Thermoelectric Element Length Research Articles

Electrical response of thermoelectric generator to geometry variation under transient thermal boundary condition

A three-dimensional numerical model is applied in this study to illustrate the electrical response of a thermoelectric generator (TEG) during transient heat flux at the hot side. In this work, various types of thermal boundary conditions are considered to evaluate the performance of the TEG. Thus, a TEG under pulsed heat flux is studied numerically, and the numerical model is verified by experimental results. With the consideration of a defined reference geometry, different heat flux frequencies are applied in order to evaluate the corresponding electrical output by the TEG. In addition, variation of the module performance for various TEG leg lengths and its cross-sectional area are studied over a wide range of heat fluxes. The results indicate that the open circuit voltage in the experiment is in a good agreement with the open circuit voltage in the simulation results. The results show that the range of power oscillation reduces at higher frequency of the applied heat flux. Furthermore, the variability of the output power increases as the thermoelectric element length increases and the area of the element reduces.

Series of detail comparison and optimization of thermoelectric element geometry considering the PV effect

Abstract This study investigates the optimum geometry for maximum efficiency of a hybrid PV-TE uni-couple using Finite Element Method. COMSOL Multiphysics is used to solve the 3-Dimensional heat transfer equations considering thermoelectric materials with temperature dependent properties. Two types of thermoelectric element geometry area ratios are considered for the range 0.5 ≤ R A ≤ 2 and 0.5 ≤ R S ≤ 2 . Nine different geometric configurations are analysed for two different PV cells. Effects of thermoelectric generator (TEG) geometric parameters, solar irradiation and concentration ratio on the hybrid system efficiency are presented. The results show that a hybrid PV-TE system will perform better with symmetrical TEG geometry ( R A = R S = 1 ) if a PV temperature coefficient of 0.004/K (Cell B) is used. This is different from the optimum geometry for a TEG only system. However, the optimum geometry of the TEG in a hybrid system will be the same as that of a TEG only system (dissymmetrical i.e. R A = R S ≠ 1 ) if a PV temperature coefficient of 0.001/K (Cell A) is used. The overall efficiency and TE temperature difference show a decreasing trend as thermoelectric element length and area increase respectively no matter the configuration or temperature coefficient value used. Results obtained from this research would influence hybrid PV-TE system design for obtaining maximum conversion efficiency.

Effect of Thermal Cycling on Zinc Antimonide Thin Film Thermoelectric Characteristics

In this study, performance and stability of zinc antimonide thin film thermoelectric sample is analyzed under transient thermal conditions. The thermoelectric materials are deposited on glass based substrate where the heat flow is parallel with the thermoelectric element length. The specimen is fixed between a heater block and heat sink cooled by the ambient. The thermoelectric element is studied under open circuit and also optimal constant loads corresponding to maximum power output. The thermal cycles are provided for five different hot junction temperatures, 160, 200, 250, 300 and 350 ᵒC. The results show that the thin film has high reliability after thermal and electrical cycles, and no performance degradation is observed.

Thermoelectric generator for industrial gas phase waste heat recovery

A technical solution recycling exhaust gas sensible heat based on thermoelectric power generation is proposed by using finite time thermodynamics. The effects of some key parameters such as exhaust gas inlet temperature, exhaust gas and cooling water heat transfer coefficient on the optimum length of the thermoelectric elements are analyzed. It is found that the gas temperature drops rapidly because of the small specific heat of the exhaust gas. Enhancing the heat transfer of gas can effectively improve the power, but not the efficiency. Exhaust gas inlet temperature and transfer coefficient have significant effects on the optimal thermoelectric element length. Due to the highest hot surface operating temperature limit 200 °C, the optimal length of the thermoelectric elements is about 2 mm. About 1.47 kW electrical energy can be produced per square meter and the conversion efficiency of 4.5% can be achieved for exhaust gas at 350 °C. The payback period of the waste heat recovery device is about 4 years for the price and performance of thermoelectric products made in China.

Multi-parameter Optimization of a Thermoelectric Power Generator and Its Working Conditions

The global optimal working conditions and optimal couple design for thermoelectric (TE) generators with realistic thermal coupling between the heat reservoirs and the TE couple were studied in the current work. The heat fluxes enforced by the heat reservoirs at the hot and the cold junctions of the TE couple were used in combination with parameter normalization to obtain a single cubic algebraic equation relating the temperature differences between the TE couple junctions and between the heat reservoirs, through the electric load resistance ratio, the reservoir thermal conductance ratio, the reservoir thermal conductance to the TE couple thermal conductance ratio, the Thomson to Seebeck coefficient ratio, and the figure of merit (Z) of the material based on the linear TE transport equations and their solutions. A broad reservoir thermal conductance ranging between 0.01 W/K and 100 W/K and TE element length ranging from 10-7 m to 10-3 m were explored to find the global optimal systems. The global optimal parameters related to the working conditions, i.e., reservoir thermal conductance ratio and electric load resistance ratio, and the optimal design parameter related to the TE couple were determined for a given TE material. These results demonstrated that the internal and external electric resistance, the thermal resistance between the reservoirs, the thermal resistance between the reservoir and the TE couple, and the optimal thermoelement length have to be well coordinated to obtain optimal power production.

