Transcritical or supercritical CO2 cycles using both low- and high-temperature heat sources

In CO2 cycles with high-temperature heat sources that are used in applications such as nuclear power, concentrated solar power, and combustion, partial condensation transcritical CO2 (T-CO2) cycles or recompression supercritical CO2 (S-CO2) cycles are considered to be promising cycles; this is because these cycles cause a reduction in the large internal irreversibility in the recuperator owing to the higher specific heat of the high-pressure side than that of the low-pressure side. However, if heat is available in the low-temperature range, the T-CO2 Rankine cycles (or fully-cooled S-CO2 cycles) will be more effective than the T-CO2 Brayton cycles (or less-cooled S-CO2 cycles) and even than the partial condensation T-CO2 cycles (or recompression S-CO2 cycles). This is because the compression work is reduced while achieving the same temperature rise by heat recovery through the recuperator before the high-temperature heater.The proposed T-CO2 Rankine cycles or fully-cooled S-CO2 cycles using both the low- and high-temperature heat sources can maximize the power output of the CO2 power cycle with the given high-temperature heat sources. Moreover, the proposed CO2 cycles combined with the low-temperature thermal energy storage offer the advantage of load leveling over other CO2 cycles, with the given high-temperature heat sources.

► We study a novel transcritical (or supercritical) CO2 cycle using both low- and high-temperature heat sources (LH T-CO2). ► We perform an energy and exergy analysis of the LH T-CO2 cycles in comparison with other CO2 cycles. ► The LH T-CO2 cycles can reduce both exergy losses of the recuperator and high-temperature heater. ► The LH T-CO2 cycles can produce more power and improve the energy and exergy efficiencies in comparison with other CO2 cycles. ► The LH T-CO2 cycles can be applied to the nuclear power plant and thermo-electrical energy storage with transcritical CO2 cycles.