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Re: How we can measure the heat loss in a solar collector? - Page 2

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Posted by Morris Dovey on January 2, 2008, 5:08 pm
J. Clarke wrote:
| Solar Flare wrote:
|| Nope. We don't, but my way was more accurate.
|| Now answer the OP's question, if you can, instead of quoting
|| physics laws.
| On a clear day measure temperature rise and flow rate--that gives
| you an approximation of the energy coming out (how close that
| approximation is depends on how accurately you can measure these
| quantities).  From a table figure the insolation per square foot at
| that location on that day (make sure to correct for snow cover etc)
| and multiply by collector area.  That gives you an approximation of
| heat input.  Subtract output from input and you've got losses.

I'm liking this much better than the WAG method. :-D

| The actual measuring of flow rate is usually the problem.
| Thermometers are easy to find--flow meters and low-velocity
| anemometers less so.

As I've been discovering. Duane Johnson has a 'heated wire' flow meter
for DIY construction at http://www.redrok.com/misc1.htm#anemometer ,
and I'm working on a measuring package with an anemometer based on a
CPU cooling fan that you can see at the link below.

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

Posted by gary on January 3, 2008, 3:49 am

For low velocity flows, the $5 Dwyer vane anemometer works well.
On a given day that looks clear, I've found that the actual solar
radiation level can vary a fair bit from the table values.
The Kestrel style wind meters work well down to about 80 fpm at about
$0.  This is good enough for most collectors.
There is also the "garbage bag" technique (see link below).

In a lot of cases, liquid flow rates can be measured very accurately
with a stop watch and a container of known volume.

I've noticed that the actual solar radiation values on what appear to
be clear days can vary a fair bit from the book values.
An Apogee pyranometer solves that problem, but at a somewhat stiff
cost of $80.

Cheap ways to measure stuff:
Any other suggestions for good and cheap measuring devices?


Posted by Niels Erik on January 5, 2008, 11:00 am
 This may work without knowing the flow:

Take two indentical collectors.
Number one is mounted at an angle of say 45 deg pointing south.
Number two is mounted at the same angle behind number one but pointing

Connect number one and number two at the top.
Pump water into the bottom of number one.
Take out water at the bottom of number two. (Take out all air in the
Measure temperature at number one input: T1
Measure temperature at the top between the 2 collectors: T2
Measure the temperature at the bottom of number 2 collector: T3
These measurements have to be done simultaniously.

Number two collector has a loss that is close to number one. In fact the los
in number two is a bit higher than number one because the mean temp. is
higher, but if the diff. between T1 and T2 is small you wil get a good
Water content of the two collectors is the same.

So if you had no loss in number one the temperature rise would have been
(T2-T1) + (T2-T3)

Example: T2 -T1 = 18 deg
T2 - T3 = 2 deg

Temp. rise without losses in number one would have been 18 + 2 = 20 deg.
Loss is 2 deg out of 20 = 10%
This loss is of course influenced by windspeed and air temperature but could
be used when comparing the effect of different kinds of insulations etc.

regards niels

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