lol, a forum with no PMs? or am I just too young of an account?
My recommendations are water cooled mini-split A/C. Stay away from traditional A/C that pulls in a significant amount of outside air.
Lets start at explaining exactly what water cooled mini-split A/C is. You have a standard A/C condensing unit hooked up directly to a heat exchanger. Shell and tube or brazed plate are the most common, with brazed plate being the most efficient. The company that invented them, Alfa Laval, makes the best and most reliable ones in my opinion. Refrigerant goes through one side and a water/glycol mix goes through the other. Why water/glycol? So it doesn't freeze in the winter. The water/glycol is pumped to and from a large reservoir(typically 5gal/ton). 225 tons means 1000-1500gals in your reservoir. Then water is pumped from that reservoir to individual fan coil units in the grow room. These are air-water heat exchangers. In this case, I would expect as many as 50. Fan coil units don't come in large sizes. 5 tons is the largest I've dealt with personally. Typical water/glycol temperatures are 42F-48F in the reservoir. The colder the water, the more efficient your fan coil units will be, but, you need more condensing units to make that happen.
Okay, that's how it works. But why this again? What makes it so special?
Well, compared to traditional mini-split A/C, you don't have to run copper. Which is really expensive to install. The water/glycol runs through PVC or ABS pipe. These installations are 20-40% the cost of copper and take half the time. Plus, you only need a plumber to install it, which are typically cheaper than HVAC techs. Also, no ducting. Ducting is large, bulky, and a pain in the ass to deal with. On top of that, ducting doesn't seal the room.
Since I assume you are going to be using some kind of CO2 generation or CO2 flooding of the room, pulling outside air is not an option. Especially if you are running a 500kw facility. The only way to get more than maybe 50% of the use of those lights is to have active CO2 generation that keeps things around 1500-1900ppm of CO2. Outside air is ~400ppm for reference. Therefore, non-minisplit systems are now out of the question.
Now, you might want to use a vented light. Which makes traditional A/C a little more attractive. Now, you'd just have to use a fan to drive the air in the winter. Well, one thing you can get with water cooled mini-split systems are what are called dry coolers. Which simply run the water/glycol through an air-water heat exchanger that has a fan on it. So, now you can get your water/glycol down to outside temperatures, of say, -20F in the winter. Doing this now means your fan coil units inside can run much less often. As I said before, they become more efficient as the water/glycol temperature drops. When it's this cold, your condensing units do not need to run, which will reduce your cooling costs by around 90%. On top of all that, bringing in -20F air to your room is not good for your plants. Not in any way. I should note, your fan coil units will not run at max capacity when the incoming water temp is that low.
The last thing you can get with a water cooled system are cooling towers. If it's not 100% RH all the time outside, you can use cooling towers, which use water, drain to waste, and evaporate it to bring your water/glycol mix down to the wet bulb, or dew point. So, when it's 40-80% RH outside in the winter, cooling towers will reduce your cooling costs by 90% or so like dry coolers. They are cheaper than dry coolers on a per ton of cooling basis, but do not work in areas of high humidity.
Right, we've gotten to relative humidity(RH) now. A water cooled mini split system can dehumidify a room as it cools. All incarnations of traditional A/C do NOT dehumidify. You need additional equipment to do that.
Now, with all these benefits, you'd think it is more expensive to get this kind of system installed, and you'd actually be wrong. Often times water cooled A/C is cheaper to install as well as cheaper to run.
