Journal of ELECTRONIC MATERIALS, Vol. 40, No. 5, 2011 DOI: 10.1007/s11664-011-1523-2 Ó 2011 TMS A Four-Quadrant Operation Diagram for Thermoelectric Modules in Heating–Cooling Mode and Generating Mode W. CHIMCHAVEE1,2 1.—Electrical and Energy Engineering, University of the Thai Chamber of Commerce, 126/1, Vibhavadee-Rangsit Road, Dindaeng, Bangkok 10400, Thailand. 2.—e-mail: wanchai_chi@utcc. ac.th The operation of a thermoelectric module in heating–cooling mode, generating mode, and regenerating mode can be discussed in terms of power, cooling load, and current. A direct current machine in motoring mode and generating mode and an induction motor in motoring mode and regenerating mode are analogous to thermoelectric modules. Therefore, the first objective of this work is to present the four-quadrant (4-Q) operation diagram and the 4-Q equivalent circuits of thermoelectric modules in heating–cooling mode and generating mode. The second objective is to present the cooling and regenerating curves of a thermoelectric module in cooling mode and regenerating mode. The curves are composed from the cooling powers and the generating powers, the input and output current, the thermal resistance of the heat exchanger, and the different temperatures that exist between the hot and cold sides of the thermoelectric module. The methodology used to present the data involved drawing analogies between the mechanical system, the electrical system, and the thermal system; an experimental setup was also used. The experimental setup was built to test a thermoelectric module (TE2) in cooling mode and regenerating mode under conditions in which it was necessary to control the different temperatures on the hot and cold sides of TE2. Two thermoelectric modules were used to control the temperature. The cold side was controlled by a thermoelectric module labeled TE1, whereas the hot side was controlled by a second thermoelectric module labeled TE3. The results include the power, the cooling load, and the current of the thermoelectric module, which are analogous to the torque, the power, the speed, and the slip speed of a direct current machine and an induction motor. This 4-Q operation diagram, the 4-Q equivalent circuits, and the cooling and regenerating curves of the thermoelectric module can be used to analyze the bidirectional current and to select appropriate operating conditions in the cooling and regenerating modes. Key words: Four-quadrant operation diagram, four-quadrant equivalent circuits, cooling and regenerating curve, heating–cooling mode, generating mode, regenerating mode, thermoelectric modules, bidirectional current INTRODUCTION Thermoelectric devices typically consist of a series of n-type and p-type semiconductors electrically (Received April 29, 2010; accepted January 17, 2011; published online February 8, 2011) connected at one end to form an n–p junction. Series connection of such n–p junctions proportionally increases the voltage of the thermoelectric device, whereas the current remains constant throughout the module. The n–p junctions and the p–n junctions can be split into two components, yielding an electrical terminal for input and output power 707 708 known as a ‘‘module.’’ If direct current is delivered to both electrical terminals of the thermoelectric module, heat is rejected on one side. This makes the other side cooler; this is known as ‘‘heating–cooling mode’’ and is required for intended uses of heating and cooling. Clearly, one side of the thermoelectric module ventilates the heat, but the other side of the module has a thermal energy input that causes a temperature difference between the hot side and the cold side. When this happens, a direct current voltage may develop across the two terminals, creating a power generator, i.e., so-called generating mode. Case studies of the generating mode have been conducted in the past. In 2004, Weiling et al. proposed that thermoelectric power generators could be used for numerous applications.1 Mikhailovsky also explored the physical characteristics of a 100-W thermoelectric power generator that was used with cooling water.2 In 2005, Bodnaruk published the results of an analysis of the characteristics and parameters of a thermoelectric module that can change thermal energy into an alternating current.3 In 2006, Rowe suggested that thermoelectric modules be used to generate power by using waste heat recovery and renewable energy sources.4 In 2007, Zhang et al. proposed an evolution of the thermoelectric module for generation of power in China.5 Lidorenko et al. did the same for Russia,6 and Kajikawa did the same for Japan.7 