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

Jeffrey Wolfe, Global Resources global at sover.net
Thu May 9 19:16:41 PDT 2002


David,

After some practical field problems, we developed the following standard 
for deciding when to install PV direct vs controller systems in cold 
climates:

 - when the SDHW is heating a tank with no other source, then we will power 
with a PV direct (unless the piping run is too long)
 - if there is any other heat source to the tank (which we do frequently, 
since we use those dreaded Thermomax tubes on quite a few of our josb), 
then we always put a controller in. We don't want to take fossil heated 
water and cool it in the collectors, and more significantly, in the 
connecting piping.

No matter what you do, you're going to get some losses in the winter due to 
pipe runs and collector warmup. This is one of the factors in the evacuated 
tube technologies superior performance in the winter.

Jeff

-----Original Message-----
From:	David Blittersdorf [SMTP:davidb at nrgsystems.com]
Sent:	Thursday, May 09, 2002 9:20 AM
To:	RE-wrenches at topica.com
Subject:	Solar Hot Water Systems - PV vs delta T control [RE-wrenches]

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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