The concept of performance "Performance (physics)") is applied to machines for generating or transforming energy, and, in accordance with the first principle of thermodynamics, its value cannot be greater than unity. Refrigeration machines, and therefore the heat pump, do not generate energy, they only transport it from a cold region to a hotter one. In this task, yields greater than 100% are obtained. To avoid the confusion that this could entail, in this type of machines the concept of performance takes the generic name of efficiency and in the case of the heat pump, the efficiency coefficient is called CoP, which is the acronym for Coefficient of Performance.

Obviously, in reversible machines there will be two efficiency coefficients, one as a refrigeration machine and another as a heat pump, known in practice as summer CoP and winter CoP. In both, the concept is the same expressed in the Carnot machine:

Where the subscripts cold focus and hot focus.

The only difference between the two is the numerator, that is, the benefit obtained. In winter, when the machine works as a heat pump, the benefit is the heat deposited in the hot source or, in other words,[2]​ the absolute temperature of the hot side, while in summer it will be the heat dissipated in the cold side, that is, the absolute temperature of the cold side. And not only this: among its most tangible benefits are savings on the electricity bill and energy efficiency, since it uses free natural sources for its operation and electricity consumption is actually minimal. It is also sustainable and helps reduce carbon emissions, thus meeting the European Union's objectives against climate change. And it is also very functional, since a single device serves to provide both hot water and cooling and heating. These benefits of savings and respect for the environment led the International Energy Agency to consider the heat pump as the best technology available for space air conditioning in 2016.[3]​.

This CoP is the theoretical maximum, which under conditions of 0 °C outside and 20 °C inside gives a value of 14.65; That is, in theory up to 14.65 kW of heat could be obtained for each kW of electricity consumed. Obviously, this is theoretical and in practice, taking into account the irreversibilities of the real refrigeration cycle, the CoP remains at 15% of the theoretical maximum.[4]​.

In the P-h diagram of the corresponding refrigerant, the cycles can be plotted and the theoretical operating CoP can be calculated exactly as is done for the refrigeration machine.

The graph shows in which concepts efficiency is lost:

On the other hand, the lower the temperature of the cold source and the higher the temperature of the hot source, the lower the efficiency, which is a serious drawback, taking into account that the heat pump is used as a heating device and will therefore have less performance the colder it is outside, that is, when more heat is needed inside. However, with a good machine, CoPs much higher than 1 are obtained, that is, the machine provides more energy than it consumes, up to outside temperatures around -10 °C.

Even with all these drawbacks, the heat pump recovers heat from the outside air. Due to this ability to extract heat from very cold sources, which otherwise would not be usable, this machine is considered renewable energy.[6]​

As in all heating systems using renewable energy, it is advisable to provide a support system with conventional energy,[7]​ to cover very cold periods. It must be taken into account that when the CoP decreases to 1, the advantage of the heat pump has been completely lost, since its effectiveness is the same as that of any electric stove, which, due to the Joule effect, produces one thermal kW per electrical kW consumed.

One approach to the second principle of thermodynamics is that it deals with the quality of energy. Energy quality is understood as the greater or lesser willingness that it offers to be converted into work.[8]​ With this criterion it can be said that electrical energy is of high quality, since when applied to an electric motor, it can be converted into work on the shaft with a performance greater than 90%. The quality of thermal energy depends on its thermal level, that is, its temperature. In a conventional thermal power plant, high-temperature steam is produced from a fuel, which is transformed into electrical energy by a turbine, with a total efficiency of around 30%,[9]​ that is, approximately 3 thermal kW produce 1 electrical kW. With this premise, using 1 kW of electricity to obtain 1 kW of heat does not seem like a very coherent consequence.

The production of heat from electrical energy is only admissible by means of a heat pump (apart from electric radiators) and even then, after a feasibility study that verifies, that at all times the heat production will be higher than the consumption and that the CoP guarantees both economic and energy savings.

To have an idea of ​​the profitability that a heat pump can give us as a heating device, the seasonal CoP is used, which is a quotient between the energy provided by the machine throughout the winter and the energy consumed in the same period, that is, the amount paid for heating in the winter season bills.

