[Paper Review] Daytime radiative cooling under extreme weather conditions

Jaesuk Hwang|arXiv (Cornell University)|Oct 11, 2023

Thermal Radiation and Cooling TechnologiesEngineering3 citations

TL;DR

This study demonstrates daytime radiative cooling in extreme tropical conditions using a highly thermally insulated system in Singapore, showing sub-ambient cooling of up to 8 °C by leveraging dynamic sky emissivity and cloud base as a radiative heat sink, even under humid, cloudy, and rapidly changing weather.

ABSTRACT

Radiative cooling, taking advantage of the coldness of the sky, has a potential to be a sustainable alternative to meet cooling needs. The performance of a radiative cooling device is fundamentally limited by the emissivity of the sky, therefore depends heavily on the regional weather conditions. Although the sky emissivity is known to increase with the dew point temperature, this interdependence relates only to clear skies and does not cover extreme weather conditions. Therefore, the feasibility of radiative cooling remains elusive in the equatorial tropical climate. A case study of Singapore is presented where the weather is humid, cloudy and constantly changing. We point out that a high degree of thermal insulation of the radiative cooling system can be effective under such extreme weather conditions. A new method to characterise dynamic sky conditions is presented, namely to measure the sky window emissivity in the zenith direction. We show that a sub-ambient cooling up to 8 °C is possible during daytime and that the cloud base is not a complete blackbody and can be used as a heat sink for radiative cooling.

Motivation & Objective

• To evaluate the feasibility of radiative cooling in equatorial tropical climates characterized by high humidity, persistent cloud cover, and rapidly changing weather.
• To investigate how thermal insulation affects radiative cooling performance under extreme meteorological conditions.
• To develop a new method for characterizing dynamic sky conditions using zenith-direction sky window emissivity.
• To assess the radiative cooling potential of cloud bases, treating them as effective heat sinks despite their non-ideal emissivity.
• To quantify the cooling performance of radiative cooling systems under real-world tropical weather variability.

Proposed method

• Measuring sky window emissivity in the zenith direction to characterize dynamic sky conditions under varying cloud cover and humidity.
• Using a highly thermally insulated radiative cooling system to minimize conductive and convective heat losses.
• Analyzing field measurements from Singapore to correlate sky emissivity with ambient and surface temperatures.
• Modeling the cloud base as a radiative heat sink by estimating its effective emissivity and thermal coupling with the cooling surface.
• Applying a radiative transfer model to estimate cooling potential under different sky conditions, including broken and overcast clouds.
• Validating results against measured temperature differentials between the cooling surface and ambient air.

Experimental results

Research questions

• RQ1 Can radiative cooling achieve sub-ambient temperatures in a tropical climate with high humidity and persistent cloud cover?
• RQ2 How does thermal insulation influence the performance of radiative cooling systems under extreme weather conditions?
• RQ3 To what extent can the cloud base act as an effective radiative heat sink in daytime cooling?
• RQ4 How does sky window emissivity vary dynamically under rapidly changing tropical weather, and how does it affect cooling potential?
• RQ5 What is the maximum achievable cooling temperature difference under real-world tropical conditions?

Key findings

• Sub-ambient cooling of up to 8 °C was achieved during daytime in Singapore despite high humidity and cloud cover.
• The cloud base, though not a perfect blackbody, functions as an effective radiative heat sink due to its high effective emissivity.
• Sky window emissivity in the zenith direction is a reliable metric for characterizing dynamic sky conditions in real time.
• Thermal insulation significantly enhances cooling performance by reducing conductive and convective heat transfer.
• Radiative cooling remains viable under extreme tropical weather when system design accounts for dynamic sky and thermal losses.
• The study demonstrates that radiative cooling is feasible in equatorial tropical climates, challenging prior assumptions about its limitations.