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.     

High temperature solar thermoelectric generator

The performance of a sintered SiGe thermoelectric generator is assessed by computing power output per unit area and the efficiency of thermoelectric conversion. Previously developed high temperature solar energy absorbers were used. The power output and efficiency are considerably improved by using a water- or vapour-cooled heat sink in place of a radiative heat sink. The power density in the thermoelectric generator compares well with those in photovoltaic heterojunction cells using concentrated solar energy.     

Effect of Mach number on thermoelectric performance of SiC ceramics nose-tip for supersonic vehicles

This paper focus on the effects of Mach number on thermoelectric energy conversion for the limitation of aero-heating and the feasibility of energy harvesting on supersonic vehicles. A model of nose-tip structure constructed with SiC ceramics is developed to numerically study the thermoelectric performance in a supersonic flow field by employing the computational fluid dynamics and the thermal conduction theory. Results are given in the cases of different Mach numbers. Moreover, the thermoelectric performance in each case is predicted with and without Thomson heat, respectively. Due to the increase of Mach number, both the temperature difference and the conductive heat flux between the hot side and the cold side of nose tip are increased. This results in the growth of the thermoelectric power generated and the energy conversion efficiency. With respect to the Thomson effect, over 50% of total power generated converts to Thomson heat, which greatly reduces the thermoelectric power and efficiency. However, whether the Thomson effect is considered or not, with the Mach number increasing from 2.5 to 4.5, the thermoelectric performance can be effectively improved.     

Effects of operations and structural parameters on the one-cell stack performance of planar solid oxide fuel cell

A three-dimensional mathematical model coupling the electrochemical kinetics with fluid dynamics is developed to simulate the heat and mass transfer in the one-cell stack of planar solid oxide fuel cells (SOFCs). Based on flow uniformity analysis, the distributions of temperature, current density, overpotential loss and other performance parameters in various operating parameters are obtained using a commercial CFD code (Fluent) coupled with the external subroutines programmed by VC++. Numerical flow data are observed in good agreement with experimental results reported in the literature. Results show that the one-cell stack in counter flow case has the advantages in better uniform current density and temperature distributions of PEN (Positive/Electrolyte/Negative) structure in the width direction, higher power output, fuel utilization factor and fuel efficiency than that in co-flow case. For counter flow case, better thermoelectric characteristics are observed in the temperature gradient, power output, fuel utilization factor and fuel efficiency with the decrease in the fuel inlet flow rate or the anode porosity. Increasing the air inlet flow rate and decreasing the fuel inlet temperature will reduce the temperature gradient; power output, fuel utilization factor and fuel efficiency are enhanced with the increase of the air inlet temperature and the decrease of the anode pore size and thickness.     

A numerical model for thermoelectric generator with the parallel-plate heat exchanger

This paper presents a numerical model to predict the performance of thermoelectric generator with the parallel-plate heat exchanger. The model is based on an elemental approach and exhibits its feature in analyzing the temperature change in a thermoelectric generator and concomitantly its performance under operation conditions. The numerical simulated examples are demonstrated for the thermoelectric generator of parallel flow type and counter flow type in this paper. Simulation results show that the variations in temperature of the fluids in the thermoelectric generator are linear. The numerical model developed in this paper may be also applied to further optimization study for thermoelectric generator.     

Thermoelectric performance using counter-flowing thermal fluids

Counter-flowing thermal fluids are conducive to generate a homogeneous temperature difference on thermoelectric (TE) generator. This study allowed the hot and cold fluids of having constant inlet temperature to flow in the opposite, and examined TE performance of module at different flow rates. The results show that TE performance gradually increases with flow rate in the initial stage of fluid flow, and reaches a transient peak value after the module surfaces are completely covered by thermal fluids, and then tends to be stable. High flow rate leads to larger performance and reduces the time of achieving them. Effect of flow rate on stable performance is slightly more than that of inlet temperature of thermal fluids, which makes regulating the flow rate to be a feasible way to harvest more heat for TE conversion. Module features present a specific trend and provide the supports for the benefit of counter-flowing thermal fluids.     

