Water heat storage for solar hot air system.
 

I have an idea for coupling my planned 150 sqft solar hot air collector to a EDPM lined water tank for thermal mass storage. I'm wondering if the heat exchange method I'm thinking of will be effective.

The hot air panels will be connected to 3 or 4 in pvc tubing powered by a 500 cfm blower. Im wondering if it would be realistic to plumb this tubing in a zig-zag pattern at the bottom of a 500 gal. or so EDPM rubber lined water storage tank and have maybe aluminum fins siliconed to the pvc pipes underwater to aid in heat transfer. Could heat be absorbed into the water fast enough in this setup? How many linear feet of pvc pipe (or similar material) in the bottom of the tank would be necessary to get a decent temperature drop for the air returning to the panels?

I just hope there's a way to make this work as the design is pretty elegant IMO. You could simply raise or lower the water level in the tank to tune the thermal mass amount, and put in a coil of PEX tubing floating just below the surface for hot water preheating.

Unfortunately, PVC has too low a melting temperature to use it with a hot air panel.

Also, if you want to store the heat energy, I'd recommend just going with hot water panels, not air.

if you're going through the trouble of having a water tank why not make the collectors water based?

If you really really need to do this, use a short stack of car radiators, but even then, I and everyone else has to ask why you are bothering with heating air if you want to heat water? air holds so little heat that it makes a really lousy transfer fluid.

Well, menaus2 is planning to use air for his transfer medium. It does have certain advantages, one of which is that if it leaks, it doesn't wreck everything.

So, he could store heat in a rock bin. It would take more space, but it has been shown to work, and it is highly reliable.

Something that may not be obvious is that if a rock bin is used, the preferred method is to circulate the air DOWN through the bin rather than up, because if circulating up, warm paths will become the channels of least resistance and the hot air will self-select those warm channels, thus reducing efficiency & capacity. If circulated down, the bin will self-select cooler paths through the rock bin and thus self-select even warming.

As I recall, fist-sized rocks are the preferred size for maximum capacity of daily heat storage and heat release.

Good luck, menaus2!

-AC_Hacker

I've heard of problems with rock heat storage and mold growth. Hot and humid air make a pretty good place for that stuff to grow.

Hi,
This is a good writeup on using air collectors to heat water:
http://www.builditsolar.com/Projects...aterHeater.pdf


While I like the EPDM lined tanks, maybe a metal tank is better for air heating collectors.

Al did a hybrid air and water heating collector:
DIY Solar Air Heating Collector with Water Heating

I think that the car radiator scheme that was mentioned above would also work.


Gary

Wow some really great ideas here! After doing some thinking, I don't think that seasonal air heat storage makes as much sense as I first thought it would. Water is by far the superior transfer fluid and will work best at cooling the panels in the summer. Due to the huge storage requirements for space heating, it makes the most sense imo just to use the heat when it's produced. In the long run, it's gonna save me a lot of headaches and frustration to go with something already proven, so I'm gonna go with Al's hybrid air and water heating setup. My lower panel is only maybe 8ft from where the pump will go so that will work nicely. The upper panel is about 2 stories up, so I'll just push the air directly into the apartments and just vent them during the summer. I don't think pumping that high is worth the cost. Now to plan it up for the city permit :P I'll keep everyone updated on the project's progress!

Hey everyone its been a long time, but I want to bring everyone up to speed on the state of the apartment solar project.

I've broken the project into 3 large panels (roughly 3ft wide by 2 stories high each) on the southern exposure of the building.

I've finished work on the first panel. Its an aluminum screen type hot air collector that feeds directly into one of the lower apartment units.

https://lh6.googleusercontent.com/-y...0915163846.jpg
Here is the unit before any work was done. The panels will go the tall spaces between the windows on the far left, right middle and far right.

https://lh6.googleusercontent.com/-x...0922153510.jpg

https://lh5.googleusercontent.com/-H...0922164625.jpg
I decided to build the panel directly on the side of the building as I had no means to hoist up a completed panel. The sides are 2"x8"s that i screwed into the wood siding. The lower air intake part was built over the stone foundation.

https://lh5.googleusercontent.com/-F...0927164644.jpg
I had to break a hole through the foundation (an ordeal in itself) and ran 4" metal ducting into the basement where the blower will be situated. The hole in the foundation and gap between plywood backing and the foundation was filled in with spray foam. The inside of the panel was covered with 3/4" polyiso insulation. It was sealed with silicone caulk and painted with flat black primer spray paint.

https://lh6.googleusercontent.com/-c...0927164629.jpg

Intake closeup. Not pretty, but it will be painted black and covered by the baffle anyways :P

Wow, super tall collectors. I'm quite interested in seeing how this turns out!

I assume you're planning on using forced convection through them? I'd think you'd have super hot air coming out of the top of the collector otherwise. Also, what do you plan on using for glazing?

