A Finnish development, much mentioned on BBC radio news yesterday morning. I won’t put a link in; just search for “Polar Night Energy”.
Obviously storing renewable-energy-derived heat for district heating over long periods in a huge well insulated container full of very hot sand will work; but thermodynamically the method is so disappointing. If you are heating sand (or anything else) to 500 – 600C, when you harvest the stored heat why just use it to produce hot water ? As there will no doubt be a demand for electricity in the winter, surely better to use your high temperature resource to run a steam (or gas) turbine, and then use the lower temperature heat available from the condenser (or equivalent) for space heating? Siemens-Gamesa are developing their Electrical Thermal Energy Storage (ETES) along these lines.
