Air Conditioning Systems in Vehicles: Approaches and Challenges

Abstract

Automotive air conditioning systems improve passenger comfort and safety while keeping pace with changing environmental and technological requirements. This review evaluates the historical development, technological progress, and future trends of automotive air conditioning systems, primarily focusing on passenger vehicles, where cabin comfort and individualized thermal control are essential. The analysis examines the transition from early, energy-intensive cooling systems typically operating at a coefficient of performance (COP) of around 1.5 to modern, environmentally friendly alternatives that achieve COP values of approximately 3.0 or higher, highlighting the impact of regulatory measures such as the Kigali Amendment. A particular focus is placed on comparing refrigerants, especially the transition from HFC-134a to HFO-1234yf, with a discussion of their ecological impact and compliance with regulations. Innovative technologies, including adsorption cooling, AI-enhanced climate control, and the integration of renewable energy, are being explored as potential solutions to current challenges. Initially, 121 articles were reviewed, with 84 chosen for detailed examination based on their relevance, methodological soundness, and contributions to the field. The results reveal the trade-offs among efficiency, cost, and sustainability, highlighting the need for ongoing innovation to balance energy usage and environmental stewardship. Future studies should focus on creating refrigerants with extremely low global warming potential, improving battery efficiency in electric vehicles, and utilizing AI for tailored climate control. By tackling these issues, the automotive sector can offer more sustainable and efficient air conditioning options that align with consumer expectations and environmental regulations.

1. Introduction

Air conditioning systems in vehicles play a pivotal role in ensuring passenger comfort and road safety. By maintaining optimal cabin temperatures, these systems reduce driver fatigue and enhance alertness, particularly under extreme weather conditions. Recent advancements have shifted focus towards sustainability, energy efficiency, and personalized comfort, aligning with global environmental and technological trends [1].

Historically, integrating air conditioning systems into vehicles has undergone significant evolution from basic cooling mechanisms to sophisticated, eco-friendly solutions. Early energy-intensive systems, particularly those introduced in the 1930s, relied on chlorofluorocarbons (CFCs) such as R-12, which had a global warming potential (GWP) of approximately 10.900 [2]. These were later replaced by hydrofluorocarbons (HFCs) like HFC-134a in the 1990s as part of ozone-protection efforts. Today, alternative refrigerants like HFO-1234yf and innovations such as adsorption cooling systems and solar-powered air conditioning are reshaping the field [1,2].

Moreover, technological advancements in materials, sensors, and intelligent control systems are enhancing the efficiency and functionality of modern air conditioning systems. For instance, integrating nano lubricants has optimized performance and reduced wear in critical components [3,4]. Integrating composite nano-lubricants such as Al₂O₃-SiO₂ in PAG base oil has been shown to increase the coefficient of performance (COP) by up to 10% and reduce component wear, enhancing thermal efficiency and mechanical durability in automotive air conditioning systems [4].

Climate control systems in vehicles are undergoing a rapid transformation in response to increasing environmental concerns, regulatory pressures, and the global shift toward electrification [1,2]. While effective in maintaining cabin comfort, traditional vapor compression systems contribute significantly to vehicle energy consumption and greenhouse gas emissions [3,4]. As the automotive industry embraces electrification and digitalization, the role of air conditioning systems has become increasingly complex, extending beyond thermal comfort to include energy optimization, intelligent control, and system-level integration [5].

This article comprehensively reviews current and emerging HVAC (Heating, Ventilation, and Air Conditioning) technologies in the automotive sector, focusing on sustainability and innovation. What distinguishes this work from previous reviews is its integrated perspective: we evaluate the thermodynamic, environmental, and practical trade-offs between conventional vapor compression systems and alternative cooling cycles such as absorption, adsorption, thermoelectric, and magnetocaloric systems [6,7,8,9]. In parallel, we examine the potential of AI-driven control strategies to optimize energy use and enable predictive maintenance, especially in electric vehicles where HVAC load directly impacts driving range [10,11,12].

This article combines technological trends, system-level challenges, and sustainability considerations to offer a holistic outlook on the future of vehicle climate control. Our goal is to identify the limitations of existing approaches and the opportunities for novel architectures and materials to enable more efficient, environmentally friendly, and intelligent HVAC systems [13,14].

Air conditioning systems pose unique challenges in electric vehicles (EVs), significantly impacting battery range and energy consumption. Research highlights the need for energy-efficient solutions, such as resorption working pairs and advanced thermal management strategies [5,6].

This review draws on extensive studies and recent findings to explore the evolution of automotive air conditioning systems, their current applications, and their future potential. It highlights their role in addressing environmental concerns while meeting consumer demands for comfort and efficiency [7,8].

2. History of Automotive Air Conditioning Systems

2.1. Technological Evolution of Air Conditioning Systems

The technological evolution of vehicle air conditioning systems began in the 1930s with the introduction of basic cooling mechanisms in luxury cars. These early systems were rudimentary and relied heavily on inefficient and bulky designs. Over the decades, significant advancements were made, transitioning to compact, efficient, and environmentally friendly solutions [9,10].

One of the most notable milestones was the replacement of ozone-depleting chlorofluorocarbons (CFCs) such as R-12 with hydrofluorocarbons (HFCs) like HFC-134a. Although HFC-134a posed no risk to the ozone layer, its high global warming potential (GWP ≈ 1430) raised new environmental concerns, prompting the introduction of low-GWP alternatives such as HFO-1234yf [15]. The adoption of this refrigerant aligns with the European MAC Directive 2006/40/EC, which mandates refrigerants with a GWP below 150 in new vehicles [4].

