CO₂ cycles

CO₂-based (both transcritical and supercritical) cycle systems have emerged as a promising option for power generation thanks to their robust thermodynamic performance as well as advantages offered by CO₂ as a working fluid, which is nontoxic, nonflammable, and robust to decomposition at high-temperature conditions. Good thermodynamic performance in these systems is promoted by the good thermal match that can be achieved between the cycle and heat source(s), again as a consequence of the thermodynamic properties of CO₂. Heat from fossil-fuel combustion as well as solar, geothermal, biomass heat and waste-heat recovery are all potential application areas for CO₂ cycle systems, covering heat-source temperatures over a wide range from 300 °C to 1200 °C, with a thermodynamic efficiency of 20%–65%. When the turbine inlet temperature is ~500 °C the thermal efficiency of supercritical (s-CO₂) cycle systems reaches ~30%, but a thermal efficiency of 60% can be achieved when the turbine inlet temperature reaches 1200 °C. Moreover the high density of CO₂ in the supercritical region allows compact component and system design, which is particularly advantageous in space-limited applications. Although the technology has not yet been deployed widely, economic performance projections of s-CO₂ cycle systems have been performed. A variety of such assessments have predicted that (1) the specific investment cost of s-CO₂ cycle systems will fall in the range 900–1650 $/kW in waste-heat recovery applications, (2) the levelized cost of energy (LCOE) of coal-fired CO₂ power plants can be as low as ~70–90 $/MWh, (3) the unit cost of electricity of s-CO₂ cycle systems in solar applications can reach 0.07–0.09 $/kWh, and (4) a total cost saving of up to 30% can be achieved by CO₂ cycle systems relative to steam Rankine cycle systems. Research on CO₂ cycle systems is extensive and spans diverse areas from component (especially turbomachine and heat exchanger) design, cycle innovation and optimization, thermodynamic and economic analyses, prototype construction, and experimental testing, all aimed at overcoming the challenges associated with high-temperature/high-pressure operation and the significant variations in the working fluid properties near the supercritical region. This chapter aims to summarize previous numerical and experimental studies on CO₂ power cycle systems, present comparisons with other power generation technologies, propose future research directions, and provide valuable information and guidance for the development and demonstration of this promising technology in suitable practical applications.