Multi-mode analysis and comparison of four different carbon dioxide-based combined cooling and power cycles for the distributed energy system

Abstract

To meet the diversified demand of power and cooling for the distributed energy system, this paper proposes two combined cooling and power (CCP) systems combining the supercritical carbon dioxide cycles with a carbon dioxide ejector refrigeration cycle, which can switch between full power mode (Mode-P), combined cooling and power mode (Mode-PR), and full refrigeration mode (Mode-R) according to the energy demands of users. In the proposed CCP systems, the recompression cycle and modified dual-stage compression cycle are used to replace the regenerative cycle and basic dual-stage compression cycle in two reference systems respectively. The mathematical models of systems are established and verified. Detailed parametric analysis is conducted to investigate the effects of key parameters on the performances of proposed systems and reference systems. Moreover, the modified systems and reference systems are optimized and compared at three modes. Multi-mode analysis is also applied to study the ability of the four CCP systems to convert power into cooling. Finally, detailed exergy analysis and thermoeconomic analysis are performed for the optimal system. The results show that the modified systems can achieve higher exergy efficiency by recovering partial waste heat before the pre-cooler. Under the refrigeration condition (the evaporation temperature is 0 ℃), the exergy efficiencies of the modified single-stage compression (M−S) system are 13.71% and 14.08% higher than those of the reference system at Mode-P and Mode-R, respectively. The cooling capacities of the systems at Mode-PR and Mode-R are positively correlated with their power outputs at Mode-P. The M−S system has the best multi-mode performance owing to its highest exergy efficiency (64.01%) at Mode-P, minimal energy loss at Mode-PR, and the maximum cooling capacity to heat input ratio (1.714 at 0 ℃ and 0.964 at −20 ℃) at Mode-R. Under the freezing condition (the evaporation temperature is −20 ℃), the single-stage compression systems generally perform better than the dual-stage compression systems. During the process of switching from Mode-P to Mode-R, the total product unit cost of the M−S system increases from 12.24 $·GJ−1 to 44.74 $·GJ−1 under the refrigeration condition and from 12.24 $·GJ−1 to 38.15 $·GJ−1 under the freezing condition.