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Posted by Gary on November 29, 2005, 3:48 pm
 
Hi Iain,

I am in the process of building some solar space heating for my house:
http://www.builditsolar.com/Experimental/InWorkshop/SolarShed/solarshed.htm

It will have a "hole in the ground" style 500+ gallon heat storage
tank in the garden shed floor (well insulated).

I have a simulation, and have run it with a 500 gallon tank and a 1000
gallon tank.  The rough results are as follows:

Climate: SW Montana (8000 deg-days)
Collector area: 240 sqft
House heat loss:  350 BTU/hr-F
House effective thermal mass: 6000 lb
Collectors: Flat Plate, selective coating

Solar Fractions by month for the two tank sizes:

Month       500gallon    1000 gallon
10        0.81      0.94
11         0.36      0.39
12        0.23      0.23
1         0.24      0.24
2        0.31      0.32
3        0.48      0.5
4        0.62      0.64

The larger tank does a bit better in the warmer months of the heating
season.  But, when you look at total BTUs produced by the system with
the two tank sizes, its not very great -- about 5%:

Total solar BTUs:
with 500 gallon tank:  20.4 MBTU
with 1000 gallon tank: 21.2 MBTU

I've also run it with evac tube collectors, and the differences are
small.
The simulation uses the hour by hour TMY weather file for Helena, MT,
and the collector output is calculated based on the efficiency curve,
sun in, Tambient, and Tstorage.

So, I am not seeing much difference between 500 gallons of storage and
1000 gallons of storage.
I am wondering how you got to 10,000 gallons?  Is there that much
difference because of climate or other factors? Or, did I screwup
somewhere?

I should add that since the pictures at the link above were taken, we
got 14 inches of snow over Thanksgiving :-)

Gary



Iain McClatchie wrote:

www.BuildItSolar.com
gary@BuildItSolar.com
"Build It Yourself" Solar Projects









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Posted by Iain McClatchie on November 29, 2005, 7:21 pm
 
Gary,

Nice writeup.  I hope to do something like that for my house.

How did I get to 10k gallons?
  - Northern California climate, 2000 degree-days/year
  - 1300 ft^2 of R-2.1 windows.  That's a lot.
  - CA Title 24 requires expensive windows, small windows, or
    solar thermal, and solar thermal is cheaper than expensive
    windows.
  - few hundred cfm of fresh air
  - house loses ~700k BTU/day in January, 1450 BTU/hr-F
  - 35x 40 ft^2 flat plate collectors = 1400 ft^2 area.  The
    house roof is designed for them.  Angle is too low: 22
    degrees... but that fits in the height limit (28 feet).
  - weather station shows that we see 2-week stretches of
    cloudy weather during the winter at this site

At $/gallon, the best tank size for this house is about 2000 gallons
(about two nights worth of heat).  But if the incremental cost of the
tank is around $.30/gallon, the best tank size rises to around 4000
gallons.  This analysis presumes current fuel costs.

It is very expensive to rip out a built tank and build another larger
one.  If fuel prices rise, the optimal tank size rises too.  The
question
I asked myself is, how much am I willing to pay for a hedge against
future fuel price increases?  The answer is: a couple thousand
dollars.  Now note that the extra tank volume has some usefulness
even now, so that my marginal loss for extra tankage is pretty low.
A couple thousand dollar hedge ends up being a pretty big tank.

This could all be quite stupid if the big tank ends up losing lots of
heat because of insulation failures.

The collectors cost too much as well, and only make sense because
they heat the pool for seven months of the year as well.  Maybe
more, as I've tried to be conservative with the analysis.

Feedback on your design:
  - Under "pros", you claim not to need an expansion tank.  Why not?
    Are you assuming your storage tank will be only partly full, and
the
    ullage space will act as your expansion tank?
  - Given your collector area and house losses, you need only store
    heat overnight.  Extra tankage just makes your collectors a little
    better in midwinter, and helps for a few weeks in spring and fall.

How steep are the walls of your hole in the ground?


