Solar Hot Water Systems - PV vs delta T control [RE-wrenches]

[Tom Simko]

David,
  I have several systems operating pv direct during the cold Idaho winters,
(Hartell MD10-HEH and Dankoff SunCentric pumps) with the heat exchangers (a
simple 60' coil of 1" type K copper) of the glycol loop immersed in 500
gallon stainless tanks. These are space heating systems. Some of the
Heliodyne panels are mounted over 60' away from the storage tank, the supply
and return lines being well-insulated of course. I install "Chek-it" brand
digital thermometers (Grainger # 2T323) on all my thermal jobs, the brushed
stainless readout panel can be located up to 1000' away from the sensor, and
it mounts in a single gang elec box. It operates for a couple years off of a
9 v. battery. I often mount it right next to a Tri-Metric meter on off grid
jobs, and like the Tri-Metric I put it in the kitchen or other heavily used
room in the home. Also like the Tri-Metric its great fun to watch the
numbers roll over as the sun comes out!

 What I have found is the minor 1 or at most 2 degree momentary temp loss in
the storage tank as the system comes up to temperture is a small price to
pay for the simplicity. Once the system is "woken up", the sun disappearing
behind clouds off and on doesn't seem to matter, the slight loss is a one
time early morning thing. To me the diff control v. pv direct is somewhat
like the PV tracker/PVfixed array argument. Points can be made for both
sides, I figure simple is ALWAYS better. These 6 panel (Heliodyne 4x8's)
systems raise the 500 gallon tank temps over 50 degrees (starting at about
80 or 90, ending up at around 140 or so) during the short winter days, which
handles the homes heat load through the night with a simple radiant floor
heat system and good insulation. Is this the most efficient way to go? I
don't know, I do know its maintenance free and it works!

Tom Simko
Skyline Solar
www.skylinesolar   


on 5/9/02 7:20 AM, David Blittersdorf at davidb at nrgsystems.com wrote:

> Question:
> 
> We are in Vermont which gets real cold in the winter and SHW systems are
> antifreeze 
> loop types. I have a Heliodyne system with AC pumps and a differential
> temperature 
> sensor controller. I am looking at the trade-offs of going to the PV powered
> pump 
> system vs the delta T control. I like the temperature sensor system as it will
> not turn on 
> the pump when the stored hot water is warmer than the collector temperature. I
> have 
> noticed in the very cold winter months that the pumps cycle on and off to make
> sure the 
> incoming collector water is at a higher temperature than the tank.
> 
> I can forsee a problem with the PV powered pump happly pumping cold water into
> a hot 
> tank because the PV panel is very efficiently running the pump (cold temp.)
> but the 
> collectors are not warm yet. I also notice that my PV panels shed snow quicker
> than my 
> hot water collectors.
> 
> Any experience out there?
> 
> Thanks,
> 
> David Blittersdorf
> NRG Systems, Inc.
> 110 Commerce Street
> Hinesburg, VT 05461 USA
> Tel: 802-482-2255
> Fax: 802-482-2272
> email: davidb at nrgsystems.com
> Web:http://www.nrgsystems.com
> 
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