| dc.description.abstract | The performance of an intermediate-temperature proton-conducting solid oxide fuel cell (pSOFC) hybrid system is investigated in this work. The hybrid system consists of a 20-kW pSOFC, a micro gas turbine (MGT), and heat exchangers. Heat exchangers are used to recover waste heat from pSOFC and MGT. The performance of the system is analyzed by using Matlab / Simulink / Thermolib. Flow rates of air and hydrogen are controlled by assigning different stoichiometric ratio (St). St considered in this study is 2 – 3.5 for air, and 1.25 - 1.45 for hydrogen. System operation pressure is controlled from 1 to 10 atm. Results show that the operation temperature of pSOFC affects the output power greatly. As fuel St increases, although the cell efficiency drops due to unreacted fuel, the higher cell operation temperature causes more electric power output from system. As air St increases, the cell efficiency does not change too much, but the system output power is reduced because of decreased operation temperature. Increasing operation pressure leads to better pSOFC performance and larger MGT output power, but it also increases the consumption power of compressors of air and fuel. The CHP efficiency can be increased by increasing air St or operation pressure, but it makes the system efficiency lower because of larger consumption power of air compressor. System operability can be enhanced effectively by attaching fuel and air bypass designs. Stack temperature can be adjusted by changing bypass ratio and keep the system performance. Air bypass design also enhances system efficiency via better heat utilization.
In addition, this paper also investigates the heat transfer characteristics of high-temperature components. The effects of temperature dependence of radiative properties of a medium on radiation and natural convection interaction in a rectangular enclosure are studied. The radiative transfer equation is solved using the discrete ordinates method, and the momentum, continuity, and energy equations are solved by the finite volume method. Effects of the conduction-to-radiation parameter (Nr), Rayleigh number (Ra), and optical thickness are discussed. Results show that temperature dependence of radiative properties affects the temperature gradient, and hence the energy transport even in relatively weak radiation condition. As Nr is decreased or Ra is increased, the effects of temperature dependence of radiative properties become more significant. Lower Nr means larger weighting of thermal radiation contribution than convection. Larger Ra means stronger natural convection, and the local temperature difference is enhanced due to convection vortex. This, in turns, enhances the effects of different radiative properties. Effects of radiative properties on convection flow diminish when the medium is either optically thin or optically thick. However, optical thickness affects thermal convection greatly, and the two types of media show completely different trends. In case A, thermal convection is suppressed as the optical thickness is increased. In case B, thermal convection is enhanced when radiation effects become obvious. | en_US |