Research Excellence

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.


Development of Passive Cooling

1970s

The technology of passive cooling was developed, but it functioned only at night and therefore had limited impact.

2014

A group of researchers from Stanford University, published in Nature. They renewed the energy available in the day, demonstrating radiative cooling under direct sunlight using a photonic radiative system1.

2016

Some of these Stanford researchers introduced a photonic radiative cooler, which could lower the temperature by 37 degrees Celsius during the day, with a maximum temperature reduction of 42 degrees Celsius. The technology was found to be rather expensive.

2017

Researchers from the University of Colorado at Boulder and the University of Wyoming published another method of passive cooling in Science. This method used a polymer cooler that required substantially lower time and cost and had greater efficiency, with a cooling power of 93 W per square meter at noon.

2017

Using another polymer cooler, Kou et al revealed the skyrocketing cooling power of 127 W per square meter and a temperature reduction of 8.2 degrees Celsius in daytime operation with a silicon dioxide (SiO2) panel coated on both sides with polydimethylsiloxane (PDMS) and silver.

2017 - Current Development

Despite the great success in the United States, a survey conducted by Tso and his team in Hong Kong revealed that the surface temperature of photonic radiative coolers remained higher than the ambient temperature in daytime, although it dropped below ambient temperature at night.

Tso and his team have demonstrated how a radiative cooler can succeed in lowering temperatures under climatic conditions of high humidity. They are in the process of applying for patent rights based on their research results, which provide a solution to the problem faced by past researchers.

1The cooler came with a thermal emitter made of a 7-layer structure alternating SiO2 and HfO2 backed by an Ag solar reflector. It displayed high solar reflectance of 0.97 and high 8–13 μm emittance, producing a cooling power of 40 W per square meter and a temperature reduction of nearly 5 degrees Celsius below ambient.

\\  This finding allows us to achieve unprecedented efficiency in space cooling and takes us closer to our goal of creating smart green buildings.  \\
 

How is the Technology Used? The Application of Passive Cooling

Passive cooling is a part of a larger initiative: Smart Green Buildings. Building experts use a range of technologies to construct buildings that are environmentally friendly, efficient in cost and energy consumption, and comfortable to live in. Tso is on the team led by Prof Christopher Chao, Chair Professor and Head of Mechanical and Aerospace Engineering at HKUST. The team, which comprises researchers from HKUST, City University of Hong Kong and Nanyang Technological University, Singapore, was granted HK$7.33 million Collaborative Research Fund from the Research Grants Council of the Hong Kong Government to work on cooling effects for smart green buildings.

A primitive radiative cooler created by Tso and his team.

Research in Hong Kong: A Business Model

Collaboration with Industry Partners

A few years ago, Tso earned a patent through one of his research findings on a type of refrigerant—an adsorbent that produces large cooling power. A sister project focuses on a smart window that can control heat flows in accordance with indoor temperature. These technologies together will vastly change our living environment.

Throughout the research process, Tso and his team have found collaboration opportunities with a number of partners in China, including Guangzhou Wanbao Group, a household device manufacturer with a Chinese refrigerator brand. Further collaboration opportunities are under negotiation. Industry partners work together with researchers to gain access to the newest technologies. With a view to potential profitability, some even invest funds for research projects.

A collaborative model between research institutions and enterprises has thus been developed, advancing Hong Kong’s research and benefiting China’s flourishing economy through research and development. Tso’s earlier research on adsorption cooling is close to becoming a commercial product. His finding on radiative cooling is new, but it may later be commercialized.

HKUST as a Platform for Basic Research

HKUST has been preparing for innovation and opportunities, and research facilities have been set up in China. To facilitate researchers’ work and to amplify its impact, basic research is conducted at HKUST, while HKUST’s China branches, Building Energy Research Center, Fok Ying Tung Graduate School in Nansha and Guangzhou HKUST Fok Ying Tung Research Institute, support applied research and knowledge transfer. University networks support the introduction of research products to the market in collaboration with partners in industry. This collaboration generates funding that supports research.