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.
