I hope you're on board for a lengthy explanation, but I sense you might appreciate it.
Wattage is unknown, but I ran a few numbers to give at least my best estimate. The compressor itself is rated by the manufacturer, but those are in controlled conditions and nothing like what a real system observes.
For example, if you ever look at a datasheet, I can guarantee you that there aren't many systems out there running with a 0 degree evap temp and 40 degree return gas temperature. Assuming that the liquid is sub-cooled when it leaves the TXV you're adding way too much superheat if you have 40 degree return gas. So those ratings are really only a general idea as to what you can expect. My estimates were based on the actual figures with some estimated capacities based on observed tendencies.
The lower the return gas temp, the slightly higher the capacity. The table below gives a good idea of what type of capacity I might expect. Given that return gas temperature is lower, heat of compression will be a bit lower, and thus we'll have better capacity. Don't forget that this is still dependent on how much of that return gas temp is superheat. If its 30F, count on the compressor being overwhelmed and the windings to be the opposite of cooled.
The efficiency of my evaporator is pretty good considering the "homemade" design. When temps are low, the Delta T gets lower, but it stays above 6F for the most part. At times it gets as high as 10F in mid range temps from 40 - 18F or so. Though, when you get lower in temperature, the refrigerant is not as plentiful at the lower pressures and isn't receiving as much heat, and as pressures drop it isn't able to accept as much. I would like to have higher flow rates, as that would boost efficiency, but overall I'm satisfied with the results.
Again, the above is just an idea. It may be higher or lower, but given the variable load of the components, it will be more than enough. At the lowest evaporator temperatures I'll still be higher in capacity than wattage supplied to the loop most likely.
As for temps, it is quite clear that I could get this sucker to pull the solution down to -30F in time. Though, with a 50/50 PPG/Water solution, the minimum advisable temperature is around -20F (it freezes at about -30F), and its not advisable to go higher than 50% unless for some reason I want to reach the pump's shutoff head. If I wanted to go that low I would have ultimately had to use ethylene glycol, and I just don't want to use a toxic chemical given future plans. PPG has low acute toxicity. A side effect is the higher pumping head that results.
Another problem with going that low is that plastics become brittle, and that pump has a Ryton housing. Polypropylene would be better, and I'm considering the change. In any event, stress on the plumbing must be completely alleviated to ensure that it does not crack. At -30, its far too risky. At -5, its manageable. That said, using any corrosive chemicals as a heat transfer fluid requires that you ensure all of your seals and materials are compatible. Any incompatibility and plastic goes KERSPLIT! For example, Polypropylene plastic is susceptible to UV light. It yellows, and cracks with over-exposure.
If you are doing light chilling, you should be fine using a glycol such as ethylene glycol. You must, however, be absolutely SURE that it does not leak and it is not accessible by any critters or creatures in your house. I would say the same for ethanol, but ETG is probably more compatible. I would also say that you should investigate Propylene Glycol for this reason if you ever go sub-freezing, but not sub-zero. At 30% PPG, you have less viscosity. That said, you still would need a bigger pump. So if toxicity really isn't a problem (and I stress really), then ETG is a good way to go. You'll sacrifice a bit of the heat transfer ability of PPG, but you'll have less head loss than with PPG. Ethanol isn't bad, but it is hungry and likes to eat plastic and rubber seals.
It is worth mentioning that Tygon is not recommended for use with Ethanol. It will exhibit a moderate reaction to the Ethanol. It, however, has no measurable effects on PVC tubing. It doesn't react with Nickel alloys and only has minor reaction to Aluminum. Its actually worse with Brass. But as with Tygon, its a moderate effect. You also need to make sure that your O-Rings are Nitrile or FKM for best results. Silicon or EPM rubber will exhibit a minor effect.
In conclusion, for your purposes of a budget, SS and Water Chillers are a bit pricey. Though, you can scrounge together some materials for the cheap too. You can make it as basic as you want. I designed mine differently than most, and it cost me some $$$. Though, it pulls a 5-6 Gallon Reservoir from 65F to 25F in less than 15-20 minutes. For the volume, it worked out very well. However, I suspect its not your cup of tea. It requires a lot of patience too, and research. I've got a lot of folks around here that contributed some knowledge as well.
If you ever do go this route, you must be careful to heed the HVAC Gods and NOT violate the principals of Superheat and Sub-Cooling. The demands of SS and Chillers are somewhat different. An SS is a direct application, while the chiller is indirect with a more constant and decreasing load. The principles are the same, however. If you have gas coming back to the compressor that is superheated by 30 degrees, the compressor will ultimately fail sooner rather than later. The same goes for too little SH. Patience.... yes..... Patience.
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