Experimental Characterization of Thermoelectric Modules and Comparison with Theoretical Models for Power Generation

The interest in thermoelectrics for power generation applications has dramatically increased over the past decade as a result of recent advancements in thermoelectric materials. Although measuring thermoelectric properties of materials has received significant attention, measuring thermoelectric module (TEM) power generation performance has received less attention. Characterizing TEMs is vital for validating module-level models used in optimizing TEM designs. Measurements of module performance can also be used for the optimal incorporation of TEMs into power generation systems. A TEM test apparatus has been developed and characterized to test current and future modules under a wide range of temperature and loading conditions. In addition to temperatures and electrical performance metrics, heat rates, and mechanical loading conditions are monitored. The developed technique extracts module parameters, which can be used for system-level design, to measure performance of advanced TEMs, and to validate theoretical models for module design optimization. Experimental results are compared with standard analytical TEM models and a newly developed model.     

Investigation of the Effect of Electrical Current Variance on Thermoelectric Energy Harvesting

The performance of thermoelectric modules for energy-harvesting applications is investigated, and a model is presented to predict module performance. Derived from energy conservation equations, the model predicts module performance by solving for the temperatures at both ends of the thermoelectric materials within a module. Unlike traditional methods, the model accounts for the effect of electrical current with respect to the load resistance by considering additional heat transfer by Joule heating and the Peltier effect. This establishes a nonlinear quadratic form of temperatures which can be solved by an iterative numerical solution. The model is extended to predict the performance of energy-harvesting systems, which may include connection of multiple thermoelectric modules in series to meet the necessary power requirements. However, a key issue with multiple module connection is the power reduction that arises when there are significant differences in module properties and/or the corresponding external conditions to which each individual module is exposed. Power reduction is thus investigated, as in some cases the overall power output for multiple modules can be less than the power output of a single module. For validation and comparison of the model, experimental support is provided for the case of two commercial thermoelectric modules connected in series. The model also provides optimum load resistances, and a system optimization of the number of modules for a designated heat sink to maximize power generation. The overarching goal of this work is to provide performance prediction and optimization considerations for actual thermoelectric energy-harvesting systems.     

Study of Complete Thermoelectric Generator Behavior Including Water-to-Ambient Heat Dissipation on the Cold Side

Reduction of the thermal resistances of the heat exchangers of a thermoelectric generation (TEG) system leads to a significant increase in TEG efficiency. For the cold side of a thermoelectric module (TEM), a wide range of heat exchangers have been studied, from simple finned dissipators to more complex water (water–glycol) heat exchangers. As the Nusselt number is much higher in water heat exchangers than in conventional air finned dissipators, the convective thermal resistances are better. However, to conclude which heat exchanger leads to higher efficiencies, it is necessary to include the whole system involved in the heat dissipation, i.e., the TEM-to-water heat exchanger, the water-to-ambient heat exchanger, as well as the required pumps and fans. This paper presents a dynamic computational model able to simulate the complete behavior of a TEG, including both heat exchangers. The model uses the heat transfer and hydraulic equations to compute the TEM-to-water and water-to-ambient thermal resistances, along with the resistance of the hot-side heat exchanger at different operating conditions. Likewise, the model includes all the thermoelectric effects with temperature-dependent properties. The model calculates the net power generation for different configurations, providing a methodology to design and optimize the heat exchange in order to maximize the net power generation for a wide variety of TEGs.     

Thermoelectric-Generator-Based DC–DC Conversion Networks for Automotive Applications

Maximizing electrical energy generation through waste heat recovery is one of the modern research questions within automotive applications of thermoelectric (TE) technologies. This paper proposes a novel concept of distributed multisection multilevel DC–DC conversion networks based on thermoelectric generators (TEGs) for automotive applications. The concept incorporates a bottom-up design approach to collect, convert, and manage vehicle waste heat efficiently. Several state-of-the-art thermoelectric materials are analyzed for the purpose of power generation at each waste heat harvesting location on a vehicle. Optimal materials and TE couple configurations are suggested. Moreover, a comparison of prevailing DC–DC conversion techniques was made with respect to applications at each conversion level within the network. Furthermore, higher-level design considerations are discussed according to system specifications. Finally, a case study is performed to compare the performance of the proposed network and a traditional single-stage system. The results show that the proposed network enhances the system conversion efficiency by up to 400%.     

Modeling of TE cooling of pump lasers

The pump laser is a key module in optical amplifiers for long-haul fiber optic telecommunication systems. Its core component is a semiconductor laser diode mounted on a thermoelectric cooler. It is of crucial importance to maintain the laser diode temperature in a narrow range during operation in order to achieve satisfactory performance and reliability of the module. Therefore, a proper thermal management solution is very important to the pump module design. In this paper, a three-dimensional finite element analysis on thermoelectric cooling is presented. The modeling results show good agreement with the experimental results obtained by IR thermometry. When the heat source has a high power dissipation and a small footprint compared to the size of the heat sink, the spreading resistance becomes important. To analyze the maximum performance of the heat sink, both single and dual pump module configurations are considered.     

