A Cooling Solution with Zero Electricity Consumption
IAS Junior Fellow and Research Assistant Professor of Mechanical and Aerospace Engineering Dr Edwin Chi Yan Tso Studies New Model of Passive Cooling
Frost is a natural example of passive radiative cooling. Radiation leading to heat loss results in a temperature below the freezing point, and therefore small pieces of ice are formed.
Heat and humidity in places like Hong Kong have made space cooling essential to every household. Refrigeration, space cooling and heating account for more than 38% of energy consumption in commercial buildings in Hong Kong, compared to about 20% in the United States. By modeling natural phenomena such as frost, Dr Tso and his team have created an environmentally friendly device using a passive radiative cooling technique that can cool down indoor space without using electricity.
The Theory behind Passive Radiative Cooling
Traditional approaches to space cooling consume enormous amounts of heat and drive up demand for electricity. Passive radiative cooling seeks to provide a solution for smart green buildings by reflecting solar radiation and thus requiring zero electricity input.
Frost can be examined as an example of natural radiative cooling. Moisture may freeze on land surfaces at night even though the surrounding temperature may be above the freezing point. This is due to radiation being reflected from the land surface, lowering its temperature. Passive radiative cooling simulates this effect to achieve efficient cooling power. A series of experiments in the United States have had tremendous success.
However, researchers conducting these experiments have been challenged by changes in the environment and expensive materials. A limitation of various types of coolers is that cooling power is undermined in places with high humidity. Because humidity and sky clearness play a crucial role in cooling performance, perpetual radiative cooling in a highly humid environment, under overcast skies or in a subtropical climate remains challenging. When humidity is high, the transparency of the atmospheric window is lost. Due to the scattering of water molecules, mid-infrared radiation originating from the cooler will be blocked, absorbed and re-emitted by the atmosphere. By strongly reflecting solar radiation and emitting thermal radiation to the cold universe through and atmosphere window capturing 8-13 μm of the electromagnetic spectrum, surfaces exposed to the sky could be cooled below ambient temperature.
A solution proposed by Tso and his team is to restore cooling capacity by combining a radiative cooler and an asymmetric electromagnetic transmission (AEMT) window.
Reflecting Solar Radiation to the Universe
The AEMT window is a planar optical device. Within a finite bandwidth, lights are transmitted in an imbalanced biased manner between illumination in the forward and backward directions to the reflecting surface. In radiative cooling applications, the outgoing transmission of thermal radiation from the cooler concentrated in a bandwidth of 8–13 μm is permitted by the window, whereas incoming radiation of the same wavelengths from the atmosphere is reflected away. Contrasted radiative transmission within 8–13 μm realized by the AEMT window will result in net heat removal from the surface and enhanced cooling power.
The mechanisms in the AEMT window are also found in a number of natural phenomena. Recently, solar reflective mid-infrared emissive hairs on Sahara silver ants were found to have similar effects. They are crucial to unloading excessive body heat from the ants, preserving their body temperature below a critical point. Frost that forms on surfaces at night despite ambient temperatures above the freezing point also illustrates this condition.