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stmbtwle

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Reply with quote  #11 
Greg:  
In an ARETHA the air never leaves the collector.  It just goes round and round and through the radiator HEX, which cools the air. while heating the water.  The only thing going "in" or "out" is the water.

Thor:
The "V" shape will be a pain to build.   Make it a "wedge" shape and put your HEX assembly at one end.  Yes you can put the thick end at the bottom, it would sit well against a wall that way.  

Another way would be to build Greg's standard ZP, but run the air through a separate "HEX Box" then back to the collector.  It might be easier to maintain as the fan is not in the collector, but there might be some losses due to the ductwork.  See diagrams in the next post.

My ARETHA is very similar to FIG 2, except I have a Cinefoil absorber instead of screens.  Screens might be better.

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stmbtwle

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Reply with quote  #12 
Here is the ZP with a separate heat exchanger to heat water, and a ZP ARETHA.  See the similarities?  Drawings are simplified and some parts are omitted. Not to scale.

ZP ARETHA HYBRID.png 


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Thor

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Reply with quote  #13 
Thanks Wille- the only reason I put the fan exchanger in the middle was to balance out the push/pull of the fan and to have an effiecient as possible flow of air.... I guess it can go anywhere with the right ducting.
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joebehr

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Reply with quote  #14 
What is a "zero pass/ARETHA" collector and can someone post a photo or sketch? Thanks. 
stmbtwle

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Reply with quote  #15 
A "zero-pass" is a type of screen collector where the air passes BETWEEN two layers of screen, as opposed to passing THROUGH them.  It has had good reviews:
http://www.simplysolar.supporttopics.com/post/the-zero-pass-hot-air-collector-6611426?&trail=10

An ARETHA is an "assembly" of a hot air collector with a heat exchanger so the hot air circulating inside the collector heats water. The output is hot water, the air never leaves the collector. As opposed to a water-tube collector, it's quick, easy and relatively cheap to build. It was originally designed for third-world hospitals.  
http://www.simplysolar.supporttopics.com/post/aretha-project-hot-air-collector-with-car-radiator-6882647?trail=10.  
As a water collector it shows lots of promise but not many have been built, and the design has not been optimized.  Many have problems understanding how it works, but it does, quite nicely.

The "zero-pass/ARETHA" is still just a concept, as far as I know none have been built.  It would combine the efficiency and ease of construction of the zero-pass air collector with the heat-storage advantages of a hot-water collector.  See the drawings above.



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gbwillson

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Reply with quote  #16 
I think you could build an ARETHA and combine it with any type of air heater. What volume of air does the ARETHA work best?

Greg in MN where it was -17˚F outside this morning!!![eek][eek][eek]


stmbtwle

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Reply with quote  #17 
I dont know, it's hard to determine as the air never leaves the collector. I think a variable speed would be best, depending on the temperature differential. My experiments indicated it was less than I originally thought as I got full cooling with about half power on the fans, but I can't give you a CFM.

I tried to adjust fan speed so the air temp leaving the HEX was as close as possible to the water temp coming in. To me this indicates max heat transfer. Any more IMO is wasted, and may cause excessive heat loss thruogh the glazing.

Paolo may know more.

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Thor

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Reply with quote  #18 
hey willie- do you have a make/model of the HEX you used?


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stmbtwle

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Reply with quote  #19 
Thor:

The HEX isn't critical.   Paolo uses auto radiators, probably from a small car.   He also uses the fan that goes with them.  If you look at the ARETHA thread you'll find a description and some photos.  

I used an oil cooler from the auto parts store, it's a common aftermarket transmission oil cooler.  I had one on the shelf so I used it, it's 10" x 14" I think.  When I started experimenting I bought a second identical one for side-by-side testing.  Eventually I put BOTH of them in a single collector, see diagram: Hex.png 
The way it works is the hot air from the collector enters HEX #1 and heats the water.   HEX #2 is not "necessary" but DOES help heat transfer.  It "scavenges" leftover heat that HEX #1 missed and uses it to "preheat" the incoming water.   Then the cooled air goes to the fan and is returned to the collector.   I use the side-by-side configuration due to space limitations inside the collector box, and I always put the fan on the "cool" side of the HEX, so it isn't subjected to as much heat.  

I used the oil coolers because they were compact and would fit in my 2' x 4' experimental collector.  I doubt they're big enough for a 4' x 8', and the serpentine configuration of the cooler restricts water flow.  The auto radiator is apparently big enough for an 8' x 8' or larger.  Stacking two radiators as above would be even better.

I also think an air-to-water heat exchanger for a furnace would work well and be quite durable as the water passages are copper, and already arranged in a "stacked" configuration.   However they're not cheap.  I'd start with an auto radiator and matching fan.


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Thor

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Reply with quote  #20 
i know its not critical... just curious what you used, to try and gauge how efficient it is, I will most likely pick up some finned piped and make my own, just not sure on sizing yet.
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