Cold Tube Draws The Heat

If you live anywhere near the tropics, air conditioning isn’t a luxury but a necessity. The problem however is that humid climates can cause conventional air conditioners to draw more power to dehumidify the air than it requires to just cool it, which increases the power needed to run the unit. Back in 1963, there was a proposal to create a cooling system that didn’t foster condensation and couple it with different methods of removing humidity. Researchers in Singapore have now created such a system. It uses a membrane that is permeable to infrared radiation but prevents condensation around the cooling unit.

You can see a video of the apparatus in a pavilion in the Singapore heat in the video below. Chilled water runs through tubes behind a membrane that passes thermal radiation. Since the tubes are not exposed to the ambient atmosphere, condensation is minimal. But heat radiates from the warmer area to the much colder area of the tubes.

Desiccant passively dries the air. Does it work? The paper mentions testing in January which sounds odd to people from chillier latitudes, but Singapore is so near the equator that it isn’t really what others might call  winter there even in January. A quick search reveals the average temperature in January is 87F or about 30.5C. People found the area to be subjectively cool and comfortable.

We wonder of course if you could cool electronics with a scheme like this in applications where airflow or vibration from a contact surface could be an issue–for example, under a microscope. You can build a classic air conditioner out of odd parts if you like. Swamp coolers work well, but not with humidity.

63 thoughts on “Cold Tube Draws The Heat

        1. HaD is ad-funded, so in a way: yes you should.

          You’re still paying – it’s just that the bill goes the long way around when the advertisers add the cost to the price of their products.

          The unethical bit is that you’re still paying whether you even know of the existence of this site, or of any other like it, and there’s no way to opt out.

          1. Seriously? Advertising is unethical?? Yeah, how dare those companies make this own decisions about how to spend their money. Obviously it would all be much better if you decide where all the money goes.

            Damn, I’ve seen you write some absolute nonsense in the comments here but this one is a whole new level.

          2. The unethical bit is ad agencies are not reputable, and often deliver viruses. I used to whitelist tech sites and such that I approved of, but when websites like Ars Technica started reliably sending viruses my way via their banner ads, I decided to block all advertising until the entire industry reforms and becomes unimpeachable.

    1. Hey, whip out both your FLIR camera and your johnson sometime. The ol’ chap is a potent heat radiator. That’s the key to this thing’s efficiency: you’ll have to direct your (bare, preferably half-mast) tallywhacker at it at all times. Even more effective if you lift it up and pin the bellend under your belt, exposing the full surface area of your bollocks as your body moves blood through those hectares of thin, permeable, corrugated flesh. Much like a spacecraft pumps coolant through accordion-fold vacuum radiators aimed away from the sun.

      Spacecraft can also vent coolant as a far more effective open-cycle system; it wastes precious mass and has a limited capacity, but if it’s particularly sweltering you could always take the piss.

      1. The ones they show in the video are not dry-able. The pellets melt when in contact with moisture and the membrane on top keeps the liquid inside. You then have to cut them open to pour out the liquid before recycling the now empty plastic container and inner pieces.

        1. The box is mostly sealed, so the dessicant is only there to mop up any humidity that manages to get inside (e.g. after opening the box for maintenance.) It’s not for dehumidifying the outside air. As such, it will last a long time, and since it’s just bentonite clay it’s cheap and non-toxic.

      1. That would still defeat the point of increasing efficiency. It would waste even more energy dehumidifying instead of cooling if it were in the muggy outdoors… with a non-enclosed volume of air that it could never possibly dry out to a point of homeostasis.

    1. The trick is that rather than cooling people by cooling the air around them (which contains the moisture which then condenses) they’re making the walls cold so that heat radiating from people standing next to them is caught and not reflected back at them. This provides extra comfort without needing nearly as much energy input.

      1. I’m extremely skeptical it would use less energy or provide extra comfort that way, but I’d love to see in person. Have you ever felt radiative cold from a wall before? Like a basement wall that’s chilly despite the air in the room being muggy? Radiative heat, sure, but heat is a thing and cold is merely the absence of it. But maybe there’s a way to work the physics out beyond my anecdotal assumptions, I’ll always be wrong about things.

