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To face the climate change and carbon neutrality, reductions of energy-uses and carbon emissions are imminent. Pipe-encapsulated phase change material (PenPCM) wall is a novel envelope system with renewable energy utilizations in buildings. It combines the PCMs, nocturnal radiant cooler and gravity heat-pipe with the wall to achieve the automatic heat insulation and removal without the mechanical energy. Parameters may have influences on system performances and the parametric analysis is understudied. In this study, the influences of parameters including phase-change temperature, PCM thickness, pipe-spacings, cooler areas and orientations are investigated. The system applicability for different climate regions is also studied. Results show that, when the phase-change temperature is 27–29 °C, the ratio of the cooling load reduction to the cooling load of the conventional wall reaches largest with 17.4%. Appropriately increasing the PCM quantity can improve the thermal-storage capacity and decrease the wall temperature fluctuation. Enlarging the cooler area can increase the energy-saving effect of the PenPCM wall system. For different climate regions, the PenPCM wall system is applicable. Compared with conventional wall, the ratio of the cooling load reduction from June to September can reach 11.7%, 16.1%, 21.1%, 28.6%, 44.2% respectively for Hong Kong, Wuhan, Beijing, Harbin and Kunming. [ABSTRACT FROM AUTHOR]

Abstract Aerogel insulating panels (AIPs) exhibit extensive prospects for application in aerospace, industry and buildings as excellent energy-saving thermal insulators owing to their ultra-low thermal conductivity. This work aims to numerically and experimentally investigate the thermal performance of AIPs in building energy efficiency. The AIPs were prepared and used as an insulation layer in an insulating cell. Three insulating cells with three different insulating materials as the insulation layer were prepared to measure and comparatively analyse their thermal performances. A resistance-capacitance thermal network model to predict the thermal performance of the insulating cells was developed and validated with the experimental data. The thermal indices of cells, i.e. the time lag, decrement factor and daily heat loss, were explored under periodic disturbances of exterior air temperature. Furthermore, the thermal performance of AIPs for exterior walls was predicted by adopting the typical wall structure in the hot summer and warm winter zone of China. The results showed that the AIP wall has decreases of ∼20% and ∼40% in the fluctuation amplitude of the internal temperature and heat flow, respectively, compared with the traditional insulating walls. Highlights • A method to indicate thermal performance of AIP for energy efficiency is proposed. • Three insulating cells with EPS, GF and AIP are prepared and compared. • AIP cell shows twice time lag, 40% less decrement factor and 35% less daily heat loss. • AIP wall has ∼20% and ∼40% decreases in amplitudes of temperature and heat flow. [ABSTRACT FROM AUTHOR]

Roof is a weak part of the building envelope due to the high intensity of solar radiation in summer. Improving thermal performance of roof is crucial to reduce energy consumption. In this study, the thermal performance of a new ventilation roof with shape-stabilized phase change material (SPCM) is investigated. The dynamic heat transfer model for this roof is established by coupling the number of transfer units (NTU) heat exchange model with the resistance-capacity dynamic heat network model and used to analyse the heat transfer. The accuracy of the model is validated by experiment. The indoor air temperature, internal surface temperature of the roof, and cumulative cooling load in summer are simulated and analysed in Wuhan. The results show that PCM and night ventilation have great energy saving potential. When the 30 mm PCM is applied, the peak indoor air temperature and peak internal surface temperature decreased by 2.9 °C and 5.5 °C, respectively, and the cumulative cooling load of the building decreases by 19.2%. When night ventilation is applied (v = 3 m/s), the average latent heat utilization rate of the PCM layer increases, and the cumulative cooling load decreases by 22.9% and 37.5% compared with the PCM roof and the reference roof, respectively. [ABSTRACT FROM AUTHOR]

Ground heat exchanger takes the soil underground as heat source or sink to supply cooling or heating. It has been widely used in building heating and cooling systems due to high efficiency and environmental friendliness. This paper reviews the latest research on ground heat exchangers from several new perspectives and demonstrates their potentials in achieving zero energy buildings. Firstly, ground heat exchangers are classified into water-based and air-based ones based on the heat transfer medium. They can be used in a passive or active approach. Associated research and projects for each approach are introduced and analysed. Then the integration of ground heat exchangers with various cooling and heating technologies and related studies are reviewed. These technologies include solar thermal collectors, cooling towers, nocturnal radiative cooling technology, solar chimney, etc. Finally, a technical route for ground heat exchangers to help realize zero energy buildings is presented, which provides a promising solution to improve energy efficiency of buildings. [ABSTRACT FROM AUTHOR]

