Thermodynamic analysis of a Brayton cycle and Rankine cycle arranged in series exploiting the cold exergy of LNG (liquefied natural gas)

Abstract

The LNG (liquefied natural gas) regasification process is a source of cold exergy capable of being exploited to improve the efficiency of energy conversion. This paper presents a novel power plant consisting of a combination of a CBC (closed Brayton cycle) with a SRC (steam Rankine cycle), arranged in series with regard to the power source, while exploiting the cold exergy available in the regasification process of LNG. The power plant is fuelled by a combustion system of natural gas where the flue gases firstly yield heat to the CBC, then to the SRC and finally to the combustion air by means of a heat recovery process.The LNG cold exergy is exploited to cool the He used in the CBC to cryogenic temperatures at the compressor inlet as well as for generating electric power through direct expansion. The power plant is thermodynamically modelled and simulated using EES (Engineering Equation Solver). An energy and exergy analysis is carried out to evaluate the effect of some key parameters on the efficiency such as the temperature at the compressor inlet, the compression ratio, the temperature at the CBC turbine inlet and the LNG pressure during the regasification process. The outcome is a high efficiency power plant.