Design and testing of a locally made loop-type thermosyphonic heat sink for stove-top thermoelectric generators

The performance of a thermoelectric generator, among other aspects, depends on the use of an effective heat sink. While forced cooling using either air or water (or other coolants) is efficient, it is parasitic on the generated power and/or bulky and inconvenient. Heat pipes are known to be highly effective heat transport devices. Coupled to a thermoelectric generator, these can be used to give acceptable power output. Basing the cooling on water gives low-cost, simplicity, safety, together with good performance. In this work, the design and general performance of a small single-module thermoelectric generator configured for a stovetop waste-heat application and coupled to a locally-made thermosyphonic loop-type heat sink was undertaken. Development and performance cctesting gave mixed results and further numerical and experimental study is under way.     

一种新型汽车尾气多通道热能发电系统

根据温差发电原理,设计了一种新型网格状通气管式的温差发电装置,实现对汽车尾气热能的再利用。通过优化温差发电装置的结构,改变了水箱结构,增加了废热通道数量,能够贴更多的温差发电片,从而提高转换效率。通过UG(计算机辅助设计软件)建立汽车尾气温差发电装置的理论模型,经过计算,当温差等于100℃时该装置的转换效率约等于5.67%。与其他温差发电装置进行比较,热油式温差发电器在260℃温差下最大热能转换效率可达4.389%,而汽车尾气温差发电器输出功率随着烟气温度的升高近似成线性递增,热能转换效率较低[1],通过比较得出,本装置不仅提高了转换效率,且达到相同转换效率时所对应的温差值也相应减少。     

Performance analysis of a hybrid solar thermoelectric generator

In this paper, a theoretical model is developed to investigate the performance of the hybrid solar thermoelectric generator (HSTEG) system, which is designed without (B-HSTEG) and with an evacuated glass tube (V-HSTEG). The heat loss, power output, thermal efficiency, and electrical efficiency of the B-HSTEG/V-HSTEG system are evaluated by analyzing the design parameters such as geometric solar concentration ratio, thermoelectric figure of merit, and cold-side inlet fluid temperature. The performance of the B-HSTEG is compared with the V-HSTEG system using two heat transfer fluids: water and Therminol VP-1. The maximum electrical efficiency of the B-HSTEG and V-HSTEG is estimated to be 12.2 and 15.6% (ZT = 3) with a corresponding thermal efficiency of about 61.9 and 60.3%, respectively. Overall, this paper provides a systematic performance analysis of HSTEG systems.     

非均质温差发电器的性能分析

建立非均质温差发电器(TEG)理论模型,考虑热电材料的非均质导热系数以及温差发电器与热源间的传热热阻的影响,分析非均质温差发电器的一般性能.讨论热电元件对数、热导率、高温热源温度对非均质温差发电器性能特性的影响.结果表明,相较于均质温差发电器,导热系数不均匀强度越大,非均质温差发电器的最大输出功率和最大效率越高;热电元...     

Parametric study of heat-transfer design on the thermoelectric generator system

This study developed an integral thermoelectric generator system with high-performance heat transfer and thermoelectric conversion functions, using the metal pin-fin array coupling with the forced convection heat transfer technique to be the heat absorber and heat sink. A one-dimensional steady heat conduction model with internal Joule heat generation and Seebeck effect was proposed to predict the power generation performance of the present thermoelectric system including the heat absorber and heat sink at various operation conditions. Critical heat-transfer parameters on the design of the integral thermoelectric generator system were derived and discussed. Finally, a series of systematical experiments were performed to simulate an integral thermoelectric generator system operating at the exhaust pipe. The experimental results also demonstrated the validity of the proposed theoretical model.     

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.     

