Cooling by Solar heating air?

  1. I have an airtight container, the "hot-box", filled with air from outside.
    I close the two valves to the outside air, and let the sun heat it up.
    Once it reaches a certain temperature/pressure: I open/unlock a one-way check-valve that leads to a shaded/water-immersed coil of copper tubing. The tube is open to (indoor) air on the other end.
    For simplicity, let’s say the copper coil is long enough that hot & high-pressure air from the container, while passing through it, will be cooled to room-(outside)-temperature by the time it reaches the open end.
    When the air finally escapes from the tube, it will expand from high-pressure to atmospheric pressure, further cooling the gas.
    The amount of time it takes the hot-box chamber to stop pushing air (1atm), depends on the friction of air in the copper tube, and size of the hot-box.
    When the hot-box reaches 1atm, or close-to-it* (no longer pushing high pressure air); we close/lock the valve to the copper tube, and open the two valves (top and bottom) exposing it to open air so the hot air inside will be replaced with fresh air, and we can repeat the process.

    Is all of this actually correct? Am I forgetting/ignoring something important? Like; will significant pressure be lost due to friction in tube? Or, do I need to consider the air already in the tube if << the air in hotbox?
    If this does work, I assume such a device like this already exists, what is it called? ( I saw similar, but not the same, devices here,

    If I wanted to use this as a small cooling unit (which would probably require a round-robin array of heating chambers, with temperature controlled valves for each, leading into to the coil, and so forth), will the cooling effect even be noticeable? Or should I be asking, how big or hot would my hot-box need to be for the effect to be noticeable? (I live in rural high desert: powerful sun, low atmospheric pressure, lots of space)
    I'm getting a bit lost trying to figure out what’s going to happen to the pressure in the hot-box chamber (and the tube's output), when the air is leaving via the open end of the copper tube, AND still being heated by the sun. E.g. will the decrease in pressure be linear, or exponential? *I think this info will be important in determining when a "cycle" should end, and the hot-box flushed with fresh air.

    Thanks in advance for your feedback!
  2. jcsd
  3. mfb

    Staff: Mentor

    This is roughly the basic idea of a fridge or machines that produce liquid air, just in the first step the heating gets replaced by compression as this is faster and does not heat the medium so much.
  4. Indeed, AC's and Fridges are where the idea originated. I'm trying to make it even more KISS, and reduce electric use to powering valves open and closed, rather than running a compressor.

    But is it too simple to work WELL, and be worth the effort of making it? (I live off grid, and need to save my PV generated electric for lights and computers, so "worth the effort" has a much lower threshold than for most: just needs to cost less than new PV cells to power an AC unit, and mounting equipment.) Size of those hot-boxes will really effect price. How can I determine how big I'll need to make them?
  5. mfb

    Staff: Mentor

    I doubt you get much out of it. Air has a tiny density, has a bad absorption both for visible and infrared light (so heating a lot of it takes a while), and I guess you won't achieve high pressures. Your cooling pipe cannot be placed inside (otherwise the setup heats your room), so a part of the temperature drop just goes to the difference between outside and inside air temperature before any cooling effect starts. And your overall setup would be another heat bridge where heat gets conducted to the inside.
  6. >>has a bad absorption both for visible and infrared light
    That's why I would make the"hot-boxes" out of metal, and paint them black. Perhaps even add mylar reflectors to direct more sunlight on them.

    >>and I guess you won't achieve high pressures
    I think I see your point, this is indeed probably the biggest issue. Just to confirm I have PV=nRT right, I would need to DOUBLE the air temp (in Kelvins), to get just 2ATM of pressure? That does indeed seem difficult (75F -> 620F!)

    >>so a part of the temperature drop just goes to the difference between outside and inside air temperature before any cooling effect starts
    I'm actually thinking about using a small outside POND that I have, for cooling the tube. The pond will be the average daily temp, which includes those cold desert nights. Evaporation will also help keep it cooler than hot daytime air (and probably even cooler than my house, which does NOT have any AC yet.)

    >>And your overall setup would be another heat bridge where heat gets conducted to the inside.
    Do you mean the copper pipe will thermally conduct the heat from the hotbox to inside? This I don't understand, wont any given section of the tube be the temp of the air inside that section of tube? If we can cool the heated air inside the tube down to outside-temp, wont the section of tube containing that cooled air also be the outside-temp?
  7. mfb

    Staff: Mentor

    I would paint the bottom and make the top out of glass - that way, you trap more light. And convection works better if you heat it at the bottom.

    Unless the pond is directly at the house, there is a pipe in between that can get hotter than the pond temperature. And your pipe needs some connection to the wall, which might not be 100% airtight, and so on.
  8. What about circulating the pond water through a heat exchanger in the house?
  9. sophiecentaur

    sophiecentaur 13,292
    Science Advisor
    Gold Member

    Did you ever consider a cooling tower style of cooler? A tall black chimney will have a vertical flow of air through it. The air flowing in at the bottom is drawn over a 'wick', wetted by the pond water and in good thermal contact with the pond. The hotter the sun, the stronger the updraft and the more evaporative cooling.
    No moving parts either. Top up the pond occasionally.
    The chimney / tower may need to be quite tall, of course but I have seen a reference somewhere to such a system in PF which works. I will try to find it.
  10. I thought about evaporative too, but it would depend on the size of the pond and the frequency if rain or whatever method keeps the pond full... Wouldn't want to empty the pond I assume, as they are off the grid, they may use a well for water, which would use power to fill back up...
  11. sophiecentaur

    sophiecentaur 13,292
    Science Advisor
    Gold Member

    Evaporation could be a problem; true. For a pond that's not too large, there's always the possibility of using plant cover. Plants can be good at regulating temperature by efficient use of water by transpiration.

