Energy Consumption and Efficiency of Heat Pumps

What Does the COP of a Heat Pump Mean?

The energy consumption of various heat pumps is expressed by the Coefficient of Performance, or COP. Regardless of the type, whether it's an air-source heat pump or a geothermal heat pump, efficiency is measured based on the COP. But what does this COP actually mean?

A common mistake is to evaluate heat pumps based on a single COP value. A high COP does not necessarily imply that a heat pump is economical. To make fair comparisons, the integrated value (SCOP) plays a crucial role. It is also important to understand how the heat pump performs under partial load.

The Origin of the Heat Pump: How Does It Work?

A heat pump extracts heat from water or air, with various systems distinguished based on the heat source. Virtually any heat-retaining medium, obtained through solar energy or not, can serve as a source, such as ground heat, surface water, outdoor air, ventilation air, or waste heat from industrial processes. The heat is upgraded and transferred to air or water for use in space heating or domestic hot water. Electricity is required to initiate and maintain this process.

What Does the COP of a Heat Pump Mean?

The COP is the ratio between the amount of heat released and the electricity consumption (absorbed power) of the heat pump. This can be best explained through an example.

For instance: for every 5 kWh of heat a heat pump produces, 4 kWh comes from the source (air or water), and 1 kWh comes from electricity. In other words, for every 5 kWh of heat, the COP in this example consists of 4 kWh from the source and 1 kWh from electricity. So, the COP here is 5. In a calculation, it looks like this:

COP=Output HeatInput ElectricityCOP=Input ElectricityOutput Heat​

A higher COP implies less electricity consumption and indicates more economical operation of the heat pump. A heat pump with a high COP has a shorter payback period, and fewer solar panels are needed to offset the consumption. This makes the COP a relevant figure.

Efficiency of a Heat Pump

In the example mentioned earlier, the COP is 5, where only 1 part of electricity is needed for 5 parts of heat. This results in an efficiency of 500%, which is exceptional in the heating sector. For comparison, here are the efficiencies of electric heating and a gas boiler.

COP and Efficiency of Electric Boiler or Electric Radiators

In fully electric heating, like an electric boiler or an electric radiator, the efficiency is not as high. In the best case, a fully electric heater produces 1 kWh of heat for every kWh of electricity it consumes. 

and the efficiency is 100%. The energy input equals the output. More importantly, a heat pump is 5 times more efficient with electricity than electric heating, possibly because a heat pump extracts and upgrades heat from a source rather than relying solely on electricity. The majority of the energy comes not from electricity but from the heat in the air or water.

COP and Efficiency of a Gas Boiler

A gas boiler deals with losses. The heat released from burning gas cannot be fully transferred to the water being heated, resulting in heat loss and a loss of efficiency. Additionally, during the process of heating water in the boiler, water vapor is produced, which, along with flue gases, escapes through the chimney. In the case of a high-efficiency (HR) boiler, the water vapor is condensed, resulting in less loss and higher efficiency. Nevertheless, the efficiency can never be 100% or higher. The average efficiency of a boiler is around 90%, resulting in a COP of 0.9. This is more than 5 times less efficient than a heat pump. Some boiler manufacturers claim efficiency up to 107%, but this claim is disputed on the following website: cvketelkiezen.nl.

COP of a Heat Pump Is Not Fixed

The COP of a heat pump depends on various factors:

1. The efficiency of the heat pump itself is influential; the more efficiently the heat pump produces hot water or air, the higher the COP. This includes the use of inverter compressors and electronically controlled expansion valves.

2. The temperature of the source (air or water) also affects the COP, as well as the temperature of the produced water/air. The higher the produced temperature, the higher the consumption, and the lower the COP. This is logical; it takes more energy to heat water from 15˚C to 80˚C than from 15˚C to 35˚C.

COP of an Air/Water Heat Pump

An air/water heat pump literally extracts heat from the outside air warmed by the sun, even in freezing conditions. This heat is upgraded to a usable level and transferred to water for use in space heating in underfloor heating or radiators and/or for producing domestic hot water.

This heat pump usually consists of an outdoor unit and an indoor unit. The outdoor unit is placed outside and draws in air and expels it, but cooler. The temperature difference between them represents the heat absorbed by the heat pump.

You can imagine that when it is colder outside, the heat pump has to work harder to heat water than when it is warmer. After all, a greater difference needs to be bridged. This also means that the COP fluctuates at different outdoor temperatures. The colder it is outside, the lower the COP at the same output temperatures.

