Phase Change Materials (PCM) absorb heat from the human body, ensuring thermal comfort for the wearer. As PCM undergoes a phase transition from solid to liquid, it absorbs excess heat through conduction from the body. Common PCMs include paraffin waxes, hydrated salts, and fatty acids, often embedded in textiles to regulate the thermal behaviors between the textile and the human body and provide comfort to the wearer. The PCM packets are placed inside vest pockets, with or without a Velcro fastener, to maintain close contact with the wearer for optimal heat transfer and cooling efficiency (Mekrisuh et al., 2025). PCM enables the efficient transfer of thermal energy without energy consumption. Thus, developing effective strategies to integrate PCM thermal storage technology with wearable devices is crucial for achieving energy sustainability and efficient personal thermal management systems. For instance, Gu et al. (2025) developed a novel cooling vest with PCM that is strategically placed based on the body’s mapping of thermal sensation and perspiration. Promising results showing that the sustained cooling effect (lasting for up to 2 h) with improved thermal comfort and lower self-perceived level of fatigue by the subjects were presented. However, the limitations of PCM, including low thermal conductivity and leakage issues, have hampered its development and applications used for human cooling. From the user survey conducted (refer to Figure 3) by the authors here, issues relating to weight, condensation, and the long time required for recharging were also highlighted. Researching further beyond this, composite PCMs with enhanced thermal conductivity and stable structure appear to have good potential for human cooling (Yang et al., 2024).

Evaporative cooling technologies mainly rely on water or moist air as the coolant to dissipate heat. This technology has the advantage of high heat dissipation, owing to water’s high latent heat capacity and eco-friendly nature. The most straightforward of them all is to use cooling towels or a vest that uses the effect of water evaporation to draw heat away from the body. The performance of such evaporative cooling vests was investigated by Dehghan and Reza in 2023 (2023). It was found that the use of such evaporative cooling vests can reduce the perceptual and physiological heat strain of construction workers working under hot and dry conditions. Wettable or hydrogel-based fabrics can also be used to enhance evaporation to cool the body down. Although they offer efficient cooling, their performance and efficiency tend to deteriorate when subjected to prolonged sunlight. In addition, vests that make use of evaporative cooling tend to require the vest to be wet, which will therefore also cause the user to be wet and the vest to be heavy. The wet sensation can be uncomfortable for many. Lastly, evaporative vests’ performance tends to deteriorate in humid environments, when the evaporation rate is reduced. To this, innovative techniques that can reduce the humidity in the microclimate around the vest need to be introduced. An example is the M-cycle evaporative cooling vest designed by Raad et al., which has a separate channel outside the wet channel to induce air flow, increasing the rate of evaporation (Raad et al., 2019). Separately, innovative, nature-inspired, and smart materials are being developed that can enhance evaporation, which can also contribute to the advancement of such an evaporative cooling vest. For example, membrane-based and vacuum desiccant-based cooling garments are two widely used evaporative methods (Xue et al., 2024) that are used to enhance evaporation. This will be elaborated on in Section 2.1.4.

Radiative cooling and heating offer significant energy savings for human thermal comfort, with excellent energy efficiency (Li et al., 2022). Human skin, with an emissivity over 0.95, effectively radiates heat in the mid-infrared range (7–14 μm) (Cai et al., 2018; Xue et al., 2024). By designing fabrics to control optical properties like emissivity and reflectance, passive thermal regulation can be achieved based on ambient conditions. However, challenges remain with the reflective appearance and limited comfort of these materials.

