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solar air heater review and a few questions - Page 2

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Posted by gary on September 23, 2008, 7:06 pm
 
Hi Nick, ...

We have a nice sunny day going today, so took some measurements of
collector output at various vent sizes.

Cross section area per half bay = (5.75)(23.25) = 133,7 sqinches
Vent when full open = (4)(18) = 72 sq inches -- just a touch above 50%
of cross section

Condition     Vent velocity  Col delta T   Vent area   Product
Relative
Full open        130 fpm        29F            72
271.4K    base
Half blocked    158 fpm        34F           36             193.4K
71%
3/4 blocked     180 fpm        50.5F         18             163.6K
60.2%

The "Product" column is (vent velocity)(delta T from inlet to outlet)
(Vent area)  -- so, proportional to collector heat output.
Temperatures in F, velocities in fpm, areas in sq inches.

Test started at 11:15am and went through 12:24 pm  -- started with 4
measurements with full open vents at about 2 minute intervals, then 4
with half blocked vents at 2 minute intervals, then 3 at 3/4 blocked
at 2 minute intervals, then 3 back at full open.
Not a lot of variation from reading to reading -- pretty consistent.
Used the Kestrel wind meter for velocity and just did the center of
vent velocity (seems to me this favors the small vents with higher
perimeter to area ratio) -- checked readings against the Dwyer Vane
Meter.
For temperature, just used 2 hardware store alcohol thermometers with
the plastic cut away from bulb area so they would not block flow --
checked to make sure they read the same before starting, although it
would not make much dif for this test.
The azimuth on the collector was zero at 12:14.
Appeared to be very clear with some breeze.
Tambient was about 60F.

Note that collector output is still modest at this time of year
because sun is still fairly high (45 deg) and no snow field in front
of collector.  On a mid winter sunny day, it routinely produces 50 to
60F temperature rise with 140fpm, and this is with a colder ambient.
Vertical collectors are the deal for space heating.

Seems to me the penalty for small vents is not so small -- wish it
were the other way round.

Seems like there is something that could be learned by studying this
vent thing in more depth -- for example, effect of nicely shaped
turning vanes at the vents and closer attention to a lower drag
configuration inside the collector.  The numbers indicate to me that
not nearly all the flow resistance is due to the absorber?

Gary




Posted by gary on September 24, 2008, 2:24 am
 
Just to clear up the badly formatted table of results in the last post
-- here are the results from the three tests.
The first test is fully open vents, in the 2nd, half the vent area is
blocked with cardboard, and in the third 3/4 of the vent area is
blocked with cardboard.

Test 1: Vents full open
---------------------
Vent area = 72 sq inches
Vent velocity = 130 fpm
Delta T from lower vent to upper vent = 29F
Product of Area*dT*Velocity = 271.4K
Relative performance = base

Test 2: Vents half closed
------------------------
Vent area = 36 sq inches
Vent velocity = 158 fpm
Delta T from lower vent to upper vent = 34F
Product of Area*dT*Velocity = 193.4K
Relative performance = 71% of full open

Test 3: Vent 3/4 blocked
-------------------------
Vent area = 18 sq inches
Vent velocity = 180 fpm
Delta T from lower vent to upper vent = 50.5F
Product of Area*dT*Velocity = 163.6K
Relative performance = 60.2% of full open

Gary

Posted by schooner on September 24, 2008, 10:15 am
 Gary - Do you have a fan in your unit?

Based on your numbers I wonder if moving more air through the unit would
further increase the performace by getting more air through the system and
in turn heated.  Its really trying to find a balance between the temp
increase and the airflow to max out the overall heating performance.




Posted by gary on September 24, 2008, 2:21 pm
 Hi Schooner,
I don't have a fan -- its designed as a thermosyphon collector.

I tested it last winter with pyranometer, temperature logger, and two
kinds of airflow monitoring, and its efficiency is quite good.
With the vents that are on it, it actually achieves the airflow that
is recommended for fan forced collectors.
I like the idea of not having to buy, power, and maintain a fan and
controller.

I really like the way it responds to low sun conditions -- as soon is
there is enough heat to warm the inside of the collector just a bit, a
very small airflow starts -- you can just barely see the back draft
damper move.  As sun increases, the flow increases accordingly.
With a fan forced collector and controller, setting the controller is
always a compromise -- if you set it to respond to very low sun
conditions you may be using more energy running the fan that the
collector produces, and if you set it to wait for more sun, you are
not collecting the energy from low sun conditions (albeit small).

I am a "fan" of well designed thermosyphon collectors -- their
simplicity, low cost, no maintenance, and subtle control system are
amazing (to me).

Gary



Posted by schooner on September 24, 2008, 2:49 pm
 Gary thanks for the info.  Ya that setup is certainly the best and most
efficient.

I guess I'm just use to working with our designs for mounting on our house,
where we don't have the option to do such a wide inlet and outlet, which
forces us to use a single in/out from the collector boxes with the fan to do
the work to move the air, but overall it has worked well.  For one of my
panels the inlet and outlet are both at the bottom due to the need to have
it heat the basement, no real other option when the access points need ot be
so low.  Also handly if the panel needs to be mounted from the house a bit
and so on for best sun.

Hi Schooner,
I don't have a fan -- its designed as a thermosyphon collector.

I tested it last winter with pyranometer, temperature logger, and two
kinds of airflow monitoring, and its efficiency is quite good.
With the vents that are on it, it actually achieves the airflow that
is recommended for fan forced collectors.
I like the idea of not having to buy, power, and maintain a fan and
controller.

I really like the way it responds to low sun conditions -- as soon is
there is enough heat to warm the inside of the collector just a bit, a
very small airflow starts -- you can just barely see the back draft
damper move.  As sun increases, the flow increases accordingly.
With a fan forced collector and controller, setting the controller is
always a compromise -- if you set it to respond to very low sun
conditions you may be using more energy running the fan that the
collector produces, and if you set it to wait for more sun, you are
not collecting the energy from low sun conditions (albeit small).

I am a "fan" of well designed thermosyphon collectors -- their
simplicity, low cost, no maintenance, and subtle control system are
amazing (to me).

Gary



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