How to find the COP of a refrigerator

Are you seeking guidance on how to calculate the COP of a refrigerator? Did you know that you can easily determine the maximum possible efficiency (COP) of your fridge by using a thermometer that can measure both below 0 degrees Celsius and above? Simply record the temperatures of the condenser and the evaporator coil, and you’re good to go!

COP of a refrigerator

Have you ever wondered how to calculate the COP (Coefficient of Performance) of a fridge? It’s actually a fairly straightforward process! All you need to know are two temperatures to calculate the maximum possible COP.

However, if you want to calculate the actual COP of a fridge, it’s a bit more complicated. In addition to the temperatures, you’ll also need to know the type of refrigerant that’s being used.

Let us discuss how to calculate the maximum possible COP and the actual COP!

Maximum COP of a refrigerator

Did you know that the maximum possible COP of a refrigerator is only achievable if it works on a reversible cycle, like the Carnot cycle?

Assuming your refrigerator works on the reversed Carnot cycle, you can determine the condenser and evaporator temperatures in Kelvin, and then use the formula To/(Tk-To), where “To” is less than “Tk”.

By substituting the respective temperatures into the formula, you can determine the maximum COP that your refrigerator can achieve!

For example, let’s say you have a domestic refrigerator that produces refrigeration at -25°C (273+(-25)=248 K) (evaporator temperature), and heat is rejected to the ambient at 60°C (273+60=333 K) (condenser temperature).

The maximum possible COP of this refrigerator would be (COP) max= To/(Tk-To) = 248/(333-248.85) = 2.9.

This means that the refrigerator would produce a maximum of 290 W of refrigeration or cooling effect per 100 W of power consumption. Remember, the higher the COP number, the more efficient the refrigerator is.

You might be wondering why the maximum possible COP in the above example is only 2.9 and why in that refrigerator, efficiency higher than 2.9 is not possible, am I correct? To know the answer to it click/tap here.

Actual COP of a Refrigerator

If you want to check the actual efficiency or COP (Coefficient of Performance) of your refrigerator, you’ll need to know which refrigerant is being used, as well as the condenser and evaporator coil temperatures.

Normally, our household refrigerators use the vapour compression cycle to cool the food items inside them. This cycle consists of several thermodynamic processes, including isentropic compression, desuperheating and condensation, isenthalpic expansion, and evaporation.

To calculate the COP of your refrigerator, you’ll need to refer to the pressure enthalpy chart of the refrigerant being used in the refrigerator to obtain the values of h1, h2, h3, and h4 for the corresponding evaporator temperature “To” and the condenser coil temperature “Tk”.

Once you have these values, you can use the formula COP = Refrigeration effect / Work done, where Refrigeration effect = h1 – h4 and Work done = h2 – h1. After substituting these values into the formula, you’ll be able to calculate the COP of your refrigerator.

Keep in mind that the COP of a refrigeration system tends to decrease and power consumption tends to increase as we go to lower and lower refrigeration temperatures.

Operating conditions that affect the COP of a refrigerator

Did you know that the efficiency or COP (Coefficient of Performance) of a refrigerator is influenced by various operating conditions such as temperatures, condenser pressure, and evaporator pressure? Let me explain in brief how these conditions affect the COP of a refrigerator.

Effect of evaporator Pressure on COP of a refrigerator

If you reduce the evaporator pressure of a refrigerator, it can lead to a decrease in the COP (Coefficient of Performance). This is because the reduction in pressure can result in various negative effects such as a

• decrease in the refrigerating effect,
• an increase in the specific volume of the suction vapour,
• a decrease in the volumetric efficiency, and
• an increase in the compressor work.

Effect of Condenser Pressure on COP of a Refrigerator

If you increase the condenser pressure of a refrigerator, it can lead to a decrease in the COP (Coefficient of Performance). This is because the increase in pressure can cause a reduction in the refrigerating capacity, refrigerating effect, and volumetric efficiency of the refrigerator.

Therefore, it is not advisable to increase the condenser pressure or lower the evaporator pressure in an attempt to increase the COP of a refrigerator.

Effect of Operating Temperature on COP of a Refrigerator

To Obtain the maximum Possible COP in any application

1. The refrigeration temperature To should be as high as possible
2. The heat rejection temperature Tk should be as low as possible

Is it possible to improve the COP of a refrigerator?

Shall we take a moment to discuss the impact of subcooling, regenerative heat exchanger and superheating on the COP of a refrigerator? These methods are often used to improve the efficiency of vapour power cycle!

Effect of subcooling

Did you know that subcooling can improve the COP of a refrigerator? However, there’s a more efficient way to achieve this without a separate sub-cooler.

Instead, you can combine the functions of the condenser and sub-cooler in the condenser itself by slightly oversizing it. This means that the condenser will have a larger surface area, which allows for more heat exchange and subcooling, resulting in an improved COP for the refrigerator.

Effect of suction vapour superheating

It is worth noting that while superheating may be effective in increasing the COP for some refrigerants like Freon 12, it may not be beneficial for all refrigerants.

Effect of using liquid vapour regenerative heat exchanger

The use of a vapour-liquid regenerative heat exchanger can be effective in improving the COP of a refrigerator for some refrigerants like freon 12, but this method also may not work for all refrigerants.

References

Refrigeration and Air Conditioning by C P Arora