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PanelCrafters
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Posts:1329
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| 12/06/2007 6:40 PM |
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Posted By billmh on 12/06/2007 6:20 PM
So, that said, is there a resource that I can use that would for example, tell me how large a storage tank would be required to provide X number of BTU's for Y number of hours.
Bill,
It's pretty simple: Your Heat Loss per hour x 24 hours is what you'll need for one day. Then calculate the amount of heat in your storage medium. If you are using water, and 90° is the lowest temperature that you can use, and you have 1 gallon at 100°, you have: 10° x 7.5(pounds of water) = 75 BTU's of heat stored in that gallon.
A good explaination from Purdue U is here.
Good Luck! |
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....jc
If you're not building with OSB SIPS(or ICF's), why are you building? |
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billmh
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Posts:37
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| 12/06/2007 7:10 PM |
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Thanks, that is what I was looking for. I remember reading about it somewhere.
Are there manufactureres of standard large size tanks for solar purposes? I ask this for when I was doing a superficial search I saw either the typical smaller 80 gal tanks that look like the standard hot water tanks or custom made for all sorts of purpses. If we go larger and need, lets say, 800 gals and 3 coils for input/output, would this tank have to be custom made?
Posted By PanelCrafters on 12/06/2007 6:40 PM
Posted By billmh on 12/06/2007 6:20 PM
So, that said, is there a resource that I can use that would for example, tell me how large a storage tank would be required to provide X number of BTU's for Y number of hours. Bill, It's pretty simple: Your Heat Loss per hour x 24 hours is what you'll need for one day. Then calculate the amount of heat in your storage medium. If you are using water, and 90° is the lowest temperature that you can use, and you have 1 gallon at 100°, you have: 10° x 7.5(pounds of water) = 75 BTU's of heat stored in that gallon. A good explaination from Purdue U is here.Good Luck!
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PanelCrafters
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Posts:1329
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| 12/06/2007 7:54 PM |
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Posted By billmh on 12/06/2007 7:10 PM
Are there manufactureres of standard large size tanks for solar purposes? I ask this for when I was doing a superficial search I saw either the typical smaller 80 gal tanks that look like the standard hot water tanks or custom made for all sorts of purpses. If we go larger and need, lets say, 800 gals and 3 coils for input/output, would this tank have to be custom made?
I'm sure that there are companies that make larger tanks, but cost usually dictates a 'custom made' solution. You can find used tanks, or for more $$$ buy a large tank(possibly heavy plastic). The tank wouldn't need to be 'custom' but the interface with the coil would be.
You might look at 1 smaller tank for DHW and a larger tank for space heating. Each tank should only require 1 coil(this is the closed loop that travels to and from the collectors). Of course, there is more than 1 way to do anything, so be creative!
Good Luck! |
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....jc
If you're not building with OSB SIPS(or ICF's), why are you building? |
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Quantum
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Posts:262
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billmh
Registered Users
Posts:37
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| 12/06/2007 8:04 PM |
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| Yes, that was the way the system in my last house was configured, but that only provided house hot water and a few radiators during the winter. Now adding radiant flooring and using either water/gycol I would expect that a larger source than 80 gals may be required. But I am only guessing, so it's time to get the old calculator out and see just what may be required. |
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PanelCrafters
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Posts:1329
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| 12/06/2007 9:06 PM |
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Posted By billmh on 12/06/2007 8:04 PM
Now adding radiant flooring and using either water/gycol I would expect that a larger source than 80 gals may be required. But I am only guessing, so it's time to get the old calculator out and see just what may be required.
Absolutely. Now you know(or will) why SH is using a 6,000 gallon tank. But, also realize, that he's producing 100% of his heating(and electrical with PV) needs. And, the cost of fuel will just continue to escalate. |
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....jc
If you're not building with OSB SIPS(or ICF's), why are you building? |
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dennismedeiros
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Posts:10
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| 12/06/2007 11:44 PM |
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Hi All,
I have been reading this thread with much interest, especially since I started it. Perhaps I can answer some questions.
As someone pointed out, a systemdesign starts with calculating the heat lost from the house plus the energy used for hot water. In a well insulated house this might be as low as 10000 btus/hour or in a large house with nominal insulation 36000 btus /hr. Average. The actual loss is computed knowing the average yearly outside temperature and the desired indoor temperature. In sizing a solar system we use what is called degree days. One degree day is when the average temperature for the day is one degree below the desired indoor temperature. If you want to keep your house at say 70 deg F and the ouside average temperature is 30 degrees for 10 days straight then the number of degree days is (70-30)*10days or 400 degree days. The NASA Surface Meteorology site ( http://eosweb.larc.nasa.gov ) maintains historical records of heating degree (inside temperature at 65 degs) days for any latituude and longitude over a full year. The significence of a years data showing the heating degree days is that we can compute the average loss from your house month by month. Knowing the insulation properties of your home called UA value and degree days, the product is a good indicator of the btus loss per day. More on this if anyone is interested. Do not cofuse this method with Design Temperature which calculates a worst case day and is used to size your boiler.
the formula BTUs = #gallons* 8.33 lbs/gal (weight of water)* Temperature change gives the heat energy stored in a tank
Using the losses we stated above a 10000 btus/hour is a daily loss of 24*10000 or 240,000 BTUs per day If the lowest usable temperature of the tank is 100 degs F and the highest temperature after a full day of solar charging is 180 degs F then a change of temperature is 80 degs. Putting these in the formula and solving for gallons gives 360 gallons required. The higher loss rate of 360,000 BTUs /hr would require a tank of 1295 gallons.
