Ben's Heat Exchanger #2

[bennelson]

Hi everyone,

Last summer I got to play around just a little bit with some solar hot water. I managed to build a tube-in-a-tube single-wall heat exchanger.

It was a fun soldering/plumbing practice project. The general consensus was that it was a nice little unit, but it didn't have much surface area to transfer the heat through.

A professional solar installer I spoke with told me that style is typically only used to tap off solar hot water from a larger solar space-heating setup.

Also, my house has some special limitations that I have to work around to design a solar hot water system.

1) I have REALLY hard water - very high iron content
2) I have a crawlspace (cement block walls, concrete floor) which is only 22" tall
3) Available space in my utility room would be about that of a 40 gallon hot water heater, but event that would block my existing gas water heater.
4) I live in a cold climate where freeze protection would be required

So, these limitations mean that I really can't use an on-demand water heater without a very expensive filter system. My solar hot water tank can NOT be bigger than about 20" wide if in the utility room, or more than 22" tall (including lid & insulation) in the crawl-space. On the other hand, it could be pretty TALL in the utility room or WIDE in the crawl-space.

Freeze protection means that it needs to use either a closed-loop anti-freeze system, or a drainback system. I originally sort of ruled out a drain-back, but rethinking it, it does have some advantages.

The main one is that doesn't use anti-freeze. That is actually TWO advantages. 1) Water is a more efficient heat exchanger than glycol. 2) Single-wall heat exchangers are more efficient than double-wall exchangers (which is what you are supposed to use with anti-freeze systems, toxic or not.)

I am planning on a 4'x10' solar hot water panel, ground-mounted horizontally, on the south-east corner of my house. There is exposed foundation there, and the ground slopes down and away from the house, so there isn't a real big height difference between there and room in the crawlspace. As long as there is enough difference in height for drain-back to work properly, it would be minimal head, so the pump would not have to work very hard at all to initially fill the collector.

HEAT EXCHANGER #2, Idea A: The BOX OF WATER

For a drain-back system, the solar storage tank could be easily custom made, as it doesn't need to be pressurized. It could literally be an insulated plywood box with a rubber liner. A long coil of copper pipe could go from my cold water line from the well, into the box of water that is heated by the sun, and then up to my existing gas water heater. This would be a very simple, single-wall, heat exchanger. I am going to call this setup a "Box of Water" for simple reference.

Another advantage to the "box-of-water" method is that it would be easy to add another coil of copper pipe it as either another input or output of heat to the system. For example, I could run hot water from my woodstove to a copper coil in the box of water to heat the water in the cloudy, cold, winter.

Also, the box of water could be used with either a drain-back OR a closed-loop system. For the closed loop, there would simply be TWO coils of copper pipe in the box of water. This would then be a double-wall heat exchanger, with the water in the box transferring heat between the two pipes and protecting against contamination if there was a leak. It would be a little less efficient than the single-wall/drain-back setup.

HEAT EXCHANGER #2, Idea B: BOA CONSTRICTOR WATER HEATER

If I put the solar hot water storage tank and heat exchanger in the utility room, it could not be a drain-back system, as the storage tank would be above the solar collector. (But what would it take to make a Thermosiphon and eliminate a circulating pump?)

As a closed-loop system, it really should be a double-wall heat exchanger. If I repurposed an old electric hot water heater tank as a storage tank, some insulation could be stripped away, and copper pipe could be wrapped around the OUTSIDE of the water tank, and though it were being attacked by a boa constrictor. The copper pipe would then be re-covered with insulation. The cold water would run (with line pressure) in to and out of the water heater tank in the typical manner, but while it is in there, it would be heated to the temperature of the copper pipe wrapped around it.

The boa constrictor water tank wouldn't heat exchange as efficiently as the box of water, but it would be easier to access and service than something in the crawlspace. If I build a custom tank, it could be built as a taller cylinder to take advantage of a small footprint, but floor to ceiling space, or possibly a standard tank, but up on a stand to make thermosiphoning a real possibility?

I guess mostly, I would just like to experiment and try it out.
A friend of mine has some coils of copper pipe I can borrow to play with for a box-of-water exchanger. I also have an old electric water heater tank that I found on large garbage pickup day. The heating elements were burned-out, and there was lots of crud in the tank, but it doesn't leak. I got it for free, so I wouldn't be afraid of cutting off the outside and stripping insulation on it.

Also, a drainback system would need a bigger (noisier?) pump, while a closed-loop system could use a smaller PV/DC pump. I'm not sure exactly what would be needed for a closed-loop thermosiphon - just big pipes and height, I think.

For an example of the "Boa Constrictor" heat exchanger tank, please visit:
Doug's Thermosyphon Solar Water Heater
and scroll down to the second photo.

For a "Box of Water" example, please visit
$1000 solar water heating system -- storage tank
Which shows building an entire setup in a crawl-space (although one MUCH taller than mine!!!) using plastic pipe. I would use copper for better temperatures and heat exchange efficiency.


So, which system do you think I should start playing around with next, and why?

-Ben