A combined cooling and power system of supercritical/transcritical CO₂ cycle with liquefied natural gas as cool source

To improve the efficiency of low-temperature waste heat recovery for the supercritical CO₂ Brayton cycle(SCO₂ cycle), a cooling and power system combining recompression SCO₂ cycle with transcritical CO₂ cycle(TCO₂ cycle)and with liquefied natural gas as the heat sink was established to yield electricity and cold capacity. A TCO₂ cycle was employed as a bottoming cycle to recover the waste heat in the topping recompression SCO₂ cycle, and liquefied natural gas (LNG) was adopted to condense the CO₂ in the TCO₂ cycle to improve the heat recovery efficiency. Exergy analysis was performed and the effects of several key thermodynamic parameters on the system performance were examined according to the performance criteria, including net power output, refrigeration output, overall cycle thermal efficiency and exergy efficiency. The results show that the lower condensation temperature in the TCO₂ cycle could improve the heat recovery efficiency, with the thermal efficiency of 54.47% under given conditions when LNG was adopted as heat sink. Moreover, an increase in the LNG inlet temperature can lead to a reduction in exergy loss of the system. Furthermore, both thermal and exergy efficiency increase when the high-temperature recuperator efficiency increases; when the SCO₂ turbine expansion ratio increases, the thermal efficiency declines while exergy efficiency increases; with the increase of TCO₂ turbine inlet pressure, both thermal and exergy efficiency increase first, and then declines and increases at last; as the condensation temperature increases, the thermal efficiency deceases and exergy efficiency increases firstly and then declines.