Aerothermal energy is the thermal energy that a heat pump extracts from ambient air. It is legally considered energy from a renewable source in accordance with Directive 2009/28/EC of the European Union, in force, relating to the promotion of the use of energy from renewable sources and by which Directives 2001/77/EC and 2003/30/EC are modified and repealed. It is considered renewable as it does not reach the minimum COP (coefficient of performance, performance) (4.5) except under laboratory conditions.
In aerothermal systems, the heat pump is of the air-air or air-water type. The first of the two terms indicates the external medium (air) with which the machine exchanges heat, while the second indicates the internal medium.[3] To provide 100 units of thermal energy (heat), aerothermal energy needs to inject around 30 units of electrical energy.[4] The exact amount will depend on the device (some manufacturers produce more efficient devices than others), its state of maintenance (a properly maintained device will be more efficient) and the temperature difference between the outside and the inside. (The efficiency is greater the smaller the temperature difference).[5].
When it works for heating, the aerothermal device extracts heat from the outside air and injects it inside. When operating for cooling, it extracts heat from the indoor air and releases it to the outdoors. In this mode of operation, most air conditioning units (air conditioners) also extract (for comfort reasons) water vapor from the indoor air and expel it outside in the form of liquid water. Operating for heating in optimal conditions, aerothermal energy consumes (variable cost) only 25% of the electrical energy[6] that conventional electric heating (resistance) would consume, although the fixed cost of the device is much higher. In addition to heating, the aerothermal device can produce domestic hot water (DHW).[5].
Origin and impact in the air conditioning sector
Aerothermal energy has its theoretical roots in the principles of thermodynamics developed in the century by Nicolas Léonard Sadi Carnot and Lord Kelvin.[7] The first functional heat pump system was built in 1856 by the Austrian engineer Peter von Rittinger, who used it for brine drying, marking one of the first practical applications of these concepts.[8].
Aerothermal system
Introduction
Aerothermal energy is the thermal energy that a heat pump extracts from ambient air. It is legally considered energy from a renewable source in accordance with Directive 2009/28/EC of the European Union, in force, relating to the promotion of the use of energy from renewable sources and by which Directives 2001/77/EC and 2003/30/EC are modified and repealed. It is considered renewable as it does not reach the minimum COP (coefficient of performance, performance) (4.5) except under laboratory conditions.
In aerothermal systems, the heat pump is of the air-air or air-water type. The first of the two terms indicates the external medium (air) with which the machine exchanges heat, while the second indicates the internal medium.[3] To provide 100 units of thermal energy (heat), aerothermal energy needs to inject around 30 units of electrical energy.[4] The exact amount will depend on the device (some manufacturers produce more efficient devices than others), its state of maintenance (a properly maintained device will be more efficient) and the temperature difference between the outside and the inside. (The efficiency is greater the smaller the temperature difference).[5].
When it works for heating, the aerothermal device extracts heat from the outside air and injects it inside. When operating for cooling, it extracts heat from the indoor air and releases it to the outdoors. In this mode of operation, most air conditioning units (air conditioners) also extract (for comfort reasons) water vapor from the indoor air and expel it outside in the form of liquid water. Operating for heating in optimal conditions, aerothermal energy consumes (variable cost) only 25% of the electrical energy[6] that conventional electric heating (resistance) would consume, although the fixed cost of the device is much higher. In addition to heating, the aerothermal device can produce domestic hot water (DHW).[5].
Origin and impact in the air conditioning sector
Aerothermal energy has its theoretical roots in the principles of thermodynamics developed in the century by Nicolas Léonard Sadi Carnot and Lord Kelvin.[7] The first functional heat pump system was built in 1856 by the Austrian engineer Peter von Rittinger, who used it for brine drying, marking one of the first practical applications of these concepts.[8].
During the following decades, systems based on vapor compression cycles were perfected. In Switzerland, between the 1940s and 1950s, the first heat pumps were installed on an industrial scale. Interest in these technologies intensified after the energy crisis of the 1970s, due to the need for more efficient and electrically driven energy sources.[9].
The specific development of air-water technology, on which modern aerothermal energy is based, began to consolidate itself in the second half of the century as an effective solution for the air conditioning of spaces and the production of domestic hot water (DHW). This type of heat pump allows thermal energy to be extracted from outside air even at low temperatures, which has facilitated its integration in various climates. [10].
