Drainback vs. Closed Loop Circulators [RE-wrenches]

[Jeff Clearwater]

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Hey Travis,

This is excellent info!  Thanks, I really appreciate the real world data!

Best,

Jeff C.


>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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-- 
~~~~~~~~~~~~~
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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