Radiant cooling provides better thermal comfort, operates quietly, and offers energy and space savings, making it a promising supplementary to conventional air-cooling methods. However, the use of radiant cooling in hot and humid climates poses challenges because the radiant cooling surface is also the air-contact surface. To increase the cooling capacity and satisfy cooling demand, the radiant cooling temperature should be lowered, but this increases the condensation risk due to humid indoor air. To address this issue, a condensation-free ceiling radiant cooling unit (condensation-free CRCU) that uses an infrared-transparent and conductive-resistant (ItCr) layer to cover the radiant cooling surface was proposed and prevent it from contacting with air. This reduces the condensation risk and simultaneously increases the cooling capacity.

The design and application of radiant cooling systems should consider thermal comfort and cooling load characteristics. Utilizing the condensation-free CRCU with a low radiant cooling temperature range of -2.3 to 14.7ºC can result in a significant enhancement (26.6%‒159.9%) in cooling capacity per unit area compared with traditional CRCU with a high cooling temperature of 18.3ºC. A computational fluid dynamics (CFD) model was developed to investigate the thermal environment created by a condensation-free CRCU inside a small chamber. Four cases with different radiant cooling temperatures (18.3°C, 14.7ºC, 9.5ºC, and -2.3ºC) and radiant cooling areas (9.0 m², 7.2 m², 5.4 m², and 3.6 m²) were investigated. The thermal environment was evaluated based on temperature and velocity distributions, and thermal comfort was evaluated using the predicted mean vote (PMV), predicted percentage dissatisfied (PPD), and percentage dissatisfied (PD). The results indicated that the thermal environment created by the condensation-free CRCU under low radiant cooling temperatures was as uniform as the traditional CRCU, and thermal comfort satisfied the comfort criteria of the ASHRAE Standard.

An analysis model suitable for both condensation-free ceiling radiant cooling units (condensation-free CRCUs) and traditional ceiling radiant cooling units (traditional CRCUs) was developed to investigate the cooling load characteristics of a thermal chamber conditioned by a condensation-free CRCU based on thermal environment parameters. The total, radiative, and convective heat flux, as well as the heat exchange with a thermal manikin and walls, were analyzed under different radiant cooling surface temperatures of condensation-free CRCUs. The effect of the emissivity of the thermal chamber's external wall on the cooling load was also studied. The results indicated that the cooling load created by the condensation-free CRCU was slightly smaller (~2.0%) than that by the traditional CRCU when the same operative temperature was created. Furthermore, by decreasing the infrared emissivity of the inner surface of the exterior wall, the total cooling load created by the condensation-free CRCU could be decreased by 8.4% which was slightly higher than that created by the traditional CRCU (7.4%).

To enhance flexibility in regulating the microthermal environment over a wide range of radiant surface temperatures, a condensation-free personalized radiant cooling unit (condensation-free PRCU) with an air-layer integrated radiative cooling unit (AiRCU) was proposed. The condensation-free PRCU separates the air-contact surface from the radiant surface, which simultaneously improves cooling capacity and reduces condensation risk. The condensation-free PRCU allows individuals to regulate the microthermal environment for specific body parts to meet their comfort requirements, rather than regulating the entire conditioned space with an all-air conditioning system. Additionally, the condensation-free PRCU is an efficient technique for energy saving, allowing higher ambient temperatures (e.g., ambient temperature of 30ºC) under the same comfort requirement. The purpose of this section is to experimentally study, and obtain the microthermal environment model and thermal comfort zone under the regulation of condensation-free PRCU designs. The results would be useful for building designers, HVAC engineers, and researchers interested in sustainable building design and energy-efficient heating and cooling systems.

The microthermal environment model of the condensation-free PRCU was established by using 360 sets of experimental data and a linear curve fitting method to examine the relationships between various environmental variables (e.g., membrane surface temperature, local air temperature, local mean radiant temperature, and operative temperature) and the radiant surface temperature in different ambient environments. The findings revealed that the air layer provided sufficient thermal resistance to prevent condensation. For instance, when the ambient air temperature was 31.3ºC and the average radiant surface temperature was 5.3ºC, the average membrane surface temperature could reach as high as 20.8ºC. In addition, the condensation-free PRCU exhibited a notable capability to lower the local mean temperature (e.g., an almost 4.0ºC decrease), but it demonstrated limited capability in reducing the local air temperature (e.g., a decrease of approximately 2.0ºC), when the radiant surface temperature was around 5ºC at an ambient temperature of 31.3ºC, resulting in an operative temperature above 26ºC with a PRCU size of 1 m ×0.7 m. To further enhance the capability of the condensation-free PRCU to adjust the microthermal environment, the effect of PRCU sizes on the microthermal environment was investigated. Results revealed that the operative temperature could be further decreased by 2.1ºC from 27.4ºC to 25.3ºC by enlarging the PRCU dimensions from 0.7 m ×1 m to 1.2 m ×1 m when the radiant surface temperature was approximately 5ºC at an ambient temperature of 30ºC.

Based on the findings derived from the occupant survey and physical measurements, a thermal comfort model for the condensation-free PRCU was developed. A total of 1880 data sets were utilized, employing a linear curve fitting method to establish quantitative relationships between environmental variables, such as radiant surface temperature and operative temperature, and thermal comfort perceptions encompassing thermal sensation, thermal comfort thermal acceptability, and thermal preference. The aim was to predict how these environmental variables contribute to the creation of a comfortable microthermal environment. The results indicated that the local operative temperature comfort range observed in the present study was 25.4-27.3ºC, slightly higher than the range of 22.6-25.8ºC in Fanger's model. Furthermore, physiological response variables including mean skin temperature and local skin temperature were employed to predict overall body and local thermal comfort perceptions for different body parts. The range of mean skin temperature that achieved a comfortable sensation was found to be 33.2-33.8ºC.