Making solid oxide fuel cells more efficient
An alternative design for a recirculation device can make small solid oxide fuel cells more efficient. Idha Valeur finds out more.
A 10% efficiency increase in domestic 6kWe solid oxide fuel cells (SOFCs) has been achieved with a new take on an already established concept. Researchers at École Polytechnique Fédérale de Lausanne (EPFL), Switzerland, said it could lead to an uptake of the SOFC technology in Europe as improvements in efficiency and lifespan would result in lower manufacturing costs.
To improve the SOFCs, the team designed a new anode off-gas recirculation device. EPFL Scientist in the Laboratory for Applied Mechanical Design, Patrick Wagner, told Materials World how the systems convert fuels such as natural gas or biomass to hydrogen and then to electricity and heat. He added that in re-designing the recirculatory system, the team used a steam turbine to drive the cell, which omits explosion risk due to lack of electronic parts or motor.
‘Secondly, we used dynamic steam-lubricated bearings, which can operate at very high speeds without contact to the stator part. Additionally, these types of bearings are oil-free, which is particularly interesting for SOFC systems or catalytic systems in general. Oil or grease could drastically decrease the lifetime of the steam reformer and/or the cells in the SOFC stack,’ Wagner said.
He explained that in this new design the team used herringbone-grooved journal bearings and spiral-grooved thrust bearings. ‘According to my knowledge, it was the first time that such bearings were tested at high temperatures, up to 230°C, and with steam.’
SOFCs are more common in countries such as Japan and Germany, mostly because their governments offer subsidies. Wagner believes that the further increase in lifespan and the cell’s efficiency rate would decrease the levelised cost of electricity and make mass-manufacturing more economical.
How it works
To increase efficiency, the team passed the gases through the cell a second time by joining the outlet to the inlet, in comparison with normal practice where the SOFCs convert approximately 80-85% of the input fuel and it only passes through once.
‘The anode off-gas recirculation fan recirculates the unreacted hydrogen and the steam – hydrogen reacts with an oxygen ion to form water vapour – to the steam reformer, downstream of the anode inlet. The steam is needed for the steam reforming reactions. The addition of hydrogen to the anode inlet increases the hydrogen content,’ Wagner said.
‘Thus, more hydrogen reacts in the SOFC system, which increases the global fuel utilisation of the system and makes it more efficient. On the other hand, the increase of hydrogen at the anode inlet, leads to a decreased local fuel utilisation in the stack, leading to a higher cell potential and to a lower cell overpotential. The cell overpotential is one of the main drivers of degradation process in the cells – the higher the overpotential, the faster the degradation process,’ he added.
Testing showed an increase in efficiency of 10% in part-load of 50%. ‘The local fuel utilisation dropped from 75% to 61%,’ Wagner said. ‘As a result, the cell potential increased from 0.72 V to 0.80 V. The overpotential was thus decreased by 0.08 V. Here, we can both increase efficiency and lifetime. In part-load, at a global fuel utilisation of 85%, we measured up to 67% electrical gross direct current (DC) efficiency and 61% in full load.’ In future tests, the researchers aim to achieve up to 93% of global fuel utilisation, resulting in the gross DC efficiencies of up to 73% and 67%.
Since the initial research, a patent has been filed for the new SOFC system, with plans to commercialise through start-up company, OTurb. ‘Since the research proof-of-concept was successfully realised, the next step is to integrate the steam-driven anode off-gas recirculation fan into a “push-button” commercial and certified system,’ Wagner said. ‘OTurb wants to manufacture the recirculator and sell it to a SOFC manufacturer.’