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25 Watt Passive Cooling Solution
By Amallore
This guide will walk you through the construction and use of a low power passive cooling solution that does not use atmospheric kits or wall coolers. I've tested this cooler in a full glass greenhouse with a 100% CO2 atmosphere at a pressure of 101 kPa. This cooler has successfully maintained interior temperatures even with ten ferns planted and fully grown.

Power consumption for the only active component of this cooler is 25 Watts (According to Silent1's cheat sheet) with manual control. Using a thermostat made of logic chips increases total power draw to ~70 Watts depending on design.
Gather supplies!
To build this cooler you'll need to gather the following supplies.

The cooler:
1 x Pipe Digital Valve
20 - 30 Pipe Radiators
Minimum 40 sections of pipe. (May vary based on your design and space constraints)
A full canister or portable tank of Pollutants (X)

Filling apparatus:
1 x Tank connector or gas tank storage based on preference
1 x Pipe Volume Pump
Extra pipe if needed.
1 or 2 pipe meters if you hate having to grab the tablet all the time.

Paint everything if you want to get funky with it. I do so to identify what pipe contains which gas or gasses at a glance.
Principles of Operation
For those unfamiliar with how an air conditiong system works I'll give a brief explanation here. (I teach college classes on the subject but I won't bust out the powerpoints.)

I'll be using these terms throughout the rest of this guide so please take the time to read these definitions.

An air conditioner is used to collect heat from one space and transport it to another separate space to be dissipated. In this case we wnat ot take heat from our greenhouse and move it to the outside environment of space. There are four main components that accomplish this.

Evaporator: Collects heat inside the greenhouse and transports it through....

Refrigerant: This is used to carry the heat from inside to outside. Once we load it into the system it won't leak out. Set it once and forget it. We're using pollutants in this case because they can move a lot of heat very quickly compared to other gasses. See Silent1's cheat sheet on the Unofficial Wiki for more information if you're a huge nerd like me.

Condensor: Located outside (in space) and dissipates heat collected in the greenhouse.

Digital Valve: This component divides the system into Hot and Cold sides. With the valve closed, everything in the evaporator heats up and everything in the condensor cools down. When the valve is opened, the temperature between both sides will equalize, allowing the heat to be moved outside. It can be seen in the pictures above.
Choose a location

Pick a location that meets the following requirements:
  • Ample room
  • An exterior wall to pass your plumbing through
  • Floor or wall space to mount your gas filling equipment
  • Make sure the evaporator will be in the path of airflow for best results
  • Power


Basic construction involves laying paralell lengths of pipe to build the condensor. Make sure that the ends are connected to each other instead of making one kong continuous pipe. This allows the heat passed to the condensor to be dissipated as fast as possible. See the picture below. NOTE: There should be only ONE pipe that passes through the wall and connects to the digital valve. If you connect both sides of the condensor to the evaporator this won't work.

Place radiators on the paralell lengths, the more the merrier. On this large cooler I used 24 because the greenhouse gets particularly hot. On a smaller setup for my fabrication room I used 15 since it stays relatively cool. We need this many radiators to dissipate the heat put into the condensor as quickly as possible.

For best results: Don't place the condensor in the sun as it will cool less effectively during daylight hours. If necessary you can build a shade using walls as seen in the picture.


This is built inside and is very similar in construction to the condensor. Make sure to place it in the path of airflow for the best efficiency. It's not necessary though, as convection will take over and move the air through the room naturally (Yay physics!). Another thing is that you'll likely want to have more radiators on the condensor than the evaporator. It's best to remove heat from the condensor as quickly as possible. In addition to that, the fewer radiators on the evaporator are much more effective because of the atmosphere around them.

Again. There should only be one pipe connected to the digital valve.

Your setup should look something like this when you're done. Just install the digital valve in-line with the only pipe going from the evaporator to the condensor.

Refrigerant fill

Connect your fill euipment (bottle and pipe volume pump/pressure regulator) and fill the condensor and evaporator to around 500 - 600 kPa. Make sure to turn on the digital valve so both sides reach this pressure. Pollutants work well as refrigerant. Water and CO2 would both be fairly effective but have other uses that are far more important.

It doesn't matter which side you use to fill the system. Just make sure the digital valve is on. After you reach 500 - 600 kPa, stop your filling equipment and turn off the digital valve. Experiment with the pressure if you like but I've found that this is fairly effective. You should start to notice a temperature change pretty quickly if your room is already sealed and heated. You'll find the condensor getting colder and the evaporator getting warmer. This is normal. It's setting up the temperature differential needed for the system to operate. Let the system sit and cool for a good long while before using it.

Here's my smaller system in my fabrication lab. There are only 10 radiators indoors and 15 outside. The condensor is underground as well which seems to help tremendously with the temperature.



