Seri / Tp - 35 - 253 Uc Category : Uc - 64 Analysis of a Heat Exchanger - Thermoelectric Generator System Jon Henderson

Analysis of a thermoelectric generator (TEG) in an ocean thermal energy conversion (OTEC) application is presented. An analytic model is developed for describing the heat exchanger-TEG interactions. This model is used to illustrate limitations of applying conventional fixed junction temperature assumptions to systems experiencing significant temperature drops across the heat exchanger surfaces. Design methods are developed for determining the thermoelectric element geometry that produces maximum output power. Results show that a heat exchanger-TEG system may deli'ver about 100 W/m2 of heat exchanger surface. This compares favorably with conventional OTEG schemes. RECENT WO~~ performed at the Solar Energy Research Institute has considered thermoelectric generators (TEGs) for electric power generation from low temperature solar thermal sources, such as ocean thermal layers or salinity gradient ponds (1*). These applications consider a TEG sandwiched between the warm and cold flow channels of a plate-fin or parallel-plane heat exchanger. Mathematical analysis of TEG systems is relatively straightforward; however, no previous investigations addressing this type of heat exchangerTEG system were uncovered by the author. Most conventional design methods assume known, fixed temperatures at the warm and cold junctions. This approach fails to address the temperature dependence between ~,e energy flux passing through the TEG and the associated temperature drops across the heat exchanger surfaces. Since these temperature drops strongly affect power generation and are sensitive to TEG design, fixed temperature techniques are inadequate for this analysis. Fixed heat input TEG analysis considers variations in junction temperatures ~ut is intended for systems significantly different from ocean thermal energy conversion (OTEC) or other fixed-temperature-source applications. This paper describes an analytic !!Iodel suitable for predicting performance of a heat exchanger-TEG system. The model is used to illustrate the limitation of applying conventional fixed junction temperature analysis when significant temperature drops occur across the heat exchanger surfaces. An OTEC performance analysis is included that shows the relationship between the ratio of thel"llloelectric element area to length and the output power generated by a TEG using standard maxilJum efficiency and maxh~um power fixed junction temperature design constraints. For OTEC applications, the thermal energy source is free. Thus, the cost of power Is controlled by the cost of the conversion equipment. The cost of electrical energy for a given heat exchanger is minimized *Numbers in brackets des ignate References at end of paper. 1 by maximizing the TEGpower generation. The method of Lagrange multipliers is used to solve numerically the heat exchanger-TEG model for the maximum power generation condition. This technique is rather tedious; therefore, a simplified approximate technique to design a TEG that delivers maximum power within a heat exchanger-TEG system is also developed. NOMENCLATURE A,B,C,D,E,F heat exchanger-TEG constants defined in text COY