What is a glycol chiller?
This term generally refers to a chiller that consists of a reservoir of cooled liquid. Typically, the liquid consists of a mixture of propylene glycol and water.
The cooled reservoir solution is then pumped through a heat exchanger (cooling loop) to chill a target solution or the surrounding atmosphere (air conditioning).
In many applications, the chilled reservoir is cooled to a temperature that is much less than that of the desired target temperature. A control system is utilized to adjust the flow into the heat exchanger to manage the target temperature. By chilling the reservoir to a low temperature, the system provides a large heat capacity and allows it to quickly react to large changes in the target temperature.
An additional feature of a glycol chiller, sometimes found on chillers for brewing systems, is that they are able to control multiple target temperatures independently. For each target, a separate pump and control system is utilized to control the target temperature. For instance, two loops could be utilized with one loop controlling the temperature of a RDWC reservoir and the other loop feeding an air heat exchanger (air conditioning).
What is propylene glycol?
Propylene glycol is a synthetic, water-soluble, generally non-toxic, and easily metabolized chemical that has a variety of uses. One use is that it acts as an anti-freeze and is used, such as in this application, to reduce the freezing point of water. It acts as a safety feature to prevent freezing of the reservoir and potential damage to the chiller itself. It also has anti-bacterial properties and acts to buffer metals against corrosion. Do NOT confuse this with ethylene glycol which is quite toxic if ingested by humans, pets, etc.
The experimental set-up
In order to build a glycol chiller, there are a several pieces of equipment that are needed
- a chiller with sufficient capacity to handle the maximum expected heat load (in this case a TECO aquarium chiller)
- an insulated reservoir (camping cooler)
- a heat exchanger (such as a beer wort coil, use stainless steel for anything metal contacting the nutrient solution)
- a temperature PID controller with thermocouple
- submersible aquarium pump(s)
Other miscellaneous items include
- hose, tubing, or pipe for interconnecting the components
- interconnect fittings as needed
- bulkhead fittings for the reservoir (see Extra Long Bulkheads)
- gylcol (optional) + water
This is the chiller I choose to use:
The glycol reservoir is simply a modified camping cooler (the better the cooler, the better the efficiency):
Connections to the chiller are made and insulated (somewhat) with some neoprene:
And, the tubing is routed to a couple of bulkheads installed in the reservoir (camp cooler). The holes for the bulkheads were drilled using a step drill bit (a large one):
Within the reservoir we have:
In this case, you’ll notice that there are two small aquarium pumps. The TECO chiller does not have an internal pump. One of the pumps purpose is to recirculate the glycol reservoir solution through the chiller. This pump currently operate in a continuous manner. The chiller itself monitors the temperature of the solution flowing through the chiller heat exchanger and will cycle on/off automatically based on the set-point. I have the set-point set to 48 degrees Fahrenheit. This will be the temperature of the glycol reservoir solution.
The second pump is used to pump the chilled glycol solution to an external heat exchanger (located in the RDWC reservoir). This pump is controlled by a PID controller which will cycle the pump on/off (PWM). PID controllers are “closed loop” controllers which “learn” how the system is reacting to changing conditions. The PID controller adjusts the on/off times and durations in order to precisely control the temperature. The PID controller which I’m using is an Inkbird (controller works but there are better options):
The pump to the external heat exchanger is connected to a “wort” chiller coil located in the RDWC reservoir:
The semi-transluscent tubing is an air line that is described in another topic, here: Airpumpless Sluckets
And, the interior of the RDWC reservoir:
The glycol solution never comes into contact with the nutrient solution and, likewise, the nutrient solution never leaves the RDWC reservoir. The heat exchanger (wort chilling coil) does the work of removing heat from the nutrient solution and transferring it to the glycol solution. Notice that the heat exchanger coil which is in contact with the nutrient solution is stainless steel. Avoid other common metals such as copper as these can slowly dissolve into the nutrient solution eventually lead to plant toxicity.
In this first graph you’ll see the environmental temperatures. The red trace is the temperature in the grow area. The green trace is the temperature outside of the grow area. For this test, the lights, in a closed room, were ramped to full power and then ramped down. You can see how it affected the room temperature:
In the second graph, you can see the RDWC solution temperature at the point where it returns to the RDWC reservoir (system set-point is 68 degree Fahrenheit):
You’ll also notice how the PID controller reacted to the change in heat load. When the heat load is removed (lights off), the temperature overshoots then undershoots while the PID controller tries to learn the new situation with the solution temperature.
Here is another example where the lights are being cycle on/off and random intervals:
And, the solution temperature:
In both cases, the temperature set-point is 68 F and is controlled to within +/- 0.5 degree Fahrenheit. Additional tuning of the PID loop or using a better PID controller can improve the performance further.
Do you need this level of control? Probably not, but if you are running experiments this appears to be a good way to control the temperature variable.