This study presents the use of radiant cooling in Singapore as a method for people to feel more comfortable and cool in warm and humid climates. The authors claim that radiant cooling is a viable alternative to traditional systems despite the widely held beliefs that the low temperatures required for radiant cooling to work would cause condensation, and the air in contact with the chilled pads would become chilled and thus reduce the effectiveness of the radiant cooling. This study provides a proof-of-concept where the researched constructed a "cold tube" with several membranes that would provide radiant cooling at temperatures below the dew point while having participants walk through the cold tube and provide a satisfaction rating. The cold tube resembles that of a non-airtight room with two doors constructed outside. Despite having little to no air flow and a relative humidity of roughly 66.5%, 37 participants reported their experience in the cold tube as satisfactory. No dew was ever observed forming on panels in the cold tube and the water used to chill the membranes was roughly 17 °C which kept the membranes at 23.9 °C.
To demonstrate their proof-of-concept that radiant cooling could be effective indoors without having to worry about condensation or significant loss in efficiency. The main goal was to show that radiant cooling can be used independently from air conditioners, de-humidifiers, and fans to provide comfort cooling.
The system used in this study was able to make participants feel "cool" in the tropical conditions of Singapore despite the relatively high humidity and lack of air flow. Upon a review of roughly 80,000 data sets of indoor climate measurements there has not been data points made using the system in this study. In order to make sure the membranes don't just chill the air around them, they need to be isolated to allow the thermal radiation to pass through the air and membranes so it could the occupants. In order to isolate the chilled surfaces from the surrounding air the researchers used a membrane transparent to longwave thermal radiation with a range from 5-50 micro meters that was made of low-density polyethylene (LDPE). This acted thin film of LDPE acted as a convection shield and isolated the chilled surface from the surrounding air. This would prevent any unwanted transfer of heat.
Such a system was tested without treating the air surrounding the membranes as a way to test the effectiveness of just the radiant cooling membranes. This is of particular importance as the electrical cost to modern AC units is rather strenuous on our current electric grid and the demand is only predicted to climb. Similarly, modern systems both chill and dehumidify at the same time, but that does not work well in humid climates leading to a variety of issues. Therefore, a system like the ones the researchers devised offers many distinct advantages over a traditional system particularly in the tropics.
The system allows radiation to pass, while preventing air and humidity thereby blocking condensation and loss of efficiency. Cold water is circulated through interconnected mats inside the panels. To further test this system, it was run at a temperature below the dew point. The dew point was 23.5 °C while the air temperature inside the cold tube was 29.3 °C and the water temperature circulating in the panels was 10.8 °C this resulted in a MRT of 19.9 °C, where MRT is the mean recorded temperature of the objects surrounding the panels. With the radiant cooling panels were active in the cold tube, 80% of people tested were satisfied in comparison to when they were just able to be in the shade.
The construction of such a cold tube has never been done before. The cold tube demonstrates a first step in the utilization of radiant cooling technology, but in its current state may not be enough to be used independent of other systems. Similarly, the results demonstrate that as the panels get colder the amount the person is chilled increases as well despite a constant air temperature, which indicates radiant cooling is active.
This system was designed to cause a cooling sensation without chilling the air or causing condensation The cold tube created in this paper is a step in the right direction for the future of radiant cooling, but requires more studies.