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Abstract: Low-grade energy sources such as geothermal energy, favorable ambient air and industrial waste heat etc. exist widely. Sufficient utilization of these low-grade energy sources may reduce our daily dependence on high-grade energy sources such as electricity resulting in reduced emission of green house gas for environmental conservation. Active pipe-embedded structure as floor/ceiling usually with water as the medium to carry heat or coolth may utilize these low-grade energy sources for providing space air-conditioning. Compact arrangement of pipes in the structure may significantly enlarge heat transfer surface between the slab mass and water in the pipe allowing substantial heat flows even for relatively small temperature differences. Application of the heat or coolth storage capacity of this structure for preheating or pre-cooling is also one among the advantages of this structure for shifting load and exploiting the nighttime cheap electricity tariff in some regions. This paper presents the technology of the active pipe-embedded structure for utilizing widely existing low-grade energy sources following by a comprehensive review on the heat transfer calculation models of this structure and its practical applications in real building systems for space air-conditioning. This review shows that more works on the active structure, especially simple and transient models for dynamic and accurate performance prediction and easy integration with existing building energy simulation packages, are worthwhile for further promoting the practical application wherever the low-grade energy sources are favorable. [ABSTRACT FROM AUTHOR]

Radiant cooling in buildings, as an energy-saving and comfort cooling method, is very promising in creating a green and healthy indoor thermal environment in buildings. However, radiant panels are prone to dew condensation and have low cooling capacity; thus, their application in hot and humid regions is limited. Covering the surface of the radiant cooling panel with a high transmittance low conductivity material (HTLCM), and, separating the air contact surface and the radiating surface, to form a new decoupled radiant cooling technology is an effective way to address the deficient radiant cooling capacity and easy condensation. Radiation heat exchange occurs between the radiant cooling surface and the indoor heat source surface through the HTLCM, while the indoor humid air undergoes convective heat exchange with the higher temperature HTLCM surface. The optical, and thermal properties of the material determine the cooling capacity and condensation resistance (thermal performance) of the radiant cooling air conditioning. This review provides a summary of the research advances in HTLCM-assisted radiant cooling air conditioning (RCAC) in recent years. First, the classification and optical and thermal properties of the currently used HTLCMs for radiant cooling are presented. Second, the heat transfer mechanism of RCAC with HTLCM is introduced. Finally, the research advances in the application of HTLCM-assisted radiant cooling are summarized, and the challenges and future development in this field are provided. The practical engineering application of HTLCM-assisted radiant cooling units in hot and humid regions is highlighted. • Material, mechanism, application of HTLCM-assisted radiant cooling are discussed. • Structural design, thermal performance, and cooling effective of RCAC are presented. • HTLCM improves condensation resistance, cooling capacity, temperature range of RCAC. • An updated review and discussion of HTLCM-assisted RCAC is provided. [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]