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stmbtwle

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Reply with quote  #1 
I finally got a cpvc water-tube collector built, to compare with the single-screen Aretha type I had already built. 

Both collectors are 2' x 4' overall, glazing area 7.265 sq ft.  Both are pine, with 6mm twinwall glazing, 1/8" Thermoply backs and 3/4" Polyiso insulation on the back.  The sides are not insulated.  Both have identical 4 lph powered from a 12v battery.  Both started with 1 gallon of water (including the water inside the collector).

The Aretha is 7-1/2" deep (needed that depth to accommodate the fan and heat exchanger), while the cpvc collector is 3-1/2" deep. 

The Aretha is painted flat black inside, the back of the cpvc collector is left reflective as the back is not exposed.

The Aretha has a single "solar" screen, a 10 x 14" aluminum heat exchanger (oil cooler) and 2, 40 cfm computer fans to circulate the air through the heat exchanger.  The cpvc collector has 11 vertical tubes of 1/2" cpvc siliconed to aluminum flashing, and sprayed flat black.
IMG_0180.JPG 

Water temperatures monitored with Taylor oven meters. Solar insolation was monitored with a "Dr Meter" SM206.  Readings every 10 minutes. Test started at 1700, start water temperature was 95F.  Water mass 8.3# for each collector.

"The proof is in the pudding"... After 1 hr the CPVC water reservoir had reached a temperature of 153F (58F rise or 481 BTU),  while the Aretha had reached a temperature of 147F (52F rise, 432 BTU).

Clearly the water-tube collector is more efficient by about 11%.  As the insolation was the same, the difference had to be in the losses.   I think a considerable amount was lost through the sides of both (both were noticeably warm to the touch), but the Aretha being deeper had more surface area.  Also the Aretha has a fan circulating (hot) air over the glazing, sides, and back which can't help, but unfortunately the air flow is needed to transfer heat to the water.  I need to figure out a way to keep the air off the shell and glazing of the unit. 

Here's the data,

 Aretha collector vs CPVC tube collector 8/17/2014    
      
Start time: 1700 End: 1800 Run: 1 hrSq ft: 7.265
Start temp: 95F     
  Data   
TimeArethaCPVC tube Insolation (btu/hr/sq ft) 
17009595  326
1710113111  349
1720126126  351
1730136140  350
1740144147  335
1750147153  265
1800147153  240
      
Total rise52F58F Avg BTU/ft316
Water Mass8.38.3 sq ft7.265
Total BTU432481  2300
      
Eficiency18.78%20.91%   
      

Thanks for the help G_H!


Thoughts, anyone?


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Willie, Tampa Bay

Garage_Hermit

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Reply with quote  #2 
Hi, Willie !

Good test (I just *love* the cart [smile]).

Actually, *my* first impression, is, how *CLOSE* they are !

Anyhow, I'd be wary of relying on a single one-hr test to compare two very different technologies...

First thing that springs to my mind, is difference in volume of WOOD on the Aretha - don't know the thickness of your deals, but the Aretha probably contains 100 percent more wood in the side structure, so this will be absorbing heat at a different regime than the CPVC...  I reckon that that fact alone would cater for most of the difference, but this is just a guess...

Nice test, anyhow !

G_H


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(1)  "Heat goes from hot to cold, there is no directional bias"
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stmbtwle

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Reply with quote  #3 
Yes it does have twice the wood and that's part of the problem.  Wood absorbs a lot of heat and the increased surface area of course radiates more heat.  I think I'm going to build up the back on the inside to reduce volume and amount of structure exposed to the hot air.  Also an absorber plate over a reflective back to reduce losses through the back of the collector.  Some insulation on the sides should help too even though it will reduce the collector area.

I want to try a single screen right below the glazing as per Gbwillson's zero pass collector. That should help keep the hot air away from the twinwall.

Also a speed control for the fans to see if I can find a "sweet spot" between too much circulation and not enough.   Or a bypass so that I have plenty of air going through the heat exchanger but less circulating around inside the collector.  

My original plan was to compare two Aretha collectors and try to improve the design but I needed the comparison with an EXISTING technology hence the water-tube.   I think I'll stick with the water tube as a reference, as it gives me a benchmark everyone else can understand.

I like the Aretha as it's quick and easy to build while the water-tube is a PIA, but if I can't get the efficiency up to near par I'll give it up.

Yes the 1 hr test was short and the water mass small but that was all the time I had and I needed quick results.   I used plastic jugs for reservoirs and I know a lot of heat was lost through the sides of the jugs, with a corresponding loss in overall efficiency.  I'm going to go back to 6 gallon styrofoam coolers and let the test run all day, next time.