Moreover, Anatychuk analyzed and developed a material used to produce the thermoelectric module by considering other problems and provisions.8 Pustovalov also proposed an evolution of the thermoelectric module that has been used to generate power in space for the past 40 years.9 In addition, Strutynska et al. proposed a physical model and analyzed the basic characteristics of a thermal source power generator with a small thermoelectric module that used gas and liquid fuel.10 It was intended to be used for analysis of power generator systems. Anatychuk et al. also analyzed the efficiency of a thermal source power generator system with a thermoelectric module that used gas and liquid fuel.11 In 1997, Lau et al. calculated the performance of a thermoelectric power generator by analyzing a finite-element model.12 Lineykin et al., in 2005, analyzed a model of a thermoelectric module capable of cooling and generating power by simulating a Simulation Program with Integrated Circuit Emphasis (SPICE)-compatible equivalent circuit.13 Furthermore, Lobunats analyzed the performance of a thermoelectric module that could generate heat found in the heating–cooling system.14 In contrast, Mitrani et al. studied the impact of temperature on the lumped parameters of a material used for producing thermoelectric modules. This was done by simulating with a SPICE model.15 Additionally, Chen et al. studied how the heat transfer properties of the thermoelectric module affect the heat absorption within the module, which is useful Chimchavee information when modeling power generation with mathematical calculations.16 In the case of the heating–cooling mode, it appears that most of the research studies use thermodynamic theory and mathematical methods.17 Nevertheless, Chimchavee analyzed a thermoelectric module in cooling mode using an electrothermal model, the so-called CHIM model. This proposed model is based on Kirchoff’s laws and a PSPICE program.18 In 2003– 2009, Chimchavee also analyzed a sine wave temperature generator along with the concept of a thermoelectric module plus analysis of thermoelectric modules in heating–cooling and generating modes using electrical circuit theory.19,20 The main points of interest are the operation of a direct current machine in motoring mode and generating mode at a steady state. In motoring mode, the voltage input (V) is equal to the sum of the electromotive force (Em) and the voltage drop from the resistance of the conductor (IR) (Eq. 1). The power input (Pin) is equal to the sum of the power output (IEm) and the power loss of the conductor (I2R) (Eq. 2). In generating mode, the load voltage (VL) is the difference between the electromotive force (Eg) and the voltage drop from the resistance of the conductor (Eq. 3). The load power (PL) is equal to the difference in the power output (IEg) and the power loss of the conductor (Eq. 4). The electromotive force created by the motor and the generator is the product of a constant value for the machine (K), the magnetic flux (U), and the speed (Eq. 5). The basic equation given in Eq. 6 is the relationship between the power (P) and the speed (x) for the operation of an electrical machine. From the perspective of a motor, the torque and speed are the outputs and the power is the input (T = P/Px). From the perspective of a generator, the speed and the torque (T) are inputs and the power is the output (P = Tx). In Eqs. 2 and 4–6, the torque and power equations of a direct current machine can be written in motoring mode and generating mode, as shown in Eqs. 7 and 8. The relationship between input and output can be explained by a four-quadrant (4-Q) operation diagram as shown in Fig. 1. The first quadrant represents the forward motoring mode; the torque and power are positive, and the speed is positive. The second quadrant represents the forward generating mode; the torque and power are negative, and the speed is positive. The third quadrant represents the reverse motoring mode; the torque and power are negative, and the speed is negative. The fourth quadrant represents the reverse generating mode; the torque and power are positive and the speed is negative. In the case of the motoring mode, V ¼ Em þ IR; (1) Pin ¼ IEm þ I 2 R: (2) A Four-Quadrant Operation Diagram for Thermoelectric Modules in Heating–Cooling Mode and Generating Mode Fig. 1. 