Starting in the 1950s and 1960s, several companies specialized in air conditioning began to incorporate this technology into their product catalogs. For example, Daikin developed one of the first reversible heat pump systems for residential use in 1958[11]. Subsequently, other brands such as Mitsubishi Electric, Panasonic, Bosch, Vaillant or Saunier Duval promoted the expansion of aerothermal energy with ranges designed to improve energy efficiency, adapt to different climatic zones and integrate with other renewable sources.
These companies have played a relevant role in the expansion of aerothermal energy in markets such as Europe, where its implementation has been favored by community directives that promote the use of clean energy in buildings, such as Directive 2009/28/EC and its updates within the framework of the European Green Deal. [12].
Currently, aerothermal energy is a booming technology in the air conditioning sector,[13] both in new construction and rehabilitation, due to its ability to reduce greenhouse gas emissions and its compatibility with medium-term climate neutrality objectives.
Appliance structure
Normally an aerothermal device is made up of an outdoor unit (compressor) and an indoor unit, connected by electrical cables and fluid tubes.[5] It also has an ACS storage unit, which can also be included in the indoor unit, and with a heating and/or cooling system.[14].
Difference with a heat exchanger
The difference between a heat exchanger and a heat pump is that the former transfers heat from a hotter fluid to a colder fluid (in the normal sense of heat flow), while the latter transfers heat from a colder fluid to a hotter fluid, taking advantage of the property of gases to release heat when they expand and consume it when they are compressed. But to carry out this transfer contrary to the normal flow you need to inject energy.
Applications of aerothermal energy in air conditioning systems
Aerothermal heating
Aerothermal heating, also known as aerothermal heat pump, is a heating system that uses outside air as a source of thermal energy. The fluid (in gaseous state) from the closed circuit is absorbed by a compressor, which increases its temperature through a compression process. The gas (hot and at high pressure) is transferred to a heat exchanger, where it heats the water of the central heating system[15](secondary circuit). When the temperature of the gas (primary circuit) drops, it liquefies. Hot water (from the secondary circuit) is distributed throughout the home using radiators or underfloor heating. The warm, high-pressure liquid from the primary circuit passes to the outside, where it expands (within the primary circuit) and becomes a gaseous state. In this process it absorbs a lot of energy from the outside environment.
Aerothermal underfloor heating
Aerothermal underfloor heating has a series of advantages over other heating systems, such as hot water radiators or hot air systems. Firstly, it is much more energy efficient, since the heat is transmitted more uniformly and directly to the environment. This means that less energy is required to heat the room, which in turn reduces your heating bill.
Secondly, underfloor heating does not produce drafts, making it ideal for those who suffer from allergies or asthma. Hot air can dry the throat and chest, aggravating the symptoms of these conditions. Underfloor heating, on the other hand, emits gentle, constant heat that does not aggravate these problems.
Another advantage of underfloor heating is that they can be installed under any type of floor, including wooden, linoleum or tile floors. This makes them ideal for those people who want to maintain the style and aesthetics of their home. They are also a very safe option, since there is no type of flame or fire involved in their operation.
Aerothermal energy can present some problems such as acquisition cost or maintenance, which must be carried out by someone specialized.
Split type systems and air-air/air-water solutions [16]
Air-thermal heat pumps allow air-water and air-air configurations, operating with both split-type emitters and fan coils and distributed indoor units. In addition, many brands offer systems that allow both heating, cooling and DHW, thus adapting to various construction typologies and needs.
Hybrid systems
They combine an aerothermal heat pump with a gas or diesel boiler. This approach makes it easy to maintain high efficiency even in extreme cold conditions, automatically switching between sources based on thermal demand and outdoor temperature. [16].
Integration with low temperature radiators.
It allows the use of water circuits with impulses between 30°C and 45°C, compatible with radiators designed for low temperatures.[17] It represents a practical solution in the rehabilitation of buildings where it is not feasible to install underfloor heating.
These applications offer comprehensive air conditioning (heating, cooling and domestic hot water) adapted to different types of buildings and climatic contexts, demonstrating the flexibility and energy efficiency of aerothermal energy.
Advantages and disadvantages
Advantages
The most relevant advantages of aerothermal energy are:[18][19].
Disadvantages
The main disadvantages or challenges to consider are:[20][21].
Energy efficiency: COP and SCOP
Contenido
La eficiencia energética de las bombas de calor aerotérmicas se evalúa mediante dos indicadores fundamentales: el Coeficiente de Rendimiento (COP) y el Coeficiente Estacional de Rendimiento (SCOP). Ambos permiten comparar el rendimiento de distintos equipos y tecnologías en función del consumo eléctrico requerido para generar energía térmica.[22].
