Heat pumps are used to heat a storage container called buffer, a water container of 200l or more of water. This is used to run the heat pump when energy is cheap to warm up the buffer. That is for example in daytime when energy is available from the photovoltaic. If you don't have photovoltaic you can also have a good energy price when running this at night.
Also the storage of the heat in summer is nothing new. These systems are also commercially available for years. It is only that they are so expansive that many companies have backed out of those.
The idea here is that in summer time you heat up a big tank in the ground like with a thermal system on your roof. Then in winter time you use that "saved" energy to run the heat pump. At the end of the winter the water in the tank will likely be freezing. That state change of the water will give an additional energy boost, because it is the same amount of energy you need to boil water. All this is well known and working and commercially available for years.