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

[Daox]

Personally, I'd much rather do a box of water type. Simpler and more efficient. Plus, once its working you won't need to mess with it.

[Heynow999]

Do you want to build everything yourself?

If it were me, I would use off the shelf hardware and just piece it together. I would use a Viessman collector, grundfos pump, flat plate heat exchanger then maybe PV power for the pump in place of an expensive controller.

Just a note about electric hot water tanks. You can usually order a tank in a "tall" that is obviously taller than a regular tank. I think this would be better as a storage tank as it would allow the hot water to stratify better. It may also help with your space problems

[bennelson]

If you have any specific recommendations for parts, please let me know.

I already have a 4'x10' flat plate collector.

I have concerns about mineral build-up in flat-plate heat exchangers. (I think they might require a second pump as well?) Also, I wouldn't be able to simply add in alternate heat sources like the wood stove. Additionally, the flat-plate heat exchanger has no solar storage capacity, I still need a separate storage tank.

On top of that, I don't have too much money to spend on projects like this. They need to save money in the long-run, but I am still looking at used/salvaged parts and DIY skills to save costs up front.

For pumps, I have considered the "El Sid", which is available in pump rates appropriate for a simple solar hot water system, and can be directly powered off a PV panel.

Thanks for the note on the "tall" water heaters - I hadn't heard of them before.

[strider3700]

Just a note on the copper wrapped around a hotwater tank boa idea. The tanks are usually glass lined and heat transfer is painfully bad that way. I built a small test one and it was hopeless. If you like the idea I'd recommend ditching the hotwater tank and just getting a stainless tank then wrapping that. It would be far more efficient. Having said that I don't have serious space limitations at my new house and am taking the box of water approach.

[bennelson]

A while ago, a neighbor had several item on the curb, with a sign marked “Free”. Most of it was typical large junk, a projection TV, an old fridge, but was WAS appealing was a steel pressure tank in good condition!

This was the “old-fashioned” style with no rubber bladder. Instead this “maintenance-style” tank has to be occasionally refilled with air, as the tank air dissolves into the house water over time. I assume that the tank was being thrown out because of being replaced with the newer style tank.

I snarfed-up the tank, not sure exactly what I was going to do with it. At a bare minimum, I would need an expansion tank for the glycol loop in my solar system, even though this one was overkill, the price was right.

I have had an old electric water heater for some time that I was originally planning on using as my solar tank. But the more I looked at it, the more I disliked it. There was still a burn mark from where the wiring shorted, there was plenty of calcium build-up on the inside of the tank, and the heating element ports would require a special adapter to run pipes into. The advantage of the water heater was that it was well sealed up, and completely insulated.

I had already removed some of the insulation from the water heater where I was planning to wrap soft copper pipe around the outside, to act as a double-wall heat exchanger. Once stripped down to the bare tank, I realized the water heater and the pressure tank looked like they were almost the same diameter. I measured both circumferences – sure enough, almost exactly the same size!

I also noticed that the expanded foam insulation on the water heater isn’t directly on the tank itself. It is really inside two layers of plastic bag material. Once the sheet metal jacket of the water heater is removed, it’s pretty easy to take off the insulation as well.

So, here’s where I had the idea. Why not use the Pressure Tank, but with the water heater’s insulation on it? The pressure tank is in great condition, and has nice big, standard-sized pipe-thread ports on it. All I would have to do is wrap the insulation around it, and BAM! – solar water tank!

First, I needed to clean up the ports. The pressure tank has several 1&1/4″ ports on it. One on the back, two on the front, and one on top. I cleaned up the threads with an awl (there was just a little rust on some of the threads.) The back port and middle port on the front both got plugged up with 1&1/4″ stoppers. There was also a little port near the top front with a 1/4″ diameter. I plugged that as well, but it might be a good point for a temperature gauge or some other use.

