An online simulator for thermoelectric cooling and power ... - nanoHUB

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software. The simulation tool solves the heat balance equations at the top and bottom ... taking into account heat flow and electrical energy supplied to a load.
An online simulator for thermoelectric cooling and power generation Je-Hyeong Bahk, Megan Youngs, Kazuaki Yazawa, Ali Shakouri Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907, USA [email protected] Abstract— We present an online simulator that can be used to teach the principles of thermoelectric energy conversion, and analyze the detailed performance of Peltier coolers or thermoelectric power generators with simple user interfaces [1]. The simulation tool is implemented on nanoHUB.org, so it can be run on any web interface without the need to install commercial software. The simulation tool solves the heat balance equations at the top and bottom sides of the thermoelectric device using 1D thermal network model and the electric circuit model to analyze the steady-state temperatures of the device and the thermoelectric energy conversion efficiency. Both cooling and power generation modes can be solved upon user’s input. Using this simulator, users are able to optimize the performance of a thermoelectric device with a variety of different design parameters such as the device dimensions and material properties. In particular, this simulator can be very useful to teach the importance of the thermoelectric figure of merit, ZT, of the material used on the performance of the device. This simulation is also instructive to show that as material properties improve Carnot limit can be achieved at negligible output power, while efficiency at maximum output power converges to CurzonAhlborn limit. Keywords— online simulator; thermoelectric; cooling; power generation;

I.

INTRODUCTION

As the world strives to solve the energy problem, there has been a growing interest in strengthening education in renewable energy technologies and related fields [2]. Since most of the energy technologies are interdisciplinary, conventional approaches to the higher-level teaching of such subjects may not be applicable. For example, the efficiencies of state-of-the-art energy conversion systems are determined not only by the materials used, but also by the design of deviceand system-level electrical circuits connected to it. For this, an interdisciplinary education encompassing a broad range of science and engineering fields is necessary to meet the everrising demand of energy technology engineers and researchers. An online simulator is a very useful method for energy technology education. First, it can provide students an easy access to the complicated energy systems in a virtual environment. Second, the operation principles or performances of the systems can be easily understood and evaluated for undergraduate students without directly solving the difficult mathematical equations. Third, online simulators can be

Oxana Pantchenko Baskin School of Engineering, University of California, Santa Cruz, CA 95064, USA [email protected]

directly used in real research activities for graduate students where the system optimization and evaluation in terms of various design parameters for the system are crucial. One of the great examples of an energy system that can take full advantages of an online simulator for education purposes is thermoelectric (TE) energy conversion system. Recently thermoelectric energy conversion has received much attention for waste heat recovery and for micro-chip cooling applications [3]. More than 55% of energy generated in the society is wasted in the form of heat so thermoelectrics could play an important role in future energy landscape [4]. Analysis of a thermoelectric device is complicated because one needs to solve the coupled thermal and electric transport equations taking into account heat flow and electrical energy supplied to a load. An online simulator for TE devices may be very useful for students to have an easy access to devices that couple electron and heat transport and energy conversion applications. In this paper, we present an online simulator that can be used to teach the principles of thermoelectric energy conversion, and analyze the performance of thermoelectric devices. The simulation tool can be used on any web interface without the need to install MATLAB or other commercial software, as it is implemented on nanoHUB.org with simple and easy user interface. We are currently developing several course modules at UC Santa Cruz and Purdue University that utilize this online simulator to teach undergraduate students the principles of thermoelectric energy conversion devices. Lastly, we will also show the methodology to propose student project using the online TE simulator as well as the expected outcomes and evaluation plan. II.

SIMULATION OF THERMOELECTRIC DEVICES

A. Principles and modeling of thermoelectric devices The underlying physics of thermoelectric cooling is Peltier effect. When a current flows through an interface between two dissimilar materials, thermal energy is absorbed or dissipated depending on the direction of current flow in order to compensate the difference in thermal energy transported by electrons in the two materials. The thermal energy Q transported by a current I in a material having a Seebeck coefficient S at absolute temperature T is given by Q=STI. Thus, the absorbed or dissipated heat at the interface between material 1 and material 2 by the Peltier effect becomes the

