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jjackstone

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Reply with quote  #1 
Hi folks,
You'll note that I am a new member. I have read through a large number of posts, topics and designs. I've also performed searches for the efficiency of some of the air collectors and I see an occasional number for one here and there but no inclusive list. My question is, is there one page where everyone has listed the calculated efficiency for their system so that eveyone else can use those numbers for comparison?

JJ

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SolarInterested

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Reply with quote  #2 
I don't believe so. The closest would be the solar air panel comparison page by Scott:
http://www.n3fjp.com/solar/comparisonhotair/comparisonhotair.htm

Scott links to the companion BuildItSolar page by Gary Resa:
http://www.builditsolar.com/Experimental/AirColTesting/Index.htm

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jjackstone

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Reply with quote  #3 
I've read the comparison pages and they seem like well thought out tests but I don't remember seeing any of them stating an actual efficiency rating for a particular design build. 
Most of them show calculated BTU/hr based on temperature change vs. flow rate and the other more or less "constants". Looking at Gary's? comparison @BIS between a pop can system and a screen absorber he appears to have efficiencies of around 60%. I'm using 300 to 340 btu/sq ft as the input because it depends on where you read for the solar radition number. He has the two, 6sq ft systems pumping out between 1100 and 1200 btu/hr. I'm rounding to 200 btu/sq ft. On the other hand I have seen input numbers as low as 254 w/sq ft which assume an input of 800 w/sq meter. That would put the efficiency of these panels at the much higher number near 80%.

I've looked at and used the hot air efficiency calculator but I have some questions about its use.

1. For collector area. On this site are most people using the total area of the box including the plenums , or is it just the absorber area for calculations?

2. I don't see the fan power use listed anywhere. While that is not a direct reflection of the heating unit, it is part of the overall efficiency of the system. In my case of a 32 sq ft collector, I am using 27 watts to push the air through the heater. I realize that's only about a watt/sq ft, but everything counts.

3. I have seen sites(not this one) claiming efficiencies claiming upper 80's to 90%. This seems extreme unless the glazing is near perfect and almost every bit of heat can be extracted from the collector. From the many posts I've read, most of the glazing material being used is no more than 80 to 90% efficient at heat transfer itself. Are these experimenters using some type of unobtainium for their glazings?

I guess overall I'm just trying to find out is if there is a standard that most people here are using. Thanks for the input.
JJ

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stmbtwle

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Reply with quote  #4 
Solar radiation varies from day to day, hour to hour and sometimes even from minute to minute. Unless you have the means to measure it in real time, any efficiency calculations are going to be nothing but a wild-ass-guess.

I'd LOVE to get some of that unobtanium glazing! Does anyone know what it's R-value is?

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gbwillson

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Reply with quote  #5 

JJ-

I stopped using efficiency calculations to evaluate the effectiveness of a given collector more than 5 years ago. Not for the reason Willie mentioned, which is still valid, but I’m interested in the actual BTU’s being collected.  If someone asked me about how my solar heater performs, BTU’s is something most people understand when compared to a small electric space heater. 


To answer your specific questions:

1. The physical size of the collector is commonly used for reference, since designs may vary greatly. The wall and insulation thickness will also vary, depending on the climate. It would be hard to compare the actual heat collecting area of a screen collector to a serpentine or can collector. 

2. You bring up a good point about how much electricity is used to move heat through an active collector. It is something that should be noted, and if a more efficient fan can be found than a current fan, switched out. But many of the fans used for DIY collectors are salvaged of re-tasked, so they are often free. Spending $100 for a new, energy efficient fan to save 100 watts of electricity will take a long time to recover the cost of the fan, but may save money over time. Someday I may come across a free solar panel and pay nothing to operate my collector fans. 

3. As mentioned, the BTU output of a collector is what is most important. If I build a collector that is nothing more than an empty black box it might only be 30% efficient. But if it provides me with the BTU’s I need, than it is not only efficient for my needs but cost effective. Let’s say I need twice the heat of the empty box heater. I can increase the physical size and increase the heat output. Or I can make a more effective unit within the same space by adding an absorber to help transfer heat to the air as it passes through the collector. Almost any well built solar heater should be able achieve 50% efficiency. And this is more than double the efficiency of a PV panel. There are plenty of solar heater claims shouting outrageous output temps. But these will likely be less efficient as much of the heat will be lost before it enters you home. Lots of lukewarm/warm output air is far more efficient than a trickle of hot air. An empty black box, which would be the least efficient design, with adding an effective absorber inside the collector being the key to maximizing performance. 

The standard 2-screen collector is what is generally used as a reference standard when comparing solar heater designs. If you want to test or compare the effectiveness of a given heater this is the control unit to use. We use it because it is not only cheap and easy to build, but one of the best heater designs. Enough of them have been built where we generally have a pretty good idea of the heat output you can expect. 