A breakthrough in this area is metafabrics, which enhance cooling by radiating heat to the environment. Yan et developed a bioinspired metafabric with a dual-gradient Janus design for personal radiative and evaporative cooling (Yan et al., 2025). The dual-gradient Janus design enhances sweat-wicking and evaporative cooling, with a minimal sweat consumption of only 0.5 mL/h needed to maintain a comfortable skin temperature, preventing excessive sweating. Its hierarchical fiber structure achieves an impressive solar reflectance of 99.4% and a mid-infrared emittance of 0.94, leading to a 17.8 °C skin temperature reduction under intense sunlight. Mesoporous silica nanoparticles embedded in the fibrous network absorb moisture from the atmosphere during humid nights and release it during hot days, providing an extra 2.5 °C cooling effect. Additionally, the metafabric offers excellent wearability and a wide range of color options, circumventing the issues raised earlier in terms of appearance and ergonomic comfort for the wearer. Similar findings were also observed in another study by Xue et al. (2024), where their dual-mode Janus Metafabric was designed to be used for both radiative cooling and solar heating. The cooling side, featuring a cellulose acetate@Al₂O₃ porous coating, boasts a solar reflectance of 92.12%, an infrared emissivity of 83.22%, and a net cooling power of 55.85 W·m⁻². The heating side, made of multi-walled carbon nanotubes, achieves a high solar absorption rate of 88.4%. The metafabric also offers excellent ultraviolet resistance, durability, and washing performance, making it ideal for practical use. By simply flipping the fabric, users can easily switch between cooling and heating modes to adapt to changing ambient temperatures. In the drive for sustainability, researchers have also begun looking at radiative cooling materials that are sustainable. A sustainable and biodegradable laminated membrane with integrated radiative cooling and unidirectional sweat absorption capabilities was developed for personal moisture-thermal management (Zhao et al., 2025). This membrane is constructed through a layer-by-layer assembly process using cellulose and hydrophobically modified hydroxyapatite nanowires as the primary building blocks. The design features an asymmetric wettability structure, with a hydrophobic hydroxyapatite inner layer and a hydrophilic cellulose outer layer, enabling unidirectional sweat transport from the skin, through the inner layer, and out to the environment, ensuring moisture comfort for the wearer.

Janus fabrics with unidirectional liquid transport are effective in removing excess sweat and lowering skin temperature (Min et al., 2024; Xi et al., 2025). However, when the hydrophilic surface reaches its absorption limit, sweat buildup occurs, slowing sweat transport and making the fabric heavy and clingy. Inspired by the Namib Desert beetle and drip tips, (Min et al., 2024), developed a design with circular Coolmax patterns for sweat extraction near the skin and triangular Coolmax patterns for sweat removal on the exterior. This design enhances continuous sweat transport, diffusion, and shedding, reducing skin temperature by 1.3 °C compared to conventional cotton fabric. Additionally, the low adhesion (∼0.2 mN cm⁻²) between wet skin and fabric improves comfort.

The porous structure and fiber spacing in fabric design significantly impact comfort and mechanical properties (Hu et al., 2024). Ke et al. (2018) hypothesized that materials capable of transmitting thermal infrared radiation could provide passive cooling without energy input. A nanoporous polyethylene membrane was developed for passive cooling clothing, lowering skin temperature compared to traditional cotton. Meng et al. (2025) created a sandwich-structured nanofibrous (SNF) textile combining radiative cooling, moisture-wicking, and self-cleaning functions. The cooling mid-layer, made of hydrophilic PAN-PVA nanofibers, has a solar reflectance of 97.9% and infrared emittance of 90.8%, while the hydrophobic PPy-PVDF-HFP layer absorbs solar heat with 96% efficiency. Outdoor tests showed that the cooling side of the SNF textile kept simulated skin 8.5 °C cooler than conventional fabric. This combination allows for directional sweat transport, keeping the skin dry and comfortable.

The porous structure, fiber spacing, and network structure in fabric structure design directly affect the comfort and mechanical properties. (Hu et al., 2024). Ke et al. (2018) hypothesized that materials capable of transmitting thermal infrared radiation could provide passive cooling without energy input. A nanoporous polyethylene membrane was developed for passive cooling clothing, lowering skin temperature compared to traditional cotton. Meng et al. (2025) created a sandwich-structured nanofibrous (SNF) textile combining radiative cooling, moisture-wicking, and self-cleaning functions. The cooling mid-layer, made of hydrophilic PAN-PVA nanofibers, has a solar reflectance of 97.9% and infrared emittance of 90.8%, while the hydrophobic PPy-PVDF-HFP layer absorbs solar heat with 96% efficiency. Outdoor tests showed that the cooling side of the SNF textile kept simulated skin 8.5 °C cooler than conventional fabric. This combination allows for directional sweat transport, keeping the skin dry and comfortable.

