This is my first post. I have lurked here for a while and am calling this research rather than netbased prevarication!

We live in a large converted barn with solid stone walls a metre thick, double glazing and a thick insulated roof. The setting is rural but not excessively isolated.
The Welsh mountain climate is probably more like the Pacific North West: mild, damp and cool with warmish summers and generally mild winters. If it snows or we get frosts, generally minima are around -10 or -15/20C should the snow persist for a few days (rare, but not unknown).
We’ve mains power and water from a spring. We replaced the oil boiler. It was on its last legs and we don’t feel comfortable burning fossil fuels. Its replacement is a computerised biomass pellet boiler. This was an expensive, albeit subsidised, mistake. It is pointlessly complex, badly engineered and unreliable. This winter we spent more on repairs than fuel. It’s miserable being in a house where you can see your breath.
We have solar PV cells which are great. They divert spare power to heat our domestic hot water tank. This works fine from late March to early October. Building up a larger reservoir of heat for water based central heating in a thermal store would make sense. This would be heated by the boiler and the solar array.  It would also give us breathing room should the pellet boiler go off line.
We have woodburning stoves to provide back up heat, and I will install a rocket mass heater but this wouldn’t work in the side of the property that we rent out to holiday makers: in the UK they are still seen as niche and unlikely to pass local building regulations or insurance.
Commercial thermal stores are expensive. We are researching whether we could use stacked IBCs - intermediate bulk containers - as a practical DIY alternative. IBCs are sturdy, readily available and cheap. We would insulate these with straw bales. They’d sit on a wood framework on a hard standing with a damp proof membrane and clad with rough wood lapping (possibly even breathable lime plaster) and a roof to keep it all dry and pest free.

So here are my questions:
Has anyone got any hot tips on using straw bales as insulation? What sort of insulation do they give in the real world? Has anyone arrived at "U" or "R" values for them?
Are there any issues we should take into account using IBCs?
I’ve worked out that the house needs something like 26kw/ 89000btu to keep it snug.
Any thoughts on what sort of tonnage of water we should aim for?
Commercial boiler people are saying something around 2,000 to 2,500 litres of water. Given that another 2 IBCs would cost less than £100 delivered (+/- 130 USD) would we lose anything if we made this bigger? Space isn't an issue, we have land we can play with.

Sorry if this all sounds garbled, but I find the more I research and plan, the more questions I generate.
I am aiming for a system which is cheap, bomb proof and DIYable, avoiding C02 emissions and zero fancy tech just using with standard plumbing parts.

Any input or points to consider would be very welcome.

Welcome to the forums!
I love your post,  IBC are a great resource.

So,  you would want these to be outside the building envelope?
Is your home at all insulated?
Clearly it has lots of thermal mass,  but that may not be enough.
I wonder if glazing a wall could create a solar collector of sorts.

Would you run non-potable water  in a loop,  or would this be part of the potable water system?

What ways are you planing on heating the water.
Storing enough PV as  heat to get thru an entire winter on seems unlikely, though it would be awesome.
It's hardly low tech,  but some solar advocates suggest heat pump furnaces and water heaters as the most efficient way to use PV .

I have no experience with straw bales,  but we do buy hay for our rabbits and the rodents love it.

A south facing(?) wall of IBC's could have a solar  thermal collector on the face of it.
Two parallel walls could be the walls of a building.
Bunching them together in a cube  would minimize surface area, heat loss and insulation expense.
Stack them along the wall of your home and they could shield that wall from cold air and summer sun, and the insulation would serve double duty.

Keeping the water thermally balanced might require a more elaborate plumbing scheme than just storing water.

Consider that  an above ground pool type tank would require simpler plumbing,  have an even more minimal surface area than a cube, and offer a roof for solar.
Cost per gallon should be lower as well.
Here is a link to a low cost  DIY tank build
https://www.instructables.com/id/How-to-Build-a-6000-gallon-Water-Tank/

First off thanks for the input and the welcome.

William Bronson wrote:
So,  you would want these to be outside the building envelope?

Yes. The boiler is outside in a shed and the feed goes underground to the house. I would build these near the house.

William Bronson wrote:
Is your home at all insulated?

Yes. The walls are a mix of airbrick, rubble and faced with stone. It is slightly better than solid but still feels cold to the touch in winter.

William Bronson wrote:
Clearly it has lots of thermal mass,  but that may not be enough.
I wonder if glazing a wall could create a solar collector of sorts.

That would be nice but would be rather difficult given the planning regime where we are. We are in a national park and the planners like things to look traditional eg a barn needs to stay looking like a barn. My tendency is to not involve them unless it is absolutely necessary.

William Bronson wrote:
Would you run non-potable water  in a loop,  or would this be part of the potable water system?

Yes it is all indirect and completely separate from the drinking water not least because it would have inhibitor in to prevent limescale build up.

William Bronson wrote:
What ways are you planning on heating the water.
Storing enough PV as  heat to get thru an entire winter on seems unlikely, though it would be awesome.

Wouldn’t it just! ;-)

William Bronson wrote: I have no experience with straw bales,  but we do buy hay for our rabbits and the rodents love it.

The dogs keep down the rabbits but the mice and rats will get at everything given half a chance. I’d make sure it was all much proof.

William Bronson wrote: A south facing(?) wall of IBC's could have a solar  thermal collector on the face of it.

The main part of the house is all on the north side. Barns round here were designed to minimise solar gain.

William Bronson wrote: Bunching them together in a cube  would minimize surface area, heat loss and insulation expense.

Great minds think alike!

William Bronson wrote:Stack them along the wall of your home and they could shield that wall from cold air and summer sun, and the insulation would serve double duty.

