Thermodynamic assessment of integrated heat recovery system combining exhaust-gas heat and cold energy for LNG regasification process in FSRU vessel

A thermodynamic assessment of an integrated heat recovery system, which simultaneously recovers both the cold energy of LNG released into seawater and the exhaust gas heat of diesel generator released into ambient air during the regasification process in a LNGFSRU vessel, has been carried out. For the LNG regasification unit consisting of two-stage heat exchangers, a primary Rankine cycle was applied as a typical power cycle of the type A for recovering cold energy to the first-stage heat exchanger. A secondary Rankine cycle of the type B was serially inserted between the first-stage and the second-stage heat exchangers for recovery of the remaining cold energy of preheated LNG. Then, in the type C, the exhaust gas, which had a relatively high temperature, was applied as the heat source of the secondary Rankine cycle, instead of seawater. In such a sequential procedure, the type C was finally suggested as an integrated heat recovery system, in which the seawater and exhaust gas were combined as the heat sources. When the net outputs produced from each heat recovery system were maximized by changing the pressure and mass flow rate of working fluid, the thermal efficiency of the integrated heat recovery system of the type C was ηI,EG = 0.0741. The results showed an improvement of approximately 13.3% (25.6%) in the thermal efficiency compared to the value of ηI,SW = 0.0654 ( ηI = 0.0590) for the conventional cold energy recovery system of the type B (the type A), which only used seawater as the heat source. Based on this finding, a possibility of utilizing the integrated heat recovery system with the combined cycle within the LNG-FSRU was confirmed.