Low-grade heat conversion into electricity by thermoelectric and electrochemical systems

Abstract

Developing cost effective technologies that convert low-grade heat into electricity is essential to meet the increasing demand for renewable energy systems. Thermoelectric and recently emergent electrochemical heat conversion devices are promising candidates for this purpose. However, current performance and cost of these devices limit their widespread application. In this thesis, we investigate design guidelines for heterostructured thermoelectric systems and electrochemical heat energy harvesters to address these challenges. Material cost and scarcity of elements in state-of-the-art thermoelectric materials are current limitations. Conductive polymers has become an attractive alternative to those materials, however they suffer from low Seebeck coefficient. Nanoscale composites of inorganic semiconductors with conductive polymers could improve low Seebeck coefficients and power factors of conductive polymers, however quantitative understandings on the mechanisms lying behind the enhancements were often missing. In our research, thin film heterostructures of a conductive polymer, PEDOT:PSS / undoped Si or undoped Ge were selected as templates for mechanistic investigations on thermoelectric performance enhancements. With the combination of experiments and simulation, it was determined that p-type PEDOT:PSS transferred holes to the interfaces of adjacent Si and Ge, and these holes could take advantage of higher hole mobility of Si and Ge. This phenomenon called modulation doping, was responsible for thermoelectric power factor enhancements in Si / PEDOT:PSS and Ge / PEDOT:PSS heterostructures. Another technology to transform low-grade heat into electricity is electrochemical heat conversion. Traditionally, the electrochemical heat conversion into electricity suffered from low conversion efficiency originating from low ionic conductivity of electrolytes, even though high thermopowers often reaching several mV/K has been an alluring advantage. Recently developed breakthrough on operating such devices under thermodynamic cycles bypassed low ionic conductivity issue, thereby improving the conversion efficiency by multiple orders of magnitude. In this study, we focused on improving efficiency by increasing thermopowers and suppressing heat capacity of the system, while maintaining the autonomy of thermodynamic cycles without need for recharging by external sources of electricity. These detailed interpretations on nanoscale composite thermoelectric systems and electrochemical heat harvester provide insights for the design of next-generation thermoelectric and electrochemical heat energy harnessing solutions.