Hello,
so forget the idea of a 20-30m³ storage tank again quickly. The size of the buffer tank and the required area of solar panels is determined by the house residents.
If the solar system is also to be used for heating, then your house needs at least a low energy standard (airtightness, very good thermal insulation, good glazing and window frames, etc.) or better yet passive house standard.
What I would recommend to you:
a) Low energy house:
The heat sources that come into question are:
1.) solar panels: For warm water and possibly additionally to other energy sources also for heating. Alone it is too little for heating!
2.) Surface or deep collectors for the use of geothermal energy
3.) Heat pump to amplify the energy gained by 1. and 2.
Here we are talking about low-temperature systems since the heating temperature is only slightly higher than the desired room temperature (compared to radiators which can have 70-80°C).
As "distribution systems" for the low energy standard:
1.) Wall and/or underfloor heating
2.) Controlled residential ventilation
Now for the explanation:
With deep or surface collectors, hoses are either laid horizontally in the ground (surface collector) (partly several hundred meters long distributed over a large area) or, as with a well, vertically at great depth (>80m, depending on necessity), a few hoses (usually 2 circuits) are inserted into a small borehole. Deep collectors have the advantage that they require only a small amount of space on the property compared to surface collectors, but you can read all about this on Wikipedia and other sources if you are interested.
With these collectors, a constant temperature level can be extracted from the ground all year round. However, this temperature level is still far too low for heating and must be raised to a usable temperature level by a heat pump.
The heat pump works like a refrigerator: A compressor compresses a gas which heats it in this area => this temperature can be transferred via a heat exchanger to, for example, underfloor heating. The water that has then flowed under the floor to be heated and returns is colder because it has given off heat to the room. This cooler water cools the warm side of the heat pump again (so it always circulates in a loop). The cooler gas in the heat pump, which is still compressed, is cooled even further by expansion at a valve (the opposite of compression). At this point, the cold side of the heat pump takes up the warmed energy level from the deep or surface collector and through compression it becomes usable again, thus starting the heat pump cycle over again. The cooled medium of the collector (usually a brine solution => saltwater) is pumped through the hose and absorbs more and more heat from the ground/groundwater along the way and arrives warm again at the heat pump.
I know, it's awkward to describe this in writing, but there are some videos on YouTube regarding the operating principle.
The solar collectors themselves are mainly responsible for heating water for the hot water circuit for cooking, washing, etc., which is why the number of residents is mainly of interest for sizing.
In a passive house, you can really only use controlled residential ventilation as a distribution system, because with this you send the "old" warm exhaust air through a heat exchanger before it leaves the house so that the heat energy is transferred to the cold fresh air and thus warmed. Of course, there can be no 100% heat recovery, but since the people living in the house also release heat (cooking, sweating, etc.) and geothermal energy in connection with the heat pump also heats, this is enough to easily maintain the temperature level in the house. Additionally, you always have fresh air in the house because it is regularly automatically changed and the system also works in summer to cool the house like an energy-saving air conditioner.
To separate the passive house from the low energy house, you also need photovoltaic panels which cover the resulting energy demand of the system and usually somewhat better insulation and at least triple glazing of the windows.
The concrete calculation and dimensioning of the system should be done by a professional (who also has experience with it). In terms of cost, you have to reckon with about €25,000-35,000 additional costs for such a system on top of the low energy standard compared to a standard house.
The passive house costs somewhat more again due to the photovoltaic system etc.
However, it must be said: these systems mostly pay off after about 15 years (with constant energy prices). If you then consider the price increase of fossil fuels or electricity, the period is definitely shorter.
If I have mixed something up somewhere or you do not understand anything, feel free to ask.
But please forget the idea of 20-30m³ => huge storage volume needed and I can think of many other reasons against it that would quickly kill the project (storage costs, construction implementation, and much more). Better stick to the "conventional" construction techniques, they have long proven themselves and are well thought-out.
Regards Thomas