Most cooling fabrics offer static thermal regulation, limiting their adaptability to fluctuating ambient temperatures. Therefore, there is a need for temperature-responsive fabrics that can adapt to dynamic outdoor environments. Thermal management textiles, with ultralight insulating layers and adaptive cooling technologies, improve temperature regulation in extreme conditions. Moisture management textiles use advanced structures for one-way transport and breathable membranes, enhancing comfort in activewear and outdoor gear (Tian et al., 2025). Xue et al. (2025) developed a fabric that has dynamic thermochromism (visible light regulation capability of 59%) and reversible moisture transport. Under hot environmental conditions, the fabric reflects sunlight and shifts to a hydrophobic inner and hydrophilic outer state, promoting sweat conduction and evaporation to keep skin dry and comfortable. Lan et al. (2025) designed a kirigami-inspired fabric that captures solar energy for heating in low sunlight and creates heat-dissipation channels during perspiration, demonstrating superior adaptability compared to static fabrics.

Air-cooling garments (ACGs), illustrated in Figure 5a, use small battery-powered fans or air circulation units to create a microclimate between the body and clothing. These systems circulate air around the body to remove heat and moisture, promoting heat dissipation. ACGs are lightweight and portable, with no need for excessive tubes or pumps, making them suitable for various occupations and applications. The cooling performance of ACGs depends on the ventilation unit, air gap between the skin and fabric (Zhao et al., 2022), eyelet design, fabric permeability (Zhao et al., 2023), and the integration of fan units to ensure efficient air movement and moisture transport. Proper fan placement and airflow distribution across the body enhance evaporative heat loss and cooling (Del Ferraro et al., 2022). However, fan placement may result in uneven cooling, and ACGs may be less effective in extremely humid environments (Wenfang et al., 2024).

There exist air-cooling garments in the market today, exemplified by the Venture Heat fan vest surveyed as presented in Figure 3. However, it is unlikely that air-cooled units can provide sufficient cooling for thermal comfort by themselves, as also shown with the user survey conducted in Figure 3. Rather, the integration of air-cooling technology with other technologies, such as TEC (to be elaborated in Section 2.2.3) or passive cooling technology such as phase changing material or evaporative fabric design (see Section 2.1) seems to be more promising. An example is the wearable cooling and dehumidifying system (WCDS) designed by Lou et al. (2022). The team showed that, as compared to pure air-cooling garments, WCDS provided an additional 4 to 6 ℃ drop in cooling temperature, and a mean cooling power of more than 5 times that provided by air-cooling garments. In their review paper published in 2024, Xu et al. also noted the superior performance of hybrid vests incorporated with PCM packs and air-ventilation fans (Xu et al., 2024). Numerous experimental studies have validated the superior performance of such hybrid vests by enhancing evaporative cooling and convective heat transfer using air ventilation within the clothing microclimate. (Lu et al., 2015; Chan et al., 2017; Wang et al., 2020). Hence, it is expected that future research in this area will likely centre on the optimization of air-cooling unit designs in combination with other technologies. Factors such as weight and vibration stemming from the incorporation of air-cooling fans must also be addressed (Xu et al., 2024).

Liquid cooling garments (LCGs), which normally use water, can be more effective in removing heat when compared to air cooling garments due to the greater heat conductivity and thermal capacity of water than air. The cooled liquid will be circulated through a network of tubes over the skin surface to absorb the body’s heat dissipation, as seen in Figure 5b. Originally designed for astronauts in hostile space conditions, the weight of the entire garment was not a consideration during the design process (Tokizawa et al., 2020). Increasingly, this has been used in other applications, such as for pilots (Xu et al., 2023), soldiers (Jovanović et al., 2012), firefighters (Yang et al., 2023), and for other uses such as protective gear. Results show that using such garments helps maintain significantly lower body skin temperature (Cvetanović et al., 2021) and reduces the physiological strain caused by physical exertion, with significant benefits in nearly all the physiological parameters tested (Jovanović et al., 2012; Zheng et al., 2022; Yang et al., 2023; Zhang et al., 2024a). LCGs are always dependent on the liquid’s temperature and flow rate, accompanied by a complex configuration and network of tubes embedded within the clothing to circulate and distribute the chilled liquid (Xu et al., 2023), effectively reducing body and skin temperature. These systems typically rely on a refrigeration unit, pumps, or ice packs to maintain the cooling effect. By varying the liquid temperature and arrangement of the tubes, Xu et al. (2023) have observed that localized cooling at specific parts of the body can be optimized.