Here is a simple formula to estimate the BTU’s per hour collected of a solar heater:

BTU’s/Hour = CFM x 1.065 x Delta T

Once you have the BTU output, you can easily calculate the efficiency percentage, if needed.

 

Greg in MN

jjackstone

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Reply with quote  #6 
Ok guys. Thanks for the info. The math behind this is fairly simple. And I understand that there a number of variables to consider. I've been reading Gary's site off and on for years. Been following solar PV for decades. I'm not sure why it took me so long to realize that solar thermal was so much more efficient than PV. Maybe because I had never seen a kwhr comparison in the past. I'm used to electrical numbers. The main reason efficiency concerrns me is that when I start giving these things away later in the year I want to be able to tell the recipients what they are getting. However you may be right in that they would understand BTUs better.

Not sure what size(power usage) most fans are that are being used but just as a quick calculation say it took a 75 watt fan to produce 1500 watts out in a 3 sq. meter system. If my math is right that is an additional 5% of energy use compared to the output. Not at all bad compared to using a gas or electric heater. 
If we wanted output in BTU/hr the conversion factor I have seen most frequently is 3410 BTU/hr per kilowatt. So 4710 Btu/hr. Using an additional 256 btu/hr for the fan.

Unobtainium. In this case unidirectional with an infinite R value for heat already trapped.[wink]

JJ

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gbwillson

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Reply with quote  #7 
JJ-

Below is an Excel file that has a field that shows the WATTS the collector saved(minus any watts the fan uses) from my 4x16 ZP collector. The numbers are rounded for simplicity, but they are a fair representation of the output.

The numbers in the orange fields are the only ones you need to change. So in this example, the current output of the solar heater would have needed 5,000 watts of electricity to produce the same number of BTU's, which is a bit more than if you had three electric space heaters running. And yes, these numbers are very typical of a well designed solar heater. 

xxx.jpeg 

Here is the Excel file:
xls EZ Solar Heat Calc.xls     


Greg in MN




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Reply with quote  #8 
Those numbers are impressive, but how big a collector did you have to build to get them? Without some reference to size, they're pretty much meaningless.

A word about efficiency or even BTUs, there's more to it than that. What if it's summer or you live in the south? A collector that's shut down (for whatever reason) is ZERO % efficient and produces zero BTUs.

So if you have a collector that's 60% efficient, but you only use it 3 months a year, then on an annual basis it's only 15% efficient... about the same as PV, which runs year 'round.

PV can not only provide heat, but it can also cool and light your home, chill your beer, cook your food, pump your water, run your TV and computer, charge your phone, and maybe even your car. Go figure.

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gbwillson

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Reply with quote  #9 
Willie-

Oops!
Yeah I should have included that tiny nugget of vital information in the above post. I have corrected the information in post #7. The numbers above are from my 4x16 ZP, and while rounded for simplicity are a fair representation of the BTU output. Subsequent ZP's people have built have performed even better, as they have the larger screen gap. A solar air heater is nothing more than an oversized space heater.

I know you can do SO much more with PV. But from my perspective where the heating season lasts 6 months, solar heat is a cheap and easy to build solution to the major portion of my natural gas bill.  As far as efficiency, I've never calculated a solar air heater on an annual basis. NASA states that the most efficient PV panels availabe are just over 20% efficient. Is that a snapshot of a panels ability to capture a percentage of the sun, or does that number represent something else? 

Again, my perspective comes from long, frigid winters. And you can't chill your beer with a solar heater. But that's why God gave us snow the majority of the year up here in the frozen tundra.[wink]

Greg in MN



jjackstone

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Reply with quote  #10 
"So if you have a collector that's 60% efficient, but you only use it 3 months a year, then on an annual basis it's only 15% efficient... about the same as PV, which runs year 'round."

That's what is known as a capacity factor. A little different than efficiency.

The net capacity factor of a power plant is the ratio of its actual output over a period of time, to its potential output if it were possible for it to operate at full nameplate capacity continuously over the same period of time...From wiki. Now that number is going to be low for all solar since it can't function at night.

Even though I am in a fairly moderate climate(Sacramento), I spend a lot more money heating in the winter than I do cooling in the summer. It would be even worse if I had electric heat. Hence the interest in solar heat. I am fortunate to get a regular cool breeze in the summer nights to cool the house down with minimal AC use. 

"As far as efficiency, I've never calculated a solar air heater on an annual basis. NASA states that the most efficient PV panels availabe are just over 20% efficient. Is that a snapshot of a panels ability to capture a percentage of the sun, or does that number represent something else?"
Yes, that is what a good PV panel will convert in full sun at around 25 degreeC or 77F. Efficiency drops at higher temperatures and rises with cooler temps.

Your output is impressive Greg. Hopefully I'll approach this soon. 
JJ

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