Very cool, keep us informed on how it goes!

https://lh5.googleusercontent.com/-m...0928164159.jpg

Baffle in place.

https://lh6.googleusercontent.com/-R...0929161759.jpg

Baffle insulated on inside and painted. You can see the 3/4" metal conduit support bar as well. The side 2x8s are painted with several coats of primer and gaps with the siding are being caulked in.

https://lh6.googleusercontent.com/-v...0928180312.jpg

One side of the two layer screen absorber. because of the height of the panel the absorbers are placed in a "sideways V" arrangement. Heres what it looks like from the right side view:
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https://lh3.googleusercontent.com/-L...0929132216.jpg

Closeup of the screen absorber. This is just the first layer above the baffle.

https://lh4.googleusercontent.com/-p...1004142329.jpg

Detailing the sides. The clear suntuff type polycarbonate will go over this so i want it to look nice :)

https://lh3.googleusercontent.com/-E...0929161806.jpg

Another shot of the support bar. You can also vaguely see the 2 layers of screen in place.

https://lh5.googleusercontent.com/-C...0930181200.jpg

Polycarbonate starting to go up. Its sealed with silicone caulk and secured with rubber washer roofing screws. There is a vertical wood bar up the middle to secure the 2 polycarb sheets together.

https://lh4.googleusercontent.com/-2...0930182901.jpg

filling in the top.

https://lh3.googleusercontent.com/-x...1019165441.jpg

Top done. The top is insulated on the inside with 2 layers of polyiso. The exhaust wraps around the side of the house into the units kitchen. the ducting is currently insulated with a polyiso box filled with sprayfoam (i like sprayfoam :) ) The very top is also insulated with spray foam. The exhaust boxing and angled top detailing will wait till spring.

The blower is a Suncourt TF104. It's controled by a plug-in thermostat (commonly used in greenhouses). It has an electronic sensor so i simply extended it from the furnace room into the panel with some thermostat wiring. The exhaust has a hinged dryer vent to prevent backflow.

Have you considered using some air valve to prevent heat lost during night and periods without sun?

Yup I made sure to do that :) I simply put one of those dryer exhaust vents with the little flaps on the inside wall where the panel exhausts to prevent backflow of the air.

My renter said that it gets pretty toasty when the panel is going. The sensor is placed about halfway up the panel and set to turn on at 100 deg. F, so the bulk of the air is probably hotter than that. I haven't timed it, but on a good sunny day the blower comes on in about 15-20 second intervals.

I'd say it adds about 5 degrees in the apartment. I'm basing this off the temperature differential between this and the adjacent unit. The thermostat is located in the apartment with the panel, which unfortunately means the adjacent unit gets down to 64-65 :(... Guess I'll just have to build another panel! I'm wondering if it would be worthwhile to heat the basement with solar so it heats more evenly? :/

Just an update on the system. I resolved the thermostat issue by simply raising the set temp a degree or two higher. Other than that I've been very pleased with the system's performance. The heating season here in Wisconsin is basically over, so I've unplugged it for the summer. Also, I was able to fully depreciate it on my taxes as it was an energy property.

Quick question though for everybody, would you be concerned about high stagnation temps? Would the polycarbonate glazing have enough heat loss to keep it from self destructing? I have foam wiggle strips I can take out at the top for ventilation, but I would rather not have to get up on a ladder twice a year if I can avoid it.

I'm also looking at getting a new temp. controller from myDTC. I'm unsure if I'd be able to use 3 sensors: one each for panel, indoors and outdoors. So, say It'll turn on when the panel is over 100 f, it's under around say 70f inside and outside. I also cant find any material if the relays can control the 120v ac blower.

Hi,
My experience has been that single glazed vertical collectors are OK in through the summer stagnated.

The one I have on my shop does not go above 185F when stagnated.

Still not a bad idea to monitor the temperature for the first year.

Warm Fall days may produce higher temperatures than mid-summer as the sun get lower and you still are not using the heat.

Gary

I know this year was really warm, and hard to compare to previous years. But, do you think you can provide any additional details on how the system works? Like what kind of benefit it provided, or any other semi-measurement of its performance? How often, or under what conditions did it turn on?

Hi Doax,
I wish I had some hard data to go off of, but I haven't really had a chance yet. Maybe that's something I could spend a sunny afternoon logging the temp rise. Circumstantially, the renter mentioned on on sunny days the apartment would get pretty toasty, and on one day with outdoor temps around the low 70s it got around 78 inside. I think this pretty consistent with the 5 or so degree rise I've seen from the heater. I want to put up another hot air panel this summer, so I'll have records to compare what that does.

If I install solar hot air panels in the other 3 units, I think I might have to install TRV's (Thermostatic Radiator Valves) to prevent overheating in the rooms and generally be able to realize heating gains from the Central heater.

Hi, great project but I thought you were supposed to circulate the air in and out of the same room, you seem to be taking air from the basement into the kitchen, does this make a difference to the performance?

Hi,

The blower actually pulls the air from the same room, it's just that the ducting runs through the basements for a few feet. It's a more convenient location for the blower.

https://lh4.googleusercontent.com/-4...s95picture.jpg

Just an update: Some finishing trimwork around the foam insulated exhaust pipe.

You had a relay issue to turn on the blower and i wonder if you resolved it. Many relays can come with 24vac or dc coils or with 120vac coils to switch 120/240vac loads.

Just an update on the system. It's been operating for 9 years or so without a hitch although there have been some changes made to it years ago. First, I closed off the space between the collector and the roof since pigeons liked roosting up there. Second, instead of heating the apartment, I rerouted the ducts so air is drawn from and dumped in the basement. The apartment was heating up too much, and since the whole house thermostat for the boiler is located in that apartment, it was throwing off the temperature for the other apartments. This way heating is more even and helps prevent pipe freezes down there.

In the next year or two I would like to replace the glazing with twinwall polycarbonate. It will give it better performance in colder weather and a more professional look. The corrugated suntuff glazing has held up great, but with the roofing screws it has a ghetto-steampunk vibe that the building could do without.

Do you really think the twin wall poly would help to create more heat?
I didn't think it would make much difference from your original poly?

I agree it will look better (thinking of changing mine as well over to twin wall.) I have 2 hot air collectors in place at my house similar to yours.

Click here https://ecorenovator.org

and I have to tell ya.. when I saw you were sending all that heat into a kitchen first thing that came to mind was overheating the kitchen. :) but you have everything under control now including the pigeons. :thumbup: Good luck with your projects!