Due to their weight penalty and reliance on pumps or compressors, LCGs have not been extensively explored. Thus, LCG still has much space for improvement and optimization (such as the inlet liquid temperature, conductivity properties of the pipes, and ability for intermittent cooling). Zheng et al. (2022) in their numerical work proposed that intermittent cooling could be an effective way to be employed, given its lower power consumption while ensuring that thermo-physiological comfort can still be maintained. Zhang et al. (2022) proposed to cool the liquid by using a thermoelectric conductor–showing potential that similar cooling effects can be achieved, but at a much lower weight penalty. This highlights the potential of hybrid PCD systems with the incorporation of liquid cooling technology.

Thermoelectric cooling (TEC) garments (TCGs) use the Peltier effect, where an electrical current creates a temperature differential that absorbs heat and dissipates it externally. Thermoelectric devices (TEDs) are solid-state refrigeration devices offering adjustable cooling power, a lightweight design, and temperature control (Feng et al., 2024; Yusuf et al., 2022; Ang et al., 2022). CGs provide advantages over air-cooling and liquid-cooling garments, including portability, zero emissions, precise temperature control, no noise pollution, and no need for liquid media (Chen et al., 2022; Sun et al., 2022). A sample TCG is seen in Figure 5c. TEDs are evaluated on parameters like coefficient of performance, cooling capacity, temperature difference, and cooling heat (Sun et al., 2022). While promising for wearable personal thermoregulation, challenges remain, particularly in dissipating heat from the TED’s hot side, which can affect cooling performance. Optimizing TED integration in garments and ensuring efficient heat dissipation are key areas for improvement. Effective placement of TEDs targeting key body areas has shown notable cooling benefits (Wenfang et al., 2024). However, achieving optimal design and testing for TED-based garments in high-temperature environments remains crucial for enhancing comfort and cooling capacity (Wang et al., 2025).

In the following subsections, we highlight major technological gaps identified in this area. First, there is a need to redesign the structure of TEC for effective cooling against human skin. Second, the primary challenge is their lack of flexibility, as most TECs are built on rigid substrates, making them unsuitable for wearable applications. Additionally, TECs must efficiently dissipate both parasitic Joule heat and pumped heat to sustain the Peltier effect on the cold side. While bulky heat sinks can improve heat dissipation and enhance cooling performance, their size and weight make them impractical for personalized wearable devices. Lastly, the thermal efficiency of TEC is notoriously low. It is envisaged that a revolutionary increase in thermal efficiency is only possible through the discovery of new thermoelectric materials to increase the materials’ Seebeck coefficient, while maintaining high electrical conductivity and low thermal conductivity.

Magnetocaloric cooling relies on the magnetocaloric effect (MCE), where certain materials experience a temperature change when subjected to a varying magnetic field. This phenomenon has gained significant interest due to its potential for environmentally friendly and energy-efficient refrigeration applications (Das, 2024). Among magnetocaloric materials, gadolinium and its alloys have been widely studied due to their strong magnetocaloric properties. However, challenges such as high cost, oxidation susceptibility, and limited temperature range have led researchers to explore alternative materials, such as La–Fe–Si family and Mn–Fe–P–As compounds, which exhibit large entropy changes and enhanced performance (Mellari, 2023). The key advantages of magnetocaloric cooling include the absence of refrigerants, reduced energy consumption, high reversibility, and near-silent operation, making it a viable alternative for industrial and household cooling applications (Owolabi, 2023; Hamad and Alamri, 2024).

Computational models have played a crucial role in optimizing magnetocaloric materials. Techniques such as support vector regression and extreme learning machines have been employed to predict magnetic entropy changes and relative cooling power, allowing for better material selection without extensive experimental testing (Shamsah, 2024). These advancements are paving the way for the practical deployment of magnetocaloric cooling systems.

For practical implementations of magnetocaloric cooling, researchers should focus on advancements in high-entropy alloys, the use of rare-earth-free magnetocaloric materials, and nanostructured composites to improve the magnetocaloric effect and thermal conductivity. Optimizing regenerative cycle designs, such as active magnetic regenerators, and integrating novel heat transfer fluids can enhance cooling efficiency. Developing low-cost, scalable synthesis methods and improving the durability of materials will support commercial adoption. Additionally, hybrid cooling approaches that combine magnetocaloric and other solid-state cooling techniques are being explored to achieve higher coefficients of performance (COP) and broaden application potential. Figure 6a shows a simplified magnetocaloric cooling cycle.

