Carbon dioxide power cycles using liquid natural gas as heat sink

Liquefied natural gas (LNG) is recognized as a source of usable cryogenic exergy for power cycles. The performance of conventional cycles are calculated. A binary steam–Organic Rankine Cycle (ORC) at 550 °C has an efficiency of about 52%, somewhat higher than that of a nitrogen Brayton cycle (50.7% at 700 °C). Carbon dioxide is recognized as an almost ideal medium for implementing single fluid condensation cycles. Its proven practical use both at low temperature (by the refrigeration industry) and at high temperature (by the nuclear reactor industry) makes it suitable for direct utilization without any extended preliminary research. A carbon dioxide cycle in its basic configuration featuring a pump, a regenerator, a heater and a condenser is much simpler than the binary steam–ORC cycle but has a lower efficiency (around 47%). All condensing cycles exhibit a limited capability of exploiting the whole cryogenic exergy of LNG in that they cannot heat the natural gas at temperatures above the condensation temperature. This drawback is fully overcome in nitrogen Brayton cycles which can heat LNG up to ambient temperature. Slightly modifying the basic cycle so that it can partially use free thermal energy from sea water increases efficiency to 51%. Multiple condensation cycles allow a better overall performance at the cost of a more complex layout. Compound cycles, featuring also a gas compressor, exhibit an improved thermodynamics by reducing the temperature difference within the regenerator, with the result of increasing the overall efficiency at values better than those of both binary and Brayton cycles. At 600 °C top temperature, for example, a compound cycle at 100 bar maximum pressure has an efficiency of 55.3% (52.3% for a binary steam–ORC cycle at 550 °C, 150 bar steam parameters; 46.5% for the nitrogen cycle at 600 °C top temperature).