Resumen de Electrically tunable thermal conductivity and exhaust heat recovery applications of thermoelectric materials
Applications involving the use of thermoelectric materials can be found in many different areas ranging from thermocouple sensors and portable coolers, to solar power generators. Generally, they can be subdivided by the sense of the energy conversion. While the Peltier effect is used in solid-state refrigeration, the Seebeck effect is responsible for the conversion of temperature differences into electrical voltage in energy harvesting systems.
The current work is focused on two areas related to thermoelectricity: (i) the study of the capability of controlling the thermal conductivity of TEMs and (ii) the development, testing and improvement of an automotive thermoelectric generator (ATEG).
The first part of the thesis proposes a novel approach to the use of thermoelectric couples, treating them as variable insulators in thermal systems. Here, it is demonstrated that thermal conductivity in thermoelectric materials can be externally controlled by electrical parameters such as electrical load or DC voltage in passive and active systems, respectively. The active mode is a good solution when a complete insulation or a high control of thermal conductivity is needed. The passive mode permits a thermal conductivity increment of 1+ZT times with respect to semiconductor initial thermal conductivity. Results open new opportunities for the application of thermoelectric materials.
The second part of this investigation is focused on exhaust heat recovery development. In the first place, author describes a method to help ATEG design and to predict its performance in terms of fuel economy. The procedure is divided into two main parts. The first one consists in predicting the ATEG performance by using the finite element/volume model (FEM/FVM) methodology. The present study validates the theoretical FEM/FVM -based model comparing its output values with those obtained experimentally by an ATEG installed in an exhaust system of a gasoline engine. The goal is to determine the feasibility of FEM/FVM on predicting its performance. The second part proposes a method, based on results obtained in the FEM/FVM analysis, to predict the expected fuel economy of the ATEG. Results show the consistency of the simulation tool, revealing an agreement of about 97% between simulation and experimental data. In addition, the method applied to the ATEG developed in the present thesis, predicts a maximum fuel economy value of 0.18%.
In the second place, the work gives some ATEG design recommendations to obtain a better performance for real driving conditions. Many models and prototypes have been developed and validated with, a priori, very promising results. The majority of them have been tested under steady-state engine conditions. However, light-duty vehicles operate under wide variable loads, causing significant variation of the ATEG performance. The purpose of this study is to and to analyze an ATEG under different steady-state engine conditions and under the transient New European Driving Cycle (NEDC). Data obtained show that both thermal inertia and pressure drop play a key role in designing an ATEG for real applications. Variations on the exhaust gases temperature and mass flow rate prevent the achievement of the thermal steady state. Consequently, the total energy generated during the NEDC is lower than that expected from a steady-state analysis. On the other hand, excessive pressure loss on the exhaust considerably reduces the engine performance. Results show that the overall power generation of the ATEG can be significantly improved by maximizing the heat transfer through TEMs using a finned geometry, employing lower temperature thermoelectric materials and including a hot-side temperature control.
