There are so many ways to store energy. Ranging from chemical to mechanical and even thermal.
However, only one of those 3 methods hasn’t seen any headway beyond cool lab results. Thermal energy is just one of those solutions that hasn’t shown much promise. In terms of estimated market size for thermal ESS it is around $4.35 Billion and the two really leading the charge in trying to bring it to market are Siemens and EnergyNest. Large enough brands that give the technology a fighting chance.
NREL and other labs are also pushing forward on evaluating thermal energy. Companies like Brayton Energy & Echogen have also been participating in various grants to develop a thermal energy storage system with a target price of $0.05/kWh
As of today – much like flow batteries – thermal energy storage isn’t a commercially viable solution.