menaus2's 240sqft Hydronic Thermal collector

[menaus2]

Hi All! It's been a while since I've documented a major project on the forum, so I'll be starting from the planning process going forward.

Last year I installed a 36sqft drainback collector with a 100gal. storage tank on our house. I havn't gotten around to posting the details yet, but hopefully they'll be forthcoming. Suffice to say it works great (roughly 5,000 btu/hr), except our hot water usage keeps it in the 60-90F range. So, it's time to expand big time!

I have a woodshed about 100' away from the house with roughly 720 sqft of wall available, my propane furnace also has a plenum water-air heat exchanger from the wood boiler we removed available as well. So I would like a system that can keep the solar tank above 120 often enough to kill bacteria problems and also use the furnace for space heating when there is excess heat being produced. The collector design should be freeze-proof and have the option to be expanded in the future in a modular way.

I was inspired by Scott Davis' 194sqft pex collector and will also use a gycol loop with copper coils in the tank. I am looking to improve upon the design by using a manifold in the collector with 3 pex-al-pex runs to get better flow, I also plan to use twinwall polycarbonate sheets to get higher collector temps for better heat exchange.

I still have a lot of details to work out that are beyond my experience: relays & pumps and integrating that with the furnace. As well as sizing the collector copper coil heat exchangers in the tank.

I've attached a mock-up of the collector and a possible expansion of it in the future. Also a materials list and cost estimation.

[menaus2]

Also some rough diagrams of the system as well as my mostly uninformed wiring for the space heating controls.

[menaus2]

The ground will still be frozen for a few months yet, but I'm going to start with the inside work. First on the list are the heat exchanger coils in my 100 gal. tank. I already have a 60' 1" copper coil for the DHW preheat. That leaves about 24" diameter circle by 25" height inside the coil to work with. Since I potentially have a lot of collector area and want to be able to work around say a 10F delta T, I want to maximize surface area and keep a pressure drop low as possible. I picked up 4 coils of 50' x 1/2" OD refrigeration copper to accomplish this. I think 4 coils in parallel should leave a similar area as the 1" pex pipe. 1/2" cu should be easier to bend in the confines space. Values to calculate the heat transfer are hard to find. Engineering toolbox gives anywhere from 50-200btu per sqft of pipe surface area per degree f... hmmm....

[menaus2]

Heat exchanger coils are wound in the tank. It wasn't easy hanging over into the tank for a few hours, but it got easier as I worked my way higher in the tank. I had a 24" diameter by 30"H space to work with. The 1/2" OD refrigeration line was easy to bend, and I only had a couple spots where there was a minor dimple from bending it too tight. I used 14ga copper electric wire to space the coils & keep everything tidy.

From what I've researched with wort chillers used for brewing, it's a good idea for coils to be separated into different "zones". The first coil is laid out as a disk at the bottom of the tank. The second is a cylinder at the bottom half, then the third coil in a similar way above that. The fourth is a tighter coil in the middle from bottom to top. Currently I'm building the manifolds out of PEX and will attach it to the lines with flare fittings. With the tight space it should be a challenge!

[SDMCF]

Originally Posted by menaus2

From what I've researched with wort chillers used for brewing, it's a good idea for coils to be separated into different "zones". The first coil is laid out as a disk at the bottom of the tank. The second is a cylinder at the bottom half, then the third coil in a similar way above that. The fourth is a tighter coil in the middle from bottom to top.

Can you write more about why that layout is a good one? Are you primarily aiming to achieve good/best heat transfer?
I have a similar project planned for the summer, but so far my tank layout ideas have concentrated on improving stratification.

[menaus2]

The layout is trying to make the best out of limited space. I built the tank last year with only Domestic Hot Water in mind. The tank is only 100 gallons, and the temperature probes have never shown much stratification anyways. If you're starting from scratch, I would recommend a bigger tank & maybe look at PVC stratification tubes (I think that's what they're called?).

As far as my setup the idea is that splitting it up into "zones" the hotter section of pipes are in different areas of the tank, so more exposure to cold section of tank and more even heating. In wort chillers it cools the entire bucket faster so same idea but with heating. I also feel like putting it in zones was the easier option to wind the coils. If you have a stratified tank, putting the coils in the cold bottom would be most efficient. At least that's my $0.02

[redrok]

Hi menaus2;
I would like to hear how much the collector costs?
I suspect electric PV panels cost much less for an equivalent system.
Your 5000btu/hr = about 1465W of power.
PV panels are quite cheap today. It is easy to get them for about $0.30/W
for a PV panel cost of about $440. You can also get cheap used panels for about
$0.10/W.
Does it make sense to use hydronic collectors today when PV is so cheap?
redrok
redrok@redrok.com

[menaus2]

Hi Redrok!

According David Poz (great youtube channel btw). Solar is more efficient per dollar & solar thermal is more efficient per area. I think he used commercial collectors so idk how a diy panel compares in terms of cost, but we'll sure find out! I'm estimating somewhere between $10.00 and $16.00 per sqft. Besides costs, I want to maximize the use of the area on the shed it's mounted on & get the most heat out of it. I would like to keep space for a PV array elsewhere dedicated for electric in the future. PV cost & storage will only come down with time so it makes sense to postpone that until later. Reducing my electricity usage is the lower hanging fruit on the electric side. Mostly though I just like building thermal systems & I have experience with it. For other people & their goals/ constraints PV heat might make more sense for them.

[menaus2]

With a good portion of the inside plumbing done, I've shifted to working on the collector frame. I was able to scramble & get all the materials before the Beer Virus cases started taking off so I guess I'll have extra time to work on it.

The frame is treated 2x6s wraped around 1/2" treated plywood sheets and secured with brackets. This way I can use full twinwall polycarbonate sheets for the glazing. It's a lot of work, but it will be well worth hauling less wood for years to come!

[menaus2]

The world slows down, but construction continues! Framing has been completed. The shed wall isn't exactly plumb, but it's level. Good enough! I siliconed (black silicone) the wood to keep moisture out on back side. Next is the 1" polyiso board (staggered). Then pine 3/4"x 1.5"s to finish the glazing support. Glazing support is set back 1/2" to allow the polycarbonate to be siliconed on the outside. I put 2" strips of polyiso on the inside edge of the frame to finish up the insulation. Currently working on putting up the 1/2" OSB panels over the polyiso. All the piping/ collector fins will be eventually stapled to the OSB.

The underground insulated pex I got is the reflexive bubble wrap inside of drain tile style. Some online sources point out they can lose a ton of heat if water gets into the pipe. Does anyone have experience with wood boiler pipe like this? I was thinking I might install sock drain tile pipe beneath it just incase the pipe ever got punctured to keep it dry enough. Possibly a drain hole at the low point of the underground boiler pipe just to give the water an escape route in case water ever did get in?