COP (Coefficient of Performance)
The COP is an instantaneous measure of efficiency that expresses the relationship between the useful thermal energy produced and the electrical energy consumed under standard operating conditions. For example, a system with a COP of 4 provides 4 kWh of heat for every kWh of electricity used. This value is determined in accredited laboratories under standardized conditions (constant exterior and interior temperature), so it represents a reference data, not necessarily a reflection of real performance throughout the year.[23].
SCOP (Seasonal Coefficient of Performance)
The SCOP reflects the seasonal efficiency of the heat pump during a complete annual period, taking into account the temperature variations and the typical climatic conditions of a certain geographical area.[24] It is a parameter that is more representative of the real behavior of the equipment in everyday situations. Systems with a SCOP greater than 3.4 are classified within the highest energy categories according to the European energy labeling regulations (Delegated Regulation (EU) 811/2013).
Directive 2018/2001 of the European Parliament and of the Council recognizes as renewable those heat pump systems whose performance exceeds a minimum energy efficiency threshold. In this way, compliance with certain COP and SCOP values is key to consider aerothermal energy as a renewable energy source and for its eligibility in public subsidy programs or energy rehabilitation of buildings. [25].
[7] ↑ Erlichson, Herman (1 de enero de 1999). «Sadi Carnot, `Founder of the Second Law of Thermodynamics'». European Journal of Physics 20 (3): 183-192. ISSN 0143-0807. doi:10.1088/0143-0807/20/3/308. Consultado el 8 de agosto de 2025.: https://doi.org/10.1088/0143-0807/20/3/308
[25] ↑ Directiva (UE) 2018/2001 del Parlamento Europeo y del Consejo, de 11 de diciembre de 2018, relativa al fomento del uso de energía procedente de fuentes renovables (versión refundida) (Texto pertinente a efectos del EEE.) 328, 11 de diciembre de 2018, consultado el 11 de agosto de 2025 .: http://data.europa.eu/eli/dir/2018/2001/oj/spa
During the following decades, systems based on vapor compression cycles were perfected. In Switzerland, between the 1940s and 1950s, the first heat pumps were installed on an industrial scale. Interest in these technologies intensified after the energy crisis of the 1970s, due to the need for more efficient and electrically driven energy sources.[9].
The specific development of air-water technology, on which modern aerothermal energy is based, began to consolidate itself in the second half of the century as an effective solution for the air conditioning of spaces and the production of domestic hot water (DHW). This type of heat pump allows thermal energy to be extracted from outside air even at low temperatures, which has facilitated its integration in various climates. [10].
Starting in the 1950s and 1960s, several companies specialized in air conditioning began to incorporate this technology into their product catalogs. For example, Daikin developed one of the first reversible heat pump systems for residential use in 1958[11]. Subsequently, other brands such as Mitsubishi Electric, Panasonic, Bosch, Vaillant or Saunier Duval promoted the expansion of aerothermal energy with ranges designed to improve energy efficiency, adapt to different climatic zones and integrate with other renewable sources.
These companies have played a relevant role in the expansion of aerothermal energy in markets such as Europe, where its implementation has been favored by community directives that promote the use of clean energy in buildings, such as Directive 2009/28/EC and its updates within the framework of the European Green Deal. [12].
Currently, aerothermal energy is a booming technology in the air conditioning sector,[13] both in new construction and rehabilitation, due to its ability to reduce greenhouse gas emissions and its compatibility with medium-term climate neutrality objectives.
Appliance structure
Normally an aerothermal device is made up of an outdoor unit (compressor) and an indoor unit, connected by electrical cables and fluid tubes.[5] It also has an ACS storage unit, which can also be included in the indoor unit, and with a heating and/or cooling system.[14].
Difference with a heat exchanger
The difference between a heat exchanger and a heat pump is that the former transfers heat from a hotter fluid to a colder fluid (in the normal sense of heat flow), while the latter transfers heat from a colder fluid to a hotter fluid, taking advantage of the property of gases to release heat when they expand and consume it when they are compressed. But to carry out this transfer contrary to the normal flow you need to inject energy.
Applications of aerothermal energy in air conditioning systems
Aerothermal heating
Aerothermal heating, also known as aerothermal heat pump, is a heating system that uses outside air as a source of thermal energy. The fluid (in gaseous state) from the closed circuit is absorbed by a compressor, which increases its temperature through a compression process. The gas (hot and at high pressure) is transferred to a heat exchanger, where it heats the water of the central heating system[15](secondary circuit). When the temperature of the gas (primary circuit) drops, it liquefies. Hot water (from the secondary circuit) is distributed throughout the home using radiators or underfloor heating. The warm, high-pressure liquid from the primary circuit passes to the outside, where it expands (within the primary circuit) and becomes a gaseous state. In this process it absorbs a lot of energy from the outside environment.
