To address the issues of poor air quality and the lack of fresh air in traditional variable refrigerant flow （VRF） systems， fresh air units are commonly supplemented in practical engineering applications. However， these systems are typically controlled independently. To achieve coordinated operation between VRF systems and heat-recovery fresh-air units， a new integrated VRF fresh-air system was developed， with the aim of reducing system energy consumption and improving indoor comfort and air quality. Based on the full-condition performance model of the VRF system and the characteristics of the heat-recovery unit， simulations were conducted to evaluate the energy-saving effects of conventional VRF systems and of three types of integrated VRF fresh-air systems in residential buildings in Nanjing and Beijing. The results indicate that， compared with conventional VRF fresh-air systems， the system equipped with a constant air volume heat-recovery unit significantly reduced the operational energy consumption， achieving annual energy savings of 20.7% and 30.6% in Nanjing and Beijing， respectively. The energy-saving performance was positively correlated with the severity of cold-climate conditions. During the heating season， the new integrated system prioritizes the use of the heat-recovery mode and minimizes the fresh air volume. During the cooling season， the energy-saving performance can be further improved by introducing a bypass branch and combining it with a variable air-volume control strategy. Compared to the primary baseline system， the combined VRF fresh-air system achieved cooling-season energy-saving rates of 2.77% and 15.31% for Nanjing and Beijing， respectively.

The refrigerant charge significantly influences the system's cooling and heating performance across all operating conditions. However, operating complex systems with a fixed refrigerant charge remains the standard. This approach is inherently suboptimal, leading to significant efficiency losses across most real-world conditions. To investigate the impact of refrigerant charge variation on system performance under different operating conditions and determine the optimal refrigerant charge for each scenario, this study establishes a distributed-parameter simulation model for a direct expansion air conditioning system. Using a specific VRF system as a case study, the model calculates the refrigerant charge that maximizes the COP for both cooling and heating modes under typical outdoor conditions and compressor frequencies. The accuracy of the model is validated against experimental data. Furthermore, the model is applied to analyze the refrigerant distribution under different operating conditions of VRF system. The results indicate the following: (1) In both heating and cooling modes, as the refrigerant charge increases under different scenarios, the system's COP initially rises and then declines. This demonstrates that an optimal refrigerant charge exists for each operating condition to maximize system performance. (2) The optimal refrigerant charge in heating mode is generally higher than in cooling mode. (3) In heating mode, the optimal refrigerant charge decreases as the outdoor temperature decreases, increases as the compressor frequency increases. Conversely, in cooling mode, the optimal charge increases with rising outdoor temperature, decreases with rising compressor frequency. This research can be applied to guide the development of control logic for next-generation variable refrigerant charge systems.

The high electricity consumption of building cooling systems poses a significant challenge to carbon neutrality. Low-grade thermal energy-driven ejector refrigeration technology presents a promising alternative to mitigate this reliance on electricity. However, the efficiency of this technology is often limited, owing to conventional design methods that rely heavily on expert experience and thus tend to yield suboptimal solutions. To address these challenges, this study employs the intelligent GraPHsep method to construct a novel ejector cycle using low-grade thermal energy. This method integrates graph theory with heat exchanger network optimization, enabling the synergistic optimization of cycle structure and operating parameters. Through this approach, a novel zeotropic ejector refrigeration cycle incorporating a vapor-liquid separator (NSERC) was constructed. A progressive comparative framework spanning from the basic ejector refrigeration cycle (BERC) to the NSERC was developed for elucidating the thermodynamic advantages and pinpointing the sources of performance enhancement. Under baseline conditions, the NSERC using R290/600a (50/50 wt%) achieved a coefficient of performance (COP) 17.9% higher COP than BERC. Concentration optimization identifies a R290 mass fraction of 0.1 for peak cycle performance. Furthermore, parametric analysis at this concentration across a wide range of conditions (heat source: 70–95 °C; ambient sink: 20–40 °C; chilled water outlet: 5–15 °C) demonstrated a COP improvement of 11.9% to 30.4% for the NSERC.

