Detailed study of integrating solid oxide fuel cell-gas turbine hybrid system for marine applications

Abstract

This thesis presents the computational evaluation of the system configuration and optimisation of a recuperated SOFC-GT system with combined heating cooling and power (CCHP) or trigeneration for marine applications. A comprehensive configuration analysis of a SOFC-GT system is needed to characterise the performance of different system configuration across a range of operating conditions, in order to choose a design point with optimum performance, and carry out off-design analysis. Then, a sensitivity analysis of the effects of changing the components, ambient temperature and air utilisation and fuel utilisation within a safe operating range is carried out. The fuel cell module within the Matlab model simulates methane reforming reactions; and fuel cell electrochemical reaction with the use of Voltage-current (V-i) curve from Siemens Westinghouse experimental SOFC data. The trigeneration model was calculated based on the outlet temperature of the SOFC-GT system as well as the inlet flow rates into the system. A number of system configurations of air conditioning system with conventional Heating Ventilation and Air Conditioning (HVAC) coils, absorption chiller and desiccant wheel are integrated with the existing SOFC-GT system, in order to extract waste heat from the SOFC-GT system exhaust for heating and cooling purposes for the ship. It is found that the recuperated SOFC-GT system is the optimum system configuration. The system efficiency and specific power are both high, when the compressor is operating at 4 bar pressure, with 1250K of turbine entry temperature, fuel cell operating at 1273K, current density at 300mA/cm2, corresponding to 0.704V of fuel cell voltage. When the compressor and turbine designed by the National Technical University of Athens are used in the system with power turbine, the overall thermal efficiency at design point is 59.7%. The hybrid system can operate from 31% to 100% of design point power, when running the system in off-design with air utilisation between 0.1 and 0.25. The choice of compressor and turbine will lead to variations in operating range for off-design. The operating range of the system is bounded by a safety range of air utilisation, which has major effect on the efficiency, total specific power, and gas turbine power split; and fuel utilisation, which is negligible effect on system performance criteria. The ambient condition changes have little effects on the total specific power. However, at higher temperature, the operating line moves closer but not near to the surge line. By using variable geometry compressor and turbine, the operating line can be moved even further away from surge, and this is useful in maintaining system stability when operating in tropical areas. There are also additional benefits of extending the operating range and increasing overall system efficiency, by a maximum of 3% and 1.5% respectively. The Trigeneration model results show that the double effect absorption cooler is the most energy efficient heat recovery unit to be integrated with the SOFC-GT system. When there are fewer occupants in the ship, running fewer HVAC units than designed can reduce the volume of hot air from outdoor, hence requiring less electrical energy for cooling and dehumidification, increasing overall system efficiency. When the number of HVAC units in operation is reduced from 7 to 3 in Ship 1, the maximum number of people allowed indoors (with 18 litres/s per person of air flow rate to ensure freshness of air) is reduced from 138 to 59 persons, but the indoor heat needed to be removed is reduced from 321kW to 242kW. The absorption chiller removes nearly 50% of the heat from the indoor environment when 3 HVAC units are in operations. Hence, the net overall efficiency of the 250kWe Combined Heating, Cooling and Power system is increased from 43.2% for 7 units to 64% for 3 units. Moreover, the net electric power (after air conditioning) available for base load is increased from 74kW to 116kW.