The Cold Tube: Membrane assisted radiant cooling for condensation-free outdoor comfort in the tropics

Air conditioning demand is projected to increase rapidly over the next 50 years, particularly in already hot and humid climates. Radiant cooling can be an energy efficient strategy to mitigate comfort energy demand with high air temperatures, thereby reducing both sensible and latent loads in spaces. We have built an outdoor radiant cooling pavilion, the Cold Tube, which is able to produce a mean radiant temperature up to 10 °C below the air temperature in hot and humid Singapore. It avoids condensation and unwanted air cooling by separating cold surfaces from the outside air with a membrane transparent to the radiant cooling heat transfer. This strategy eliminated unwanted convective losses in the form of sensible (air conditioning) and latent (condensation) losses. Controlling the system to avoid condensation was a major feature of the research, and the results show that as cooling demand increases due to warmer air temperatures, the cooling capacity of the Cold Tube also increased to compensate, providing comfortable setpoints to all measured ambient conditions over the duration of the experiment. For ambient air conditions on site in Singapore of 31°C and 65 %RH, we were able to maintain a 22°C mean radiant temperature inside of the pavilion. The additional cooling increased heat flux from exposed human skin to 156 W m⁻² and was successful at avoiding condensation. While this study was conducted outdoors, this demonstration and evaluation will help inform subsequent applications of the technology, such as augmenting comfort in naturally ventilated indoor environments.