4-Q operation diagram of a direct current machine. Fig. 2. Torque, power, and slip speed curves of an induction motor. In the case of the generating mode, VL ¼ Eg IR; (3) PL ¼ IEg I2 R: (4) The basic equation of an electrical machine is Em ¼ Eg ¼ KUx; (5) P ¼ Tx: (6) Therefore, in motoring mode, T ¼ KUI ¼ ðPin I 2 RÞ x; (7) and in generating mode, P ¼ KUIx ¼ PL þ I 2 R : 709 (8) The first objective of this paper is to present the 4-Q operation diagram and the 4-Q equivalent circuits for thermoelectric modules at steady state in heating–cooling mode and generating mode, which are analogous to the 4-Q operating diagram of a direct current machine at steady state in motoring mode and generating mode. For an induction motor, the input and the output at steady state in motoring mode and regenerating mode are important; the power is related to the slip speed (S) when there is constant power loss of the rotor conductor (I22R2). The slip speed is the difference between the synchronous speed (xs) and the rotor speed (xr); to be valuable, the maximum value of the slip speed should be equal to one unit when the rotor speed is equal to zero, as shown in Fig. 2 and Eqs. 9 and 10. The power and slip speed curves are defined at the origin where the slip speed is equal to zero or the synchronous speed. The induction motor is operated in motoring mode when the slip speed is positive and in regenerating mode when the slip speed is negative. The first quadrant represents the motoring mode; the rotor speed is slightly more than the synchronous speed, the power is positive, and the slip speed is positive. The third quadrant represents the regenerating mode; the rotor speed is more than the synchronous speed, the power is negative, and the slip speed is negative.21 For example, an electric car accesses the motoring mode when operated in acceleration mode, and it accesses the regenerating mode when it is operated in deceleration mode. The energy produced in the regenerating mode can be stored in batteries. The second objective of this paper is to present the cooling and regenerating curves of the thermoelectric cooling system at steady state in cooling mode and regenerating mode to discuss the relationship between the power, the cooling load, and the current analogies to the power and slip speed curves of an induction motor at steady state in motoring mode and regenerating mode. ð1 SÞ ; S (9) S ¼ ðxs xr Þ=xs : (10) P ¼ I22 R2 The analogies between the mechanical system, the electrical system, and the thermal system used in this paper are presented in Table I.22 The power, the cooling load, and the current are the inputs and the outputs of thermoelectric modules, being analogous to the power, the speed, and the slip speed, which are the inputs and the outputs of a direct current machine and an induction motor. These analogies can be used to design a system with an optimized capacity for storing energy in the battery and to protect the thermoelectric module from thermal oscillations. 710 Chimchavee Table I. Electrical–thermal–mechanical analogies Mechanical System Electrical System Thermal System Torque (T) Speed (x), slip speed (S) Power (P) N/A N/A Voltage (V) Current (I) Temperature (T) Heat flow (Q) Power (P) Conductivity (K) Resistance (R) N/A Conductivity Resistance (R) BASIS FORMULA AND MODEL The thermoelectric module consists of a heating– cooling mode and a generating mode. The basic device comprises a power source or a thermal source, the thermoelectric modules, and the heat exchanger on the cold side and on the hot side (Fig. 3). The analysis will neglect the leakage loss (Qa, W). When Qin is the thermal input (W), PL is the power output (W), and Tin is the temperature input for the generating mode, when expressing temperatures in Kelvin (K). Pin is the power input for the heating–cooling mode (W). In the heating– cooling mode, an electrical current (I) introduced to the thermoelectric module at the cold junction that the cooling load (Qc) will be pumped from the atmosphere (Ta) to the thermoelectric module via the heat exchanger (Rc) and emit heat to the atmospheric at the hot junction with the other heat exchanger (Rh). On the other hand, in generating mode, thermal energy (Qh) is inputed to the thermoelectric module on the other side, which creates a temperature increase on the surface (Th) via the heat exchanger (Rh) and the heat ventilator (Qc) on the other side of the thermoelectric module. The temperature at the surface (Tc) rises and reaches atmospheric temperature through the thermal resistance of the heat exchanger (Rc). This may cause a difference in temperature between the two sides of the thermoelectric module, which generates a direct current to the load. Equations 11–23 illustrate the relationship between the two modes.23,24 In the heating–cooling mode, the power input is the sum of the Seebeck power (aIDT) and the electrical loss or Joule heat (I2R) in the thermoelectric