Effect of Convection Heat Transfer on Performance of Waste Heat Thermoelectric Generator

Efficiency of energy conversion processes can be improved if waste heat is converted to electricity. A thermoelectric generator (TEG) can directly convert waste heat to electricity. The TEG typically suffers from low efficiency due to various reasons, such as ohmic heating, surface-to-surrounding convection losses, and unfavorable material properties. In this work, the effect of surface-to-surrounding convection heat transfer losses on the performance of TEG is studied analytically and numerically. A one-dimensional (1-D) analytical model is developed that includes surface convection, conduction, ohmic heating, and Peltier, Seebeck, and Thomson effects with top and bottom surfaces of TEG exposed to convective boundary conditions. Using the analytical solutions, different performance parameters (e.g., heat input, power output, and efficiency) are calculated and expressed graphically as functions of thermal source and sink temperatures and convection heat transfer coefficient. Finally, a two-dimensional (2-D) mathematical model is solved numerically to observe qualitative results of thermal and electric fields inside the TEG. For all calculations, temperature-dependent thermal/electric properties are considered. Increase in thermal source temperature results in an increase in the power output with adiabatic side wall conditions. A change in boundary condition to convection heat transfer from adiabatic boundary has a large impact on thermal efficiency.     

Effects of air excess control in a heat storage solid fuel-fired household furnace

Due to a significant increase in electricity prices during the last decade and insufficient production capacity of the electric power industry in Serbia, many households that are currently using electric heat storage furnaces for heating have been forced to find an alternative solution for heating. A possible solution is replacing electric heating appliances with similar solid fuel-fired ones. Existing solid fuel-fired furnaces are often unsatisfactory with respect to their efficiencies and flue gas emissions. A prototype of a new concept of heat storage, solid fuel-fired furnace has been developed to meet these growing needs, providing electricity saving together with considerable environmental benefits. In order to examine furnace performance, efficiency and environmental aspects, and to assess the influence of air excess control in the furnace on the efficiency and flue gas emissions, numerous experimental tests were conducted. The amount of combustion air, the flue gas flow rate and the fuel feeding regime have been adjusted in order to keep the flue gas oxygen content in a relatively narrow range, thus obtaining controlled combustion conditions and, correspondingly, lower carbon monoxide emission and higher furnace efficiency. In this way, the furnace was made able to respond to the changes in heating needs, fuel quality and other parameters, which is considered to be advantageous in comparison with similar solid-fuel fired furnaces.     

基于太阳能光伏热利用的光伏/热电(PV/TV)建筑一体化试验研究

将太阳能电池板、集热器、热电发电片结合起来,设计并制成了一套光伏/热电(PV/TV)系统,在利用太阳能电池发电的同时,收集热量并利用其发电。在北京地区进行了该系统的室外模拟试验,测试并讨论了该系统在不同结构和不同环境下的性能,探讨该系统在光伏建筑中的应用。试验结果表明,与单纯的光伏发电系统或太阳能热水系统相比,PV/TV系统具有占地面积小、综合效率高等优点。     

Heat pipes thermoelectric solar collectors for energy applications

This study investigates the load characteristics of heat pipe thermoelectric solar collector (HPTSC) in practice. Heat pipe thermoelectric solar collector converts the heat generated by the Sun directly into electrical energy and produces hot water as well. The maximum power in HPTSC is obtained when the internal resistance of the thermoelectric module is equal to the load resistance. It has been observed to be possible to produce both hot water and electricity by improving available solar collectors or producing new generation HPTSC. While it is possible to generate an electrical power of 160 W from a HPTSC of one square meter using the thermoelectric method, the power produced with an average photovoltaic panel with the same area is only 132 W. Accordingly, HPTSC is a superior alternative not only to available solar collectors, but also to available PV panels. HPTSC, involving three different technologies, is environmentally friendly and certainly a product that allows for more efficient use of solar energy.     

