Inverter AC efficiency at different speeds

[coolguy4]

CoolGuy4

It depends what type of compressor you have Screws and traditional Centrifugal's work best at speeds above 80% as a rule, Scrolls are always most efficient at a fixed speed and there some great ways of capacity controlling these. However here I think you are asking about reciprocating compressors , these work much better with frequency inventors but there are lots of other factors to consider , lower speed mean lower performance effectively making your evaporator and condenser oversize this may well increase efficiency, but will it meet your load requirements ?

The most efficient compressor I have used , and I have used them a lot is a Danfoss Turbocore. This is an oil free two stage centrifugal that tops out on performance between 40% and 70% . We have used these for larger AC chillers and DX from 250KW and up to 5600KW ( chillers) . Reduced maintenance ( ie No mechanical ) enhanced performance and 30% less energy usage compared with traditional Screws ad Centrifugal makes it my first choice.

Great information, thank you very much. Let's say we have 3 completely identical rooms, all of which have 12000 BTU of heat gain. Room 1 has a 12K BTU inverter mini split that is maxed out at 100% capacity, room 2 has an 18K BTU inverter mini split at 67% capacity, and room 3 has a 24K BTU inverter mini split running at 50% capacity. With all else being equal, which unit will use the least energy to maintain the set temperature under those conditions? Thanks in advance!

[frank]

Room 1 has a 12K BTU inverter mini split that is maxed out at 100% capacity, room 2 has an 18K BTU inverter mini split at 67% capacity, and room 3 has a 24K BTU inverter mini split running at 50% capacity.

You must take into account the input power for the different size systems, not just the cooling duty, i.e. the smaller system will have a lower running current, and assuming that your room load is never satisfied by this unit, it will run 24/7.

If the larger 24K BTU unit draws more power than the 12K BTU unit when running at 50% duty then the 12K BTU unit will be more energy efficient. Also, as the 24K BTU unit is larger than the load, there will be thermostat off periods and multiple start ups which consume more power.

[coolguy4]

You must take into account the input power for the different size systems, not just the cooling duty, i.e. the smaller system will have a lower running current, and assuming that your room load is never satisfied by this unit, it will run 24/7.

If the larger 24K BTU unit draws more power than the 12K BTU unit when running at 50% duty then the 12K BTU unit will be more energy efficient. Also, as the 24K BTU unit is larger than the load, there will be thermostat off periods and multiple start ups which consume more power.

Thank you do much for your help! However, I am not sure if you understand what I am asking. I am not asking about a single speed 24K BTU unit running 50% of the time, rather I am asking about an inverter (variable capacity) 24K BTU unit that is only cooling at a capacity of 12K BTUs to match the load. Again, thank you very much for your help.

[Blueboy]

Thank you do much for your help! However, I am not sure if you understand what I am asking. I am not asking about a single speed 24K BTU unit running 50% of the time, rather I am asking about an inverter (variable capacity) 24K BTU unit that is only cooling at a capacity of 12K BTUs to match the load. Again, thank you very much for your help.

You could be using the same energy for all three , if the COP was the same on each unit .

If you hypothetical room is fixed load then possible the most efficient is the unit at max not using the inverter at all. However in the real world that does work as the load will be bigger or smaller at certain times than you have calculated. It also depends what ambient you calculate at. Say you assumed ambient of 28c and its 30 outside where you located the system now your smallest unit is too small on that day or in winter its 5 C and you still need cooling now it's too big - for this reason I would select the middle unit allowing for more and less is by far the best way to go . Energy is a good yard stick but not at the sacrifice of cooling on the worst day of the year .

[coolguy4]

You could be using the same energy for all three , if the COP was the same on each unit .

If you hypothetical room is fixed load then possible the most efficient is the unit at max not using the inverter at all. However in the real world that does work as the load will be bigger or smaller at certain times than you have calculated. It also depends what ambient you calculate at. Say you assumed ambient of 28c and its 30 outside where you located the system now your smallest unit is too small on that day or in winter its 5 C and you still need cooling now it's too big - for this reason I would select the middle unit allowing for more and less is by far the best way to go . Energy is a good yard stick but not at the sacrifice of cooling on the worst day of the year .

That makes sense, thank you! I wish the manufacturers provided an efficiency curve for their units like power supply manufacturers do. So presuming the minimum capacity of both units is roughly the same, there would not be any disadvantage to oversizing the unit besides initial cost?

Also, what effect would overloading and undersizing the unit have on its longevity? I remember hearing somewhere that since inverter AC units are variable speed, it isn't great for their longevity to be running at maximum capacity all day every day, similar to driving a car at full throttle all day every day- It's made to handle it, but longevity will likely be adversely affected. Is there any truth to that? Thanks in advance!