Aerothermal underfloor heating
Aerothermal underfloor heating has a series of advantages over other heating systems, such as hot water radiators or hot air systems. Firstly, it is much more energy efficient, since the heat is transmitted more uniformly and directly to the environment. This means that less energy is required to heat the room, which in turn reduces your heating bill.
Secondly, underfloor heating does not produce drafts, making it ideal for those who suffer from allergies or asthma. Hot air can dry the throat and chest, aggravating the symptoms of these conditions. Underfloor heating, on the other hand, emits gentle, constant heat that does not aggravate these problems.
Another advantage of underfloor heating is that they can be installed under any type of floor, including wooden, linoleum or tile floors. This makes them ideal for those people who want to maintain the style and aesthetics of their home. They are also a very safe option, since there is no type of flame or fire involved in their operation.
Aerothermal energy can present some problems such as acquisition cost or maintenance, which must be carried out by someone specialized.
Split type systems and air-air/air-water solutions [16]
Air-thermal heat pumps allow air-water and air-air configurations, operating with both split-type emitters and fan coils and distributed indoor units. In addition, many brands offer systems that allow both heating, cooling and DHW, thus adapting to various construction typologies and needs.
Hybrid systems
They combine an aerothermal heat pump with a gas or diesel boiler. This approach makes it easy to maintain high efficiency even in extreme cold conditions, automatically switching between sources based on thermal demand and outdoor temperature. [16].
Integration with low temperature radiators.
It allows the use of water circuits with impulses between 30°C and 45°C, compatible with radiators designed for low temperatures.[17] It represents a practical solution in the rehabilitation of buildings where it is not feasible to install underfloor heating.
These applications offer comprehensive air conditioning (heating, cooling and domestic hot water) adapted to different types of buildings and climatic contexts, demonstrating the flexibility and energy efficiency of aerothermal energy.
Advantages and disadvantages
Advantages
The most relevant advantages of aerothermal energy are:[18][19].
Disadvantages
The main disadvantages or challenges to consider are:[20][21].
Energy efficiency: COP and SCOP
Contenido
La eficiencia energética de las bombas de calor aerotérmicas se evalúa mediante dos indicadores fundamentales: el Coeficiente de Rendimiento (COP) y el Coeficiente Estacional de Rendimiento (SCOP). Ambos permiten comparar el rendimiento de distintos equipos y tecnologías en función del consumo eléctrico requerido para generar energía térmica.[22].
COP (Coefficient of Performance)
The COP is an instantaneous measure of efficiency that expresses the relationship between the useful thermal energy produced and the electrical energy consumed under standard operating conditions. For example, a system with a COP of 4 provides 4 kWh of heat for every kWh of electricity used. This value is determined in accredited laboratories under standardized conditions (constant exterior and interior temperature), so it represents a reference data, not necessarily a reflection of real performance throughout the year.[23].
SCOP (Seasonal Coefficient of Performance)
The SCOP reflects the seasonal efficiency of the heat pump during a complete annual period, taking into account the temperature variations and the typical climatic conditions of a certain geographical area.[24] It is a parameter that is more representative of the real behavior of the equipment in everyday situations. Systems with a SCOP greater than 3.4 are classified within the highest energy categories according to the European energy labeling regulations (Delegated Regulation (EU) 811/2013).
Directive 2018/2001 of the European Parliament and of the Council recognizes as renewable those heat pump systems whose performance exceeds a minimum energy efficiency threshold. In this way, compliance with certain COP and SCOP values is key to consider aerothermal energy as a renewable energy source and for its eligibility in public subsidy programs or energy rehabilitation of buildings. [25].
[7] ↑ Erlichson, Herman (1 de enero de 1999). «Sadi Carnot, `Founder of the Second Law of Thermodynamics'». European Journal of Physics 20 (3): 183-192. ISSN 0143-0807. doi:10.1088/0143-0807/20/3/308. Consultado el 8 de agosto de 2025.: https://doi.org/10.1088/0143-0807/20/3/308
[25] ↑ Directiva (UE) 2018/2001 del Parlamento Europeo y del Consejo, de 11 de diciembre de 2018, relativa al fomento del uso de energía procedente de fuentes renovables (versión refundida) (Texto pertinente a efectos del EEE.) 328, 11 de diciembre de 2018, consultado el 11 de agosto de 2025 .: http://data.europa.eu/eli/dir/2018/2001/oj/spa