Theoretical and Experimental Research on Air-layer Integrated Radiant Cooling Systems


Student thesis: Doctoral Thesis

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Awarding Institution
Award date4 Dec 2023


Compared to traditional air cooling, radiant cooling has gained increasing popularity in recent years because of its many advantages in application, such as reduced energy consumption, improved thermal comfort, simple and effective zone control, space-saving, and low operating noise. Application of radiant cooling in hot and humid climates is limited because of concerns about condensation risks and the associated limited cooling capacity, which also exists in Europe. Under a hot and humid climate, the radiant surface temperature should be reduced to enhance the cooling capacity for the typically high demand. However, a low radiant surface temperature will rapidly lead to condensation on the radiant surface when the humidity is not reduced accordingly. The dilemma is increasing cooling capacity with higher condensation risk or avoiding condensation without enough cooling. This dilemma affects the potential commercialization of the radiant cooling approach. Therefore, this thesis presents a study to overcome this problem by developing an air layer integrated radiant cooling unit (AIRCU) based on the radiant cooling structure proposed by Morse. The unit uses an infrared transparent (IRT) membrane to seal a layer of dry air between the membrane and the radiant cooling surface, separating the radiant cooling surface from the room air.

Firstly, the selection of the IRT membrane was investigated. The properties of the membrane can influence the heat transfer between the cooling panel and indoor space significantly. Therefore, numerical simulation and experimental studies were conducted to select the proper materials. In the simulation, the two-flux method can provide accurate solutions to the temperature distribution inside the membrane by solving a radiative transfer equation coupled with thermal conduction inside a participating media. From the simulation, the influence of the membrane's optical, thermal and physical properties on the performance of the AIRCU was comprehensively investigated. The extinction coefficient K, the refractive index n, and the thickness of the membrane D are recommended to be as small as possible, which can increase the cooling capacity of AIRCU. Besides, the infrared transmittance of several common membrane materials was examined experimentally, and POF, BOPP, and LDPE could be considered as the materials for the IRT membrane. The results can guide the selection of suitable IRT membrane materials. In addition, the heat transfer model was combined with the TRNSYS building model, and a building simulation methodology was developed, which showed a good agreement with the heat transfer model.

Secondly, the thermal environment model of AIRCU was developed. Due to the lack of systematic investigation into the relationships between the cooling panel temperature and membrane surface temperature, a thermal environment model was developed to estimate the membrane surface temperature and assess the condensation risk, thereby guiding the setting of the feasible range of the radiant cooling surface temperature without condensation. The cooling capacity of the AIRCU was also investigated in a typical radiant cooling environment with different levels of humidity and then compared with those of the conventional radiant cooling system in different installations, which gives the temperature range of the radiant cooling surface to improve the cooling capacity and prevent condensation in different humidity environment. In a typical office thermal environment with AUST= 26℃ and RH= 60%, the feasible cooling panel temperature range should be 5–14℃ for the downward installation, 5–16℃ for the upward installation, and 9–14℃ for the vertical installation to prevent condensation and improve cooling capacity. The maximum cooling capacity improvement could reach 18%–159% compared to conventional RCUs. This work lays the foundation for implementing the AIRCU in hot and humid climates.

Thirdly, the thermal environment and thermal comfort of the indoor space created by AIRCU were investigated. An environment chamber was built, and the AIRCU was installed on the chamber's ceiling. The influence of the cooling panel surface temperature, heat source, supply water temperature, and water flow rate on the thermal environment was studied in the experiment. The thermal comfort indices of the environment created by the AIRCU were compared with the traditional RCU and air cooling system. One method to evaluate the thermal comfort of AIRCU was developed, and the equivalent radiant temperature was defined to calculate the PMV and PPD. The feasible room air state points (with different air temperatures and humidities) were obtained when AIRCU was applied by keeping -0.5<PMV<0.5. Therefore, the thermal comfort zone in the psychrometric chart was derived for different permitted lowest radiant cooling temperatures of AIRCU.

    Research areas

  • Radiant cooling, Heat transfer model, Cooling capacity, Infrared transparent membrane, thermal performance, thermal comfort