Heating and cooling: white cubes for ground water store

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- CommentAuthorMikeRumney
- CommentTimeAug 10th 2010
Has anyone tried using those 1m3 plastic storage cubes (that come with a metal cage) for underground water storage?
(We use these already for rainwater collection for garden use as they're about Г‚ВЈ50/1000l delivered which compares very well with any other volume containers that I've come across.)
Cutting these up to make two open-topped containers of 400-ish litres from each cube would lend itself to the dimension of our "AGS" store area ... giving us about 6000l in a "cellular" set up. (see pic)

Comments welcome on "lidding" given that above there will be intervleaved layers of insulation & soil up to the slab ... (the frames metal would be available as they would no longer be needed for transport protection or lateral support) .........
- CommentAuthordjh
- CommentTimeAug 10th 2010
The cubes are called IBCs. "AGS" makes it sound like you're planning to store hot water, which is rather different to just "water". Check the temperature limits carefully; you may well find they're too low.
- CommentAuthorMikeRumney
- CommentTimeAug 10th 2010
Sorry yes I forgot their name ... I didn't think the temperatures involved would be that high ... this is not as in an indoor thermal store for taking off DHW etc, it's for slow collection and release of heat in the order of 20 degrees as opposed to 50 plus ...
- CommentAuthordickster
- CommentTimeAug 10th 2010
Wow! Are you really going to do it?

I thought long and hard about same and backed away from the idea as the thought of storing warm water under where we lived sounded a bit too bacteria/ill health inviting.

Surely you'll need 10 x storage volume to see you through a winter?
- CommentAuthorfostertom
- CommentTimeAug 10th 2010
Don't know what's wrong with me - I cd never make SketchUp work reliably at any level of complexity, like this.
- CommentAuthorCWatters
- CommentTimeAug 10th 2010
All those IBC would need connecting together with pipe work that would probably be inaccessible once built. Perhaps arrange for the pipe work to "join up" above ground so you can isolate a leaking pipe or IBC later on?

Removing the frames would be possible if adjacent IBC or walls provide support but would that be possible on the side with the valve? I'd just leave the frames on. They usually come on a wood, plastic or metal pallet. I guess the plastic one would have the longer life.

There are several different outlet sizes and threads. We got our adaptor and tap from an ebay seller based in Germany as there didn't seem to be one in the UK to match the IBC we had... so try and ensure you buy 6 identical IBC or at least check adaptors are available for the thread size on the IBC you are buying.

Don't forget to unscrew the lid or drill air holes in it.
- CommentAuthorSteamyTea
- CommentTimeAug 10th 2010
They will leak sooner or later, or the pipework will. As the water will be warmer than the surrounding air and humid, evaporation/condensation may well be a problem. Water has the advantage of being dense, high SHC and a liquid (most of the time), but it is not known as the 'universal solvent' for no reason. I would advice against this if it is inaccessible.
- CommentAuthorMikeRumney
- CommentTimeAug 12th 2010
Thanks for all your comments ...

Fostertom: I was thinking about setting up a pre-Archictect's plans "budget visualisation" service for extensions using sketchup to build models from survey measurements. Want to do a free road-test in the South West?!
(Feasibility test for me as much as anything)

As for the rest ... Maybe I didn't describe the set up properly?
It's just the cheapest (& greenest?) way I could find to make up water storage.
All the existing openings would be cut away, hence the 2 x 400 mm deep trays from each cube, so no leak points or fittings to get etc.
Also the overall store needs to be relatively shallow to allow room above for layered insulation to force a longer journey for the heat to the floor slab, so the 400 mm depth works well for us.
The pipe work would be a joint free run underneath each one (or more depending on lengths available and routes to be taken) and yes, any joints would be accessible above slab level.
The cells will be packed in soil so no need for lateral support.
The water is supposed to be sealed in by the "lid" ... which is where my original question came in really ... so no evaporation etc to the outside.
I'm now thinking of using the frames as a mesh for a slab cast-in-situ over a celotex-or-similar lid for the "cells".
The water would be sterilised although I'm not sure how best to achieve that. (Distillation? Chlorine? ...)
I'm assuming I would need to leave a small amount of air in each cell to better allow for expansion and contraction.
Any further comments still welcome.
Many thanks.
- CommentAuthorSteamyTea
- CommentTimeAug 12th 2010
Mike

Will you be having the storage tanks vented to atmosphere? I think you will need it.
- CommentAuthorMikeRumney
- CommentTimeAug 13th 2010
Posted By: SteamyTeaMike

Will you be having the storage tanks vented to atmosphere? I think you will need it.

