# Bulk solar water heaters



## Runners (Nov 6, 2003)

Has anyone played around with bulk solar water heaters?

The big black tank laid on it's side, reflector behind it, around it, inclosed in glass or a polycarbonate panel... The sun heats the tank and you use a heat exchanger to temper your incoming water to your "real" water heater.

I keep looking at these 2 round tanks I got, 300gal & 600gal, and other than small grain storage bins, I wonder how they might work for preheating water. Another possibility could be those 275gal fuel oil tanks...

Any ideas or experience?


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## mightybooboo (Feb 10, 2004)

Its been mentioned here before using an old refrigerator for the box.Put on a glass cover(Like a sliding glass door for example},insulate it well.Open the door and reflectorize it to aim more light in.Close the door at night so it doesnt freeze.You lay it on its side and angled for best solar gain.

BooBoo :gromit:


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## WisJim (Jan 14, 2004)

A long thin (smaller diameter) tank works better, more surface are to absorb heat per gallon of tank. The large tanks that you mention will get a bit warm, but probably won't raise the temp much. Of course, as just preheating ground water (probably 50 degrees out of the well?), they will make some difference. But a smaller tank, or tanks, in series, will give you a higher water temp, even to the point of not needing other water heating some of the year.


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## SolarGary (Sep 8, 2005)

Hi,
As WisJim says, those big tanks have a lot of volume for the surface area they have. If you put such a tank in a glazed box it would heat up some, but not a lot. 

You could improve the performance by including reflectors that direct more sun onto the tank. The reflectors could be inside or outside of the glazed box.

The rough rule of thumb is that you want something like 1 sqft of solar collection area (i.e. the front of the glazed box) for each 2 gallons of water -- so a 275 gallon tank would want about 140 sqft of glazing (about 12 ft by 12ft!), and probably some reflectors inside the box to direct the collected sun at the tank.

I think several smaller tanks might be a more manageable arrangement?

Lots of good ideas on these types of collectors here:
http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm#Batch
and
http://www.builditsolar.com/Projects/WaterHeating/ISPWH/ispwh.htm

Another thing you might look at is the one called a "horizontal pond domestic water heater" -- its about half way down this page:
http://www.builditsolar.com/Experimental/experimental.htm
For your case, where you want to heat lots of water, it could just be scaled up, and would probably be cheaper than the above solutions. Make sure the pipe coil in the pond holds enough hot water in the coil itself to meet one deamand, because the heat transfer from the pond to the coil is not lighting fast.

Gary


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## Jim S. (Apr 22, 2004)

You can improve performance even more in large tanks by installing a heat loop system wherein the tank is physically located above a finned radiator/solar collector painted black. The cold water falls from tank into radiator, where it is heated and rises back into the tank, forming a loop by convection. It is ideal if you have a small hill or dirt mound to locate the tank on top and the collector below.

You can make a solar collector to serve this purpose. See:

http://www.bigginhill.co.uk/solar.htm

about building DIY solar collectors.

You can spray foam insulate the tank, then paint the foam or cover it with plastic to improve heat retention. Or locate the tank in an insulated shed building made from scrap. The whole thing works better with vertical baffles in the tank, but you can't be picky when scrounging for surplus.

You may get free hot water if you scrounge hard enough for all the materials. Good luck! I wish I had 900 gallons of storage!


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## Runners (Nov 6, 2003)

..this will require more investigation, for sure! I have 8 kids and a 40gallon electric heater - far from enough!

I got a 11' fiberglass dish I was thinking of running a couple of 1" tubes up to the focal point, into a 4" pipe (copper?) and putting a small pump on it. Pump it back to a heat exchanger, expansion tank, etc. Or some alternative kind of heat transfer liquid...?

When adding up the pump and tracking mechanism - I wonder if the heat gain would pay off?

Does anyone have pics of a WORKING home built solar collector and a little data they could post? Thanks for the ideas so far, this kind of thing might be a good project for me and the kids!


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## Runners (Nov 6, 2003)

I wonder what would be easier way to maintain a steel tank's integrity.

a. clean the inside good and coat it with paint or some kind of plastic or epoxy (it would need to withstand temperature cycling.

b. leave it alone and make it a closed system w/expansion tank, relief valve etc., add an oxygen scavenger (chemical used in boilers) and purge it with CO2 after a few cycles.

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Another question - I noticed some of the heat exchangers (from the collectors) were set off to the bottom & side of the tank. Is to to promote more even heating of the 'bulk storage water' - it would circulate easier.... prevent 'stacking' (hot layers of water on top)? 

Or, would it be better to try and create stacking within the tank, if the domestic heat exchangers were mounted near the top?

Lastly, is there any advantage of using a long vertical tank over a short fat tank - if so (either), or hasn't anyone tried it?

I'm thinking really hard about the weight load of 2500lbs or 5000lbs and what's gonna happen to my basement floor - even though I could spread it out over a 6x6' or 8x8' area with timbers & insulation... that's A LOT of weight! Maybe outside in an insulated pit? 

Or cutting the end off the 600 gal, putting the 300 gal inside it and packing insulation around it - probably have to rig up legs for the 300 gal to keep it from compressing the insulation though...

One MAJOR problem - our area has acidic water, copper is dissolved in a couple of years when subjected to heat.

I appreciate the ideas, and want to use as much of the stuff I have on-hand!


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## SolarGary (Sep 8, 2005)

Hi,
Not really sure about the answers to most of your questions, but here are some examples of tanks that are similar to what you are considering (or at least very simple):

http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm#Storage
This includes Eric (from this forum) whose tank is just an insulated carefully dug hole in the ground -- this seemed to work fine.
The STSS tanks are basically just sheet metal formed into a cylinder, and then insulated and lined with and EPDM liner on the inside.

Paul's system uses a reclaimed oil storage tank:
http://www.builditsolar.com/Projects/SpaceHeating/Paul/PaulSolShed.htm
There are pictures of it toward the bottom -- he basically cut one end off.

If you make it a closed system, you will have to have some way to make sure the pressure never gets very high -- tanks like this are not made to withstand much internal pressure.

The books say that temperature stratification in thermal storage tanks is good. I think the idea is that if the collectors only partially heat the tank, the energy is more useable if it is a hot layer on top of cooler water rather than a uniform small increase over the whole tank.

Gary


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## MELOC (Sep 26, 2005)

no real expertise in boilers or closed systems here, but i have been using my hot water heating system for many, many years and i only notice a small amount of oxidation. there is no real rust. the water gets a black tone and the oxidation seems to progress no further. the water will only react so much with the metal before it can no longer react without added water or air.


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