Technological advances have also introduced variable displacement compressors, electronically controlled expansion valves, and energy-optimized architectures to improve system performance and environmental compliance. These enhancements reduce energy consumption and improve reliability and passenger comfort in diverse climate conditions [10,16].

2.2. Early Systems vs. Modern Technologies

Early air conditioning systems were characterized by high energy consumption and the use of refrigerants with a high global warming potential (GWP), such as chlorofluorocarbons (CFCs) and hydrofluorocarbons (HFCs). Modern systems, in contrast, utilize low-GWP refrigerants, such as HFO-1234yf, and incorporate advanced features, including intelligent climate control and nanotechnology-enhanced lubricants [1,4].

Early vehicle A/C systems were mechanically driven, operated with minimal control logic, and consumed considerable engine power, reducing overall vehicle efficiency. These systems offered limited adaptability to varying environmental and cabin demands, resulting in suboptimal performance.

In contrast, modern air conditioning systems are built around integrated, intelligent platforms—especially in electric vehicles (EVs), where thermal management is crucial for battery longevity and driving range. Electrically driven compressors and advanced HVAC control algorithms have enabled more precise regulation and reduced parasitic losses [4,5,6,7,8,9].

In recent years, research has focused on integrating nano-lubricants and intelligent sensors to enhance thermal performance further. For example, the addition of Al₂O₃-SiO₂ nanoparticles to PAG base oils has shown improvements in the coefficient of performance (COP) and wear reduction in compressor components [4]. AI-based predictive systems are also being explored for their potential to reduce unnecessary HVAC loads and extend the range of EVs under variable ambient conditions [7].

2.3. Impact of Regulations and Market Demands on Development

Global environmental regulations, such as the Kigali Amendment to the Montreal Protocol, have accelerated the transition towards eco-friendly refrigerants. This international agreement has not only mandated the phase-down of high-GWP refrigerants. Still, it has also stimulated significant innovation, with companies such as Honeywell and Chemours developing HFO-1234yf, now widely adopted in automotive systems, and Daikin promoting R-32 as a lower-GWP alternative to R-410A in residential units. Moreover, research groups across Europe and Asia have intensified efforts to advance natural refrigerants like CO₂ (R-744) and hydrocarbons, further diversifying the portfolio of environmentally friendly cooling solutions [8]. Market demand for energy-efficient and cost-effective solutions has driven innovation, leading to systems that balance performance and sustainability. For example, the adoption of adsorption cooling systems and advancements in solar-powered air conditioning reflect the industry’s response to these pressures [2,8].

Table 1. Advantages and disadvantages of automotive air conditioning systems.

Advantages/Disadvantages	Details	References
Advantages	✓ Progressive increase in efficiency and reliability. ✓ Wider accessibility for all vehicle classes. ✓ Enhanced thermal comfort and driving experience.	[1,10]
Disadvantages	✓ Initial technologies were energy-intensive and polluting. ✓ High costs associated with past innovations. ✓ Environmental concerns related to refrigerant emissions.	[6,11]

presents a comparative overview of the advantages and disadvantages of various automotive air conditioning systems, offering insight into the trade-offs between traditional and emerging technologies regarding efficiency, cost, and environmental impact.

3. The Operating Principle of Air Conditioning Systems

Automotive air conditioning systems are predominantly based on the vapor compression cycle due to its proven efficiency and reliability. However, other refrigeration principles, such as absorption, adsorption, thermoelectric, and magnetocaloric systems, are also being investigated as potential alternatives for specific use cases [13].

3.1. Description of Main Components: Compressor, Condenser, Evaporator, Fan

The automotive air conditioning system relies on five key components working in harmony: the compressor, located in the engine bay, acts as the core by compressing the refrigerant and increasing its pressure to enable circulation; the condenser dissipates heat by converting the high-pressure refrigerant from a gas to a liquid; the expansion valve reduces pressure and temperature before the refrigerant enters the evaporator; the evaporator absorbs heat from the cabin air as the refrigerant evaporates; and the fan ensures efficient airflow across the heat exchangers [1,2].

3.2. Refrigeration Cycle

The refrigeration cycle in automotive air conditioning systems efficiently regulates cabin temperature through critical steps that ensure passenger comfort under varying environmental conditions. Initially, the compressor compresses the refrigerant gas, significantly raising its pressure and temperature. This high-pressure, high-temperature gas then flows into the condenser, dissipating heat to the surrounding environment, and condenses into a high-pressure liquid. Subsequently, the liquid refrigerant passes through an expansion valve, causing a rapid decrease in pressure and temperature, which prepares it for the evaporator. Within the evaporator, the low-pressure refrigerant absorbs heat from the vehicle’s interior air as it evaporates, producing the cooling effect that lowers the cabin temperature. The refrigerant vapor then returns to the compressor, completing the cycle. This vapor compression cycle is the backbone of most modern automotive air conditioning systems due to its reliability and consistent performance across diverse climates and driving conditions [17,18].

Table 2. Advantages and disadvantages.

