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Modeling and Power Conditioning for Thermoelectric Generation

Lihua Chen Dong Cao, Yi Huang, and Fang Z. Peng Michigan State University/Electrical and Computer Engineering Department, East Lansing, USA

Abstract—In this paper, the principle and basic structure of the thermoelectric module is introduced. The steady-state and dynamic behaviors of a single TE module are characterized. An electric model of TE modules is developed and can be embedded in the simulation software for circuit analysis and design. The issues associated with the application of the TEG models is analyzed and pointed out. Power electronic technologies provide solutions for thermoelectric generation with features such as load interfacing, maximum power point tracking, power conditioning and failed module bypassing. A maximum power point tracking algorithm is developed and implemented with a DC-DC converter and low cost microcontroller. Experimental results demonstrated that the power electronic circuit can extract the maximum electrical power from the thermoelectric modules and feed electric loads regardless of the thermoelectric module’s heat flux and load impedance or conditions.

I. INTRODUCTION

Thermoelectric modules (thermoelectric devices) aresolid state devices that can convert thermal energy from a temperature gradient into electric energy as powergenerators or vice versa (convert electrical energy into a temperature gradient across the modules) for cooling applications [1]. Unlike the thermocouples used fortemperature measurement, the thermoelectric generation (TEG) modules can generate enough electricity for many applications [2]. Compared to other energy conversion methods, TEG modules are reliablewithout any noise or vibration and are attracting more attention as renewable energy sources where temperature gradients are available. Because of the low efficiency and cost issues, currently the TEG modules have been restricted to special applications, such as powering the onboard electronics of a satellite in deep space, and providing electricity from nuclear heat for a submarine in deep waters [3, 4]. Recently, due to environmental concern and global warming issues, TEG modules have received more attention as a method to convert wasted heat to electricity. For example,converting exhaust heat in antomobiles to power accessory electric loads in a vehicle and put the energy back to the wheel to increase efficiency [5, 6].

As shown in Fig. 1, TEG modules usually consist of an array of 2N pellets (n- and p- type semiconductorthermoelements) that make up N thermoelectric couples thermally in parallel and electrically in series to achieve high output voltage and high power [7]. Usually these pellets are connected by high conductive metal strips and sandwiched between thermal conducting while electrically insulating metalized ceramic plates. The

pellets, tabs and substrates thus form a layered assembly, which is shown in the Fig. 2 [8].

However, the TEG module output voltage and power will change distinctly with the temperature gradient, which means it cannot output stable voltage or constant current and needs a power conditioning circuit as interface between the TEG module and loads. In orderto solve this problem, the electrical characteristic of the TEG module should be modeled for circuit anaylsis and design. Some modeling method has been previously proposed [8],[9]. Some DC-DC converter circuits have been investigated to output stable voltage and improve the TEG module power condition for more applications [10, 11]. And some control methods such as Maximum Power Point Tracking (MPPT) and constant voltage have also been investigated to apply to the TEG module [12]. A TEG module was also used for battery charging and reported in [13].

Fig.1 TEG module configuration

Fig.2 typical TEG module assembly

This paper presents an equivalent electrical circuit model of TEG modules and this model can be easily embedded into simulation software like Pspice or Saber for circuit analysis and design. The issues associated with the application of the TEG models is analyzed and pointed out. Power electronic technologies provide power conditioning solutions for thermoelectric generation with features such as load interfacing, maximum power point tracking and failed module

978-1-4244-1668-4/08/$25.00 ©2008 IEEE

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bypassing. A maximum power point tracking algorithmis developed and implemented with a DC-DC converterand low cost microcontroller.

II. CHARACTERIZATION OF THERMOELECTRIC

GENERATION MODULES

At present, the power conditioning technology is mostly based on high frequency switching DC-DC converter and .or inverter designs with high frequency switching device (MOSFET, IGBT etc.) used. So the high frequency characteristics and dynamic response of TEG module are becoming major concerns for the power conditioning circuit design. In order to design a suitable switching dc-dc converter especially for TEG module use, the high frequency electrical characteristics of the TEG module in steady state and in transient have to be known. In the following sections, a method will be proposed to test the TEG module’s terminal electrical characteristics in steady state and in transient, and a high frequency electrical model will be put forward to help design the switching power supply DC-DC converter.

