Performance study of supplying cooling load and output power combined cycle using the cold energy of the small scale LNG

Highlights: • A combined cycle of supplying cooling load and output power is presented. • The combined cycle utilizes the cold energy and the pressure energy of LNG. • R717 is applied as the working fluid of refrigeration cycles. • The exergy efficiency and performance coefficient of combined cycle are analyzed. -- Abstract: The gasification process of small flows of liquefied natural gas also releases large amounts of cold energy and pressure energy. In order to effectively utilize the cold energy and the pressure energy, a combined cycle for supplying cooling load and output power was proposed. In the combined cycleone refrigeration cycle for food frozen storage, two refrigeration cycles for food cooling storage, two chilled water cycles for air conditioning, R717 is used as working substance of refrigeration cycles. The exergy efficiency and performance coefficient of the combined cycle were analyzed. The exergy efficiency of evaporator is the lowest, the exergy efficiency of each expander and condenser is higher than that of the pump in refrigeration cycles. The exergy efficiency of evaporator, expander, pump and condenser increases with the increase of evaporation temperature of R717 refrigerant. The power consumption of the each chilled water pump for air conditioning is higher than that of the R717 pumps in refrigeration cycles. LNG pump power consumption is lower than that of two chilled water pumps. As the outlet pressure of the NG expander decreases, and the pressure at the inlet of the NG expander increases, the total pump power consumption and the system total expansion output power increases. Among the system total expansion output power, NG expander accounts for more than 2/3. With the increase of the outlet pressure of the NG expander, the system exergy efficiency and the performance coefficient of combined cycle decreases. With the increase of the inlet pressure of the NG expander, the exergy efficiency of the system increases. However the inlet pressure of NG expander rises resulting in reduction in the performance coefficient of the system. And at the same inlet and outlet pressure of NG expander, the mass flow rate of NG increases, the exergy efficiency and the performance coefficient of the system hardly changes. When the mass flow rate of NG is 1 kg/s, and the inlet and outlet pressures of NG expander are 10000 kPa and 600 kPa, the exergy efficiency and the performance coefficient of the system reaches 85.19% and 6.525 respectively.