Low-Cost High Performance Daytime Passive Radiative Cooling in Building Applications

Abstract/Technology Overview

Electricity usage for space cooling contributes a significant amount of a building's energy consumption. Passive radiative cooling, which does not require any energy input, offers a great potential to conserve energy use in buildings. A passive radiative cooler reflects almost all incident sunlight, and simultaneously emits thermal radiation strongly and selectively through an atmospheric transparency window. Therefore, thermal energy of an object can then be transferred to the extremely cold universe, providing cooling effect to the object. The technology provider has developed a low-cost high performance passive radiative cooler. An ambient air temperature reduction of 6°C is achieved experimentally during the peak daytime in Hong Kong. A model has also been developed to predict the indoor ambient air temperature reduction inside an apartment by incorporating the coolers with HVAC systems. The results show that a 100m2 passive radiative cooler covered on the rooftop of a building can reduce the indoor ambient air temperature by 3°C when 5 people reside in the room, saving 20-40% electricity consumption of a traditional air-conditioning system.

Technology Features, Specifications and Advantages

Radiative cooling is one of the advancing passive cooling technologies, in which cooling is performed by longwave (infrared) radiation emission. Night-time radiative cooling has the best performance in a clear night sky. However, in daytime, the cooler is exposed to strong sunlight so the heat gain from the sun is significant. Therefore, the performance of the radiative cooler in daytime is not satisfactory. An engineered-structure, which can reflect a significant amount of sunlight and strongly emit the thermal radiation in 8-13 µm atmospheric window, is used for the radiative cooler. Due to these two effects, the radiative cooler can passively generate cooling effect under direct sunlight and can be potentially applied as a cooling source.


  • Electricity-free operation: the cooler does not require any energy input for operation
  • Environmentally friendly: the cooler does not require any refrigerants for operation
  • Lost-cost: the cooler is made by a relatively cheap polymeric material and it is easy to fabricate

Potential Applications

  • Building HVAC systems for space cooling: Installing the passive radiative cooler on a rooftop of a building to perform cooling (i.e. reducing the indoor air temperature for energy-saving of a traditional HVAC system)
  • Energy-free industrial cooling for aviation, marine, electronic and photovoltaic applications
  • Water harvesting: the passive radiaive cooler can be used to collect the water vapor from the ambient for water harvesting due to the condensation effect
  • Personal thermal managment: the passive radiaive cooler can be integrated with a textile to provide cooling to our skin without any electicity input

Customer Benefit

  • Obtain cooling without much energy consumption

Contact Person

Maggie Mak


City University of Hong Kong

Technology Category
  • Thermal System
  • HVAC - Cooling System
Technology Status
  • Available for Licensing
Technology Readiness Level
  • TRL 4

Passive Radiative Cooling, Heat transfer, Thermal Radiation, Thermal Management