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  1. #1
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    Inverter AC efficiency at different speeds



    I have a dumb question about the efficiency of an inverter AC, such as a ductless mini split. Basically, my question is in general at what capacity is an inverter AC generally most efficient? I am sure the answer will vary a bit based on the design of the specific unit, but what would be “typical”? I have not been able to find any efficiency curves at different capacities, or any actual data on efficiency at different capacities.

    I have heard that the unit’s efficiency is highest when the unit is running at the lowest speed because the heat exchangers are largest compared to the unit’s capacity. This makes sense to me.

    However, I have also heard that the unit doesn’t operate very efficiently at minimum capacity because the compressor is inefficient below a certain capacity, which also sounds reasonable.

    So in general, which would be correct? Or would the unit’s best efficiency be somewhere between minimum and maximum capacity? Thanks in advance, I have been wondering about this for a while and I have not been able to find any answers



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    Re: Inverter AC efficiency at different speeds

    I would assume that it is most efficient when it is at low speeds, as this state only occurs as the system approaches set point. As set point nears, the system running at 'tick over' seems to me to be more economical then reaching set point and then having to re-start.
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    Re: Inverter AC efficiency at different speeds

    Some articles in general.
    I think also important that unit is sized correctly so it runs in low to mid range, most of the time.
    The higher room temperature set point the better.
    Lower speeds of motors & fans, like you say as heat exchanger effectively oversize.


    https://www.hindawi.com/journals/ace/2018/7867128/


    https://www.sciencedirect.com/scienc...52484717300562
    Last edited by RANGER1; 23-05-2019 at 08:58 PM.

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    Re: Inverter AC efficiency at different speeds

    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.
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    Re: Inverter AC efficiency at different speeds

    Quote Originally Posted by Blueboy View Post
    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!

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    Re: Inverter AC efficiency at different speeds

    Quote Originally Posted by coolguy4 View Post
    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.
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    Re: Inverter AC efficiency at different speeds

    Quote Originally Posted by frank View Post
    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.

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    Re: Inverter AC efficiency at different speeds

    I do understand what you are asking, but I assume that you think the 24K BTU unit will draw less than the 12K Btu unit in input power, purely because it is oversized for the load.

    What I am trying to point out is that this may not be the case, just because it is twice the duty of the required load, inverter driven, and running at 50% load, doesn't mean that it will only require 50% input power
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    Re: Inverter AC efficiency at different speeds

    Quote Originally Posted by frank View Post
    I do understand what you are asking, but I assume that you think the 24K BTU unit will draw less than the 12K Btu unit in input power, purely because it is oversized for the load.

    What I am trying to point out is that this may not be the case, just because it is twice the duty of the required load, inverter driven, and running at 50% load, doesn't mean that it will only require 50% input power
    Oh okay that makes sense. I wasn't sure how the load/capacity of the unit effects efficiency since I can't find any data on the efficiency of inverter AC units at different capacities. I am thinking that the unit's efficiency probably isn't very good at minimum capacity, but it probably also starts to drop as it gets close to 100% capacity, so the unit's best efficiency is likely achieved somewhere in the middle of its capacity range, but I could be wrong.

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    Re: Inverter AC efficiency at different speeds

    Quote Originally Posted by coolguy4 View Post
    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 .
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    Re: Inverter AC efficiency at different speeds

    Quote Originally Posted by Blueboy View Post
    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!

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    Re: Inverter AC efficiency at different speeds

    Most manufacturers of inverter driven compressors suggest that optimum performance is between 40-70% duty and whilst the machine is capable of running at 100% duty, you are no longer running within the optimum efficency band. Some manufacturers will prevent the compressor from starting unless there is a minimum of 30% capacity demand, so the selection of a suitable unit, taking into consideration as previouisly mentioned extremes of ambient is not quite as clear cut as just looking for the potentially most power efficient unit.
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    Re: Inverter AC efficiency at different speeds

    Quote Originally Posted by hyperion View Post
    Most manufacturers of inverter driven compressors suggest that optimum performance is between 40-70% duty and whilst the machine is capable of running at 100% duty, you are no longer running within the optimum efficency band. Some manufacturers will prevent the compressor from starting unless there is a minimum of 30% capacity demand, so the selection of a suitable unit, taking into consideration as previouisly mentioned extremes of ambient is not quite as clear cut as just looking for the potentially most power efficient unit.
    That makes sense, thank you! If the unit is undersized and overloaded and running at 100% capacity all day trying to keep up, will the longevity of the unit be compromised?

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    Re: Inverter AC efficiency at different speeds

    Trying to establish the longevity of a inverter drive unit is no longer an easy calculation due to the complexities of the combined data relating to the running time, operational speed and operational duty whereas for a fixed speed drive compressor, the data is more directly related.
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    Re: Inverter AC efficiency at different speeds

    Quote Originally Posted by hyperion View Post
    Trying to establish the longevity of a inverter drive unit is no longer an easy calculation due to the complexities of the combined data relating to the running time, operational speed and operational duty whereas for a fixed speed drive compressor, the data is more directly related.
    Thanks. I have no evidence to back this up, but I would think that a compressor will wear out faster when it is running at maximum speed constantly, even though it is made to handle it. If nothing else, the compressor theoretically has a finite amount of revolutions it can make before it fails, and spinning it faster would use up those revolutions faster.

    Maybe a bad analogy, but I look at variable speed compressors sort of like car engines: If the engine (compressor) is too small or not powerful enough for the vehicle (the load on the AC system) and is running at full throttle constantly to keep up, the engine (or compressor) won't last as long as if it was correctly sized and wasn't running at maximum speed constantly.

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