Variable speed compressors and efficiency changes in COP

[ctscasemod]

Hey everyone,

For educational purposes I am trying to understand efficiency figures in variable speed units.

I understand that, with lower compressor speeds there is a lower energy consumption overall. But I don't fully understand why efficiency increases at part load. I'm guessing now both the evaporator and condenser are oversized, since there is less refrigerant for a given surface area, which would presume heat can be transferred with a lower differential pressure - hence less strain on the compressor - But if this was all, why wouldn't manufacturers just offer variants with larger coils?

Also, with a lower differential pressure / head pressure - what would be the performance where the inside unit is expected to release heat, such as a heat pump in the winter?

What about the fans?
Is it best to throttle the fans back together with the compressor, or have them run full speed to make better use of the available coil surface area?

Thanks for any clarifications :-)

[Rob White]

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The system is not static, it's changing as load changes and so one compressor
is either working or it is not (normal comp) but with a variable speed compressor,
as the load changes the compressor changes in speed accordingly.

The thing to do with efficiency is not to go too high or too low on the respective sides.
If the condenser needs 15DegC difference to work efficiently then you don't need to be at 20DegC.
If the evaporating temperature needs to be at +5DegC then it doesn't need to be down at 0DegC.

That is where the efficiency comes in, running the compressor with frequency control, so it just
meets the demand for load on the high and low side but as you state in heat pump mode
the high side (output) needs to still be 15DegC higher than the supply air temperature so in principle
there is no real difference to cooling mode.

The biggest difference in heat mode is the heat generated by the comp is now put back into the room.
In cooling mode the head from the compressor is put outside into the ambient air, in heating mode the
heat from the compressor is put into the room so that adds to the efficiency.

Rob

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