Thermoelectric Generators for Automotive Waste Heat Recovery Systems Part II: Parametric Evaluation and Topological Studies

A comprehensive numerical model has been proposed to model thermoelectric generators (TEGs) for automotive waste heat recovery. Details of the model and results from the analysis of General Motors’ prototype TEG were described in part I of the study. In part II of this study, parametric evaluations are considered to assess the influence of heat exchanger, geometry, and thermoelectric module configurations to achieve optimization of the baseline model. The computational tool is also adapted to model other topologies such as transverse and circular configurations (hexagonal and cylindrical) maintaining the same volume as the baseline TEG. Performance analysis of these different topologies and parameters is presented and compared with the baseline design.     

System Modeling and Validation of a Thermoelectric Fluidic Power Source: Proton Exchange Membrane Fuel Cell and Thermoelectric Generator (PEMFC-TEG)

To facilitate the co-design and co-optimization of fluid or combustion systems and thermoelectric devices, a three-dimensional (3D) thermoelectric generator (TEG) model has been proposed and implemented in a computational fluid dynamics (CFD) simulation environment. The model includes all temperature-dependent characteristics of the materials and nonlinear fluid–thermal– electric multiphysics coupled effects. In this paper, the device-level model is first extended to the module level by taking a general geometry, identifying regions such as positive and negative thermoelements, and assigning properties to them. The system-level model is then demonstrated by coupling the module-level model with a fluidic–thermal system model in a single CFD simulator to predict the generation performance based on the thermal equilibrium that is achieved. The linked models are validated experimentally at the system level using data from three real thermoelectric modules installed on the surface of an exhaust pipe-like rig, where the temperature profile as well as the electricity generated can be measured and compared with the simulation results. The rig is intended not only to verify the proposed system model but also to mimic a practical exhaust recovery apparatus for a proton exchange membrane fuel cell (PEMFC). Based on the data obtained from the system-level test rig, a novel low-temperature low-cost application for auxiliary electric power appliances based on the waste heat of the PEMFC can be envisaged. Within the common simulator, it is shown that the thermoelectric model can be connected to various continuum-domain CFD models of the fuel cell itself, thus enabling further possibilities to optimize system efficiency and performance.     

Progress Towards an Optimization Methodology for Combustion-Driven Portable Thermoelectric Power Generation Systems

There is enormous military and commercial interest in developing quiet, lightweight, and compact thermoelectric (TE) power generation systems. This paper investigates design integration and analysis of an advanced TE power generation system implementing JP-8 fueled combustion and thermal recuperation. In the design and development of this portable TE power system using a JP-8 combustor as a high-temperature heat source, optimal process flows depend on efficient heat generation, transfer, and recovery within the system. The combustor performance and TE subsystem performance were coupled directly through combustor exhaust temperatures, fuel and air mass flow rates, heat exchanger performance, subsequent hot-side temperatures, and cold-side cooling techniques and temperatures. Systematic investigation and design optimization of this TE power system relied on accurate thermodynamic modeling of complex, high-temperature combustion processes concomitantly with detailed TE converter thermal/mechanical modeling. To this end, this paper reports integration of system-level process flow simulations using CHEMCAD? commercial software with in-house TE converter and module optimization, and heat exchanger analyses using COMSOL? software. High-performance, high-temperature TE materials and segmented TE element designs are incorporated in coupled design analyses to achieve predicted TE subsystem-level conversion efficiencies exceeding 10%. These TE advances are integrated with a high-performance microtechnology combustion reactor based on recent advances at Pacific Northwest National Laboratory (PNNL). Predictions from this coupled simulation approach lead directly to system efficiency?Cpower maps defining potentially available optimal system operating conditions and regimes. Further, it is shown that, for a given fuel flow rate, there exists a combination of recuperative effectiveness and hot-side heat exchanger effectiveness that provides a higher specific power output from the TE modules. This coupled simulation approach enables pathways for integrated use of high-performance combustor components, high-performance TE devices, and microtechnologies to produce a compact, lightweight, combustion-driven TE power system prototype that operates on common fuels.     

A Natural-Gas-Fired Thermoelectric Power Generation System

This paper presents a combustion-driven thermoelectric power generation system that uses PbSnTe-based thermoelectric modules. The modules were integrated into a gas-fired furnace with a special burner design. The thermoelectric integrated system could be applied for self-powered appliances or micro-cogeneration. A mathematical model for the integrated energy system was established that considered irreversibilities in the thermal-to-electric energy conversion process. The electric power output and electrical efficiency of the system were simulated using the established model. A prototype system was developed and its performance was investigated at various operating conditions. Applicability of thermoelectric devices to self-powered heating systems was demonstrated. The thermoelectric integrated combustion system could provide the consumer with heating system reliability and a reduction in electric power consumption. The integrated system could also offer other advantages including simplicity, low noise, clean operation, and low maintenance.     

