Oh no doubt, I think anything can be accomplished with some thought and foresight. Expensive chiller vs an inexpensive chest freezer. Running the numbers for your particular scenario would dictate whether it would work well and be durable.
If it’s specified or if one could estimate the BTU and duty cycle rating of the compressor, that would be useful. Or, on the other-hand, simply jury rig it up. Don’t know which path would be more work in the long run. Experiments are always usually fun.
Once it got cold, the compressor just had to remove whatever heat got through the insulation. If you turned it off and didn’t open it, it would probably stay frozen for a week. Keeping it full to the brim is better than being half empty because you have a larger thermal mass and so more stability. Keeping stuff cold takes loads less energy than making it cold because of the insulation.
Most freezers have a rating for how many kilos of room temperature ‘stuff’ they can freeze in 24 hours.
I just googled it and the first one that popped up could freeze 3.5Kg per day.
If we estimate that to do so you take it from +20c to -20c then that is (3.5*40)=140 kilo-degrees, meaning you could chill 140Kg of water by 1 degree in 24 hours. I feel certain someone would be able to work out how many BTUs that is.
If your tank is 70 litres, you could remove 2 degrees of heat from it every 24 hours and stay within the freezer’s capacity. If your lights heat your solution more than that you will exceed the heat removing capability of that particular freezer (it was a vertical freezer in a 54cm wide fridge/freezer so there may be better ones for this purpose) and run the risk of burning out the compressor.
Follow on to @MicroDoser comments:
Rough estimation is 8.33 BTU per US gallon per degree Fahrenheit.
So, a 15 degree drop for, say, 40 gallons would require a total of 4998 BTU. Convert to total Watts = 4998 * 0.293071 = 1465 Watt/hour. Convert this to tons, we’d have 0.0003 * 1464 = 0.44 tons. Convert to horsepower = 4.6989 * 0.44 ~= 2 horsepower. This, is for cooling the solution 15 degrees in 1 hour. For a 4 hour cooling specification, we’d have around 1/2 HP. Rough calculation only, so check this math.
But, this ignores two important aspects, the efficiency of the cooling system (probably ~50%) and any heat input while cooling. So, we’d likely have to double the size of the system based on the real work calculation at minimum if we desire : temperature pull-down over a defined time span.
I think, (real work to be done) divided by the (determined / estimated the efficiency), and convert to HP. This gives you a compressor size required for 1 hour specification. Divide by the number of hours to achieve your cooling goals and be certain that the compressor can handle a 100% duty over that time span. I think that’d work. I also think the compressor HP will be larger than one would casually assume if, for instance, you are also heating the system while trying to pull-it down, etc.
On efficiency:
For refrigerators, AC units, etc according to http://www.wc101.com/guides/refridgeration/page4.htm:
That TK1000 chiller you are using is only 1/4 HP, 315 watts - and it does a great job for you, even though you are doing a two stage process - which has to be somewhat less efficient.
Remember, you will only be removing the heat the water gains while its in the grow tent. With a recirculating system, especially if you insulate the pipes and buckets, I dont think it will be nearly as much heat load as your calculations suggest.
A bit of apples and oranges between the OP and the example calculation which was geared more towards the re-purposing comments. Also, this may be confusing since the glycol system in the OP is a slightly different beast. The example calculation serves to illustrate what kind of power you’ll need in order to transfer a certain amount of energy over a certain time span. This is the physical minimum/truth no matter the system. The same amount of work is being done but with a glycol reservoir we are “storing” that work temporarily. If we were to calculate everything out ignoring losses, we’d see the amount of energy being expended as being exactly the same.
