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Solar collectors plumbed Parallel vs. Serial

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Posted by Nolan Tucker on October 15, 2004, 11:34 pm
I'm trying to find out some information on the pros and cons of plumbing two
solar collectors for a hot water system in parallel verses serial.  I can
understand parallel would make the pump work less, but it seems to me the
water would heat more in serial since the water would have twice as much
time in a collector.  Am I wrong?  Any links that explain this?


Posted by Graham Parkinson on October 16, 2004, 4:28 am
Don't forget that when in series the collectors would operate at higher
temperatures so the collection would be less efficent (ie some back
radiation out of the collectors).  So the best configuration depends on how
good the insulation of the collectors is, and what the final temperature of
the heat store needs to be.

There is a distinction between moving the most heat (parallel collectors
with lower temperatures but lots of water flow ) and getting the hottest
water ( hotter but less water in series)

Posted by Nolan on October 16, 2004, 9:25 am
 In my case I'm not concerned about the amount of heat 'lost' as much as the
heat 'gained' by the hot water system.  The hotter the better as more heat
will be transferred to the hot water.  Bringing the hot water (heat store)
up to 100 degrees would be great.

Posted by nicksanspam on October 16, 2004, 10:36 am

You need to understand the difference between heat and temperature.
The heat power transferred is equal to the temperature difference
times the flow rate.

If the heat store is 90 F, will that happen faster if you add
a) 2 gpm of 110 F water or b) 1 gpm of 120 F water?


Posted by Bert Menkveld on October 18, 2004, 11:59 am
 Hello Nolan,

Plumbing the panels in series instead of parallel will increase flow
resistance, resulting in a lower flow rate for the same amount of pump
power.  The lower the flow rate, the greater the temperature rise will be in
the panels.  While this may seem like a good thing, it usually is not.

If the panels were "perfect" (ie. no losses), then you would collect the
same amount of heat regardless of flow rate.  That is, if you got a 10
degree temperature rise at 1GPM, you would get a 20 degree rise at 0.5 GPM.
You do *not* collect more heat at lower flow rates, even with these
hypothetical perfet panels.  Rather, you get less water at a higher

Unfortunately, real solar collectors do lose heat, and they lose more heat
as they get hotter.  Therefore, the panel that gave you a 10 degree rise at
1GPM might only give you an 18 degree rise at 0.5 GPM.  That's why it's
usually best to run the panels at the lowest possible temperature.

If you plumb your panels in parallel, you will minimize flow resistance,
hence minimizing the amount of pump power required to move water through the
panels at a given rate.  If you decide later that you want to run at a lower
flow rate to get a greater temperature rise, you can always lower the flow
rate by adding a valve to restrict the flow.

Bert Menkveld


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