A novel Brayton cycle with the integration of liquid hydrogen cryogenic exergy utilization

Stored or transported liquid hydrogen for use in power generation needs to be vaporized before combustion. Much energy was invested in the H2 liquefaction process, and recovery of as much of this energy as possible in the re-evaporation process will contribute to both the overall energy budget of the hydrogen use process, and to environmental impact reduction. A new gas turbine cycle is proposed with liquefied hydrogen (LH2) cryogenic exergy utilization. It is a semi-closed recuperative gas turbine cycle with nitrogen as the working fluid. By integration with the liquid H2 evaporation process, the inlet temperature of the compressor is kept very low, and thus the required compression work could be reduced significantly. Internally fired combustion is employed to allow a very high turbine inlet temperature, and a higher average heat input temperature is achieved also by internal heat recuperation. As a result, the cycle has very attractive thermal performance with a predicted energy efficiency over 73%. The choice of nitrogen as the working fluid is to allow the use of air as the oxidant in the combustor. The oxygen in the air combines with the fuel H2 to form water, which is easily separated from the N2 by condensation, leaving the N2 as the working fluid. The quantity of this working fluid in the system is maintained constant by continuously evacuating from the system the same amount that is introduced with the air. The cycle is environmentally friendly because no CO2 and other pollutant are emitted. An exergy analysis is conducted to identify the exergy changes in the components and the potential for further system improvement. The biggest exergy loss is found occurring in the LH2 evaporator due to the relatively high heat transfer temperature difference, dictated by the fixed temperatures of the LH2 and of the ambient combustion air, which are far apart. The exergy efficiency is 45%. The system has a back-work ratio only 14 of that in a Brayton cycle with ambient as the heat sink, and thus can produce 72.7% more work, with the LH2 cryogenic exergy utilization efficiency of 50%.