No good heat reser­voir available?

I was search­ing for a good heat reser­voir. Unfor­tu­nate­ly it seems that all on the marked are far from state of the art (they are prob­a­bly in their class, but see below).

Most of the devices use water to store the ener­gy. I found one (in Europe/Germany) which is using phase change tech­nol­o­gy instead of water to store more heat in the same stor­age place (but you need to ask how much it costs and how long they need to deliv­er, which prob­a­bly means that it is a lot more expen­sive (part­ly due to lim­it­ed amount of pro­duc­tion quan­ti­ty) than water based heat reser­voirs). I have read a lot about phase change mate­ri­als (PCM), and it seems there are dif­fer­ent kinds of sil­i­ca or wax (or oth­er mate­ri­als) which are bet­ter suit­ed to store heat ener­gy, but the only mass-market tech­nol­o­gy seems to be water based ones.

This looks strange to me. When I look at his­to­ry, oth­er mate­ri­als than water where already used a lot in the past (e.g. stones where heat­ed and then they were used in a press­ing iron or as some­thing which is replaced now by a hot-water bot­tle or an elec­tri­cal heat­ing cush­ion in the bed; yes, all this does not involve a change in the phys­i­cal state of the mate­r­i­al, but the point is that oth­er mate­ri­als than water where already used in the past), so I do not under­stand what is pre­vent­ing to let PCM based heat reser­voirs going mainstream.

Except for choos­ing the right PCM and obtain­ing it, it does not look hard to build such a heat reser­voir. You can add a heat-exchanger in the bot­tom and feed solar-power there for long-term heat­ing the PCM, anoth­er heat-exchanger at the top to heat the use-water and/or heating-water from the heat stored in the PCM, and a 3rd heat-exchanger (placed at the top too) which you con­nect to your central-heating if you need a lit­tle bit of quick short-term heat­ing of the PCM. I do not know if you need to add some  heat-layers (e.g. by putting a big cheat of a non-PCM mate­r­i­al between the long-term heat­ing part and the short-term heat­ing part), but  it should be easy to test if some­thing like this is ben­e­fi­cial or not. If you have a fire­place which you want to con­nect to the long-term heat­ing of the PCM, it may also be ben­e­fi­cial to have a 4th heat-exchanger togeth­er with the solar-one, but maybe there is anoth­er solu­tion to do this with the 3‑heat-exchangers-setup (I have not inves­ti­gat­ed this pos­si­bil­i­ty at all).

If some­one knows some inter­est­ing prod­ucts in Europe or has some help­ful infor­ma­tion (any­thing which can be inte­grat­ed into exist­ing heat­ing sys­tems with­out much ren­o­va­tion of a lot of rooms), please write a comment.