I live in a country where summers are warm and winters are cold. One idea I've thought about is why we don't just have massive water tanks under our houses. En

I live in a country where summers are warm and winters are cold. One idea I've thought about is why we don't just have massive water tanks under our houses. Energy can be dumped in during the summer for cooling, and taken out during winter for heating. It would be more efficient that ground source geothermal as it's a closed loop system, so heat won't leak away. My house needs around 2500kWh per year, so you'd need a tank of around 100,000 litres. That is 8x8x1.5 metres - basically a small basement. To get that energy out youd need to heat it to 60c in the summer, and it would cool to 30c in the winter. You could even do that with passive water flow, no heat pumps needed.