Is there any cheaper way of storing solar energy rather than batteries?

I believe the cheapest way to store energy from a source such as PV, is domestic hot water. For most homes, domestic hot water accounts for about a third of the total energy consumption in the home. If excess daytime PV generation is used to raise the temperature in the hot water tank high enough that it will remain sufficient for the household needs until the next day, then the excess PV energy will be stored for later use especially after sunset. Automatically turning off the water tank’s heating element at sunset, unless the temperature in the tank is too low, will also reduce the night time load taken from the grid. UN 50 gallon tank heated 50 degrees hotter than normal will store about 6 kWh. That is about 20% of a typical home’s energy load for a day. That system requires no batteries, and only minor modification of an existing hot water tank. It would require a tempering valve (if the tank doesn’t already have one) and it would require control electronics and sensors to manage the storage.

For the benefit of the utilities the tank could be controlled remotely by them to add load when there is excess generation and flatten load peaks by turning it off (those manipulations would probably not be the same as those made by a homeowner trying to limit net metering by storage on the home side of the meter). Utilities frequently have excess generation at night from base-load generation that can’t be reduced. They would like to spin the customer’s meter at night.