started my solar preheat tank

[strider3700]

Ok I decided to use some of the cold evenings and work in the shop building what is hopefully the start to my solar hot water setup.  this type of heat exchanger was chosen because I had most of the stuff necessary lying around making it cheap,  it was within my building ability, I needed a double wall because I'll be using antifreeze and in the end it should also physically fit easy enough.  I previously asked and was told a preheat tank probably is the best choice as well.

Anyway I started with an old electric 20 gallon tank which isn't that old and still worked but was replaced when I redid my bathroom this summer.  Basically I stripped the outer metal skin,  cut out the insulation from the middle, wrapped 3oz copper foil around the middle and then wrapped two seperate 25' long coils of 3/8 refridgeration tuning around the tank.  I soldered the coils to the foil in a few places to partially hold the tubing in place and also to hopefully help transfer the heat to the tank.

I've got two seperate coils because I would like to connect the lower one to a solar collector and the upper one to the woodstove someday.

So here's what she looks like today.

I'm looking for suggestions on ways to increase the heat transfer to the tank some more.  soldering every inch of tubing down will cost a bunch in solder and probably won't be the easiest.  As you can see soldering isn't my strong point.  I remember seeing someone used plaster of paris to help absorb heat with a heating coil on a woodstove before.  Would that work in the opposite direction as well? any other recommendations?

The next step after deciding the heat transfer is good enough is to insulate, wrap it in reflectix and then build a box for it with the fittings protruding.  It's going in my crawlspace so I need to keep the mice out of it...

[Volvo farmer]

I don't think that's going to work very well. You are trying to transfer heat from copper, into a steel tank, then into the water. Your tubing isn't going to give up much of it's heat before it makes the trip back to the solar collector.

I'd suggest trying to build something like this so that you are transferring heat directly into the water.

http://www.butlersunsolutions.com/html/pr_solar_heat_exchagne_wand-46-reg.html

Either that, or look into those heat exchangers that folks have homebuilt using 1/2" copper inside of 3/4" copper. That system requires two pumps but is home-buildable.

[zeusmorg]

  Volvo's right, transferring heat in the manner you're doing isn't going to be very efficient. You need your coils in contact with the water, also at the lowest point where the cold water will sit, is preferrable to the middle.

 With an electric hot water tank, it's possible to pull out the old heater elements and pass your coils through those flanges, building new ones that your copper coils can pass through. You want your coil surface as large as your pipe surface in your collector for the best efficiency.

You can also use an external heat exchanger and loop pump it,that wouldn't require an additional storage tank.

[ZooT]

Quote:

That system requires two pumps but is home-buildable.

Would it require two pumps if perhaps the entire heat exchanger coil unit was located lower than the holding tank?

I guess what I'm asking is if convection/thermosiphoning could take the place of a second pump...

[strider3700]

the issue with the sun wand is that they are around $300 and none of them will work with my tank with the connectors on the side not the top.

The issue with the home made pipe in pipe exchangers is almost none of them are double walled so the run the risk of contamination should the smaller tube fail. It also won't pass code around here should someone check. also two pumps gets real expensive when it comes to the solar panels to run them.  I have 45 watts(which deliver about 30 most of the time) of panels already and would like them to run the entire system.

I'm aware that the current design is not very efficient but it is my attempt at copying a design by rheem.  I read in homepower that they are the only manufacturer producing a model like this  but they listed no downsides other then the high cost and only 2 sizes (80 gal & 120 gal) available.

If it's going to be an efficiency concern I can hook the top and bottom coils together giving me 50' of coil to transfer the heat with.  Also remember the water coming in is in the 6-8 degree range.  There is a huge temperature difference which should help.  The goal of the system is to reduce costs but it doesn't necessarily need to provide 100% of the heat. I most likely won't be shutting off the electric tank.  

[Old F]

Here is what I used for my heat exchanger for

my out door wood

stovehttp://oldf.homestead.com/Woodmasterishere.html

Old F

[GaryGary]

Hi,

There is a company called Thermon that makes a variety of heat transfer compounds.

They are intended for applications in which a large pipe is kept from freezing by "tracing" it with a small pipe that carries steam.  The compound is used between the two pipes to increase thermal coupling.  Some of their products come in caulking tube containers, so they might be pretty easy to apply?

I've never actually tried any of them, but it might be worth looking into.

www.thermon.com

Also, insulating well over the copper pipe coil would be good.

Gary

[DanG]

I know how much I detest 'after-the-fact-nay-sayers' though subjecting oneself to them is a right of passage around here. Doing something is good but presenting raw materials for suggestions before one starts doesn't seem to go over well, there is always something better to be suggested. Perhaps a new forum section called 'Brainstorming' is needed! : )

I am assuming the tank is a glass or glass lined hot water tank.

As it stands now the previous poster's idea of a thermally coupling compound would vastly increase the heat transfer - who knows, maybe the metal filings from a brake drum truing machine wetted with just enough epoxy to make it pack and adhere to the tank and coils would work, but there will be a cold 'stagnant' zone beneath the coil wraps. Having the cold water enter the bottom of the tank would help mix it up some and drawing off the hot water from the very top would still save energy since it is a tempering tank!!

If you are into tearing up what you have done, to maximize convection in a thermally stratified tank how about restoring the tank to a horizontal mount and running the coils lengthwise on the lowest 25% of circumference?

[electrondady1]

i can't see why it wouldn't work as long as there is good contact between the copper sheet and the tank.

the plaster of Paris idea is interesting.

maybe brush on a first thin coat so it gets in all the nooks and crannies.

 mix  some type of fiber (glass insulation) into the plaster to help it hold together.

 

[fcfcfc]

Hi: No, use two pumps pumping the fluid in opposite directions to set up a counter flow heat exchanger. Thermosphoning has to low a transfer rate. You want to maximize heat transfer. The head loss will be small through each side so you can use low head, low flow energy efficient pumps. Use TACO or Grundfus rated in the 30 to 40 watt range. If your collector loop is closed, you can use cast iron on the array side and bronze or SS on the potable side....

.....Bill

[zeusmorg]

 Well, I guess I'm going to have to get a bit technical as to why this is inefficient.

 Heat transfer is based on the thermal conductivity of the materials involved. see

http://en.wikipedia.org/wiki/List_of_thermal_conductivities for a list of their properties of various materials.

 This is using a steel tank with a glass lining, the glass lining does two things help holds corrosion down within the tank, and also acts as the first layer of insulation. Also, the coldest part should be used for transfer, heat rises it's as simple as that so the coldest water is in the bottom.

 Also this is not enough coil space given the size of the tank, common usage of heat exchangers declares that the area with large storage should be at least the size of the collector to get maximum efficiency. This doesn't apply to flowing heat exchangers just stationary, like tanks.

 Dual wall heat exchangers are quite common, the easiest to build is just wrap soft copper pipe around a larger one. of course a pump needs to be used with that design, low flow pumps don't use much energy.

 If you wish to call me a nay-sayer, feel free. However I am just offering the facts,trying to save someone headaches down the road before they get too far along.