Author Topic: Solar water heating with heat exchanger  (Read 1057 times)

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madlabs

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Solar water heating with heat exchanger
« on: February 23, 2017, 12:14:31 PM »
Hi All,

In my relentless pursuit of off grid hot tub perfection, I have gotten a heat exchanger so that I can run glycol through my evacuated tube array instead of water. Brominated or chlorinated water isn't good for the copper manifold. It is an extra hassle but the array wasn't cheap.

Of course, having a closed side to the system requires some kind of consideration for expansion and over pressure relief. So, my first thought on how to go about this is to get a automotive radiator cap and do it like they do in cars. Have a container for over flow and pressure release if needed, as well as allowing coolant back in when everything cools down and contracts.

Opinions? Think this will work? And if so, what is the easiest way to get or make a radiator cap seat? I don't have rigs with a brass neck any more and I can't seem to find one yet. EDIT: Found one easy peasy.

Thanks!

Jonathan
« Last Edit: February 23, 2017, 12:30:29 PM by madlabs »

OperaHouse

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Re: Solar water heating with heat exchanger
« Reply #1 on: February 23, 2017, 03:46:21 PM »
Well, all the new vehicles have expansion tanks and that would solve two problems.  Besides a space to expand to it gives you something to screw that cap on.

Ungrounded Lightning Rod

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Re: Solar water heating with heat exchanger
« Reply #2 on: February 24, 2017, 08:00:59 PM »
Doesn't the cap have to be at the high point, to remove air bubbles?

And then doesn't the overflow reservoir need to be only a feet below it, so the refill on cooling doesn't cavitate and the vacuum pulled isn't strong enough to cause damage?  (It will pull about one PSI of vacuum per two feet of height difference between the pressure cap and the surface of the liquid in the reservoir.)

I think you'll need to position the line and the reservoir so they are shaded, so the sun doesn't rot the hose or the plastic (which are designed to live in the engine compartment, which is shded all the time.)

Other than that it should work just fine.

(I was thinking about using the same approach for a chillwater line from an external chiller to a heat exchanger in the furnace air plenum.)

Eraser3000

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Re: Solar water heating with heat exchanger
« Reply #3 on: February 25, 2017, 07:25:28 AM »
I run my closed system with a few expansion tanks.
The just need to be sized properly for the amount of volume of fluid you have in your system.  This way you can run with no oxygen being added every cycle.

Eraser

tanner0441

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Re: Solar water heating with heat exchanger
« Reply #4 on: February 25, 2017, 11:55:29 AM »
Hi

Look at expansion tanks and accumulators for unvented central heating systems. they run at one and a half Bar. They consist of a sealed tank with a diaphragm across making two chambers. The lower chamber goes to the circuit used for heating, the upper one has a schrader valve on the top to prepresurise the system. You could still need a pressure relief of some sort but they will operate at 1.5  to 2 Bar all day.

I am not sure what pressure you want to run at to prevent the glycol solution forming vapour locks. I would also have a pressure gauge somewhere on the system, also are you running as a thermosyphen  or pumped?

Brian

Ungrounded Lightning Rod

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Re: Solar water heating with heat exchanger
« Reply #5 on: February 25, 2017, 09:31:55 PM »
Look at expansion tanks and accumulators for unvented central heating systems. they run at one and a half Bar. They consist of a sealed tank with a diaphragm across making two chambers. The lower chamber goes to the circuit used for heating, the upper one has a schrader valve on the top to prepresurise the system. You could still need a pressure relief of some sort but they will operate at 1.5  to 2 Bar all day.

I like it.

I have similar devices in the well water system at the ranch and in the white water system in the travel trailer, to hold pressure and smooth out the water pressure changes as the pumps start and stop.  Once the pressure is set up initially they run fine for years with no attention.  (The one at the ranch has been in service for about 17 years.)

tanner0441

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Re: Solar water heating with heat exchanger
« Reply #6 on: February 26, 2017, 03:42:02 PM »
Hi

If you look on the dow corning site it gives the freezing and boiling point at atmospheric pressure. Boiling is only 110 C for a 60% solution.

So at +1 to +1.5 Bar could put it up into the range where you should get a more efficient heat transfer. If you have a variable speed pump you could use the temperature to control the pump speed.  We had a similar setup on the condensing unit on the AC units I was involved in.

Brian