Thank you! It sounds like you might be interested in my notes and calculations on thermal energy storage in phase change materials, some of which are listed at

Thank you! It sounds like you might be interested in my notes and calculations on thermal energy storage in phase change materials, some of which are listed at https://dercuano.github.io/topics/phase-change-materials.html https://dercuano.github.io/topics/phase-change-materials.htm.... But I think TCES systems are likely to be more significant because of their technical advantages, among other things for managing indoor humidity and possibly even for seasonal thermal stores; some of my notes on the topic are at https://derctuo.github.io/notes/desiccant-climate-control.html https://derctuo.github.io/notes/desiccant-climate-control.ht... and https://dernocua.github.io/notes/shower-heating-tces.html https://dernocua.github.io/notes/shower-heating-tces.html. Various kinds of thermal energy storage do seem to beat batteries on cost by around three orders of magnitude; some of my relevant notes are listed at https://derctuo.github.io/topics/thermal-storage.html https://derctuo.github.io/topics/thermal-storage.html. I agree with you that there is no real prospect of batteries catching up with thermal energy storage in the foreseeable future. With respect to the particular problem you mention with needing expensive propylene glycol in your heat transfer fluid to keep it from freezing, ice rinks commonly use brine systems instead, despite the corrosion problems you mention. Brines are very cheap, some like dipotassium phosphate are minimally corrosive, and the commonly used ones are pretty nontoxic.