module. In the generating mode, the power output is the difference between the Seebeck power and the electrical loss in the thermoelectric module. In the case of the heating–cooling mode, COP ¼ Qc =Pin ; (11) 1 Qc ¼ aITc DTK I 2 R; 2 (12) 1 Qh ¼ aITh DTK þ I 2 R; 2 (13) Fig. 3. Configurations of thermoelectric modules in heating–cooling mode and generating mode. V ¼ aDT þ IR; (14) Pin ¼ Qh Qc ¼ aIDT þ I2 R: (15) In the case of the generating mode, g ¼ PL =Qh ; (16) Qh ¼ PL þ Qc ; (17) 1 Qh ¼ aITh þ DTK I 2 R; 2 (18) 1 Qc ¼ aITc þ DTK þ I 2 R 2 (19) VL ¼ aDT IR; (20) PL ¼ aIDT I22 R: (21) The following relationships were assumed: Voc ¼ aDT; (22) VL ¼ IRL : (23) The COP is the coefficient of performance of the cooling mode. g is the generation efficiency of the generating mode. Qc is the cooling load and the ventilation heat (W) at the cold side. Qh is the rejected heat and the thermal input at the hot side (W). a is the Seebeck coefficient of the thermoelectric module (V/K). Th is the temperature at the hot side (K). Tc is the temperature at the cold side (K). DT is the difference in temperature between the hot side and the cold side of the thermoelectric module (K). K is the thermal conductance of the thermoelectric module (W/K). R is the inner electrical resistance of the thermoelectric module (X). V is the voltage input (V). Voc is the Seebeck voltage and the open-circuit voltage (V). VL is the load voltage (V). A Four-Quadrant Operation Diagram for Thermoelectric Modules in Heating–Cooling Mode and Generating Mode I is the input and output current (A). RL is the resistance load (X). FOUR-QUADRANT OPERATION ANALYSIS This study designed and analyzed a 4-Q operation diagram for a thermoelectric module operated in heating–cooling mode and generating mode by referencing basic electrical theory and the concept of an equivalent circuit (Fig. 3 and Eqs. 11–23). It was assumed that the ground was 0 K. The approach can be modeled as an electrical equivalent circuit in heating–cooling mode and generating mode with electrothermal coupling by making use of analogies between the thermal and electrical systems. Figure 4 shows that, for the electrothermal model of the thermoelectric heating–cooling system, the CHIM model is the equivalent circuit of the thermoelectric module.18,19 Figure 5 shows the analysis model of the thermoelectric generating system. The electrothermal coupling is the power coupling between the thermal domain and the electrical domain. Therefore, the Seebeck power and the electrical loss can be transferred to the electrical domain. Rl is the thermal leakage resistance (K/W) between the hot side and the cold side of the thermoelectric module. Rc is the thermal resistance of the heat exchanger on the cold side (K/W). Rh is the thermal resistance of the heat exchanger on the hot side (K/W). Rpc is the thermal resistance of the cooling load (K/W). Qj is the Joule heat of the thermoelectric module (W). G is the thermal gain factor. In Figs. 4 and 5, aITc equals Rpc, aITh equals GTh, and I2R equals Qj. These relationships can be drawn as the analysis model in heating–cooling mode and generating mode, which is shown in Fig. 6.20 The heat flow and the electrical current appear to move in different directions, or bidirectional current. Their relationships are given in Eqs. 11–23. In Fig. 6, in heating–cooling mode, when a voltage (V) is supplied and heat (Qh) is rejected, I and Qc have an inner flow direction, whereas Qh has an outer flow direction. In generating mode, when heat (Qh) is supplied and heat (Qc) is rejected, I and Qc have an outer flow direction. These variables represent the relationship between the voltages, the power, the cooling load, and the current of the thermoelectric module. The voltage and power Fig. 4. Electrothermal model for the thermoelectric heating–cooling system. 711 inputs are shown in Eqs. 12, 14, and 15 in heating– cooling mode. The voltage and power outputs are shown in Eqs. 18, 20, and 21 in generating mode. Equations 14 and 15 of the thermoelectric module in heating–cooling mode are analogous to Eqs. 1 and 2 of the direct current machine in motoring mode. Equations 20 and 21 of the thermoelectric module in generating mode are analogous to Eqs. 3 and 4 of the direct current machine in generating mode. To visualize the analogy between the thermoelectric module and the direct current machine, Figs. 3 and 6 can be used to create the 4-Q operation diagram and the 4-Q equivalent circuits for the thermoelectric module in heating–cooling mode and generating mode, which are illustrated in Figs. 7 and 8, respectively. In Figs. 7 and 8, the vertical