CO2-Based Power Cycles: What Effect Does Additive Molecular Complexity Have on the Cycle Layout?

Abstract

Since their inception, CO2 power cycles have gained prominence for their superior performance and compactness. However, the efficiency of the simple supercritical CO2 cycle is hindered by relatively large temperature differences in the recuperator, leading to increased exergy destruction. Although complex cycles like the recompression or precompression cycles can reduce recuperator irreversibility, their higher complexity and additional equipment requirements raise the cost of the power plant.

This paper aims to demonstrate that recuperator irreversibility in a simple recuperated transcritical cycle can be alleviated using CO2-based mixtures, without resorting to complex cycles. This is achieved by comparing the efficiencies of simple and recompression cycles using CO2-based mixtures with nine additives of various molecular complexities: H2S, SO2, C3H8, C4H10, C5H12, C6H6, C4H4S, TiCl4, and C6F6. The effect of additive molar fraction (ranging from 0.05 to 0.5) on the efficiency of both cycles is examined.

Thermal efficiency optimisation reveals a correlation between the efficiency difference of the simple and recompression cycles and the molecular complexity of the working fluid. The reduction in recuperator irreversibility is attributed to the decrease in the difference in the isobaric specific heat capacities between the streams in the recuperator with the use of complex additives. Consequently, the advantage of a recompression cycle diminishes as the aggregate molecular complexity of the working fluid increases. Simple additives like H2S, SO2, and C3H8 result in recompression cycles outperforming simple recuperated cycles by 4% to 8% in terms of absolute thermal efficiency, depending on the additive and its molar fraction. Conversely, more complex additives like C4H4S, TiCl4, and C6F6, exhibit thermal efficiencies in simple recuperated cycles comparable to those of recompression cycles. The additive molar fraction at which both cycles achieve similar performances depends on the molecular complexity of the additive; the more complex the additive, the lower the additive molar fraction required to create a complex working fluid. Moreover, the split fraction in recompression cycles exhibits a similar correlation with molecular complexity as observed in the efficiency difference, suggesting that recompression cycles will morph into simple recuperated cycles as molecular complexity increases.

In conclusion, the use of additives provides an additional dimension through which the efficiency of CO2 cycles can be optimised, enabling improved performance without the need for complex cycles.