A General Model for the Electric Power and Energy Efficiency of a Solar Thermoelectric Generator

A general model for the electric power and energy efficiency of a solar thermoelectric generator is discussed, considering the influences of the input energy, the thermal conductivity, the absorptivity and emissivity of the heat collector, and the cooling water. The influences of these factors on the performance of the thermoelectric device are discussed, considering the thermoelectric generator as a whole, including the heat collector, the thermoelectric device, and the cooling. Results show that high input energy, and high absorptivity and low emissivity of the heat collector, are helpful for obtaining a high-performance thermoelectric generator. A high thermal transfer coefficient of the cooling water can increase the temperature difference across the thermoelectric device but results in greater accessory power requirements if increased further.     

Design and Numerical Simulation of a Symbiotic Thermoelectric Power Generation System Fed by a Low-Grade Heat Source

All liquid heating systems, including solar thermal collectors and fossil-fueled heaters, are designed to convert low-temperature liquid to high-temperature liquid. In the presence of low- and high-temperature fluids, temperature differences can be created across thermoelectric devices to produce electricity so that the heat dissipated from the hot side of a thermoelectric device will be absorbed by the cold liquid and this preheated liquid enters the heating cycle and increases the efficiency of the heater. Consequently, because of the avoidance of waste heat on the thermoelectric hot side, the efficiency of heat-to-electricity conversion with this configuration is better than that of conventional thermoelectric power generation systems. This research aims to design and analyze a thermoelectric power generation system based on the concept described above and using a low-grade heat source. This system may be used to generate electricity either in direct conjunction with any renewable energy source which produces hot water (solar thermal collectors) or using waste hot water from industry. The concept of this system is designated “ELEGANT,” an acronym from “Efficient Liquid-based Electricity Generation Apparatus iNside Thermoelectrics.” The first design of ELEGANT comprised three rectangular aluminum channels, used to conduct warm and cold fluids over the surfaces of several commercially available thermoelectric generator (TEG) modules sandwiched between the channels. In this study, an ELEGANT with 24 TEG modules, referred to as ELEGANT-24, has been designed. Twenty-four modules was the best match to the specific geometry of the proposed ELEGANT. The thermoelectric modules in ELEGANT-24 were electrically connected in series, and the maximum output power was modeled. A numerical model has been developed, which provides steady-state forecasts of the electrical output of ELEGANT-24 for different inlet fluid temperatures.     

Robust Finite Element Model for the Design of Thermoelectric Modules

Thermoelectric devices for power generation have been receiving increased attention as an emerging sustainable energy technology because of recent advances in thermoelectric materials and the tremendous thermal resources available. Little focus has been given to the effective implementation of thermoelectric materials in power generation modules and efficient module design. With recent exploration into new module configurations, it is imperative that a comprehensive model be developed as a design tool. A new three-dimensional, device-level, multiphysics modeling technique is developed for the purposes of designing and evaluating thermoelectric module configurations. Using the new model, we identify and explore several geometric parameters which are critical to module performance. The impact on device performance of solder, ceramic interface, and electrical contact thickness, as well as the leg spacing, is evaluated for a standard unicouple configuration. Results are compared to the standard one-dimensional constant property models commonly used in thermoelectric module design.     

Thermal Modeling of a Hybrid Thermoelectric Solar Collector with a Compound Parabolic Concentrator

In this study radiant light from the sun is used by a hybrid thermoelectric (TE) solar collector and a compound parabolic concentrator (CPC) to generate electricity and thermal energy. The hybrid TE solar collector system described in this report is composed of transparent glass, an air gap, an absorber plate, TE modules, a heat sink to cool the water, and a storage tank. Incident solar radiation falls on the CPC, which directs and reflects the radiation to heat up the absorber plate, creating a temperature difference across the TE modules. The water, which absorbs heat from the hot TE modules, flows through the heat sink to release its heat. The results show that the electrical power output and the conversion efficiency depend on the temperature difference between the hot and cold sides of the TE modules. A maximum power output of 1.03 W and a conversion efficiency of 0.6% were obtained when the temperature difference was 12°C. The thermal efficiency increased as the water flow rate increased. The maximum thermal efficiency achieved was 43.3%, corresponding to a water flow rate of 0.24 kg/s. These experimental results verify that using a TE solar collector with a CPC to produce both electrical power and thermal energy seems to be feasible. The thermal model and calculation method can be applied for performance prediction.     

