permaculture

[John Schinnerer <eco_living@yahoo.com>]

Aloha,
 
> I first looked at doing a ferrocrete tank outside on the polewards
> end of the greenhouse to block winter winds. But since I need 20K
> gallons of water, caught over the entire year and stored for Feb -
> June use, I figured the tank would be too big to make cheaply. I
> became concerned about leaks or structural failure in something that
> size. 

It might not be a DIY project, but there are plenty of ferrocement
tanks that size (and bigger) that work fine.  FC can be cheaper, even
with contractor-sprayed cement, than anything else (metal, poly, etc.)
that size.  Sprayed concrete is likely to be more cost-effective on a
bigger tank than a small one (contractor has same overhead, transport,
etc. regardless of tank size).

> I also need the water to be of a usable temperature for
> irrigation, so an outside aboveground tank didn't seem to make much
> sense because it would be frozen when needed most.

If you put "enough" of the tank below grade, or berm earth up around
"enough" of it after building it on grade, it won't freeze.  The water
may still be pretty cold, though - don't know what is "warm enough" for
your plants.  How much earth-burying/berming is "enough" depends on
your winter temp. range, frostline, etc.

> One smart friend noted that with such a huge heat sink (20K gallons
> of water weighs what, 80 tons?) that I might have problems heating
> the greenhouse. Is there any worry there? The tank will be covered
> and below ground, so I am unclear about why the water would be any
> more of a heat sink than the ground itself. I don't have enough
> understanding about thermal mass, so maybe someone here knows more
> and can explain.

Water has a higher heat capacity than earth (about double), so that's
one reason the water would be more of a heat sink than the earth it
displaces.  And the water will be outdoors-cold coming into the
storage, right?  So it will be somewhat of a factor if it's inside the
greenhouse.

To go further with this I'd need to know how the foundation of your
greenhouse is going to be built and insulated from surrounding frozen
ground in the cold season.  There is potential to use the earth (and
water if you store it there) under the structure as a heat reservoir to
moderate temperatures, etc. depending on how the foundation is built
and a whole bunch of other things. 

In any case where the water is cold coming in, you could perhaps warm
it up a bit before applying it to plants (or storing it) by running it
through some long piping runs in the warmest part of the greenhouse. 
This isn't "free" heat but it's simpler than a separate heater of some
kind...


=====
John Schinnerer, MA
--------------------
- Eco-Living -
Cultural & Ecological Designing
Food - Shelter - Community
john@eco-living.net
http://eco-living.net

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