# Solar heated water - Evac tube specs - Ur thoughts?



## Solar+Wood (Apr 20, 2012)

Hello all.

Been gone for a while, but am back at it again 

Have some specs on some evacuated tubes to heat water. Was hoping to incorporate them into the slab heat system going in. I have these in storage and am kind of stuck with them either way.

Does anyone have any idea how they rank in regards to heating water?

Has anyone here ever use evacuated tubes to heat water?

Anyone use them in a in-floor slab system?

I am going to go back and look at an old thread I started a year or two ago and re-read all.

Either way....
If I understand what I'm reading on the spec tag, which I may not, looks like they are supposed to create 3,942 MJ/Year... about 3,736,295 BTU/Year... which breaks down to about 10,236 BTUs per panel per day. The other attachments are from a study.

I have NO idea what they are basing a "day" on in regards to solar irradiation. I assume the numbers include longer days in the summer... dunno?

Anyone here have any thoughts?

Cheers,
S+W


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## jimbom (Apr 21, 2012)

Solar+Wood said:


> ... am kind of stuck with them either way...
> Anyone here have any thoughts?


If you have them and you are going to use them, consider testing one at your location, azimuth, and tilt. Gary at Build-it-Solar has a great collection of his work on line:
http://www.builditsolar.com/References/Measurements/CollectorPerformance.htm
You will be better off with approximate numbers from your own test than all the fancy information published to market those tubes.


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## woodgeek (Apr 21, 2012)

Looks like 500W per panel in the sun.  How many of these panels are we talking about?  You can use PVWatts to get an estimate of sunny hours at your location broken down per month, and correcting for solar path and panel tilt, to estimate BTU/s per month.  In Colorado, IIRC your winter resource should be decent.  If you are going for an active system with storage, it seems that you would want it sized to meet a lot of your demand.  Or you could just use it to keep up a wood fired system in the summer for DHW?

Of course, after you crunch the numbers, you might decide that the panels in storage are dogs, you could sell them to someone else, and buy/build flat panels.  Gary will show you the way!


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## Solar+Wood (Apr 22, 2012)

Now why did the obvious escape me.... doh! I'm not so bright at times.  Hope to get one of them up in the next month.... that would be kool!

FWIW, I have 20 panels to create 30 m2 (322 ft2) of area....

After looking at the spec tag I came up with about 240,000 BTUs a day without any looses for glycol, length of run, etc.

I am looking at attaching a wood boiler (outdoor or indoor) as an option, but have NO idea about them and am starting to look into them. 

THX!
S+W


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## woodgeek (Apr 22, 2012)

Solar+Wood said:


> Now why did the obvious escape me.... doh! I'm not so bright at times.  Hope to get one of them up in the next month.... that would be kool!
> 
> FWIW, I have 20 panels to create 30 m2 (322 ft2) of area....
> 
> ...


 
That's a lot of panels!  If you built enough storage for few days, you could meet most of your space heating and DHW if you wanted (and had room to mount them).  A wood boiler would be good backup, and you would likely never need to fire it in the summer (and could heat a pool with your excess).


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## jimbom (Apr 22, 2012)

Store the heat in the slabs. Start at page 305 for a discussion of this idea.
http://www.byg.dtu.dk/upload/institutter/byg/publications/rapporter/byg-r091.pdf

Here is a French system. http://www.iea-shc.org/task26/publications/data01.htm

These are from your neck of the woods. http://www.radiantsolar.com/solar_heating_options.php
http://solarhowto.us/SimpleElegantSolarHotWaterDrainBackSystem.html

Slab storage makes a lot of sense to me. We use slabs for floors. We use storage for solar hydronics. Why not combine the two.


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## GaryGary (Apr 22, 2012)

Hi,
The efficiency curve (the last of the three plots you attached) looks like a pretty typical evac tube to me.  The intercept value is 57%, which is quite typical of evacs, and the down slope of the curve is low, meaning it maintains its efficiency well as the difference between outside temp and collector temp gets larger.    You can look at the SRCC site in the OG-100 collector ratings page, and compare it to other collectors: https://securedb.fsec.ucf.edu/srcc/...end_date=&results_per_page=400&submit=Summary
compare your aperture area curve to the values given on the SRCC reports.  But, I think you will find your collectors are fine.