To characterize TEG modules, a heat exchanger capable of outputting 100 W electrical power was designed and fabricated. Fig. 3 shows the decomposed structure of the heat exchanger. Heat elements are distributed evenly within the hot plate to generate a uniform hot side temperature. A total of twenty TEGmodules, G1-1.4-219-1.14, from Tellurex® Corp.[14],are installed on both sides of the hot plate, ten pieces on the top of the hot plate and ten pieces on the bottom of it. Two radiators are clamped on both sides to cool the TEG modules’ cold surface. When a temperature gradient exists between the hot plate and the radiator, electric power is generated by the TEG modules as aresult of heat flow through them. A Thermal couple was used as temperature sensor that was installed in the hot plate and the heat sink to monitor the temperature between hot side and cold side during the tests.

Fig. 3 Heat exchanger configurations

Fig. 4 shows the TEG module heating and testing system flowchart. And this system was used to measure the steady-state and dynamic response of TEG modules. By taking advantage of the thermocoupler as

the temperature sensor, a temperature PID controllercan be used to adjust a solid state contactor to control the power flow from the AC power supply to the heating elements of the TEG modules. In this way, the temperature of the hot plate can be accurately controlled. By using this temperature closed loop control, a stable hot plate temperature can be achieved when the steady state and dynamic response of TEG modules are tested.

Fig. 4 TEG module heating and testing system

Fig. 5 shows the measured steady-state electrical output characteristics of a single TEG module. Three thermal conditions were tested and the V-I curves are plotted. During tests, the heat exchanger hot planetemperature is controlled to 150°C, 115°C, 79°C; the heatsink temperature are 27°C, 29°C, and 31°C, respectively. From the three curves shown in Fig. 4, the internal resistances of the TEG modules under thosethermal conditions are calculated and are 3.5Ω, 3.9Ω, and 4.2Ω, respectively.

Fig.6 shows the test circuit of the electrical characteristics of the TEG module’s dynamic behaviors. Fig.7 shows the captured single TEG module output voltage and current waveforms. Test results reveal that the TEG module has a very fast dynamic response and it will not influence the converter design. Because the TEG module output current can increase in the range of nanoseconds and the switching speed ofDC-DC converter is usually in the range of 50K~1MHz, the switching period will be 1us~20us. Atthis time period the maximum electric power can be drawn from the TEG module nearly instantaneously.

Fig. 5 TEG module steady-state testing results

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Fig. 6 TEG module dynamic test circuit

Fig. 7 TEG module output waveforms

From the above test results, the TEG modules can bemodeled as a voltage source in series with an internal resistance. Both the internal voltage and resistance are highly temperature-dependent [15, 16]. Similar test are also detected when characterizing other TEG modules, e.g. G1-1.4-219-1.14 from Tellurex®. The TEG module equivalent circuit can be embedded into simulation software like Pspice or Saber. It is also a convenient tool for engineers to design electric interface circuits [17].

III. APPLICATION ISSUES OF THERMOELECTRIC

GENERATION MODULES

In practice, the application of TEG modules faces many challengings, mainly because of the low outputvoltage and power, high internal inpedance and hightermature gradient dependence. The characterizing results show that the TEG module’s behavior has similarities with a battery. Both the output voltage and internal resistance of the TEG modules are functions of the temperature gradient between the hot side and cold side. In practice, thermal conditions applied to individual TEG module could be quite different, forinstance, TEG modules mounted on a truck exhaust pipe to convert waste heat to electricity. Therefore, the

TEG modules cannot be simply connected either in series or in parallel to achieve high power output if their electrical output characteristics are different. Otherwise, maximum power output can not be achieved, enven under worst case, the electric energy will circulate among them and some modules may absorb electric power. To facilitate understanding, this problem can be compared to loading new batteries and used batteries in a flashlight. Because of the problems mentioned above, a power electronics circuit is usually necessary for the TEG application for power conditioning, such as a DC-DC converter used to interface the thermoelectric modules and loads, and this DC-DC converter acts asroles of load match, maximum power point tracking, and failed TEG module bypassing

The actual output power of the TEG modules is heavily dependent on the load impedance since its internal resistance is relatively large in comparison with that of the regular batteries. The maximum power can be extracted only if the load impedance matches theTEG module internal impedance. Maximum power point tracking methods should be employed in the applications of the TEG modules [17]. Fig.8 shows the maximum power point tracking algorithm. The TEG output power curve has a maximum power point and bycomparing two sampled power points, the maximum power point can be decided. This allows the converter to be controlled to always work at the maximum power point using a closed loop control as shown in Fig.9.

