Hackaday Prize Semifinalist: Portable A/C

Many people with Multiple Sclerosis have sensitivity to heat. When the core body temperature of an MS sufferer rises, symptoms get worse, leading to fatigue, weakness, pain, and numbness. For his entry to the Hackaday Prize, [extremerockets] is finding a solution. He’s developing a wearable, personal cooling device that keeps the wearer at a comfortable temperature.

The device is based on a wearable shirt outfitted with small tubes filled with a cooling gel. This setup is extremely similar to the inner garments worn by astronauts on spacewalks, and is the smallest and most efficient way to keep a person’s core body temperature down.

Unlike a lot of projects dealing with heating and cooling, [extremerockets] isn’t working with Peltiers or thermoelectric modules; they’re terribly inefficient and not the right engineering choice for something that’s going to be battery-powered. Instead, [extremerockets] is building a miniature refrigeration unit, complete with a real refrigeration cycle. There are compressors, valves, and heat exchanges in this build, demonstrating that [extremerockets] has at least some idea what he’s doing. It’s a great project, and one we can’t wait to see a working prototype of.

The 2015 Hackaday Prize is sponsored by:

61 thoughts on “Hackaday Prize Semifinalist: Portable A/C

  1. It looks like a miniature commercial scroll pump? But battery powered? These things get hot and are pretty heavy. Not that I have a better or easier way of cooling somebody, that takes a good deal of energy.

    Reading your updates: “With the mini compressor I will be putting roughly the same amount of power into the system as with the TEC’s but I will be getting 360 watts of cooling power. This will be a vast improvement in power efficiency because I will not need to run the compressor all the time to reach my cooling temperature goals. The mini compressor is going to run 24V with a max current consumption of 10amps.”

    How are you getting 360 watts of cooling output by only putting in 24V x 10 amps or 240 watts?

    AN fittings generally work well when clean and tightened but there is a reason HVAC fittings are usually brazed……

    1. A heat pump can pump about 3x (max) the amount of energy that it consumes.
      Same with a heat pump for heating, you get about an C.O.P of 4, 1x the electricity consumed by the pump (which is also delivered as heat to the house) and 3x that amount of heat pumped from outside to inside.
      The theoretical limits are depending on temperature differences, and practical limits depend on the efficiency of the pump, and heat exchangers, but having more cooling or heating power than the electricity used by the pump is certainly possible.

        1. They can be pretty good real world, any off the shelf air conditioner or refrigerator is 2+ times efficient – the energy cost is for moving energy not making it eg: I am being warmed by a 2kW electrical heat pump outputting around 7kW of heat (Its making 0C (40F) air outside in 10C ambient). 1.5 x energy to heat movement seems pretty possible.

          1. Operating an AC unit indoors with no exchange to the outdoors would literally just heat the room though as you are not moving heat, you are netting 0 exchange and adding inefficiencies to the equation. With this “personal cooler” example, how are you effecting heat transfer when you and the environment are not really separated?

          2. What’s the r-value of a thin shirt though? 0.05? Plus, it does almost nothing to help against convection, radiation and air infiltration. It’s really not a separate, isolated environment. Now maybe if you had a true dry suit with something like aerogel insulation, then sure.

          3. “What’s the r-value of a thin shirt though? 0.05?”

            It doesn’t really matter. Air is such a poor conductor of heat.

            Take a glass of cold water and leave it on the table – see how long it takes for the water temperature to rise. Then calculate the heat flow per area – it’s not much. You can easily remove more heat from the body which is in better contact with the tubes, than the tubes are heated by the air around them.

        2. The CoP depends on the temperature differential. If you’re only looking to drop the temperature by a few degrees, the CoP can actually be above 10 with a well designed system.

          Commercial air-air heat exhangers for homes can take -10 C air from outside and pump +20 C air inside at a CoP of 2, and when the temperature is nearer to 0 C their CoP typically rises to about 4-5.

      1. i have 2 vortex tubes in front of me, they are rated at 400 watts of heat removal, he’s looking at 360. the big issue is that they take air at 100 psi, and have a flow requirement of 15cfm. you’d need a multi horsepower engine to keep up with that demand and it would be loud as hell.

      1. I know it would surely be expensive, but why wouldn’t it scale? Also why do you think the solution is already too expensive, I can’t imagine small vapor compression refrigeration parts being very costly.

  2. I’m curious as how he plans on making this quiet enough to actually use in public places. It might be technically allowed but few people are going to use a device that annoys the piss out of everyone around them.

    1. Volume shouldn’t be much of an issue. It starts at 40 dBA and his drawings show that it will be placed in a backpack, so the volume should drop further. It should only be noticeable in quiet rooms. I’ve never been bothered by the sound of refrigerator compressors, they have a mostly soft and soothing hum. If he was operating a Harbor Freight air compressor it would be a different story.

  3. Just how far can efficient refrigeration be scaled down?
    A desktop cold one anyone! A beer: cold. Lifesaving drugs in 3rd world situations. Hand crank cooled!
    But if you’re wearing clothing for the wrong weather? That “shirt” sounds like a warmer or a sweat item of clothing. For those tubes to do any good they would have to swaddle you.

    1. After reading about how they pumped cold water around the actor that played Hagrid in the Harry Potter movies, I wrapped some aquarium tubing around myself in side a sweater, spaced like three inches apart, safety pinned in place, ‘pumped’ water through with a funnel and a cup of water into the aquarium, cooled me off just fine. The sporadic designed shirt is goofy, and he should have at least one line go around the shoulders (I know my body likes to sweat in my arm pits), but it should work fine if the flow rate and relative temperature differential is high enough.

  4. I have started on a project like this but i’m using Peltiers.

    Me I cant take the heat anything over 85f my body starts to get all messed up and it can take me up to 2 week to get well again.