The very top and lowest 1&1/4″ ports will become the in and out ports for my household water supply. Since the main waterline in my house is 3/4″ copper pipe, using the same size for these ports makes sense. I bought two reducing bushings that would shrink the size of the ports directly down to 3/4″.



I prepare to slice off the outer shell of the water heater
Now it’s time for surgery on the water heater. I had already found that a battery-powered reciprocating saw with a metal-cutting blade works great to go through both the sheet metal outside AND the insulation of the tank, all in one fierce cut. I sliced down the front of the tank – through the sheet metal, and through most of the insulation. I then finished cutting the insulation with a long drywall saw, and the insulation split right open. It felt like the whole shell was ready to come right off, except for the original Temperature and Pressure Relief Port blocking the way. A quick cut of the utility knife and a little prying, and the tank slid right out of its skin.


The naked tanks, side by side.
With both de-nuded tanks side by side, the condition of the tanks was a little easier to see. although the water heater is taller, it has a bottom that curves up – concave as viewed from the outside. The pressure tank has a convex bottom. I think both tanks must have a very similar capacity. The heater tank is plainly marked 50 GALLONS, and I think the pressure tank is the same.

The ports on the pressure tank are different than those on the heater tank, so I did have to do some minor cutting of the insulation to allow for it. Then, I wrapped the heater insulation around the pressure tank, and temporarily held it in place with a strap.


The pressure tank gets a coat and belt.

The insulation doesn’t make it perfectly around. When I cut the insulation off the heater tank, it “popped” as if under pressure when attached to itself. I believe that when the water heater tanks are manufactured, the tank is placed inside the sheet-metal skin, and then expanding foam insulation is injected between the two. The insulation is sort of “pressurized” on there its whole life. because of that, I am never going to get the insulation that tight onto the new tank. There is about a half-inch gap. I think I will simply fill that in with Great Stuff or some similar expanding foam. When I do that, I will first cover the tank and ports with plastic. That way, if I ever need to access one of the unused ports, I simply need to cut away the foam and plastic – the foam will NOT be stuck directly on the metal.

I will also need to cover that very top with insulation, and let’s not forget the bottom either!

What to do next? I still need to find a good circulating pump – something that can be PV powered, and with an appropriate flow rate. I also need to buy soft copper pipe to wrap around the tank. I haven’t been able to find any great prices on that yet. The best I have come up with is 60 feet of 1/2″ copper for $90. I hope that 60 feet is enough for a good heat exchange.

I will also need to hack into my existing plumbing and add in a diversion and several shut-off valves. I am really not looking forward to that – two many chances for disaster – and most of my plumbing is nearly inaccessible – directly behind the furnace!

Well, I’ll keep chipping away at it. As near as my estimates go, I will have a $6000 solar heating system for a cost of only about $400 total when I am all done. Plus I get all the experience of figuring it out myself to boot!

[bennelson]

I got a chance to work on this project at little more.

Last night, I stopped at the store on the way home, and picked up a 60 foot roll of 3/8" soft copper pipe.



I strapped down one end of the pipe and then wrapped it around the bottom of the pressure tank. I then secured the other end with another ratchet strap.



I have a small fountain pump, which has a 3/8" output on it, and just sucks from the bottom, so it has to be submerged. I put it in a bucket and ran some tube to either end of the copper pipe wrapped around the tank.



I bought some reducing bushings and barb connectors to hook the 4'x10' solar panel up to the pump. The pump has a good flow rate, but low head. I couldn't use it to fill the solar panel. Instead, I had to fill it with a garden hose. I then had to siphon the output end of the system to get the water flowing. Once it did, the pump ran fine.


I also filled the pressure tank halfway up with water. (Couldn't fill it all the way because I didn't have a plug for the side port!) Then I covered the copper with some aluminum bubble-wrap insulation.



After that, I covered the rest of the tank with the expanded foam insulation.


I will try running this system tomorrow if we get some daylight again.

One thing that I have learned so far - it is really hard to wrap copper pipe around the outside of a drum! It's springy and bendy and difficult to get good surface area connecting. It really does need some sort of heat transfer compound.