Electrocaloric cooling is another promising solid-state cooling technology based on the electrocaloric effect (ECE), where dielectric materials undergo temperature changes under an applied electric field. The discovery of giant ECE in ferroelectric ceramics and polymers has fueled research into electrocaloric refrigeration as a viable alternative to vapor compression systems (Najmi et al., 2022). Electrocaloric materials, such as Ba (ZrxTi1-x)O₃ ceramics, have demonstrated significant temperature variations, making them ideal for compact and efficient cooling devices (Qian et al., 2018). The absence of moving parts, high coefficient of performance (COP), and compatibility with microelectronics highlight the potential of ECE-based cooling for applications ranging from wearable devices to industrial refrigeration (Bai et al., 2023).

To enhance electrocaloric cooling efficiency, advanced thermal management techniques, such as active regenerative (AER) cycles and microfluidic cooling, have been proposed. These approaches optimize heat transfer, allowing for continuous cooling cycles and improved energy utilization (Shi et al., 2019). Further integration with energy recovery systems and scalable device architectures could drive the widespread adoption of electrocaloric cooling in future refrigeration technologies (Cai et al., 2023; Metzdorf et al., 2024).

Some of the research directions are on developing electrocaloric materials with higher adiabatic temperature changes and improved thermal stability through advanced ferroelectric polymers, ceramics, and multilayer structures. Enhancing the energy efficiency of electrocaloric cooling cycles by optimizing electric field application, minimizing hysteresis losses, and improving thermal management techniques is also crucial. Additionally, research is directed toward scalable fabrication methods for practical implementation, integrating electrocaloric cooling into hybrid cooling systems, and exploring miniaturized solid-state cooling applications for electronics, biomedical devices, and wearable technology. Figure 6b shows a simplified electrocaloric cooling cycle.

Elastocaloric cooling is based on the stress-induced phase transformation of certain materials, primarily shape memory alloys such as Ni-Ti-based alloys. When an elastocaloric material is subjected to tensile or compressive stress, it undergoes a phase transformation from austenite to martensite, releasing latent heat. Upon unloading, the material reverts to the austenitic phase and absorbs heat, resulting in a cooling effect. This process is analogous to adiabatic demagnetization in magnetocaloric materials, but operates through mechanical stress rather than magnetic fields (Schmidt et al., 2015). The performance of elastocaloric cooling largely depends on the material’s mechanical and thermal properties. Ni-Ti-based shape memory alloys are among the most studied due to their large latent heat, high mechanical strength, and excellent reversibility. Other promising materials include Cu-based SMAs and high-entropy alloys that exhibit improved fatigue resistance and efficiency (Cissé and Asle Zaeem, 2020). One effective approach to enhancing elastocaloric performance is optimizing material composition and microstructure, with a focus on minimizing mechanical hysteresis and improving energy efficiency (Zhang et al., 2022).

Several prototype elastocaloric cooling systems have been developed to evaluate their practical feasibility. Most systems employ cyclic loading and unloading mechanisms, such as rotary or linear actuators, to drive the elastocaloric effect. The heat exchange efficiency of these systems is a critical factor influencing their overall performance. A study has shown that a better coefficient of performance can be achieved by a multistage elastocaloric refrigeration system (Snodgrass and Erickson, 2019). Despite its promise, elastocaloric cooling faces several challenges that must be addressed to enable widespread adoption. One major issue is fatigue failure, as repeated stress cycling can lead to material degradation and reduced performance over time. Researchers are investigating strategies to enhance the fatigue resistance of SMAs through alloy design and surface treatments. Another challenge is the development of efficient heat exchange systems to maximize the cooling effect and minimize energy losses. Future research directions include exploring new materials with higher elastocaloric effects, integrating elastocaloric cooling systems with other solid-state refrigeration technologies, and developing scalable manufacturing processes. Additionally, efforts to model and optimize the thermomechanical behavior of elastocaloric materials using machine learning and computational simulations are expected to accelerate progress in the field (Gao et al., 2024). Overcoming challenges related to mechanical fatigue, cost, and scalability will be crucial for the commercialization of elastocaloric cooling in refrigeration, electronics, and thermal management applications. Figure 6c shows a simplified elastocaloric cooling cycle.

