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Rick H Parker

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Reply with quote  #51 
Yea but you would only get 100 watts of heating power max if you heat the water directly with the PV.

In message #12, you calculated 82 BTU/sq ft/ hr.   82 BTU/hr = 24 watts of power, take that 24 watts per sq ft and multiply by your total sq footage. Your already getting more heating power from the Aretha then you would get from a 100w PV alone. If you can get the efficiency of this Aretha up, your be doing even better.

Solar thermal panels can average 50 - 70 watts per square foot annualized.

Rick H Parker



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Rick H Parker
Kansas, USA
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Reply with quote  #52 
Not around here they can't, they're shut down a good part of the time, and a collector that is not running has 0% efficiency. Annualize that.

Total sq footage of my ARETHA is just over 7, x 24 = 168 watts. Currently I'm using a 20w panel to run it. That's a "net" output of 148 watts. If I add in the pv output, I've generated 188w for an output of 168w.

Install the bigger fan and a 100w panel to run it and my cost goes up nearly double. Assume some increase in efficiency, maybe I get 200w, but my "net" drops to 100 watts. With the pv output it's 300/200. With a bigger collector, it might make sense.

That's the drawback with ARETHAS, it takes power to get that hot water, and you have to be careful how much you use or your efficiency goes to hell.

Two 100w panels could produce the same amount of power for about the same cost and a whole lot less aggravation, and I could use that power for something else when the tub didn't need it.

It's a fun hobby, but at least here in Florida I think Solar Thermal is an obsolete, cost-inefficient technology. The folks in the cold country can get a good benefit from their air heaters, but they're pretty useless here. Hot water isn't a whole lot better, and a whole lot more complicated and expensive to implement. It's possible to be "net zero" with PV, I don't think it is with thermal.

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Rick H Parker

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Reply with quote  #53 

Went back over the numbers in #12. Insolation per square meter 322 = 30 watts per sq ft. If your getting 24 watts per sq ft, that is a conversion efficiency of 80%.  It does not get any better then that. Perhaps you need to rethink your assessment or double check your math. 

80% conversion efficiency is an A+.

Rick H Parker

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Rick H Parker
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Reply with quote  #54 
80%?  

I wish it were, but that's NOT 322 watts/sq meter, it's 322 BTU/sq ft/hr. "per meter" only means I used a meter to measure it.  (a "Dr Meter" SM206)

So 83 btu/322 btu = .258, or 26 percent.  [frown] 

I think the new HEX is doing slightly better but don't have a measurement on it yet.

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Rick H Parker

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Reply with quote  #55 
Insolation is the power received on Earth per unit area on a horizontal surface in watts. Did you convert watts to btu/hr?

Rick H Parker


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Rick H Parker
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Rick H Parker

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Reply with quote  #56 
322 Btu/hour Sq meter = 94.368 watts sq meter = 8.770 watts sq ft, no way you can get 24 watts per sq ft out of 8.770 watts per sq ft. Something not right in the numbers, units or the math.

Rick H Parker


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Rick H Parker
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Reply with quote  #57 
You're comparing apples to oranges.  I am not working in watts per sq meter, I'm working in BTU/hr per sq FOOT.  Collector output was measured in pounds/min x 60 x temp rise in Fahrenheit.  Pump was tested at .6 gpm, or 4.98 #/min. rise was 2F

So 4.98 pounds x 60 minutes x 2 degrees F = 597 btu/hr total, divided by 7.2 sq ft = 83 btu/sq ft/hr

The SM206 was secured to the top of the collector, with the sensor of the meter exactly parallel to the face of the collector. It has a setting for btu/sq ft/hr, which read I directly, 322.  83/322 is still 26%.  Sorry, I WISH it were 80%, I'd go into production! [smile]

The new pump tests at .9 gpm or 7.5 pounds/min and I'm getting about the same delta T, so I'm guessing efficiency has improved a bit, but I'll have to put the meter on it, recheck the flow, and get a more accurate delta.


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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
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Reply with quote  #58 
 Test of 2' x 4' zero pass Aretha collector. Tampa FL
      
 Date03/20/17   
      
 Pump rate: (Usgpm)0.858.33 #/gal7.06# / min423# / hr
 Pounds/hr423   
 Water temp diff: (F)2.55   
 Total btu/hr1078   
 Collector area sq ft7.03   
 btu/sq ft153.3   
      
 Insolation btu/sq ft353   
      
 Efficiency43%   

Now that's a little better!  [smile]   (check my math please)

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Rick H Parker

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Reply with quote  #59 
Water weight 8.3 lbs/ US gallon
0.850 gpm * 60 = 51 gph.
8.3 lbs * 51/hr = 423.3 lbs/hr.

It takes 1 btu to raise 1 lb of water 1° F

° F * lbs = btu

2.55° F * 423.3 lbs/hr. = 1079.415 ° F lbs/hr. 1079.415 btu/hr.

(1080 btu/hr ) /7 sq ft =  1079.415/ 7 * btu/sqft/ hr = 154.202 btu/sqft/hr.

(154.202 btu/sqft/hr) / 353  btu/sqft/hr * 100% = 43.683 %, rounds to 44%.


Look good to me.


Rick H Parker



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Rick H Parker
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Reply with quote  #60 
I think one of the reasons for the high efficiency is the low temperature this particular Aretha is operating at.  Output only needs to be above hot tub temperature which isn't that far above ambient, so less is lost through the skin and glazing.  If the same unit were in DHW service at 150F or better, efficiency would be less simply due to losses.

You guys with your air collectors and big insulated ducts might bear that in mind... a 1/2" pipe is a LOT easier to insulate from the elements than a big duct (also easier to hide from the wife).  Once in the house you could use a fan coil or baseboard heater to distribute the heat.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
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