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Geo NR Gee 01-21-19 02:33 PM

Calculating liquid heat storage
I have two 375 gallon insulated steel storage tanks. They were used for storing hot water generated from 180 evacuated solar tubes. I am trying to figure out if I should keep them and use them with the 90 evacuated solar tube system for space.

They are tall vertical tanks and could fit next to the
collectors. The tanks do not have any heat exchangers in them. They do have several ports for temperature probes and other fittings. This unit has a manway for easy access.

On a sunny day, the evacuated tubes generate 185* F (85*C) or more. They are supposed to have a snap disc to limit the heat to 195* F.

I don't know how to figure out the amount of heat energy that the storage tanks could store. I contacted the manufacturer and they don't have much information on them besides a simple installation, startup and maintenance procedures. These tanks are thermally coated with a spray foam. I believe it is 2 inches.

The plans are to use the heat generated from the collectors for space heating in my house and garage, dhw heating, radiant floor heating and the rest goes down in the ground for charging my future geothermal system.

Does someone have a calculator or know of one to figure this out?

We could just pour a small concrete pad, set the tank/s, plumb them and see what kind of temperatures they would store and if it would be enough to get us heat for the night or for a few cloudy days?

randen 01-21-19 03:20 PM


782 Kbtu of usable heat


Geo NR Gee 01-21-19 03:53 PM

Thank you Randen, 782 Kbtu of usable heat. How did you get to those numbers? Is that per 375 gallon tank or total of them?

I own a 1750 Sq. Ft. house. Lets say we want to heat that space with the 782 Kbtu of usable heat. How do you determine how many hours those storage tanks would put out heat? I suppose things like temperature line loss from the tanks going to the heat exchanger, would be a factor. Area of the heat exchanger another.

I have a 3 ton heat pump at the house. It can produce around 30 Kbtu/h (per hour) at 47* F lets say. Not sure exactly, but just a number. We need to heat the home every hour for 24 hours. That would be 24 (hours) x 30,000 = 720,000. So the tank/s would heat for 24 hours in this scenario?

Trying to wrap my head around this.

Geo NR Gee 01-21-19 04:25 PM

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At one point I was told these were 375 gallons each, but the manufacturer says they are only 240 gallons. Here is the spec sheet they just emailed.

randen 01-21-19 04:34 PM

You’ve got it. Back of the napkin has it at 24 hrs for 750 gallons but now your at 480 gallons. So heating your house for 16 hrs and hopefully next morning with sun to heat it all again. Pretty neet eh??!!

Geo NR Gee 02-19-19 10:48 AM

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Thanks Randen. I'm still toying with the idea of incorporating the tank or tanks. They are rather large. The Drakes Landing community idea with these tanks included seems like it would be a small challenge.

Geo NR Gee 02-21-19 04:10 PM

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Today it is sunny and a good day to be outside. I added a small solar electric pump and a small solar panel to power it. The pump delivered the water from the 30 gallon (10 gallons at first) water tank to the top of the collectors after about 30 seconds or there abouts.

The temperature of the water before it entered the tank was in the 220*F range then dropped to about 130*F. I let it go for about 10 minutes. Satisfied that it was going to work, I filled the tank the rest of the way and pumped from the middle section of the tank to the house.

The 1" copper insulated pipe going to the house is just laying down on top of the ground to test the system. It goes into the garage and is in a loop with a pressure and temperature gauge to see the line losses (for the time being).

The pipe is covered with a fiberglass industrial insulation wrap, but ended up getting wet in spots. I am sure since the insulation is wet it is not allowing the heat to stay in the pipe. Maybe after a few hours at 100* it will dry out? Time will tell.

After filling the tank, and turning on the pump to the loop to the house it showed 80*F inside the garage loop. The temperature coming into the top of the tank from the collectors dropped down to about 120*F.

After about 30 minutes, I went back out and see that it is up to over 130*F. and in the garage loop it is now 95*F and climbing. Maybe it will dry out and insulate sooner than later?

gadget 12-30-19 10:55 PM


Originally Posted by Geo NR Gee (Post 60497)
The plans are to use the heat generated from the collectors for space heating in my house and garage, dhw heating, radiant floor heating and the rest goes down in the ground for charging my future geothermal system.

I would love to hear more about your geo thermal storage. I have been doing allot of research on the subject and wondering if you have done anything yet?

GaryGary 01-09-20 01:59 PM

Don't know if this is still a current project for you, but a little info on your questions...

Heat Stored in Water:

Heat Stored = (specific heat of water)(Thi - Tlow) (weight of water)

Thi is the temp the tank gets to, Tlow is the lowest temp you can use for heating

Specific heat of water is 1 BTU/lb-F

So, for 480 gallons with a high temp of 180F and low temp of 80F, the energy stored is:

Energy Stored = (1 BTU/lb-F)(180F - 80F)( 480 gal * 8.33 lb/gal) = 400K BTU

House Energy Use:
How much energy your house uses in a day depends on how big it is, how well its insulated and how cold it is.
You can estimate this with this calculator:

For 1700 sf home with typical insulation on a 30F day -- maybe 25,000 BTU/hr or 600K BTU/day
But, again, this depends a lot on insulation levels, temps etc....

So, the tanks probably could store enough heat on a sunny day to provide a good part of your heat needs.

Does the tank size match your collector size?
A rule of thumb on how much storage tank capacity you need to work well with a collector of a given size is that the tank should provide about 1.5 to 2.5 gallons per sqauare foot of collector area.
Not sure how large your collector is, but the 480 gallons would be a good match for a collector area of about 240 sqft.
Its not critical to hit this right on the money, but a tank that is much too small will get up to its max temperature early in the day and the rest of the days solar would be wasted, and for a tank that is too large, the tank will have a hard time heating up to a temperature that is useful for heating.

Hope that helps.


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