Electrochemical cooling, utilizing endothermic ion transport and redox reactions, presents an innovative alternative with the potential for compact, efficient, and flexible integration into wearable systems. As a solid-state cooling solution, it operates by utilizing ion migration and reversible redox reactions to absorb and dissipate heat. The core mechanism involves an external voltage driving ion movement between electrodes via an electrolyte, with heat absorption occurring at one electrode and heat dissipation at another (Abdollahipour and Sayyaadi, 2022). By carefully selecting electrode materials, separators, and electrolytes, the cooling effect can be optimized for prolonged and controllable thermal regulation.

The electrode materials play a crucial role in facilitating redox reactions and ion exchange, with commonly used options including metal oxides such as vanadium oxide (V₂O₅), manganese oxide (MnO₂), and nickel oxide (NiO). Carbon-based materials like graphene, carbon nanotubes (CNTs), and activated carbon offer high conductivity and flexibility, making them suitable for wearable applications. Conducting polymers such as polyaniline (PANI), polypyrrole (PPy), and PEDOT also serve as promising alternatives due to their tunable electrical and thermal properties. Electrolytes act as the ion transport medium, with aqueous solutions like lithium chloride (LiCl), sodium sulfate (Na₂SO₄), and potassium hydroxide (KOH) offering high ionic conductivity and environmental safety. Ionic liquids, including imidazolium- and phosphonium-based compounds, provide excellent electrochemical stability, while polymer gel and solid-state electrolytes, such as PVA-H₃PO₄ and PEO-based systems, enable flexibility in wearable applications. Separators are essential to prevent short circuits while maintaining ion flow, with commonly used materials including polymer membranes like polyethylene (PE), polypropylene (PP), and polyvinylidene fluoride (PVDF), as well as nanoporous materials such as electrospun nanofibers and porous ceramic membranes. For wearable cooling devices, materials must be flexible, lightweight, biocompatible, and mechanically stable. Carbon-based electrodes, polymer gel electrolytes, and reinforced separators are particularly suitable for integration into textiles and flexible systems.

Unlike thermoelectric coolers, which require significant electrical input to generate a temperature gradient, electrochemical cooling can achieve comparable effects with lower power consumption(McKay et al., 2019). This makes it suitable for battery-powered or energy-harvesting systems, reducing dependency on large external power sources. Additionally, electrochemical cooling devices can be designed using lightweight and flexible materials, enhancing comfort and wearability without compromising performance. Thus, electrochemical cooling can be regarded as one of the sustainable solutions for heat stress mitigation in various real-world applications.

Despite its potential, several challenges must be addressed to enable the widespread adoption of electrochemical cooling in wearables. The longevity of electrochemical cooling systems is a critical concern, as repeated redox cycling may lead to electrode degradation and electrolyte instability. Additionally, ensuring skin safety and the biocompatibility of materials is essential for direct-contact applications. Research efforts are currently focused on developing novel electrolytes with high ionic conductivity and low volatility, as well as electrode materials with enhanced durability and thermal responsiveness.

Thermionic solid-state cooling is an emerging cooling technology that utilizes thermionic emission to transport heat across a potential barrier. Electrons are thermally excited and emitted from a hot cathode, traverse a vacuum or semiconductor gap, and are collected at a cooler anode, thereby transferring heat and enabling efficient thermal management (Lee et al., 2000; Huffman, 2003). Thermionic coolers can function without moving parts, leading to increased reliability and reduced maintenance. Additionally, high-efficiency thermionic coolers can be achieved by reducing parasitic heat conduction.

Thermionic cooling relies on carefully selected materials for efficient electron emissions and heat transport. Common cathode materials include cesium-coated tungsten (Cs-W), which lowers the work function for enhanced emission, and lanthanum hexaboride (LaB₆), known for its low work function (∼2.7 eV) and high thermal stability. Barium strontium oxide (Ba-Sr-O) is widely used in dispenser cathodes, while graphene and other 2D materials emerge as promising alternatives due to their ultra-low work functions and superior electron mobility. For the anode, materials such as molybdenum (Mo), graphene, and silicon carbide (SiC) are commonly used to efficiently collect emitted electrons while maintaining high thermal and electrical conductivity. The interlayer or gap between the cathode and anode can be a vacuum, ensuring free electron transport, or a semiconductor such as gallium nitride (GaN) or silicon carbide (SiC) to mitigate space-charge effects. Additionally, work function reduction coatings, including alkali metals like cesium and rubidium, as well as oxides such as BaO, SrO, and CaO, are applied to further enhance electron emission (Bellucci et al., 2021). Recent advancements in nanostructured materials, heterostructures, and composite materials aim to optimize thermionic cooling efficiency at lower operating temperatures, making the technology more viable for practical applications.

