Computational Study on Temperature Control Systems for Thermoelectric Refrigerators

Thermoelectric refrigeration has the outstanding advantage of allowing accurate temperature control. However, on the market there are thermoelectric refrigerators which include on/off temperature control systems, because of their simplicity and low cost. The major problem with this system is that, when the thermoelectric modules are switched off, the heat stored in the heat exchanger at the hot end of the modules goes back into the refrigerator, forming a thermal bridge. In this work, we use a computational model, presented and validated in previous papers, to study alternative control systems. A new system is introduced based on idling voltages; that is, once the temperature of the refrigerator reaches the lower limit, the thermoelectric modules are not switched off but supplied with minimum voltage. Computational results prove that this system reduces the electric power consumption of the refrigerator by at least 40% with respect to that obtained with on/off control systems, and the coefficient of performance increases close to the maximum provided by any other control system.     

Thermal Comfort Study of a Compact Thermoelectric Air Conditioner

This paper evaluates the cooling performance and thermal comfort of a compact thermoelectric (TE) air conditioner. The compact TE air conditioner is composed of three TE modules. The cold and hot sides of the TE modules were fixed to rectangular fin heat sinks and fans. Thermal acceptability assessment was performed to find out whether the cooled air met the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Standard-55’s 80% acceptability criteria. A suitable condition occurred at 1 A current flow with a corresponding cooling capacity of 29.2 W, giving an average cooled air temperature of 28°C and 0.9 m/s cooled air velocity. The coefficient of performance was calculated and found to be ∼0.34. Economic analysis indicates that the payback period is 0.75 years when one compact TE air conditioner unit is used instead of a 1-ton conventional air conditioner.     

Influence of Thermal Aging Phenomena on Thermoelectric Properties of Al-Substituted ZnO

The thermoelectric properties and stability of Al-substituted ZnO as a potential high-temperature n-type material were studied in heating?Ccooling cycles. Zn_1?x Al _x O (x?=?0.02, 0.06) was prepared by soft chemistry and solid-state reaction synthesis methods. Cycling during the thermoelectric measurement leads to an increase of the electrical resistivity and Seebeck coefficient values. The reason for this aging phenomenon can be assigned to a change in composition due to oxygen uptake along with modification in the defect concentrations. The aging is enhanced if the cycling is performed in oxygen. ZT value of 0.21 is reached at 1275?K for samples with 2% Al substitution made by soft chemistry synthesis.     

Tuning Thermal Transport in Chain‐Oriented Conducting Polymers for Enhanced Thermoelectric Efficiency: A Computational Study

Thermoelectric polymers should be electron‐crystal and phonon‐glass to efficiently interconvert heat and electricity. Herein, by using molecular dynamics simulations, it is demonstrated that engineering phonon transport in conducting polymers by tailoring its degree of polymerization can effectively improve the energy conversion efficiency. This is based on the separated length scales that charge carriers and phonons travel along the polymer backbone. By tuning the chain length and the crystallinity of chain‐oriented poly(3,4‐ethylenedioxythiophene) fibers, a dramatic decrease of the axial thermal conductivity to 0.97 W m^?1 K^?1 has been observed in rationally designed polymer fibers with the crystallinity of 0.49 and the relative molecular weight of 5600. The dimensionless thermoelectric figure of merit at 298 K has been enhanced to 0.48, which is approximately one order of magnitude higher than that in crystalline polymers.     

Efficiency Study of a Commercial Thermoelectric Power Generator (TEG) Under Thermal Cycling

Thermoelectric generators (TEGs) make use of the Seebeck effect in semiconductors for the direct conversion of heat to electrical energy. The possible use of a device consisting of numerous TEG modules for waste heat recovery from an internal combustion (IC) engine could considerably help worldwide efforts towards energy saving. However, commercially available TEGs operate at temperatures much lower than the actual operating temperature range in the exhaust pipe of an automobile, which could cause structural failure of the thermoelectric elements. Furthermore, continuous thermal cycling could lead to reduced efficiency and lifetime of the TEG. In this work we investigate the long-term performance and stability of a commercially available TEG under temperature and power cycling. The module was subjected to sequential hot-side heating (at 200°C) and cooling for long times (3000 h) in order to measure changes in the TEG’s performance. A reduction in Seebeck coefficient and an increase in resistivity were observed. Alternating-current (AC) impedance measurements and scanning electron microscope (SEM) observations were performed on the module, and results are presented and discussed.     

