Xinhua Xu

Highly efficient heating systems are crucial for energy saving and carbon emission reduction in residential buildings. However, radiant floor (RF) heating systems for residential buildings coupled with air-source heat pumps (ASHP) in hot-summer and cold-winter (HSCW) regions have not been thoroughly investigated as the frosting and defrosting processes are rarely taken into full consideration. This study presents the performance evaluation of a RF system of an ordinary residential building with an ASHP in a hot-summer and cold-winter region. A detailed model of the ASHP is developed to characterize the overall system performance. The simulation platform of the heating systems is established in TRNSYS. The results reveal that the RF system offers more stable thermal environment when comparing with a fan coil unit (FCU) heating system. Seasonal performance indicates that the RF system consumes 1355 kWh with a COP (coefficient of performance) of 2.62, which is 24.2% higher than the FCU system. The RF system with ASHP emits 35.0% and 24.1% less carbon emission when compared with the FCU system with ASHP and the system with a gas boiler. These advantages highlight its potential as an energy-efficient and low-carbon heating solution for residential buildings in the HSCW region.

Frequency-priority and unit-priority strategies are two common sequential control methods for cooling towers in central air-conditioning systems. The former ensures operational stability but often leads to inefficient operation of a few units at high frequencies under partial loads. The latter improves energy efficiency but suffers from instability due to frequent fan ON/OFFs. To balance energy efficiency and operational stability, this study proposes an optimal sequential control strategy. Three calculation approaches are established to determine the key sequential control parameter of the proposed strategy, namely the Airflow Equivalent Method (AEM), Energy Consumption Equivalent Method (ECEM), and Airflow Equivalent & Median Energy Consumption Cooperative Method (AE-MECM). The AEM enables continuous regulation of the total system airflow during tower switching, thereby eliminating sudden fluctuations in heat dissipation capacity caused by fan ON/OFF operations. The ECEM realizes smooth variation in system energy consumption when putting additional cooling towers into service. As an improved dual-dimension collaborative method, the AE-MECM introduces median values to guarantee the continuity of both total airflow and overall energy consumption. The test platforms were established based on two actual central air-conditioning cooling water systems to validate the control performance of the proposed control strategy. The results show that the proposed optimal sequential control strategy achieves a remarkable reduction in energy consumption of cooling tower fans while maintaining identical cooling performance. Under low load conditions, the energy consumption decreases by over 50% compared with conventional control strategies. This control strategy does not rely on precise system models, has low data requirements, and features high dynamic adaptability, which presents an efficient and generalizable optimized sequential control scheme for cooling tower systems.

Energy-saving renovations of existing central air-conditioning systems have increasingly received recognition. Theoretical energy consumption model (TECM) is commonly used to estimate the baseline energy consumption of water pumps for comprehensive benefit assessments. Nevertheless, the calculated results often exhibit significant discrepancies compared to the actual values. This study proposes a novel enhanced theoretical energy consumption model (EN-TECM) for improving the calculation accuracy of pump energy consumption, focusing on two dimensions: the number and frequency of operating pumps. Then, 576 sets of pump operation data from 29 actual central air-conditioning systems are used as samples to determine the model key parameter. Further, three actual engineering cases are employed to evaluate the application performance of the EN-TECM. The model regression results show a significant reduction in calculation errors across all operating conditions, with the mean relative errors within ±1%. The analysis results of all engineering cases demonstrate that the relative errors of the cumulative total energy consumption decrease by more than 60% compared to the TECM. This research offers a practical and effective method for accurately calculating the energy consumption of chilled water pumps in field-operated central air-conditioning water systems, which will facilitate the advancement of energy-saving retrofitting projects for existing buildings.

Radiative sky cooling can be effectively integrated with pipe-embedded wall systems to reduce building cooling loads. However, the energy-saving and carbon reduction potential of this technology varies according to climatic conditions and the method of integration, requiring quantification. To address this gap, a revised degree-hour method of evaluating energy efficiency for an integrated system is proposed and validated, and a global potential map is developed. The proposed method can be used to predict the energy-saving and carbon reduction potential of radiative sky coolers under different climatic conditions. Compared to physical model prediction methods, the revised degree-hour method is faster and more accurate, with an evaluation error of approximately 5%. The results indicate that the integrated system performs well in most regions with cooling demand. The system’s energy-saving potential is highest in cities in tropical savanna and desert climate zones, achieving energy savings of approximately 53.96 kWh/m² and reducing carbon emissions by approximately 22.99 kgCO₂/m² during the cooling season. Its performance is reduced in subtropical monsoon zones, with savings of 8.36 kWh/m² and 3.56 kgCO₂/m². Furthermore, the system’s energy-saving potential generally declines as the cold-water temperature of the radiative sky cooler increases, especially in tropical regions. This work provides a rapid assessment tool and global reference data to support low-energy building design.

To achieve energy savings, reduce consumption, and support the “dual-carbon” strategy in China, this study applies digital twin technology to investigate the centralized air-conditioning water system of a metro-station HVAC installation and develops a high-fidelity digital twin model to reveal the thermal delay and thermal attenuation characteristics of the pipeline network. Using the noncausal modeling approach of the Modelica language, a full digital twin representation of the centralized air-conditioning water network is constructed by covering chillers, cooling towers, pumps, terminal units, the pipeline network, etc. The model is validated against real operation data to ensure high fidelity. Validation shows the predicted chilled water flow rate of the digital twin model agrees well with the measured chilled water flow rate with an RMSE of 0.27 kg/s. Validation also shows the difference is about 0.3 °C between the digital twin prediction and the measurement in the main pipe. Based on the validation digital twin model, the thermal delay and thermal attenuation characteristics of the centralized air-conditioning water system are seriously evaluated. The results indicate that branch K3, due to its longest transport distance, exhibits a delay of 227 s. The overall thermal delay of the system reaches 7.5 min. The temperature attenuation of this water system is about 0.2 °C due to heat loss through pipe walls. The findings may offer theoretical support for the optimal regulation and control, fault detection, and anomaly identification of this centralized air-conditioning water system.