Advantages/Disadvantages	Details	References
Advantages	- Efficient operation under most conditions. - Easy integration into various vehicle types.	[1,19,20,21]
Disadvantages	- Environmental concerns related to refrigerants. Requires regular maintenance.	[6,18,22,23]

summarizes the advantages and disadvantages of the vapor compression refrigeration cycle in automotive applications. Key benefits include its high operational efficiency under a wide range of ambient conditions and the ease with which the system can be integrated into various vehicle types, from compact cars to larger commercial vehicles. These benefits contribute significantly to passenger comfort and vehicle safety by maintaining stable cabin temperatures. On the other hand, the system presents environmental concerns primarily linked to refrigerants, some of which have high global warming potential and ozone depletion effects. Furthermore, regular maintenance is necessary to prevent refrigerant leaks and ensure the system’s long-term functionality and efficiency. The table provides a balanced overview of the vapor compression cycle’s strengths and challenges within automotive climate control technology [1,6,18,19,20,21,22,23].

The refrigeration cycle ensures consistent cooling performance, regardless of environmental conditions, making it reliable for diverse climates. This efficiency stems from the well-coordinated functioning of the compressor, condenser, expansion valve, and evaporator [19,20,21]. The system is designed to integrate seamlessly into various vehicle types, from compact cars to large commercial vehicles, showcasing its flexibility in meeting diverse automotive requirements [20].

A significant drawback of earlier systems was the use of refrigerants that contributed to both global warming and ozone layer depletion, such as CFCs and HFCs. In contrast, modern refrigerants like HFO-1234yf have a global warming potential (GWP) of less than 1 and an Ozone Depletion Potential (ODP) of zero, making them more environmentally friendly alternatives [21]. While modern systems employ more eco-friendly options, such as HFO-1234yf, challenges persist regarding their proper handling and disposal due to the refrigerant’s mild flammability, the need for specialized recovery systems, and the environmental persistence of degradation products like trifluoroacetic acid (TFA) if improperly released [18]. Regular upkeep is essential to prevent leaks, maintain efficiency, and ensure long-term reliability. This includes checking refrigerant levels, cleaning components, and addressing wear and tear on critical components, such as the compressor [22].

While the refrigeration cycle is a cornerstone of automotive comfort, the trade-offs between its high efficiency and environmental and maintenance challenges underscore the importance of continuous innovation. Developing sustainable refrigerants and low-maintenance systems is crucial for addressing these limitations and advancing automotive air conditioning technology [15,23,24,25,26,27,28].

From a thermodynamic viewpoint, the vapor compression refrigeration cycle presents notable inefficiencies that can be better understood through exergetic and entropic analyses. The compressor consistently exhibits the highest exergy destruction among the main components, accounting for approximately 30% of the total system irreversibility due to internal friction, heat losses, and deviations from ideal isentropic behavior [27]. Similarly, the expansion valve introduces substantial losses as it functions through a throttling process, which is inherently non-recoverable and generates significant entropy. Studies suggest replacing traditional expansion devices with ejector-based systems or isentropic expansion mechanisms could improve system efficiency [17,27].

The condenser and evaporator also contribute to exergy losses, particularly when a large temperature gradient exists between the refrigerant and the surrounding air. This highlights the importance of optimizing heat exchanger surface area, airflow dynamics, and refrigerant charge levels to reduce thermal irreversibility and improve heat transfer efficiency [29,30,31,32].

A systematic exergetic analysis can guide the development of more sustainable and efficient air conditioning systems by targeting these key components for improvement.

4. Modern Types of Air Conditioning Systems

4.1. Traditional Air Conditioning

Compression-based air conditioning systems are among the most widely used cooling systems, characterized by a well-defined refrigeration cycle. This type of air conditioning works by compressing a refrigerant, which is then cooled and converted into a liquid. The liquid evaporates to extract heat from the space’s interior and provide cooled air [27,28,29,30].

In traditional air conditioning systems, the refrigerant is compressed in its gaseous form by the compressor, increasing its temperature and pressure. The hot gas is then cooled by a fan in the condenser, causing it to condense into a liquid. This liquid refrigerant is passed through an expansion device, where its pressure is lowered. Finally, the liquid refrigerant absorbs heat from the indoor air in the evaporator and evaporates, thus cooling the air. These systems are widely used in residential and commercial applications due to their efficiency and reliability [31,32,33,34] (See

Table 3. Advantages and disadvantages of traditional air conditioning systems.

Technology Type	Key Advantages	Major Disadvantages	Quantitative Indicators	Application Area
Traditional (Compression-based) Air Conditioning	High efficiency for various types of spaces. Installation flexibility (central or individual systems). Long-term reliability and easy maintenance. Precise temperature control tailored to user needs.	High initial installation costs. Environmental impact due to refrigerants with high global warming potential. Regular maintenance required. High energy consumption, especially in extreme climates.	Cooling Capacity: Typically 3–7 kW GWP (HFC-134a): ~1300 CO2-equivalent	Passenger cars, commercial vehicles, residential, and commercial spaces

).

4.2. Adsorption-Based Systems

Adsorption-based air conditioning systems rely on the physical process of adsorption, in which a refrigerant such as water is adsorbed onto the surface of a solid material like silica gel or zeolite. Water is preferred in these systems due to its zero global warming potential (GWP), non-flammability, chemical stability, and safety compared to other working fluids like ammonia, which is toxic and corrosive [16,35].

The adsorption-based cooling cycle operates through a series of key steps. First, during the adsorption phase, the adsorbent material binds to the refrigerant, extracting heat from the environment and generating a cooling effect. Next, heat is applied to the adsorbent material in the desorption phase, causing it to release the refrigerant in vapor form. The refrigerant vapor then undergoes condensation, returning to a liquid while releasing heat. Finally, in the evaporation phase, the liquid refrigerant absorbs heat from the surrounding air as it evaporates, completing the cycle [36].