Glad you like the cart.  It allows me to move the apparatus around easily and it'll also fit through a door.  The collectors are on hinges and the angle is adjustable.   It should be longer but I used what materials were at hand.

Again thanks for the help with the spreadsheet.  I still have some formatting problems but I'll get them worked out.


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Willie, Tampa Bay
stmbtwle

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Reply with quote  #4 
The problem with testing this way is the temperature change over time. It's really difficult to measure the temperature change in a single pass through the collector. It only holds a few ounces and the specific heat of water is such that the temperature change may not be measurable with a common thermometer. So I have to use a larger mass of water and run that test over a longer period. But with a larger mass there are heat losses through the container which will skew the readings. I try to keep the containers and plumbing exactly the the same so that the losses, while they cannot be discounted, are at least equal. It is a COMPARATIVE test after all. Once I have a significant spread between the two collectors, I have my answer. The other issue is that with a small water mass the temperature may increase 50F-60F in a couple hours. The change in temperature brings with it increased heat loss and a corresponding loss in efficiency. With the 1-gal containers I reached a point where neither collector could not raise the temperature further, system efficiency had dropped to zero. Larger container have their own issues, mainly heat loss and cost. So a relatively short test with relatively small masses works best, for me at least.
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Willie, Tampa Bay
solardan1959

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Reply with quote  #5 
Willie,
   A little late to this party but why not switch the fan around so it blows from the glazing to the back?  The effect should be about the same and the benefit is your not blowing hot air towards the glazing.

Dan
stmbtwle

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Reply with quote  #6 
Either way I'll have air coming out of a hole and hitting the glazing with or without the fan, as it IS a dual pass, recirculating collector. The air coming off the fan and hitting the glazing is the coolest air in the collector as the fans are right after the HEX which cools the air. If I reversed the flow two things would happen: 1, the fans would be subjected to the HOTTEST air flow, and 2, this hotter air would also be passing along the glazing. I think it would be a net loss, and would also increase the risk of heat damage to the fans.

A compromise would be to put the HEX at one end of the collector and the fans at the other, but it would take a complete redesign. Still, the air would be coming off the HEX and hitting the glazing, and the fans would be exposed to more heat.

I think a single ZP screen between the glazing and the airflow might help, but I haven't tried it yet.

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Willie, Tampa Bay
Garage_Hermit

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Reply with quote  #7 
Er, Willie, how about encasing the radiator in a sheetmetal fairing...

The fan is upstream of the fairing, and blasts hot air into the latter.

The radiator (inside the fairing...) picks up as much heat from the air as it can, and the exhausted air then exits the fairing through a tail pipe or set or pipes, to rejoin the collector and get cycled around again...
The screen cloth (or Cinefoil etc.) are configured around this return air stream, in order to get as much heat back into the air ASAP.

G_H

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(1)  "Heat goes from hot to cold, there is no directional bias"
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stmbtwle

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Reply with quote  #8 
That's pretty much how it's already set up. My collector is a dual pass. The "first" pass is between the absorber and the glazing, and picks up heat from the absorber. The "return" pass goes behind the absorber, and picks up still more heat. At the end of the return pass is the HEX. The cinefoil is configured such that the air cannot go around the HEX but MUST go through it, giving up its heat to the water in the HEX, which is then pumped away. After the HEX the air goes through the fan (which is doing the work). After the fan there is a deflector (fairing if you like) which turns the flow 90° while keeping it away from the glazing. Then the air goes around again, and again, and again, ad nauseum, picking up heat from the absorber and giving it up to the water in turn. It's a recirculating system. As the water picks up a little heat each pass, it gets hotter and hotter. I've gotten water temperatures of 160F (70C).

Cinefoil tubes might be better. I tried a backpass to keep the air off the glazing but results were disappointing.

I've contemplated building an 8'x 8' collector, but I'd also need to build a much larger insulated storage tank, and heat exchangers etc to heat the house with. I'm not sure it would be cost effective, for only two months a year. I think PV would be a better investment here, as it works year round.

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Willie, Tampa Bay
Garage_Hermit

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Reply with quote  #9 
OK, thanks for the explanations !

Those water temps look very nice !

Thanks for doing all of the hard work, I'm sure lots of folks than me appreciate it [cool]

G_H

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(1)  "Heat goes from hot to cold, there is no directional bias"
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stmbtwle

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Reply with quote  #10 
It would be interesting to see someone else pick up on this... I think the Aretha shows a lot of promise for a hot-water collector.  Last I heard, Paolo is building them in Italy... His are a front pass, using the existing corrugated metal roof of the barn for an absorber.  Heck, half the collector is already built! 
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Willie, Tampa Bay
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