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.
The Laboratory for Applied Mechanical Design (LAMD) designed,
manufactured, and experimentally tested a novel steam-driven anode
off-gas recirculation (AOR) fan for solid oxide fuel cell (SOFC) systems
up to 10 kWel. Due to the dynamic steam-lubricated bearings,
the AOR unit is expected to have a high lifetime, even at elevated
rotational speeds and temperatures. Additional, the unit is oil-free,
explosion-proof, very compact, and cheap to manufacture. The AOR fan
diameter is at 19.2 mm and the nominal rotational speed is 175 000 rpm.
The unit was coupled to a 6 kWel SOFC system, reaching
electrical gross DC efficiencies, based on the fuel lower heating value
(LHV), of 66 % in part load (4.5 kWel gross DC) and 61 % in full load (6.3 kWel)
for a global fuel utilization of 85 %. To the best of the authors’
knowledge, this was the first time that a steam-driven AOR fan was
demonstrated in-situ with an SOFC system.
Dr. Patrick Wagner will give the talk (paper nr° 91361 ):
“Theoretical and Experimental Investigation of a Small-Scale, High-Speed, and Oil-Free Radial Anode Off-Gas Recirculation Fan for Solid Oxide Fuel Cell Systems”
at the ASME Turbo Expo 2019 conference in Phoenix, Arizona, USA. The presentation is scheduled for the track “26-6 Microturbines and Turbochargers: Emerging System and Application” in the location: 221-A at 08:30 am, June 20 (Thursday). More information on the ASME website. If you are at Turbo Expo, you are kindly invited to join.
The Laboratory for Applied Mechanical Design (LAMD) designed, manufactured, and experimentally tested a novel recirculation fan for a 10 kWe solid oxide fuel cell (SOFC). The fan uses oil-free bearings, more specifically herringbone-grooved journal and spiral-grooved thrust gas bearings. The radial inducer-less fan with a tip diameter of 19.2 mm features backward-curved prismatic blades with constant height. Prior to coupling the recirculation fan with the SOFC, the fan was experimentally characterized with air at 200 °C. At the nominal point of 168 krpm, the measured inlet mass flow rate is 4.9 kg/h, the total-to-total pressure rise 55 mbar, the isentropic total-to-total efficiency 55 %, and the power 18.3 W. This paper compares the experimental data towards a computational fluid dynamic simulation of the full fan impeller and volute suggesting an excellent correlation at the nominal point what validates the numerical approach. However, the heat flows crossing the fan fluid domain, have an increased effect at off-design conditions, thus the experimental results need careful consideration. The fan backface leakage has negligible impact on the measurements.
Within the five-year RECOGEN project, the Group of Energy Materials (GEM) in partnership with the Laboratory for Applied Mechanical Design (LAMD) from the Ecole polytechnique fédérale de Lausanne (EPFL) and the industrial partner SOLIDpower designed, optimized, and experimentally realized a patented intermediated-temperature solid oxide fuel cell (SOFC) system with steam reforming (i.e., natural gas to hydrogen). The novelty of this system is a thermally-driven anode off-gas recirculation (AOR) fan, the so-called fan-turbine unit (FTU). The AOR has the advantage of higher global fuel utilization, and thus higher efficiencies and/or lower local fuel utilization, increasing the fuel cell stack lifetime. Additionally, the waste heat of the SOFC stack can be used for local cogeneration in heating or cooling applications. The absences of a water supply is another advantage in comparison to the state-of-the-art direct-steam supply SOFC system.
The FTU consists of a small-scale
AOR fan, a steam turbine, and a shaft with gas film bearings. The radial
inducer-less fan with a tip diameter of 19.2 mm features backward-curved
prismatic blades. The radial-inflow, partial-admission (21 %), and low-reaction
(15 %) steam turbine has a diameter of 15 mm. It consists of 59 prismatic rotor
blades with a radial chord of 1 mm and a blade height of 0.6 mm. 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 a temperatures of 220 °C and a rotational speed of 175 krpm. The entire unit
is manufactured by milling, turning, and surface finishing operations (i.e.,
grinding and honing) exclusively. The unit design is focused on low
manufacturing cost, high lifetime, as well as oil-free and explosion-proof
A first proof-of-concept of this novel SOFC system was experimentally realized in the facilities of SOLIDpower in Yverdons-les-Bains, Switzerland, from December 13 to 14, 2018. The FTU was coupled in-situ to a 6 kWel SOFC system, reaching electrical gross DC efficiencies, based on the lower heating value (LHV), of 66 % in part load (4.5 kWel) and 62 % in full load (6.4 kWel) for a global fuel utilization of 85 %. At constant fuel mass flow rate and a constant global fuel utilization of 75 %, the AOR improved the electrical gross DC efficiency by 5 percentage points (from 52 % to 57 %), compared to a direct-steam supply SOFC system. The stack local fuel utilization dropped from 75 % to 61 %, suggesting an increase of the stack lifetime while improving its efficiency. To the best of author’s knowledge, this is the first time that such an SOFC system was demonstrated. It can contribute to make the SOFC technology more efficient, more reliable, more cost-effective, and thus more competitive. This research will be continued within the EU-funded project “Blaze”.