Building energy systems account for about 30% of global energy consumption and play a crucial role in energy transition and carbon neutrality. Residential buildings contribute a significant share of building energy consumption. An energy storage system equipped with domestic hot water tank and electrochemical battery is one of the most potential type of future residential building energy storage systems, which could offer substantial flexibility for renewable energy integration in residential buildings. However, the inherent complexity and stochastic nature of residential building energy storage systems require predictive control to enable effective demand response. Although Model Predictive Control (MPC) is widely applied in building energy systems, its performance is challenged by model and load uncertainties as well as limited controller transferability. In practice, data scarcity and constrained computational resources further complicate deployment. Existing approaches, such as hierarchical MPC, robust MPC, and learning-based MPC, often suffer from suboptimality or inadequate data resources. To address these challenges, this study proposes an adaptive model predictive control framework with an online-updated model based on real-time operational data. By combining an online-updated heat pump model with model order reduction and an adaptive probabilistic error correction scheme, the proposed method reduces computational complexity while preserving accuracy and enhancing controller generalizability. Case studies demonstrate that AMPC achieves a 15.2% cost reduction compared to rule-based control and a 12.1% saving relative to time-varying MPC. Under uncertain conditions, it improves comfort levels by 80%. And its validated transferability across different systems highlights its potential for large-scale deployment.

To address the issue of poor indoor air quality caused by the lack of fresh air in traditional variable refrigerant flow (VRF) systems, a combination of VRF and fresh air systems is increasingly adopted in practical engineering applications. However, these two systems are often operated independently, which may result in increased energy consumption. To enable coordinated operation, this study proposes a novel integrated VRF–fresh air system that aims to reduce total energy consumption while ensuring adequate indoor air quality. Based on a full operating condition model, this study evaluates the energy performance of the integrated system with different fresh air unit configurations. Results show that during the heating season, heat recovery effectively reduces the heating load on the VRF system, achieving seasonal energy savings ranging from 28.47% to 45.46%, with greater benefits observed in colder regions. In the cooling season, the combination of heat recovery and bypass control can deliver up to 19.98% seasonal energy savings, especially when outdoor air enthalpy lies between the minimum and maximum indoor setpoints. These findings underscore the importance of optimizing the design and control of fresh air units to enhance the overall energy efficiency of integrated systems.

Ice slurry cold storage technology using supercooled water is considered one of the most efficient methods. In traditional systems, to prevent ice crystal blockage in the supercooled water exchanger, the water from ice tank is typically heated to 0.5°C to melt ice crystals , resulting in a 15%–20 % cooling loss and a potential blockage risk. Although heat regeneration method has been introduced to mitigate cooling loss, the internal heat-cold offset still limits system efficiency. In this study, a novel system integrating both heat recovery and heat regeneration is proposed to improve system efficiency. In the system, return chilled water is used to melt ice crystals, enabling cooling energy recovery and eliminating the heat-cold offset, while the regeneration exchanger is used to decrease the inlet temperature of the supercooled water exchanger, reducing the cooling loss. A mathematical model of the whole system is developed and the system performance under rated operating condition is analyzed. The results indicate the proposed system enhances ice-crystal melting, increases the effective utilization of cooling capacity by 7.14 %, and improves the ice-making COP by 6.1% compared with the traditional system, and increases the system COP by 18.3% and 12.2% compared with the traditional and regeneration systems, respectively.