Thermoelectric power generation driven by blast furnace slag flushing water

Focusing on the low recycling rate of low temperature waste heat in China's iron and steel industry, this study presented a technical solution recycling blast furnace slag flashing water sensible heat based on thermoelectric power generation. The physical and numerical models are established. The effects of some key parameters such as slag washing water temperature, the thermoelectric element length, the packing factor of the thermoelectric module and heat exchanger flow passage length on the performance of the thermoelectric power generation device are analyzed. The results showed that for blast furnace slag flushing water at 100 °C, water temperature drops 1.5 °C per meter, about 0.93 kW electrical energy can be produced per area and conversion efficiency of 2% can be achieved. The cost recovery period of the equipment is about 8 years.

Internal and external simultaneous optimization of an irreversible thermoelectric generator for maximum power output

A generalized finite-time thermodynamic model of irreversible multi-element thermoelectric generator is established taking into account inner and external factors. The inner effects include Seebeck effect, Fourier effect, Joule effect and Thomson effect. The irreversibility is caused by the finite-rate heat transfer between the heat reservoirs and the device. The theoretical iterative functions of the hot and cold junction temperatures and energy equations are obtained. The model is applied to the analysis of a multi-element thermoelectric generator, which is made of typical thermoelectric materials. It is found that, for given other parameters, there is an optimal electrical current, an optimal length of thermoelectric elements and an optimal ratio of thermal conductance allocation corresponding to the maximum power output. Thus, internal and external simultaneous multivariable optimizations are performed for a maximum power output. The effects of several important parameters on the optimal variables are analyzed in detail. The comparison between the optimized power and the non-optimized power shows that the multivariable optimization is necessary and effective for various working conditions. The model and optimization conclusions obtained herein can be applied to not only the analysis and optimization but also the design of thermoelectric generators.

The Compatibility of Thin Films and Nanostructures in Thermoelectric Cooling Systems

The compatibility of low-dimensional thermoelectric materials in forms such as thin films and nanowires for use in thermoelectric coolers is examined. First-order thermoelectric theory predicts that the cold and hot junction temperatures of a thermoelectric circuit are governed solely by the nondimensional figure of merit, ZT. Performance predictions based on this traditional theory have been more broadly applied to the performance of thermoelectric cooler systems, thereby implying that these coolers may be miniaturized without loss of performance and that system performance is dictated principally by ZT. A nondimensional thermoelectric system model for a cooler is developed and typical performance metrics for thermoelectric coolers are presented along with predictions from traditional theory. Performance is examined as a function of thermoelectric element length for representative system conditions. This system study shows that cooler performance may drop significantly when miniaturized, particularly if the cooling elements are realized at the scale of many recently proposed thermoelectric thin films and nanostructured materials. The system theory illustrates that performance is governed by three nondimensional parameters: an effective thermoelectric figure of merit, ZeTa, the relative ability for heat to be drawn into the cooler, and the relative ability for heat to be rejected from the cooler to the ambient environment. As cooler performance depends both on material properties (ZeTa) as well as the relative scale of the materials with respect to system thermal conductances, the applicability of some low-dimensional forms of materials such as thermoelectric elements may require reevaluation. The realization of high performance coolers based on thermoelectric effects must rely on developing high quality materials realized at an appropriate, application-dependent scale.

Supercooling of Peltier cooler using a current pulse

The operation of a Peltier cooler can be temporarily enhanced by utilizing the transient response of a current pulse. The performance of such a device, using (Bi,Sb)2Te3-based thermoelectric elements, was examined from −70 to 55 °C. We establish both theoretically and experimentally the essential parameters that describe the pulse cooling effect, such as the minimum temperature achieved, maximum temperature overshoot, time to reach minimum temperature, time while cooled, and time between pulses. Using simple theoretical and semiempirical relationships the dependence of these parameters on the current pulse amplitude, temperature, thermoelectric element length, thermoelectric figure of merit and thermal diffusivity is established. At large pulse amplitudes the amount of pulse supercooling is proportional to the maximum steady-state difference in temperature. This proportionality factor is about half that expected theoretically. This suggests that the thermoelectric figure of merit is the key materials parameter for pulse cooling. For this cooler, the practical optimum pulse amplitude was found to be about three times the optimum steady-state current. A pulse cooler was integrated into a small commercial thermoelectric three-stage cooler and it provided several degrees of additional cooling for a period long enough to operate a laser sensor. The improvement due to pulse cooling is about the equivalent of two additional stages in a multistage thermoelectric cooler.