Long Term Heat Storage for our House (Hot Water 4)
Project Start: February 2012
Our old "Thermonuclear" hot water system (Hot Water 2) has been going
really well for 9 years or so now, but it has some drawbacks which we'd
like to rectify. We've also been looking at the idea of storing
heat in a LARGE tank, so that excess heat collected in summer can be
stored and used in winter for central heating.
I'll put the figures and design lower down on this page, but in the
meantime, here's the idea:
Solar Panel: 12m
x 3m (36sq.m) using LDPE pipe as the collector. Similar to our Cottage Central Heating system, but with a
rigid clear plastic covering instead of greenhouse film. This
panel should provide at least 10kW for 4-5 hrs per day in summer, and
7kW for 3 hrs per day in winter. That's 40-50kWh per day in
summer, 20kWh in winter. Lots. Storage Tank A: 1,000L
square tank (1x1x1 m) which is fed directly by the Solar Panel at 100
litres/hr. This tank is used with a heat exchanger to supply hot
water to the house. It will be made of insulated wallboard, 250mm
thick (Steel sheet enclosing a polystyrene sandwich), and lined on the
inside with black plastic film. It will take 2 days to fill this tank
with hot water at 70deg.C. Storage Tank B:
50,000L rectangular tank, (yes, that's right - 50 tons of water!!!!),
also made of insulated wallboard. We haven't settled on the
dimensions yet, but it'll probably be about 3m x 8m x 2m tall,
and supported by a wooden frame with ribs, like a rectangular
Viking boat, and also lined on the inside with black plastic
film. It should take about 3 months to heat this tank to 60deg.C
with the overflow from Tank A in summer. The water in this tank will be
used for central heating in our house. NOTE: We've decided to
make a circular tank (well, 14-sided, actually) 5.5m diameter and 3m
tall.
The idea of having two tanks is so that we can separate the two
functions - hot water for showers, kitchen, etc, vs. hot water for
central heating. The 50,000 litre central heating tank has lower
priority, so it receives the excess hot water from the 1,000 litre
tank, which would otherwise go to waste. (It does go to waste at the
moment, except when we fill our spa pool.)
Update 5 February 2012: Building the
Collector Panel
The holes are all dug, the posts concreted in,
...and the rafters are up.
The panel on the right is the 9-yr old
one which will be demolished when the new one is working. (We
will continue to use the old one in the meantime - it works pretty
well, and we don't
want to demolish our hot water supply just yet!)
Update
August 2012: (it's been a
while!)
We laid the chicken wire and building
paper...
Then the corrugated
iron (painted black,
of course!)
...and then the polycarbonate clear
plastic covering over the pipes
(I can't find any photos of the laying
of the pipes - they seem to have disappeared!) The bucket of the
tractor was a really good way to reach the face of the panel.
Update
October 2012:
We started digging the foundations for the concrete pad which will
support the 50,000 litre water tank.
We've decided to build a cylindrical tank instead of a rectangular one,
because it will be much easier to brace by strapping it with plastic or
steel straps round and round the circumference. We worked out
that the water pressure near the bottom of the tank will produce a
force of one tonne per square meter pushing the panels outwards!
And we couldn't figure out a way to brace a rectangular tank to resist
that force.
More digging. Do you notice how I
manage to get my wife and daughter involved?
The boxing goes up...
Raking
and levelling to an even depth of 100mm
The concrete truck arrives
... and the
pad gets a good smoothing
over
(What? you didn't think we were going to mix
it all by hand, did you?)
You can see the polystyrene sandwich
board panels which
we're going to use for the construction of the tank. They are
250mm thick, with steel sheet on both sides. It's going to be
interesting cutting them to shape!
Update
Jan/Feb 2013: Final Steps!
Stitching the polystyrene floor panels
together
Laying the carpet on the floor (2 layers)
The first panel goes up
And now there's only 1 to go!
Straining the first wire
The roof goes on temporarily (it's going to rain!)
The raincoat goes on top of the outside layer of carpet
This is the view
from the dining room
And this is the view from the inside (carpet on walls)
Laying
out the inner liner - gosh it's HUGE!
Draping the inner liner over the top
Slowly does it....
We centred the liner using water in the bottom
This is the main rafter for the roof (upside
down)
The tank is surrounded by LOTS of wire
Filling with water now...
The copper coil heat exchanger in the bottom of the tank
Finishing off the lid panels in
the workshop
A final swim before the roof goes on...
The completed tank!
I'll put some results of temperature measurements, etc. here later on.
Schematic
Diagram:
We are really lucky to have a continuous supply of
water from a spring in the hills above us, so we can happily waste 100
litres per hour as shown in the diagram above. This gives us 100
litres per hour of hot water continuously while the sun shines.
(There is a valve which turns off the supply automatically when the sun
goes
away!) If this water supply were not available, we'd have to
recirculate the waste back to the solar
panel, with a small solar-driven pump (which runs when the sun shines,
of course!)
Long
Term Heat Storage Support Documents: Here are two documents which you may find useful.
1. Long Term Heat Storage Notes - an
analysis of the heat gained, stored and used in a 50,000 litre tank.
2. Long Term Heat Storage Calculations - a
spreadsheet which calculates the figures used above
