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TECHNICAL PAPERS: Power Engineering

The Maximum Power Operating Point for a Combustion-Driven Thermoelectric Converter With Heat Recirculation

[+] Author and Article Information
Richard B. Peterson

Department of Mechanical Engineering, 204 Rogers Hall, Oregon State University, Corvallis, OR 97331richard.peterson@oregonstate.edu

J. Eng. Gas Turbines Power 129(4), 1106-1113 (Apr 11, 2007) (8 pages) doi:10.1115/1.2747261 History: Received June 26, 2006; Revised April 11, 2007

Abstract

A model is developed for determining the ideal operating point, based on maximum power output, for a thermoelectric conversion (TEC) element coupled to a combustor. In the analysis, heat recirculation from the combustor exhaust is included. Results presented here are relevant to the operating characteristics of small, combustion-driven energy systems. The model is composed of a TEC element, a combustor, a counterflow heat exchanger, and a thermal shunt resistance to the surroundings. Including the shunt is necessary due to the increased importance of this effect in small-scale thermal systems. From this combination of components, an optimal combustor operating temperature is found giving maximum power output and efficiency. The model is used to determine ideal performance figures as a function of system parameters such as the effectiveness of heat regeneration, loss of heat by conduction, and the parameters describing the thermoelectric conversion element (the so-called ZT parameter). Although a high degree of idealization is employed, the results show the importance of heat recirculation and the significance of thermal losses on system operation.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

Figure 1

System model diagram of the TEC element combined with the combustor having heat recirculation and a thermal shunt. Combustor operates as a well-stirred system with a temperature of Tp.

Figure 2

Plot of the relative power output and overall system efficiency as a function of combustor temperature where ε=0 and λ=0. Each curve is generated using Z values that vary from 0.0001 to 100. Using the temperature average between the maximum power point and the surroundings, ZTave for each curve is (starting with the lowest ZT value): 0.0525, 0.507, 4.8, 46.2, 453, and 45,300. The last value listed gives performance that is essentially Carnot limited in converting combustor heat to power.

Figure 3

Plot of the relative power output and overall system efficiency as a function of combustor temperature where ε=0.75 and λ=0. Each curve is generated using Z values that vary from 0.0001 to 100. Using the temperature average between the maximum power point and the surroundings, ZTave for each curve is (starting with the lowest ZT value): 0.117, 1.05, 8.51, 73.5, 706, and 70,600. The last value listed gives performance that is essentially Carnot limited in converting heat from the combustor to power.

Figure 4

Combustor temperature at the maximum power point as a function of ZTave. Curves are distinguished by the degree of heat regeneration through the HEX effectiveness. The adiabatic flame temperature and surroundings temperature are held constant at 1200K and 300K, respectively.

Figure 5

Comparison of the three solutions for efficiency. In each of the three cases, the efficiency of the TEC element and the overall system efficiency are plotted as a function of ZTave. The three solutions are: (1) the exact solution, given by filled or open circles, where a numerical computation is necessary, (2) the case where ZT⪡1, and (3) the case where ZT⪢1. The curves are generated for the base case of ε=0 and λ=0.

Figure 6

TEC element efficiency (a) and overall system efficiency (b) as a function of ZTave for a range of HEX effectiveness values. The conduction parameter is set to zero.

Figure 7

Combustor temperature (a) and overall system efficiency (b) as a function of ZTave for a HEX effectiveness of 0.85. The conduction parameter varies from 0.0 to 1.0. Conductive heat loss as a percent of heating rate at the high ZT limit for each λ value is 0%, 8.7%, 15.9%, 22.8%, and 30.1%.

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