Simulation Research On Organic Rankine Cycle And Cold Warehouseutilizing LNG Cold Energy

Liquefied Natural Gas(LNG) under ambient presure can release cold energy of approximately 830 k J/kg while regasifying to Natural Gas(NG) with normal temperature. Normally this cold energy is disspated by seawater, which not only increases power consumption with respect to seawater pump, but also leads to “cold frog” near the regasification equipment. Therefore, LNG cold energy utilization can promote energy-saving and emission-reduction. Furthermore, its various applications(including air separation, power generation, cold warehouse, etc.) can avoid the demerit of conventional tec hnologies, of which some applications have a vast market prospect.Two LNG regasification processes(Process A: LNG is pressurized before regasified; Process B: LNG is regasified before pressurized) are discussed. Quantitative comparison based on Hysys simulation showed that the power concumption of Process B was 21.87 times as much as that of Process A. Besides, LNG cold energy, cold exergy and UA(heat transfer coefficient multiplies heat transfer area) of LNG exchanger under different pressures are invesitigated. The simulation results indicated that as the regasification pressure increased, LNG cold energy released showed a linear decreasing, LNG cold exergy reported a quadratic reduction, and the UA value of LNG exchanger experienced climed up and then declined presenting an inverse U curve.Using the predefined unit operations in process simulator Hysys, the regenerative ORC(Organic Rankine Cycle) model can be promptly established, compared with the common model building process including a large array of mathematical equations for each part of the ORC system. Propane was chosen as the working fluids after analyzing and comparing 14 fluids. In the simulation part, the influences of vaporization pressure, condensation pressure, the degree of superheat of expander inlet gas, working fluid flowrate and regenerator efficiency on system performance(including flue gas discharging temperature, LNG outlet temperature, system net output power, system exergy efficiency) were analyzed respectively. Results showed that the influence of evaporation pressure on system performance in subcritical ORC system and supcritical ORC system were totally different. In the optimization part, the net output work was chosen as the objective function, and genetic algorithm was used in this optimization.Compared with the conventional ammonia compression refrigeration system, the cold warehouse using LNG cold energy uses pump instead of compressor to drive the refrigeration cycle, which can save a large amount of power consumption. By quantitative analysis using Hysys simulation, with the same cooling load of 279.3k W and design temperature of-20°C, cold warehouse utilizing LNG cold energy could save approximately power consumption of 135.9k W compared to ammonia compression refrigeration system, and the exergy efficiency of the former was twice as much as that of the latter. Specifically, the refrigeration system was established using ammonia as the intermediate working fluid and the refrigerating room and freezing room were parallelly connected. Thermodynamic analysis showed that the system exergy efficiency was 80.2%, and the exergy loss in LNG exchanger constituted 79.8% of the total exergy loss in the system. Besides, as the LNG regasification pressure increased, both the COP and exergy efficiency of the cold warehouse system rose. Economic analysis showed that the dynamic payout period of investment was 4.83 years.