axis shows the cooling load (Qc) and the power generation (PL). The horizontal axis shows the current input (I) and the output. The first quadrant represents operation in the forward heating–cooling mode, which supplies the electrical current (I) to a positive pole, resulting in heat flow to the top. The cooling load (Qc) has a positive direction, passing through the thermoelectric module and the heating and venting (Qh) to exit at the bottom. The third quadrant represents operation in the reverse heating–cooling mode, which supplies the current (I) to a negative pole, causing the heat to flow to the bottom, which has a negative cooling load (Qc). This will pass the heat ventilation of the thermoelectric module (Qh) to the top. The fourth quadrant represents operation in the reverse generating mode in which heat (Qh) is Fig. 5. Analysis model of the thermoelectric generating system. Fig. 6. Analysis model in heating–cooling mode and generating mode. 712 Chimchavee Fig. 7. 4-Q operation diagram of thermoelectric modules. Fig. 8. 4-Q equivalent circuits of thermoelectric modules. input to the top and heat is ventilated (Qc) to the bottom. This will generate an electrical current (I) that flows to a negative pole and a load that has positive power, whereas the second quadrant represents operation in the forward generating mode in which heat (Qh) is input to the bottom and A Four-Quadrant Operation Diagram for Thermoelectric Modules in Heating–Cooling Mode and Generating Mode heat is ventilated (Qc) to the top, causing a current (I) to be generated. The current flows to a positive pole and to the load that has negative power. We present a 4-Q operation diagram and a 4-Q equivalent circuit for thermoelectric modules at steady state (Figs. 7, 8) and discuss the relationship of the power, voltage, cooling load, and current in heating–cooling mode to the analogous power, voltage, current, and speed which can be represented by a 4-Q operation diagram of a direct current machine in motoring mode and generating mode (Fig. 1). Equations 12, 14, 15, 18, 20, and 21, which describe the voltage, power, cooling load, and current, are the inputs and outputs of a thermoelectric module, and are analogous to Eqs. 1–4, 7, and 8, which describe the voltage, current, power, and speed as the inputs and outputs of a direct current machine. EXPERIMENTAL RESULTS FOR COOLING AND REGENERATING CURVES Application of several thermoelectric modules that use a parallel pattern in thermal oscillation will always occur in the cooling mode and the regenerating mode. Both phenomena may cause energy to be transferred between the inner modules of the system. Therefore, a control system is required to protect the thermoelectric module, such as n–p or p–n connectors, from melting. Moreover, the large thermoelectric cooling system can store energy when the current stops charging the thermoelectric module. This causes a temperature difference that is capable of generating power. If a control system is used, it may stop heat ventilation on the hot side, and it may emit heat on the cold side. This is the regenerating mode of the thermoelectric module. An experiment is used to discuss the cooling and regenerating curves, analogous to the torque, power, and slip speed curves of the induction machine. The experimental design consists of a laboratory-scale thermoelectric module, i.e., Melcor CP 1.4-127-045L (40 mm 9 40 mm 9 3.3 mm, 713 Imax = 8.5 A, Vmax = 15.4 V, Qmax = 72 W, DTmax = 67 K, n = 127 couple, g = 0.171 cm, a0 = 22,224, a1 = 930.6, a2 = 0.9905, r0 = 5112, r1 = 163.4, r2 = 0.6279, k0 = 62605, k1 = 277.7, k2 = 0.4131). Three sets were used (each set is equal to one module). The first thermoelectric module (TE1) was operated as the cooling load (QcTEC) for the cooling mode and as the heating ventilator (QcTEG) for the regenerating mode, and it was used to control the temperature on the cold side. The next thermoelectric module (TE3) was operated as a heating ventilator (QhTEC) for the cooling mode and as a thermal source (QhTEG) for the regenerating mode, and it was used to control the temperature on the hot side of the final thermoelectric module (TE2). TE2 requires testing of the cooling mode and the regenerating mode. Figure 9 shows a block diagram of the thermoelectric module used for testing the system. The testing system can control the operation of TE1 and TE3 to keep the temperature difference between the cold and hot sides (DT) of TE2 constant by alternating the electrical pole charges on both thermoelectric modules. To test the system in cooling mode, a current was inputed to the pole of TE2. In regenerating