In this video, Mr.Vedant Sutaone answers one of the questions we get frequently asked by our clients – “What is the lowest outside temperature your heat pumps can operate at?”. Kindly watch to find out the answer and if you have any other questions that you would like us to answer, please let us know in the comments section below.

What is the lowest outside temperature your heat pumps can operate at?

An air-source heat pump works on a principle similar to that of an AC and a refrigerator. The schematic below shows the working of an air source heat pump.

With ambient air as a heating source, the refrigerant absorbs heat from the ambient air and ejects heat to the processing fluid whose temperature is required to be raised. More the ambient temperature, the higher the heat ejection from the refrigerant to the processing fluid. Heat pumps are designed to operate at a certain ambient temperature. The heat pump will meet the heating demand only at that ambient temperature or more than that ambient temperature. If the ambient temperature falls, one cannot achieve either the heating demand or the desired temperature output. We cannot meet the heating demand at low ambient temperature and vice versa.

Now the question that arises is, “what happens if the temperature decreases below the designed ambient temperature? If the ambient temperature decreases, will the Coefficient of Performance (COP) of the system also decrease?”

To understand this let us consider an example. Let’s assume that there are two 28 kilowatts air source heat pumps that are installed at two different locations and at location 1, the ambient temperature is around 20°C whereas, at location  2,  the ambient temperature is around 5°C. As you can see, the temperature lift at location 1 is around 60°C whereas, at location 2, it is around 75°C.

Since we use fixed-speed compressor, the work done by the compressor is also fixed. As the temperature lift at location 2 is higher than that of location 1, the compressor has to do more work compared to the compressor at location 2 which means that the efficiency of the system at location 2 is less compared to the efficiency at location 1.

COP variation with the ambient temperature

Now let us understand the heat pump performance at different ambient temperatures. As you can see in the table below, the ambient temperature, the heating capacity, the input to the compressor, and the COP of the system are all correlated.

Ambient Temperature(℃) Heating Capacity (kW) Heating Input(kW) COP
5 14.15 8.45 1.67
10 21.56 8.3 2.6
20 28.15 9.07 3.1
35 33.22 10.15 3.27
43 41.59 10.96 3.8

At 5°C ambient temperature, the heating capacity is around 14.15 kilowatts and the COP is 1.67. When we increase the ambient temperature to 10°C, the heating capacity increases to 21.56 kilowatts  and the COP increases to 2.6. If we further increase the ambient temperature to 20°C, the heating capacity increases to 28.15 kilowatts, and COP increases to 3.1.

From this table, we can conclude that if we increase the ambient temperature, the heating capacity, as well as the COP of the system, increases accordingly.

The Heating Capacity Curve

If we plot a graph between the ambient temperature and the heating capacity, then we get a curve known as a ‘heating capacity curve’. We can see from the curve that when we increase the ambient temperature, the heating capacity also increases accordingly.

A close up of a map

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Summary

Heat pumps are usually designed to operate at a certain ambient temperature range and hence, they will only meet the heating demand at a certain designated minimum ambient temperature or temperatures above that. Since the efficiency of the heat pump mainly depends on the ambient temperature, the higher the ambient temperature, the higher the system efficiency will be.

I hope we have answered the question. If you have any other question, you can write to info@aspirationenergy.com

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