热电堆型激光功率计设计与仿真

针对紫外、可见、红外等激光器输出功率测试需求,提出一种由传热体、吸收层、绝缘层和热电偶构成的热电堆型激光功率计.结合热效应和塞贝克效应理论,采用Solidworks三维设计软件构建不同关键结构尺寸的模型,通过ANSYS Workbench仿真软件建立了热电耦合仿真分析模型,分析关键结构尺寸参数对输出电压以及温度分布的影...     

延伸波长InGaAs探测器封装用二级热电制冷器性能研究

真空封装技术是延伸波长InGaAs探测器的主要封装方法之一,热电制冷器可为延伸波长InGaAs探测器焦平面提供低温环境。测试了基于真空封装技术无热负载条件下二级热电制冷器的性能,研究了二级热电制冷器在不同输入电流(功率)时冷、热端温差与热负载的关系,测试了二级热电制冷器在低温工况下的制冷性能以及二级热电制冷器处于不工作状态时的表观热导率。结果表明,热沉温度为274 K时,冷端可以达到221.4 K并实现77.5 K的冷、热端温差;当输入电流一定时,随着热负载的增加,冷、热端温差呈线性趋势减小,且斜率随着输入电流增大而增大;二级热电制冷器冷、热端温差在较高温度时更大,即制冷性能更好;当温度分别为233.1 K 和249.8 K时,表观热导率分别为11.30 W/(m·K)和8.29 W/(m·K)。     

Encapsulated Thermoelectric Modules and Compliant Pads for Advanced Thermoelectric Systems

The described encapsulated thermoelectric (TE) module consists of a vacuum-tight stainless-steel container with dimensions of 55 mm × 50 mm × 11 mm in which a SiGe or BiTe TE module is placed. This construction enables maximum performance and durability because: (1) the thermal expansion mismatch between the hot and cold sides of the container can be accommodated by a sliding sheet that is present in the container; (2) the TE module inside is always kept in a vacuum environment so that oxidation cannot occur; and (3) the difference in pressure between the inside and outside of the container reduces the thermal contact resistance inside the container. The design also includes a compliant pad made of porous material that is infiltrated with braze filler material. If heated to a temperature above the melting temperature of the braze filler, the thermal gap conductance of the interface is enhanced due to the strong affinity of the braze filler to an adjacent member. It is possible for this interface to slide as long as the braze filler is in the liquid state. This design strategy provides high-flux, direct conduction paths to the heat source and heat sink as well as a large temperature gradient across the TE module itself (1.7 times that in the case of the conventional design strategy); therefore, the module can provide a power output that is three times greater than that of a conventional module.     

Thermoelectric Analysis for Helical Power Generation Systems

The performance of a three-dimensional helical thermoelectric generation (TEG) system is examined by exposing it to a temperature difference with hot and cold sources. The helical paths for the two thermal fluids give the TEG device the potential to efficiently convert thermal energy. The characteristic performance of the helical system is numerically analyzed by using the finite-volume method in a compact system. The helical system is compared with a straight system in which all the thermoelectric (TE) elements present equivalent geometry. The difference in the TE performance between the two systems is not significant when the TE surfaces are maintained at constant temperatures. Both the electromotive force and the current in the TEG system increase linearly with the temperature difference ΔT applied at the two module surfaces. The current preferentially flows through a main path determined by the geometry of the TE element. The merits of the helical design are its compactness, space saving, and smooth fluid flow due to gravity, compared with the straight system.     

A Four-Quadrant Operation Diagram for Thermoelectric Modules in Heating–Cooling Mode and Generating Mode