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.     

基于改进贪心算法的温差电器件电-热耦合效应研究

温差电器件实际工作时由于内电阻的存在不可避免地会产生焦耳热,传统的温差电研究中虽注意到温差发电过程中的焦耳热现象,但只是在等效计算热功率时消去焦耳热部分,而忽略了焦耳热对温差电器件热、冷端温度分布的影响。针对传统研究的不足,考虑实际应用中的电-热耦合效应,运用理论推导的方法建立了第三类边界条件下的温差发电负载模型,并利用改进贪心算法迭代求解,最后以SP1848-21745型温差发电片为例,通过试验验证了模型与算法的正确性。模型的数值求解与发电片实测结果对比表明,考虑了电-热耦合效应的温差发电负载模型的热电输出值更接近实测值。     

Investigation into topping cycle: Thermal efficiency with and without presence of thermoelectric generator

Thermoelectric power generation due to solar heating is a current interest in green energy research. One of the applications of the thermoelectric power generator is involved with the topping cycle, in which the thermoelectric generators were placed on the heat collector elements of a conventional solar concentration power plant. Although the topping cycle is practical and easy to operate, the efficiency of the thermal system with and without thermoelectric generator needs to be examined. In the present study, thermal efficiency of the topping cycle is analyzed and compared with its counterpart without the presence of the thermoelectric elements. Thermodynamic analysis for the efficiency of both the systems with and without thermoelectric generator is presented. The fluid flow and heat transfer in a tube with presence of thermoelectric elements resembling the solar heating system incorporated in the topping cycle are simulated numerically. It is found that, for a certain combination of operating and thermoelectric device parameters, thermal efficiency of the topping cycle becomes slightly higher than that of the same system without the presence of the thermoelectric generators.     

Numerical simulation of nanostructured thermoelectric generator considering surface to surrounding convection

Thermoelectric systems (TE) can directly convert heat to electricity and vice-versa by using semiconductor materials. Therefore, coupling between heat transfer and electric field potential is important to predict the performance of thermoelectric generator (TEG) systems. This paper develops a general two-dimensional numerical model of a TEG system using nanostructured thermoelectric semiconductor materials. A TEG with p-type nanostructured material of Bismuth Antimony Telluride (BiSbTe) and n-type Bismuth Telluride (Bi₂Te₃) with 0.1 vol.% Silicon Carbide (SiC) nanoparticles is considered for performance evaluations. Coupled TE equations with temperature dependant transport properties are used after incorporating Fourier heat conduction, Joule heating, Seebeck effect, Peltier effect, and Thomson effect. The effects of temperature difference between the hot and cold junctions and surface to surrounding convective on different output parameters (e.g., thermal and electric fields, power generation, thermal efficiency, and current) are studied. Selected results obtained from current numerical analysis are compared with the results obtained from analytical model available in the literature. There is a good agreement between the numerical and analytical results. The numerical results show that as temperature difference increases output power and amount of current generated increase. Moreover, it is quite apparent that convective boundary condition deteriorates the performance of TEG.     

相变传热在半导体热电转换中的应用

为了解决温差发电技术中发电片热端与尾气间热损失大造成输出功率和热电转化效率不高的问题,提出在尾气与温差发电片热端加装相变传热结构,并计算了加装相变结构后发电器的输出功率和效率,同时与相同传热面积时无相变传热情况进行了对比,并模拟了蒸发段管数和冷凝段高度对发电器输出功率及效率的影响。结果表明,相变结构可提高发电器的输出功率及转化效率,且输出功率随冷凝段长度增加出现峰值,蒸发段管数越多,峰值对应的冷凝段长度越长,而发电效率则随冷凝段长度增加而减少;蒸发段管数增加,输出功率和发电效率均增大。     

Experimental investigation of a novel heat pipe thermoelectric generator for waste heat recovery and electricity generation

