To further investigate the reason why the convective flux of radiant cooling with a-DIMs is lower than that of radiant cooling with DIMs. Figure 9 shows the comparison of the factors affecting convective heat transfer at different scenarios of radiant cooling. It can be observed that, for the chilled ceiling with low-emissivity (such as 0.2 emissivity), the convective heat transfer coefficient and air-contact surface temperature of the radiant cooling with DIMs are much higher than the values of the conventional radiant cooling and radiant cooling with a-DIMs. The main reason is that higher air-contact surface temperature can result in greater heat exchange temperature difference, higher average temperature as well as corresponding physical parameters of the interlayer air.
To further evaluate the effect of radiant cooling with a-DIMs, Figure 10 shows the influence of the radiative properties parameters of the radiation-cooling membrane on the cooling capacity of the chilled ceiling with emissivity of 0.95, 0.6, and 0.2, respectively. It can be observed that the variation of cooling capacity with the radiative properties parameters of the radiation-cooling membrane is the same under different cooling temperature. For the chilled ceiling with high-emissivity, the maximum cooling capacity occurs when the reflectivity of the radiation-cooling membrane is less than 0.05, with little effect of the transmittance or emissivity. For the chilled ceiling with low-emissivity, the maximum cooling capacity occurs when the emissivity of the radiation-cooling membrane approaches 1.0.
Effect of Infrared Radiative Properties of Air-Contact Membrane on Cooling Capacity of Condensation-Free Radiant Cooling with a-DIMs
Figure 11 shows the temperature of air-contact surface of radiant cooling with a-DIMs for various radiant cooling temperature and transmittance of air-contact membrane. It can be observed that when the radiant cooling temperature increases from 1 to 170C, the temperature of air-contact surface increase linearly. However, the change of air-contact surface temperature is slight with the increase of the transmittance of the air-contact membrane. For example, the change of air-contact surface temperatures are both within ± 0.10C when the cooling temperature is set at 1 and 170C. This indicates that the transmittance of air-contact membrane does not determine the condensation at an appropriate cooling temperature.
Figure 12 shows the cooling capacity of radiant cooling with a-DIMs for various radiant cooling temperature and transmittance of air-contact membrane. It can be observed that the cooling capacity increases as the radiant cooling temperature is decreased from 17 to 1°C. Moreover, the cooling capacity increases more rapidly when the cooling temperature decreases from 2 to 1. The reason is that the interlayer air changes from conduction to natural convection as the temperature difference increases. The convective flux and the cooling capacity increases as well. It can be also observed that when the transmittance of the air-contact membrane is improved from 0.1 to 0.9, the cooling capacity increases from 46.5 to 67.1W/m2 at cooling temperature of 80C, and increases from 46.5 to 67.1W/m2 at cooling temperature of 150C. In conclusion, with the increase of the transmittance, the temperature of the air-contact surface changes slightly and the cooling capacity increases significantly. Therefore, the transmittance as close as possible to 1.0 should be selected for the air-contact membrane.
Effect of the Thickness of the Interlayer on the Cooling Performance
Figure 13 shows the effect of the interlayer thickness on the cooling performance when the radiant cooling temperature is set at 80C. It can be observed that the temperature of air-contact surface increase from 15.5 to 16.80C as the thickness of the interlayer is increased from 5 to 20 mm. However, the change of radiant and convective heat flux is slight with the variation of the thickness of the interlayer. This indicates that increasing the interlayer thickness can improve the safety of condensation-free radiant cooling with a-DIMs at the same cooling temperature.
Evaluating Cooling Performance of Condensation-Free Radiant Cooling with a-DIMs for Various Environmental Humidity
Humidity limits the cooling performance of radiant cooling by affecting the dew point temperature. For the conventional radiant cooling, the cooling temperature is not allowed below dew point of ambient air. On the other hand, under the condition that the air-contact surface temperature is controlled higher than the dew point temperature of ambient air, the cooling capacity of the condensation-free radiation cooling with a-DIMs can be increased by reducing the radiant cooling temperature, and the maximum cooling capacity can be obtained at the lowest allowable cooling temperature.
Figure 14 shows that the cooling capacity and the performance improvement of condensation-free radiant cooling with a-DIMs under different RH conditions and infrared radiative parameters of each surface as shown in Table 3. For instance, when the relative humidity is 75%, the corresponding dew point temperature is 21.20C, and the lowest allowable cooling temperature can be set at 14.90C. The maximum cooling capacity of radiant cooling with a-DIMs can reach 70.1W/m2, 2.4 times performance improvement compared to conventional radiant cooling by using the cooling temperature of 21.20C, which is equivalent to the dew point temperature. It can be observed that the cooling capacity of radiant cooling with a-DIMs decreases with the increase of relative humidity due to the augment of dew point temperature affected by RH. However, the performance improvement compared to conventional radiant cooling enhances with the increase of relative humidity. When the relative humidity is increased from 50 to 95%, the cooling capacity of radiant cooling with a-DIMs decreases from 134.5W/m2 to 30.3W/m2 and the performance improvement compared to conventional radiant cooling increases form 2.1 times to 2.9 times.
In order to solve the problem of insufficient cooling capacity of metal chilled ceilings with low emissivity, a new type of adaptive Double-skin Infrared Membranes (a-DIMs) consisting of one layer of high-emissivity membrane and one layer of high transparent membrane has been proposed in this article. Compared to the conventional metal chilled ceiling with low emissivity of 0.2, the cooling capacity of condensation-free radiant cooling with a-DIMs can be improved by 2 times. Moreover, compared to double-skin infrared-transparent membranes presented in previous publication (Du et al., 2021), the cooling capacity with a-DIMs can be further improved by 25%. It is resulted from that high emissivity membrane in the a-DIMs was used to replace conventional metal low emissivity chilled ceiling and high transparent radiation cooling membrane in the previously presented radiant cooling with DIMs. The high emissivity radiation-cooling membrane in the a-DIMs significantly improved radiant heat flux from cooling load to cooling source.
For the radiant cooling with a-DIMs, the maximum cooling capacity is achieved when the emissivity of the radiation-cooling membrane approaches 1.0. Several kinds of carbon materials are feasible to be selected as the radiation-cooling membrane, which have been verified that the emissivity can reach more than 0.9. As for the air-contact membrane, the low-density polyethylene material could meet the infrared transparent requirement for the condensation-free radiant cooling with a-DIMs. However, high performance infrared-transparent membranes with high strength and air-tightness could be better choices, such as PE aerogels and infrared transparent inorganic ceramics.
Further investigations under various RH conditions demonstrated that significant improvement of cooling capacity above 2 times compared to conventional low-emissivity metal chilled ceiling by using the radiant cooling with a-DIMs. It will be of great guidance for high-performance radiant cooling design with condensation-free and improved cooling capacity especially for low-emissivity metal chilled ceiling.
Ke Du is a Student / Intern at Guangzhou University, Guangzhou, China
Huijun Wu is a Professor at Guangzhou University, Guangzhou, China
Yanling Guo is from the Guangdong Provincial Key Laboratory of Building Energy Efficiency and Application Technologies, Guangzhou University, Guangzhou, China
Gongsheng Huang is from the Department of Architecture and Civil Engineering, City University of Hong Kong, Hong Kong, China
Xinhua Xu is from the Department of Building Environment and Energy Engineering, Huazhong University of Science and Technology, Wuhan, China
Yanchen Liu is a Senior Lecturer at Guangzhou University, Guangzhou, China
© Du, Wu, Guo, Huang, Xu and Liu.
This article was first published in the Frontiersin Journal. Published with due permission.