To reduce greenhouse gas emissions and improve the performance of space heating systems, a transcritical CO₂ heat-pump heating system using an ejector and integrated mechanical subcooling (EJ-IMS) is proposed. A thermodynamic model of the system is developed with the coefficient of performance (COP) as the objective function, and the subcooling degree and discharge pressure are optimized. The winter heating performance of the system located in cities located in different climate zones was evaluated. The results show that the EJ-IMS system has a maximum COP, corresponding to the optimum subcooling degree and the optimum discharge pressure. The optimum subcooling degree is 26.44%-39.21% lower than that of the integrated mechanical subcooling system, and the optimum discharge pressure of the EJ-IMS system is 0.27%-9.37% lower than that of the baseline system and ejector system. The COP and exergetic efficiency of the EJ-IMS system are 6.09%-37.74% and 6.75%-46.02% higher than those of the three conventional systems, respectively. The heating seasonal performance factor of the EJ-IMS system is 6.89%-29.61% higher than those of the three systems, indicating that the EJ-IMS system is suitable in cold regions. This study provides a theoretical reference for the construction of efficient CO₂ space-heating systems.

To meet the need for energy efficiency improvement and substitution of traditional refrigerants in the field of supermarket refrigeration, a supermarket booster refrigeration system using the eco-friendly zeotropic refrigerant CO₂/R1234yf is proposed in this study. A thermodynamic model is established and compared with that of a pure CO₂ booster refrigeration system. The results show that the maximum COP (1.40) of the system using CO₂/R1234yf was obtained under the optimal CO₂ mass fraction (0.94) and discharge pressure (8.81 MPa). The COP of the CO₂/R1234yf booster refrigeration system was significantly improved compared with the pure CO₂ system, which increased by 7.25% when the ambient temperature was 35 ℃. The APF improvement of the CO₂/R1234yf booster system was 2.68%-4.72%. The APF increased with an increase in the latitude of typical cities. The exergy efficiency of the CO₂/R1234yf booster system first increased and then decreased with increasing CO₂ mass fraction. The CO₂/R1234yf booster refrigeration system exhibited the highest exergy efficiency (0.18) when the CO₂ mass fraction was 0.95, which was 4.62% higher than that of the CO₂ system.

A CO₂ data center cooling and heating system integrated with dedicated mechanical subcooling and dual-temperature evaporation technology (DMS-DE) is proposed to realize green and efficient cooling for data centers and improve comprehensive energy efficiency. A thermodynamic and carbon emission performance system model was established and compared with the basic CO₂ system (Base) and a single evaporating-temperature CO₂ system with dedicated mechanical subcooling (DMS-SE) . The results demonstrated that the DMS-DE system had the maximum coefficient of performance (COP) , optimal subcooling degree, and discharge pressure. Therefore, adopting the DMS-DE can significantly increase the system COP and exergy efficiency. Compared with Base and DMS-SE, COP increased by 14.1% and 9.0%, and the exergy efficiency increased by 13.24% and 4.31%, respectively. The life cycle carbon emissions of the DMS-DE system were reduced by 16.1% and 9.3% compared with Base and DMS-SE, respectively. This study can provide a technical reference for the highly efficient and clean operation of combined heating and cooling utilization for data center scenarios.

In this study, a new method for passive thermal management of lithium-ion batteries based on paraffin/expanded graphite/bamboo charcoal composite bilayer phase-change materials is proposed. To solve the problem of the limited temperature-control range of existing phase-change materials, a dual phase-change temperature (30 ℃/50 ℃) gradient structure is constructed, and a composite phase-change system with dual phase-change temperature regulation is developed by combining the high thermal conductivity of expanded graphite with the porous adsorption properties of bamboo charcoal. Based on these results, at 40 ℃ ambient temperature and under 5 C large multiplication rate, the temperature increase of the battery constructed using the double-layer phase-change material was 37.8% lower than that of the non-phase-change material group (43.3 ℃ vs. 69.6 ℃, respectively); at low ambient temperatures (-10 ℃ and 0 ℃), the double-layer phase-change material extended the battery's effective working temperature range through the synergistic effects of the latent heat release of the phase change materials and the heat storage in the pores. The composite phase-change system realized intelligent thermal management across a broad temperature spectrum (-10-40 ℃) via the dual-phase-change mechanism, providing an innovative solution for the thermal safety regulation of batteries, which has significant engineering application value.

To resolve the issues of significant throttling loss and performance degradation of a baseline transcritical CO₂ air-source heat pump system (Base) for building space heating, an ejector subcooling transcritical CO₂ air-source heat pump system (ESH) driven by the waste heat of the compressor discharge gas is proposed in this paper. The thermodynamic performance of the ESH system was optimized and analyzed using four working fluids. Five typical cities were selected for comparison, and the heating season performance factor (HSPF) was analyzed. The results reveal that the coefficient of performance (COP) of the ESH system first increased and then decreased with the increase of discharge pressure, and an optimal discharge pressure exists. Compared with the Base system, the ESH system using R1234ze (Z) displayed the most remarkable improvement in COP (with the highest increase of 20.64%). The discharge pressure of the ESH system reduced by up to 9.20% compared with that of the Base system, and the exergy efficiency increased by 7.13%-18.61%. The HSPF of the ESH system was 9.68% to 14.21% higher than that of the Base system. The performance improvement was higher when it was used in severely cold regions.