This work was supported by the Center for Energy Efficient Materials, one of the Energy Frontier Research Centers of The Office of Science, U.S. Department of Energy (J.-H. B., K.Y. and A.S.), NSF TUES grant 1238565 (O. P. and A.S.), and Purdue SURF program (M.Y.). 2013 IEEE Frontiers in Education Conference 978-1-4673-5261-1/13/$31.00 ©2013 IEEE 1757

difference of the transported thermal energies or Q1-Q2=(S1S2)TI. In addition, there are other thermal transport effects: Joule heating and thermal conduction. The Joule heating is I2R in a material with electrical resistance R. This heat is equally divided and propagated to the two ends of the material. When a temperature gradient is created through a material, heat conduction Qcond occurs from the hot side to cold side proportional to the temperature difference ΔT, so that Qcond = ΔT/ψ, where ψ is the thermal resistance of the material. Fig.1(a) shows a typical structure of a thin film TE device fabricated on a conducting substrate. A top metal contact is deposited on top surface of the thin film TE element, and a ground contact is deposited at the bottom of the substrate, so that both electrical current and heat flow vertically through the whole structure. In our simulator, we solve the heat balance equations obtained at the top surface of the TE element and at the interface between the TE element and the substrate from the 1D thermal network model shown in Fig. 1(b). The heat balance equation depicts that the Peltier heating/cooling, the Joule heating, and the heat conduction are all in balance. It is important to include the heat and current 3D spreading effect in the substrate, because the substrate is much larger than the TE device on it. A closed-form thermal spreading resistance in a substrate is adopted from Lee et al. [5] with assumption of an infinitely large substrate and a perfect heat sink underneath it. The electrical spreading resistance is obtained from Vashaee et al. [6], which is based on ANSYS finite element simulation. B. Simulation method and boundary conditions A thermoelectric device works as both a cooler and a power generator. In the cooling mode, a current is injected into the TE device. Two boundary conditions can be simulated: one is that the cooling power Q1 is known, from which T1 and T2 are calculated by solving the coupled heat balance equations. The other boundary condition is that the top surface temperature T1 is known, from which T2 and the cooling power Q1 are calculated. Then the coefficient of performance (COP) is obtained by

COP =

Q1 , W

(1)

where W is the work done by the electrical current to achieve the cooling performance given by W = IVOC + I 2 Ri .

(2)

where Voc is the open-circuit voltage induced internally in the device by the Seebeck effect, Voc=STE(T1-T2)+Ssub(T2-Tamb), and Ri is the total internal resistance of the device, Ri=Rc+RTE+Rsub. In the power generation mode, a heat energy Q1 is injected into the device from the top, which creates a temperature gradient across the device and generate a voltage by the Seebeck effect. A load resistance RL is connected to the device, so that a current flows through the load to generate a power output, Pout= I2RL. The current is obtained by an electrical circuit model, in which the open-circuit Seebeck voltage source has an internal resistance and is connected to the external load.

Fig. 1 (a) Schematic of a thin film TE device on a substrate and (b) the corresponding 1D thermal network model

There are two boundary conditions for the power generation mode as well. One is that the heat input Q1 is known, and then we calculate T1, T2, and I by solving the coupled heat balance equations. The other option is that T1 is known, and we calculate Q1, T2, and I. Then the thermoelectric energy conversion efficiency η is obtained by

η=

Pout . Q1

(3)

In both the cooling and power generation modes, users can select an option that there is no substrate. A practical TE module consists of multiple n-type and ptype TE elements that are connected electrically in series, and thermally in parallel [3]. The online simulator presented in this paper has also the capabilities of simulating multi-element modules having both n- and p-type elements. Users are asked to input the dimensions and material properties of the n- and ptype elements as well as the number of the elements. C.

Implementation on nanoHUB.org We have implemented the simulator on nanoHUB.org. NanoHUB provides an easy and simple Java-based graphical user interface. We wrote the simulator core in MATLAB script, and integrated the code with the user interface of nanoHUB using Rappture toolkit [7]. Users can access and run the program by simply visiting the website. Upon the start of the program, an introduction page is shown up to give users a brief overview and instructions on how to use the simulator as shown in Fig. 2(a).

Then, users can select from a pull-down menu a simulation option in either cooling or power generation mode for single element simulation or multi-element module simulation. In the second page of the simulator (Fig. 2(b)), users are asked to choose an independent variable, for which all the outputs are calculated and plotted as its function, and the range of the independent variable to simulate, and enter material properties and dimensions of the TE elements. Then simulation is performed when the simulate button is pressed by user, and the resulting output data are plotted as a function of the independent variable as shown in Fig. 2(c) as an example.

