Investigation of passive radiative cooling in tropical climates

• Main Author: Han, Di
• Other Authors: Ng Bing Feng
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Mechanical engineering::Alternative, renewable energy sources
• Online Access: https://hdl.handle.net/10356/164702
• Institution: Nanyang Technological University

Sub-ambient cooling can be achieved through radiative coolers that selectively emit radiation within the atmospheric window (8-13 μm) to outer space and suppress absorption/emission of other wavelengths. The use of radiative cooling for daytime cooling has only just picked up momentum and there is much more to be pursued-performance analysis through experiments and simulations, parametric investigations, fundamental material design, as well as practical application. It should be highlighted that the experiments conducted thus far are geographically situated in the mid-latitude temperate regions (latitudes between 23.44° and 66.56°) with low humidity, weak solar radiation and minimal cloud cover, where cooling is not welcomed when the ambient temperature is already low for almost half the duration of the year due to change in seasons. However, the potential of daytime radiative cooling under hot and humid climates has remained doubtful as the amount of solar radiation received and atmospheric water vapor can significantly influence the efficiency of radiative cooling. In fact, it is in the tropics (latitudes between 0° and 23.44°) that experiences summer throughout the year where radiative cooling can be fully utilized and to reap the greatest economic benefits. If deployed on a large scale, radiative cooling could become an alternative or solution to overcome the large quantity of energy required for cooling of indoor spaces in tropical cities. Yet, the inherent nature of sub-tropical and tropical climates brings about extremely hot and humid environments that limits the potential of daytime radiative cooling to reach sub-ambient temperatures. This study first explores the feasibility of adopting radiative cooling in the hot and humid climate of Singapore through both numerical and experimental approaches. A theoretical simulation based on the heat transfer balance is first proposed to obtain the cooling power of the radiative cooler considering different solar spectral irradiance and total water vapor column. The larger solar irradiance in Singapore, especially within the ultraviolet (UV) and visible light (VIS) spectrum where the absorbance of the material is relatively high, could counteract its cooling effects. Moreover, the increased atmospheric radiation induced by higher humidity and ambient temperatures in Singapore could worsen cooling performances of the radiative material. Also, experimental investigations were conducted by measuring the steady-state temperatures of two radiative coolers (photonic radiative cooler and enhanced specular reflector film) under three typical weather conditions in Singapore, namely clear, partly cloudy and cloudy skies. While both radiative coolers were unable to achieve daytime cooling performance on a clear day, the enhanced specular reflector (ESR) film with higher solar reflectance can reach sub-ambient temperatures on a cloudy day. When it comes to night-time, the steady-state temperature of the photonic radiative cooler and ESR film was about 3.5 °C and 5 °C lower than ambient, respectively. Subsequently, to achieve sub-ambient cooling in tropical daytime, further improvements to present materials are required to reduce solar absorption (especially in the UV and VIS range) and to increase infrared emission within the atmospheric window. Herein, we propose a highly reflective polymeric coating with BaSO4 particles dispersed in P(VdF-HFP) matrix for radiative cooling in the tropics. Through the strong Mie scattering of sunlight and intrinsic bond vibration, the substrate-independent average solar reflectance and infrared emittance within the 8 to 13 μm atmospheric window could reach 97% and 94.2%, respectively. For the first time, surfaces could maintain sub-ambient temperatures under direct exposure to the sky and surroundings even when the solar intensity was 1000 W/m2 and downwelling atmospheric radiation was 480 W/m2, while separately achieving 2 °C below ambient during night-time with an effective cooling power of 54.4 W/m2. With a scalable fabrication-process, our cost-effective single-layer coating can be easily applied to diverse substrates, which is suitable for real-world applications in the tropics. Next, to propose a guideline to achieve cooling in tropical daytime for fundamental material designs and practical applications, the criteria to achieve sub-ambient temperatures in the challenging conditions of tropical daytime is recommended based on the solar reflectance and infrared emittance of an effective broadband radiative cooler. Briefly, the total solar reflectance of the material should be above 97% with an infrared emittance of over 80% to achieve sub-ambient temperatures. To illustrate, the feasibility of daytime radiative cooling under Singapore’s tropical climate is investigated using a material that meets these criteria. The highly reflective coating has solar reflectance of 98.4% and infrared emittance of 95% within the 8 to 13 μm spectrum, which achieved a sustained daytime sub-ambient temperature of 2 °C under direct exposure to the solar intensity of 1000 W/m2 and downwelling atmospheric radiation of 450 W/m2 and simulated cooling power limit of around 35 W/m2, with the potential to provide over 30 MJ/m2 of cooling energy per month. Then, the ability to achieve dual-mode thermal regulation for switchable heating and cooling on a single platform has thus far been challenged by the availability of suitable materials. The materials need to possess both high solar reflectance and transmittance, necessitating large and small thicknesses in the same coating layer, respectively (i.e., the thickness constraint). Herein, we report for the first time, a single-layer coating made in a facile one-step process, which exhibits rapid switch between high solar reflection (~ 96.6%) and high solar transmission (~ 86.6%). In the dry state, high solar reflectance and infrared (IR) emittance (> 96% from 8 to 13 μm) enables passive radiative cooling, resulting in all-day near/ sub-ambient temperatures in the demanding conditions of the tropical climate. Upon wetting, high transparency in the broadband range (0.3 to 2.5 μm) allows solar heating, leading to switchable thermal regulation. Such unprecedented performances are achieved through a unique hierarchical porous structure comprising of vertically aligned microscale pores in nanoscale pore matrix. This breaks the thickness constraint and broadens its applicability, in particular for seasonal areas with large temperature variation throughout the day. Finally, to realize further enhancement on the cooling performance under tropical climates, the integration of passive radiative cooling and evaporative cooling was explored using a metagel by dispersing barium sulfate (BaSO4) particles into a typical polyvinyl alcohol (PVA) hydrogel matrix. A stable sub-ambient (4-6 °C) passive cooling in all the different tropical climate conditions of sunny, cloudy, and rainy days (both outdoor and indoor) was realized by a rational integration of passive radiative cooling and evaporative cooling. The metagel cooler adaptively coordinates various passive cooling strategies according to ambient conditions, resulting in a stable sub-ambient temperature regardless of the fluctuating ambient conditions.