Open to advice on this ... thought it might not be necessary though.
What's the main reason? Expansion/contraction? "Servicing"?
- CommentAuthorCWatters
- CommentTimeAug 13th 2010
The tanks are semi rigid - you can't get the water out without allowing air in to replace it.
- CommentAuthorSteamyTea
- CommentTimeAug 14th 2010
Colin has said it, and movement will weaken the polyethylene fairly quickly I suspect, even though it is fairly thick on those containers.
- CommentAuthorCWatters
- CommentTimeAug 14th 2010 edited
We have one on our allotment. Once someone did the lid up tight and we didn't notice until the flow from the outlet valve started to reduce. The top of the container and the metal bars across it were bent down 4" by the 1 ton of water "hanging" from it. I've since drilled holes in the lid but I wouldn't be surprised if the local oiks didn't tape them up. Some people have already had eggs in their water butts.
- CommentAuthorSteamyTea
- CommentTimeAug 14th 2010
Colin
was it one tonne of water hanging or one tonne of air pressure pushing?
- CommentAuthorCWatters
- CommentTimeAug 14th 2010
Well I think I know which it is, but scientists are making "new discoveries" all the time...

http://eprints.qut.edu.au/31098/8/31098a.pdf

"A very common misconception is that siphons work through atmospheric pressure pushing
water through the tube of the siphon."
- CommentAuthorSteamyTea
- CommentTimeAug 14th 2010
Standing on the shoulders of giants is the cornerstone of new scientific discovery
- CommentAuthorMikeRumney
- CommentTimeAug 15th 2010
Thanks for your comments ... Having put in a lengthy reply to each one I've just been timed-out and lost the lot so will take them one or maybe two at a time, and a bit "curtly", to avoid the same thing happening again, so please forgive the "tone" if you think you detect one!

Posted By: CWattersThe tanks are semi rigid - you can't get the water out without allowing air in to replace it.

I wasn't planning on taking the water out as this would take the heat out with it.

Posted By: SteamyTeaColin has said it, and movement will weaken the polyethylene fairly quickly I suspect, even though it is fairly thick on those containers.

The diagram above has the soil as transparent so the position of the cells can be seen. They will be supported all around by soil of fairly stable water content, surrounded by reinforced blockwork. It's the lidding of these cells I'm really trying to sort out
- CommentAuthorMikeRumney
- CommentTimeAug 15th 2010
Posted By: CWattersWe have one on our allotment. Once someone did the lid up tight and we didn't notice until the flow from the outlet valve started to reduce. The top of the container and the metal bars across it were bent down 4" by the 1 ton of water "hanging" from it. I've since drilled holes in the lid but I wouldn't be surprised if the local oiks didn't tape them up. Some people have already had eggs in their water butts.

Isn't this a bit like the card-under-a half-full-glass-of-water trick? The fixed volume of air holds the card and water up as it doesn't want to expand.
In your tank the reducing volume of water and fixed volume of air work together as a unit, combining with normal atmospheric pressure on the outside to pull down what proves to be the weakest point ... the lid.
MI'm surprised about the bars though .... maybe your friendly local "oiks" jumped on it for good measure and that's what tightened the lid? (I just can't see them thinking of tightening the lid if rotten eggs is their level!
- CommentAuthorMikeRumney
- CommentTimeAug 15th 2010
I need to go offline but plan to re-phrase the lid question
- CommentAuthorSteamyTea
- CommentTimeAug 15th 2010
Mike.
You could make GRP lids (with vents or not). The real issue will be with sealing them. Polypropylene is a bugger to get anything to stick to, why it is used. The usual method is to weld it. Alternatively a large surface area and a high modulus silicone sealant.
Back to the venting, banging on a bit here I know. The warmer the water gets the faster the molecules will move, this causes evaporation, it is why even ice evaporates. Water vapour has a habit of begin able to pass though the tiniest of holes, microscopic in fact, and due to h2o really wanting to be a gas it will. This will quite rapidly cause a volume loss and as Colin says the mass and air pressure will deform the containers, leading to a weakening somewhere. It is not the same as an unvented water tank in a thermal store. These are really not unvented at all, its a terminology thing. The fluid in these is under pumped pressure (or gravity). When you open the tap the pressure tries to equalise, with the aid of the expansion tank. The pressure is kept higher than atmospheric at all times because of the pump and flow rates i.e higher potential flow in than out, leading to a pressure increase. You can calculate the changes using C=PVT^-1 C needs to stay the same, P=Pressure, V=Volume and T=Temperature.
Hope that helps.
- CommentAuthorMikeRumney
- CommentTimeAug 15th 2010
Steamy tea: .... NOW we're getting somewhere! Thank you, that really helps a lot. I had indeed fallen for the "un-vented" terminology trap.
So when a great big purpose-built heat store tank is put in the ground and insulated:
1) presumably the plastic is different or at very least much thicker that in these IBC's?
2) there's some sort of equivalent to a header tank + gravity to maintain volume under slight pressure?

