Aircon alternative - Cold Tubes

[tanuki.himself]

Having seen a number of articles covering a paper on Cold Tube radiant cooling i wonder if it could be adapted for use in campers

‘Cold Tube’ Invented to Beat the Summer Heat More Efficiently Than Air Conditioning

Chilled panels use less energy than conventional A/C and work in open spaces. Many people beat the summer heat by cranking the air conditioning. However, air conditioners guzzle power and spew out millions of tons of carbon dioxide daily. They’re also not always good for your health—constant expo

scitechdaily.com

as far as I can tell its basically using cool water circulating in matrix to absorb infra red radiation from the human body

I'm definitely on the lookout for some form of cooling so i can sleep in the camper at night without adding 35kg of traditional air conditioning unit on the roof and having to carry and run a generator to power it at night, or adding more heavy, expensive batteries. I've done some initial tests making a swamp cooler with an aquarium pump and a couple of 10w computer fans and i think that is something worth building, but could this be an alternative, especially when its humid?

I'm thinking of a ceiling-mounted grid of narrow PVC cooling tubes above the bed with a small pump to circulate the water, covered in a PVC membrane to keep the humidity from condensing on them. Lightweight and cheap to fit, and you only need to add the weight of water when its in use, and then you could chill an insulated reservoir of water through the day when there is plenty of solar power available, either in the fridge or running a separate heat pump like a fridge compressor.

thoughts on if its worth trying and how best to build?

 

[Rando]

I would strongly suggest you do the math first to figure out how long X kg of water would stay cold in this situation. I am guessing that unless you evacuate the area around the tubes that the water in the tubes will warm up very quickly through conduction/convection before you get much of a difference from absorbed radiation.

Also, do yo have an IR thermometer? If so, do a little experiment. Go out at night in a location similar to where you will be using this and point your thermometer at the sky. If the temperature of the sky is < 0C then sleeping outside will be more effective than the 'cold tube' panel.

 

[luthj]

Stopping condensation is not possible when the dew point is above the comfortable temperature (anywhere its hot and humid). The main advantage of cooling the air, is that you are also lowering the humidity. As humans sweat to cool themselves, this improves comfort even at a higher temperature.

Regardless of how you cool a space, energy is energy. That whole "tube" concept is very light on actual details. You still need a method to cool the water, and its not going to be any more efficient than circulated air. There is no free lunch, you need to move the heat somewhere. Using a water filled heat absorber will also weight a lot more.

In the end you need to get your cooling BTU/kw-hr from somewhere. Either by using a heat pump to cool yourself directly, or the air. If you want to save power, it only takes 200W of cooling output (80W of power input) to keep a single person cool wearing a chilled suit or vest. I was working through a mattress chilling pad. It has some downsides, as your bed can end up condensing ambient moisture over time.

There are micro AC units that cool air or water. So you could use one to cool a space large enough for someone to sleep in. Say just a bed or interior tent space.

 

[jonyjoe101]

I been doing tests with a heater core to circulate the water and cool the air before it goes in to the swampcooler. It does cool the air but less then 5 degrees, but 5 degrees in my situation was good enough especially during the hottest part of the day. The colder the air is before getting to the evaporater pad, the more cooling effect you get.

The heater cores (20 dollars) while it worked good, I seen how they get when you run water through them without antifreeze. I decided to get a 8"x8" air to water heat exchanger (55 dollars) and will use that on my next build. It has copper tubing throughout and will hopefully work better with water flowing though it.

With a heatwave running though our area while running my test, one thing I did notice was running 2 swampcoolers at the same time actually kept the inside of my heavily insulated van cooler, 2 swampcooler running 89 degrees, one swampcooler running it got up to 93 degrees. I wasn't expecting any difference between 1 or 2 swampcoolers.

I was just running room temp water through the heater core to get a small temp drop, but if you can chill the water that would definitely improve the performance, I can see a situation where using a small 12 volt fridge to keep water cool and pump it through a radiater might work. Maybe use a timer to cycle the pump on and off every few seconds to keep the water from losing too much cold.

heater core on top
8"x8" air to water heat exchanger bottom

 

[tanuki.himself]

The research paper is light on details but does say that the air gap between the IR membrane and the coolant does help to reduce heat transfer by conduction and convection, and hence reduce condensation - I suppose you would get a little from the air in the gap but if that can't circulate with the rest of the space then it should be reduced. And as long as the coolant reservoir and the pipework to the panel are well insulated most of the heat the system will absorb will be coming from the space i am trying to cool.

