Filling the thermal buffer all the way to full, only if wether report is calling for a longer no sun phase is not very reliable. This way the effective buffer capacity is reduced. Even if the next few days have not much excess energy predicted one would have to keep the buffer full as there might be a cold phase after that. So not filling up the buffer increases the risk of running out of heat, or one would need a larger buffer or PV. It is only if the predicted weather allows to fill up the buffer all the way later, that one could safely not put the energy in the buffer, without increased risk of running out of heat. With something like a 5 days reasonable reliable weather forecast this won't happen very often in winter.
As mentioned before having the buffer at around 75 to 80% most of the time is OK and if weather forecast (that unfortunately is not always reliable) predicts multiple consecutive bad days then buffer can be charged to 90 even 100% even if less comfortable for that one or two days.
Without much control over heat release it already is a problem, that a very hot buffer will send out quite a lot of heat and will thus initially loose its temperature faster than normally needed. So the upper temperature part is not that effective for storage. This might not be a problem if there is another thermal buffer that has much better control for the heat release, so initially one can use heat from the concrete only and use the more controllable buffer only later.
To get the heat out of the concrete buffer one would need it to be considerably warmer than the room. So using the energy from cooling to 16 C would only work if the room temperature really falls down, maybe to 12 C - maybe Ok for a few rooms, but not very comfortable. Even when using the floor for preheating fresh air, one would still need a second heat source to compensate the other heat loss. So I think the buffer capacity of the concrete floor is overestimated, especially at the lower temperature end. Without a heat pump (could be used for cooling too), temperature lower than room temperature (plus a little extra) has a low value. Possible use like air preheating or maybe water preheating would need extra investment.
I understand now what you try to say. My house ambient air temperature is always the same as concrete floor temperature with no more than 0.5C difference between ambient air and concrete floor.
The amount of loss is extremely predictable and only based on delta between inside temperature and outside.
So if the concrete floor gets to 16C then ambient temperature is also at least 15.5C
So when I was mentioning absolute max range of 12C delta in the example I considered the +16C to +28C floor temperature range that will equate to a 15.5 to 27.5C ambient temperature unless you affect that by opening a window or something like that and after you close the window the ambient will stabilize again at around the floor temperature in no more than 1 to 2h
The wall that are 10cm solid wood and only thermally insulated on the outside additional mass that I did not added in the example to keep things simple but is not insignificant.
The wall is made out of about 8.5 cubic meter of wood that has a thermal heat capacity of about 0.35kWh per cubic meter per degree Celsius so almost additional 3kWh per degree Celsius additional to the concrete floor that has 8kWh per degree Celsius.
So while 12C delta for concrete floor may be a bit highs for comfort the storage capacity of the house is still probably at around 100kWh when also wood in the walls and Drywall are included at probably just 8C delta.
The example was just that showing how to calculate the thermal mass storage and the fact that a large floor can have significant amount of thermal storage at no additional cost since is part of the structure anyway and has the role of floor and foundation.
I would more expect the concrete buffer part to be between 20 C and maybe 35 C (if the structure allows). The higher temperature part would have a small draw back: To compensate for to much heat released to the building one would need to cool be something like extra ventilation. So there would be extra heat loss if the buffer is near full, but still some extra heat could be stored for a short time, with little extra costs (ventilation for cooling).
It could be a good idea to have something like a thick carpet to slow down the heat release in a way that could be removed in a worst case period. By design the structure is only a short time storage, especially if not isolated against the room. Ideally coupling is variable - but this costs extra.
Anyway the concrete buffer should only be a part of the thermal buffer - maybe half, so you have at least some heat available you can control and keep for longer than maybe a 3-4 days.
As mentioned above there is no significant difference between concrete floor and actual indoor air temperature.
I just dislike carpets that is why plan was always for ceramic tiles and in floor heating.
I will say Concrete floor can easily be all the thermal storage needed even if you need to add more concrete than minimum required for structural integrity.
Yes a smaller 15 to 20% of the main thermal storage can make the ambient more constant for better comfort.
This is the reason why underground molten salt storage can reach claimed 99% annual thermal efficiency once a large enough size is reached. Volume is indeed important and it's actually advantageous to increase the heat buffer size in addition to using the highest specific heat possible (basically water), maximizing both minimizes both the temperature delta needed and also the percentage lost at the surface, of course insulation is still warranted.
For a ~130m^2 home in cold climate I don't think even 20kW PV is enough heat buffer, one really does need thermal collectors even assuming $.50/watt panels, also should be possible to use about half the area if it's 34% vs 17% efficient.
Water allows easy heat transfer in and out when needed. If PV really is afforded for such a large cold climate heat buffer, place heating elements inside the water tank. That should be more economical than a heated slab, you'll still have the inside watersource handler heatpump but you need, at least want, an airhandler anyways. Circulate water to an insulated hole lined with a pool liner to make a tank (or use a tank). Basically no loss at night. Run the inside heatpump as needed.
You couldn't tell by a single data point (i.e. device of only one size tested) or even the heat flux formula, but due to the 2/3 power law that applies to well everything physical, scaling anything becomes slightly non-linear. This can add up, in a good way hopefully.
I do not doubt that you can create a large seasonal thermal storage with realy good efficiency outside the house but seasonal thermal storage makes no economical sense with inexpensive PV solar.
Maybe I already mentioned this in this forum and are sure present in my paper but here are the economical reasons.
This are just rough numbers based on 25 year amortization period.
PV panels
2.4 cent/kWh (based on 80 cent/Watt acquisition cost, 25 years amortization and amount of solar energy at my location) it assumes you will be able to use all available energy and in my case I will be using only around 40 to 45% in average over a year.
LiFePO4 battery is a bit harder to calculate since it depends on model and how well it is used but a realistic number will be around
25cetn/kWh (a bit lower with DMPPT)
And last is thermal storage short therm few days estimated at
0.5cent/kWhBest from an economic perspective is to maximize the thermal storage and PV array and reduce as much as possible the LiFePO4 storage.
Considering my example total cost for heating around $12k big part of that is the PV array at about 8k then much less thermal storage 2k and wires for heating and power transmission + DMPPT and some other accessories make up the rest.
Now thermal storage is say good for around 3 to 5 days and if you want seasonal storage then that will need to be able to store energy for 60 to 90 days and that means a thermal storage that is in a simplified way 15 to 30x more expensive and that means a cost of 30k to 50k for a seasonal thermal storage with savings of just 4k by reducing the PV array needed to half.
Of course that will not make sense for me in my climate and probably will also make no sense for anyone in any climate.
Even if you have horrible solar compared to my location adding extra PV capacity will still probably be a better option.
My house is 65m^2 and in a realy cold climate so a house that is build at same standards as mine ad same location that is 130m^2 will need probably just 16kW of PV since the larger the house the more efficient it is assuming the same level of thermal insulation.
As you seen in my comparison thermal collectors are actually more expensive than PV panels (just the panels) and when you consider the entire system that gap gets even larger so the only advantage is reduced collector area needed to half but at significant extra cost and complexity also reduced reliability to name just a few.