Losing a small amount of water to the surrounding soil shouldn't matter too much as damp soil with hold heat better than dry.
I'm beginning to think that the vagaries of water + plastic + pipework etc make this IBC cell idea un-viable.
I don't have the option to dig a big hole to take a purpose built tank.
Maybe I would be better off with an all-soil version, even if it IS less efficient as a store.
The lower cost and maintenance might mitigate against the lower efficiency?
This brings me back to another question I've floated before about the relative storage capacities of different materials ...

On site I'll have available:

Trench soil
Shingle
Reclaimed bricks

They are all equally "green" in terms of reducing transport away and I can't legislate for how well they would be used once gone, so it's all down to their heat storage capacity.

Heat will be delivered "wet" (pipe loop) in one area and "dry" (vented) in another and there are supposed to be plenty of thermistors dotted about for taking comparative readings.
Maybe I should just divide up the store as well to get even more comparisons going?
(Equal sized areas of collector supplying equal volumes of store)
Not sure I can stop these from interacting with eachother a bit but presumably that can be taken into account or be set up to make most discrepancies cancel eachother out?
- CommentAuthorViking House
- CommentTimeAug 15th 2010 edited
Hi Mike
The Dutch are saying there is no need to divide up your Thermal Store as it will automatically stratify anyway. Here's our latest developments http://www.viking-house.ie/zero-energy-victorian-renovation.html and the one I told you about in Cork http://www.viking-house.ie/hydro-thermal-energy-store.html
Are you thinking of pumping excess heat in the form of air into the heat store as well? The densest material will store the most heat, is that the shingle or the soil?
- CommentAuthorSteamyTea
- CommentTimeAug 15th 2010 edited
You can look up the Specific Heat Capacities of most materials via Kaye and Laby (on the NPL site) and Engineering Toolbox is pretty useful too.
http://www.kayelaby.npl.co.uk/
http://www.engineeringtoolbox.com/

The rest I shall think about in the morning, but suspect that the trade off between dampness and insulation would lead to better performance with a dry store rather than a wet one (soil wetness here, not water or stone/soil).
- CommentAuthorMikeRumney
- CommentTimeAug 18th 2010
Posted By: Viking HouseHi Mike
The Dutch are saying there is no need to divide up your Thermal Store as it will automatically stratify anyway. Here's our latest developmentshttp://www.viking-house.ie/zero-energy-victorian-renovation.html" rel="nofollow" >http://www.viking-house.ie/zero-energy-victorian-renovation.htmland the one I told you about in Corkhttp://www.viking-house.ie/hydro-thermal-energy-store.html" rel="nofollow" >http://www.viking-house.ie/hydro-thermal-energy-store.html
Are you thinking of pumping excess heat in the form of air into the heat store as well? The densest material will store the most heat, is that the shingle or the soil?

Hi Seamus, I couldn't get your links to work I'm afraid ...
I was thinking of using air vented to a shingle store as it allows permeation without the dust problem associated with chippings.

The information I found on one of the links suggested by Steamytea shows water ahead of the field, as expected, (1.00 cal/gram degrees C) and wet mud (0.6) as being a better store than Granite (0.19) or sandy clay (0.33)
http://www.engineeringtoolbox.com/specific-heat-capacity-d_391.html
I would have thought, like you, that the denser Granite would be better than wet mud etc ... Maybe I've mis-contrued yet again ... ?