I totally agree that energy is energy and it has a cost - the question is where can i get it from and how can i store it. To run a normal compressor air conditioner you really need it to be running at the time you want to cool the space - in my case when I am trying to get to sleep. Cooling the air in the camper all day while i have solar power available isn't really going to help as its not going to stay there - as soon as i open the camper door to get in its going to come tumbling out, and by the time i need the cooling hopefully ambient temperature will have dropped some so replacing what is in the camper with fresh air from outside will help. To store that solar power as electrical energy in the form of batteries becomes heavy and expensive. If I can store it in the form of cool water its cheap and, compared to batteries i think still quite lightweight - its been many decades since I was taught physics but i figure that a small volume of liquid with a higher temperature differential can hold as much energy as a large volume with a small temperature variation. And a smaller volume of liquid will be easier to store insulated until I need it.

Its also about weight distribution. A roof mounted air con on a slide in camper doesn't do much for my COG and handling on what is already a tall vehicle. If i can keep a chiller unit lower down or use the fridge which is at floor level then that helps - there are a couple of cubby holes that i think i could mount another fridge compressor in with airflow through the outside wall and pipework to a liquid reservoir elsewhere. If i end up going the hacked micro spilt AC route like luthj this is where i would mount that compressor, but that option brings me back to the energy source and storage.

I'm not expecting this thing to cool the whole space all the time, just to take enough energy out of me and the Mrs for an hour or two so we can get to sleep - once we are there then the human body usually drops its temperature a bit anyway - thats why many single room aircon systems have a sleep mode that raises the temperature a couple of degrees after an hour or two.

And persuading the Mrs to give up her sprung mattress, insect-meshed windows, blackout blinds and en-suite facilities in favour of a camp cot outside is going to be a hard sell......even if the stars are out

 

[rruff]

I totally agree that energy is energy and it has a cost - the question is where can i get it from and how can i store it. To run a normal compressor air conditioner you really need it to be running at the time you want to cool the space - in my case when I am trying to get to sleep.

The cold tubes will be the same. You need to run it while you use it.

And you need a large radiant surface. It's 8" thick with a metal frame and a bunch of tubes. It looks heavy.

 

[rruff]

I was just running room temp water through the heater core to get a small temp drop

If the water is already at ambient, then what is cooling it?

 

[Rando]

Quick order of magnitude calculation:

1kg of water cooled 1C absorbs 4.2kJ of energy. Assuming you want the camper to be 25C and you can cool water to near freezing (0C), then water can store ~ 100kJ/kg, or about 30Wh/kg. I have no idea how much cooling you need, but lets assume 1000W (that would be equivalent to a 300 - 500W electrical power AC unit). That would mean you would need about 33kg of cold water per hour. If you just want to cool for an hour or two in the evening, this could work, but it would be tough to go all night.

 

[rruff]

1kg of water cooled 1C absorbs 4.2kJ of energy.

Melting ice absorbs 333 kj/kg. If you want efficiency and something that lasts all night, I think that's the way to go. But I don't see anything commercially available that would use ice.

I did find this. It circulates cool water through a sleeping pad. It uses a plug-in cooler, but you could maybe buy just the pad and rig up an ice chest and a pump. Or just use as is; it would surely be a more energy efficient way to cool a person while sleeping than running AC: https://www.amazon.com/ChiliPad-Cube-2-0-Temperature-Enhancement/dp/B07GTBWQKZ?th=1

 

[tanuki.himself]

The cold tubes will be the same. You need to run it while you use it.

And you need a large radiant surface. It's 8" thick with a metal frame and a bunch of tubes. It looks heavy.