Countries with hot climates, particularly those near the tropics, face significant challenges related to high cooling demands. The Gulf Cooperation Council (GCC) region, which includes Saudi Arabia, Kuwait, Bahrain, Oman, Qatar, and the United Arab Emirates, experiences extreme heat conditions, leading to high electricity consumption. In the GCC, air conditioning accounts for approximately 70% of household electricity consumption in Saudi Arabia, 60% in Oman, and 36% in the United Arab Emirates. The region’s cooling demand is expected to triple by 2030 compared to 2010 levels (Al-Nini et al., 2023).

Urban areas with high population densities benefit significantly from DCS due to their energy-intensive environments. The implementation of centralized cooling plants helps mitigate peak electricity demand during heat waves, reducing strain on the power grid. According to the International Energy Agency (IEA), space cooling accounts for 20% of building electricity consumption, responsible for approximately 1 Gton of CO₂ emissions annually. By 2050, the cooling demand could increase by 100%, with two-thirds of households worldwide equipped with air conditioning systems (Neri et al., 2024).

Enhancing DCS efficiency requires integration with renewable energy sources, such as solar or geothermal energy, along with thermal energy storage (TES) systems. TES tanks, combined with chillers, enable better load management, reducing peak electricity demand. Free cooling sources, such as cold-water basins or waste heat from industrial processes, can also be incorporated to improve energy savings. Fifth-generation district heating and cooling (5GDHC) is an emerging approach that promotes heat sharing among prosumers through a bidirectional, low-temperature network (Gjoka et al., 2025). This system reduces reliance on fossil fuels by enabling energy exchange among buildings with complementary heating and cooling needs. Decentralized heat pumps are employed to bridge temperature gaps, reducing operational costs and improving efficiency. Studies suggest that 5GDHC systems can achieve electricity savings of up to 40% compared to conventional setups (Chaudhry et al., 2024; Qin and Gosselin, 2024).

An alternative district cooling approach involves using natural sources such as winter snow. Seasonal snow storage systems have been implemented in countries like Sweden, Norway, and Japan, where snow is collected and stored in insulated tanks for use during warmer months. This method offers significant energy savings, as melting snow provides a free cooling source with high latent heat of fusion. Additionally, integrating snow cooling with solar photovoltaic (PV) electricity can further reduce the carbon footprint of such systems (Sreenath et al., 2024).

Phase change materials (PCM) also offer a viable option for seasonal thermal energy storage. PCMs store thermal energy in the form of latent heat, allowing for effective load shifting. However, selecting appropriate PCM parameters, such as melting temperature and storage size, is crucial to optimizing performance. Recent studies highlight that PCM-based storage, when coupled with photovoltaic integration, can enhance CO₂ emission reduction efforts (Yang et al., 2024).

District cooling technology has evolved through multiple generations.

First Generation: Pipeline refrigeration systems utilizing decentralized evaporators and centralized condensers.

Future research directions in district cooling focus on enhancing energy efficiency, integrating renewable energy sources, and optimizing system design through advanced modeling and control strategies. Adopting low-carbon and waste heat utilization technologies, such as absorption and adsorption, is gaining traction to reduce reliance on conventional electricity-driven cooling. Thermal energy storage (TES) systems, including phase change materials (PCMs) and chilled water storage, are being explored to improve load flexibility and peak demand management. Integrating artificial intelligence (AI) and digital twins enables real-time monitoring, predictive maintenance, and dynamic optimization of district cooling networks. Additionally, the development of next-generation cooling fluids and high-efficiency heat exchangers aims to minimize environmental impact and improve performance. Future research also emphasizes smart grid integration and district-wide energy sharing, enabling interconnected cooling systems to enhance sustainability and resilience in urban environments.