The operation of a thermoelectric module in heating–cooling mode, generating mode, and regenerating mode can be discussed in terms of power, cooling load, and current. A direct current machine in motoring mode and generating mode and an induction motor in motoring mode and regenerating mode are analogous to thermoelectric modules. Therefore, the first objective of this work is to present the four-quadrant (4-Q) operation diagram and the 4-Q equivalent circuits of thermoelectric modules in heating–cooling mode and generating mode. The second objective is to present the cooling and regenerating curves of a thermoelectric module in cooling mode and regenerating mode. The curves are composed from the cooling powers and the generating powers, the input and output current, the thermal resistance of the heat exchanger, and the different temperatures that exist between the hot and cold sides of the thermoelectric module. The methodology used to present the data involved drawing analogies between the mechanical system, the electrical system, and the thermal system; an experimental setup was also used. The experimental setup was built to test a thermoelectric module (TE₂) in cooling mode and regenerating mode under conditions in which it was necessary to control the different temperatures on the hot and cold sides of TE₂. Two thermoelectric modules were used to control the temperature. The cold side was controlled by a thermoelectric module labeled TE₁, whereas the hot side was controlled by a second thermoelectric module labeled TE₃. The results include the power, the cooling load, and the current of the thermoelectric module, which are analogous to the torque, the power, the speed, and the slip speed of a direct current machine and an induction motor. This 4-Q operation diagram, the 4-Q equivalent circuits, and the cooling and regenerating curves of the thermoelectric module can be used to analyze the bidirectional current and to select appropriate operating conditions in the cooling and regenerating modes.     

Evaluation of the Thermoelectric Module Consisting of W-Doped Heusler Fe2VAl Alloy

Power generation performance of a thermoelectric module consisting of the Heusler Fe₂VAl alloy was evaluated. For construction of the module, W-doped Fe₂VAl alloys were prepared using powder metallurgy process. Power generation tests of the module consisting of 18 pairs of p–n junctions were conducted on a heat source of 373–673 K in vacuum. The reduction of thermal conductivity and improvement of thermoelectric figure of merit by W-doping enhanced the conversion efficiency and the output power. High output power density of 0.7 W/cm² was obtained by virtue of the high thermoelectric power factor of the Heusler alloy. The module exhibited good durability, and the relatively high output power was maintained after temperature cycling test in air.     

Thermoelectric Energy Harvesting from Transient Ambient Temperature Gradients

We examine a thermoelectric harvester that converts electrical energy from the naturally occurring temperature difference between ambient air and large thermal storage capacitors such as building walls or the soil. For maximum power output, the harvester design is implemented in two steps: source matching of the thermal and electrical interfaces to the energy source (system level) followed by load matching of the generator to these interfaces (subsystem level). Therefore, we measure thermal source properties such as the temperature difference, the air velocity, and the cutoff frequency in two application scenarios (road tunnel and office building). We extend a stationary model of the harvester into the time domain to account for transient behavior of the source. Based on the model and the source measurements, we perform the source and load matching. The resulting harvester consists of a pin fin heat sink with a thermal resistance of 6.2?K/W and a cutoff frequency 2.5?times greater than that of the source, a thermoelectric generator, and a DC/DC step-up converter starting at a total temperature difference of only ??T?=?1.2?K. In a final road tunnel field test, this optimized harvester converts 70?mJ of electrical energy per day without any direct solar irradiation. The energy provided by the harvester enables 415?data transmissions from a wireless sensor node per day.     

热管式半导体热电冷水机参数影响分析与性能优化

利用热管具有高效的导热能力,而半导体制冷具有无噪音、环保、制冷迅速等独特的优点,提出了一种基于热管散热的半导体热电冷水机结构.基于有限时间热力学理论,考虑包括汤姆逊效应在内的各种内部效应,建立了详细的计算模型.分析了关键运行参数和设计参数对装置最优电流和最优性能的影响,并给出了协调最大制冷率和最大制冷系数的最优参数区间.优化结果表明:当温差为20 K、电流为2.5A时,最大制冷率和最大制冷系数分别可达29.49W和1.47,相比于优化前,分别提升了55.3％和47.0％.     

Design,Modeling, Fabrication,and Evaluation of Thermoelectric Generators with Hot-Wire Chemical Vapor Deposited Polysilicon as Thermoelement Material

This paper presents the design, modeling, fabrication, and evaluation of thermoelectric generators (TEGs) with p-type polysilicon deposited by hot-wire chemical vapor deposition (HWCVD) as thermoelement material. A thermal model is developed based on energy balance and heat transfer equations using lumped thermal conductances. Several test structures were fabricated to allow characterization of the boron-doped polysilicon material deposited by HWCVD. The film was found to be electrically active without any post-deposition annealing. Based on the tests performed on the test structures, it is determined that the Seebeck coefficient, thermal conductivity, and electrical resistivity of the HWCVD polysilicon are 113 μV/K, 126 W/mK, and 3.58 × 10^?5 Ω m, respectively. Results from laser tests performed on the fabricated TEG are in good agreement with the thermal model. The temperature values derived from the thermal model are within 2.8% of the measured temperature values. For a 1-W laser input, an open-circuit voltage and output power of 247 mV and 347 nW, respectively, were generated. This translates to a temperature difference of 63°C across the thermoelements. This paper demonstrates that HWCVD, which is a cost-effective way of producing solar cells, can also be applied in the production of TEGs. By establishing that HWCVD polysilicon can be an effective thermoelectric material, further work on developing photovoltaic-thermoelectric (PV-TE) hybrid microsystems that are cost-effective and better performing can be explored.     

