Experimental characterization of a solid oxide fuel cell coupled to a steam-driven micro anode off-gas recirculation fan

David Constantin, Stefan Diethelm, Jürg Alexander Schiffmann, Jan Van Herle, Patrick Hubert Wagner, Zacharie Wuillemin
2020
Article

Résumé

While the global fuel utilization of solid oxide fuel cells (SOFCs) is limited by the stack aging rate, the fuel excess is typically used in a burner, and thus limiting the system electrical efficiency. Further, natural-gas-fueled SOFCs require treated water for the steam reforming process, which increases operational cost. Here, we introduce a novel micro anode off-gas recirculation fan that is driven by a partial-admission (21%) and low-reaction (15%) steam turbine with a tip diameter of 15 mm. The 30 W turbine is propelled by pressurized steam, which is generated from the excess stack heat. The shaft runs on dynamic steam-lubricated bearings and rotates up to 175 krpm. For a global fuel utilization of 75% and a constant fuel mass flow rate, the electrical gross DC efficiency based on the lower heating value was improved from 52 % to 57 % with the anode off-gas recirculation, while the local fuel utilization decreased from 75% to 61%, which is expected to significantly increase stack lifetime. For a global fuel utilization of 85%, gross efficiencies of 66% in part load (4.5 kWe) and 61% in full load (6.3 kWe) were achieved with the anode off-gas recirculation. The results suggest that the steam-driven anode off-gas recirculation can achieve a neutral water consumption.

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https://infoscience.epfl.ch/record/274340?ln=fr

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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.

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Theoretical and Experimental Investigation of a 34 Watt Radial-Inflow Steam Turbine with Partial-Admission

Jürg Alexander Schiffmann, Jan Van Herle, Patrick Hubert Wagner

A micro steam turbine with a tip diameter of 15 mm was designed and experimentally characterized. At the nominal mass flow rate and total-to-total pressure ratio of 2.3 kg/h and 2, respectively, the turbine yields a power of 34 W and a total-to-static isentropic efficiency of 37 %. The steam turbine is conceived as a radial-inflow, low-reaction (15 %), and partial admission (21 %) machine. Since the steam is limited in the system (solid oxide fuel cell), a low-reaction and high-power-density design is preferred. The partial-admission design allows for reduced losses: The turbine rotor and stator blades are prismatic, have a radial chord length of 1 mm and a height of 0.59 mm. Since the relative rotor blade tip clearance (0.24) is high, the blade tip leakage losses are significant. Considering a fixed steam supply, this design allows to increase the blade height, and thus reducing the losses. The steam turbine drives a fan, which operates at low Mach numbers. The rotor is supported on dynamic steam-lubricated bearings; the nominal rotational speed is 175 krpm. A numerical simulation of the steam turbine is in good agreement with the experimental results. Furthermore, a novel test rig setup, featuring extremely-thin thermocouples (0.15 mm) is investigated for an operation with ambient and hot air at 220 °C. Conventional zero and one-dimensional pre-design models correlate well to the experimental results, despite the small size of the turbine blades.

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Theoretical and Experimental Investigation of a 34 Watt Radial-Inflow Steam Turbine with Partial-Admission

Jürg Alexander Schiffmann, Jan Van Herle, Patrick Hubert Wagner

A micro steam turbine with a tip diameter of 15mm was designed and experimentally characterized. At the nominal mass flow rate and total-to-total pressure ratio of 2.3 kgh-1 and 2, respectively, the turbine yields a power of 34W and a total-to-static isentropic efficiency of 37%. The steam turbine is conceived as a radial-inflow, low-reaction (15%), and partial admission (21%) machine. Since the steam mass flow rate is limited by the heat provided of the system (solid oxide fuel cell), a low-reaction and high-power-density design is preferred. The partial-admission design allows for reduced losses: The turbine rotor and stator blades are prismatic, have a radial chord length of 1mm and a height of 0.59mm. Since the relative rotor blade tip clearance (0.24) is high, the blade tip leakage losses are significant. Considering a fixed steam supply, this design allows to increase the blade height, and thus reducing the losses. The steam turbine drives a fan, which operates at low Mach numbers. The rotor is supported on dynamic steam-lubricated bearings; the nominal rotational speed is 175 krpm. A numerical simulation of the steam turbine is in good agreement with the experimental results. Furthermore, a novel test rig setup, featuring extremely-thin thermocouples is investigated for an operation with ambient and hot air at 220°C. Conventional zero and one-dimensional pre-design models correlate well to the experimental results, despite the small size of the turbine blades.

2020

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EPFL2019