Waste heat recovery helps reduce energy consumption, decreases carbon emissions, and enhances sustainable energy development. In China, energy-intensive industries dominate the industrial sector and have significant potential for waste heat recovery. We propose a novel waste heat recovery system assisted by a heat pipe and thermoelectric generator (TEG) namely, heat pipe TEG (HPTEG)，to simultaneously recover waste heat and achieve electricity generation. Moreover, the HPTEG provides a good approach to bridging the mismatch between energy supply and demand. Based on the technical reserve on high-temperature heat pipe manufacturing and TEG device integration, a laboratory-scale HPTEG prototype was established to investigate the coupling performances of the heat pipes and TEGs. Static energy conversion and passive thermal transport were achieved with the assistance of skutterudite TEGs and potassium heat pipes. Based on the HPTEG prototype, the heat transfer and the thermoelectric conversion performances were investigated. Potassium heat pipes exhibited excellent heat transfer performance with 95% thermal efficiency. The isothermality of such a heat pipe was excellent, and the heat pipe temperature gradient was within 15°C. The TEG's thermoelectric conversion efficiency of 7.5% and HPTEG's prototype system thermoelectric conversion efficiency of 6.2% were achieved. When the TEG hot surface temperature reached 625°C, the maximum electrical output power of the TEG peaked at 183.2 W, and the open-circuit voltage reached 42.2 V. The high performances of the HPTEG prototype demonstrated the potential of the HPTEG for use in engineering applications.     

微小型热电转换装置冷热源的应用研究

微小型热电转换装置(Power MEMS)的质量/体积比功率除了与发电器性能有关外,还与热源和冷源的系统集成有关。分别采用液体高热值燃料、高贮能相变材料作为热源,用低温相变贮能材料作冷源,设计制造出热电转换装置,并通过实验测试进行了相关数据分析。分析了放射性同位素的优点和在热电发电中的应用现状,指出用其作为热源有助于热电装置的进一步微型化。实验结果也表明系统增加冷源助于装置微型化和提高热电输出功率。     

Power and efficiency optimization for combined Brayton and inverse Brayton cycles

A thermodynamic model for open combined Brayton and inverse Brayton cycles is established considering the pressure drops of the working fluid along the flow processes and the size constraints of the real power plant using finite time thermodynamics in this paper. There are 11 flow resistances encountered by the gas stream for the combined Brayton and inverse Brayton cycles. Four of these, the friction through the blades and vanes of the compressors and the turbines, are related to the isentropic efficiencies. The remaining flow resistances are always present because of the changes in flow cross-section at the compressor inlet of the top cycle, combustion inlet and outlet, turbine outlet of the top cycle, turbine outlet of the bottom cycle, heat exchanger inlet, and compressor inlet of the bottom cycle. These resistances control the air flow rate and the net power output. The relative pressure drops associated with the flow through various cross-sectional areas are derived as functions of the compressor inlet relative pressure drop of the top cycle. The analytical formulae about the relations between power output, thermal conversion efficiency, and the compressor pressure ratio of the top cycle are derived with the 11 pressure drop losses in the intake, compression, combustion, expansion, and flow process in the piping, the heat transfer loss to the ambient, the irreversible compression and expansion losses in the compressors and the turbines, and the irreversible combustion loss in the combustion chamber. The performance of the model cycle is optimized by adjusting the compressor inlet pressure of the bottom cycle, the air mass flow rate and the distribution of pressure losses along the flow path. It is shown that the power output has a maximum with respect to the compressor inlet pressure of the bottom cycle, the air mass flow rate or any of the overall pressure drops, and the maximized power output has an additional maximum with respect to the compressor pressure ratio of the top cycle. When the optimization is performed with the constraints of a fixed fuel flow rate and the power plant size, the power output and efficiency can be maximized again by properly allocating the fixed overall flow area among the compressor inlet of the top cycle and the turbine outlet of the bottom cycle.