[fulafel]

I think it's more like 90C in (winter), 40C out. The temps have been going down in newer infrastructure as it's more energy efficient. See eg https://www.shcbysweden.se/wp-content/uploads/2019/11/Best-P... p. 7

And of course it's still a win if you can heat the return water half of the way to spec with the battery, it's not necessary to have the battery heat it all the way to the plant outgoing temp.

District heating systems have been happily using ~90C water based heat batteries for a long time.

[Ekaros]

Other limitation that I know is that for tap water minimum is 55C. So the last building in loop should get at least that much. So 40C is only acceptable back at power plant.

In the end it comes to balance with cost, simplicity, capacity and such. Heat pumps do allow extracting heat from colder storage medium. But on other hand electric heating elements and heat exchangers are very much simpler and cheaper.

[fulafel]

In the good systems there's two pipes, return gets its own (like eg the earlier linked PDF describes), so it's fine for buildings to output cooler water to the return pipe.

But yes heat pumps are used in some parts as well in DH.