A hollow block ventilation wall (HBVW) is an innovative construction where the block cavity of the wall is used as a ventilation duct. Low-grade energy can be used to remove heat stored in the structure in summer and warm the structure in winter. Therefore, the cooling and heating loads of buildings can be significantly reduced. Therefore, an experimental study was conducted to investigate the thermal performance of HBVW under different airflow temperatures and rates. The temperatures and heat flux of HBVW surfaces and the airflow temperatures in cavity were tested on a typical summer day. The results show that the air velocity in the cavity has a great influence on the thermal performance. Compared with a non-ventilation wall, the thermal resistance increases from 0.637 m 2 K/W to 1.61, 3.06, 3.86, and 4.12 m 2 K/W when the air velocity is 0.3, 0.9, 1.5, and 1.9 m/s, respectively, and the heat flux transferred through the inner surface can be decreased by 40.5%, 73.3%, 79.4%, and 82.6%. The average temperature of the inner surface can be respectively decreased by 1.8, 2.8, 3.0, and 3.1 °C. The airflow temperature has little effect on the equivalent heat resistance, but it does greatly affect the inner surface temperature. [ABSTRACT FROM AUTHOR]

Renewable energy is becoming increasingly popular in the development of low-carbon societies. Radiative sky cooling (RSC) technology utilizes long waves in the atmospheric window to radiate heat into outer space; it promises a good cooling effect and wide applicability. This paper reviews RSC technology in terms of its background, materials, heat transfer and influences, and applications in buildings. High-performance radiative cooling materials have been proposed to realize RSC during both the day and night. Using these materials, sky radiative coolers and cooling systems have been fabricated for different scenarios. Radiative and non-radiative heat transfer and cooling performances can be calculated or measured using the radiation law. The correlations between the sky effective radiative temperature and effective emissivity are summarized. Environment conditions affect the RSC performance, and the influences are systematically analyzed. Owing to its advantages of cleanliness, accessibility, and extensive resources, RSC technology can be applied to building envelopes and heating, ventilation, and air conditioning systems, to develop low-carbon and zero-energy buildings. Passive and active applications are also discussed. Finally, challenges of applying RSC technology to building systems are briefly discussed. This review should help promote the practice and implementation of RSC technology. • Spectrally selective radiative cooling materials and cooler forms are reviewed. • Heat-transfer parameters, performances and influences of RSC are summarized and analyzed. • Applications of RSC in building envelopes and HVAC systems are presented. • Challenges on RSC applied to buildings are discussed. [ABSTRACT FROM AUTHOR]

Aiming at reducing the heating and cooling loads for built environments, the thermal resistance and capacitance (R C) model has widely been acknowledged as an effective method for predicting the heat flux through building walls by simplifying wall structures and their properties into R and C allocations. This paper demonstrates an inverse optimization method based on particle swarm optimization (PSO) for determining the optimal R C allocation that minimizes the heat flux through building walls. A thermal R C model with three resistances and two capacitances was used as an example to search for the optimal building R C allocation in the hot summer and warm winter zone of China. The inverse optimization could be efficiently accomplished in 2.5 h with an ordinary computer, approximately 0.12% of the time consumed by using the exhaustive search method. The optimized R C allocation was composed of three resistances of 0.43, 0.18 and 0.39 and two capacitances of 0.50 and 0.50. Compared to the three typical thermal insulation (e.g., internal, external, and internal/external), the optimal R C allocation could reduce the heat flux into/from buildings by 17.3%–44.3%. The proposed inverse PSO method shows an effective and efficient capacity in searching for the optimal R C allocation of thermal R C models. • An inverse PSO method for the optimal RC allocation is proposed. • Optimization shows fewer steps and less time cost. • An optimal RC allocation is obtained. • Heat flux is reduced by 17.3%, 34.9% and 44.3% than three traditional RC allocations. • The effect mechanism of RC allocation on heat flux is explored. [ABSTRACT FROM AUTHOR]