Integrating two-pipe variable refrigerant flow (VRF) systems with heat-pump-based domestic hot water (DHW) production offers a promising solution for improving energy utilization in residential buildings. However, system-level experimental evidence for such integrated systems remains limited, and their operating characteristics under different modes are not yet well understood, which hinders the development of effective control and optimization strategies. This study experimentally investigates a residential two-pipe VRF system with partial heat recovery for DHW production. The system achieves coordinated space conditioning and DHW production through five operating modes without requiring a complex system configuration. A prototype was developed and tested under standard laboratory conditions. The performance of each operating mode was evaluated under representative conditions and comparatively analyzed, with particular attention given to the partial heat recovery modes. The results indicate that, in the simultaneous cooling and DHW production mode, condenser heat recovery can reduce auxiliary electric heating demand and increase the overall coefficient of performance by up to 30.78% compared with conventional heat-pump cooling combined with electric resistance water heating.

The significant consumption of electricity and hydrofluorocarbons in refrigeration, air conditioning, and heat pump systems substantially contributes to the sector's impact on global climate change. A key strategy for reducing carbon emissions in this sector is the development of efficient vapor compression systems using low global warming potential (GWP) refrigerants. The selection of refrigerant and the design of cycle structure jointly affect system performance. Constructing efficient low-GWP systems is a complex multivariate problem that is difficult to solve empirically, calling for computerized and intelligent methods. The authors' previously proposed GraPHsep method overcame limitations of existing computer-based methods, like an incomplete cycle structure search range and co-optimization issues with the external heat exchanger network (HEN). To develop efficient low-GWP vapor compression systems, this paper further develops GraPHsep to co-optimize refrigerant selection, cycle structure, and system parameters. The developed method was used to construct a new efficient relay-evaporation air conditioner using R32/R1234yf. A prototype was developed, and experiments showed its seasonal energy efficiency was 14.8 % higher than the conventional system. The study shows GraPHsep's effectiveness in developing low-GWP high-efficiency vapor compression systems, which can contribute to the refrigeration sector's decarbonization.

The global shift toward low-carbon energy underscores an urgent need for efficient heating in cold regions. CO₂ heat pumps, leveraging eco-friendly refrigerants and excellent low-ambient performance, show promise but suffer from significant throttling losses due to CO₂’s low critical temperature, limiting cycle efficiency. Existing optimizations (e.g., recuperators, two-stage cycles) rely on expert-guided modifications and fail to holistically integrate refrigerant selection, cycle configuration, and heat exchange networks. To overcome this, we propose the intelligent GraPHsep method, which: (1) generates compressor/throttling valve topologies via graph theory; (2) constructs refrigerant flow structures using splits, junctions, and phase separators; (3) synthesizes heat exchangers by coupling refrigerant branches with heat sources/sinks and optimizing layouts via pinch analysis combined with a genetic algorithm. Applied to space heating at −30 ℃ ambient/45 ℃ supply water, GraPHsep designed a novel CO2 cycle achieving a coefficient of performance of 2.17, which is 77.9

Cleanroom air conditioning in microelectronics requires high energy input， high precision， and continuous operation， accounting for 40%–60% of the total plant energy consumption. Most reviews concentrate on a single technology and lack systematic classification and engineering adaptation analysis. Thus， they fail to support low-carbon manufacturing for advanced processes and dual carbon goals. Focusing on systematic energy savings， this study used bibliometric analysis and inductive review to summarize key technical routes， application fields， and research developments across four aspects： air systems， cold and heat sources， intelligent control， and energy recovery. Technical limitations are also analyzed， and future development trends are predicted. The results indicate that improvement of air systems， rational allocation of cold and heat sources， intelligent control techniques， and energy recovery technologies can achieve an energy-saving rate of 10%–39%. Multi-technology integration and cooperation， digital twins， and low Global Warming Potential （GWP） refrigerants have emerged as dominant research themes. The industry still faces significant challenges， including low technology implementation rates， unresolved coupling mechanisms， a lack of comprehensive life-cycle evaluations， and divergent industry standards. This study builds a complete energy-saving technology framework and clarifies the potential energy-saving and optimization directions， with the aim of providing a systematic theoretical framework and an engineering foundation for the energy-saving design， operational optimization， and technological upgrading of cleanroom air conditioning systems in microelectronics.

