Integrated Design, Optimization, and Experimental Realization of a Steam-Driven Micro Recirculation Fan for Solid Oxide Fuel Cell Systems

This thesis presents the results of the design and experimental investigation of a patented 10 kWel solid oxide fuel cell (SOFC) system with a thermally-driven anode off-gas recirculation (AOR) fan, the so-called fan-turbine unit (FTU). The system has the advantage of higher global fuel utilization, and thus higher efficiencies and/or lower local fuel utilization, increasing the fuel cell stack lifetime. Electrical net DC efficiencies, based on the lower heating value (LHV) of 65 % for a global fuel utilization of 92 % are demonstrated with a system simulation and optimization. This correlates to an electrical gross DC efficiency based on the LHV of 69 %. Additionally, the waste heat of the SOFC stack can be used for local cogeneration in heating or cooling applications leading to a net utilization ratio (cogeneration of heat and electricity) of 90 %. Other advantages of this system design include the absence of a water supply, a simplified water treatment system, and the reduction of the evaporator and pump component size, which reduces the system's initial and maintenance costs. A first FTU proof-of-concept was designed, manufactured, and extensively experimentally tested. The radial inducer-less fan with a tip diameter of 19.2 mm features backward-curved prismatic blades with a constant height. Prior to coupling the recirculation fan with the SOFC, the fan was experimentally characterized with air at 200 °C. At the nominal operational point of 168000 rpm, the measured inlet mass flow rate was 4.9 kg/h with a total-to-total pressure rise of 55 mbar, an isentropic total-to-total efficiency of 55 %, and a power of 18 W. Although the consumed fan power is very low, this FTU can increase the net efficiency of a 10 kWel SOFC system by five percentage points. The fan and shaft are propelled by a radial-inflow, partial-admission (21 %), low-reaction (13 %) steam turbine with prismatic blades. This turbine has a diameter of 15 mm and consists of 59 rotor blades with a radial chord of 1 mm and a blade height of 0.6 mm. At the design point, it has a total-to-total pressure ratio of 1.9, an inlet mass flow rate of 2.1 kg/h, and an isentropic total-to-static efficiency of 38 %, yielding a power of 36 W. To the best of the author's knowledge, it is one of the smallest steam turbines in the world. The shaft features one single-sided spiral-grooved thrust and two herringbone-grooved journal gas film bearings. Nominally, these bearings operate with water vapor at temperatures up to 220 °C. The bearings were tested with ambient air, hot air, and water vapor to rotational speeds up to 220000 rpm, suggesting very stable operation (rotor orbit less than 0.002 mm). Within this work, the recirculation unit operated for more than 300 hours. In a final step, the FTU was coupled in-situ to a 6 kWel SOFC system, reaching electrical gross DC efficiencies, based on the LHV, of 66 % in part load (4.5 kWel) and 62 % in full load (6.4 kWel) for a global fuel utilization of 85 %. To the best of the author's knowledge, this was the first time that a steam-driven AOR fan was demonstrated in-situ with an SOFC system.