    Alternatively, what about a reflective floating cover?
  12. Air heaters

    The tank could be fitted internally with finned air heaters powered by photovoltaic cells.This will increase tank pressure substantially.Small dia orfices at the outlet end will reduce the temperature.
  13. I've heard of "swamp coolers" before, but never heard of one using a chimney to power airflow like that. But I'm a bit confused about that idea, won't the sun on the black chimney make the air hot, rather than cool? Is the ouput \ top of the chimney supposed to feed into the area I'm cooling?

    I DO use a well for water, so there IS a power cost to pump it up, but pumps are one of the things I don't mind using my PV electric power for. Of course I'd rather power pumps differently, but that's another post.
  14. I think the Pr drop inside the box will be close to exponential anyway as it will take longer to keep increasing the Pr due to heat than the Pr drop because of opening the valves.
    I think there are 2 main problems for this approach to be able to produce a useful cooling effect:
    -Air as the refrigerant. Reason why they have been using other gases to increase efficiency of refrigeration systems instead of air.
    -Open refrigerant circuit.
    Perhaps you can still use solar energy for the heating and use a system similar to absorption refrigerators Having a close refrigerant circulation circuit, will allow you to use a much more efficient refrigerant than air.
  15. sophiecentaur

    sophiecentaur 13,292
    Science Advisor
    Gold Member

    If you want to go down the refrigeration route then use a 12v compressor-based system, fed from a large PV array. Proper compressor systems have by far the best performance coefficient. Even doing it this way, the problem is that you need to shift 1kW worth of solar energy falling on every sq metre of pond area with what the PV array can produce. Allowing for inefficiency of the whole refrigeration power chain, that means a pretty massive area of PV.
    It is far better to stop the sunlight from falling on the pool in the first place - if that's practicable.
  16. "...Proper compressor systems have by far the best performance coefficient..."
    That's why they are used more than anything else. Apparently he wants to use sunlight directly in the first place. Since he wants it for lightly cooling a room and not refrigerate goods, perhaps he does not need so much sun power to get the cooling effect he needs. On the other hand, the conversion from sunlight to electricity in the PV, is pretty inefficient anyways and most of the sunlight power will be lost right there. Maybe its better to use sunlight to directly heat the absorption refrigerator system. It could be less expensive also. I don't know.
  17. Great input, thank you!
    I read that wiki article on the absorption refrigerator, but I don't think I'm quite clear on it yet: I see how the thermodynamics of it are supposed to work, but I'm not quite clear how the fluids and gases are moved around between the different chambers, does it use a pump, or does the heat gradient itself generate this flow, like a convection cell?
    So you think I can buy a commercial Absorption Fridge that runs off, say... propane, and convert it to heat the internal-chamber with solar reflectors instead? I'll need fans to get the cool air into the room I want it, but that should be pretty low power usage. I can even recirculate back to the cooling unit over and over, for better cooling over time. I like this solution.

    Regarding the pond, which actually has yet to be dug, it's going to be a DEEP and long, kind of like a stream (minimal surface), with lots of plants growing on the banks, and floating potted plants in the water itself to provide SOME shade to reduce evaporation. It may not be enough.

    Regarding PV: We have high winds in this area, so mounting a PV panel, costs about as much as the PV panel itself, maybe even more! This can get really pricey really quick, which is why I'm shying away from PV where possible.
  18. -No pump required to circulate the refrigerant, heat does the job.
    -Yes, I think you can buy a commercial unit and heat the system with sunlight reflectors, etc. I would suggest you build a temp control system of some kind controlling the amount of sunlight to avoid the thing getting overheated and over pressurized which can make it explode. An overpressure release valve will also be a good idea. Generally speaking, provide safety means to avoid overpressure of the refrigerant which might occur by overheating, leading to explosions.
    -I agree, you can circulate the air using a simple fan which takes very little power to operate.
    -PVs are expensive and inefficient in energy conversion. I would avoid using them if not really needed.
  19. mfb

    Staff: Mentor

    No, this would be a method to cool the pond. Or a smaller amount of cooling water.

    Why do we have to cool the pond now?
  20. "...but I'm not quite clear how the fluids and gases are moved around between the different chambers, does it use a pump, or does the heat gradient itself generate this flow, like a convection cell?..."
    Check kerosene or propane refrigerators. They work very similar to compressor driven ones, that is they have a close refrigerant circulation circuit evaporator and so on. They don't operate based on heating air; but the refrigerant inside the close circulation refrigerant system which when boils in the evaporator, absorbs heat from the surroundings. The chambers (I guess) refer to the evaporator, condenser and so on, not the cooling chambers inside the machine where the food is placed and air circulates by convection.
  21. If your area is not very humid, swamp coolers work alright for cooling. All they are are a box with some absorbant material through which water flows, usually from top to bottom. At one end of the box there is a fan which blows through the box into the room. People usually mount them in windows and they have a small hose which feeds the top of the box.

    As i mentioned they are good in hot, dry climates, not humid ones, so your usage may vary.

    Have a great morning! :)
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