Posted by Gary on November 30, 2005, 2:01 am
 Hi Iain,


Wow -- those are big numbers.
My house has about 450 BTU/hr-F loss rate now, and I am going to try
to get it down to about 350 with more insulation etc.  -- its hard to
fathom 1450 BTU/hr-F and 1300 sqft of window.
That makes the 10000 gallons make a lot more sense.

...


What kind of collectors are you planning to use?


The system will be set up like a solar DHW drainback system. The
storage tank will be vented to ambient just like the drainback tank on
a DHW drainback system.  I am a bit concerned about the fact that this
might introduce some air into the system, but the simplicity of it
seems worthwhile.  I may have to use non-iron pumps.
The system has one circulation loop that goes to the collectors, and
drains back to the storage tank when not collecting (for freeze
protection).  And, a 2nd circulation loop that goes from the storage
tank to the heaters in the  house.  So there are two pumps and couple
valves, but thats all.  No heat exchangers, expansion tanks, purge
valves, vacuum breakers...

I don't quite fully trust the drainback controller to never fail, so,
I am thinking about wiring a thermal snap switch in series with the
pump.  The snap switch would live in the collector space, and open at
temps below about 80F.  I think this will provide a fully independent
insurance against the differential controller (or sensors) failing,
and the system freezing up.

I'm also working on a simple system to vent the collectors during the
non-heating part of the year so they don't see stagnation conditions
all summer.


Right -- thats how I sized the tank.  I think that the simulation only
showing a 5% improvement for doubling the tank size tends to confirm this.


For my soil, I think that about 60 to 70 degrees is going to be good,
but the tank is not very deep.  I've done this before for the "coil of
pipe" DHW system that Nick and I are working on, and the most
difficult thing is getting the hole sides uniform enough that the
insulation board bears against it well, and also the cutting of the
beveled insulation board pieces -- not actually all that difficult,
but time consuming.

 From your other note:
Two other questions about your design:

- How much insulation around the pipes taking water to
   and from the house?  Have you calculated the losses
   here?

The insulation is shown here:
http://www.builditsolar.com/Experimental/InWorkshop/SolarShed/trench.htm
Its about R15 (3 inches of Polystyrene extruded foam board).  I tried
hard to use insulation that would hold up underground, and to insure
that the pipe and insulation string were kept in the dry.  The R15 is
not giving any credit to the gravel and dirt around the pipe string
that will remain dry, and should help.  It also does not take any
credit for the fact that the two pipes (supply and return) see each
other (and not ambient) for part of the way around their heat loss
circle.  I plan to put a data logger sensor down in the trench to get
some idea what the temp settles out at 4 ft down, but I know it will
be above the outside ambient -- so that should also help some.
I did a heat loss for times when there is flow in the pipe, and its
came out pretty low -- don't remember the exact number.


- How much insulation around your tank?  I found I needed
   a huge amount.  You should get by with less, but I'm
   still curious what you got.

I'm debating between 4 inches and 6 inches.  I did a heat loss for
R30, and it seemed fine.  I'll probably go with 6 inches, since its
not all that much more money for my size tank.  Most of the tank is
buried, so it should see a surrounding temp above ambient, and get a
little benefit from the insulation value of the dirt.  I also plan to
pile some fiberglass insulation left over from another project over
the top -- so it might be more like R50.



Please keep us posted on the progress of your project.

--


Gary

www.BuildItSolar.com
gary@BuildItSolar.com
"Build It Yourself" Solar Projects









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Posted by Ecnerwal on November 30, 2005, 3:29 am
 

Please tell me they see each other though a layer of insulation. We've
already had more than enough instances of people creating heat
exchangers by insulating around parallel supply and return pipes, but
failing to insulate the supply and return from each other. This leads to
poor performance - please don't repeat the mistake...

--
Cats, coffee, chocolate...vices to live by

Posted by Gary on November 30, 2005, 4:55 am
 Ecnerwal wrote:

Good point.
There is a bit over an inch of insulation between the supply and
return pipes.
Seems like that should be OK.

--


Gary

www.BuildItSolar.com
gary@BuildItSolar.com
"Build It Yourself" Solar Projects









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