The "stratification" is more of a "heat path elongation" ... I'm trying to get the heat delivery point as low and central as possible on the basis that it will then radiate out in all directions but more so upward.
The interleaved insulation is supposed to make the distance from "core" to "floor" long enough to make this interseasonal as opposed to diurnal.
(We're going to build a mass stove for diurnal winter heat)

Posted By: SteamyTeaYou can look up the Specific Heat Capacities of most materials via Kaye and Laby (on the NPL site) and Engineering Toolbox is pretty useful too.
http://www.kayelaby.npl.co.uk/" rel="nofollow" >http://www.kayelaby.npl.co.uk/
http://www.engineeringtoolbox.com/" rel="nofollow" >http://www.engineeringtoolbox.com/

The rest I shall think about in the morning, but suspect that the trade off between dampness and insulation would lead to better performance with a dry store rather than a wet one (soil wetness here, not water or stone/soil).

Does the table of values on the page linked above run counter to what you have been thinking?
My "intuitive" thought was that dry dense brick would store better than shingle or wet soil but maybe it becomes more complicated than simply comparing densities when water becomes involved?

Perhaps the only way I'll find out is by setting up the comparison and hoping that the experiment is sufficiently well designed to not have massive errors built in
- CommentAuthorSteamyTea
- CommentTimeAug 18th 2010
Mike
Make sure you are looking at Specific Heat Capacity and not Heat Capacity. Water is very good at storing energy, stone isn't oddly enough. The other part of the equation is conductance. Some material conduct very well and others do not, think Copper and Air. Granite does not store that much kilo for kilo but it does conduct well, so if it is in an insulated envelope it can be a responsive heat store. It also weighs a fair bit, so more can be packed into a smaller space.
Water both stores and conducts well, why we use it for cooling (and heating).
- CommentAuthorGavin_A
- CommentTimeAug 19th 2010
when comparing materials heat capacity for this you need to be looking at it in terms of volume, not weight of the material as presumably it's the volume that's the limiting factor not the weight.

I really don't understand how you've planning to do this with no joints, and taking the tops of the IBC's and then expecting to be able to somehow attach a new none leaking top to them.... nice idea, but I predict lots of frustrated attempts almost inevitably followed by giving up in frustration. Either that or you're building in a perfect breeding ground for mould, and will probably end up giving your family pleurisy or legionella or something and having to dig it all back out a few years down the line.

If you're going to do it I'd think that a big thick pond liner inside the insulation filled with sand and probably heated / heat extracted via a circuit of warm air pipes (eg 32 / 40mm upvc pipes solvent welded together) so that if anything does leak it's not a problem. The only thing I can think of you'd want to be careful of would be the potential for insects (or other bigger creatures) to get in and start calling it home, but that should be simpler to stop than steam leaking as they tend to need bigger holes.

I'm pretty sure this technique is used fairly commonly elsewhere, but can't remember where I've seen it... maybe canada?
- CommentAuthorSteamyTea
- CommentTimeAug 19th 2010
Gavin

It the density of the material not the weight. Kilo of air, stone, water is a kilo, but they take up different volumes. I am sure it was just a slip of the keyboard

A plastic pipe heat exchanger is a good way to go, could still use the plastic containers filled with water and make your own exchangers from copper pipe. Joints can then be easily make.
- CommentAuthorGavin_A
- CommentTimeAug 19th 2010
yeah, but my point was that he's using figures to compare them that are based on weight not volume - (1.00 cal/gram degrees C). what he needs is the specific heat capacity per volume of the material, figures which are available somewhere via google because I've used them in the past myself.
- CommentAuthorGavin_A
- CommentTimeAug 19th 2010
oh yeah, what I came back to add was that one or more continuous lengths of MDPE plastic water pipe like the stuff used by farmers / festivals would be preferable, so there's no inaccessible joints anywhere to go wrong.

using water in such an inaccessible place just seems like a massive gamble, with so many potential problems to it for something that should ideally be lasting a lifetime, not having to have the floorboards ripped up every few years to attempt to find and fix the latest leaks.
- CommentAuthorSteamyTea
- CommentTimeAug 20th 2010
That is the Heat Capacity C=JK^-1, as opposed to the Specific Heat Capacity c=Jkg^-1K^-1.

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Heating and cooling: white cubes for ground water store