I am much for a permie approach, but obviously that does not scale up to town or city size. Own minimalist setup, a hundred watt of solar electric was plenty for lighting and computer/small appliances, though I didn't get around to making an extra low consumption freezer converted to fridge which on paper at least would have fitted. This is south of Spain mind. What I found was that at our locality, everything was made more difficult by bureaucracy, it really seemed like an individual or independent approach was to be discouraged, even the humblest of kind. Spain has a herd like mentality though, if you stand out somehow equally you get picked on. Same goes for monopolising energy supply and introducing hurdles for independent alternate there , and at EU level we have similar by increased taxing not too long ago on solar panel imports. Actually I remember a story from Oz of someone who set themselves up to be self sufficient, and felt secure with that until there was some kind of riot that went on, and he realised that without a strong community all his effort would be for nothing in certain circumstances. In Spain it was the authorities themselves that eventually crossed the line of "completely unacceptable", and we left.
Anyway, there I made a heat exchanger based on
https://makezine.com/projects/heat-exchanger/
with room sealed enough to work with only one 12cm (if I remember) 12v computer fan. Inlet and exit was at top at ends, those could be placed any side. The one I made was polycarbonate housing, simple foil folded back and forth (5 mm or 10mm as channels, I don't remember which). That was left powered on year round, hardly uses electricity, and fan would need replacing maybe once a year (they are about 5 pounds). Figuring a water run off channel (from condensation) is nescessary also.
The result was always clean air in the 20m sq room, no humidity. I had made it because in winter temps go down to under 10 centigrade, towards freezing, for part of the year, and I wanted ventilation for evening use of kerosene heater, and to reduce condensation at night, while keeping heat in. As it was it ended up keping the room warm enough without having to use the heater, and no condensation at all. The building itself was not even very insulated. So there is a simple idea for some circumstances.
NOTE You would not want to use that setup in a sealed room with combustion going without also a natural air inlet open, a failsafe second vent, CO alarm or combination of those etc., and should follow recommended guidelines or due diligence for that. The fan stopping for any reason would otherwise lead to a perilous circumstance, even considering that many heaters now have a low O2 cutoff threshold.
Lower temperature ground heat storage is also an idea that has been used in the past, in the south of europe it is pretty much the other way round though, where building thermal mass helps keep temperatures down inside in summer. I had wanted to try a water tank thermal storage arrangement for extra heat, but we left before I got around to that.