How many btus in a tank of water?

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Old 03-15-17, 08:25 AM
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How many btus in a tank of water?

I want to figure out how many btus I can store in a tank of water. I understand that it takes 1 but to raise 1 pound of water 1 degree F. Is it possible to reverse the formula to achieve my results?

To explain in more detail, I want to store heat in a tank of water for later use. Let's say I can store the water at 120F. I want to be able to use that energy to heat my home at a later date. Being able to calculate how many btus are available would give me and idea on size.
 
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Old 03-15-17, 09:51 AM
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This is the general concept used by a water source heat pump. They remove the heat from water and use it to heat your home. There are closed loop systems that used stored water just as you propose (usually commercial application, not residential because of volume of water required), and open loop systems that use ground water from a well that is then returned to the ground in another well. Or a large pond can be used.

Here's a link that talks about the flow rate needed:

Q: How much water does an open-loop geothermal heat pump system require? – Wellowner.org

Now they are talking about water in the 50 degree range (ground water) and having hotter water will reduce the flow rate required, but they require a *lot* of water. 1.5-3 gallons per minute, for every 12000 BTU/hr heating load. In New Jersey, it would not be unusual to need 50,000 btu/hr for heating, although highly energy efficient homes can do much better.
 
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Old 03-15-17, 10:30 AM
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Yes, you can reverse the formula. How much of the BTU energy you can recover and put into your home depends on your system's efficiency. Just quickly throwing numbers If you have a hundred gallon tank of water and raise it's temperature 100 degrees f then you've put 80'000 btu's into the water. Flip that and you could pull 80'000 but back out of the water but what you end up in your home will depend on efficiency losses.
 
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Old 03-15-17, 12:18 PM
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It sounds like you are in the planning stages. There are things that come into consideration with a project like this.

How are you going to heat the water ?
Heating with fossil fuel for a later date won't save energy.
Heating with solar for later reuse is viable.

120° water won't yield much heating power.
 
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Old 03-15-17, 03:43 PM
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Yeah. This is definitely more of a mental exercise, and I'm well aware that this is unlikely to be practical.

For heating, I was thinking thermal solar collectors and/or picking up heat from my yet to be asphalt driveway during the summer. I'll have to remember to get some temperature readings when the hot weather comes for some better info on the asphalt method. I'm hoping to pick up higher temps than what a roof collector would provide.

Tell me more about your thoughts on the 120 degree water? I don't need water much hotter than that for my coldest days. Honestly, I probably could do with 120. I will have to do some calculations again.

I expect to have to use a heat exchanger between the tank and the heat source. And I probably should use one between the tank and the heating system in my house. So I think that will contribute quite a bit of loss having the two exchangers.
 
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Old 03-15-17, 03:53 PM
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Your thinking the wrong way.

If its hydronic heat you want youll go by the emmiters and the BTU they produce at a given temperature.

copper finned tube for example 550 btu per ft @ 180 f. Its not linear so use the chart.

[ATTACH=CONFIG]78347[/ATTACH]
 
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Old 03-15-17, 03:57 PM
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So if heating a 10x10 room thats 100 sq ft

100x25 = 2500 btu to heat the room.

120f water nets 220 btu per foot. 2500/220 = 11.3 so youll need 12 ft of baseboard in that room.
 
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Old 03-15-17, 04:46 PM
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Mike, you are misunderstanding what I am trying to do. I am looking to store solar energy in the summer for use in the winter.
 
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Old 03-15-17, 04:53 PM
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I am looking to store solar energy in the summer for use in the winter.
That you cannot do. There isn't a way I know of in an above ground tank to store water with no heat loss.

You would look into solar hot water heating for heating the water during the day and using it that night.
 
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Old 03-15-17, 05:04 PM
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Mike, you are misunderstanding what I am trying to do. I am looking to store solar energy in the summer for use in the winter.
You need to be more specific..

Solar hydronic is one thing... You must think about electric use to for the pumps to run. Whats your payback for your project? What are you trying to accomplish?

Electric solar can charge batterys. Use at a later date. Batterys can run a invertor that may run electric heaters. But you need some good AH batts. Like 6 volt golf carts.

But all this is money..

What are you trying to heat?
 
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Old 03-15-17, 05:16 PM
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Old 03-15-17, 05:43 PM
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That you cannot do. There isn't a way I know of in an above ground tank to store water with no heat loss.
It absolutely can be done and has been done. With an above ground tank? No. But That wasn't what I was suggsting. And whether it makes sense for me to do is another story. But I thought I made that clear in post #5.
 
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Old 03-15-17, 06:23 PM
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OK, I'll bite. If it can be done how do you store water for months (summer to winter) and retain the heat for winter use?
 
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Old 03-15-17, 06:23 PM
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No. But That wasn't what I was suggsting. And whether it makes sense for me to do is another story. But I thought I made that clear in post #5.
Since I am technical minded and need to know all details ( Which you have not provided IMO) , I probably cant help you.

But it seems your question below is all you want answered.. ( I was just trying to read between the lines) which it seemed I failed at..

Heres all the info I got.

I understand that it takes 1 but to raise 1 pound of water 1 degree F. Is it possible to reverse the formula to achieve my results?
Im sure you read this..


