Author: Zhong, Ziwen
Title: Mitigating the condensation risk of radiant cooling panels by superhydrophobic surface materials
Advisors: Niu, Jianlei (BEEE)
Degree: Ph.D.
Year: 2022
Subject: Condensation
Surfaces (Technology)
Hong Kong Polytechnic University -- Dissertations
Department: Department of Building Environment and Energy Engineering
Pages: xxiii, 189 pages : color illustrations
Language: English
Abstract: Radiant cooling systems have merits of energy-saving, quiet operation, and good thermal comfort level, but its application is limited due to condensation risks. To avoid condensation, methods based on air dehumidification and the control of radiant surface temperature have been proposed, but the cooling capacity of the system is sacrificed as surface temperature must be controlled higher than the air dew point to prevent condensation. Previous research found that condensation droplets formed on superhydrophobic surfaces can leave the surface autonomously with a tiny size before growing into large ones. With an imperceptible size, falling droplets may not be a concern for space occupants. Since the potential of applying superhydrophobic surface materials in radiant cooling panels has not been completely investigated, this work aims to explore the possibility of using superhydrophobic surface materials to mitigate the condensation risks of radiant cooling panels and to find a suitable approach to determine the cooling capacity of such panels with superhydrophobic surface treatment.
The contents presented in the thesis consist of four parts: 1) a literature review on condensation control methods in radiant cooling systems and the progress of droplet jumping condensation as well as condensation heat transfer of superhydrophobic surfaces; 2) an experimental study on the condensation mitigation performance of a superhydrophobic aluminum surface in typical conditions of panel system applications in buildings; 3) an experimental study on the condensation heat transfer between a superhydrophobic aluminum surface and indoor moist air under natural convection; 4) the development of an approach for determining the cooling capacity of a superhydrophobic radiant cooling panel with latent heat transfer.
Very little evidence on the condensation mitigation of superhydrophobic surface materials in radiant cooling panels was found from literatures, except for one study which investigated the condensation performance of a copper-based superhydrophobic surface. In the thesis, aluminum is selected as the substrate to fabricate superhydrophobic surfaces as aluminum is commonly used to assemble radiant cooling panels. A scalable approach for preparing superhydrophobic surfaces on aluminum substrates is developed. The superhydrophobic sample is installed on a ceiling radiant cooling panel in a climate chamber. A series of eight-hour condensation experiments are performed in various air conditions. The growth and self-removal dynamics of condensation drops on the superhydrophobic surface are observed by a camera with optical magnifier. Large condensation drops are observed in some fixed positions in a large view area in experiments with different conditions. Whereas, in a smaller focused view area the diameter of the largest condensation drop is found to firstly increase over time and then reaches around 80 µm with an occasional size of 152 µm. All these were in contrast with the continuously growing drops simultaneously observed on an ordinary aluminum sheet, which reaches 4 mm. The largest size of the droplets in the small focused area increases with increasing humidity, but changes little with the air change rates. The finding provides experimental evidence that the size of condensation drops formed on the superhydrophobic aluminum surface can be constrained below people's perceivable range in a long-time condensation in typical indoor air conditions.
With mitigated condensation risks, a superhydrophobic radiant cooling panel can be operated with surface temperature lower than the air dew point. The cooling capacity of the panel can be enhanced by additional condensation heat flux, which requires the condensation heat transfer between superhydrophobic surfaces and indoor air to be known. Thus, the condensation heat transfer between the superhydrophobic aluminum surface and indoor moist air under natural convection is investigated. The fabricated superhydrophobic surface is vertically positioned in quiescent moist air to perform condensation experiments. The air temperature and relative humidity are controlled in a range of 24°C - 30°C and 63% -65% respectively. The surface is cooled by chilled water to achieve a subcooling degree between 3°C and 16°C. A hydrophobic surface, a hydrophilic surface, and a superhydrophilic surface are fabricated as a comparison. The condensation regime on each sample is visualized. The condensation mass rate of the superhydrophobic surface is consistent with other surfaces and can be predicted by the empirical correlation of natural convection heat and mass transfer with a deviation of 12%.
The condensation heat transfer of a ceiling positioned superhydrophobic aluminum surface in moist air is further investigated. The empirical method is found able to predict the condensation heat transfer with an error within 15%. Based on the finding, the total heat flux of radiant cooling panels with superhydrophobic surfaces is estimated by considering the sensible heat flux by thermal radiation and convection as well as the latent heat flux by condensation, as a superhydrophobic radiant cooling panel can be operated with a surface temperature lower than the air dew point temperature but without much condensation concern. The results indicate a cooling capacity enhancement of 3% -500% for the latent cooling panel compared with a sensible cooling panel. This is realized by the limited cooling capacity of a sensible cooling panel in humid conditions due to the relatively high panel temperature, and the less limited cooling capacity of a latent cooling panel due to the lower panel temperature as condensation risks can be mitigated by superhydrophobic surfaces.
Rights: All rights reserved
Access: open access

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