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menaus2 09-14-11 12:40 PM

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.

Daox 09-14-11 01:30 PM

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.

strider3700 09-14-11 03:31 PM

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

Ryland 09-14-11 07:36 PM

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.

AC_Hacker 09-15-11 11:53 AM

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!


Daox 09-15-11 01:28 PM

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.

GaryGary 09-15-11 07:38 PM

This is a good writeup on using air collectors to heat water:

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.


menaus2 09-20-11 10:50 AM

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!

menaus2 11-08-11 10:23 AM

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.
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.
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.
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.

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

Daox 11-08-11 10:32 AM

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!

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