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undergraduate classes in physics and electrical engineering departments are good places to use this simulator, in which students do not have to use commercial engineering software such as MATLAB or Mathematica, but simply go on the website and run the simulator. Also, this simulator can be useful as a supplementary software in conjunction with handson thermoelectric energy conversion experiments in introduction to renewable energy classes [8]. Students should have basic knowledge of classical physics on thermal transport and thermodynamics prior to using this simulator. Understanding of basic electrical circuits is also necessary. The simulator is very versatile and it allows students to study the limit of thermoelectric power generation as the material properties are improved. Carnot limit (1- Tcold/Thot) can be achieved at negligible output power, while efficiency at maximum output power converges to Curzon-Ahlborn limit (1√Tcold/√Thot) [9].

Fig. 2 (a) Introduction page of the online TE device simulator on nanoHUB.org, (b) material properties and simulation option page, and (c) an example of simulation results (net cooling vs. current) for various sizes of TE devices.

III.

PRACTICES, EVALUATION AND FUTURE WORK

This online simulator is built and designed for use in classes where students are taught the principles of TE devices and the detailed performance analyses of the system. For that purpose, we offer a variety of independent variables for users to choose from in this simulator, so that students can do performance optimization and comparison of TE devices in terms of the selected independent variables in a simple and easy way. For example, a user can choose the load resistance as an independent variable in the power generation mode, and find out that the power output is maximized when the load resistance is equal to the internal resistance of the device, the so-called load-matching condition. For another example, the material properties such as the Seebeck coefficient S, electrical conductivity σ, and thermal conductivity κ can be chosen as an independent variable for a user to learn how the thermoelectric energy conversion efficiency is determined as a function of those properties. This way, students can learn the importance of the figure or merit ZT=S2σT/κ of the material for enhancing the efficiency of the device. This simulator is suitable for undergraduate-level students who are beginning to learn the principles of thermal transport physics and thermoelectric energy conversion, as well as for graduate students who are doing research on their own design of TE devices and systems. In particular, students from materials and chemical engineering departments who grow or synthesize their new TE materials can benefit from using this simulator as they want to predict realistic performance of the future devices that utilize their materials. Additionally, the

A suggested method of evaluating what students have learned from the simulator is to give each student a questionnaire before and after the use of simulator, which contains a set of questions relevant to the subject of the class. Also, students can take an on-site quiz in a computer lab with the internet access at the end of the academic term, where they run the simulator in a real time and solve the problems related to the performance of TE devices. The simulator provides users functionalities that export the simulation results into text or MS Excel files. Students can generate plots with this simulator. The online simulator has been installed for public use on nanoHUB.org [1]. It will be constantly upgraded. We are planning to use this simulator in classes at UC Santa Cruz and Purdue University and continue to upgrade it based on the feedback received back from students, researchers and instructors. REFERENCES [1] [2] [3]

[4] [5]

[6]

[7] [8]

[9]

Our online simulator can be found at https://nanohub.org/tools/thermo. P. Jennings, “New directions in renewable energy education,” Renew. Energ., vol. 34, pp. 435–439, July 2009. L. E. Bell, “Cooling, heating, generating power, and recovering waste heat with thermoelectric systems,” Science, vol. 321, pp. 1457-1461, Sept. 2008. Lawrence Livermore Nat. Lab., “Estimated U.S. energy use in 2011,” https://flowcharts.llnl.gov/, Oct. 2012. S. Lee, S. Song, V. Au, and K. P. Moran, “Constriction/spreading resistance model for electronic packaging,” Proceedings of the 4th ASME/JSME Thermal Engineering Joint Conference, Vol. 4, pp. 199206, 1995. D. Vashaee, J. Christofferson, Y. Zhang, A. Shakouri, G. Zeng, C. LaBounty, X. Fan, J. Piprek, J. Bowers, E. Croke, “Modeling and optimization of single-element bulk SiGe thin film coolers,” Microscale Thermophysical Engineering, vol. 9, pp. 99-118, 2005. https://nanohub.org/infrastructure/rappture. O.S. Pantchenko, S. Shahab, D. Tate, P. Matteini, M. Isaacson, Shakouri A., "Work in Progress - Enhancing Students Learning Through Instructional Videos during Hands-On Laboratories on Renewable Energy Sources", Frontiers in Education Conference, October 12-15, Rapid City, South Dakota USA, 2011. K. Yazawa, A. Shakouri, "Optimization of power and efficiency of thermoelectric devices with asymmetric thermal contacts", Journal of Applied Physics 111, 024509, 2012.

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