Transparent building envelopes significantly increase energy demands due to low thermal resistance and solar heat gain, while conventional double-skin facades may lead to overheating and high cooling loads in the summer. This study proposes a novel earth-to-air heat exchanger (EAHE)-assisted ventilated double-skin facade (VDSF) system utilizing low-grade shallow geothermal energy for year-round thermal regulation of transparent building envelopes. A numerical model of this coupled system was developed and validated to estimate the thermal performance of the EAHE-assisted VDSF system in a hot-summer-and-cold-winter climate. Parametric study was conducted to investigate the impact of some key design parameters on thermal performance of the EAHE-assisted VDSF system and further reveal recommended design parameters of this coupled system. The results indicate that the EAHE-VDSF system reduces annual accumulated cooling loads by 20.3% to 76.5% and heating loads by 19.6% to 47.1% in comparison to a conventional triple-glazed, non-ventilated facade. The cavity temperature of the VDSF decreases by 15 °C on average in the summer, effectively addressing the overheating issue in DSFs. The proposed coupled EAHE-VDSF system shows promising energy-saving potential and ensures stability and consistency in the thermal regulation of transparent building envelopes.

Cooling water, a crucial component of the central air conditioning setup, exerts a relatively minor direct impact on the thermal comfort of building indoor environments while it has a great effect on the system’s energy efficiency. Numerous studies exist on the cooling water system, particularly focusing on the process by which the cooling tower system operates, but the linkage between the chiller and the cooling tower is typically overlooked. When the connection is long and the passage environment for the pipeline is not conventional, it cannot be neglected for the optimal control for system efficiency improvement and energy consumption reductions. Throughout this research, a control strategy of the cooling water system for deep subway stations with long pipelines is presented. This cooling system was connected with outdoor cooling towers through a corridor about one hundred meters long. In this process, the cooling water temperature is influenced by the corridor’s thermal environment. For this study, an online control strategy optimizes the cooling water temperature, and a simulation platform of the air conditioning cooling water system of the deep subway station was also developed to evaluate the energy-saving potential of the control strategy of this cooling water system. Atop this platform, a simplified heat transfer model of the pipe corridor was created to determine the cooling capacity provided by the cooling water pipe in the corridor. The outcomes suggest that, as opposed to the conventional control mode, the energy-saving ratio of the optimal control strategy during a typical day may reach 4.1%, and the cooling source system’s Coefficient of Performance (COP) might see an increase of about 4.2%. The energy consumption of the water system throughout the whole cooling season may decrease by 9778 kWh, and the energy-saving rate is 4.1%. The results also demonstrate that the cooling water pipes release heat to the air in the corridor most of the time, and the released heat is larger than the absorbed heat. The maximum heat dissipation to the air in the corridor from the cooling water supply and return pipe can be up to 24.3 kW. The cooling effect of the corridor of subway stations with large depths below the ground surface cannot be ignored when optimal control is considered for the cooling water system.

Air-permeable building envelopes (APBEs) utilize the infiltrated or exfiltrated airflow within porous materials to directly change their temperature distribution to reduce heat loss/gain. APBEs effectively integrate building ventilation and heat recovery to achieve excellent thermal insulation while improving indoor air quality. This paper presents a comprehensive review of the fundamentals and classifications, historical evolution over time, opportunities and benefits, and future views on APBEs. It can be treated as a responsive building envelope that enables building envelopes to dynamically change the U-values by varying the infiltrated or exfiltrated airflow rate within a porous material. Previous studies have indicated that the U-value of 0.1 W/(m²·K) can be realized by employing APBEs. Moreover, some research demonstrates that APBEs could act as high-performance air filters that reduce over 90% of particulate matter within fresh, ventilated air. Some factors, such as airflow rate, thickness, and thermal conductivity of porous materials, have a significant influence on the effectiveness of APBEs. For practical applications, integrating the APBE with passive building ventilation can help reduce the initial cost and facilitate decarbonization in buildings. Moreover, advanced control strategies could collaboratively optimize the operation of ABPEs and build energy systems to maximize their energy-saving potential.

This study investigated the air quality and ventilation systems of 22 public toilets in 10 railway stations in China. Approximately 80% of public toilets meet the standard for ammonia concentration in Class I toilets, while 20% exceed the standard. It was found that the concentration of pollutants is mainly related to the number of toilet users and the ventilation system. In 20% of public toilets, the change in ammonia concentration was delayed by about 1 to 2 h with the change in hourly service number. In order to improve the air quality, a design method for calculating the number of toilet cubicles was proposed. Results show that the service capacity of the cubicle per hour (SCCH) of a female toilet is 12, the SCCH of male toilets is related to the ratio of squatting pans to urinals (RSU), which is suggested to be 1:1~1:0.8, and the corresponding SCCH is 16~20. Then, the effect of different ventilation forms was simulated by computational fluid dynamics (CFD) 2019 software. The results show that the bottom exhaust was better than the top exhaust and that the fresh air supply system is unnecessary. The recommended ventilation rate for toilets is 20 air changes per hour (ACH). The scale design method of toilets proposed in this paper was meant to address the gender imbalance and avoid queuing and provides a reference for the renovation and design of public toilets.