The Kigali Amendment to the Montreal Protocol establishes a clear phase-down restrictions on the production and consumption of hydrofluorocarbons. As the largest consumer of hydrofluorocarbons, the refrigeration industry faces a considerable challenge in terms of refrigerant substitution. The variable-refrigerant-flow (VRF) air-conditioning system is the most popular air-conditioning type in non-residential buildings and has a remarkable trend of sustained growth. Refrigerant substitution pathways of the VRF directly affect the implementation of the Kigali Amendment. Therefore, the refrigerant substitution pathways of the VRF were planned by adopting a systematic approach in this paper. Firstly, a model for predicting the future development of the VRF air-conditioning system was put forward, and a hydrofluorocarbons consumption and emission assessment model for the VRF air-conditioning system sector was developed using a bottom-up approach. Based on them, the substitution pathways of refrigerants for VRFs are planned using optimization method under the constraints of Kigali Amendment. The findings indicated that VRF air-conditioning systems utilizing alternative refrigerants must be made available for sale as soon as possible. Projections indicate that a 58% emission reduction effect can be achieved by 2050. However, if the energy efficiency of these systems is further improved, this synergistic emission reduction scenario could achieve an even greater reduction of 61% by 2050.

Expansion work recovery is an important part of enhancing the energy efficiency of air conditioning refrigeration systems, and to reduce greenhouse gas emissions and mitigate climate change. This paper investigates a novel expansion work recovery device with development potential—the pressure exchanger to improve the energy efficiency of vapor compression systems and to enhance the overall efficiency of refrigeration systems. The pressure exchanger, serving as a mechanical substitute for an expansion valve, concurrently fulfills the functions of both an expander and a compressor. It not only captures the expansion pressure energy from high-pressure fluids but also utilizes this energy to compress low-pressure fluids. In this paper, the fundamental physical structure of the pressure exchanger was constructed, and the basic thermodynamic principles of the pressure exchanger were analyzed. To simplify the problems and facilitate validation, models were established using ideal gases and incompressible liquids as the study subjects. Besides, for a pressure exchanger, various factors affecting efficiency were proposed, among which the compression ratio was specifically discussed, and methods to improve efficiency were summarized. When the compression ratio is reduced from 10 to 1.5 while keeping all other conditions constant, the overall efficiency of the pressure exchanger may increase from 31.5 to 80.4

In year-round high-humidity regions, the demand for flexible temperature and humidity control is particularly pronounced. However, conventional air conditioning systems typically regulate only temperature, while dedicated temperature-and-humidity independent control systems often involve complex configurations and high costs. Moreover, there remains considerable room for further improvement in both cooling and dehumidification efficiency. Based on our previously proposed flexible heat-recovery VRF system, this study performs system sizing and modeling for residential applications in high-humidity regions. A residential building model with realistic operating schedules is established for Chongqing, China, and the system's seasonal performance is thoroughly evaluated for the cooling and transitional seasons. Meanwhile, the performance of the proposed system is compared with that of a conventional heat-pump VRF system that relies on terminal electric reheat. The results show that: 1) Compared to the conventional system, the proposed system achieves a 4%–10% decrease in energy consumption and a 2%–30% increase in dehumidification efficiency during the cooling season. 2) During the transitional seasons, the SMER is approximately 5% higher under isothermal dehumidification conditions, and the overall energy consumption is significantly lower, achieving energy savings of 57%–70%. 3) Considering both seasons together, the proposed system achieves a total energy saving of 37.23%. This study provides a viable solution for achieving energy-efficient, high-comfort air conditioning in high-humidity regions.

