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solar hot water with HUGE tank

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Posted by cam on April 24, 2008, 6:17 am
hey guys

im new to the group - ive been looking into solar hot water for my new
i currently have 2  commercially made solar water panels 12ft x 6ft
and 12ft x 4ft.
I actually plan to install a 20000Litre tank beside my house insulated
with 200mm polystyrene all round. I plan to collect from both panels
as well as plumbing black Polyethylene solar safe piping down each
corrugation run behind my roofing iron(black) - approx 40m2 total if i
run them down each 3rd or 4th rib.Ill insulate behind these pipes to
help put the heat into the piping. Ive got a friend who will design me
a controller for multiple sensors ,pumps etc or i may run an old pc to
do it.
what i want to know is

Is volume enough?
I want to heat the floor with infloor water (only about 100m2) from
the tank also - this will be isolated from ground by 100mm polystyrene
and edge insulation also.
Im considering heating my house hot water (not floor) by running
copper heat exchange coil in big tank but thinking that maybe i need
an actual hot water cylinder so i can transfer the water from the big
tank into the smaller one if it needs extra heating.
considering also using a heat pump hot water heater in tandem with

any comments help etc gratefully appreciated

new zealand

Posted by Solar Mike on April 24, 2008, 10:37 am

"cam wrote"

Hi Cameron

My suggestion if you are going to diy the system and controller is:
Use a separate hot water cylinder with a backup element, setup the
controller to heat this cylinder first from your 2 panels  With that area 12
sqm shouldnt take too long. That way you will get useable heat quickly. If
there is no sun then the controller could turn the element on. Use a big
cylinder > 360L with the element 1/3 rd from the top.

When it gets up to a suitable temp say 60-65 C. the controller then diverts
heated water to your huge storage tank -sounds a bit big, will take many
days to heat up. The pipes running down the roof iron will not be as
efficient unless you encapsulate that section of roof with a thermal lid eg
clear polycarbonate mounted on wooden frame bolted to roof.

I was going to use 30 sqm panels to heat a 3000L outdoor tank for indoor
space heating, trouble with a very big external tank is the losses, takes
days to heat and during that time you are loosing heat also. If the tank was
located under your house then at least the heat lost goes into the house.

Christchurch is not the sunniest place in the winter - had any frosts yet ?.
A ground source heatpump connected to your large tank as the heat source
could extract the low order heat and heat your slab. If the water was
sufficiently hot then you could bypass the heatpump altogether - unlikely in
the dead of winter.

I was looking at an energy efficient house last week not far from where I
live (Whitby - Wellington), they had buried 300m of piping 1 metre under the
insulated slab and were using the ground as a heat store. After the main hwc
was heated, the controller - pump diverted the water into this mass of pipes
in the ground. Dont think I would do that, too many variables, an insulated
water tank buried in the ground would seem a better bet.

Mike (NZ)

Posted by cam on April 25, 2008, 10:26 am
yeah i had planned a ring main fully insulated system on pump .

 The pipes running down the roof iron will not be as

entire NNW 20m long 40deg roof is going to be clear polycarb
hopefully- with solakote coated black metal sandwiched between
polycarb and roof batterns running manifold of black PE (maybe welded
joints for security) every 2-3 corrugations and of course some kind of
back insulation.
a pump to circulate to the storage tank and yes probably a primary
cylinder, may buy a 30 set vacuum tubes for the high end heat of
topping up the cylinder if the storage tank runs at 40deg appx year
figured that it would take at least 6 weeks in summer to get to temp
but thought that it
will hold temp through jun july aug and then start recovering sept and
not have to worry after that.

only got a 450sqm site so not so keen on groundsource.
this concept has been done in belgium year round hot water but i cant
find the website for it

Posted by Solar Flare on April 25, 2008, 1:29 pm
 Yeah, exactly I was


maybe next time

you could

so that your message

was more readable

as if you cared.

Posted by Joe User on April 25, 2008, 1:29 pm
 On Wed, 23 Apr 2008 23:17:22 -0700, cam wrote:

Water is an excellent material for storing heat.

But, it might be a lot cheaper to use insulated soil or rock to store the
heat, and use water as a heat transferring fluid.  Maintenance and repair
would probably be cheaper, too.

You could dig a trench, insulate it, concrete it, install heat transfer
pipes, and backfill it.  Then, you would cover it with insulative,
load-bearing material.  When you have a leak, you just open the trench,
dig up the leaking pipe, patch it, and close the trench up.  In the US,
it would probably be easy to use one or two prefabricated septic tanks, or
concrete drainage tiles, in the construction of the heat store.

The heat store would have to be much larger than 20 cubic meters, but it
would still be much cheaper and easier to build, I think.

    The study of history is a powerful antidote
    to contemporary arrogance. It is humbling to
    discover how many of our glib assumptions,
    which seem so plausible, have been tested
    before, not once, but many times and in
    innumerable guises, and discovered to be,
    at great human cost, wholly false.
        -- Paul Johnson

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