Efficiency and performance of heat pumps
In energy engineering, the term energy efficiency refers to the ability of a physical system to achieve a given result using less energy compared to other systems with lower efficiency.
Efficiency, Performance, and Proper Use of Energy Parameters
In the field of energy engineering, the term energy efficiency refers to the ability of a physical system to achieve a given result using a smaller amount of energy compared to other systems performing the same function.
This concept is central to evaluating modern technologies for heating and cooling environments, particularly heat pumps.
Operation of Heat Pumps
Common domestic heat pumps—particularly inverter-type systems—operate according to two main modes:
- cooling of environments
- heating of environments
In both cases, the system uses electrical energy to operate the refrigeration cycle and exploits heat exchange between the indoor environment and an external source (air, water, or ground).
Performance Indicators: EER and COP
The performance of a heat pump is expressed using two distinct parameters:
- EER (Energy Efficiency Ratio): used in cooling mode
- COP (Coefficient of Performance): used in heating mode
The COP is defined as the ratio between the thermal energy delivered to the environment being heated and the electrical energy consumed to operate the machine:
[
\text{COP} = \frac{\text{useful thermal energy produced}}{\text{electrical energy consumed}}
]
In domestic applications, both EER and COP typically have values around 3.
This means that for every 1 kWh of electrical energy consumed, the heat pump can deliver approximately 3 kWh of thermal energy to the environment.
Of these 3 kWh:
- 1 kWh comes directly from the electrical energy consumed;
- The remaining 2 kWh are drawn from the external source (air, ground, or water), which serves as a renewable and free energy source.
Considering that electrical energy is generally produced with an average efficiency of around 36%, the overall efficiency of the system is significantly higher than that of traditional thermal machines, reaching equivalent values above 100% from the perspective of total energy use.
Alternative Systems and Performance Comparison
By using a TFC system instead of a traditional heat pump, with the same installation, it is possible to achieve significantly higher efficiencies, with COP values approaching 6—equivalent to an overall efficiency of around 220%.
An additional advantage of these systems is their lower sensitivity to external temperature variations, which significantly affect the performance of air-to-air or air-to-water heat pumps.
For comparison, the best traditional boiler systems, powered by gas or fossil fuels, have efficiencies that rarely exceed 90%.
Influence of the External Heat Source
The COP of a heat pump depends strongly on the temperature difference between:
- the environment to be heated;
- the external heat source being used.
The smaller this difference, the higher the system’s efficiency.
Indicatively:
- with external temperatures around 7 °C, the COP remains close to 3;
- Below 2 °C, performance drops rapidly, resulting in increased electricity consumption.
Usable renewable heat sources
The main renewable sources that can be utilized by heat pumps are:
- Air
- Water
- Earth
Heat pumps that use the ground or groundwater effectively harness geothermal energy that spreads from the Earth’s core to the surface. These sources offer greater thermal stability compared to air, allowing the system to operate with a more consistent COP throughout the year.
The subsoil, below the uppermost surface layers, actually maintains an almost constant temperature, making it an almost ideal source.
Similarly, groundwater or lake water has relatively stable temperatures, provided that extraction is technically and economically feasible.
Access to these resources is achieved through:
- vertical geothermal probes,
- horizontal geothermal collectors,
- spiral geothermal probes.
Although these solutions involve higher installation costs, they ensure superior performance and greater long-term reliability.
Using air as a heat source remains the simplest and most widespread solution, as it is available everywhere and requires no regulatory permits; however, strong seasonal variations, especially during the winter months, significantly reduce its efficiency precisely when heat demand is highest.
Final considerations on the use of COP
Applying the concept of COP as a universal efficiency indicator to thermal machines that do not use an external renewable source constitutes a significant conceptual error, one into which even experienced operators often fall.
The laws of physics dictate that energy cannot be created, only transformed; every transformation inevitably involves losses, generally in the form of heat.
The electrical efficiency of a device therefore indicates exclusively how much of the energy consumed is converted into useful work, and how much is dissipated.
Sometimes the COP is expressed in economic terms, as the ratio between the unit cost of electrical energy and that of the thermal energy produced (€/kWh). However, this representation is of limited usefulness, since the parameter retains its full physical meaning when expressed as an energy ratio (output energy or power relative to input) or, alternatively, in percentage form.