        1. It works roughly symmetrical (not really because there is a 4th power involved but for these kinds of temperatures radiation is linear enough). So if the panel is colder than your body, it will absorb the heat radiating of your hot body better that a hot wall, allowing your body to loose more heat and you to feel colder. Try by standing in front of an open fridge/freezer. Not so close that you would feel the cold air. I think it’s ingenious, especially when avoiding all or most of the condensation energy losses.

      2. but they are providing the unit with chilled water… I can’t see how that would happen without a typical refridgeration unit somewhere and there is the whole idea of cooling people OUTSIDE that makes no sense to me at all especially in the context of energy efficient anything. I mean seriously… what is this thing doing the rest of the 80% plus time when no one is standing near it? Cooling the surrounding air of course:-p Sorry that is beyond ridiculous. I may also point out that whilst one side of the person is getting some marginal non reflection the other side is certainly getting plenty of it. And those rubber/plastic membranes are going to last ummm about 1hr before getting slashed up. Nope sack the people responsible. Now if they would just enclose the space and set the cooling system some 5C under the outside air there will be plenty of cooling via their skin and not so much power being used. Maybe put some solar cells on the roof to not only provide shade but also power the damn thing…..

        1. It would work well in certain places – Florida, for example, has many springs that output large amounts of water at a constant 72 F/22 C year-round. Park facilities near the springs could use that water for the cooling effect.

    2. The outer layer is not actively cooled. So any occurring condensation would quickly raise it’s temperature above the dew point. It is also warmed by the room air. This works similar to a two pane window. Except that glass is not really transparent to longwave IR (heat) radiation.

    1. Also—doesn’t dehumidifying only draw excessive energy for a period of time until it reaches a balance and the room is as dry as it’ll get given that AC system? I mean assuming the building is remotely sealed and insulated; if outdoor air is just constantly wafting through, there’s your efficiency hole right there. And once it gets the humidity of that volume down, it keeps the thermal mass low as well. Furthermore, if this works by radiation, water vapor has a bad habit of catching IR and it seems like it would absorb much of it en-route to this machine. Desiccant is great, but it’s not exactly the same as good HVAC.

      1. In areas where it is always humid, you will always be dehumidifying. The idea here is that you don’t need to cool the space at all. You only need to cool the *people*. And they proved that works: “People found the area to be subjectively cool and comfortable”. They aren’t trying to keep the space cool, they are trying to make the people inside the space feel cool.
        Good HVAC is probably going to work better, and also remediate the humidity, yes, but it’s going to use more energy. This is a way to make people comfortable in a less energy-hungry way.

        1. “People found the area to be subjectively cool and comfortable”

          That’s not some monumental statement. Fans also make areas feel subjectively cooler and more comfortable. Fans are likely more effective too as they drive evaporative cooling of sweat.

          Dehumidifying isn’t a separate aspect of air conditioning, of making spaces more comfortable for people. It’s part and parcel with increasing heat transfer from our bodies to make a space feel cooler. The old phrase “it’s not the heat it’s the humidity” is flawed in that it’s both in combination, but it holds as much for an environment feel hotter as it does for making an environment feel cooler.

  1. Solar heat collector tube yield improves when the tubes are enclosed in a covered (and insulated) box to prevent airflow stealing heat from the tubes. The top performing cover material (in an early test) was a thin Teflon sheet. Let the most IR and visible pass to heat the tubes.

  2. Why not close the windows and let the dehumidifier work.

    Anyway, the daylight sky temperature is just above freezing if there are not clouds. Reverse solar collectors facing the other way will cool a fluid.

    1. “The problem however is that humid climates can cause conventional air conditioners to draw more power to dehumidify the air than it requires to just cool it”

      This is the same concept as you are describing, but the sky is replaced with cooled panels. Sometimes there are clouds though.

      1. Yes, it is. But the sky is free and even cloudy sky is 60 to 65F. Isn’t this why it can freeze at night in the desert after a blistering hot day? I have meant to experiment a bit.

  3. I seriously question how much you could actually cool something down through radiative cooling alone. It’s not even increasing the rate of emission; it’s just providing a “shadow” to catch IR radiating out in one direction instead of bouncing it back. I would think that conductive and convective (and radiative from every other angle) effects would entirely dominate. It’s not like cooling something in a vacuum. If this actually ended up being worth the amount of energy it consumes, I’d be extremely surprised. But pleasantly surprised. Maybe I’m wrong.

