Analysis of efficient hydrogen fuelled steam cycle for power production

Abstract

The share of renewable energy sources such as wind and solar are increasing rapidly in our society due to harder carbon capture policies and an increased request for green energy. Due to the fluctuating nature of these energy sources, reliable storage is needed and additionally highly efficient power cycles able to convert energy from storage back to useful energy. Hydrogen is a promising energy carrier than can be produced when energy is high, stored in gaseous or liquid phase and converted back to energy in a hydrogen fuelled power cycle when energy is low. The power cycle needs to be highly efficient to be competitive with fossil fuels and reach a high round trip-efficiency from energy to hydrogen to energy. A hydrogen fuelled power cycle similar to a Rankine cycle (steam cycle) with direct combustion of hydrogen and oxygen has been modelled and evaluated in IPSEpro in this project. Combustion of hydrogen and oxygen results in high temperature steam functioning as the working media. The aim of the project is to investigate the thermodynamics and critical parts of the steam cycle and quantify its efficiency. The modelled hydrogen cycle resulted in an efficiency based on lower heating value, LHV, of 67.7% which is lower than for other similar hydrogen power cycles studied in previous literature. These are the Graz cycle, Toshiba cycle, Westinghouse cycle and MNRC cycle. In this model, hydrogen and oxygen were assumed to be supplied ”freely” at required pressure to the combustion chamber. A sensitivity analysis of three critical parameters showed that the efficiency is increased with increased pressure and temperature of the cycle. To expand the system boundary of the power cycle, and not assume ”free” hydrogen and oxygen to the combustion chamber, the substances are instead assumed to be supplied from its production site at 40 bar. A model of liquefaction of hydrogen and oxygen together with pumping up to combustion chamber pressure was then added to the cycle. This enables liquid storage of hydrogen. The resulting model including hydrogen power cycle and liquefaction and pumping of hydrogen and oxygen resulted in an efficiency of 31.1%. The decrease in efficiency depends mainly on the high energy consumption to liquefy hydrogen as a result of its low boiling point of -252°C.