Thermal efficiency improvement of closed hydrogen and oxygen fuelled combined cycle power plants with the application of solid oxide fuel cells using an exergy analysis method

Abstract

Today’s renewables, wind and solar power, have a fluctuating nature, making the grid less stable.However, with the increasing share of intermittent sources of renewable power, novel options have to be created to stabilize the power grid. One of these options is energy storage via the conversion of excess power to hydrogen, during periods of high generation from wind and/or solar. In periods of power shortages hydrogen is converted back to power. In the literature fossil fuelled turbine cycles have been studied extensively, furthermore multiple fuelcell turbine cycles have been modelled. However, models with hydrogen and oxygen fuelled turbine cycles and fuel cell turbine cycles are scarce, leaving opportunity for further optimization of modeled hydrogen and oxygen fuelled cycles. The application of pressurized H2/O2 can lead to several improvements over conventional thermodynamic cycles and fuel cells. In this work, a number of high efficiency hydrogen and oxygen fuelled thermodynamic cycles, based upon the Graz cycle and the Toshiba Reheat Rankine cycle, both a coupled closed Brayton cycle with a Rankine cycle, are investigated. urthermore, the following thermodynamic improvements are proposed to fit the Graz cycle: an increased TIT (turbine inlet temperature), the use of a reheat combustor (the inclusion of a second combustor), condensate preheating and hybrid cooling (a combination of open loop and closed loop cooling) of the high temperature turbine blades. All upgrades together lead to an efficiency of 75% LHV. The rise of efficiency of the individual upgrades summed up is lower than the rise off all upgrades together. This is due to the fact that adding reheat and increasing TIT introduce extra high temperature turbine blade cooling needs and losses, which an improved cooling method counteracts. For this reason, the cooling method used is rather influential. Therefore, two cooling methods, open loop cooling and hybrid cooling, used in the Graz and Toshiba cycle respectively, are studied in this thesis.Next to the thermodynamic improvements, an electrochemical improvement is studied, by adding fuel cell systems to the turbine cycles. Multiple fuel cells combinations are studied: a single low temperature solid oxide fuel cell (SOFC) and a low temperature, intermediate temperature and high temperature SOFC in series. Moreover, the type of fuel cell cooling is investigated, both cooling by adding steam to the cathode and recirculating excess oxygen in the cathode are studied. The addition, of a triple SOFC cooled by oxygen recirculation, to an upgraded Graz cycle leads to a potential efficiency of 84% LHV. This efficiency is reached at a relatively low fuel cell fuel utilization of 58%. In this study the turbine cycle is not fully adapted to the fuel cell system or vice versa. Both systems are adapted to each other, leading to the unique triple fuel cell system operating in series, which is more easily coupled to a turbine cycle and can operate at an overall higher SOFC utilization, compared to a single fuel cell. However, from the fuel cell side 2 of the 3 fuel cells are less electrochemically favorable, as only one of the three fuel cells can operate in the best conditions. The overall system efficiency of the combined multiple fuel cell in series system with a turbine cycle, however is most efficient.