If i keep a reservoir of cold water that is chilled during the day, I figure the only thing needed to run it at night from the battery would be a small pump - it allows me to split off the heavy power requirement for chilling the water to when i have solar available

They built theirs for a free standing public test, so I can see why its stronger and safer. I didn't want to prejudice any build suggestions but i'm thinking of trying a lightweight ceiling mounted version above the bed. If i use a 100m roll of 4mm black PVC micro irrigation pipe at 20mm spacings over a 1.25m width it gives me 50 tubes and so a coverage of about 20%. That is the weight of the tube plus about 1.2kg of water when full. If i put a thin layer of matt black painted aluminium - chopped up beer cans - over that it gives me better coverage but slightly more weight. OK, i'll have to change my behaviour for a few days as i usually drink bottles, but i have a friend visiting next week so we should be able to process a few. Fold the aluminium around the pipes, contact adhesive it to a thin sheet of rigid foam insulation, and then double sides tape the whole thing to the already rigid and insulating ceiling. A few foam standoffs and glue a thin sheet of clear PVC film to them. This is just for a "what if" test, so i'm not bothered about dessicants or condensation drainage at this stage. Chill some bottles of water in the normal domestic fridge, then pour them into a cooler at night and run a small pump for a couple of hours and see how it feels

one key thing i didnt get from the paper was if there is any easy scientific way to measure its effectiveness or if its just a subjective "i feel cooler" when near it. I guess if you had the kit you could measure heat loss from a dummy body at 37C, but i can't think of a way to do that easily so it might just be a "lay there for a while....."

 

[tanuki.himself]

Melting ice absorbs 333 kj/kg. If you want efficiency and something that lasts all night, I think that's the way to go. But I don't see anything commercially available that would use ice.

I did find this. It circulates cool water through a sleeping pad. It uses a plug-in cooler, but you could maybe buy just the pad and rig up an ice chest and a pump. Or just use as is; it would surely be a more energy efficient way to cool a person while sleeping than running AC: https://www.amazon.com/ChiliPad-Cube-2-0-Temperature-Enhancement/dp/B07GTBWQKZ?th=1

Never seen that approach. wonder if it feels like sleeping on a water bed? expensive too.....

 

[tanuki.himself]

Quick order of magnitude calculation:

1kg of water cooled 1C absorbs 4.2kJ of energy. Assuming you want the camper to be 25C and you can cool water to near freezing (0C), then water can store ~ 100kJ/kg, or about 30Wh/kg. I have no idea how much cooling you need, but lets assume 1000W (that would be equivalent to a 300 - 500W electrical power AC unit). That would mean you would need about 33kg of cold water per hour. If you just want to cool for an hour or two in the evening, this could work, but it would be tough to go all night.

not sure if this is applicable. I don't think this will cool the overall space, but absorb more radiant heat from the hottest item near it, ie the human body. the physics says radiant heat flow will increase with a greater temperature differential, although i'm still not entirely sure how the body acts to make that happen. But having experienced the Singaporean climate anything that makes testers say they felt more comfortable has to be worth a try. Or black magic......

 

[luthj]

Radiant emissions from a body in the 90F range are pretty minor, and you aren't fully surounding yourself with this cool surface anyways. Heat transfer will still be dominated by convection and conduction from the skin.

So you will still have heat transfer to your environment via sweat/convection of air. The only difference less IR incident on your skin. And Ambient IR in room temperature range is not all the intense anyways. As opposed to the sun which is a major IR source (roughly 40% of its heat content at sea level is IR).

 

[rruff]

If i keep a reservoir of cold water that is chilled during the day, I figure the only thing needed to run it at night from the battery would be a small pump - it allows me to split off the heavy power requirement for chilling the water to when i have solar available

Too much water that needs to be kept quite cold. Ice might work, but not chilled water. Actually, just buy some Chinese LiFePo batteries and run a cooler overnight...

Never seen that approach. wonder if it feels like sleeping on a water bed? expensive too.....

Efficient though, since it cools only the person who needs it, and it's insulated on the other side.

but absorb more radiant heat from the hottest item near it, ie the human body. the physics says radiant heat flow will increase with a greater temperature differential, although i'm still not entirely sure how the body acts to make that happen.

In order for the body to reject heat (and we are always producing and rejecting heat), the skin temperature must be higher than the ambient... or you sweat to cool by vaporization. The body does this naturally.

 

[tanuki.himself]

anecdotally, this works.