Due to their energy-saving potential, these systems are increasingly being explored in hybrid and electric vehicles. By utilizing waste heat from the engine or exhaust gases to drive the cooling process, adsorption-based systems help reduce the load on the vehicle’s battery or engine, improving overall efficiency [37] (See

Table 4. Advantages and disadvantages of adsorption-based systems.

Advantages	Disadvantages
Uses waste heat, reducing overall energy consumption.	Lower cooling capacity (approximately 1–3 kW) than compression-based systems (approximately 3–7 kW) [12].
Environmentally friendly, as it utilizes natural refrigerants, such as water.	Bulkier design, making it harder to integrate into compact vehicles.
Silent operation due to the lack of moving parts.	Slower cooling process, which may not meet high-demand requirements.
Well-suited for hybrid and electric vehicles, reducing battery load.	Higher initial investment and increased system complexity.

).

4.3. Adding Functionalities of Intelligent Air Conditioning Systems

Integrating artificial intelligence (AI) in car air conditioning systems aims to optimize passenger comfort by automatically adapting the cabin conditions to individual needs and external factors [7,38].

Adaptation to external conditions: Modern systems can adjust the interior temperature depending on the position of the sun and external climatic conditions. Solar sensors placed on the dashboard detect solar radiation intensity and angle. Based on this data, the climate control unit increases cooling in the sun-exposed zones to ensure balanced thermal comfort inside the cabin [39].

Specific systems activate automatically when the driver approaches the vehicle, preparing the cabin for a pleasant experience from the moment they enter.

Integrating voice commands and touch interfaces allows users to adjust the climate settings intuitively and efficiently [40,41,42,43,44].

By optimizing the operation of the air conditioning system, AI contributes to reducing energy consumption and, implicitly, saving fuel [7].

Intelligent systems can learn user preferences and offer personalized settings that enhance the driving experience [7,45].

AI can monitor the air conditioning system’s performance, identifying potential problems before they become critical and reducing maintenance costs.

A doctoral thesis from Politehnica University of Bucharest, Romania, explores the optimization of vehicle comfort by modeling and optimizing the suspension, highlighting the importance of integrating advanced technologies to enhance the passenger experience [46,47].

Another study examines using sound-absorbing materials in railway vehicles to enhance comfort, highlighting the importance of noise control in improving passenger comfort. Integrating AI into automotive air conditioning systems represents a significant step toward more innovative and efficient vehicles, offering increased comfort and tailoring solutions to users’ needs [7,48,49,50,51].

AI integration may lead to higher upfront costs due to additional sensors, control units, and software complexity (as in

Table 5. Advantages and disadvantages of AI-integrated climate control systems.

Characteristic	Advantages	Disadvantages	Application Area	References
Energy Efficiency	Optimizes energy consumption, reducing fuel usage and emissions.	Initial setup and system calibration may require extensive development costs.	Electric and hybrid vehicles	[40,52]
Personalized Comfort	Learns user preferences for temperature and airflow, improving the passenger experience.	Privacy concerns due to data collection on user behavior and preferences.	Passenger cars with adaptive climate systems	[53]
Predictive Maintenance	Identifies potential system failures before they occur, reducing long-term maintenance costs.	May lead to over-dependence on AI, making manual troubleshooting more difficult.	All modern vehicles with connected services	[54]
External Adaptation	Adjusts based on external conditions (e.g., sunlight direction, outside temperature).	Advanced sensors and software might increase manufacturing complexity and costs.	High-end or premium vehicles	[30,45]
Intuitive Interfaces	Voice and touch controls make climate control more accessible and user-friendly.	System glitches in recognition (e.g., misinterpreted voice commands) can reduce usability.	Vehicles with human–machine interface (HMI) integration	[51]
Sustainability	Reduces environmental impact through energy-efficient operation.	Recycling and disposal of advanced AI hardware may pose ecological challenges.	Future-focused electric vehicle platforms	[52]

). However, it can also result in long-term operational savings by reducing maintenance needs, optimizing performance, and preventing system failures [39,45].

5. Environmental Impact

The environmental impact of refrigerants is critical in automotive air conditioning systems. Two commonly discussed refrigerants are HFC-134a and HFO-1234yf.

Recent research has explored capture and neutralization techniques to mitigate the environmental accumulation of trifluoroacetic acid (TFA). These include soil adsorption barriers, catalytic degradation in wastewater treatment plants, and atmospheric filtration systems that target fluorinated compounds. While still in early development, such strategies represent promising avenues to minimize the long-term ecological burden of HFO-1234yf usage [55].

Analysis of Refrigerants: HFC-134a vs. HFO-1234yf

HFC-134a has a GWP of approximately 1300, indicating a significant impact on global warming. In contrast, HFO-1234yf has a GWP of less than 1, making it a more environmentally friendly option. HFC-134a persists in the atmosphere for approximately 14 years, while HFO-1234yf has a significantly shorter atmospheric lifetime of around 11 days, reducing its long-term environmental impact [56].

Both refrigerants can degrade into trifluoroacetic acid (TFA) in the atmosphere. Notably, HFO-1234yf degrades entirely into TFA, raising concerns about its environmental effects [56].

The EU has implemented regulations to phase out refrigerants with a GWP greater than 150 in mobile air conditioning systems. This directive has accelerated the adoption of HFO-1234yf in new vehicles across Europe. This international agreement mandates the gradual reduction in HFC production and consumption, including HFC-134a, to mitigate climate change. The amendment encourages the transition to low-GWP alternatives, such as HFO-1234yf [56,57].

