How to Calculate Chiller Efficiency in HVAC

We are going to be looking at how to calculate the efficiency of a chiller now why we might want to know this well chillers are actually pretty expensive to run. So you want to make sure that the chiller is running in its optimal performance. The way to check that is to do some calculations on the chiller efficiency.

A very low efficiency means you're getting very little output for your money and a very high efficiency means that you're getting a lot of value for your money so it's a good calculation to perform on your chiller at different loads and also if you have multiple chillers test them all at different those to see which one is the most efficient and run that as the main one at that peak.

So chiller efficiency is measured in COP, the COP stands for coefficient of performance      and really the

COP = Ratio of how much refrigeration you get per unit of electricity that you put into the compressor

Now the formula for this one is very simple

 COP = Kw of Refrigeration (Output) / Kw of Electricity (Input)

So, we're going to do the calculations on this we need is the electricity in and that's measured in kilowatts now that's very easy to get, you just put on a clamp meter on supply lines to get it or maybe from your BMS or on the head end of the control panel.

You should be able to read them from so many of the electricity in will depend upon the power consumption by this unit will vary by the amount of load that is pressed onto the compressor.

Now the second part you need in this equation is the refrigeration effect so how much cooling are you getting out of this chiller now we want the units in kilowatts. If you don't know how to calculate this, then I highly recommend you read our other article just before this which is how to calculate the chiller cooling capacity.

Also Read: How to Calculate Cooling Capacity of a Chiller in HVAC

So back to the calculation we're just going to drop in the number. We'll put refrigeration effect that at the top of the division and then we'll also put the amount of electricity going into the machine as well.

 COP = Kw of Refrigeration (Output) / Kw of Electricity (Input)

COP = 2500 Kw / 450 Kw

COP = 5.5

If you want to calculate in imperial units then follow this calculation;

1 Kw = 3412.142 Btu/h

So

8,533,364 Btu/h ÷ 3412.142 = 2500 Kw

COP = 2500 Kw / 450 Kw

COP = 5.5 

You'll see that these COP comes out of five point five that's very good that's really very good so what this means is that for every one kilowatt of electricity that you put in you will get five point four kilowatts of cooling out so there you go you get five times your      money's worth of electricity out in cooling.

So like I said the COP of a chiller will actually vary throughout the amount of low that's placed on the compressor throughout the year throughout the day as well unless you've got a constant load then COP will be also be approximately constant.

There is a difference in the COP compared to a constant speed chiller or a variable speed chiller. Variable speed chillers others are more efficient usually because in normal buildings it's actually quite rare that a chiller would operate at certainly 100%.

Most of the time is operating at part load so it's only going to be few days per year when the chiller will actually be operating full load. The majority of the time it's going be      somewhere between 40% - 60%. So if you have a variable speed chiller then it's going to be much more efficient.

In the above graph for the variable speed chiller you can see here that at around 50% load we're going have our highest COP. However, as the load increases maybe in the peak of summer and we're approaching kind of 90% - 100% of the load on the chiller and you can see the COP becomes a lot lower. Whereas the constant speed chiller which is the standard chiller will provide much more efficient or economic performance the higher or greater the load that is placed on them.

But you can see in the graph at 90% - 100% load it reaches its peak performance but on normal load during the normal time of the year it's probably going to be a COP of 6 at 40% to 60% part load. Whereas at the same time variable speed chiller will give you a COP of 8 or maybe 9.

so that’s how you calculate the coefficient of performance and how efficient your chillers are do remember that you can upgrade your chillers and if you have a normal building like an office type building and you are in certainly the northern hemisphere then you really consider installing a variable speed drive on the compressor and speak to your manufacturer or service provider for that.