Design and experimental investigation of a high-temperature and oil-free compressor-turbine-unit for solid oxide fuel cell based combined heat and power

For the generation of renewable combined heat and power (CHP), coupling a biomass gasifier with a solid oxide fuel cell (SOFC) system can be advantageous due to their matching operating conditions, thus lowering the number of heat exchangers and increasing the overall system efficiency. However, practical implementation is currently limited due to the lack of suitable coupling devices, particularly at small scales. Therefore, a novel prototype of a small-scale, high-temperature, oil-free compressor-turbine-unit (CTU) is presented here. It is supported on steam-lubricated bearings and designed to valorize the system’s waste heat through a micro-cantilever steam turbine which powers a centrifugal syngas compressor, allowing safe and continuous operation of the coupled biomass gasifier-SOFC system. Under laboratory conditions (ambient conditions, pressurized air-driven), the CTU was experimentally characterized for the first time, safely reaching speeds near 120 kRPM, total-to-static compression pressure ratios of 1.27, turbine inlet mass flows of 72.68 kg h-1, and non-adiabatic full-stage efficiencies of 68.7 % and 68.8 % for the compressor and turbine, respectively. Despite operation under a wide range of off-design and non-adiabatic conditions, the obtained experimental data show a good agreement with the 1D and numerical results: Across all performance metrics (efficiency, pressure ratio, turbine mass flow), the maximum deviation from design conditions is 10.6 %, associated with the compressor pressure ratio. These results demonstrate the CTU’s feasibility and high robustness towards off-design conditions and manufacturing deviations, as well as its ability to meet the challenging requirements of this application. The validated holistic design methodology and aerodynamic models are fit to serve as guidelines for the development and testing of similar small-scale high-temperature turbomachinery for CHP systems.