Mesoscale power generation incorporating heat-recirculation, porous inert media, and thermoelectric modules

Abstract

Mesoscale power generation operated on liquid fuels has the potential for delivering higher energy density (per unit volume) or specific energy (per unit mass) than battery technology. In this study, a complete mesoscale power generation system that incorporates a novel thermoelectric module to generate electrical power from the heat released by combustion is investigated. An annular, fuel-flexible, heat-recirculating mesoscale combustor utilizing porous inert media was developed to reduce heat loss and improve structural strength. The combustor can operate at lower reactant flow rates and results in higher product gas temperatures than previous designs. Experimental and computational analyses of the combustor operated on methane demonstrated low CO and NOx emissions with over 99.7% combustion efficiency. Measurements show that the primary pressure drop occurred in the fuel injector, while the pressure drop in the combustor system including the porous media was minimal. Thermoelectric (TE) devices can offer reliable power for applications such as waste heat recovery and portable systems. Current devices provide low TE element/module/system efficiency, primarily due to the poor heat transfer between the working fluids and TE junctions. In this study, an integrated 3D Computational Fluid Dynamics - Thermoelectric (3D CFD-TE) model has been developed to analyze complex fluid-solid interactions in a TE system and to gain insight into the combined effects of fluid flow, heat transfer, and material properties on TE power generation. Computational analysis of current TE module designs in a simple counterflow heat exchanger configuration identified deficiencies related to poor heat transfer. Next, a novel TE module design was developed that extends the length of the TE legs to increase the temperature differential between the junctions. The working fluids flow between the TE legs, covered with a high thermal conductivity material to increase the heat transfer between the working fluids and TE junctions. This novel design approximately doubled the system efficiencies compared to conventional systems, with and without heat recirculation. A parametric study of the TE module design demonstrated that the performance can be improved further by limiting axial conduction on the surface of the TE legs. Computational analysis of the TE module demonstrated that thermal strategies affecting the heat transfer rate to the TE module led to greater increases in the system efficiency than improving performance by increasing the TE figure-of-merit, ZT. For the final part of this study, the computational models were incorporated to investigate a mesoscale combustor with the novel TE module design. Results demonstrated that low reactant mass flow rate and low equivalence ratio improved the overall performance by increasing the proportional heat transfer rate to the TE module and TE module efficiencies. The proposed mesoscale power generation system had higher system efficiency than current power generation systems utilizing conventional TE modules. The computational analysis demonstrated the relative importance of Joule heating, Peltier effect, and Thomson effect in the TE module and how they are affected by thermal processes. The model demonstrates the advantage of the novel TE module design which can lead to a viable mesoscale power generation system incorporating TE power generation.