Author Topic: thermosiphoning solar water heater  (Read 5004 times)

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spotlifter

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thermosiphoning solar water heater
« on: November 30, 2004, 09:19:25 PM »
I'm building a thermosiphoning solar water heater, I have two used 4' by 8' panels that I'm going to connect to a heat exchanger in my attic thats connected to two 40 gal water heaters. The horizontal distance from the panels to the heat exchanger is about 30' and the vertical distance is about 10'.

My heat exchanger is a tube in shell type with 3.6 sq. ft. of transfer area. I'm planning on connecting the the panels in parallel because I hear series has a higher pressure drop. I'm going to use pex pipeing because it's cheaper and easier than copper, but I dont know if I should use 3/4'' or 1'' pipe. The outlets on the panels and the heat exchanger are both 1''. 3/4'' is much cheaper than 1'' even with the added reducers 3/4 would be $40-$60 less than 1''. Of course 60 bucks is nothing if it's the difference between a system that works well and one that doesn't. There will also be a small thermal expansion tank connected near the heat exchanger and the fluid will be a water anti-freeze mix. I'm not sure what the percentage will be, I have to find out what is available, that is animal safe and see what the bottle says. Will 3/4 pipe require too high of a temp to circulate and thus have high heat loss? Or will they both work equally well making 3/4 better because it's cheaper and easier to insulate?  
« Last Edit: November 30, 2004, 09:19:25 PM by (unknown) »

Chagrin

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Re: thermosiphoning solar water heater
« Reply #1 on: December 01, 2004, 12:09:38 AM »
The web's wisdom suggests that a minimum 5 degree angle in the piping is required for thermosiphoning to work, so your 30' horizontal/10' vertical should be OK, assuming that the 10' vertical is to the top of the panels.


At the low flow rates you'll be seeing (maybe 1GPM?), the decrease in flow rate is going to be pretty puny regardless of 3/4" or 1" tubing - it's on the order of 10 ft/s velocity decrease per 1000 ft with 3/4" tubing. Design considerations like putting the panels in parallel are going to be much more important, and you should consider putting the tubing inside the panels in parallel as well (you know what I mean...). Any air bubbles will really screw your flow rates.


I'm not sold on the idea of using PEX inside the panels, however. Copper sandwiched between aluminum flashing should be much more efficient and require a lot less tubing. My thinking is that the insulative value of the PEX will be close to that of the panel's glazing, so what's the incentive for the heat trapped inside the panel to migrate through the pex rather than through the panel's glazing?

« Last Edit: December 01, 2004, 12:09:38 AM by Chagrin »

merkurmaniac

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Re: thermosiphoning solar water heater
« Reply #2 on: December 01, 2004, 06:50:29 AM »
I am hoping to build out my system that sounds very similar to yours.  I would not use PEX in the solar panel boxes, but I don't think that is your intention anyways (not how I read it.)  I am also planning on running PEX tubing from some solar water heaters I bought on eBay (copper internals) to a single 40 gal water heater.  I actually have a second one, but I am considering running a winter and a summer configuration.  One of the water heaters is stripped and painted black.  It will be my summer heater.  My attic gets to 120°F in the summer here in Houston, so I may run the solar heater and maybe a car radiator or transmission cooler in my attic to try and cool the attic while heating my water.  I have a second water heater that I have left insulated.  It might be my winter tank.  My attic was 40°F last night.  I was thinking about a small controller that would take water from tank as long as it was warmer than the cold water inlet to the house.


I have an on demand water heater made by Takagi that I installed.  I just love it.  It can utilize pre-heated water and just uses less gas.  I hope to pre-heat my water using solar from say... 80°F to 100°, then use the gas heater to bring it up to 120°F.  I would theoretically cut my gas demand in half.


The big problem that I face with the thermosiphon arrangment is that the panels will lay on the roof, and the outlet should come off the top and go into the roof thru a pennetration, rather than go back downhill to get under an eave.  THat kind of sucks.  Ease of piping may preclude the thermosiphon route for me.  I might have to add a small solar powered pump at a cost of about $100 which would take a long time to payoff in savings.

« Last Edit: December 01, 2004, 06:50:29 AM by merkurmaniac »

spotlifter

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Re: thermosiphoning solar water heater
« Reply #3 on: December 01, 2004, 10:57:43 AM »
The pex won't be in the panels, the panels are already built, they're copper tube on a copper absorber plate with 1' headers placed verticaly connected by 22 1/2' tubes spaced about 2' apart. All covered with a piece of tempered glass in a insulated box. 1gpm huh? If that's the case I think it will work fine, if my heat exchanger is reasonably efficent it will heat up my 80 gal storage tank in a couple hours of good sun.
« Last Edit: December 01, 2004, 10:57:43 AM by spotlifter »

Ex Mek

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Re: thermosiphoning solar water heater
« Reply #4 on: December 02, 2004, 05:47:12 AM »
  Vehicle antifreeze mix is between 30 and 50 % i would go for 50% with a corosion inhibitor in it
« Last Edit: December 02, 2004, 05:47:12 AM by Ex Mek »

spotlifter

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Re: thermosiphoning solar water heater
« Reply #5 on: December 02, 2004, 10:57:01 PM »
Put your panels on the ground if you can. thats where mine are. you will get more hight to thermosiphon and the panels will be easier to access if they need repair. avoid sensors and pumps if possible, in my experience electric gadgets always fail much sooner than solid mechanical systems.
« Last Edit: December 02, 2004, 10:57:01 PM by spotlifter »

jamespipers

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Re: thermosiphoning solar water heater
« Reply #6 on: September 13, 2010, 01:55:10 PM »
The setup of your water heater is just fine. The dropping of the flow rate is not big so has not major effect. It is a common reason that some of people want to make it in parallel.

TomW

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Re: thermosiphoning solar water heater
« Reply #7 on: September 13, 2010, 06:01:08 PM »
This post is only 6 years old. Wonder if the OP is watching?

Tom