Evaluation of thermoelectric modules for power generation

A procedure is developed to assess the potential of thermoelectric modules when used for electrical power generation. The generating performance of a thermoelectric module is evaluated in terms of its power output, conversion efficiency and reliability, while the potential for improving its performance is investigated based upon the power-per-area, cost-per-watt and manufacture quality factor. The methods employed in determining these parameters are described and used to evaluate several commercially available modules. The results show that a thermoelectric module is a promising device for low temperature waste heat recovery.     

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.     

Thermoelectric Module Integrated Fuel Cell in a Liquid Desiccant-Assisted Air-Conditioning System

Abstract

This study aims to estimate the energy performance of a liquid desiccant and evaporative cooling-assisted 100% outdoor air system (LD-IDECOAS) combined with a thermoelectric module integrated proton exchange membrane fuel cell (TEM-PEMFC). During the cooling season, recovered heat from the PEMFC was reclaimed to heat a weak desiccant solution and the generated electricity was used to operate the LD-IDECOAS. The TEM was operated as an auxiliary heater for heating the weak desiccant solution. In the off-cooling season, the PEMFC was operated to generate electricity and the recovered heat was also used to generate electricity using TEMs. In this study, a detailed energy simulation model was developed to estimate the energy savings potentials of the proposed system compared with the conventional LD-IDECOAS that uses a gas boiler and grid power without TEM-PEMFC. The result shows that TEMs can operate with a mean coefficient of performance of 2.0 when utilized for auxiliary heater in the cooling season. In addition, TEMs generate additional electricity with a mean power generation efficiency of 0.9%. Finally, the proposed system can save the 10.6% of annual primary energy compared with the conventional LD-IDECOAS. Therefore, the advantages of using TEM-PEMFC as heating and energy harvesting components were verified.     

汽车发动机尾气余热温差发电装置结构研究

半导体温差发电技术在低品位余热回收技术领域具有重要的应用价值。汽车尾气温度高,带走的热量约占发动机总量的40%,温差发电技术能直接将废热能量转化为电能回收利用。介绍温差发电装置的设计原理,结构参数对性能影响以及装置输出性能参数,并结合试验对温差发电装置的传热性能和电功率输出性能进行分析以及提出有效的改进方案。     

Experimental study on a new hybrid photovoltaic thermal collector

A calorific energy is generated during the photovoltaic conversion of the solar module which increases the temperature of the cell and will causes a fall of its electric output. This phenomenon is due, on one hand to the partial unabsorptive solar radiation which constituted the origin of the cells heating and on the other hand, with the Joule effect caused by the passage of the photo electrical current generated in the external circuit. This heating, harmful for the photovoltaic cells output involved many research efforts to limit its effects by evacuating this heat. There was also the idea to exploit this phenomenon by the combination of the photovoltaic module with a thermal system to form the photovoltaic-thermal hybrid collector (PVT) which will generate at the same time electricity and heat. In this paper we described the design of a new type of PVT collector through its experimental study. This novel collector constitutes a new technical approach to maximize the total output of conversion with lower cost compared to the traditional hybrid collectors.     

Experimental investigation of the performance of a liquid fuel-fired porous burner operating on kerosene-vegetable cooking oil (VCO) blends for micro-cogeneration of thermoelectric power

Studies related to porous burner for thermoelectric (TE) power generation have mainly focused toward achieving a specific range of power output for various applications. However, detailed analyses on the performance and emission aspects of the porous burner are lacking. In addition, physical integration between the burner and TE modules has added further complexity in this research area. Thus, this work aims to comprehend the effects of fuel–air equivalence ratio on the performance and emission characteristics of a liquid fuel-fired porous burner for micro-cogeneration of TE power. A catalytically inert Al₂O₃ porous medium was incorporated into a liquid fuel-fired porous burner operating on four mixtures of kerosene-vegetable cooking oil (VCO) blends: 100 kerosene, 90/10 KVCO, 75/25 KVCO, and 50/50 KVCO. Ten bismuth-telluride TE cells were arranged in a ten-sided polygon that, together with finned dissipators, formed a TE module electrically connected in series but thermally connected in parallel. The performance aspects at various fuel–air equivalence ratios were thoroughly evaluated with the corresponding temperature profiles, voltage, current, power output, and electrical efficiency. Results indicated that the surface temperature of the porous media was generally higher than the developed and exit flame temperature of the burner. Varying the fuel-air equivalence ratio significantly affected the electrical efficiency, with a maximum and minimum value of 1.94% and 1.10%, respectively. The power output steadily increased in the lean region, but stabilized as the fuel–air equivalence ratio slowly increased beyond the stoichiometric ratio. The CO emission was relatively lower at the lean region; however, significant amount was recorded in the rich combustion region. Moreover, NOx fluctuated between 1 ppm and 4 ppm over the entire range of fuel–air equivalence ratio.     