Most designs do not attempt to store 100% of heating needs. 40% to 60% is more practical and is usually supplemented with a tankless or conventional boiler. The reason is both technical and economic. If your system is too large many solar panels are require to generate that heat but you are only using the radient heat 6 months of the year. the other months you require just hot water (only requiring a fraction of the number of panels).
Trendsetter is one company that manufactures tanks as large as several thousand gallons. They are custom and expensive. You can make a tank yourself but you still need good heat exchangers that can transfer the large heat load into and out of the tank and you need need to insulate the tank to the max. Even with drainback tanks you need to use a heat exchanger to get heat out of the tank. If might be possible to use only a heat exchanger to separate the water you drink from the water that heats your floors. Put heated water into the tank directly from the collectors and use the water directly from the tank to heat your floors (you will need thermal mixing valves or a multi pump mixing block to control the heat to the floors)
I guess this was a little long but hope it helps
Dennis
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billmh
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Posts:37
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| 12/07/2007 5:45 PM |
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| Dennis, it was very informative. I have the ability to do the calculations as you stated, but since the house design is not finsihed I have not done so. And I agree with you that once one determins their heat requiements then a decision will need to be made as to how much solar you can afford vs conventional. That's what its all about, isn't it? |
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PanelCrafters
Registered Users
Posts:1329
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| 12/07/2007 7:18 PM |
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Posted By billmh on 12/07/2007 5:45 PM
Dennis, it was very informative.
Yes it was. However, your goals may determine what basis you use for your calculations.
And I agree with you that once one determins their heat requiements then a decision will need to be made as to how much solar you can afford vs conventional. That's what its all about, isn't it?
Sort of. Where you live(solar isolation) has a big impact on the amount of energy that can be generated. So, you need to understand that also.
One thing that can and should be done in a heating dominated area, is to Super Insulate. The cost for doing this will likely be less than any(most?) active solar installations. And, if you do utilize active solar your heating load will be considerably reduced allowing the solar to provide a greater percentage of your heat. If you looked at Solar Harvest you'll see that is what they did.
It all boils down to your goals. Once you know what you want, making the decisions becomes easier. |
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....jc
If you're not building with OSB SIPS(or ICF's), why are you building? |
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billmh
Registered Users
Posts:37
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| 12/08/2007 4:11 PM |
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| Yes, you are right about determining your basis. We will be building in NM and have decided on adobe as the wall material. Adobe has some interesting characteristics, mainly being cooler in the summer and warmer in the winter, which is sought after here in the SW. However, these properties are known but not quantifiable....yet. It has do with it's ability to absorb moisture and the resultant latent heat affect. So, we are left with treating it as concrete, which it does have similar mass numbers, but knowing that in the end less cooling and heating will be required. Include passive and active solar into the design and you get a winning combination. |
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caja_ca
Registered Users
Posts:14
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| 01/01/2008 10:46 PM |
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| I think if you are looking to retrofit what you have, with the boiler...I'd look at one of those tanks that circulates solar heat in the bottom half and the top half is assisted by the boiler. Tank is about 600 litres and if you can plumb it yourself it should be fairly reasonable cost. Just learning this stuff too. |
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Topgas
Registered Users
Posts:64
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| 02/27/2008 8:44 PM |
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| I'm with Dennis and Quantum..... |
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Dteltech
Registered Users
Posts:9
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| 02/28/2008 3:31 PM |
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I love a good debate on thermodynamics and heat transfer. They always have a happy ending and physics always wins.
The problem is to recognize all of the characters in this (thermal) calculation, where it is going to exist (local environment), the personal reasons for making this investment (payback or payforward) and are you enjoying the process!!!
To try and fail is far better than to have never tried at all.
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They lacked the knowledge of houses - Aeschylus |
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Tomsos
Registered Users
Posts:1
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| 03/16/2008 6:07 PM |
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March now and the winds in NM are blowing.
Saw the last post from HO regarding adobe.
Cool and warm adobe houses are functions directly related to the thickness of the walls.
I can show houses that have 8" -10" made on site adobe walls that are horrible to heat and cool.
I can show older (1890) houses with 24" walls that work.
NM energy code can actually require exterior insulation for the thinner adobe walls.
What are you using?
Regards. |
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