1 BTU=1 DEGREE FARENHEIT INCREASE IN ONE POUND OF WATER.

8.34 POUNDS IN ONE GALLON

BTU EXAMPLE:
Incoming water 50F outgoing water 120F 120 - 50 = DELTA T 70F
BTU's for 1 gallon of water is 70 (delta T) * 8.34 (pounds per gallon) = 583.8 BTU's
Flow Rate 2 Gallons per Minute
2 Gallons * 60 minutes = 120 Gallons per hour
120 Gallons * 583.8 BTU's per Gallon = 70,056 BTU's per Hou
r

I want to be able to use that energy to heat my home at a later date.
I was thinking thermal solar collectors and/or picking up heat from my yet to be asphalt driveway during the summer.
I don't need water much hotter than that for my coldest days. Honestly, I probably could do with 120.
You sure?

I expect to have to use a heat exchanger between the tank and the heat source. And I probably should use one between the tank and the heating system in my house. So I think that will contribute quite a bit of loss having the two exchangers.
Whats the heat source? boiler, air handler?
 
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Old 03-16-17, 04:40 AM
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OK, I'll bite. If it can be done how do you store water for months (summer to winter) and retain the heat for winter use?
Insulation .
 
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Old 03-16-17, 04:44 AM
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I don't need water much hotter than that for my coldest days. Honestly, I probably could do with 120.
You sure?
Yes.

Whats the heat source? boiler, air handler?
My home heating system is a boiler.
 
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Old 03-16-17, 06:01 AM
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"insulation"... absolutely brilliant. While you're at it how are you coming on a perpetual motion machine?
 
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Old 03-16-17, 10:54 AM
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I don't quite understand the poor attitudes here. No one is obligated to respond.
 
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Old 03-16-17, 11:50 AM
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It's not poor attitudes. You are reaching for things that are not practical.
You can't practically store large amounts of hot water. The best insulated tanks have loss.
Short time water storage is possible but not for months.

If you really want to look into energy savings.... look into geothermal heating where groundwater is used for heat.
 
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Old 03-16-17, 01:16 PM
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It is not poor attitude. You are talking of something akin to a perpetual motion machine and your only answer to this technical issue is "insulation".

Measure the temperature of the water in your water heater. Turn it off and track it's temperature over the following week. Now try insulating it however you want and track it's temperature loss. Then come back and explain your plan for storing heat in water for months.
 
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Old 03-16-17, 02:04 PM
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Let's say one has a super-insulated home with an average heating load of 10000 btu/hr. That's a very well insulated and air sealed home.

Let's say the heating season is 3 months. 3 months is about 2150 hours.

That means you need 21,500,000 btus to heat it for those 3 months.

So starting with 120 degree water and ending with 70 degree water, you would need about 37,000 gallons of water, neglecting transfer and efficiency losses.

Starting with 150 degree water and ending with 50 degree water, you would need about 28,000 gallons of water, again neglecting losses.

37000 gallons is 4921 cu feet, or the volume of a 17x17x17 cube.

It would weigh about 155 tons.

Go for it!
 
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Old 03-16-17, 02:45 PM
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My home heating system is a boiler.
OK lets speak logical for a moment.

Do you want to heat the whole home?

How many ft of baseboard element is in the home you want to heat?

How many sq ft is the home?

Im just trying to tackle this from a plumbing perspective.

I entertained the idea of a system like this for my home, and use my HWH to heat the home with circ..

Never fully investigated though..

https://www.youtube.com/watch?v=bCbg9XCoR_8
 
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Old 03-16-17, 02:52 PM
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This one too I toy with as I have 4 residential solar panels I acquired that I am doing nothing with..

https://www.youtube.com/watch?v=M1YFqBl6Cxs
 
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Old 03-16-17, 04:00 PM
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It is poor attitudes. I will refer to post #5 again. The first paragraph: "Yeah. This is definitely more of a mental exercise, and I'm well aware that this is unlikely to be practical."
 
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Old 03-16-17, 04:03 PM
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It's not even close to a perpetual motion machine. At no point did I suggest 0 energy loss. That is what a perpetual motion machine is.

I didn't invent this idea myself. It has been done before with great success. I would share the article I read, but it was several years ago.
 
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Old 03-17-17, 06:17 AM
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In order to make this mental exercise feasible you have to come up with a storage method. So far you have only gotten "cranky" and not proposed any ideas, theories or even wild guesses. So, if this is a mental exercise as you state... so far have not been playing along.

There are many industrial situations where large volumes are well insulated. None of those storage methods, even with millions of dollars spent, are efficient enough to make residential long term heat storage remotely cost effective.

Municipal power systems have used thermal storage but it's generally only for a period of hours to go from day to night or vice versa. Cryogenic gasses also use super insulated containers. The storage of liquid natural gas is probably the longest term storage of large volumes and they still have significant boil off. Vacuum flasks/dewars are quite efficient but are expensive and still have significant/measurable heat loss that is acceptable for shorter term storage but long term would leave you with very little stored energy. Even in the very good vacuum of space conduction and convection is nearly zero but a lot of heat is still lost though radiation. Just ask the astronauts of Apollo 13.
 
 

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