Development of zinc oxide/silicon dioxide semi-conductive thermoelectric generator (STEG)

Abstract

The proportion of chemical energy released as heat that is converted into energy depends on the ratio between the volume of the engine cylinder charge that is ignited and the volume when the exhaust valve is opened. In this case, there is a huge loss of thermal energy, especially in the exhaust gases. There are many ways to convert the waste thermal energy into electrical energy that has led to a significant reduction in fuel consumption of internal combustion engines, especially using thermoelectric generators. Very low conversion efficiency is one of the main problems with thermoelectric generators. This has become the main reason for limiting their use in power generation for specialized areas with large areas of application. The aim of this study is to develop a laboratory-scale thermoelectric generator that was fabricated using a ZnO / SiO2 semiconducting composite to convert waste heat from an internal combustion engine (ICE) into wealth. A parametric study of the composite, ZnO/SiO2, is done using ANSYS software. That was followed by synthesis and preparation of the composite for the TEG. A laboratory scaled samples then were fabricated and then further investigated experimentally. A simple mathematical model has been developed for the simulation. The model has been used for all conditions of the concentration of ZnO/SiO2 composite. The maximum composite thickness for the simulation has been considered 2 mm based on the thickness of the conventional TEG. Several samples with different compositions of ZnO/SiO2 have been developed for the S-TEG. Five samples are made for the composite by taking the different weight percentage (wt. %) of ZnO/SiO2:10%/90%, 5%/95%, 15%/85%, 8%/92%, and 20%/80, Each sample was divided equally into three parts for the final sample (specimen) using different composition of epoxy resin and hardener in the amount of 40 g, 35 g and 20 g, respectively. Thus, a total of 15 additional specimens were made from this study to verify their morphological structure by scanning electron microscopy (SEM) and electrical resistivity. A vacuum machine was used at a pressure of 0.8 kN / m2 for five hours to remove air bubbles from each of the samples. Four major laboratory scaled STEG were made by sandwiching the ZnO/SiO2 between Aluminium Foil (p-type), and Carbon Fibre (n-type). Each of the final samples have been sized as a surface area of 2000mm2 and a thickness of 2 mm. Differential Scanning Calorimetry (DSC) is used to measure the temperature difference of the samples based on the equivalent rate of flow of exhaust. The STEG samples have been tested using Keithley Parametric Analyzer software in the electronic lab to get the optimum composition based on the performance electric resistivity. Results showed that the lesser the epoxy resin and hardener the lower the electric resistivity. The least obtained electrical resistivity was 5.4xe08 with the composite composition of 30 wt. % ZnO/70 wt. % SiO2. Temperature gradient test results shows that the thicker the sample the higher temperature gradient. The maximum voltage recorded from the sample was 525 mV and short circuit current density of 25x10-6 A/mm2. Samples show very good improvement shifting from being very insulative to ideal semiconductor resistivity range. Sandwiching the STEG with ceramic paper to increase the overall thickness of the sample could increase the temperature gradient. The output voltage is expected to increase by about 13 V if samples are fabricated and tested at actual dimensions and exhaust temperatures. In this way, the power generation from the STEG can be used to power the vehicle's electrical system, which will reduce the engine's alternator power consumption by 10%, as well as emissions.