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stmbtwle

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Reply with quote  #11 
Greg,
I modified the spreadsheet to accept the collector dimensions and output the BTU/hr/sqft, and I have to admit your numbers were pretty impressive!  Makes me think, even if I could only use it 3 months a year. It might allow me to generate a surplus in the winter!


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Willie, Tampa Bay

gbwillson

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Reply with quote  #12 
Thanks Willie-

I don't see the spreadsheet you modified. Would you post a copy? I don't know who created this spreadsheet, maybe Scott or Gary. All I did was change a few colors, text, and blocked of a bunch of visually unneeded cells to make it easier to understand(and to keep me from accidentally clicking in the wrong place and messing up the spreadsheet).

The ROI for my ZP was less than one heating season, unless you have to make a fancy stand like Krautman or DBC. I did spent a bit more for the aluminum trim so the unit looks really nice as it is very close to the street. But a ZP could also be built on the cheap too.

I've been kinda surprised nobody has looked at using a ZP air heater to heat water. Not sure how it would be done, but I know the ZP needs a LOT of airflow to keep from overheating. And flowing water should be able to draw off enough of the heat to prevent a meltdown.

Greg in MN
gbwillson

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Reply with quote  #13 
Thanks JJ-

Hopefully you can use the spreadsheet above as an easy way to see not only the amount of heat(BTU's) captured, but what it would have cost to produce that heat you used electric heaters. My watt number above would show as if I had 3 or more electric space heaters running at the same time. That is a lot of heat and something most people can identify.

My ZP performance drops roughly 15-20% if below outdoor temps are below 0˚F  and winds are blowing out of the North. My panel sits off to the East side of the house so over 30' of ducts are exposed to the frigid winds. But even on sunny days below zero, I'm still producing a lot of cheap heat. And why do PV panels performance drop so dramatically in the hot weather? I remember last summer during very hot days in Arizona where people had no AC because their solar panels wouldn't perform in the heat.

Greg in MN


jjackstone

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Reply with quote  #14 
I knew about the electric heaters. They normally use around 1500 watts/heater. Fortunately I will never see zero degrees here. Since I moved here in 1999 the lowest I can remember is 19 or 20. Normal winter lows are in the upper 20's and lower 30's.
The PV issue is just the nature of the materials. IIRC the loss(or gain for temp decrease) is in the range of a 1/4% to 1/2% per degreeC. 
JJ

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stmbtwle

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Reply with quote  #15 
Greg, My last ARETHA is a ZP. It's a small one, intended for my hot tub, but it DOES work. https://simplysolar.supporttopics.com/post/willies-zp-aretha-8407113?highlight=zp+aretha&trail=10 I've wanted to build an 8x8 space heater of the same general design, but between the weather and my health, I just can't seem to get to it.

Here's that modified spreadsheet:

 
Attached Files
xls EZ Solar Heat Calc.xls (13.50 KB, 6 views)


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jjackstone

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Reply with quote  #16 
So looking at the updated spread sheet showing the % efficiency is where I have problems. I know you guys have been doing this for years and I'm really not try to be a smarta$$ or even a pain in the butt. But your calculations come out with an 89% efficiency. I'm assuming your efficiency number is derived from your input of output of 266.35 over your input of 300. Maybe I'm having a terminology problem of some sort. If we look at what I guess to be Gary's transmittance tests over at BIS only one of the materials he tested had a transmittance of 89%. Now he did state that he noticed that his measurements were in some cases lower than the manufacturer's stated values, but it wouldn't be the first time that a manufacturer fudged on their numbers or gave ideal values under perfect, idealized conditions. 
I guess one question would be, was your insolation meter inside or outside of the collector when measuring the energy/sq ft?
What type of glazing is used on your system?
Were there reflectors on the system or snow reflecting on the collector?

Like I said, not trying to be a pain, just trying to determine if what I am doing is accurate or if there is something I am overlooking?
Thanks, JJ

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gbwillson

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Reply with quote  #17 
JJ-

We throw solar terms and jargon around all the time. I suggested a glossary of terms a while back, but there wasn't an interest. So don't be afraid to ask if something isn't clear. As I mentioned above, I rounded my numbers ever so slightly so that the watts used cell was roughly 5,000, as an example for you. But the numbers are a fair representation of current performance.

The twinwall glazing I'm using is 6mm thick and is by stated the manufacturer to have a transmittance of 82% if I recall. I don't use an isolation meter. Never had a need for one. But I no longer look at efficiency, but BTU output. I have done some experiments on a very small scale with reflectors, but being that my collector is angled back at a 60˚ angle, with a garden, sidewalk and fence in front of my collector, it's not practical. 

Measuring the input and output temps of a collector is pretty easy. Many thermometers can be calibrated and if that is not possible, it can be compared to a thermometer that has be calibrated. Measuring the CFM can be a bit more tricky. Some people use the time it takes to fill a large bag. But on collectors that have a high velocity output, filling a bag happens too quickly for an accurate timing. An anemometer is often used to measure the speed of the air leaving the collector. But care must be taken and best practices should be followed when taking readings. Once you have the output air speed, A calculation can be made to determine the CFM output, and entered onto the spreadsheet.

Greg in MN
stmbtwle

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Reply with quote  #18 
JJ,

I just threw in that 300btu to have a number. I have no idea what his actual efficiency is, but I doubt it's 89%.

You have to be able to ACCURATELY measure the INPUT as well as the output to get efficiency. Otherwise it's just a wild guess.

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

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Reply with quote  #19 
 
Quote:
I just threw in that 300btu to have a number. I have no idea what his actual efficiency is, but I doubt it's 89%. 

You have to be able to ACCURATELY measure the INPUT as well as the output to get efficiency. Otherwise it's just a wild guess.
 


Ah, there's the answer. Agree with your statement. I do like those output numbers though.
JJ

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gbwillson

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Reply with quote  #20 
This is why I have never been concerned with, nor stated efficiency numbers, of a solar air heater...

The exact solar irradiation of a given spot on earth is a static number. So I don't know if it can accuracy be used to calculate solar air heater efficiency. Air inside an active solar heater is anything but static. We know this because the moving air is continuously adding heat from the time air enters the collector until it leaves.  

For example, let's say the irradiation is 300 BTU's/hr/sf. When air enters the first sf of the collector it might only capture a small fraction of those 300 BTU's since the air is moving on before it can collect the entire 300 BTU's of a given SF of collector. But as air moves through the collector, it continues to gather a fractional amounts for each given sf of collector. By the time the swirling air exits the collector, it has gathered many fractional parts of the static 300 BTU's/hr/sf.

You could, in fact, EXCEED the irradiation BTU's/hr/sf. This would not mean a collector is necessarily greater than 100% efficient, it simply means that a static measurement is being used to calculate the efficiency of a moving fluid, air. 

The BTU output of a solar air heater can easily be calculated without knowing the exact irradiation level , or size of a collector. 


Greg in MN
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