The example specifically illustrates pull-down (e.g. temperature pull-down over a define time span) and is a different situation than temperature maintenance. In pull-down we are talking about a fairly large swing. For maintaining temperatures we’d be talking maybe couple of degrees. In maintenance we’ve already expended the needed energy to get us to our set-point, how ever long that took. The important metric in maintenance is going to be the amount of energy (as heat) being input into the system so that you don’t overwhelm that capability of your compressor. Then, we consider the duty cycle. So, yea, you could use a smaller rated system to maintain temperature but it might take longer than you’d like to get to where you need to be in the first place. And, then you’ll want to consider that 1KW+ of heat input into the tent by the lighting, and so forth. I’ll see if I can pull up the time it took to pull-down this system, I don’t know if I still have the numbers.
And the reservoir, for the glycol system, serves like a capacitor. As such, it is capable of quickly pumping large amounts of energy but only for a relatively short duration. We still have to expend the needed energy, how ever long that takes, to store/remove the energy necessary to get to the reservoir temperature to 48F. And, then consider continuing maintenance.
If we remove the reservoir and instead directly use the chiller to cool the solution, we’d be closer to what the example calculation is trying to illustrate for pull-down.
I can say, that when I had originally calculated the needs, there is a point where this system will not be able to keep up with the demand with my current equipment. E.g. lights full power with fairly high ambient temperatures. Though, the glycol system can help ride out short lived bumps because we’ve “put in the work ahead of time”. The system acts as though it has many more horsepower than spec’d but it runs out of energy quickly, like a sprinter. Combined with a monitoring system, realtime adjustments can be made in the lighting power, for instance, to reduce the load temporarily with the glycol reservoir taking up the slack for a short period. We can determine when things are on the decline when the duty cycle of the chiller and the PID are equal or worse.
Point was, it is helpful to have the numbers estimated ahead of time. Then calculate how the system might perform overall to both maintain a load and for pulling a solution down across a large span. Duty cycle will have an effect on longevity of the compressor. Example illustrates a way to estimate the energy demands, just change the deltas, time, efficiency, etc. You don’t “need” to do that, it’s just the way I would approach it whether it’s an off-the-self chiller or a re-purposed compressor.
Those are all very good points, and I dont disagree with any of it.
However, I still think you are over estimating the amount of heat that will need to be removed once the system is stable - after the initial cool down. I also think you may be under estimating the toughness of a chest freezer.
Im quite certain our old chest freezers compressor never shut off once over a large portion of every year. Thinking back on it, it was probably even worse after my dad enclosed it in the shed. It would have been pumping all the heat right back into the room it was sitting in, with an ambient temp well over 100 for at least 18 hours a day or more.
Of course, the only way to be sure is to try it and see 
Will @HappyHemper or @SuperiorBuds be the first? 
Haha. No rush for me, but I had already purchased most of the parts as something close to this was already in my plans.
I’m doing things slightly differently, though. I have a single loop that goes through the chiller and then through the wort coil. This means I’m setting my chiller at a higher temp than @Northern_Loki. I’ve also got my chiller located in my grow room, since I need that heat to keep the room warm during winter. Switching to LEDs means no more HPS heat, so I’ve been running the baseboard heater. Plus my top-off comes directly from the well, through the RO system and into the res – and the water starts at about 50 degrees already. If needed I can relocate the chiller outside the room w/ no problem, and I’ve got plenty of spare pumps to use if I want to separate into two loops. The only thing I’m really missing is the PID controller, but those can be here in 2 days if needed. Thanks Amazon Prime for delivery into the middle of nowhere in 2 days, just like I was still in the city.
Larry LOL.
I’m not sure where you are getting this from, I haven’t provided those estimations. Nor have I said one way is better than another, just differences.
Though I have provided some simple first order formulas (substitute your numbers), things to consider (efficiency, duty cycle), etc.
I’ll consider putting some more illustrative examples together.
Cool. A nice set-up.
I’m not certain you’d want a PID for your set-up, the chiller should have a basic hysteresis based controller. You wouldn’t want the controllers competing with each other.