Some European countries are considering restrictions on per- and polyfluoroalkyl substances (PFAS), a category that includes certain HFCs and HFOs. These potential regulations could impact refrigerants like HFO-1234yf in the future. While HFO-1234yf presents a lower GWP and shorter atmospheric lifetime than HFC-134a, concerns about its degradation into TFA and potential regulatory changes regarding PFAS may influence its future use. Ongoing evaluation of environmental impacts and regulatory developments is essential in selecting appropriate refrigerants for automotive applications [55,58,59].

From a cost perspective, HFO-1234yf is substantially more expensive than HFC-134a, with price estimates ranging between EUR 40 and EUR 70/kg, compared to EUR 5 and EUR 10/kg for HFC-134a, depending on the region and regulatory pressures. This price gap impacts original equipment manufacturers (OEMs) and end-users, especially in markets with limited subsidies or mandates for low-GWP adoption. Moreover, transitioning to HFO-1234yf may involve system redesign, as older systems are not directly compatible due to pressure, flammability, and lubricating properties. These additional engineering and validation costs significantly contribute to the higher total cost of ownership. However, in regulated markets, long-term savings may result from avoiding penalties, gaining tax incentives, and ensuring compliance with Kigali and EU directives.

Consequently, while HFC-134a remains economically attractive in the short term, the overall cost-efficiency of HFO-1234yf improves when considering lifecycle compliance and environmental responsibility [55,58].

Although HFO-1234yf has a near-zero global warming potential (GWP), the total ecological impact of the air conditioning system also depends on the electricity consumed during operation [56]. A suitable approach to assess both direct and indirect effects is Life Cycle Assessment (LCA), which considers refrigerant emissions and regional electricity grid carbon intensity (gCO₂/kWh). This helps tailor HVAC strategies to local sustainability goals [55].

A comprehensive evaluation of both direct and indirect environmental impacts of automotive HVAC systems can be achieved through Life Cycle Assessment (LCA), which considers refrigerant production, leakage, operational energy use, and disposal [55].

Direct impacts include the refrigerant’s GWP, leakage rate, and product degradation, such as trifluoroacetic acid (TFA) in the case of HFO-1234yf [56].

Indirect impacts depend on the carbon intensity of local electricity grids (measured in CO₂/kWh) and the HVAC system’s energy demand, which varies under real driving conditions [29,60].

For instance, an HVAC unit operating in Norway (hydropower-based grid) will result in significantly lower total emissions than one operating in coal-reliant regions like Poland.

These insights can help develop more sustainable design choices, optimized thermal strategies, and regional policies, especially in electric vehicles, where HVAC consumption directly affects range and overall emissions [60].

Table 6. Advantages and disadvantages of refrigerants (HFC-134A Vs. HFO-1234Yf).

Characteristic	Adv./Disadv.	Details	Application Area	References
Global Warming Potential (GWP)	Advantage	HFO-1234yf has a GWP of less than 1, significantly reducing climate impact.	All modern vehicles comply with low-GWP regulations	[61,62]
Global Warming Potential (GWP)	Disadvantage	HFC-134a has a GWP of approximately 1300, contributing significantly to global warming.	Legacy and older vehicle platforms	[61,62]
Atmospheric Lifetime	Advantage	HFO-1234yf has a short atmospheric lifetime (~11 days), reducing its long-term environmental effects.	Eco-certified vehicle systems	[63]
Atmospheric Lifetime	Disadvantage	HFC-134a persists in the atmosphere for ~14 years, prolonging its impact.	High-emission conventional systems
Environmental Degradation	Advantage	HFO-1234yf degrades faster, resulting in less accumulation in the atmosphere.	Modern climate-friendly systems
Environmental Degradation	Disadvantage	HFO-1234yf degrades entirely into trifluoroacetic acid (TFA), raising concerns about water pollution.	Environmental monitoring systems	[60]
Regulatory Compliance	Advantage	HFO-1234yf complies with EU and Kigali Amendment GWP regulations.	Vehicles sold in the EU and under the Kigali targets	[62,64]
Regulatory Compliance	Disadvantage	HFC-134a faces phase-out mandates under the Kigali Amendment and EU regulations.	Pre-2017 vehicle fleets
Energy Efficiency	Advantage	Both refrigerants exhibit similar energy efficiency in most systems.	General automotive A/C platforms	[65]
Energy Efficiency	Disadvantage	Transitioning to HFO-1234yf may require new infrastructure and higher upfront costs.	OEMs and aftermarket services
Safety	Advantage	HFO-1234yf is less flammable than hydrocarbons.	Vehicles designed with enhanced safety features	[62,63]
Safety	Disadvantage	HFO-1234yf is mildly flammable and requires additional safety measures.	All vehicles using HFO-1234yf
Economic Impact	Advantage	HFO-1234yf offers long-term compliance benefits, thereby avoiding penalties associated with high-GWP refrigerants.	Regulated automotive markets	[61,62]
Economic Impact	Disadvantage	HFO-1234yf is more expensive than HFC-134a, raising initial costs for manufacturers and users.	Cost-sensitive manufacturers and markets

summarizes the advantages and disadvantages of HFC-134a and HFO-1234yf across several technical and environmental dimensions. Although HFO-1234yf has a near-zero global warming potential (GWP), the total ecological impact of the air conditioning system also depends on the electricity consumed during operation. The carbon intensity of the electricity grid in each region primarily influences this indirect climate impact. For example, using the same refrigerant in Norway (with hydropower) versus Poland (with coal-based energy) results in vastly different net CO₂ emissions. Therefore, direct (refrigerant-related) and indirect (energy-related) effects must be considered for a realistic sustainability assessment [55,60].

