One of my friends is a physicist that had the plan to install a closed cistern under his house and heat it up in summer with mostly solar energy. Looked really promising and he had all the numbers figured out to supply a family home with heating and warm water through a normal winter. Didn't do it in the end because there were some problems with building a cistern on his property and the large up front investment necessary.

I wish you luck with your endeavor. I think you are correct that the problem of energy storage is the most important one to solve to allow renewables to really take off for general power supply.

I also believe that the efficiency of storage is secondary to a degree as renewables can supply an enormous amount of energy. So loss from pumps or energy conversions are nearly insignificant. At least at the moment where energy storage is in such a bad shape. Certainly a lot more promising than having batteries everywhere.