The refrigeration and air conditioning industry consumes substantial electricity, making it a critical sector in global climate change governance. Developing waste-heat-driven ejector refrigeration technology can reduce electricity dependency while enhancing comprehensive energy utilization efficiency. Given the low efficiency of conventional ejector refrigeration cycles and the inability of experience-based construction methods to achieve globally optimized cycle structures, this study employs an intelligent cycle construction method named GraPHsep to develop a zeotropic ejector refrigeration cycle with a three-stream heat exchanger (TSERC). Energy and exergy analysis models were established to investigate the cycle's thermodynamic characteristics. Results show that TSERC features a flash separator directing low-boiling-point-enriched refrigerant to the low-pressure side to elevate evaporating pressure; a three-stream cascade heat exchanger recovering heat from the ejector outlet refrigerant to reduce generator load; and a low-pressure recuperator lowering refrigerant temperature before throttling to reduce throttling losses. Using R290/R600a and R134a/R236fa as refrigerants, the study examines effects of refrigerant composition (0.1-0.8), heat source temperature (65-85 degrees C), ambient heat sink temperature (20-40 degrees C), and chilled water temperature (3-11 degrees C) on cycle performance. Compared to conventional ejector refrigeration cycle and advanced flash-separation ejector refrigeration cycle, TSERC achieves COP improvements of 10.5%-68.0% and 5.4%-27.2%, respectively.

The rapid expansion of low-cost small logistics cold storage facilities in China, driven by rising income levels and cold chain demands, has raised concerns about their unregulated greenhouse gas (GHG) emissions. This study pioneers the first nationwide quantification of GHG emissions from China's small cold storage sector, addressing the critical knowledge gap posed by the lack of clear data on their system performance and GHG emissions that hinders effective policy formulation. Through systematic analysis of both direct and indirect GHG emissions, the inaugural emission baseline for this understudied sector is established. A hybrid methodology was adopted: direct emissions were estimated using the emission factor method, while indirect emissions were modeled through two components-a refrigeration system performance model derived from extensive field measurements and a cooling load characteristic model developed from operational surveys. The refrigeration system performance model was derived from extensive field measurements conducted over six months during 2023-2024 across 48 refrigeration systems in 16 frozen storage rooms and 34 systems in 18 chilled storage rooms, all constructed between 2019 and 2021. Results reveal that China's small logistics cold storage facilities generated 4.27 Mt CO2-eq in 2022, with indirect emissions (2.55 Mt CO2-eq, 59.7%) and direct emissions (1.72 Mt CO2-eq, 40.3%). Geographically, Shandong Province ranked highest in annual emissions from chilled rooms (55713 tons CO2-eq), while Guangdong Province led in frozen room emissions (306244 tons CO2-eq).

The building sector accounts for 28 % of global energy consumption, with more than two-thirds of household energy is used for space heating, cooling, and hot water. The development of highly efficient air conditioning and heat pump systems using refrigerants with low global warming potential is imperative. However, conventional research methodologies, such as cycle structure enhancement based on expert experience and performance investigation of the predefined systems charging different refrigerants, are challenging for the global optimization of high-efficiency systems. In this study, the intelligent construction method for vapor compression systems, namely GraPHsep, is further developed to facilitate the simultaneous optimization of refrigerant selection, cycle structure, and system parameters. The GraPHsep method is applied to construct high-efficiency air conditioning and heat pump systems using refrigerants with low global warming potential for buildings. Concurrently, the preferred refrigerants with low global warming potential are derived, and the enhanced structure-energy efficiency relationship is analyzed. The energy efficiency potential of the constructed systems is evaluated through a comparison with existing systems. The analysis indicates the potential for improving coefficient of performance is 22.3 %, 9.2 %, 12.8 %, and 24.0 %, for the four scenarios: air conditioners, chillers, air source heat pumps for space heating, and heat pump water heaters, respectively.

