Chill Out With Cold Tube Radiant Cooling

North Americans are used to being heated or cooled by moving air. That's why they are called HVAC systems: they combine Heating, Ventilating, and Air Conditioning all in one convenient system. Except in these pandemic times, it's not so convenient to combine the V with the H and the AC. Instead, you want to open your windows or bring in fresh air instead of recirculating and trying to filter the same air.

That's why this "membrane-assisted radiant cooling" system called a "Cold Tube" is so interesting. Project co-lead Adam Rysanek, assistant professor of environmental systems at the University of British Columbia's (UBC) school of architecture and landscape architecture, explains in a UBC press release:

Air conditioners work by cooling down and dehumidifying the air around us—an expensive and not particularly environmentally friendly proposition. The Cold Tube works by absorbing the heat directly emitted by radiation from a person without having to cool the air passing over their skin. This achieves a significant amount of energy savings.

Before we explain how this device works, we have to do a bit of explaining about Mean Radiant Temperature, a little-understood subject in North America. As Robert Bean of Healthy Heating explains, it is all about our skin and it's all in our head. He quotes Dr. Andrew Marsh:

One square inch of skin contains up to 4.5m of blood vessels, the contents of which is heated or cooled before flowing back to influence the deep body temperature. Thus the close relationship between radiant energy and thermal comfort.

Our skin can be cooled by evaporation, which can be increased by moving air (which is why fans work) or by radiation, the direct transfer of infrared energy from warm surfaces to cold ones. Dr. Marsh again:

Even though not in direct contact with the body, hot or cold objects still greatly affect our perception of temperature. This is because they emit and absorb radiant energy which activates the same sensory organs as conducted or convected heat.

The researchers – from the University of British Columbia, Princeton University, the University of California, Berkeley, and the Singapore-ETH Centre – have constructed a panel where chilled water is pumped through tubes and a capillary mat to maximize surface area. There is nothing new in this; we have shown radiant ceilings that are used for cooling. There is no issue of condensation and getting rained on as long as the panel is kept above the dew point, "the temperature the air needs to be cooled to (at constant pressure) in order to achieve a relative humidity (RH) of 100%" and at which temperature the water in the air condenses out. However, in really hot and humid climates like Singapore, the dew point and the ambient temperature are pretty close together.

What the researchers have done that's different is put a layer of plastic that is mostly transparent to infrared radiation six inches in front of the panel, put a desiccant at the bottom to keep the air inside the box dry, and eliminated condensation on the panel. This probably hasn't been done before because it is counterintuitive; in most air conditioning systems, you want condensation and dehumidification, which increases skin evaporation and keeps you cooler. But it takes a lot of energy to condense water, known as the latent heat of vaporization. By separating the radiant cooling from the evaporative cooling they save all the energy absorbed by condensing the water, creating some interesting opportunities. The researchers note in the study published in the proceedings of the National Academy of Sciences:

We had the goal of demonstrating that if radiant cooling is separated from comfort cooling, it can be relied upon independently as a heat-transfer mechanism to provide comfort....We aim to demonstrate the potential of it as a cooling mechanism that can be operated independent of convection-constrained air conditions, and without any mechanical treatment of the air.

Your average North American HVAC guy would say this is ridiculous, you are not changing the air temperature or the humidity of the space, things they can measure with instruments. But as Robert Bean keeps telling us, it's all in our head, in our perceptions. So you ask people what they think and feel.

To demonstrate that our system provides comfort while operating outside the conventional comfort modes, we conducted a thermal-comfort study, surveying participants to gauge the perception of the thermal environment.

They set up a room in Singapore, where the humidity and temperature are really high. It had the radiant panels on the walls and on the ceiling and had 55 people sit outside in the shade for 15 minutes to get accustomed to normal ambient conditions, and then sit inside the room for 10 minutes. Eighteen members of the group sat inside when the panels were turned off, so they were getting the same shaded condition they got outside.

The results clearly showed that it worked, that there was a far higher level of satisfaction among those who sat in the room with the panels turned on. "There was a visible segmentation between the on and off groups, which shows that this type of system has potential for augmenting comfort in naturally ventilated spaces without air conditioning."

Despite the low chilled water temperature, the air temperature inside the Cold Tube was largely unaffected, changing from 31 to 30 °C, as measured inside the Cold Tube. These data are evidence that the Cold Tube panels convectively isolated radiative cooling from convective cooling, with the large increase of occupant cooling due to radiant losses to the chilled water, not convective.

Thermal imaging also showed heat transfer, an "increase in heat flux from a person to the panel as the water temperature decreases, despite a nearly constant (close to skin temperature) air temperature, confirms that heat is being lost primarily to the panels via radiation."

This is not Air Conditioning, it is People Conditioning.

This is a big deal, especially for big rooms, auditoriums, and even outdoors.

 If fresh air can be supplied at an arbitrary rate with little or no energy or comfort penalty, fundamentally, the climate-conditioning paradigm is changed. Further, as preliminarily demonstrated with the data from the Cold Tube, strict dehumidification is also not necessary, which could reduce large dehumidification loads across humid climate regions worldwide.

This is not air conditioning; the air temperature and humidity in the space is not affected. It's people conditioning, removing the heat directly from the people in the space. It's not going to be as effective as cooling the entire space, but it takes a lot less energy and note that there are no doors closing off this room, they are irrelevant. Compare that to when you are conditioning the air, not the people.

The research was done before the Covid-19 pandemic hit, but they were quick to realize the implications. Adam Rysanek is quoted in the press release:

The COVID-19 pandemic has brought to the public’s awareness how sensitive our health is to the quality of the air we breathe indoors. Specifically, we know that some of the safest spaces in this ‘new normal’ are outdoor spaces,” said Rysanek. “As the climate changes and air conditioning becomes more of a global necessity than a luxury, we need to be prepared with alternatives that are not only better for the environment, but also our health. The idea of staying cool with the windows open feels a lot more valuable today than it did six months ago.

The air conditioning paradigm has already been changed by the pandemic; the consensus among engineers in North America is shifting to be more like the European (and Passive House) approach, where fresh air and ventilation is a separate system from heating or cooling. If North Americans finally wrap their brains around the concept of Mean Radiant Temperature and the importance of radiant heat transfer, it will change the building design paradigm as well.