@dzwiedziu@mastodon.social @infobeautiful@vis.social As for the heat storage: mid-temperature heat accumulator can store around 500kWh per 1 cubic meter of gravel. Insulated by the very same gravel turned into the mineral wool. My house heat accu (I can not build on my own land) is designed to the tune of 7MWh, co it could store heat from 30kW solar installation operating from March to September. This fits into the 6m diameter rotunde. In 2020 was expected to cost around €15000. 4/5 of that the insulation. In non-lobbied regulatory environment this insulation could be made on-site from the gravel (and some amount of aluminium) by simple machine known for two centuries now. Edit: checked project. The nearest unsorted bedding was 1700kg/m³ , 0.3kWh/°C/kg. The accu core has 38m³ of bedding (way cheaper than gravel) and can store up to 7MWh at edge temperature of 560°C (60-560). Long-term safe capacity is 7MWh. Accu core is almost cylindrical cone 4m diameter x 3m height. 150m³ insulation. Fully loaded gives ~8kW at the collector with all vents closed so heat all goes thru insulation. Empty (60°C) with all vents open outputs around 6.5kW.