Heat Transfer in Small Sustainable Thermoelectric Power Sources

1. Heat Transfer in Small Sustainable Thermoelectric Power Sources Matthew Delaney, Nathan Coussens, Russell Austin, Peter Wills School of CBEE, CHE 415-416, 2008 Schematic of Heat Transfer Behavior for Module Problem Statement A local start-up company, Perpetua Power Source Technologies, is designing small thermoelectric devices to power wireless sensors. The thermoelectric device relies on a temperature difference to generate electricity. Perpetua is seeking a model, backed by experimental data, which predicts heat transfer within the device, and allow devices to be designed with optimal fin heat dissipation with minimal trial and error. Objective Develop a theoretical model to predict heat transfer rates through a thermoelectric power source composed of varying materials in a cylindrical geometry, and confirm model with experimental results. Experimental Procedure • Test fins only with heating block and IR lamp. • Test base unit for 1-D flux with heating block. • Test unit for 2-D flux with heating block. • Test completed unit in outdoor conditions. • Test completed unit with heating block and forced convection in a wind tunnel. Ambient air Heat sink Conduction module Ground Results The Different Modes of Heat Transfer Conductive heat transfer is the transfer of energy from one stationary mass to another. Convective heat transfer describes the exchange of energy from a given mass to a moving liquid or gas. Radiative heat transfer describes the transfer of energy via particles or wavelengths. This would describe the transfer of energy from the sun. Each of these can be modeled individually. However, all three of the modes of heat transfer have some interdependence. For this reason, the conductive, convective, and radiative components need to be integrated into a single model. This model was broken into two components: the cylinder at the base of the device and the fins that are at the top of the device. These components were then combined together to create an overall heat transfer model. Experimental measurements involves an analysis of all modes together. • Radial temperature gradients obtained with current experimental methods are too small to confirm or deny theoretical models, given the 1°C precision of type K thermocouples used. Larger temperature gradient may resolve problem. • Axial temperature gradients were confirmed by the theoretical models within 20% error. • Heater coil requires larger power supply than currently used. • Solar radiation creates a significant temperature drop across the module ~3 °C. • Current Capabilities of Theoretical Programs Experimental module with swappable heat sink, electric heating element, and thermocouples. Interior view of “wind tunnel” used to model forced convection. Analysis of Experimental Data Forced Convection Conduction Test Outside Sun Experiment Temperature vs. elapsed time for various axial and radial for the experimental module in an outdoor environment with ice bath cooling and mixed sun/clouds. Region I—sunny/no fins; II—shade/no fins; III—shade/fins; IV—sun/fins. Temperature vs. elapsed time for various axial and radial positions of the experimental module (positioned in the wind tunnel seen above). Future Work Theoretical Modeling • Refine model to improve accuracy. • Integrate into MATLAB program solar radiation and forced convection. • Obtain more experimental data using a 12 V, 1500 mA power supply. Simulation models have been designed for the theoretical model of the module. These programs have been developed to allow Perpetua to find the optimal dimensions for the customer’s requested power requirements . The graphs below show the temperature gradient results of the programs with the dimensions of the current test unit. Sources MATLAB Eckert, E.R.G., Drake, R.M. Jr. Analysis of Heat and Mass Transfer. New York: McGraw-Hill., 1987. Modest, Michael F., Radiative Heat Transfer, 2nd ed., Academic Press, 2003. Rao, V. Dharma, S. V. Naidu, B. Govinda Rao, and K. V. Sharma. "Heat Transfer From a Horizontal Fin Array by Natural Convection and Radiation." International Journal of Heat and Mass Transfer 49 (2006): 3379-3391. Thomas, Lindon C. Heat Transfer – Professional Version. 2nd ed. Tulsa, OK: Capstone Publishing Corporation. 1999. Welty, James R., Charles E. Wicks, Robert E. Wilson, and Gregory L. Rorrer. Fundamentals of Momentum, Heat, and Mass Transfer. 4th ed. Hoboken, NJ: John Wiley & Sons, Inc., 2001. "An Intro to Thermoelectrics." Tellurex Corporation. 2001. 8 Mar. 2008 <http://tellurex.com/cthermo.html>. COMSOL Acknowledgements • Marshall Field (Perpetua Power Source Technologies, V.P. of Engineering) • R. Jon Hofmeister (Perpetua Power Source Technologies, President) • Dr. Philip Harding (Oregon State University, Linus Pauling Chair Engineer) • Andy Brickman (Oregon State University, Instrumentation Specialist) • Dr. Alex Yokochi (Oregon State University, Professor) • Dr. Gregory Rorrer (Oregon State University, Professor) 2-dimensional temperature profile for conduction, convection, and emissive radiation. Model shows heat flow from base to the fins. Solar radiation is yet to be implemented. 3-dimensional temperature profile for conduction only. Model shows heat flow from the base to the fins.