Waste Heat Recovery for Fuel Cells

Abstract

Fuel cells are one of the cleanest ways of generating electricity, and as they the gain in popularity, the waste heat recovery (WHR) of these systems becomes increasingly more important. This is because it's possible to reuse this waste heat that the system produces for the purpose of reaching a higher overall efficiency for the entire system. Certain fuel cells, such as a proton-exchange membrane fuel cells (PEMFC), can operate at low temperatures with an efficiency close to 60%, making them well suited for non-stationary applications such as vessels or vehicles. The approximated energy loss, of 40%, for these fuel cells are in the form of dissipated heat at a low temperature of 75 C. The aim of this project was to develop a WHR system for the low temperature waste heat dissipated from a PEMFC. For this project the waste heat of a 300kW PEMFC is used. The report documents the concept selection process, design and simulation of the selected concepts. To analyze, eliminate and select solution concepts, thorough research of the scientific literature on low temperature WHR systems was conducted and the three final concepts were compared with a Pugh matrix. The concepts were evaluated on efficiency, production cost, ambient conditions, maturity and physical dimensions. The two concepts that were deemed the most viable and practical solutions concepts were the Organic Rankine Cycle (ORC) and the Thermoelectric generator (TEG). The concepts were further developed and evaluated with the help of running simulations of the WHR systems in MATLAB Simscape and COMSOL Multiphysics, as well as designing 3D models of the systems in CATIA V5 and Autodesk Inventor. From the simulations of the selected concepts, the ORC proved to be more efficient at recovering heat with an overall system efficiency gain of 5 percentage point and an electrical output of 25kW. TEG has a comparatively low power output for its size as well as a high installation cost and requirement of low thermal fluid pressure. However, as the ORC entail more moving parts, as well as a working fluid, its operating process is more costly than the TEG's. The project concluded that the ORC is the most efficient and viable solution concept for WHR, however, the TEG concept holds the potential of achieving a much better efficiency rating with potential development in the semiconductor area and further optimization.