Manufacturers include[10]​ in their catalogs tables of thermal power and absorbed power of each model, for different indoor and outdoor conditions. The values ​​indicated therein are assumed to be real and should prevail over any estimate. A weighted average, over the winter, of these instantaneous values ​​can be a good anticipation of the seasonal CoP.

From what has been said so far, from an energy point of view, we would need a seasonal CoP of 3 to recover the 3 thermal kW that each electrical kW cost to produce. With combined cycle plants, efficiencies greater than 50% are achieved, in which case a CoP of 2 would be sufficient. From an economic point of view, the viability of the system depends on the comparison between the price of the electric kW and the thermal kW at each moment and with each fuel.

The cold source does not have to be exclusively the street air in winter, but could also be: the water of a river, an underground stream or even the ground. In the same way, the condenser can transfer heat, in addition to the ambient air, to water systems.

Depending on the source from which they take heat and to which they transfer it, the following classification of heat pumps is obtained:

These pumps, which take heat from water, have a higher CoP than air pumps, since underground water currents have an almost constant temperature. That of the rivers is more variable throughout the year, but still, it is not comparable to that of the outside air, nor is it as unpredictable. However, their use is subject to the existence of an adequate water flow in the vicinity of the facility, which is not easy and greatly restricts the application of these machines.

Aerothermal energy is a heat pump technology that extracts energy from the air to air condition spaces and provide domestic hot water efficiently. This technology, classified as renewable energy by the European Union, offers an ecological and economical alternative to gas and diesel boilers. Among its main advantages, aerothermal energy stands out for its high energy efficiency, low maintenance, and reduced environmental impact by not emitting polluting gases during its operation. In addition, the aerothermal systems are reversible, allowing both heating in winter and cooling in summer with a single piece of equipment. This type of heat pump can be installed in new and existing homes, offering flexibility in integration with traditional heating systems or by combining with solar energy sources.[12]​.

The concept of performance "Performance (physics)") is applied to machines for generating or transforming energy, and, in accordance with the first principle of thermodynamics, its value cannot be greater than unity. Refrigeration machines, and therefore the heat pump, do not generate energy, they only transport it from a cold region to a hotter one. In this task, yields greater than 100% are obtained. To avoid the confusion that this could entail, in this type of machines the concept of performance takes the generic name of efficiency and in the case of the heat pump, the efficiency coefficient is called CoP, which is the acronym for Coefficient of Performance.

Obviously, in reversible machines there will be two efficiency coefficients, one as a refrigeration machine and another as a heat pump, known in practice as summer CoP and winter CoP. In both, the concept is the same expressed in the Carnot machine:

Where the subscripts cold focus and hot focus.

The only difference between the two is the numerator, that is, the benefit obtained. In winter, when the machine works as a heat pump, the benefit is the heat deposited in the hot source or, in other words,[2]​ the absolute temperature of the hot side, while in summer it will be the heat dissipated in the cold side, that is, the absolute temperature of the cold side. And not only this: among its most tangible benefits are savings on the electricity bill and energy efficiency, since it uses free natural sources for its operation and electricity consumption is actually minimal. It is also sustainable and helps reduce carbon emissions, thus meeting the European Union's objectives against climate change. And it is also very functional, since a single device serves to provide both hot water and cooling and heating. These benefits of savings and respect for the environment led the International Energy Agency to consider the heat pump as the best technology available for space air conditioning in 2016.[3]​.

This CoP is the theoretical maximum, which under conditions of 0 °C outside and 20 °C inside gives a value of 14.65; That is, in theory up to 14.65 kW of heat could be obtained for each kW of electricity consumed. Obviously, this is theoretical and in practice, taking into account the irreversibilities of the real refrigeration cycle, the CoP remains at 15% of the theoretical maximum.[4]​.

In the P-h diagram of the corresponding refrigerant, the cycles can be plotted and the theoretical operating CoP can be calculated exactly as is done for the refrigeration machine.