This

Table 7. Side-by-side comparison of HFC-134a and HFO-1234yf.

Characteristic	HFC-134a	HFO-1234yf
Global Warming Potential (GWP)	≈1300	<1
Atmospheric Lifetime	~14 years	~11 days
Safety Classification	Non-flammable (ASHRAE A1)	Mildly flammable (ASHRAE A2L)
Environmental Impact	Long persistence; contributes to global warming	Degrades to TFA; environmental concerns remain
Cost	Low (~ EUR 5–10/kg)	High (~ EUR 40–70/kg)
Regulatory Status	Subject to phase-out (EU, Kigali)	Compliant with EU and Kigali Amendment
Energy Efficiency	Comparable	Comparable, but requires system adaptation
Infrastructure Compatibility	Widely compatible	Requires retrofit or redesign

directly compares HFC-134a and HFO-1234yf based on key characteristics relevant to automotive air conditioning systems.

6. Current Challenges

The evolution of automotive climate control systems has brought significant advancements; however, several challenges remain. As highlighted in

Table 8. Current challenges in automotive climate control systems.

Challenge	Description	Application Area	References
Refrigerant Impact	Despite the shift to low-GWP refrigerants like HFO-1234yf, concerns persist about degradation into trifluoroacetic acid (TFA), accumulating in water sources.	All vehicle types using low-GWP refrigerants	[55,62]
Compliance with Regulations	Manufacturers must adapt to international standards like the Kigali Amendment and EU directives to phase out high-GWP refrigerants.	OEMs and global vehicle manufacturers	[55,65]
Energy Consumption	Climate control systems are energy-intensive, particularly in EVs, where they reduce driving range. Heat pumps are being explored as solutions.	Electric and hybrid vehicles	[55,60]
Complex System Design	Modern systems integrate sensors, AI, and IoT, making them complex to design and maintain.	Connected and smart vehicle platforms	[61,62]
Thermal Comfort Optimization	Achieving uniform comfort for all passengers in larger or variable-occupancy vehicles is technically challenging.	Buses, SUVs, and multi-zone passenger cars	[55,62]
AI Integration	AI requires data collection and processing, increasing costs and raising privacy concerns.	AI-enabled vehicles	[59,60]
High Development Costs	Transitioning to advanced systems, such as electric air conditioning units or low-GWP refrigerants, is expensive.	Automotive R&D and production sectors	[55,62]
Retrofitting Costs	Upgrading older vehicles with modern systems is often prohibitively expensive.	Aftermarket services and older vehicle fleets	[55,64]
Consumer Accessibility	Intelligent climate control systems remain unaffordable for many consumers, creating a market gap.	Mass-market vehicle segments	[66,67]
Flammability of Refrigerants	Low-GWP refrigerants, such as HFO-1234yf, are mildly flammable, necessitating additional safety measures in vehicle design.	All platforms using mildly flammable refrigerants	[68,69,70,71,72]
System Failures	Malfunctions in AI-driven systems can cause incorrect temperature readings or sensor failures, impacting safety.	Smart HVAC systems	[73,74]
Dynamic Standards	Manufacturers must adapt to evolving refrigerant usage, energy efficiency standards, and emissions regulations.	Global OEMs and regulatory compliance teams	[75,76,77]
Global Variability	Regional differences in standards challenge manufacturers to globally standardize systems.	Multinational vehicle production and distribution	[78]
Lifecycle Impact	Climate systems’ manufacturing, operation, and recycling necessitate sustainable practices to minimize their environmental impact.	All vehicle platforms	[77]
Battery Dependence in EVs	Climate systems in EVs heavily depend on battery power, demanding innovations to improve energy efficiency.	Electric vehicles	[79,80]

, these issues span technical, environmental, and regulatory aspects.

Although HFO-1234yf complies with current low-GWP regulations, its degradation into trifluoroacetic acid (TFA) remains an environmental concern, as discussed in Section 5. Continued monitoring and developing mitigation technologies are essential for sustainable application [55].

While climate control systems are listed among the primary energy consumers in EVs, it is essential to note that HVAC usage can reduce driving range by 10% to 30%, depending on ambient temperature, humidity, and system configuration [79,80]. In particular, heating demand in cold weather, especially when using resistive heaters, can drastically lower efficiency. To address this, many EV manufacturers implement heat pumps, thermal pre-conditioning while charging, localized heating elements, and improved cabin insulation, all of which contribute to minimizing HVAC-related energy losses.

7. Future Directions

As the automotive industry moves toward more sustainable and intelligent thermal management, several emerging technologies are being explored as potential alternatives or complements to vapor compression systems. These include thermoelectric, magnetocaloric, adsorption, and absorption-based cooling systems. The following subsection provides a comparative analysis to better understand these alternatives’ feasibility.

Comparative Outlook of Emerging Cooling Technologies

Efficiency ratings in this table refer to the typical thermodynamic performance, expressed as approximate coefficients of performance (COP) where available. Vapor compression systems typically achieve COP values between 2.5 and 4.0 [4], while adsorption and absorption systems show lower COPs in the range of 0.4–1.2 [35,36]. Thermoelectric systems generally remain below 0.7 due to material limitations, and magnetocaloric systems, though still experimental, have reported COPs between 1.0 and 2.0 in laboratory conditions [81]. While absorption systems can use natural refrigerants, some (e.g., ammonia) are toxic and require strict safety measures [35]. Water/lithium bromide is safer but limited to >0 °C under vacuum. Ionic liquids and organic absorbents are being explored as safer, sustainable alternatives [55,56].