Summer sun is rarely that unwelcome here, although with climate change local patterns here are as out of whack here as they are elsewhere.

William Bronson wrote:
Keeping the water thermally balanced might require a more elaborate plumbing scheme than just storing water.

We’ve planned for that and are working on an arrangement of valves including 3 way ones.

William Bronson wrote:
Consider that  an above ground pool type tank would require simpler plumbing,  have an even more minimal surface area than a cube, and offer a roof for solar.
Cost per gallon should be lower as well.

What we were thinking about was having 4 tanks (possibly more) where one is kept as close to 70c as possible so it could deliver a hefty amount of heat to the building when there is demand. The others would work as subsidiaries. We would have sensors (arduinos) towards the top and bottom of each to monitor the system temps. There would be remotely shuttoffable valves controlling the flow between them but with the facility to turn these off manually if the IT stops cooperating. I am working on this with a tame engineer who thinks along similar lines to me, is interesting in developing this as a pilot and making the design a publicly available resource. He definitely gets the message that critical success factors include reducing carbon, having a system which is bombproof and while smart it would have the option to manually control things. Currently our computer controlled boiler has decided it doesn’t want to talk to the net, can you imagine how frustrating this is when it doesn’t have any buttons to control it beyond a couple of on/ off switches?

In the meantime, I will definitely have a look at your link.

This group uses fresnel lens to heat veggie oil beyond what water can take before vaporizing and store heat as their energy battery.

They use the heat two ways: directly to cook using metal jackets the oil moves through; then they also get electricity by running it thru their stirling engine.....oops they also use it to pump water with.

You can find the physicist in Germany and their test bed in Portugal.

Tamera.org

and this guy's thread on permies...his son saw it first hand!

https://permies.com/t/30950/Stirling-Engine-Tamera-Portugal

A few thoughts:

4 IBCs isn't a lot of mass.  Is it going to be enough heat?  I don't know ...  I know of installations in the Fairbanks, Alaska region that use 7,000-10,000 gallon tanks to pack away summer heat for the winter.  These of course are highly technical, highly sealed (and insanely insulated ... something like R100 in the ceiling) PassivHaus designs and so they can milk a LOT of comfort from a little heat.

Two concerns about IBCs.  First, the heat.  I don't know what the temperature profile is for the plastic, but repeated heating and holding at temperature has to induce some stress and shorten the life of the tank.  Second, I find the connection of IBCs to be expensive and complicated - price out what it will take to get threaded adapters, shut-off valves, unions, etc.  All those fittings seem superfluous until one of those IBCs gets a leak!   Although they seem modular I'm not convinced of the economy when I can buy new poly storage tanks for about 45 cents a gallon.

Strawbale ... insulation quality depends a lot on the density of the bales and how they are sealed.  And they're not a good as you'd like - something like a 2' bale is R50.  The bigger problem is moisture - when we were considering straw bales for an outbuilding in a humid climate there was a lot of concern about the absence of a drying heat source such as a wood stove or furnace to chase away moisture in the bales (and keep them from rotting).  I wonder if the heat of the IBCs ... and the fluctuation in temperature ... is enough to keep the bales dry if they are outside the building envelope.  I'd really prefer to have them inside the envelope, even if heavily insulated.



Sorry to raise questions instead of solving them

IBCs are plastic so how hot would you want to heat them? I know of a friend who used one as a thermal store to 60C without issues (look at legionella regimes though!), but that isn't a whole lot of heat stored.

Keep an eye out for old copper water tanks to use as thermal stores? They take them out of houses when they fit combi gas boilers and if the plumber is a larger company they leave them outside for the scrap-man (I gained one the other day this way!). They're easy enough to add more insulation to.

There is an existing thread here where IBC totes were used as thermal mass in a greenhouse: https://permies.com/t/62395/Greenhouse-build-thermal-mass-storage

Thanks for this. Brilliant feedback and more stuff to think about. Our plan in terms of valves and the like is to just use standard plumbing with compression joints. There would be powered as well as manual shut offs. Lots to chew over. I did consider using lime plaster as a sealant on the outside. Round here lime plaster, mixed with a bit of horsemuck and straw is the traditional way of finishing off walls that haven't been made of stone. This would mean the roof would need to hang over a fair distance so there isn't direct splashing on to it. In terms of temps this wouldn't be going any hotter than 70C. Given the thickness of the plastic I doubt this would cause significant deformation. The process here is as much about experimentation and learning to see whether this is feasible.

Given:
Stone Wall/Thermal mass on the outside to preserve historic/HOA
Straw-bale/insulation on the inside
Radiant Floor Heating delivery system possible with a Heat Recovery Ventilator (HRV).
Given that you have 'main's power' a 1/25hp pump with pex piping for the radiant floor heating seems good.
And a 50cfm HRV 'fan motor' would also be a light load on mainsvpower for heat recovery.
Seeing as how you have a automatic feed pellet boiler. I would run it constantly at low power to heat the water in the radiant floor heating to 90F-120F maybe 140F is it is wall radiators.

Quite Note
1,000ft of pex piping to heat a 1000sqft house will hold over 40gallons of water. So you can pretty much treat it like a boiler that powers a hot water tank. In theory you could also have a backup heat source from a heat pump (Air to Water or Water to Water)

Agreed underfloor heating would make sense
Our place is already converted. Were I doing the conversion myself it would definitely be something I would consider. However retrofitting it would be costly. I wouldn’t want to interfere with the the seal against radon (a known risk with our geology). Building something on top would also be hard as the headroom downstairs is tight: 205 cm (6’ 8”). If I was converting this place myself I would build in passivhaus levels of insulation/ heat exchange and some sort of mason stove arrangements instead of a boiler.