Computational Optimization of a Thermoelectric Ice-Maker as a Function of the Geometric Parameters of a Peltier Module

The objective of this paper is to optimize a thermoelectric ice-maker installed in a no-frost refrigerator, by means of a computational model. This model provides the electric power consumption of the Peltier module and the ice production. The Peltier module is the most important part of the thermoelectric ice-maker; therefore, it must be optimized in order to obtain an efficient ice-maker. First of all, the length of the thermocouples of the Peltier module has been optimized in order to obtain the maximum ice production. It turned out that 3.5 kg per day could be achieved if 1.5-mm-long thermocouples were used. The coefficient of performance (COP) was 0.44. Second, the ice production was expressed as a function of the number of thermocouples of the Peltier module. Given a constant electric power consumption of the module, the results showed that the maximum ice production was achieved with a Peltier module with 254 thermocouples. However, if a module with 140 thermocouples was installed, the ice production would decrease by only 1%.     

热电式MEMS微波功率传感器的热学特性研究

热电转换效率直接影响热电式MEMS微波功率传感器的性能。着重对衬底掏空结构的热电式微波功率传感器进行了研究。将热电式微波功率传感器分成三个区域,建立了傅里叶模型,研究背面刻蚀的长度与厚度对热电堆热端温度的影响,发现热电堆两端温差与背面刻蚀的长度、厚度成正比。利用有限元仿真软件ANSYS,对不同刻蚀长度、厚度的传感器进行热学仿真。结果表明,背面刻蚀尺寸越大,热电堆两端的温差越大,传感器的灵敏度得到提高。仿真结果与模型结果具有较高的一致性,验证了模型的准确性。     

Thermal and Thermoelectric Properties of Molecular Junctions

Molecular junctions (MJs) represent an ideal platform for studying charge and energy transport at the atomic and molecular scale and are of fundamental interest for the development of molecular‐scale electronics. While tremendous efforts have been devoted to probing charge transport in MJs during the past two decades, only recently advances in experimental techniques and computational tools have made it possible to precisely characterize how heat is transported, dissipated, and converted in MJs. This progress is central to the design of thermally robust molecular circuits and high‐efficiency energy conversion devices. In addition, thermal and thermoelectric studies on MJs offer unique opportunities to test the validity of classical physical laws at the nanoscale. A brief survey of recent progress and emerging experimental approaches in probing thermal and thermoelectric transport in MJs is provided, including thermal conduction, heat dissipation, and thermoelectric effects, from both a theoretical and experimental perspective. Future directions and outstanding challenges in the field are also discussed.     

Modeling of a Thermoelectric Generator for Thermal Energy Regeneration in Automobiles

In the field of passenger transportation a reduction of the consumption of fossil fuels has to be achieved by any measures. Advanced designs of internal combustion engine have the potential to reduce CO₂ emissions, but still suffer from low efficiencies in the range from 33% to 44%. Recuperation of waste heat can be achieved with thermoelectric generators (TEGs) that convert heat directly into electric energy, thus offering a less complicated setup as compared with thermodynamic cycle processes. During a specific driving cycle of a car, the heat currents and temperature levels of the exhaust gas are dynamic quantities. To optimize a thermoelectric recuperation system fully, various parameters have to be tested, for example, the electric and thermal conductivities of the TEG and consequently the heat absorbed and rejected from the system, the generated electrical power, and the system efficiency. A Simulink model consisting of a package for dynamic calculation of energy management in a vehicle, coupled with a model of the thermoelectric generator system placed on the exhaust system, determines the drive-cycle-dependent efficiency of the heat recovery system, thus calculating the efficiency gain of the vehicle. The simulation also shows the temperature drop at the heat exchanger along the direction of the exhaust flow and hence the variation of the voltage drop of consecutively arranged TEG modules. The connection between the temperature distribution and the optimal electrical circuitry of the TEG modules constituting the entire thermoelectric recuperation system can then be examined. The simulation results are compared with data obtained from laboratory experiments. We discuss error bars and the accuracy of the simulation results for practical thermoelectric systems embedded in cars.     

