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

Isn't she a great worker?     The old and the new...
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!)

Building Paper     Corrugated iron
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.

foundations 1     more digging
More digging.  Do you notice how I manage to get my wife and daughter involved?

Boxing     raking and levelling
The boxing goes up...                                                                 Raking and levelling to an even depth of 100mm

The truck arrives     smoothing the pad
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!

Building the floor    The carpet goes down
Stitching the polystyrene floor panels together                              Laying the carpet on the floor (2 layers)

The first panel     One to go...
The first panel goes up                                                               And now there's only 1 to go!

Straining the wire     The roof goes on temporarily
Straining the first wire                                                                  The roof goes on temporarily (it's going to rain!)

Carpeting the outside     view from the kitchen
The raincoat goes on top of the outside layer of carpet               This is the view from the dining room

The inside     Laying out the inner liner
And this is the view from the inside (carpet on walls)                  Laying out the inner liner - gosh it's HUGE!

draping the liner     Slowly does it
Draping the inner liner over the top                                             Slowly does it....

That's the bottom     The rafter
We centred the liner using water in the bottom                           This is the main rafter for the roof (upside down)

gripples and wire     Filling up
The tank is surrounded by LOTS of wire                                    Filling with water now...

Heat exchanger coil     Finishing the lid panels
The copper coil heat exchanger in the bottom of the tank            Finishing off the lid panels in the workshop

swimming     FINISHED!
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:

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

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