    I dont have the math down yet only playing around seeing if it kinda works first.

    1 18v 36wh dewalt battery.
    voltage converter down to 12 v
    1 fan for heat sink.
    1 12v 6w peltier
    1 home made heat extanger out of copper and soldered it together.
    1 12v pump (Wipper pump voltage dropped down to 6v)
    500ml / 1/2 quart res. for water.(not what I plan on using with the vest)

    got water down to 52f
    and it ran for 3.5 hours

    now that is with no control.
    plan on using arduino for control and censers
    .
    i think as proof it can work quit nicely.
    of course i will have to double the battery size n make my own batteries.

      1. Depends how fast it takes to generate those 100 watts. If it constantly cools as your body slowly rises, it could be fine. Ultimately, it’s always suggested to have a control knob the wearer can easily adjust, and over cooling can be compensated by lowering flow rate while overheating is the entire purpose of the cooling unit. ‘Build it to spec, then over engineer it!’

          1. Yeah what I said was stupid… N*m /s … I meant to talk more about the surface area that heated up that you worked on cooling down vs submerging your entire body in cold at the perfect temperature to keep homeostasis.

  5. How about putting dry ice in a small water bath with a heat exchanger? The water will freeze but when it thaws “enough” the CO2 it will provide large surface contact with the dry ice, so this should give a lot more cooling power than relying on simple sublimation without requiring rigorous containment measures for an alcohol bath.

  6. This is very interesting. I know people [OK one person] that make heavy, all-covering cosplay costumes, this could make their life better. As long as the unit isn’t too heavy and the heat exhaust works well… Game changing. :P If people can stick 2-3 Litres of water on their back and be happy, they can carry a tiny compressor as well.

  7. IMO This is cludgy compared to using the patient’s blood as the HX fluid. There are some extremely impressive models for portable dialysis machines that could be adapted. IT would reduce the complexity significantly while boosting COP.

  8. Who cares about efficiency? What matters for this type of build is size, weight, and cost..So just compare this system with one using peltiers and batteries. If you scale them both so that the cooling capacities match up, which weighs less, is smaller, and costs less? I’m going to guess the peltier system.

    1. What’s confused me is why we keep using batteries? Depending on the country if you’re hot there’s probably sun shining on you so why not use some solar panels to do something useful with that energy? Use it to drive a peltier, pump etc, added bonus of more sunshine creating more power for cooling so it’s kinda self-regulating.

        1. If you were going to solar power a backpack like this, it make sense to spend the extra $$$$ and use “space cells” on a light honeycomb laminate panel. In that case, 40″x40″ and <10lbs is a feasible solar array size. Still kind of big to carry around. Probably best as an add-on umbrella/parasol so you get shade/shelter with the solar energy.

  9. This is a great project. I have used small 12/24VDC compressors like this in the past and they are stellar to TEC in most non-synthetic, non-gee-whiz applications. I see there is some disagreement here. Interesting.

    1. Do you have some application links? How well can you get a heat pump to throttle or have you just made on/off systems? I currently use TECs for some small tightly regulated (+-0.01C) thermal controls, but don’t like the low efficiency.

  10. I don’t know why the need for the refridgerator.

    Body temperature is already higher than ambient, so a simple radiatior would work.

    The reason why we feel hot in hot weather is because air is a very good thermal insulator and quickly forms a boundary layer that prevents heat from escaping your body, so the layer against your skin heats up to your body temperature and your body temperature begins to rise.

    Even if the ambient temperature and the circulating liquid were +30 C they would still cool you down because you are +37 C. As long as there’s good thermal conductivity from your skin to the liquid, it will feel cold.

      1. Human temperature sense is not absolute. The common party trick is to have three bowls, one hot, one cold, and one in between. Holding your hands in the hot and cold bowl, and then dunking them in the average bowl makes it feel warm in one hand and cold in the other.

        If you were in a 30 C environment your skin would be significantly warmer than 30C, and a 30 C steel bar would feel cool because it actually is colder than your skin and draws heat away from it.

        If you are in a 20 C environment, the surface of your skin is significantly warmer but not 30 C warm, so the same steel bar would feel warm because it is warmer than your skin.

      1. Half a millimeter of air is already a very efficient insulator, and it sticks to you by electrostatic forces, and because everyone is slightly fuzzy and hairy, and we’re wearing clothes that trap air and prevent it from moving around.

        That’s why we sweat basically all the time. The evaporative cooling beats the insulating boundary layer.

    1. An evaporative cooler should be plenty in all but the hottest *and* most humid environments. Anyone who poured water all over themselves during a hot summer would understand. An evaporative cooler would work almost as well without the wet feeling.

  11. If this worked in humid climates it would be perfect for the Queensland summers. Although whether it could be made cheaper than just adding an air-conditioner to my apartment I dont know (my apartment is the one place I spend any meaningful length of time that isn’t air-conditioned)

  12. Anyone know where to buy these in single digit quantities? I found that there are several chinese manufacturers that make very similar compressors, but only way to buy them is in whole pallets (100+ units), these would be great for DIY tinkering…
    I can think of several projects that I would like to try with these, namely a mini battery powered fridge (that is about 2-3x as efficient as peltiers) and a laser cooling system, both nice and portable…

  13. I’ll be watching this project VERY closely as someone that is an avid runner and someone that has MS. Currently I try to avoid running outside on very hot days. I have been kicking a very similar solution around in my head. My personal idea involves supercooled masses(possibly dry ice) so I can get approximately 1-2 hours worth of cooling. I have been considering using gels or fluids to transfer the heat but I have also considered using the bodies circulatory system to keep the overall weight of the device down. A project is already using a vacuum to keep the blood vessels in the hand close to the surface and then cool the blood directly.

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