Electric Power Generation from Thermoelectric Cells Using a Solar Dish Concentrator

In this paper, design details, theoretical analysis, and outcomes of a preliminary experimental investigation on a concentrator thermoelectric generator (CTEG) utilizing solar thermal energy are presented. The designed CTEG system consisted of a parabolic dish collector with an aperture diameter of 1.8 m used to concentrate sunlight onto a copper receiver plate with 260 mm diameter. Four BiTe-based thermoelectric cells (TEC) installed on the receiver plate were used to convert the concentrated solar thermal energy directly into electric energy. A microchannel heat sink was used to remove waste heat from the TEC cold side, and a two-axis tracking system was used to track the sun continuously. Experimental tests were conducted on individual cells and on the overall CTEG system under different heating rates. Under maximum heat flux, a single TEC generator was able to produce 4.9 W for a temperature difference of 109°C, corresponding to 2.9% electrical efficiency. The overall CTEG system was able to produce electric power of up to 5.9 W for a 35°C temperature difference with a hot-side temperature of 68°C. The results of the investigation help to estimate the potential of the CTEG system and show concentrated thermoelectric generation to be one of the potential options for production of electric power from renewable energy sources.     

Examination of a Thermally Viable Structure for an Unconventional Uni-Leg Mg2Si Thermoelectric Power Generator

We have fabricated an unconventional uni-leg structure thermoelectric generator (TEG) element using quad thermoelectric (TE) chips of Sb-doped n-Mg₂Si, which were prepared by a plasma-activated sintering process. The power curve characteristics, the effect of aging up to 500?h, and the thermal gradients at several points on the module were investigated. The observed maximum output power with the heat source at 975?K and the heat sink at 345?K was 341?mW, from which the ??T for the TE chip was calculated to be about 333?K. In aging testing in air ambient, a remarkable feature of the results was that there was no notable change from the initial resistance of the TEG module for as long as 500?h. The thermal distribution for the fabricated uni-leg TEG element was analyzed by finite-element modeling using ANSYS software. To tune the calculation parameters of ANSYS, such as the thermal conductance properties of the corresponding coupled materials in the module, precise measurements of the temperature at various probe points on the module were made. Then, meticulous verification between the measured temperature values and the results calculated by ANSYS was carried out to optimize the parameters.     

Performance variation of dark current density-voltage characteristics for PID-affected monocrystalline silicon solar modules from the field

Potential-induced degradation (PID) has caught considerable attention in recent years because of its detrimental influence on output power of solar module and energy yield of photovoltaic power plant. In this paper, performance variation of dark current density-voltage (dark J-V) characteristics for PID-affected monocrystalline silicon solar modules dismounted from photovoltaic power plant was investigated. By measuring dark J-V characteristics of the PID-affected modules, the deterioration trend of dark J-V characteristics for PID-affected modules was found for the first time. Based on diffusion theory of metal ions, a new field-assisted diffusion model of metal ions was built, and the deterioration of open-circuit voltage and increase of recombination current density were explained for solar modules affected by PID. The thermal characteristics of solar modules with different level of PID were revealed under forward and reverse bias. There is an obvious temperature difference between the substrings neighboring the frame and the middle substring within the PID-affected modules. The results obtained in this paper give out the reasonable explanation about mechanism of PID and the deterioration trend of dark J-V for PID-affected modules.     

