The Issue
The temperature of the universe is extremely cold, reaching as low as minus 270°C. During clear nights in the tropics, the sky temperature can surprisingly drop below 0°C. Most people in the tropics do not realise that the night sky can be a source of cooling, because people don't feel the full force of the night sky cooling as they are surrounded by warm air and warm (ground) surfaces. Therefore, this radiant cooling power goes unused. This investigative project sets out to explore methods for harnessing this sky cooling phenomenon.
Our Approach
The building construction suited to harvest sky cooling is the roof, because it is facing the sky directly. So, in order to harvest the sky cooling power, a roof panel was created. The panel consists of canvas transparent to heat radiation, a desiccant chamber that dehumidifies the air. We also added some metal pipes on a black painted metal collector plate for tapping of extra cooling (nighttime) and heat (daytime), but this additional system will not be the focus of this article.
The roof panel is tilted, so it can operate purely from the principles of buoyancy, aka the principle of "hot air rising" and "cold air dropping".
Nighttime operation
During the nighttime, the warm air below the ceiling of the bedroom, rises and enters into the the desiccant chamber, where the air is dehumidified. Subsequently, the air passes beneath a transparent canvas, where the sky cooling lowers the temperature of the air. The cooler and heavy air naturally drops out of the bottom of the sky cooling panel. As a result, a buoyancy driving air circulation to the roof is established throughout the night. The sky cooling varies depending on the cloud cover. On clear nights without clouds, the sky cooling can reach a maximum cooling effect of removing 80 W/m2.
Daytime operation
Throughout the daytime, the air enters the panel from its lower section and proceeds beneath a transparent canvas, where it is heated by the sun. The heated air rises through the desiccant chamber at the top of the sky cooling panel, drying out the desiccant material. Finally, the hot air is released from the panel to the outside.
Concept validation
In order to validate the cooling effect of this concept, a CFD (Computational Fluid Dynamics) simulation was conducted using the software SimScale. The simulation aimed to analyze the temperature and airflow changes over time. Four simulations have been realized, referring to four times of the day: 10 PM, 7 AM, 10 AM, and 1 PM.
Each time of the day has different characteristics:
At 10 PM, the outdoor and indoor air temperature is 29°C.
At 7 AM, the outdoor air temperature is 24°C, and the indoor is 26°C.
At 10 AM, the outdoor air temperature is 29°C, and the indoor is 27°C.
At 1 PM, the outdoor temperature is 32°C, and the indoor is 29°C.
The simulations at 10 PM and 7 AM represent the beginning and the end of the nighttime. The simulations at 10 AM and 1 PM represent the beginning and the sunniest moment of the daytime, respectively.
During the nighttime, we consider that there is a person sleeping in the room. This person has a body temperature of 37°C and releases 75 W/m2 of heat. The cooling power of the panel during a clear sky night is about -80 W/m2.
During the daytime, there is no one in the room. At 10 AM, the heating power of the sun is about 400W/m2, and at 1 PM, it is about 900W/m2.
Our Findings
1. Nighttime:
A. 10 PM:
The cooling effect of sky cooling panel is clearly visible at the CFD simulation, with a steady stream of 24°C cool air leaving the bottom opening of the panel and being replaced by 27°C warm air entering along the top of the ceiling. At 10 PM, this steady-state CFD simulation shows a room temperature between 26 and 27°C, which is 2-3°C cooler than the outdoor temperature.
B. 7 AM:
At 7 AM, the temperature of the room has dropped to 24 - 25°C, which is 1-2°C cooler than the outdoor temperature. The sky cooling panel has air entering at 24°C and and leaving at 21°C.
C. Particle trace:
The particle trace video shows that the air flow through the sky cooling panel is fast, thanks to the buoyance principle of cold air being heavier, hence, dropping out of the bottom of the panel and into the room. Once inside the room, the cold air disperses and the air velocity inside the room itself is relatively low.
2. Daytime:
For the daytime analysis, a different design variation of the sky cooling panel was use, namely a version where the air intake comes from outside, instead of from inside the room. But the working principle is the same as per the original sectional diagram of the sky cooling panel inserted at the top of this article.
A. 10 AM:
At 10 AM, the air enters the panel at 29°C (the outdoor air temperature) and having having been heated up by the sun (400 W/m2) leaves the panel at 80°C having. In the desiccant chamber, the temperature is 70°C, which is sufficient to dry out the desiccant, so it is ready to absorb moisture from the bedroom during nighttime. The CFD shows that an irregular air flow movement inside the sky cooling panels, which is due to the big temperature differences.
B. 1 PM:
At 10 AM, the air enters the panel at 32°C (the outdoor air temperature) and leaves the panel at 130°C. In the desiccant chamber, the temperature is 120°C. Like before, the air swirls inside the panel.
C. Particle trace:
The air swirl in the panel and stays for a bit of time in it. During this time, the air is heated up and this heat dries out the desiccant. To prevent the air to become too hot, a simple thermostat controlled fan can be installed to force a higher air flow through the panel, which only has to achieve 80°C in order to dry out the desiccants. This fan should be installed at the bottom of the panel where the temperature is the lowest.
Outcome
The CFD simulations showed a proof of concept for the sky cooling system, both with respect to the buoyancy-driven cooling of the bedroom at night, and with respect to obtaining sufficiently high daytime temperatures to regenerate the desiccants at a temperature of about 80°C.
This sky cooling panel can replace air-conditioning at night and does not need energy to work, except for switching between the day and light configurations.