Roof receives heat directly from the solar radiation and outdoor air, and the heat transfer of the roof is often greater than that of any external wall. The building roof is usually overheated in summer, causing a great increase in the air conditioning load and negative effects on indoor thermal comfort. Thus improving the thermal insulation performance of the roof is essential for reducing air conditioning energy consumption and improving indoor thermal comfort. In this paper, an innovative pipe-embedded ventilation roof with outer-layer shape-stabilized PCM (named VRSP for short) was proposed. The heat gain is stored in the PCM to migrate excessive heat during the day and released through air ventilation at night. A three-dimensional transient-state heat transfer model of the VRSP system was built by CFD. The effects of phase transition temperature range of PCM, thickness of PCM layer and airflow rate in the ventilation duct on the thermal performance of the structure in five representative climate regions of China were evaluated. Results show that the optimum phase transition temperature ranges of PCM in severe cold region, cold region, hot summer and cold winter region, hot summer and warm winter region and mild region are 31–33 °C, 34–36 °C, 36–38 °C, 34–36 °C and 29–31 °C, respectively. The optimum thicknesses of the PCM layer are 25–30 mm, 25–30 mm, 30–35 mm, 25–30 mm and 20–25 mm, respectively. The suitable airflow rates are 1.5–1.9 m/s, 1.6–2.0 m/s, 2.1–2.5 m/s, 1.9–2.3 m/s and 1.4–1.8 m/s, respectively. The conclusion provides valuable guides for the application of VRSP under various climate conditions. • A pipe-embedded ventilation roof with shape-stabilized PCM is proposed. • Effects of physical properties of PCM and airflow rates are investigated. • The new system is optimized for its applications in different climate zones. • Considerable energy-saving effects are observed in Kunming, Harbin and Beijing. [ABSTRACT FROM AUTHOR]

Pipe-encapsulated PCM wall system with self-activated heat removal by nocturnal sky radiation cooler is a feasible means for building insulation and heat removal by using natural energy. In this paper, a system simulation platform including the simplified pipe-encapsulated PCM wall model, heat pipe heat-transfer model and nocturnal radiation model are established to investigate the thermal performance of the pipe-encapsulated PCM wall system with self-activated heat removal by a nocturnal sky radiation cooler. The temperature and heat flow of this system used in light-weight, medium-weight and heavy-weight walls under a dynamic boundary condition are respectively simulated on this platform. Results show that about 55.6%–82.8% of heat from the outdoor can be resisted by the pipe-encapsulated PCM wall, and 54.7%∼81.0% of that can be removed by the nocturnal radiation cooler at night. Comparing with the common wall, the proposed system can reduce by 32.4%∼55.5% of the accumulated heat entering into indoor environment. Good energy-saving potential can be achieved by this pipe-encapsulated PCM wall system. Besides, the result further reveals that the reduction of the accumulated internal surface heat transfer of this system used in medium-weight and heavy-weight walls is obvious larger than that of this system used in light-weight wall. • System platform of a pipe-encapsulated PCM wall system with heat removal. • Thermal performance of three walls (light, medium, heavy) with the PCM wall system. • 55.6%–82.8% of heat from the outdoor can be resisted by the PCM wall system. • Reduction of heat into the indoor is lager for the heavy wall with the PCM system. [ABSTRACT FROM AUTHOR]

Roof with outer-layer shape-stabilized phase change material (RSPCM) is a building construction that incorporates shape-stabilized phase change material (PCM) into the out layer of the roof. The decrement factor of the roof and the peak temperature of the inner surface can be greatly reduced by using the PCM in summer. CFD numerical simulation is used to investigate the thermal performance of RSPCM. The effects of the phase transition temperature, layer thickness and phase transition temperature radius of PCM are studied numerically. Results show that the suggested PCM thickness is 30 mm and the temperature radius should be as small as possible. The optimum phase transition temperatures are 31–33 °C, 34–36 °C, 36–38 °C, 34–36 °C, and 29–31 °C respectively in severe cold region, cold region, hot summer and cold winter region, hot summer and warm winter region and mild region. The decrement factors at the corresponding optimum phase transition temperature are about 0.030, which are lower than that of the roof without PCM by over 85%. The peak temperatures of the inner roof surface are all decreased by over 3.7 °C. These results can be used as a guideline for optimum design to improve the dynamic thermal performance of roof structure in summer. • Heat transfer model of roof with outer-layer shape-stabilized PCM is built by CFD. • Effects of phase transition temperature and radius, and PCM thickness are studied. • Decrement factor, time lag and latent heat utilization of this roof are analyzed. • PCM thickness of 30 mm is recommended. • Optimum phase transition temperature of PCM are obtained for five climate zones. [ABSTRACT FROM AUTHOR]