两级热电热机的有限时间热力学分析

建立了考虑外部传热影响的两级半导体热电热机模型,用有限时间热力学对牛顿传热规律下两级半导体热电热机的性能进行分析,导出了功率、效率与工作电流的一般关系式,得到了两侧换热器的最优面积分配和热电单元数的最优分配,并分析了多种因素对其性能的影响。     

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.     

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。     

基于单层MNCl(M=Zr,Hf)的温差发电模型仿真分析

在研究单层ZrNCl和HfNCl材料热电性能的基础上,搭建温差发电模型,研究不同规格温差发电模块的输出性能,然后与其他学者研究的温差发电模型及热电材料的热电转换效率进行对比分析.结果表明:在低温区和中温区,单层ZrNCl的热电转换效率更高.温差发电模块的输出功率随温差发电模块横截面积和热电单元对数的增大而增大.单层Zr...     

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.     

应用热化理论,实现节汽、增电双重效益

以热化理论为基础,从自行推导的三个宏观经济、技术指标入手,初浅地说明热化指标的应用意义和作用。经过生产实践检验,突破了背压机组绝对“以热定电”的束缚,促使石化企业自备热电站进一步提高全厂热电联产中热能体系的“功效率”,在热能的生产过程中最大限度地换取廉价的电能,同时获取节汽、增电的双重效益。     

Performance analysis of a double-pass thermoelectric solar air collector

The thermoelectric (TE) solar air collector, sometimes known as the hybrid solar collector, generates both thermal and electrical energies simultaneously. A double-pass TE solar air collector has been developed and tested. The TE solar collector was composed of transparent glass, air gap, an absorber plate, thermoelectric modules and rectangular fin heat sink. The incident solar radiation heats up the absorber plate so that a temperature difference is created between the thermoelectric modules that generates a direct current. Only a small part of the absorbed solar radiation is converted to electricity, while the rest increases the temperature of the absorber plate. The ambient air flows through the heat sink located in the lower channel to gain heat. The heated air then flows to the upper channel where it receives additional heating from the absorber plate. Improvements to the thermal and overall efficiencies of the system can be achieved by the use of the double-pass collector system and TE technology. Results show that the thermal efficiency increases as the air flow rate increases. Meanwhile, the electrical power output and the conversion efficiency depend on the temperature difference between the hot and cold side of the TE modules. At a temperature difference of 22.8 °C, the unit achieved a power output of 2.13 W and the conversion efficiency of 6.17%. Therefore, the proposed TE solar collector concept is anticipated to contribute to wider applications of the TE hybrid systems due to the increased overall efficiency.     

Design optimization of thermoelectric devices for solar power generation

We present an improved theoretical model of a thermoelectric device which has been developed for geometrical optimization of the thermoelectric element legs and prediction of the performance of an optimum device in power generation mode. In contrast to the currently available methods, this model takes into account the effect of all the parameters contributing to the heat transfer process associated with the thermoelectric device.The model is used for a comparative evaluation of four thermoelectric modules. One of these is commercially available and the others are assumed to have an optimum geometry but with different design parameters (thermal and electrical contact layer properties).Results from the model are compared with experimental data of the commercial thermoelectric module in power generation mode with temperature gradient consistent with those achievable from a solar concentrator system. These show that it is important to have devices optimized specifically for generation, and to improve the contact layer of the thermoelements accordingly.     

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.