Acknowledgment

F. Tavano 1, D. Ben Ayoun 2, D. Granit 2, E. Taragan 2,3, Y. Sadia 2, Y. Gelbstein 2

Thermoelectric generator modeling for a Caterpillar C7 Engine

IntroductionCaterpillar C7 engine system serves multiple kinds of armored military vehicles all over the world and the waste heat emitted can provide energy for a wide variety of support systems. Due to the latest efficiency enhancement of thermoelectric materials, there is a dramatic increase in applicative researches about Thermoeletric Generators (TEG). In order to outline the feasibility of using a TE device on a real engine, a simulation has been carried out by using Caterpillar C7 engine distinctive parameters for the determination of the maximal power that could be drawn from the device itself. More specifically, the exhaust gas of the engine has been able to reach up to 580°C, while, on average, as much as 30% of the engine maximum power (200 [kW]) is dissipated by the exhaust pipe as waste heat. Thanks to the TE generator, which has been able to achieve up to 14% efficiency by means of state of the art materials, the hybrid system would be able to harvest about 8 [kW] of extra electricity out of 60 [kW] of otherwise wasted thermal energy. Based on simulations and measurements, the expected results are promising both in economic terms, reducing fuel related costs, and improved troops safety, decreasing the exposure time of military personnel on the battlefield while waiting for refueling.

Fig. 1. Upper (a) and lower (b)thermocouple locations along

the exhaust pipe

References

Poster No. 173

We would like to thank I.T.E (Caterpillar, Israel) and RDT (Fluke, Israel) for assisting and providing us the generator and thermal camera, respectively.

[1] E. Hazan, O. Ben-Yehuda, N. Madar, Y. Gelbstein, Journal of Advanced Energy Materials, 11 (2015) pp. 1-8

In order to analyze the temperature regime of the exhaust pipe along the vertical axis, two K thermocouples (TC) have been placed on the upper (Fig 1.a), and lower (Fig 1.b) locations, where the TEG array will be mounted in future to come.The temperature regime was measured under different power loads: from lack of load up to 250[kW].

Figure 2 presents the steady state maximum temperature measured along the exhaust pipe for each load.It is clearly seen that there is a temperature gradient along the pipe. Moreover, there is a direct correlation between the load and the temperature, achieving higher potential for higher loads.

Preliminary experiments and conceptual design

Objectives1. Verification of thermal feasibility for thermoelectric generator (TEG) array by engine’s exhaust pipe temperatures modeling.2. Mechanical assembly design and materials optimization for TEG array, under nominal operation conditions.3. Integration of state-of-the-art TEG [1] and further optimization of the energy consumption for armored military vehicles.

1. Energy Engineering, Polytechnic University of Turin, Italy2. Energy Engineering Unit, Ben Gurion University of the Negev, Beer Sheva, Israel

3. NRCN, Israel

)a(

)b(

Thermal camera AnalysisTemperature Regime

Fluke thermal camera Ti300 has been used in mapping the temperature distribution on the surface of the exhaust pipe. Emissivity calibration has been obtained by comparing the thermal camera and the thermocouple measurement.

Fig. 3. Temperature mapping for a load of 250[kW]

Fig. 2. Load dependence of the temperature

Gripper Design & Thermal characterization

Fig. 4. Temperature drop trend along the designated area

A mechanical gripper has been designed as an interface between the exhaust pipe to the TEG array. Also, in order to achieve reasonably high efficiency, it is a crucial to generate an appropriate temperature gradient across the TEG.

Design considerations:- Minimizing parasitic thermal losses.- Decreasing the thermal contact resistance

between the components.- Increasing the temperature drop within the

TEG by forced convection on the cold side.- Service temperature, thermal conductivity,

thermal expansion coefficient etc. should be all taken into account simultaneously.

Thermoelectric generator implementation

So far, in order to verify the feasibility of the whole study, a commercial TEG based on bismuth telluride (5-7% efficiency) has been mounted on a Caterpillar C7 engine’s exhaust pipe, although the main driver of this research is to integrate a higher performance TEG. A recent study [1] displayed a remarkable efficiency of up to 14%, showing the highest efficiency ever reported before (Fig. 7). It is well known that high thermoelectric conversion efficiencies can be achieved by using materials with, as high as possible, figure of merit . A significant improvement of upon appropriate geometrical optimization will be applied in the next stage of the research, together with n-type Functionally Graded Materials (FGM) coupled with a phase separated p-type compound. The aforementioned approach will guarantee impressive thermoelectric efficiency while operating within cold and hot junctions temperatures of 50°C and 500°C, respectively.

Fig. 5. Gripper design and cooling ribs assembly

Fig. 7. Previous study results [1] Fig. 6. System cross section FEM analysis

p-type Ge0.87Pb0.13Te

n-type 0.055%

0.01%

500

oC

50

oC

RL

Heat Source

I

y axis

Finally, Finite Elements Method (FEM) analysis has been implemented in order to deeply investigate the system behavior under various loads and outdoor atmospheric conditions.