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.     

Detailed Modeling and Irreversible Transfer Process Analysis of a Multi-Element Thermoelectric Generator System

Thermoelectric (TE) power generation technology, due to its several advantages, is becoming a noteworthy research direction. Many researchers conduct their performance analysis and optimization of TE devices and related applications based on the generalized thermoelectric energy balance equations. These generalized TE equations involve the internal irreversibility of Joule heating inside the thermoelectric device and heat leakage through the thermoelectric couple leg. However, it is assumed that the thermoelectric generator (TEG) is thermally isolated from the surroundings except for the heat flows at the cold and hot junctions. Since the thermoelectric generator is a multi-element device in practice, being composed of many fundamental TE couple legs, the effect of heat transfer between the TE couple leg and the ambient environment is not negligible. In this paper, based on basic theories of thermoelectric power generation and thermal science, detailed modeling of a thermoelectric generator taking account of the phenomenon of energy loss from the TE couple leg is reported. The revised generalized thermoelectric energy balance equations considering the effect of heat transfer between the TE couple leg and the ambient environment have been derived. Furthermore, characteristics of a multi-element thermoelectric generator with irreversibility have been investigated on the basis of the new derived TE equations. In the present investigation, second-law-based thermodynamic analysis (exergy analysis) has been applied to the irreversible heat transfer process in particular. It is found that the existence of the irreversible heat convection process causes a large loss of heat exergy in the TEG system, and using thermoelectric generators for low-grade waste heat recovery has promising potential. The results of irreversibility analysis, especially irreversible effects on generator system performance, based on the system model established in detail have guiding significance for the development and application of thermoelectric generators, particularly for the design and optimization of TE modules.     

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.     

Optimization and Fabrication of a Thick Printed Thermoelectric Device

Thermoelectric power generation technology aims to convert thermal energy into electricity. Micromodule design optimization depends directly on the thermal environment. For low thermal energy input, optimized thermoelectric devices require 100 μm to 500 μm element thickness. These dimensions currently present a challenge for standard mass-production manufacturing techniques. In this paper, a unique printing technology for micromodule fabrication is presented. This technology is compared with a traditional bulk thermoelectric manufacturing process to highlight the advantages of the printing process to obtain scalable thermoelectric devices. Initial thermoelectric materials have been integrated in inks and then deposited by a spray technology onto a polymer substrate. A complete micromodule for application on nonplanar surfaces is also presented.     

Thermoelectric Module Design Under a Given Thermal Input: Theory and Example

Established thermoelectric theory enables direct calculation of the power output and conversion efficiency if the temperature difference across a module is given. However, in some applications such as those using a radioisotope or solar radiation as a heat source, the thermal input remains constant while the temperature difference varies with the geometry of the thermoelectric module. In this paper, a theoretical framework for thermoelectric module design under a given thermal input is presented. It provides a convenient approach for module geometry optimization. The usefulness of the theory is demonstrated through a design study, in which an appropriate thermoelement length for a solar thermoelectric system is determined by considering conflicting requirements for a longer length to obtain a greater temperature difference and for a shorter length to produce a larger power output.     

Thermoelectric Modules Based on Half-Heusler Materials Produced in Large Quantities

Half-Heusler (HH) compounds are some of the most promising candidates among the medium-temperature thermoelectric materials being investigated for automotive and industrial waste heat recovery applications. For n- as well as p-type material, peak ZT values larger than one have been published recently, and first modules have been built. The next step to facilitate the industrialization of thermoelectric module production is upscaling of material synthesis. In this paper, the latest results of the thermoelectric properties of HH compounds produced in kg batches are presented and compared with values published in the literature. The performance of modules built from these materials is analyzed with respect to power output and long-term stability of the material and electrical contacts.     

Experimental Study and Optimization of Thermoelectricity-Driven Autonomous Sensors for the Chimney of a Biomass Power Plant

In the work discussed in this paper a thermoelectric generator was developed to harness waste heat from the exhaust gas of a boiler in a biomass power plant and thus generate electric power to operate a flowmeter installed in the chimney, to make it autonomous. The main objective was to conduct an experimental study to optimize a previous design obtained after computational work based on a simulation model for thermoelectric generators. First, several places inside and outside the chimney were considered as sites for the thermoelectricity-driven autonomous sensor. Second, the thermoelectric generator was built and tested to assess the effect of the cold-side heat exchanger on the electric power, power consumption by the flowmeter, and transmission frequency. These tests provided the best configuration for the heat exchanger, which met the transmission requirements for different working conditions. The final design is able to transmit every second and requires neither batteries nor electric wires. It is a promising application in the field of thermoelectric generation.     

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.     

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%.     

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