Need help sizing storage tanks

GlennHarrison

My boss has a project that involves a heat recovery system.

What we are trying to figure out is what the fomula(s) would be to calculate the amount of water we need to store in tanks to heat the offices in the building over night.

So lets say the heat loss for the offices is 200,000 btu/hr, times 10 hours of heating time when the oven that would generate heat is offline overnight. How do I calculate the storage we would need to maintain 200,000 btu's/hr ouptut for 10 hours? (2,000,000 total)?

Robert O'Brien

The heat loss isn't going to be 200K every hour,only on a design day. If you store it at 200* and need a min temp of 120* that is an 80F DeltaT. 80 x 8.35 (lbs/gal) = 668 BTU. 2,000,000/668 = 2994 gals. Adjust for your deltaT and that's the answer. It won't be delivered in a linear manner,as the water temp drops so,will output

Robert O'Brien

Hatterasguy said:

Not quite that simple.

The tank starts with a specific volume of water at a specific temperature. All night long, the tank temperature is dropping and the capability to provide 200K per hour must be figured at the last hour of the night when the tank is at its lowest temperature.

You would need to know the type and amount of radiation in the offices and then determine the minimum water temperature necessary to still squeak out 200K at the last hour. That becomes your minimum tank temperature.

Then, you must provide the temperature of the tank at the beginning of the night. Can you get the tank up to 190°F with the boiler that you have in a reasonable amount of time?

It's going to be a very large tank.

As an example, if the tank starts at 190°F and the minimum water temperature necessary to still provide 200K per hour is 150°F, the tank can suffer a 40°F drop. Total BTU usage overnight (say 10 hours) is 2M BTU.

Required tank size: 6000 gallons


With a tank that large, it would take a very large boiler to bring it back to 190°F in a reasonable time.

The formula is: BTU=(DT)(Gallons)(8.3)


NB: You'd need to insulate such a large tank or, preferably, place it in the conditioned space. A tank of that size is a significant radiator all by itself.

We agree on the math,the assumed DeltaT is the difference. I didn't consider the heat loss from the tank over the other 14 hours,that is going to be a huge number. How many therms will a system like this offset? Maybe 24 therms on a 10 hour period at design temps? Given nat gas prices,is it even worth the capital investment?

SWEI

Heat recovery from what? Is there a heatpump involved? There are a number of parameters involved here and we would need a lot more information in order to provide even a reasonably accurate estimate.

hot_rod

There is certainly a place for buffer or thermal storage. I know Siggy has a handful of projects in the works, mainly upstate NY for pellet and chip boiler buffers.
Un-pressurized tanks are sometimes a price point option, if code allows.
Next week I can send you the pdf of Idronics 17 on Thermal Storage. There are more details and tighter formulas for determining the viability and important design points. Stay tuned.

A teaser, from I-17, which tank stores more energy?

hot_rod

Bingo, and the word for that is exergy. Both tanks contain the same amount of energy. The stratified tank has a "thermodynamic advantage, more useful energy.

The key is piping and designing to maximize that stored energy,
I-17 will show a formula to calculate equivalent temperature.

This should be useful info if installers plan on using the tank style combi boilers or heaters. Need to maximize the energy in the tank.

GlennHarrison

Thanks for the math guys.

So the customer has a factory that makes barrels. They have 2 ovens running at 450 degrees. The idea is to pull heat from the oven exhaust using a high temp transfer fluid oil, run it through a heat exchanger, then heat the water in the storage tanks during the day. At night to heat the offices they would pull water from the storage tanks to heat the offices. There of course would be a back up boiler.

Now that I see the math, 6000 gallons of storage is crazy.

We just crunched some numbers and figured a 25 to 30 year payback on the heat load of the offices.

We will talk to the customer, but it looks like now we will just go with heat recovery during the day transferred to the offices, and run a new boiler at night.

hot_rod

My thought also was to store something like 1000 gallons.

Refurbished or even new 1000 gallon LP tanks are not really that expensive. Used LP tanks around here run a buck a gallon. Refurbished, retested and stamped around 2 bucks a gallon.

Several of the wood boiler dealers offer refurbished 500 and 1000 LP tanks, with additional ports welded in. Insulation could be a 2" ridgid foam with a fireproof coating, or metal jacket.

Mark Odell is a friend in upstate NY building large pressurized tanks for storage. He has taken a lot of cost out of the tanks by eliminating the domed ends and using flat plates and stay rods. Much like a steel boiler construction. I think he has a 210 and 735 ready to go. Available with insulation and spray on fire coat.

AHONA is another source for refurbished tanks.