This means that the COP of an air heat pump is weather-dependent but also depends on the season and location. In winter, it is colder than in summer, and the heat pump has to work harder, the same as in Scandinavia, where it is colder outside than in the Netherlands.

Therefore, an air heat pump never has a single COP (see the graph below). The left axis has two values, namely the COP and the outdoor air temperature. The bottom axis shows the temperature of the water produced.

SCOP of a Heat Pump

So, what is the SCOP of a heat pump? The abbreviation stands for Seasonal Coefficient of Performance. Essentially, it is nothing more and nothing less than an average COP over a year, taking into account the seasons in a particular region.

The SCOP makes it easier to compare heat pumps, especially air heat pumps where seasons influence efficiency. When it's colder, a heat pump has to work harder, and the COP decreases, as described above.

To calculate the SCOP of an air/water heat pump, all COP values throughout the year are averaged in a specific region, such as the Utrecht region (or another part of Europe). This is done at multiple output temperatures in 5˚C increments. Then you know for that heat pump what the SCOP is when producing 30˚C water, 35˚C, 40˚C, 45˚C, and so on.

The SCOP of a heat pump is lower in Scandinavia than in Spain because it is colder there. Additionally, the heating system, such as radiators or underfloor heating, can affect the SCOP. More on this in the next chapter.

For modern air heat pumps in the Netherlands, the SCOP lies between 3 and 4. This is an average value, as no heat pump operates continuously at the same outdoor temperature and water temperature.

Air/Water Heat Pump and Low Outdoor Temperatures

To improve efficiency at low outdoor temperatures, a different type of refrigerant must be used, which can withstand lower temperatures. In addition, inverter technology plays an important role. The outdoor unit contains a frequency-controlled compressor, which can adjust its speed to the demand for heat. This means that when it is less cold outside, the compressor runs more slowly, and when it is colder, it runs faster.

This way, the compressor always operates at an optimal speed, which has a positive effect on the COP. Moreover, inverter technology prevents the compressor from starting and stopping all the time, which is less efficient. Inverter technology is not only applied to air/water heat pumps but also to other types.

COP and Efficiency of an Air/Air Heat Pump

An air/air heat pump extracts heat from the outside air and transfers it directly to the indoor air. In contrast to an air/water heat pump, no water is produced here. This type of heat pump usually consists of an outdoor unit and several indoor units.

Just like the air/water heat pump, the COP of the air/air heat pump depends on the outside air temperature. However, because no water is produced here, the COP of an air/air heat pump is generally higher than that of an air/water heat pump at the same outdoor temperature.

But, and this is a big but, an air/air heat pump is not suitable for all types of homes. Only when the home is very well insulated and equipped with low-temperature heating, such as underfloor heating or radiators with a large surface area, can an air/air heat pump be used.

COP of a Geothermal/Water Heat Pump

A geothermal/water heat pump extracts heat from the ground or groundwater and upgrades it to a usable level. The COP of this type of heat pump is less dependent on the outside air temperature, making it more stable. However, it is not entirely unaffected by the outdoor temperature.

Even in winter, the temperature of the ground at a certain depth remains more constant than the outdoor air temperature. Therefore, the COP of a geothermal/water heat pump is generally higher in winter than that of an air heat pump.

COP and Efficiency of a Hybrid Heat Pump

A hybrid heat pump combines two heat sources: a heat pump and a gas boiler. The heat pump extracts heat from the air and upgrades it. The gas boiler only switches on when the outdoor air temperature is too low for the heat pump to operate efficiently. The two heat sources are connected to a buffer tank, which supplies the heating system with hot water.

The advantage of a hybrid heat pump is that it can operate independently of the gas grid. This is useful in new homes that are no longer connected to the gas grid. An advantage of a hybrid heat pump with a gas boiler is that the gas boiler only needs to be connected in the winter months, saving gas consumption.

Conclusion

In conclusion, the COP is an essential factor in determining the efficiency of a heat pump. However, it's important to consider the SCOP, which provides a more accurate representation of a heat pump's performance over the course of a year, considering varying outdoor temperatures and demand for heating. Different types of heat pumps, such as air/water, air/air, geothermal/water, and hybrid, exhibit variations in COP and efficiency based on their heat source and technology used.

It's recommended to assess the specific requirements of a heating system, the climate of the region, and the intended use to determine the most suitable type of heat pump. Additionally, regular maintenance and optimal usage conditions can contribute to maximizing the efficiency and lifespan of a heat pump system.