial dry out.     

Numerical study of a solar thermoelectric generator with nanofluids based microcooling system

Abstract

Nanofluids have been recently gaining ever-increasing attention in solar thermoelectric applications due to their promising potentials as heat transfer fluids. This research investigates numerically the performance of a thermoelectric generator (TEG) that is cooled by Al₂O₃/water nanofluid flows in zigzag microchannel heat sinks (ZMCHS). The one-way fluid–structure interaction (FSI) tool was used to couple the thermal-electric and fluid flow tools in ANSYS 15.0. The present study focused on the effects of heat flux (2–50?kW/m²), laminar Reynolds number (5–1500), inlet flow temperature (293–303?K) and the nanoparticle concentration (1–6%) on the output electric power and the efficiency of the TEG module. The applied heat flux limitations and its relation to the thermal limitations of thermoelectric materials were considered. The results indicated that the increase of heat flux increased the output power and the efficiency of TEG. Higher Reynolds numbers (Re > 400), inlet temperature and nanofluid concentration had an insignificant impact on the TEG performance.     

Critical Heat Flux During Flow Boiling in Microchannels: A Parametric Study

The application of flow boiling in microchannels in copper cooling elements for very high heat flux dissipation from microprocessor chips is one of the promising technologies to replace air cooling and water cooling of these units, particularly in mainframes and servers. Recently, the authors have proposed a new theoretical model to predict the critical heat flux (CHF) in microchannels, and it is used here to perform a parametric study to investigate the effects of fluid, saturation temperature, mass flux, inlet subcooling, microchannel diameter, and heated length on CHF for this application. The parametric study shows that CHF is increased by: (i) decreasing channel length, (ii) lowering saturation temperature, (iii) increasing mass flux, (iv) increasing inlet subcooling, and (v) increasing microchannel diameter. The best coolant is water, but water is not feasible for the present application because of its very low saturation pressure at 30–40°C. Of the other four fluids simulated, their order of merit from best to worst is as follows: R-245fa, R-134a, R-236fa, and FC-72. FC-72, however, has a low saturation pressure (in fact, it would operate under vacuum at the saturation temperatures of 30–40°C envisioned here) and is not a candidate fluid for the flow boiling coolant here. Furthermore, the authors have also recently proposed a diabatic flow map for microchannels based on their database for R-134a and R-245fa in 0.5- and 0.8-mm channels. The new CHF model has been incorporated into their map here to predict the transition from annular flow to dry-out, which is a critical design limitation for microprocessor coolers. Importantly, this map then provides the feasible operating range of such coolers with flow boiling as the cooling process, in terms of mass flux and maximum vapor quality at the outlet to avoid CHF.     

Enhancement of heat transport in thermosyphon air preheater at high temperature with binary working fluid: A case study of TEG–water

In this research, the critical heat flux (CHF) due to flooding limit of thermosyphon heat pipe using triethylene glycol (TEG)–water mixture has been investigated. From the experiment it is found that, use of TEG–water mixture can extend the heat transport limitation compared with pure water and higher heat transfer is obtained compared with pure TEG at high temperature applications. Moreover it is found that ESDU equation is appropriate to predict the CHF of the thermosyphon in case of TEG–water mixture.For thermosyphon air preheater at high temperature applications, it is found that with selected mixture content of TEG–water in each row of the thermosyphon the performance of the system could be increased approximately 30–80% compared with pure TEG for parallel flow and 60–115% for counter flow configurations. The performances also increase approximately 80–160% for parallel flow and 140–220% for counter flow compared with those of pure dowtherm A which is the common working fluid at high temperature applications.     

Coupled thermal model of photovoltaic-thermoelectric hybrid panel for sample cities in Europe

In general, modeling of photovoltaic-thermoelectric (PV/TEG) hybrid panels have been mostly simplified and disconnected from the actual ambient conditions and thermal losses from the panel. In this study, a thermally coupled model of PV/TEG panel is established to precisely predict performance of the hybrid system under different weather conditions. The model takes into account solar irradiation, wind speed and ambient temperature as well as convective and radiated heat losses from the front and rear surfaces of the panel. The model is developed for three sample cities in Europe with different weather conditions. The results show that radiated heat loss from the front surface and the convective heat loss due to the wind speed are the most critical parameters on performance of the hybrid panel performance. The results also indicate that, with existing thermoelectric materials, the power generation by the TEG is insignificant compared to electrical output by the PV panel, and the TEG plays only a small role on power generation in the hybrid PV/TEG panel. However, contribution of the TEG in the power generation can be improved via higher ZT thermoelectric materials and geometry optimization of the TEG.