Yeah, but ... sand is dirt cheap, and will always be far cheaper than batteries. Also batteries cost grows linearly with capacity, whereas the main cost of thi

Yeah, but ... sand is dirt cheap, and will always be far cheaper than batteries. Also batteries cost grows linearly with capacity, whereas the main cost of this is probably the insulation and containment, but that cost is proportional to surface area, not volume. So the bigger you go, the cheaper it gets. Then there is the source of energy. Yes, PV is one source, but it is only 20% efficient. Evacuated solar tubes collect heat energy at around 60% efficiency, and last about a long as PV panels. And I think you may not realise how most of our electricity is used. Most of it goes into heating and cooling. (I speak from experience here. I have a house battery. Turn off the hot water, ovens, stoves, dryers, air conditioners and the remainder of our electricity use could be handled by a 2.5kW battery. How do I know. Well we lost electricity, and our 5kWh battery easily handled all what was left, including refrigeration. I estimate it would take a 15kWh to handle everything.) Yes, the COP of absorption refrigeration is poor, but the 60% vs 20% makes up for it. Finally, I suspect you haven't twigged to just how much energy this can store. Sand melts at 1700°C, so they could take it to 1200°C without hitting too many limits. At 1200°C, 1 cubic metre of sand holds 451 kWh. In comparison, right now a battery holding that much would cost AUD$300,000.