Researchers have successfully demonstrated a relatively simple technique that could prove to be a boon for cheaper residential cooling.
Circulating hot water through tubing embedded in the floor has become a common type of space heating in the U.S. Radiant floor heating systems are quiet and all but invisible, and those advantages raise a good question: Why can’t the same system be used to cool the indoors in summer?
Radiant cooling systems are, in fact, available but they are uncommon. Particularly in high-humidity areas, radiant cooling has an inherent weakness. When moisture-laden air comes into contact with surfaces that are below the dew point, water vapor starts to condense into a liquid, and when the condensing surface is a wall or a ceiling that can lead to a variety of problems.
A dedicated dehumidification system would be required in all but the driest climates in order to limit condensation. That has helped make radiant cooling relatively rare. Now, a group of researchers is proposing another way of skirting the condensation issue with a surprisingly simple workaround that represents a potential breakthrough for high-humidity, high-temperature regions.
In a paper published September 1 in Proceedings of the National Academy of Sciences, 10 researchers representing several institutions outline their work on a demonstration project in Singapore that kept people feeling comfortable even when both humidity and temperature were high.
The idea is relatively inexpensive as well as simple. Surfaces in the experimental “cooling pavilion” were chilled while a thermally transparent membrane (nothing more exotic than 2-mil polyethylene) prevented condensation and unwanted air cooling. Both characteristics, they said, would be necessary for radiant cooling panels to be used without humidity control.
“The lack of widespread commercial adoption of radiant-cooling technologies is due to two widely held views,” the study says ” 1) The low temperature required for radiant cooling in humid environments will form condensation; and 2) cold surfaces will still cool adjacent air via convection, limiting overall radiant-cooling effectiveness.”
The work comes at a time of rising energy use for conventional air conditioning systems. Radiant cooling has the potential to lower cooling costs substantially, providing that consumers are willing to rethink conventional attitudes about what “comfort” actually means.
Building an experimental pavilion
Researchers built a modestly sized outdoor structure in Singapore in which a low-density polyethylene membrane prevented the condensation of moisture from the air on the panels, even when they were cooled below the dew point. The membrane did not block infrared energy, so people standing nearby could still feel like they were cooling down.
There was nothing fancy about the locally sourced polyethylene, lead author Eric Teitelbaum said in a telephone call. The plastic sheet was 50 microns thick, about 2 mils. “I’m pretty sure the supplier in Singapore usually sells it to people who are making shopping bags,” he said. “It’s basically a garbage bag, but a little more upscale.”
More than three-quarters of the test subjects who used the pavilion said they were “satisfactorily” cool even though air temperatures were about 85°F and the relative humidity was greater than 66%. Those are conditions that would have most people in the U.S. running for the AC thermostat.
Water circulating through the panels was chilled to about 62°F. Researchers said they didn’t see any condensation on the panels even though the dew point was more than 74°F. The temperature of the water could be lowered to about 55°F without any observed condensation.
“The cooling panel operates below dew-point temperatures, but is insulated from humid air by a membrane transparent to longwave radiation,” the researchers said. “It successfully makes people feel comfortable in conditions exceeding 30°C [86°F] and 65% relative humidity without modifying the air temperature or humidity circulating around human bodies.”
Idea was proposed but never studied
The report says this idea was first proposed nearly 60 years ago, but until now a full-scale test system had never been built or studied.
Conditions in which air temperatures are high and the mean radiant temperature of surfaces are low don’t occur naturally, researchers said, because chilled surfaces act as heat exchangers and cool surrounding air. In the Singapore experiment, commercially available low-density polyethylene prevented heat transfer via convection.
Ordinarily, people associate comfort with mechanical air conditioning, which rely on air temperature, relative humidity, and air speed.
“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,” researchers wrote.
Although people often rely on fans and natural ventilation to cool down, but as air temperature and humidity go up, that becomes less effective. The aim of the experiment was to show that people could feel comfortably cool without air movement or the mechanical treatment of air.
Radiant cooling offers a way of separating, or “decoupling,” the process of lowering air temperatures (the sensible load) and lowering humidity (the latent load) that are normally combined in conventional AC equipment. In tropical areas especially, however, latent loads can be much higher than the demand for cooling.
“This study demonstrates potentials for radiant cooling to create comfortable conditions while doing nothing to the air,” the study says.
The potential for energy savings is substantial—although not spelled out directly in the group’s paper—because supplying panels with chilled water should be much cheaper than cooling and dehumidifying air with conventional AC equipment.
“In humid environments is where you’d expect to see the biggest energy reductions,” Teitelbaum, who wrapped up his PhD studies at Princeton earlier this year, said by phone. “And that is very simply because for air conditioning up to 60% of the compressor’s energy budget is devoted just to dehumidification. Dehumidification is very energy intensive. If you don’t have to do that, and you don’t have to do that for thermal comfort, that could be 60% off your energy bill.”
One of the more important implications of the group’s work, he said, is that it’s possible to introduce lots of fresh air into the space that’s being cooled without incurring a big energy penalty. With conventional AC, increasing the amount of outdoor air quickly drives up energy use because it ramps up both latent and sensible cooling loads. That’s not the case with radiant cooling.
More work lies ahead
What researchers called the “Cold Tube” in Singapore was built with 10 panels, each 4 by 8 feet, that were cooled with capillary mats. The maps were isolated from ambient air by 2 inches of polyisocyanurate insulation on one side and polyethylene on the other.
While “thin and flimsy,” the membrane had enough tensile strength and puncture resistance to last four a couple of months. Longer lasting installations would need stronger materials, but increasing the thickness of the membrane would decrease its infrared transmittance and make the panels less effective for cooling. “Future work could be spend designing better stabilized polymers,” the researchers noted.
Teitelbaum said it should be fairly easy to come up with another material for the membrane, although alternatives might not be quite as transparent to radiant energy as polyethylene.
A more difficult problem could be retraining consumers to accept that comfort doesn’t necessarily mean lower temperatures or lower humidity.
“Air conditioning is so conflated with cooling,” he said. “If you think of cooling, your go-to is not to go to the hardware store and pick up a radiant cooling panel. No, you’re going to get a window air conditioning unit and install it. Your average consumer probably has never thought about radiant cooling.”
-Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine.