Comparison of direct expansion and combined direct expansion and organic Rankine cycle for power generation by utilizing LNG cold energy.

A large temperature difference between liquefied natural gas (LNG) and the environment offers the potential for extraction of power from the cold energy that is usually wasted in many LNG regasification terminals. Direct Expansion (DE), or DE coupled with Organic Rankine Cycle (ORC) is used for this purpose at some LNG receiving terminals. In this work, effects of parameters such as working fluids (pure hydrocarbons, their derivatives and mixtures), flow rates, operating pressures and thermal sizes of the heat exchangers are evaluated through exergy analysis by simulating the system with Aspen Hysys 8.6TM process simulator. Exergy analysis identifies less efficient components in thermodynamic systems. Comparison of optimum parameters that generate a maximum exergy efficiency (implying the maximum power generation) for a desired condition (objective function) may help to arrive at a suitable process. Integration of DE and ORC have a higher efficiency than that of DE and ORC alone.