Heat pumps are not viable in a lot of cases, in an urban setting, it's probably the vast majority. So for the many cases were the alternative for heating would be burning fossil fuels, using electricity, ideally produced via renewable energy/nuclear, could be a superior alternative.

Wait, why heat pumps are not viable in an urban setting?

My urban apartment has a heat pump and it's nothing special. I even pay the heat bill and they still gave me a heat pump. I have no idea why someone would say they aren't viable. Not only are they viable, they're typical for new construction in warmer climates.

For many reasons.

First, transporting heat as opposed to electricity is very wasteful, so you only want to transport it in very short distances.

Second, typically even small single family home requires quite large volume to store the heat effectively for many months. It isn't that complex only because in a single family home setting you already have a bunch of uncontested land available so you can use the volume that is relatively flat and not too deep.

Building this on a scale of a city would be insurmountable challenge. You would have to dig deeper than the buildings are high and any kind of works like that are difficult in urban areas.

A heat pump is just a backwards air conditioner. Not whatever you are thinking it is.

I think you are thinking of something very different than what "heat pumps" actually are? They don't involve storing heat, they're just an inverse fridge.

I think guys you are all wrong.

https://en.wikipedia.org/wiki/Heat_pump

The first sentence:

"A heat pump is a device that transfers heat energy from a source of heat to what is called a thermal reservoir."

So yes, it involves energy storage.

The way this works is you store heat in the summer (warm up a lot of rock or ground underneath your house) and recover that energy in the winter by pumping a liquid through warm rock back to your house and use it as a heat source.

The air outside your house counts as a thermal reservoir as well. That it doesn't store anything to the next season is annoying and makes it not work well in cold temperatures.

I think wikipedia is wrong or confusing here. A heat pump requires a thermal resevoir (something that doesn't change temperature much when you move heat to or from it) of some sort, usually the atmosphere, or in ground source heat pumps pipes running through the ground, but it can move heat in either direction.

> While air conditioners and freezers are familiar examples of heat pumps, the term "heat pump" is more general and applies to many heating, ventilating, and air conditioning (HVAC) devices used for space heating or space cooling.

Basically what I would understand from the term heat pump in ordinary conversation would be an air conditioner intended for use in a heating dominated climate rather than a cooling dominated one, but there might be some regional differences in usage.

Thermal reservoir is a technical term in thermodynamics and as others have pointed out it doesn’t mean what you think it means. Anything remotely resembling a (reversed) Carnot cycle would involve thermal reservoirs. You can read its own Wikipedia page.

Given that the neighbouring house has one on their wall, I think I know what they are.

What a few of these "lot of cases" where they are not viable?

Anytime it is "cold" air source heat pumps don't work. Cold varies a bit, somewhere between -5C and - 25C depending on design factors. Even in the best case as you get closer and closer to the minimum temperature the worse they work (IE when you need them most!), and once you hit the cut-off you better have a backup source off heat.

You can use geothermo (ground) to work around this. I'd recommend it, but the one time install costs mean it is questionable if it is cost effective.

OK, so that's one, very well understood case. Yes, a heat pump will not work all the time. I have one and it stops working around 20F and the furnace takes over. So what? I live at the US/Canada border and my furnace runs maybe ~2 months during the year. This is still tremendous savings.

Big savings, but is it big enough to be worth the extra expense of a heat pump vs used using the furnace year round. The times when the heat pump works are the times when you least need it, since other activities of life are adding heat to the house too.