Thermodynamic modeling and experimental investigation of operating conditions for a SOFC/GT hybrid power plant

Abstract

The SOFC/GT hybrid power plant is a promising technology to answer the challenges arising from the transition of a fossil energy based and centralized power supply system to a renewable energy based and distributed power supply system. These challenges include high electrical efficiency, fuel flexibility, operational stability, security in power supply, good part-load performance and fast response to load changes. This thesis investigates operating limitations and heat transfer effects as well as stack performance and ambient conditions variations by means of a modular and computationally efficient 0D system model. The model allows for stationary and transient simulations. Model parameters are based on a real hybrid power plant that is currently under commissioning at DLR in Stuttgart. The particular component models are based on experimental data of different level of detail or factory acceptance test protocols from suppliers, where possible. If experimental results are unavailable, component parameters are based on actual design and material specifications to allow the best model parametrization and thus best prediction of operating characteristics possible. The comparison of adiabatic and non-adiabatic simulation results emphasize the importance of proper heat transfer considerations. The consecutively performed heat transfer variation simulations support this correlation. The effects on efficiency are significant, as expected, while the operating range is severely affected by heat transfer effects, as well. An electrical efficiency (HHV) loss of about 4 percentage points is noticed in contrast to the adiabatic results, whereas the operating range is expanded by about 2kW in high power range due to relaxed cooling air requirements in the non-adiabatic scenario. The electrical efficiency (HHV) remains above 0.53 in the operating range of around 17kW to 39kW, peaking short of 0.56. The stack performance variation has only moderate influence on electrical efficiency where a gain in electrical efficiency (HHV) of up to 4 percentage points is observed with stack performance increase. However, stack performance degradation imposes a significant system constraint, in particular for the high power operating range. The maximum power is reduced from 39kW down to 24kW , while the electrical efficiency (HHV) is reduced by about 2 percentage points. The ambient conditions variation refers to temperature and pressure variations, while Central European climatic conditions are assumed. The temperature variation shows a high power operating range constraint of about 5kW once very low temperatures are investigated. The investigated pressure range shows quite similar results. However, the isothermal power range is reduced by about 60% for the temperature variation while the impact of pressure variation results in a reduction of about 10% . The changes in electrical efficiency (HHV) are limited in the range below 1 percentage point. The system is exposed to a transient daily ambient temperature profile, chosen from historic weather data to form a challenging scenario. However, the system does not show a significant response to the imposed daily temperature profile, indicating high operating stability for Central European climatic conditions. Eventually, the system is exposed to a challenging combined power and temperature profile. The isothermal power reduction by about 25% is performed in less than 5min while further power reduction to 50% requires a stack temperature adaption for about 1.7h . The system follows the profile without problems, however, reaching steady state after the temperature change requires about a week’s time due to the heat stored within the system and the related surface losses. Altogether, this work allows to investigate the details of system characteristics and operating restrictions for the represented hybrid power plant. It allows to understand the effects imposed by internal and external system challenges and can predict hazardous operating regimes, to be handled with care in the real pilot power plant.