The graph shows in which concepts efficiency is lost:

On the other hand, the lower the temperature of the cold source and the higher the temperature of the hot source, the lower the efficiency, which is a serious drawback, taking into account that the heat pump is used as a heating device and will therefore have less performance the colder it is outside, that is, when more heat is needed inside. However, with a good machine, CoPs much higher than 1 are obtained, that is, the machine provides more energy than it consumes, up to outside temperatures around -10 °C.

Even with all these drawbacks, the heat pump recovers heat from the outside air. Due to this ability to extract heat from very cold sources, which otherwise would not be usable, this machine is considered renewable energy.[6]​

As in all heating systems using renewable energy, it is advisable to provide a support system with conventional energy,[7]​ to cover very cold periods. It must be taken into account that when the CoP decreases to 1, the advantage of the heat pump has been completely lost, since its effectiveness is the same as that of any electric stove, which, due to the Joule effect, produces one thermal kW per electrical kW consumed.

One approach to the second principle of thermodynamics is that it deals with the quality of energy. Energy quality is understood as the greater or lesser willingness that it offers to be converted into work.[8]​ With this criterion it can be said that electrical energy is of high quality, since when applied to an electric motor, it can be converted into work on the shaft with a performance greater than 90%. The quality of thermal energy depends on its thermal level, that is, its temperature. In a conventional thermal power plant, high-temperature steam is produced from a fuel, which is transformed into electrical energy by a turbine, with a total efficiency of around 30%,[9]​ that is, approximately 3 thermal kW produce 1 electrical kW. With this premise, using 1 kW of electricity to obtain 1 kW of heat does not seem like a very coherent consequence.

The production of heat from electrical energy is only admissible by means of a heat pump (apart from electric radiators) and even then, after a feasibility study that verifies, that at all times the heat production will be higher than the consumption and that the CoP guarantees both economic and energy savings.

To have an idea of ​​the profitability that a heat pump can give us as a heating device, the seasonal CoP is used, which is a quotient between the energy provided by the machine throughout the winter and the energy consumed in the same period, that is, the amount paid for heating in the winter season bills.

Manufacturers include[10]​ in their catalogs tables of thermal power and absorbed power of each model, for different indoor and outdoor conditions. The values ​​indicated therein are assumed to be real and should prevail over any estimate. A weighted average, over the winter, of these instantaneous values ​​can be a good anticipation of the seasonal CoP.

From what has been said so far, from an energy point of view, we would need a seasonal CoP of 3 to recover the 3 thermal kW that each electrical kW cost to produce. With combined cycle plants, efficiencies greater than 50% are achieved, in which case a CoP of 2 would be sufficient. From an economic point of view, the viability of the system depends on the comparison between the price of the electric kW and the thermal kW at each moment and with each fuel.

The cold source does not have to be exclusively the street air in winter, but could also be: the water of a river, an underground stream or even the ground. In the same way, the condenser can transfer heat, in addition to the ambient air, to water systems.

Depending on the source from which they take heat and to which they transfer it, the following classification of heat pumps is obtained:

These pumps, which take heat from water, have a higher CoP than air pumps, since underground water currents have an almost constant temperature. That of the rivers is more variable throughout the year, but still, it is not comparable to that of the outside air, nor is it as unpredictable. However, their use is subject to the existence of an adequate water flow in the vicinity of the facility, which is not easy and greatly restricts the application of these machines.

Aerothermal energy is a heat pump technology that extracts energy from the air to air condition spaces and provide domestic hot water efficiently. This technology, classified as renewable energy by the European Union, offers an ecological and economical alternative to gas and diesel boilers. Among its main advantages, aerothermal energy stands out for its high energy efficiency, low maintenance, and reduced environmental impact by not emitting polluting gases during its operation. In addition, the aerothermal systems are reversible, allowing both heating in winter and cooling in summer with a single piece of equipment. This type of heat pump can be installed in new and existing homes, offering flexibility in integration with traditional heating systems or by combining with solar energy sources.[12]​.