Off-design analysis of a CO2 Rankine cycle for the recovery of LNG cold energy with ambient air as heat source

Abstract

In this paper, a novel simple CO2 Rankine cycle to recover LNG cold energy is proposed. The system is driven by ambient air, which is regarded as one of the most convenient and economic heat source. For the reason that the ambient temperature changes with days and seasons, it is necessary to take into account the system behavior with the variations of ambient temperature and mass flow rate. In this study, a detailed off-design performance analysis based on the preliminary design of the system is carried out. A sliding pressure control method is applied to keep the system stably and safely operate at off-design conditions. And the system behavior at design condition are compared with that under off-design conditions using a statistical tool. It is noted that the design value of temperature and mass flow rate of ambient air is 313.15 K and 26.86 kg·s−1. The results indicate that the design cycle thermal efficiency, cold energy recovered from LNG and net power output is 6.75%, 1500.7 kW and 108.7 kW, respectively. With the decrease of ambient air temperature and mass flow rate, almost all the performance indicators decrease. But pump power consumption and efficiency changes slightly. Compared with the design values, cycle thermal efficiency and net power output decreases up to 47% and 60% when the ambient temperature decreases to 293.15 K, and 64% and 75% when the mass flow rate of ambient air decreases to 16 kg·s−1, respectively. Moreover, the smaller mass flow rate of ambient air has greater impact on the system performance than the bigger one.