The Impact of Nanostructuring on the Thermal Conductivity of Thermoelectric CoSb3

The high concentration of grain boundaries provided by nanostructuring is expected to lower the thermal conductivity of thermoelectric materials, which favors an increase in their thermoelectric figure‐of‐merit, ZT. A novel chemical alloying method has been used for the synthesis of nanoengineered‐skutterudite CoSb₃. The CoSb₃ powders were annealed for different durations to obtain a set of samples with different particle sizes. The samples were then compacted into pellets by uniaxial pressing under various conditions and used for the thermoelectric characterization. The transport properties were investigated by measuring the Seebeck coefficient and the electrical and thermal conductivities in the temperature range 300 K to 650 K. A substantial reduction in the thermal conductivity of CoSb₃ was observed with decreasing grain size in the nanometer region. For an average grain size of 140 nm, the thermal conductivity was reduced by almost an order of magnitude compared to that of a single crystalline or highly annealed polycrystalline material. The highest ZT value obtained was 0.17 at 611 K for a sample with an average grain size of 220 nm. The observed decrease in the thermal conductivity with decreasing grain size is quantified using a model that combines the macroscopic effective medium approaches with the concept of the Kapitza resistance. The compacted samples exhibit Kapitza resistances typical of semiconductors and comparable to those of Si–Ge alloys.     

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.     

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.     

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.     

Experimental Study of a Thermoelectric Generation System

A flat wall-like thermoelectric generation system is developed for applications in exhaust heat of kilns. The design of the whole experimental setup is presented. The essential performance of the thermoelectric generation system is tested, including open-circuit voltage, output power, and system conversion efficiency. The results illustrate that, when heat source insulation is not considered, the system conversion is efficient at hot-side temperatures between 120°C and 150°C. In addition, the nonuniformity of heat transfer is found to significantly affect the power-generating ability of the system. System-level simulation is carried out using a quasi-one-dimensional numerical model that enables direct comparison with experimental results. The results of both experiment and simulation will provide a foundation to improve and optimize complex thermoelectric generation systems.     

Methods for Enhancing the Thermal Durability of High-Temperature Thermoelectric Materials

Thermoelectric materials, for example skutterudites and magnesium silicides, are being investigated as promising materials for medium-to-high-temperature waste heat recovery in transport and in industry. A crucial aspect of the success of a thermoelectric material is its stability over time when exposed to rapid heating and cooling. In this work different aspects of the degradation of these thermoelectric materials at high temperature were examined. Initial thermal durability was studied, and several candidate coatings were evaluated to enhance durability by protecting the materials from oxidation and sublimation during thermal cycles in air for up to 500 h and up to 873 K. The samples were characterized by SEM and EDS. The results showed it is possible to reduce degradation of the thermoelectric material without compromising overall thermoelectric efficiency.     

Optimized Thermoelectric Refrigeration in the Presence of Thermal Boundary Resistance

Thermoelectric refrigerators (TEMs) offer several advantages over vapor-compression refrigerators. They are free of moving parts, acoustically silent, reliable, and lightweight. Their low efficiency and peak heat flux capabilities have precluded their use in more widespread applications. Optimization of thermoelectric pellet geometry can help, but past work in this area has neglected the impact of thermal and electrical contact resistances. The present work extends a previous 1-D TEM model to account for a thermal boundary resistance and is appropriate for the common situation where an air-cooled heat sink is attached to a TEM. The model also accounts for the impact of electrical contact resistance at the TEM interconnects. The pellet geometry is optimized with the target of either maximum performance or efficiency for an arbitrary value of thermal boundary resistance for varying values of the temperature difference across the unit, the pellet Seebeck coefficient, and the contact resistances. The model predicts that when the thermal contact conductance is decreased by a factor of ten, the peak heat removal capability is reduced by at least 10%. Furthermore, when the interconnect electrical resistance rises above a factor of ten larger than the pellet electrical resistance, the maximum heat removal capability for a given pellet height is reduced by at least 20% and the maximum coefficient of performance at low $K_{u-infty,u}/(NK)$ values is reduced by at least 50%.