Solar heater efficiency?

[jaskiainen]

Hi all!

Just got my solar heater ready and i'm wondering it's efficiency?

Inlet temperature was 3 celcius degrees and outlet was 66 degrees.

I'm using two PAPST 4412F/2GLL:n (41,2 CFM) blowers in it.

If you could point me a formula to calculate the efficiency I would be happy.

I was about to search the board first, but i'm in deeeeeeeeeeeep forest and

my internet connection is quite bad in here, so I just bother you guys to

get my answer...

Later I'll post my whole progress with this one including photos.

Best regards Jaskiainen

[Dave B]

 I did quite a bit of research before building my hot air panels and many references indicated a 40-50 degree F difference in input to output temperature to be efficient. This would be for continuous air flow.

  have a snap disk thermostat kicking on at 110 F and off at 90 F. I am pulling air from inside my home so the input temp. is usually no colder than 65 F, I have seen 120-130 F with outdoor temps as low as 15 F. With fans of less CFM these were running much hotter output but with less volume of air. This was much less efficient as I was losing heat back off the glass by not being able to move enough air.

  Keep us posted, if you have any photos that would be great. Here's mine, they are dual pane from 8' sliding doors. Very heavy.  Dave

[GaryGary]

Hi,

This page takes you through estimating the heat output and the efficiency:

http://www.builditsolar.com/References/Measurements/CollectorPerformance.htm

Its an approximate method, but it should get you in the ball park.  You will need a way to measure the airflow velocity out of the exit vent.  The CFM ratings on the fan don't mean much because they go down as the flow resistance through the collector goes up.  There are some cheap ways to measure airflow here:

http://www.builditsolar.com/References/Measurements/measurments.htm

Basically, a solar air heating collector is going to be most efficient when it has enough airflow so that the temperature rise through the collector is just enough to be useful for heating -- this is usually in the 40F to 60F range.   As a general rule, if the temperature rise is more than this, the collector does not have enough airflow, and the absorber is running too hot and losing a lot of heat out the glazing.

Its good to have about 2 cfm per sqft of collector area.

You can have a temperature rise in the right area, and still have a not very efficient collector -- the main reason would be that the air is not flowing evenly over all of the absorber.  That is, there are dead air areas that are not getting enough airflow to carry the heat away.  Not sure how you find these dead air areas, but if you (or anyone) works out a good way please let me know  -- maybe smoke?

One more thing to look at:

http://www.builditsolar.com/Experimental/AirCollectors/AirCollectors.htm

Gary

[zap]

Years ago I tested the efficiency of my panel filling a trash bag, similar to what Gary has on his site at http://www.builditsolar.com/References/Measurements/measurments.htm

With most bags it's hard to measure the exact volume so after doing the measurements a few times I ended up containing the bag inside a large cardboard box.  This made it easier to measure the volume.

I cut a flap in the box that would move out when bag filled up to let me know when to stop the time portion of the measurement.

The larger you make the bag and box, the more exact your measurements will be.

[jaskiainen]

Yep!

The sun came up and i did that trash bag test.

I got 150 liters trashbag full in about 25 seconds. I tried to find some

converter to get it in ft/min, but came up with nothing.(Should it be cubic feet?)

Mainly because this very slow connection here. It's taking forever opening pages.

My test results are following:

Duct is 0.084539 sqft

Toutlet is 150F

Tinlet is 37F

Vair is 6 litres/second (a little help with this please)

I start to think that maybe I need a second pair of those blowers?

I even saw 174F coming out from it. Anyway I think it will do okay and survive with

or without those extra blowers. The area it should heat is about 2600 cubic feet.

[wdyasq]

150. liters = 5.29720001 cubic feet
     
1. liter = 0.0353146667 cubic feet
   
   

HTH,

Ron

[GaryGary]

Hi,

6 l/sec = 12.713 cubic foot/minute

A good site for doing conversions is: http://www.onlineconversion.com

So, your heat output is:

Qout = (air weight flow)(Tout - Tin) (Cair)

Qout = (12.71 ft^3/min)(0.065 lb/ft^3)(150F - 37F)(0.24 BTU/lb-F) = 22.4 BTU/hr or 1344 BTU/hour.

Don't know how large your collector is, but lets say its 20 sqft (you can adjust the calcs to the actual area of your collector).

Solar In = (300 BTU/sqft - hr)(20 sqft) = 6000 BTU/hr

Efficiency = (Heat out)(Solar in) = (1344 BTU/hr)(6000 BTU/hr) = 0.224 (22 %)

The 300 BTU/sqft -hr is for full sun, and for the collector being pointed right at the sun.  Its best to do this test around solar noon, and aim the collector right at the sun.

The temperature rise of 150 - 37 = 113F is a lot -- the collector could use more airflow for better efficiency.

How large is the collector?

Gary

[jaskiainen]

Hi

It hit me yesterday evening:

I just put some rough made fins inside the inlet and forgot they were there.

That I think cause some problems like overheating and much less airflow.

Like yesterday the panel outlet temp was 79 Celsius degrees.

A day before it was "only" 66 C.

So I think I need to fill my trash bag again?

Maybe it will fill within 15-20 seconds. Gonna do that test next weekend.

My panel surface is 3.2 squaremeters or 34.5 squarefeets.

It is solar can heater made out of 340 beercans with double glasing.

Inner is 2mm thick polycarbonate sheet and outer is 3mm acrylic sheet.

Two layers for better performance in winter.

[GaryGary]

Hi,

So, with 34 sqft, ideally you would want around 68 cfm -- I think that's the problem.

Sounds like you may have found the solution.  You don't really have to get all the way up to 68 cfm, but I think you do want a lot more than 12 cfm.

With the beer cans, you should be getting a pretty even distribution of air through the collector as long as the manifold delivers air evenly to the can rows.

You can do a rough check on this by temporary taking off the glazing.  Let the collector stabilize in the sun with the fan on.  Measure the surface temps of the cans with one the IR gun style thermometers.  If you find fairly even temps that increase some for the intake end to the exit end, then the airflow in the cans is even and removing heat all over.  If you find hot spots, those are areas that are not getting enough airflow.  If you don't have the IR temperature gadget, you may be able to feel any hot spots just touching the surface with a finger.

How hard was it to measure the airflow with the plastic bag?  Does the bag technique seem to work pretty well?

Gary

[jaskiainen]

Plastic bag testing was kind of easy and it really is a simple technique.

I agree with the blower part. It should be a bit higher cfm. I will work on that later.

Taking off the glasing is not that tempting. I can get it off, but it's glued on.

So i prefer to keep it in place.

I'll keep you guys posted when i get my mind and on it again.

[jaskiainen]

Mind and hands...

[mbeland]

In a tight modern home with a heat recovery ventilator, how would you think a solar air heater interfere with the air flow?

In that line of thought, I see two possibilities:

1. -If you make your ventilator to run on air from the collector, you would increase the temperature of the cold incoming air. Of course, the HR ventilator probably has a much too strong air flow form the collector.
   
2. - If you run your collector from outside air you can reduce the time needed by you heat recovery fan to get rid of indoor moisture. Maybe you simply don't need it any more.
   
   

Martin