    But it would have about the same effect as covering a section of wall with aerogel or something that doesn’t bounce back IR… until it heated up at least. Even an incredibly cold surface with only radiative transfer won’t “suck” heat away any faster. At least as far as I know. The rate of radiative loss is dominated by the temperature of the hot object emitting photons. Black-body radiation and such. Unless you can superheat the human to a thousand centigrade or so (like high-performance radiators in space work) it won’t increase the rate that the body loses heat any more than it always does. And the body is not designed to shed much heat as IR. Why the hell would it be? We live in a fluid. Our pores secrete and evaporate fluid! Radiative cooling is literally the worst method we could choose from. If we didn’t have convection and conduction, we’d literally boil ourselves rather quickly with the waste heat of our own metabolism. Which is why space suits must have beefy cooling systems. You don’t freeze in space until that metabolism is halted by other forms of death. That’s a big myth.

    Or does it still cool the wall itself down enough to conduct heat out of the air; just not enough to form condensation on the wall? That seems like it would still be inefficient and at that point you may as well just force air over it like any other air conditioner evap coil, but with way more surface area so the surface temperature is higher despite it pumping the same amount of BTUs. You could just submerge the evap coil in water entirely and force air over the water. Can’t form condensation at 100% humidity. Have a big enough surface and it would move a lot of heat out without lowering the water temperature too low.

    Maybe I’m missing something.

    1. If you sit by a campfire on a cold night you will soon find that body parts facing away from the fire chill very quickly and you can feel the “cold” as soon as a you move. Survival shelters are built to eliminate the radiative losses. I suspect you could easily “feel” the cold wall when sitting in the room. The wall should have high thermal conductivity, like aluminum, and have very low conductivity like foam material elsewhere.

      The are high absorbtivity/low emissivity (in the IR) paints and coatings as well. For example the white titanium paint on an observatory dome is the opposite and will feel very cool to the touch in bright sunlight at the same time bare aluminum will burn you. You can’t get something for nothing. Can it be cooler than the air around it? I guess so if there is a good thermodynamic sink. The sky qualifies at 1 deg C.

        1. You don’t benefit from the hot air. It is all radiative transfer. That is why a bed of glowing coals warms so well – the parts facing the coals. You even have to flip your hands over to keep both sides warm.

    2. In addition to various above:
      With a sufficiently insulated tent in the desert, when the interior surface is black, it absorbs the heat radiated by someone inside the tent, instead of reflecting it back at them, making them cooler. Downside: once the heat gets through the insulation, it will radiate the interior – you need enough insulation to keep the exterior heat out during the day.
      This also turns up at Burning Man with Monkey Huts with a dark interior colour.
      It’s rather fun working out why some structures in the desert are cool inside and others hot. It’s not always what you would expect from knowing the materials, thickness, orientation and colour.

  4. So it leaves the room cool and damp? ugh.
    “Clammy” is the name for this condition, just like a mildew festering basement in the northern climate.
    Sweat never dries off of your skin in clammy room. Good way to promote skin or bed sores.
    Nope, just nope.
    Swamp coolers make building icky~nasty too. Never understood anyone wanting one of those things around either.
    Just push some ambient temp air through. Adding any moisture just defeats your bodys attempts cool off by evaporative cooling (sweating). You’ll feel drier and you wont need diaper cream to fight the jungle rot.

  5. regardless of the rest, dehumidifying for the major part is what gives the feeling of cooler temperatures. so cooling without dehumifying makes no sense, and you will have water condensing everywhere.

  6. It would never work if CO2 blocks radiant heat transfer in the atmosphere, fortunately that whole CO2 thing is a fiction, however there is a lot more to heat than long wavelength light because heat is molecular motion and that is also transferred via conduction and drives convection. If cold tubes work at all they would be the equivalent of of a window with a cold mirror coating, so not that effective but better than nothing.

      1. Try pointing an IR thermometer at the night sky when there are no clouds. I’ve seen -10C and below. If it didn’t get lower how could you get earth surface temperatures of -70C in Antartica?

    1. So this would be a supplimentary system? Humidity needs to be removed from a conditioned space, to a degree anyway. I think if one wants to put effort into saving energy, look up how much information the internet at large collects, puts on a server farm somewhere. I’ve not heard of the number mention before, it is unbelievably big, and that is per day. How much energy is being burned by those millions of servers? And, for what…. If it comes down to saving energy, and things have to go in some kind of order, get rid of those servers before making AC less ‘good’ to save energy.

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