It's true that heat pumps become less efficient at higher temperature differences. That's why these systems use lower storage temperatures. It's also true that,

It's true that heat pumps become less efficient at higher temperature differences. That's why these systems use lower storage temperatures. It's also true that, even then, the storage temperature has to by higher than the typical evaporator condensor for a heat pump, leading them to be less efficient (think 175% efficient vs. 300% efficient). You're looking at LCOS of $0.23 kWh-1 [1], which is higher than Li-ion. However, such a system may offer other advantages (better recyclability, no reliance on lithium, less risk of fire), or such systems may be cheaper in the future, which is why people are researching them. Additionally, these systems work with existing district heating pipes, which is great because it mean's you don't have to expand the grid capacity. In places with district heating, that could make a lot of sense. (I don't think anyone is proposing building new district heating systems, just retrofitting old ones). [1] https://www.sciencedirect.com/science/article/abs/pii/S019689042100248X https://www.sciencedirect.com/science/article/abs/pii/S01968... Also, Carnot efficiency increases without bound as the temperature difference decreases, so lowering the temperature of the room or increasing the temperature of the outdoors make a huge difference. The same applies vice versa for air conditioning which is why people use cooling towers.