@qrnef, I was wondering if somebody would mention PCMs as storage. As a storage medium, PCMs are great but the big advantage of water is the easy retrofit to a heat pump where the hot refrigerant pipes are cooled by the water. You get the 2 or 4 EF because you are using outdoor heat to heat your water. Typical PCM applications (SunAmp) use direct grid or solar resistive elements so there is no energy gain as such. That’s not to say you can’t use PCMs as the “water” substitute in certain heat-pump configurations. I think that’s evolving technology.
My main issue with PCMs is the practicality at scales that really matter. I’ve done calculations for my house and oil-fired boiler and determined I would need about 1,000 gallons (!) of H2O stored at regular heat-pump-capable temps of 50C. That volume goes down a bit for 80C (high temp heat pump). Meaning: perhaps a 200-250 gallon tank of PCM. That certainly seems more manageable volume-wise but there are reasons why BIG tanks favor H2O. The tank and materials are easier/cheaper to make I believe and if you get an inevitable tank failure you don’t have such a cleanup nightmare.
Almost seven years and counting … I’ve posted this before. It doesn’t mention heat pumping but it’s more about the philosophy of integrating large water volumes as storage:
@MikeekiM Something you can consider is that HPHW (Heat Pump Hot Water) tanks can be run on 120V only if you don’t have resistive elements so you could perhaps save on the electric install.