Solar Thermal storage [RE-wrenches]

[Doug Wells]

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Geoff,
I don't think I have an answer to your system design, but maybe some
thoughts to share.  It might be helpful to ask the client what temperatures
the tank has maintained in the past.  If the Tarm is bringing the tank up to
temperatures above 130-140F then the Solar Collectors efficiency will drop
way down.  This will seriously diminish the amount of energy that the
collectors contribute regardless of your heat exchange method.
In the summer, when the Tarm is no longer being fired, you are heating 800
gallons of water through 2 heat exchangers to heat probably 80 gallons of
domestic water.  Again pretty inefficient.  If you get a serious bout of
sunshine there in Ohio you also have the potential for overheating, so
summer dump would be wise.  Even with a 55-60 degree pitch on the
collectors.
A client last summer had a similar system and we decided to bypass the whole
large tank and installed a smaller domestic system.  They are seeing 140-150
degree water from 2 collectors spring to fall.  If they are using
sustainably harvested wood then...
Maybe they can use the saved money to buy some PV.
Doug Wells
Solar Works Inc.
800-339-7804 ex. 305


-----Original Message-----
From: Geoff Greenfield [mailto:Geoff at Third-Sun.Com]
Sent: Tuesday, May 10, 2005 5:33 PM
To: RE-wrenches at topica.com
Subject: Solar Thermal storage [RE-wrenches]


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I have a not so typical project, and I would really appreciate some input:

The customer has a (really complicated) Tarm wood gassifier with an 800
gallon atmospheric tank.  The Tarm has it's own treated water loop through a
double (parallel) immersed coil heat exchanger.  Also - there is a HE for
DHW pre-heat (actually it is valved to bypass the HW tank and brought down
with a mixing valve) and another HE for a closed radiant floor system. All
of these HE are 55 gallon drum sized coils of soft 3/4 copper pipe - I think
about 90' per coil.

So - all is well and the customer has a budget for (5) 48 SF flat plate
collectors - My question is about how to tie in?  I want to avoid opening
this tank - I am thinking of an external (flat plate) heat exchanger and the
appropriate check valves and pumps to make use of one of the existing coils
(Radiant) to store heat (and when radiant system is calling for heat to use
it directly).  The extra pump and HE are negatives, as is the "double" heat
exchanger loss.

Option B is to open the tank and use an HE with the storage water directly.

Option C is to charge the radiant system with glycol and integrate it with
the collectors.

Option D is to do an open drain back with the actual stored water running in
the collectors (risking a freeze should a control fail).

Option E is your better idea...

For a brighter energy future,

Geoff Greenfield
NABCEP Certified Energy Practitioner

THIRD SUN SOLAR & WIND POWER Ltd.
340 West State Street
Athens, OH 45701

www.third-sun.com
(740) 597-3111

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