The current standards for evaluating chiller-plant energy performance provide recommended energy-efficiency values only for annual or rated conditions. The continuous commissioning of chiller plants, based on real-world operating conditions, is crucial and can increase their energy efficiency; however, the existing commissioning methods are too general to provide practical steps for system retrofitting and do not grade the retrofitting steps in terms of economy and difficulty. To address this, this study proposes a stepwise commissioning method to predict the energy efficiency potential of a chiller plant from operational parameters measured using basic sensors. In this method, improvements in efficiency via retrofitting are estimated based on the optimization of the control strategy, equipment performance, and system structure. Based on this method, for the high-performance chiller plant examined as a case study, control-strategy optimization could improve the system energy efficiency ratio from 5.2 to 5.7, an 8.7 % increase; optimizing equipment performance could further improve it from 5.7 to 5.9, a 4.6 % increase. In terms of different types of equipment, replacing the chillers, water pumps, and cooling towers would achieve improvements in energy efficiency, of 2.7 %, 0.3 %, and 1.5 %, respectively, as the existing equipment is already efficiently operated. This method therefore supports improvements in chiller-plant energy efficiency considering economic feasibility and implementation difficulty.

Refrigerant screening is a crucial preliminary step in designing high-efficiency heat pumps. Based on the previously proposed life cycle expense indicator that integrates economic costs and environmental impacts, this study systematically investigates the influence of key parameters on refrigerant selection outcomes. It assesses the most promising refrigerants for moderate and high temperature heat pumps within major global carbon markets. The findings are expected to provide decision-making support for formulating refrigerant replacement policies. Results indicate that: The energy efficiency of selected refrigerants is the primary factor in refrigerant selection for all the markets under current energy prices and carbon prices; Strategic policy interventions that combine carbon pricing mechanisms with energy subsidies can effectively accelerate the adoption of environmentally friendly and high-efficiency refrigerants.

The increasing deployment of small-capacity logistics cold storage facilities has led to a notable rise in greenhouse gas emissions, driven by refrigerant leakage and energy consumption. To address both direct and indirect emissions, this study proposes an environmentally friendly CO2-based refrigeration system and introduces a novel design methodology using the GraPHsep approach. Unlike conventional methods that optimize system configuration for a single condition, this work constructs and integrates optimal refrigeration cycles for four typical scenarios-frozen and chilled storage in both summer and winter. The resulting unified system enables efficient operation across variable ambient and cooling load conditions and allows seamless switching between storage modes. A simulation model is established to analyze system performance and control strategies. Results show that the double-stage compression flash tank system achieves superior COP in summer, while a single-stage system with an internal heat exchanger is optimal for winter. Under a chilled condition with 10 kW load and 20 degrees C outdoor temperature, the system achieves a maximum COP of 3.17 in single-stage mode. A lifecycle assessment reveals an 18 % reduction in TEWI compared to a conventional R404A system, avoiding approximately 102.7 tonnes of CO2 emissions.

With the increasing demand for energy efficiency and comfort, traditional variable refrigerant flow (VRF) systems, while effective in temperature management, lack adequate humidity control functionality. This limitation becomes particularly evident in low-temperature, high-humidity conditions, where the systems often fail to deliver desired environmental control performance. Furthermore, the current system lacks sufficient mode adaptability to accommodate varying operational requirements. This study proposed a flexible heat recovery VRF system with high dehumidification capacity and energy efficiency. Through the reconfiguration of valves and pipelines within the system, the indoor and outdoor heat exchangers can be adapted to perform different functions, achieving flexible thermal energy redistribution within buildings, thereby meeting diversified zonal requirements for temperature and humidity control. The system can operate in four distinct modes: double-evaporation temperature cooling, heating, heating dehumidification, and heat recovery mode, catering to diverse environmental conditioning needs across different zones while maintaining energy efficiency. An experimental prototype was constructed to validate the feasibility of each operating mode and the environmental conditioning effectiveness of the system. Additionally, simulation models were developed for each operational mode to investigate system performance characteristics and thermal energy dispatch flexibility under varying environmental conditions and mode requirements.