Modeling, simulation and experimental work with high-temperature fuel cells, in particular solid oxide fuel cells (SOFC). We work with using biogas as fuel, as well as with detailed modeling and simulation of fluid, ion and electron conduction on the cell level. Further, we work with modeling and simulation of transient performance of SOFC, as well as with so-called hybrid systems where the SOFC and a gas turbine is integrated. We regard hybrid systems as very interesting in the long term, because of its fuel-to-electricity efficiency potential (>60%) in relatively small plants (< 5-10 MW).