Hot Water Thermal "Dump"

[WStayton]

Hi Guys!

  I've just been thinking, here, about what to do with the "excess" thermal energy that I will have in the summer time.

  I'm anticipating something like 120 sq-ft of home-made solar collector that will use heat exchangers to transfer their heat to three each tanks that have about 450 gallons each i them, so I have a total heat storage of about 11,000 BTU/degree F.

  My heat loads for domestic heating, during the winter, will easily keep this under control, but during the summer my only heat load will be for domestic hot water and I'm pretty sure that I won't need all the BTU's that accumulate to heat a couple of showers a day and a little hand-washing/dish-washing along with  load of dishes in the dishwasher every other day or so.

   So, how do I get rid of the excess?  I think it stand to reason that I have to circulate something through the panels when the sun is shining, just to keep from having a steam boiler and pop-off problems - so what do I do with the heat I collect?  I suppose I could just run the circulator 24/7 and let it radiate back off at night, but I hate to  spend the solar energy for that if I don't absolutely have to and, with my proposed system of a panel just for the circulator pump, with no battery, it will be a pain to do!  Not to mention throwing away the juice to run the pump that I could be selling to NYSEG! <grin>

  I have sort of thought about some sort of drainback system that would just let the panel(s) sit there empty if I don't need the heat, but that is yet another complication, that I'd rather not do if not necessary.

  So how do you folks solve this problem?

  Input is hereby solicited!  <grin>

Regardz,

Wayne Stayton

[Carlb]

If the panels are easily accessible you can partly cover them in the summer time.  We have two solar air heaters about 160sq feet total that we cover in the summer time.

Carl

[Tom Reed]

My system (still in the planning stage) is for a drain back system. Steam is not the only issue with over temp, the anti-freeze turns acid too. I like drain back also because it is a fail safe system. Glycol systems usually have a freeze protection valve that dumps the fluid if temps drop below a set point. I know people that have had this valve fail and their panels were destroyed. The valve is rather difficult to test for proper function too.

[WStayton]

Carlb:

   The panels are going to be on a not-too-steep sheet metal roof - the only issue is that yuou have to be careful to step on where one of the longitudinal 2" x 4"'s is, below the sheet meta, so as not to bend the sheet!

   What do you use for a cover?  Do you think that something like landscape fabric would work???

  Of course there is the issue of having to inspect them frequently witha fabric/soft covering because if the cover tore off and you didn't fix it, you could have BIG thermal problems - though in my case, with the amount of water-storage I'll have, it won't run away real quick!  <grin>

  Maybe something like a 1/4" or even 1/16" piece of plywood that would "drop-in" to the solar panel frame would be less hassle - just screw down a couple of cleats and its there for the summer???

  Then comes the question of how much should i leave open/exposed so that I still have hot water in the summer?  I see number's like 2,000 BTU's per sq-ft per day in the summer - with my home-made panels I'm thinking that i will probably be lucky to get 1,500 BTU per sq-ft per day, so if I left 16 sq-ft exposed, I could, theoretically, "harvest" 24,000 BTU per day.  If I put that many BTU's into a 500 lbs of water, I'm going to raise its temperature by 48 F, so my incoming 60F water will exit at 108F which is more than adequate for domestic hot water and I have a "cushion" of about 100% ion case I have a couple of days where I don't use much hot water.

  And, there is another benefit with a cover, it insures that I will climb up there at least twice a year for covering/un-covering and have the opportunity to give every thing a close look-over to make sure everything is okay.  If you don't HAVE to climb up there for anything, its easy to put it off and, then, suddenlyu have a problem that could have been easily fixed earlier but is now a catastrophe!   If your shower's are suddenly getting chilly, you DO have incentive to get-er-done!  <grin>

Tom:

  I, too, thought of a drain-back system, but in the interest of KISS, I was hoping for something that would let me not go to that extreme - maybe just covering up 3/4 of the panel(s) will solve my problem, without having to resort to something else in the system to fail and/or screw-up in some new and novel way???  <grin>

  Thanx for the input guys!  All comments are appreciated!

Regardz,

Wayne Stayton

[Carlb]

Percentage needed to be covered will depend on the efficiency of panels and how much storage you have and how much of the heat generated on an average day you use.  This will be trial and error.  I use a heavy weight white Poly tarp to cover my one hot air panel that is 3'x36'  and i use regular vinyl house siding to cover the other, these are both mounted vertically on the back of my house. 

[LowGear]

Where there's a way to procrastinate I'm your man.

Have the white or silver tarp ready for deployment.  (Most Costco tarps are silver on the "other" side and last at least a year here in Hawaii.)  Wait and see if you approach the magic number.  Skate by for the first season using a time tested system.  Then decide if you need the ultimate system.

Keep It Simply Smarty.

Casey

[WStayton]

Hi All!

  LowGear has an excellent point - as usual!!!  <grin>

  It is a whole lot easier to fine-tune how much of the panel you are leaving exposed if you can just turn back, or cover up, a bit more of it with what your already using rather than have to climb up there with a skill saw to shorten a plywood panel or drag ANOTHER hunk of plywood up there to add on to what you already cut too much off of!!!

  I suppose a couple of 5'x8' silver/white tarps lashed under the four foot wide panels with a few elastic straps through their side holes wouldn't be to much of a hassle (hassel??? whatever!) to install, either.

  Thanx for the heads up LowGear!  Sometimes the obvious stuff escapes me!  <grin>

  Oh, and I always heard Keep It Simple Stupid, isn't that the way it goes??? <smile>

  Oh, and I have been sitting here doing some figuring on what my actual tank capacity will be when they get built to fit in the space that exists and I found that I will only have about 7,500 ppounds of water, not 12,000 as previously advertised - but that is still enough that the response with a little piece of the panel exposed is still going to be lots less than instantaneous!  Probably have to leave it running in one condidtion for a week, or two,  to really see what its doing???

  Thanx for the input . . . all is appreciated!

Regardz,

Wayne Stayton

[LowGear]

Smarty works for me.

KC