Thermoelectric Energy Conversion Devices and Systems

The following sections are included:

• Preface
• About the Authors
• Acknowledgments
• Contents

The following sections are included:

• Discovery of Thermoelectricity
• Thermodynamic Cycles
• Refrigeration and the Coefficient of Performance
• Thermodynamics of Thermoelectrics
  • Seebeck effect
  • Peltier effect
  • Thomson effect
• Role of Heat Transfer in Thermoelectric Systems
• Scalability of Thermoelectrics
• Societal and Environmental Impact
• References

This chapter describes the thermoelectric applications in photonics and electronics, including energy-friendly cooling, device thermal management, on-demand power generation, and waste heat recovery. The examples show how thermoelectric works to accomplish individual objectives including several technology readiness levels from concept to actual implementation. The most common market of thermoelectric technology commercialization is the cooling of electronics and photonics devices. The advantages of thermoelectrics are compactness, lightweight, quick response, precise temperature controllability, and scalability for the thermal solution.

This chapter discusses important heat transfer mechanisms including major parasitic heat losses involved in thermoelectric energy conversion devices and systems. Due to the electrothermal coupling in the system, the analysis and modeling of fundamental heat transfer is indispensable for accurate performance optimization of a thermoelectric system assembled with heat sinks and thermal contacts. In addition, electrical parasitic losses are discussed as they cannot be separately determined from thermal contacts parasitic losses.

This chapter discusses the modeling and analysis of thermoelectric refrigeration primarily for electronics cooling. Refrigeration in a thermoelectric device is driven by applying an electrical bias to the device to cause an active heat pumping. The heat pumping draws heat from the target device to be cooled and releases the heat to a thermal ground, e.g. to ambient air. The heat source temperature may be higher or lower than that of the thermal ground as the active cooling can transfer heat against the passive heat conduction. The model is developed for a single thermoelectric element (leg), which is then extended to a module and a system. The working principle is discussed, and then the methodology for design optimization of thermoelectric coolers is provided. The coefficient of performance (COP) is used to evaluate the performance since it is a common metric for conventional vapor-compression-based refrigeration. There are two objectives, COP and maximum cooling capacity Q_max, for design optimization. Then, a unique beta-shape curve in the relationship between COP and Q_max is discussed. Finally, the advantages of thermoelectric coolers in comparison with vapor compression cycles are discussed.

This chapter describes detailed modeling and analysis of thermoelectric (TE) power generation for various scales of application from a single thermoelectric element and modules to systems. Starting with the working principles, we discuss the design optimization of thermoelectric modules in contact with hot and cold thermal reservoirs. Due to the inter-play between the electrical and thermal energy transport, impedance matching for both electrical and thermal resistances are required to simultaneously achieve the maximum power output. Electrothermal optimization of thermoelectric elements with external contacts allows for a cost-effective and compact design for obtaining the desired power output. With leg thickness and fill factor as two important key variables, strategies for cost-effective design for TE power generators are discussed. Then, energy payback analysis with fluid pump power taken into account for the efficient convective heat exchange at both the hot and cold sides is provided. Theoretical maximum efficiency and power output are further discussed with asymmetric thermal contacts in the context of Carnot and Curzon−Ahlborn limits. Finally, impacts of temperature-dependent material properties, and the thermal stresses caused by large thermal gradients in TE generator modules during operation are briefly discussed.

This chapter highlights the several modeling and analysis cases of thermoelectric (TE) power generation for industrial and energy applications. The focused topics include the thermoelectric topping cycle for power plants and the high-temperature waste heat recovery from grass manufacturing processes. For the middle-temperature applications, the vehicle exhaust gas heat recovery and the solar concentrate combined heat and power application are also discussed. Solar CHP includes the results from a hands-on experimental demonstration. This chapter not only describes the examples but also shows how the analytic model and the optimization of thermoelectric generators works in the applications.

This chapter specifically discusses the thermoelectric energy conversion applications focusing on wearable devices and Internet of things (IoT). These applications share a lot of characteristics, such as room-temperature operation, small temperature difference, and low heat flux. Curved surfaces of the heat source are another similarity of both applications. Thermal contact is a big challenge in this case, and it is important to maintain the external thermal resistance as small as possible since the heat flux is very small. In addition to the flow bypass and thermal bypass concepts, flexible thermoelectric modules are discussed as an approach to obtain a good thermal contact between the TE module and the heat source. Fundamentals are discussed in the early part of the chapter, and then some unique characteristics of each application is specifically discussed in the later sections. As the Internet becomes available to connect remote sensors and actuators to the remote server, many unique features become possible with newly developed software and wireless communication. Thermoelectric heat energy recovery is a suitable technology for powering the necessary electronics by using on-site waste heat adjacent to the device. In wearable applications, two different types of thermoelectric energy conversion are considered: one is body heat harvesting and another is thermotherapy for both heating and cooling of the local skin.

In this chapter, several key techniques used for characterization of individual material properties as well as devices are reviewed. The measurement techniques for the three thermoelectric material properties, electrical conductivity, Seebeck coefficient, and thermal conductivity, of thin films and bulk materials are introduced and their limitations are discussed. For device characterization, various micro/nano-scale thermal characterization techniques are reviewed. In particular, thermoreflectance imaging, impedance spectroscopy, and the Harman method are discussed in detail for thermoelectric device testing.

In this chapter, several online simulation tools for thermoelectric devices and materials are introduced. From thin-film devices to multi-element modules and systems with cost-performance tradeoff analysis, this simulation package covers a variety of thermoelectric systems for both power generation and cooling applications. Thermoelectric performances can be calculated as a function of various design parameters in these simulation tools to find optimal design conditions and learn the basic principles. For material simulation, a Boltzmann transport simulator has been developed, which offers prediction of thermoelectric material properties based on the band structure and scattering parameters of the material. Finally, we introduce an online course developed for education on topics in thermoelectrics from the principles and atomic-scale devices to large-scale systems and applications. In addition to the wide use of the simulation tools expected for individual research of engineers and scientists, the online course is also designed to utilize the tools for students learning with associated exercise problems.

The following sections are included:

• Energiology
• Distributed Power Generation and Scalability
  • Scaling to smaller dimensions
  • Off-grid power generation
  • Hybrid renewables and heat pumps for dispatchability
• Engineering the ZT
  • Figure ofmerit
  • Thermoelectric energy conversion
  • Trade-off between electrical conductivity and the Seebeck coefficient
  • Optimum band gap of thermoelectric material
  • Electrical properties of low-dimensional and 2D material thermoelectrics
  • What can we learn from nanoelectronics and nanophotonics in energy applications?
  • Polymer/Organic Thermoelectrics
• Role of Advanced Computing Power
• Material Cost/Efficiency Trade-Off
• Summary
• References

The following sections are included:

• Nomenclature
• Index