Some material on collector efficiency here: http://www.builditsolar.com/References/Measurements/CollectorPerformance.htm
You can see on the one plot how evacs and flats compare -- in most cases they end up giving similar results.

330 sf is a very worthwhile amount of collector area.  Its way more than you need for domestic water heating, so the space heating is a good idea -- it should provide a very worthwhile amount of heat over the season.  

How large and area are you heating, how well is it insulated, and what kind of climate are you in?  

There are some example systems on this page:  http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm  see the section on "Active space heating - water".

-- some of the examples  heat the slab directly and some heat a tank and then use radiant floors or other means of heat distribution.
Either method works -- the slab is nice and simple and cheaper, but you have less control and you have some limits in that you won't want to be in the room if the slab gets too hot 
Slabs that are heated need insulation under them, so if you don't have insulation under the slab, I'd go to another way of distributing the heat.  There are a bunch of ways shown here: http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm#Distribution

With any solar system, the lower the temperature you heat the water to, the more efficient it will be -- that is, its more efficient for a solar collector to heat a lot of water up by a relatively small amount vs heating a little water really hot -- this is just because the hotter the collector runs the more heat it loses out the glazing to the outside.  This is why solar and radiant floors work well together -- the radiant floors can heat with warm water rather than hot water.  This tends to be less true with evac tubes because they are more efficient at high temps, but its still good to use a distribution system that does not require high temp water if you can.

Gary


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## Tatnic Corners (Apr 22, 2012)

Wow, you have the makings of a great solar heating setup  Definitely go to Gary's build it solar site. make a tank to store the heat in, the plans on his site are really simple and easy to follow.

Look into nofossils site as well. He has a set up that you could emulate, solar to heat the water, oil back up, and his ncfs controller to control it all.
http://www.americansolartechnics.com/
Tom from American Solar technology in Maine has some really easy to construct and break down storage tanks. Pricey, but they may be exactly what you are looking for...


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## Solar+Wood (May 6, 2012)

Gents,

Thanks for the info! 

I've been tied up with builder issues and am a little more free now to start reading everything written here.

As for space...
Trying to heat 5,600sq ft of ranch style buildings. Multiple slabs. Three actually in two buildings. One will be part of a pseudo greenhouse (~600sq ft) and the other two will be split between garage and living space.  Each slab with 1 to 2" thermal break from abutting slab.

Windows and curtains will be used to capture and retain as much passive heat as possible.

Going with ICF below grade for cement work, blue foam under slab and closed cell spray foam for the walls and ceilings. Thickness will depend on how much money is left.... hope to get R-values of 4" in walls and 6" in ceiling at a minimum (R26 and R39 min). May have to spray an inch or two on all and then use pink bats to get the desired R-values.

Doors and windows are decent R-value stuff.

Will look into tanks mentioned, but am hoping my brother-in-law will be able to weld me up something to shave cost....this is gotten out of hand financially.
So.... anyone out there that has a good plan on that and is interested in sharing would be a blessing. Since that will be the heart of the storage area... I'm trying to get it right the first time. Either SS or aluminum with at least 6 in/out ports. 2 ports for fossil fuel back-ups (LP and wood), possible 2 for panels...and a spare or two for future heat sources or mx issues.. Figure glycol mix?

Actually... if anyone has an idea on what would work best in regards to tank order/priotity and use, I'd be all ears.  Maybe the links above will answer my questions and guide me. 

FWIW, I have had these two Weil McClain indirect fired Plus tanks and these panels in storage for years now with this little dream of going mostly solar for heating --and would like to see it all come together.

For those interested, the Mclain "Plus" tank has about a 17gallon non-potable side and 70 gal potable side.  The home-made tank would be probably 400gal.   Trying to figure out what to use, what not use and any order that works best in a real world application.

Off to the batcave with my aluminum foil made propeller topped hat to read all the links!
S+W


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## Solar+Wood (May 11, 2012)

Well....

Unfortunately it looks like this will have to be scaled back considerably after reading all of those links. 
I will have to focus on the living area only. MUCH smaller sq ft area to heat and more controlled envelope.
The garage and all other pie-in-the-sky dreams will have to be put on hold. 
Prob plumb it all, but wait for extra cash and get a wood and/or LP boiler in the near future.

Knew I should've played the MegaMillions <g>

S+W


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