mode, an electrical load was connected to the pole of TE2. The experimental data were measured using a Testo454 (error ±0.1°C) and a METRA Hit29s (error ±0.2%). The cooling load (Qc) was calculated by Eq. 4, which explains the cooling and regenerating curves of the thermoelectric module in cooling and regenerating mode as illustrated in Fig. 10. At the origin, the input and output current are equal to zero. The thermoelectric module is operated in cooling mode when the current is positive and in regenerating mode when the current is negative. In cooling mode, the horizontal axis is the input current (A) and the vertical axis is the cooling load (W). In regenerating mode, the horizontal axis is the output current (A) and the vertical axis is the output power (W).25 The first quadrant represents the cooling and current curves for operation in cooling mode, when the current Fig. 9. Block diagram of the thermoelectric module used to test the system. 714 Chimchavee P & Qc (W) 80 Cooling Mode 70 Rh=0.05ºK/W 60 Rh=0.20ºK/W 50 40 Rh=0.35ºK/W 30 20 10 I (A) 0 -2 0 2 4 6 8 10 -10 Regenerating Mode Fig. 10. Cooling and regenerating curves of the thermoelectric module. input for the thermal resistance of the heat exchanger at Kh is equal to 0.05 K/W, 0.20 K/W, and 0.35 K/W. The third quadrant represents the power and current curves for operation in regenerating mode, when the load resistance (RL) for DT is equal to 10 K, 20 K, and 30 K. The COP of the cooling mode was approximately 0.72 when Kh was 0.20 K/ W and I was 7 A. The system efficiency of the regenerating mode was approximately 5.12% when DT was 30 K and I was 0.26 A. The relationship between the power, the cooling load, and the current of the thermoelectric cooling system in cooling mode and regenerating mode is shown in Fig. 10. The analogy to the power and slip speed curves of an induction motor in motoring mode and regenerating mode is shown in Fig. 2. Equations 12 and 21 represent the power, cooling load, and current, which are inputs and outputs of the thermoelectric module, analogous to Eq. 9, which represents the power and slip speed and the inputs and outputs of an induction motor. CONCLUSIONS The 4-Q operation diagram and the 4-Q equivalent circuits of thermoelectric modules at steady state in heating–cooling mode and generating mode are analogous to the 4-Q operation diagram of a direct current machine in motoring mode and generating mode. The first quadrant represents the forward heating–cooling mode of the thermoelectric modules, which is analogous to the forward motoring mode of a direct current machine. The second quadrant represents the forward generating mode of the thermoelectric modules, which is analogous to the forward generating mode of a direct current machine. The third quadrant represents the reverse heating–cooling mode of the thermoelectric modules, which is analogous to the reverse motoring mode of a direct current machine. Finally, the fourth quadrant represents the reverse generating mode of the thermoelectric modules, which is analogous to the reverse generating mode of a direct current machine. The cooling and regenerating curves of the experiment in cooling mode and regenerating mode are analogous to the power and slip speed curves of an induction machine in motoring mode and regenerating mode. The first quadrant represents the cooling mode of the thermoelectric modules, which is analogous to the motoring mode of an induction machine. The third quadrant represents the regenerating mode of the thermoelectric modules, which is analogous to the regenerating mode of an induction machine. Because the electrical system and the thermal system of thermoelectric modules are analogous to the mechanical system and the electrical system of a direct current machine and an induction machine, the 4-Q operation diagram, the 4-Q equivalent circuits, and the cooling and current curves of the thermoelectric module can be used to analyze the bidirectional current and to design a thermoelectric cooling system that operates in both cooling and regenerating modes. ACKNOWLEDGEMENTS This research was completed with the support of funding from the Research Support Office at the University of the Thai Chamber of Commerce. The author would like to express special gratitude to the university. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. L. Weiling and T. Shantung, Chin. Sci. Bull. 12, 1212 (2004). V.Ya. Mikhailovsky, J. Thermoelectr. 2, 82 (2004). V.I. Bodnaruk, J. Thermoelectr. 1, 74 (2005). D.M. Rowe, Int. J. Innov. Energ. Syst. Power 1, 13 (2006). J. Zhang, B. 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