The

Table 9. Comparative outlook of emerging cooling technologies.

Technology	Efficiency	Cost	Reliability	Environmental Impact	Key Challenges
Vapor Compression (VCR)	High efficiency; proven under diverse conditions	Moderate; supported by established infrastructure	High, mature, and widely adopted	Moderate to high, depending on refrigerant (e.g., HFC-134A vs. HFO-1234Yf)	Refrigerant GWP, energy consumption, system leaks
Adsorption	Moderate; utilizes waste heat efficiently	High, bulky systems and low production scale	Moderate; fewer moving parts, longer lifecycle	Low; uses environmentally benign refrigerants (e.g., water)	Slower cooling rate, size constraints in compact vehicles
Absorption	Moderate; effective when the heat source is available	Highly complex design and corrosive fluids	Moderate; dependent on continuous heat supply	Low; uses ammonia-water or lithium bromide	Safety concerns, integration with vehicle layout
Thermoelectric	Low to moderate; limited by current material performance	High; relies on rare/expensive materials	High, solid-state system with no mechanical wear	Very low; no refrigerants used	Low cooling power, limited to niche use cases
Magnetocaloric	Potentially high, promising in lab environments	Very high; experimental and costly technology	Undetermined; requires further long-term validation	Very low; solid-state and no harmful refrigerants	Material limitations, magnetic field generation complexity

summarizes a comparative analysis of vapor compression systems with alternative cooling technologies for automotive applications. These include adsorption, absorption, thermoelectric, and magnetocaloric systems. Each technology is assessed in terms of efficiency, cost, reliability, environmental impact, and the specific engineering challenges it faces before potential widespread adoption.

Table 10. Comparative summary of automotive air conditioning technologies.

Technology	Efficiency	Cost	Environmental Friendliness	Safety
Vapor Compression (VCR)	High; proven across various climates and vehicle types	Moderate; widespread infrastructure	Variable; depends on refrigerant (HFC-134a high GWP vs. HFO-1234yf low GWP)	Generally safe; newer refrigerants may be mildly flammable
Adsorption	Moderate; uses waste heat but has a slower response	High system size and complexity limit applications	High; uses natural refrigerants (e.g., water), low emissions	Very safe; no toxic or flammable substances
Absorption	Moderate; effective with constant heat input	High; uses corrosive and potentially toxic fluids	Moderate; uses ammonia or lithium bromide	Risk of leakage; proper containment required
Thermoelectric	Low to moderate; dependent on material properties	Highly expensive semiconductors	Excellent; no refrigerants used	Very safe; solid-state, no moving parts
Magnetocaloric	Potentially high; still under experimental validation	Very high; rare materials and high production cost	Excellent; no refrigerants, zero direct emissions	Safe in principle; challenges with magnetic shielding and control

summarizes each system’s key characteristics—efficiency, cost, environmental friendliness, and safety—to provide a more straightforward overview of the comparative potential of conventional and emerging technologies. This allows for a quick assessment of their relative advantages and limitations.

While Table 9 and Table 10 provide a concise comparative overview, it is essential to examine further the thermodynamic and engineering limitations that hinder the widespread adoption of alternative cooling technologies in automotive HVAC systems.

Adsorption and absorption systems, though environmentally friendly, generally suffer from low coefficients of performance (COP < 1) due to poor heat and mass transfer. Their bulky design and slow transient response make them unsuitable for the rapid thermal demands of vehicle cabins [35,36]. However, recent developments in metal–organic frameworks (MOFs) and nanostructured porous materials with improved thermal conductivity promise to enhance dynamic performance and system compactness.

Thermoelectric systems are limited by conventional materials’ modest figure of merit (ZT), resulting in low efficiency (COP ~0.3–0.7). Ongoing research into nanostructured bismuth telluride, quantum wells, and skutterudites aims to improve ZT and, consequently, power density and cooling effectiveness [31]. Integration challenges, such as heat dissipation and power management, require advanced packaging solutions.

Although attractive due to their solid-state nature and refrigerant-free operation, magnetocaloric systems face obstacles related to the generation of strong oscillating magnetic fields, limited temperature spans, and high material costs. Promising breakthroughs include developing rare-earth-free magnetocaloric materials (e.g., La-Fe-Si alloys) and compact permanent magnet assemblies, which could improve scalability and affordability [81].

Ultimately, achieving performance parity with vapor compression systems will require a combination of advanced materials, thermal system miniaturization, and intelligent control strategies optimized for the variable conditions encountered in vehicle operation.

The future of automotive climate control systems lies at the intersection of sustainability, digitalization, and vehicle electrification. Based on our analysis of the current literature and market evolution, we propose several key trajectories likely to shape the next generation of air conditioning technologies.

First, there is a clear and urgent need to develop next-generation refrigerants with ultra-low global warming potential (GWP), minimal toxicity, and high thermodynamic efficiency. Current solutions, such as HFO-1234yf, offer significant GWP reductions compared to HFC-134a [82], but their degradation into trifluoroacetic acid (TFA) and potential inclusion under PFAS-related restrictions pose new regulatory and ecological concerns [76]. Future research should explore fluorine-free refrigerants or natural substances such as CO₂ and hydrocarbons, provided safety can be ensured through advanced containment and leak detection systems.

