I was a doing a placement at a company, and they were designing a new heat pump. I was told the more compact you can make a heat-pump, the better it is for efficiency, and they mentioned something about this was because the power density was increased.
I get there are benefits from using less material and making something lighter which would improve efficiency in that regard, but I don't get the benefit of a higher power density in a heat pump. I understand that in a heat engine like a combustion engine you want a high compression ratio so you'd want to make the thing as compact as possible, but why is a better power density also good for a heat-pump? Is it this just the thing with all types of machinery which have some sort of volume which is compressed? If so Why?
Edit: I didn't really want to say COP but rather efficiency to keep the question more general. These guys seemed pretty certain that having a small high rpm heat pump gave improved performance, whilst I assume they meant COP could be something else they are looking at. Maybe a better way to ask this is: Are there any theoretical benefits to the COP of a heat pump by making it smaller?