Fig. 8 Maximum power point tracking algorithm

Fig. 9 Maximum power point tracking control structure

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By adding a DC-DC converter outside the TEG modules, failed TEG modules can be easily bypassed.Without adding a DC-DC converter, TEG modules usually should be connected in series in order to satisfy the load voltage requirement. If one of the TEG modules failed in the series connection, the whole group will not work. If a power conditioning circuit was added, the TEG modules only need to meet the outside power rating; the voltage and current rating can be satisfied by the DC-DC converter. That means theconnection of TEG modules will not influence the power output. In order to minimize the risk of TEG module failure, parallel connection will be the best choice. Because if one of the TEG modules failed in the parallel connection, it will not influence the output voltage. Next, experiment results will demonstrate the features and advantages of the power electronic technologies applied to the TEG applications.

IV. EXPERIMENTAL RESULTS

In the following experiments, twenty TEG modules, as shown in Fig. 3, are grouped into two sets top side and bottom side. First, both groups of ten TEG modules were connected to resistive loads. Each group of ten TEG modules was connected in series with a 40 ohms resistor load, which is close to the total internalresistance of ten TEG modules connected in series. During the test, the hotplate temperature was maintained at 150°C and the heatsink temperature was held at 31°C. The total output voltage measured for the top side was 45V and the electrical power output was 50.6W based on calculation. The voltage output of the bottom side was 44V and the electrical power outputwas 48.4W.

Then, the TEG modules are connected to real loads, specifically, light bulbs. As shown in Fig. 10, the first set of TEG modules is connected to the light bulb directly. Fig.11 shows the second set is connected to the light bulb via a PE (power electronic) circuit. Fig.12 shows the electrical power output vs. ∆T (THot-TCold) curves with three thermal conditions tested. The maximum power output of the first set is only 23W although it is able to output 50.6W. This is due to the heavily mismatch of load impedance and TEG module internal impedance. However, the second set can still output electric power of 47W, which is close to themaximum output power that can be provided by the TEG module set. Fig.9 also shows that the second set can output much more electric power than that of the first one at all thermal condition. In other words, the PE circuit can extract the maximum electric power fromthe TEG modules and feed it to the load regardless of TEG module’s thermal condition.

Fig. 13 shows the schematic drawing of the PE circuit. Actually this is a boost dc-dc converter with synchronous rectification. A low-cost microcontroller (μC with embedded ADC circuit) is employed to implement the PWM control. A stimulating Perturb & Observe (P&O) maximum power point tracking

(MPPT) algorithm is developed and implemented with software programming.

Fig.10. TEG modules directly power a light bulb

Fig.11. TEG modules power a light bulb via a PE circuit

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Fig. 12 TEG modules output power curves

Fig. 14 shows the output voltage and current waveforms of the two sets of TEG modules under the thermal condition of THot=150°C and TCold=31°C. The output voltage and current of the TEG module set two have ripples of less than 5%, although actually a small sized input filter was used in the PE circuit. This is attributed to the relatively larger internal resistance and fast dynamic response of the TEG modules. Fig. 15 shows the output voltage and current of the PE circuit and the TE module set two. The TEG output voltage is

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boosted from 47 V to 112 V, and the calculated efficiency of the PE circuit is as high as 95.3%.

Fig. 13 PE circuit for Maximum power point tracking

Fig. 14 TEG modules output waveforms

I(set2), (0.2A/div)

V(set1), (20V/div)

V(set1), (20V/div)

I(set2), (0.2A/div)

Fig. 15 PE circuit output waveforms

V. CONCLUSION

In this work, the TE modules are characterized and modeled for electric circuit design and engineeringapplications. The issues associated with the application of TEG modules are analyzed and pointed out. Power electronic technologies provide solutions for theseissues and experiment results are given to demonstrate the features of load interfacing, maximum power point tracking and power conditioning. A thermoelectric generator capable of 1 kW electricity power output is under construction and will be used for converting waste heat in a vehicle’s exhaust system to electricity to charge the car battery and power vehicle accessory loads, and experimental results will be reported infuture.

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