Second, integrating renewable energy sources will play a pivotal role, especially in electric vehicles (EVs), where energy efficiency is critical. Solar panels mounted on vehicle rooftops or hoods may partially power HVAC components, reducing dependency on battery power during idle phases or in urban stop-and-go traffic [74,83,84]. We foresee the emergence of hybridized HVAC architectures, combining traditional refrigeration cycles with auxiliary thermal storage units or phase-change materials to buffer peak loads.

Third, solid-state cooling and magnetic refrigeration are increasingly being explored as disruptive alternatives to vapor compression cycles [82]. While still in early stages of development, these technologies offer promising advantages in terms of miniaturization, energy efficiency, and refrigerant-free operation. Their application in automotive systems, especially in compact or autonomous vehicles, could reshape current design paradigms by eliminating moving parts and enabling zonal or personalized cooling solutions.

From a systems perspective, climate control will evolve toward innovative, adaptive, and context-aware configurations. Artificial intelligence (AI) and Internet of Things (IoT) platforms will not only optimize energy use based on passenger load and ambient conditions but also predict component failures, manage cabin microclimates, and personalize comfort settings [7,77,84]. However, these benefits come with challenges: the complexity of software integration, sensor calibration, and data privacy concerns must be addressed to ensure robustness and user trust [68].

Moreover, affordability and lifecycle sustainability remain pressing issues. The adoption of intelligent HVAC systems will depend heavily on cost reduction strategies and modular design that allow scalability across vehicle classes. At the same time, sustainability must extend beyond operational emissions. Material choices, system recyclability, and end-of-life processing should be considered from the design phase to minimize the environmental footprint [78].

In summary, future developments in automotive air conditioning will not be driven solely by regulatory mandates but also by innovation across thermodynamics, data science, and systems engineering. By anticipating these multidisciplinary shifts, the automotive industry can accelerate the transition to climate-responsible, user-centric thermal management solutions.

Additionally, safety concerns related to system failures and the flammability of certain refrigerants highlight the need for rigorous testing and targeted innovations in system design, such as sealed electric compressors, integrated leak detection sensors, automatic system shutdown protocols, and active ventilation in refrigerant compartments, all of which enhance both safety and reliability [84].

To ensure the safety and reliability of next-generation HVAC systems, rigorous testing protocols should include refrigerant leakage detection, flammability evaluation (per ASHRAE A2L standards) [85], crash response validation, thermal cycling and endurance testing, and vibration, shock, and functional performance tests, particularly under variable environmental and driving conditions.

Despite significant progress, several research gaps remain, particularly regarding the long-term performance of AI-driven HVAC systems, the real-world integration of alternative cooling technologies, and the practical application of renewable energy sources in automotive thermal management. Promising directions include the development of flexible photovoltaic panels integrated into vehicle rooftops capable of generating up to 200 Wp to support pre-conditioning or ventilation functions without draining the main battery, especially during idle phases or in urban traffic conditions.

Advanced energy management strategies are becoming essential in electric vehicles, where energy efficiency directly impacts range and performance. Future HVAC systems must closely coordinate with battery thermal management, powertrain cooling, and heat pump systems, ensuring optimal energy distribution under varying driving and ambient conditions.

Intelligent control algorithms, possibly supported by AI, can dynamically allocate thermal loads and prioritize cabin or battery cooling based on context, helping extend range without compromising comfort.

Additionally, novel system architectures such as zonal cooling, which targets specific cabin areas instead of the whole interior, and direct refrigerant cooling, which minimizes energy losses by eliminating intermediate heat exchangers, offer promising paths to achieve deeper integration and efficiency improvements.

Embedding HVAC within a vehicle-wide thermal network could unlock substantial gains in overall system optimization. However, alternative cooling cycles still face core thermodynamic challenges. Adsorption and absorption systems have low COPs and slow dynamics; thermoelectrics are limited by low material efficiency (ZT < 1); magnetocaloric systems require strong magnetic fields and expensive materials. Ongoing research into MOFs, nanostructured thermoelectrics, and rare-earth-free alloys aims to improve their viability and power density for automotive use.

Advanced energy management strategies can intelligently coordinate HVAC operation with battery thermal control, powertrain cooling, and heat pump systems to reduce total energy consumption and extend EV range. Novel architectures such as zonal cooling and direct refrigerant systems can further enhance integration and efficiency.

8. Conclusions

The journey of automotive climate control systems is continuous improvement, balancing the competing demands of environmental responsibility, technological advancement, and user satisfaction. By addressing current challenges and embracing future opportunities, the industry can develop efficient and effective systems aligning with sustainable mobility’s broader vision. Collaboration among manufacturers, researchers, and policymakers will be crucial in driving this transformation and shaping the future of automotive climate control. Future research should prioritize the development and integration of refrigerants with zero global warming potential (GWP), such as carbon dioxide (R-744) and natural hydrocarbons like propane (R-290) and isobutane (R-600a). While these refrigerants offer significant environmental benefits, their implementation poses technical and safety challenges. For example, R-744 operates under very high pressures, requiring robust components and precise control systems. In contrast, hydrocarbons are flammable and require strict safety protocols, including advanced leak detection and proper ventilation [63]. Enhancing the energy efficiency of thermal systems in electric vehicles will also be critical, particularly through integrated battery climate management platforms. HVAC systems are expected to evolve into fully connected, AI-driven subsystems integrated with vehicle electronics and energy management. These systems will optimize energy use, increase comfort, and play an active role in next-generation vehicles’ overall thermal and functional performance.

In this context, further research should investigate key parameters such as AI algorithms’ long-term accuracy and adaptability, sensor degradation, real-world energy savings, and the robustness of predictive maintenance capabilities under varying operational conditions.