Synthesis and Characterization of New Ceramic Thermoelectrics Implemented in a Thermoelectric Oxide Module

Novel thermoelectric oxides were developed, produced, and characterized to demonstrate their promising thermoelectric conversion potential in a thermoelectric converter. Four-leg thermoelectric oxide modules were fabricated by combining p- and n-type oxide thermoelements made of pressed polycrystalline GdCo_0.95Ni_0.05O₃ and CaMn_0.98Nb_0.02O₃, respectively. In these modules, the p- and n-type thermoelements were connected electrically in series and thermally in parallel. The materials were joined by electrical contacts consisting of a Ag/CuO composite material. Fairly good thermal contacts were ensured by pressing the thermoelements between alumina substrates. Cross-sections of the alumina/Ag–CuO mixture/thermoelement interface were investigated by scanning electron microscopy. The temperature distribution across the module was monitored using K-type thermocouples and a micro-infrared (IR) camera. The open-circuit voltage and the load voltages of the module were measured up to a temperature difference of ΔT = 500 K while keeping the temperature of the cold side at 300 K. The output power and internal resistance were calculated. The characteristics of the module evaluated from electrical measurements were compared with respective values of the p- and n-type leg materials. An output power of 0.04 W at ΔT = 500 K led to a power density of ~0.125 W/cm³, where the volume of thermoelectric material was determined by a cross-section of 4 mm × 4 mm and a leg length of 5 mm.     

接触效应对小型半导体温差发电器性能的影响

针对小型半导体温差(TEG)发电器中接触热阻和接触电阻的影响进行了分析研究.结果表明,接触热阻和接触电阻只在2mm以内的电偶臂长度内有明显影响;在电偶臂长度小于1mm时,输出功率和热电效率均有一个急剧上升的变化阶段;当长度超过5mm后,输出功率和热电效率均趋于定值;在冷热端温度分别为283和383K,Z=0.0024K-1、电偶臂长为2mm、接触热阻比0.2和接触电阻比0.1条件下,热电功率约为4mW/mm2,热电效率约为3.5%,而理想无接触热阻和电阻的热电效率约为4.2%.由此可知,半导体温差发电器中接触热阻和接触电阻的影响不可忽视.     

Achieving Maximum Power in Thermoelectric Generation with Simple Power Electronics

A thermoelectric generator typically delivers a relatively low power output, and hence it is of great practical importance to determine a design and operating condition close to those which can provide the maximum attainable power. To maintain a favorable condition for the maximum power output, power electronics circuits are usually applied. One of the simplest methods is to control the operating voltage at half the open-circuit voltage, assuming that the typical impedance-matching condition, in which the load and internal resistances are matched, yields the maximum power output. However, recent investigations have shown that, when external thermal resistances exist between the thermoelectric modules and thermal reservoirs, the impedance-matching condition is not identical to the condition for the maximum power output. In this article, it is argued that, although the impedance-matching condition is not the condition for maximum power output, the maximum power is still achievable when the operating voltage is kept at half the open-circuit voltage. More precisely, it is shown that the typical V–I curve for thermoelectric generators must show approximately linear behavior, which justifies the use of a simple strategy in thermoelectric power generation applications. The conditions for the validity of the approximation are mathematically discussed, supported by a few examples. Experimental evidence at room temperature is also provided.     

Model for Increasing the Power Obtained from a Thermoelectric Generator Module

We have developed a model for finding the most efficient way of increasing the power obtained from a thermoelectric generator (TEG) module with a variety of operating conditions and limitations. The model is based on both thermoelectric principles and thermal resistance circuits, because a TEG converts heat into electricity consistent with these two theories. It is essential to take into account thermal contact resistance when estimating power generation. Thermal contact resistance causes overestimation of the measured temperature difference between the hot and cold sides of a TEG in calculation of the theoretical power generated, i.e. the theoretical power is larger than the experimental power. The ratio of the experimental open-loop voltage to the measured temperature difference, the effective Seebeck coefficient, can be used to estimate the thermal contact resistance in the model. The ratio of the effective Seebeck coefficient to the theoretical Seebeck coefficient, the Seebeck coefficient ratio, represents the contact conditions. From this ratio, a relationship between performance and different variables can be developed. The measured power generated by a TEG module (TMH400302055; Wise Life Technology, Taiwan) is consistent with the result obtained by use of the model; the relative deviation is 10%. Use of this model to evaluate the most efficient means of increasing the generated power reveals that the TEG module generates 0.14 W when the temperature difference is 25°C and the Seebeck coefficient ratio is 0.4. Several methods can be used triple the amount of power generated. For example, increasing the temperature difference to 43°C generates 0.41 W power; improving the Seebeck coefficient ratio to 0.65 increases the power to 0.39 W; simultaneously increasing the temperature difference to 34°C and improving the Seebeck coefficient ratio to 0.5 increases the power to 0.41 W. Choice of the appropriate method depends on the limitations of system, the cost, and the environment.