Panel is made of 25mm styrofoam sheet 600 x 1250. I routed grooves in at 28mm centres. Diffusion plates are cut from disposable oven trays - beer cans were too brittle and cracked as I tried to fold them around the pipe - sprayed matt black. Pipe is 4.6mm black PVC micro irrigation pipe in a 50m roll. 20mm wooden frame around that to give an air gap and some PVC sheet stuck on to that with double sided tape. Pump is an 18w submersible transfer pump - smaller aquarium pump i had for the test swamp cooler wasn't powerful enough to push water around the whole length of pipe. Total cost around 50 euros, of which the tube and pump are reusable



I clamped the whole panel to the cupboard frame above the sofa as its almost the same height as the ceiling above the bed, but a little easier to get in and out of for the test audience. It also meant i could stick the cooler with the water reservoir in it at more or less the same height in case the pump was struggling to raise the water



This panel had 19 pipe runs in a spiral pattern so used about 25m of the pipe, the rest was just curled up in the cooler to try and minimise heat loss, but as you can see it was still capable of attracting significant condensation from the surrounding air.

Ambient temperature in the camper was 30c when we started, measured from a thermocouple taped to the aluminium diffuser panel about 5 runs in from the feed. We started testing at 2:00pm which is close to local solar noon, and the roof and that side of the camper are in full direct sunlight. Figured these conditions were as warm as we are still going to get here this year. Humidity was reported at 51%

I pre-filled the pipe with ambient temperature water then added 3 litres of chilled water and 2 x 1kg blocks of ice from the domestic fridge freezer, and fired up the pump. Within about 15 minutes the panel temperature was down to 15c, and after about an hour the lowest it dropped to was 12c. The camper door was open and the roof vent was partially open, but there were no fans or other forced air circulation, and we were not seeing any real air movement on the loose edge of the plastic sheet.

Myself, a visiting friend and then the Mrs each took 30 minutes laying on the sofa directly under the panel, two of us just in shorts, the Mrs in a sundress, so lots of exposed skin. We had all been sitting in the shade outside beforehand and were feeling like we were sweating slightly. By the time the half hour under the panel was up we all agreed we had stopped perspiring and felt more comfortable. The Mrs said she felt like there was a slight cooling breeze on her face and arm nearest the panel, but she couldnt see any indication of air movement. She actually managed to fall asleep.

After a little over 90 minutes the ice had fully melted and the temperature of the panel started to rise slowly, so we concluded the test and switched off the pump. I tore the membrane open and there was no sign of condensation on the pipe or diffuser plate inside the panel, so it looks like the membrane did its job of keeping the ambient air from contact with the cold source. We each then went back outside and sat in the shade with a slight breeze, and all felt like we started warming up and sweating again within a short time.



The panel wasn't completely airtight, and there was heat loss from the excess pipe in the cooler evidenced by the condensation there. It looks to me like the system was absorbing heat from somewhere to melt the ice and for the temperature to them start rising, and it could be that the cooling effect we felt was just convection of cooler air dropping down from the panel, but even so its dropping where we want it to anyway, with no sign of condensation that would cause the bed to become damp. Within 30 minutes of turning off and tearing open the membrane the temperature was reading 28c.

I am convinced that the basics of this system work, at least enough for me to reuse the tube and pump i have paid for and build a better panel into the ceiling above the camper bed. I was always planning to put extra insulation panels up there to cover up wiring and give more heat retention and sound deadening for when its raining, so no real extra space or expense needed, the foil roasting trays seem a good source of cheap aluminium that can be bent to fit around the pipe and is uncoated so seems to transfer the heat and take a coat of matt black paint quite well.

I'll probably try and build the panel completely from foam so need to find some glue that doesnt melt it, or better quality double sided tape, and i'd like to find some form of white/cream visually opaque membrane that is still IR transparent - the test of "can you use the TV remote through it" worked for the membrane I had.

If I can keep the liquid volume down to 5 litres including the pump i can get a vacuum flask big enough to act as a reservoir - pump action catering coffee pots are about 18 euros.

For a chiller the best idea i can come up with so far is a counter top ice maker. Most of them seem to be based on a 150w compressor system that can make around 12kg of ice a day and they weigh around 10-12kg. I have 700w of solar panels so should be able to run one of these through the daylight hours and still recharge my battery and cope with other loads - assuming when its dull and cloudy it will be cool so I won't actually need the cooling system. And we can always also use ice in the cool box, drinks, etc, and it can pack away low and forward when not in use for weight distribution, or stay home. If I can find one that is higher power without a huge weight increase, or that can make the ice colder than just frozen that would probably be a better choice, but i think a 400w industrial one is just going to be too big and heavy.