Drainback vs. Closed Loop Circulators [RE-wrenches]

Travis Creswell, Ozark Solar ozsolar at ipa.net
Mon Jun 5 12:52:50 PDT 2006


Hello Jeff,

We just did a drain back system with six 4x10 flat plates.  Calculations
showed that we had to use Grundfos 26-96 to overcome the head and total
pressure drop.  I carefully added up every foot of pipe, valve, elbow etc to
make sure I didn't oversize it.

It's rated at 215 watts but we choked the flow down to ~9 gpm (1.5 per
panel) and dropped the wattage to 135 as measured with a Brand power meter.

The pump would do almost 20 gpm with out reducing flow which was higher then
the charts said it would do so I assume there was a siphon effect adding to
the flow.  But you can't factor that into the pump sizing as you have to
overcome the head initially.  And the next smaller pump left no room to
safely overcome the head plus it didn't use much less wattage.

I'll gladly use the extra energy to get all the benefits of drain back.  In
the morning I saw a 20f delta t at 9 gpm.  Of course the Delta T drops as
the days goes on but stays above 10.  That's 90,000 BTU's of heat for 460
BTU's worth of electricity.  So the average of the day that's 1% of the
energy collected is used to pump.  Share that with the "thermal engineer"
and see what he says.

The common circulation pump is centrifugal which is not a positive
displacement type.  Very low wattage positive displacement pumps are
available that can handle the temp but their flow rate limits them to a max
of several panels.  Ken Schaal of Commonwealth Solar can speak with direct
knowledge of them.

Best,

Travis Creswell
Ozark Energy Services


-----Original Message-----
From: Jeff Clearwater [mailto:clrwater at earthlink.net] 
Sent: Friday, May 19, 2006 7:56 AM
To: RE-wrenches at topica.com
Subject: Drainback vs. Closed Loop Circulators [RE-wrenches]


Hi All,

We have been getting back into offering Solar DHW - mostly due to two 
factors that make it a doable business model in terms of 
installation, service and liability.

The first is switching to evacuated tube collectors.  They are light, 
easy to ship and handle and easy to install - I don't have to worry 
about cranes or major racking weight and it eases safety factors with 
my installers.

Second is going with drainback instead of closed loop glycol.   This 
eliminates having to service the glycol every 2-3 years and actually 
gives a chance that the system might live on when the folks sell the 
house etc.

However, I recently suggested drainback, closed loop to a low income 
zero energy home development we are working with and the thermal 
engineer came back with data saying that the drainback circulators 
used up to 1/3 of the energy harvested by the system in terms of 
electrical cost vs. gas saved!  I find that hard to believe. He 
claimed the closed loop circulators use less but still was showing a 
significant percentage.  I'm not sure whether he's just finding an 
excuse to go with what he is used to or whether there is a 
significant difference here.

So the questions for my esteemed wrenches are:

1)  Does anyone have KWH use figures for solar DHW circulators?
2)  Does anyone know of the most efficient circulators?  (are 
circulators positive displacement pumps?) - any models to suggest for 
drainback?
3)  Do folks recommend using PV direct pumping for drainback?  What 
controller would one use then?

Thanks!

Jeff C.

-- 
~~~~~~~~~~~~~
Jeff Clearwater
Village Power Design
Sustainable Energy & Water Solutions for Home & Village
http://www.villagepower.com
gosolar at villagepower.com
NABCEP (tm) Certified Solar PV Installer

530-470-9166
877-SOLARVillage
877-765-2784
72 Baker Rd.
Shutesbury, MA 01072
425 Nimrod St.
Nevada City, CA 95959
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~`~


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