Coolant Storage/Thermal Battery Sizing

[AR2490]

I'm trying to size a tank for my cooling system. Worst case scenario I'm looking to dissipate 20,000,000 BTU/hr for 3 hours. I have a 1000 GPM pump and a heat exchanger that can dissipate the heat I need if supplied with 40 deg F 50/50 ethylene glycol (returns 66 deg F) at the given flow rate. As you may imagine this would require a lot of cooling power, and during the summer months it becomes almost intractable. Since I don't constantly need the cooling I wanted to look into building a storage tank to hold a mass of coolant precooled rather than attempt to dissipate the heat all at once and spend infinite money. The system can be simplified as a tank cooled to -10 deg F containing an as-to-be-determined volume of coolant. Assuming ideal heat transfer the initial state will be 1000 GPM of the coolant leaving the tank at that temperature, and 1000 GPM of fluid entering the tank at approximately 35 deg F. For simplicity I'm going to assume perfect mixing, the tank is going to be well insulated (underground, thick walled, and insulated on the outside) and the time period is relatively short so no external heat transfer. I'd like to figure out how big I'd need to make the tank in order to only reach 40 deg C in the tank after three hours of run time. I also plan to stall for time by running the chiller/cooler while this is occurring (but this has yet to be sized, so I'd like it to be considered in the equation), but that could likely just be modeled by reducing the heat I'm looking to dissipate.

I understand this would be kind of tricky to work through, and I'm not looking for a solution, just for someone to point me in the right direction. I'm fairly certain I'm looking for a form of differential equation, and I'm capable of deriving one if I can find some resources with similar problems. Most examples I have found online and in textbooks deal with mixing concentrations of materials, but few involve mixing fluids with different temperatures in a large volume. I also have a feeling I am overthinking this and it can be done without worrying about flows and simply considering heat transfer rates. Can anyone help point me in the right direction? Thank you!

 

[IRstuff]

20000000 BTU/hr*3hr = 6.33E10 J Divided by 1 cal/gm-K * 50K = 3E5 kg of water --> ~300 cubic meters of water

5.8 MW is nothing to sneeze at.

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[EdStainless]

There are high rise commercial offices that chill water at night when power costs less. I have seen a system that has six tanks each about 35m3 that they nearly freeze at night in order to provide cooling the next day.
They still run chillers, but at much reduced power. And they can shut them down completely to avoid peaking charges.
One trick is using mixing valves so that you circulate water at the desired temperature. After all you want to keep your cooling circuit nearly constant T regardless of load.

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P.E. Metallurgy, Plymouth Tube

 

[Compositepro]

It is far more efficient and practical to store "cold" in the form of ice rather than in the form of chilled water.

 

[AR2490]

Yeah, it would be more efficient. Can you direct me to an ice pump?

 

[EdStainless]

They form slush in the tanks. usually the cooling coils are fastened to plates so that ice that forms can be wiped off by stirring or scraping. This is where the careful blending becomes key when using the cold source.

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P.E. Metallurgy, Plymouth Tube

 

[AR2490]

Okay, that makes a lot more sense. Rough calculations make it seem like I can double the amount of heat this can store if I get a third of that frozen. I'm curious about the coils you're describing though. Do you know a supplier that provides tank mount coils?

 

[EdStainless]

You can find it by searching HVAC storage systems.
They are actually derived from food processing equipment.

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P.E. Metallurgy, Plymouth Tube

 

[FreddyNurk]

I suspect this will be a bit small compared to the numbers proposed, but I've come across this previously, which indicates use of frozen water as an energy storage medium.

I've got no idea how well they work though.

 

[lilliput1]

Check out Trane Ice Storage System and also CALMAC Ice Storage System on the internet.