Once the water was flowing, it was easy to feel with my hand that the copper around the tank was warm, and the pressure tank of water was cold, but who knows how much heat transfer is going through from one to the other!!!

[GaryGary]

Hi,
One advantage of the big coil of PEX as the heat exchanger is that the PEX coil itself holds about 10 gallons of water. Since most hot water demands are less than 10 gallons, and the 10 gallons in the coil is already heated up to full storage tank temp, its a 100% efficient heat exchanger most of the time. After the first 10 gallons, its still pretty good because while PEX is much less conductive than copper, it has much more surface area.
The system in the link you gave is my system, and while I have no issues at all with copper heat exchangers, I have to say I'm really pleased with the performance of the big coil of PEX.

If you used the crawl space and allowed 2 inches below and 2 inches above for insulation, you have about 18 inches left -- enough for a big coil of PEX, but you would have to build the tank around it. The 22 inch crawl space does not sound like a lot of fun to work in, but if it allowed a simple drain back system it might be worth it.

These kinds of EPDM lined tanks can also be built in in cylindrical shape out of a single piece of sheet metal wrapped in a circle -- like the tanks that STSS sells. This might be easier to work with in such a limited space.



Some of the other people who have built the $1K solar water heating systems have use other approaches --
Solar Water Heating Projects and Plans
Mostly with copper, but some mixed systems.

I think that Chad's system has a tall skinny tank that might fit your space.

Gary

[bennelson]

I moved on to test the "box of water approach".

I was able to get some really high temperatures going through the water panel with the copper pipe wrapped around the steel tank, but that actually isn't a good thing - it means heat wasn't transferring well INTO the tank.

I would imagine that with the copper completely submerged in water, it should get excellent heat transfer. I unwound the copper pipe from the steel tank, and found a large plastic tub. I wound the copper small enough to fit down into the tub, and then filled it with water. The tub didn't have any volume markings on it, but I think it is between 10 and 15 gallons.



Once I got water flowing through the panel again, I stuck my arm into the frigid bucket of water to feel the copper. The pipe had a gentle warmness to it. I also noticed that right where the pipe came into the water, it was very warm, but the output end was almost the same temperature as the bucket water. That means that almost all the heat is being transferred to the water in the bucket.



It is mostly cloudy today, so I am not expecting to get too much out of this system right now. Also, please note that since I am currently planning a glycol closed-loop system, a second loop of pipe would go into the "box of water". One would be for the glycol, and the other would be for the household water going to the water heater.

This would be considered a "dual-wall" heat exchanger, so no problems with contamination in case of a leak. However, plain water should act as an excellent heat exchange medium, especially if both loops are copper pipe.

[bennelson]

Yesterday, some of the guys came over to help on my Diesel S10 conversion project. While they were here, I also showed them what I was doing in my solar experiments, what I had to work with, and what the limitations are.

Everyone really seemed to like the steel pressure tank, in that it is in good condition, will fit in my utility room, and sure was hard to beat the price.

I really like the “box of water” concept of a double-walled heat exchanger. It is cheap, safe, and effective. It also relies on simple water pressure for the domestic side, eliminating a pump.

If I run the glycol loop on a small PV panel and 12VDC pump, I won’t need any other controls or electronics – a simple, reliable setup.

So what’s next? I think it’s time I build an ACTUAL Heat Exchanger Storage Tank. I plan to cut the top off the steel tank, wind two coils of copper pipe, install both into the tank (one lower, one higher) put the lid back on, insulate it, and fill it with water.

I also need to figure out EXACTLY where I am going to tap into the existing cold water line in my utility room, and where and how many shut-off valves and tees I will need. I don’t see that as particularly complicated, but I don’t have that much plumbing experience, and it is tight quarters. I also still need a pump!

Over at the EV club, we have a plasma cutter. The next meet isn’t for two weeks though. Maybe at that time I will have some video of cutting the tank.

-Ben