You'll notice that the temperatures are drastically different between the two. This happens when the valve is closed. Basically the valve is separating the two sides of temperature and pressure. This keeps them from equalizing. Once the digital valve is turned on, however, the sides will equalize and heat will flow out to the condensor where it will dissipate. The evaporator will cool off very quickly and cool the air around it.

Since this depends heavily on the cold temperature of the condensor, the colder you can make that component the better.

Cool! It's built! But now what...?
Connect the digital valve to your favorite logic thermostat or operate manually. There are several good guides on automatic thermostats. I've built my own but that's a guide for another time. Below are some pictures of the temperatures and pressures during operation.

Before valve opens

The temperature is reaching the limit I set on the thermostat. The valve is about to open. Notice that the evaporator is at the same temperature as the room.

Valve open

The valve has opened. Notice the evaporator is now veerrry cold. This will cool the air flowing across it and bring down the room temperature drastically. The valve will close shortly after this depending on where your atmosphere sensor is located. However the evaporator will continue cooling your space until the temperatures equalize.

30 seconds after valve closed

Notice the sharp drop in ambient air temperature. That's 4 degrees Celcius in 30 seconds! For reference, I ran this test in a 3x5 sealed room. 3 sides and the ceiling are all glass and this is the middle of the day. You'll also notice that the evaporator temperature is almost equal to ambient and the cycle is ready to begin anew.

One thing you may want to avoid is having the valve spend too much time 'ON'. If this happens you'll notice the cooling efficiency decline sharply. So I'd suggest using a thermostat with a buffer. I've designed my own but I'll post that guide another time.
So this sucker can absorb a ton of heat as it is. If you increase the size of the evaporator and condensor it will only increase the ability for this setup to remove heat.

I left the valve open in my greenhouse for an extended period just to test things and the temperature went from 20 C to 6 C in less than half a day. I'm not sure how cold it would have gotten but I'll probably find out soon enough.

Anyways, hope you like this guide and find it useful. Again I'll be writing a guide for my thermostat with built-in cycle delay soon. I'll post a link here when it's finished.
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LexaMaslakov Mar 19 @ 2:21am 
Hello, guys
A few remarks about my system that based mostly on this idea.
1. It's good to know how much moles contain the gas in your garden. In my case, it's 334 moles/square (104 kPa, clean CO2), 12 squares.
2. It's good to add GasAnalyzer to cooler from the side of the garden.
3. More than 1MPa pressure is OK. Just you have to calc energy ratio in the garden Air and in the cooler. In my case, it's approx 1.04MPa pressure, X as a cooler, 670moles. So, result is (334 * 12 * 28.2) / (670 * 24.8) = 6.8 as a ratio.
4. The logic then is very simple. We have
StartCoolingTemp (38 C)
AimTemp (36 C)
CoolerActualTemp (from p.2)
If CurrentAirTemp >= StartCoolingTemp, switch on CoolerDigitalValve and CoolerGasAnalyzer
Wait until CurrentAirTemp - AimTemp + (CoolerActualTemp - AimTemp) / ratio <= 0
Switch off DigitalValve and CoolerGasAnalyzer. Wait long enough (15 seconds for example)

PS I have a system with 5 radiators inside, 8 radiators outside
Amallore  [author] Apr 15, 2018 @ 9:52am 
Divinepower, higher pressure is indeed better. But if you set the pressure too high the refrigerant takes longer to return to the temperature of the evaporator or condensor.

I've found that 1 MPa works pretty well. It pulls a lot of heat but doesn't take terribly long to recover.
DivinePower Apr 15, 2018 @ 8:27am 
Great work. May I ask why should we set the pressure of coolant 500~600kPa? Isn't the higher pressure the better?
AnonSliver Apr 8, 2018 @ 3:38am 
well put guide, also adding this to the wiki:steamhappy:
Amallore  [author] Feb 20, 2018 @ 6:40pm 
Amario you can give it a shot.

I know the initial design drops a 3x6 room 10 degrees off of a 2 second on time. That's at 101 kPa 73/25/2 N2/O2/CO2 mix.
Tosh AT Campfire Feb 20, 2018 @ 6:20pm 
Amalloe If I where to make an air tunel with this I could cool all my gases befor they to to prossings?
Amallore  [author] Feb 20, 2018 @ 7:07am 
KoJak that's a HUGE area! I didn't think this concept would work for such a large space.
KoJak Feb 19, 2018 @ 10:28pm 
Its works great. converted over today and works like a charm. i used a 21(Condenser) by 12 (evap) system with a space 10 x 10 x 2 and if maintains temp all day.
Amallore  [author] Feb 6, 2018 @ 12:24pm 
@BeeSkee I'm not sure on that. Haven't really tried to downsize it. I'd imagine that since it acts like a heat sink that shrinking it would considerably affect the performance.

You may try making it smaller (10 rads external, 5 internal) and increasing the pressure of pollutants in the pipes. The second system I built is charged to 1 MPa and is working great.
BeeSkee Feb 6, 2018 @ 12:17pm 
How small can this go?