Exactly why I hadn’t thought of that in the first place. I’m running a 1/10hp Hydrofarm Active Aqua right now…
It’ll be interesting to compare the two setups though. I’m betting your setup with lower temps and dual loops will be more efficient with the chiller working less, but we’ll see. 
I love these types of experiments…
Ignore me. You are quite right. I was misinterpreting what you said. Reading while stoned is my only excuse
I know this has been a while, but I wanted to circle back w/ my experience so far.
I am still using a single loop though I did set it up differently then I originally planned. I am using a low-flow pump out of the glycol reservoir, from there it goes to the stainless wort chiller submerged in the nutes, then out of that and into the chiller, before finally dumping back into the res. I ordered more insulated tubing so I can adjust this later on if needed, but so far it seems to be working well.
Still playing with temps to get everything dialed in. Right now I am running my chiller at 66 degrees and the heater at 64, which this is keeping the nutes right around 68-69 degrees. Changes do take a while to make it through the system like you said @Northern_Loki – but I am ok with that as long as it says in the proper range. Time will tell as we are just now starting to really heat up outside.
Here’s a video showing the complete res/chiller loop
This res has been running for months so I was able to ensure the heating side of things is working properly, even in extreme cold, now I am just making sure the cooling side works. So far it looks like I finally have a single system that will work for winter and summer w/ no changes.
One additional thing that also helps is I am now running my RO via a float that keeps the res topped off at all times. Each time the HPA refills and pulls 2G into the accumulator it will be refilled from the well with 45 degree water. So even in the summer the heater is needed to ensure I don’t shock the plants, but it only takes a few minutes for it to adjust the water temps back up into range.
I do like the idea of having both heating and cooling capacity. Keep us up to date how things look as the external temperatures approach the extremes. Would serve as a good datapoint.
I need to do this, as well. Good idea.
Good stuff!!
Where I live in oz there is most definetly no need to heat anything as even in the midst of the coldest winter the lowest we get is 10 C. Heat on the other hand, temps over 100F are regular. I had been thinking of simply running a plate heat exchanger like this
With a rez of glycol in a mini freezer and the nutrients simply being pumped through the exchange and temp simply controlled by a PID as I do with my brew wort. I have looked at chillers, but anything like that is as expensive as poison here. In my brewing I have found these plate exchange’s to be LOADS more efficient than the copper coil I used previously.
Expensive here as well, if can you get away with a mini-freezer all the better.
Would like to hear more once you get it up and running. I like these type of projects.
I can’t recall the cost of the stainless steel coil I used but it was something under $100 I think. Still not cheap but a one time purchase, ahh well.
Depending on how you plumb the exchanger up, would the nutrients at any point contact the copper (try to avoid copper contact, if possible)?
An excellent point I had not even considered! The one I use for brewing has copper plates, though I am not certain if they are chrome plated or not…
After a bit of a look I found these guys locally that carry them with stainless exchange plates, they are more expensive but not hugely… I’ll sus it out a bit more…
@slain Keep your eyes on gumtree mate. I picked up a hallier(sp?)400 with a 3ft tank, a few pumps and filters for 200 dollars two weeks ago.
Chiller has been working good. Next trial experiment, a slapped together air conditioner / dehumidfier connected to the chiller reservoir.
Similar concept to a split A/C. Heat exchangers are a couple of 8 inch iceboxes connected to a cloudline fan.
The iceboxes will be liquid cooled from the same reservoir handling the RDWC solution temperature. The fan controller will be interconnected to an Advantech PLC controller … which I’m also using for all the sensor inputs and control outputs for this set-up. I should be able to control the temperature / humidity towards a good VPD range.
Each icebox has a condensate drain, which; I’ll likely just run into a condensate pump of some sort.
Will be interesting to see if the additional load will overrun the thermal capacity of the reservoir…
What happened to this?
The rez chiller noted in the OP works awesome.
I haven’t actually plumbed in the A/C with the icebox, though. Just need to get a small pump … which I’ve been lazy and distracted with some other priorities.
