Enhancing Dewpoint Indirect Evaporative Cooling with Intermittent Water Spraying and Advanced Materials: A Review

Abstract

Dewpoint indirect evaporative cooling (DIEC) offers an energy-efficient, eco-friendly alternative to conventional air conditioning by using water and air to lower temperatures. With the rising demand for sustainable cooling solutions—especially in regions facing water scarcity and high energy costs—optimizing these systems for real-world conditions is more important than ever. One major challenge is ensuring that DIEC systems perform well when water is supplied intermittently rather than continuously. In this review, we examine how intermittent water supply affects the cooling performance and overall efficiency of DIEC systems. We discuss recent studies that highlight the importance of key factors such as the properties of heat exchanger materials, design modifications, and control strategies. Our analysis reveals that while innovative materials like hydrophilic membranes and adaptive design features can improve performance, their widespread use is often limited by cost and scalability. We also point out critical research gaps, particularly in applying intermittent water spraying to non-porous heat exchangers. Overall, our findings underscore the need for integrated water management strategies in DIEC design. We advocate a cross-disciplinary approach—bridging fluid dynamics, material science, and environmental engineering—to develop more resilient and sustainable cooling technologies.

1. Introduction

Energy consumption for air conditioning (AC) has increased dramatically over recent decades due to improvements in living standards [1,2], urbanization [3,4], and climate change [5]. The reliance on AC systems to maintain thermal comfort has led to significant resource depletion and increased carbon emissions [6]. Globally, buildings account for approximately 30–40% of total energy consumption and a comparable share of CO₂ emissions [7]. Among the various contributors, Heating, Ventilation, and Air Conditioning (HVAC) systems consume nearly half of a building’s total energy use [8].

With the global population spending more time indoors, the demand for efficient and sustainable cooling solutions is increasing significantly. This is especially true in rapidly developing regions such as China, where cooling-related electricity demand in non-residential buildings is projected to rise from 166 TWh in 2015 to 564 TWh in 2050 [9]. The Middle East and North Africa (MENA) region faces increasing cooling demands, with space cooling energy use growing by 396% from 1990 to 2016 [10]. Also, in the U.S., research indicates that rising temperatures and humidity levels are projected to increase cooling needs by up to 8–15% with 1.5–2.0 °C of global warming [11].

However, the challenge of space cooling is multifaceted. It is not only the increasing temperatures and humidity [12] that pose a problem but also the rising costs of energy. If immediate action is not taken, global energy consumption for cooling is projected to more than double by 2050, putting immense pressure on power grids and increasing greenhouse gas emissions [13].

To address these challenges, cooling solutions can be categorized into passive and active methods, as illustrated in Figure 1. Passive cooling techniques focus on reducing indoor temperatures without the use of mechanical systems. These methods are often integrated into the initial design of buildings and urban landscapes to create naturally cooler environments. These methods, when implemented correctly, can significantly minimize the reliance on active cooling [14,15,16]. As they do not consume energy during the building lifetime, they should be implemented as the first option. However, sometimes, this is impossible due to existing architecture or other spatial or policy restrictions. Then, active cooling solutions (the right side of the figure) can be implemented. They involve mechanical and technological systems that actively regulate indoor temperatures by removing heat from buildings. These systems typically require electricity or other energy inputs, making them more resource-intensive than passive solutions. However, they provide precise thermal control and are widely used in various climates and building types. Among those, conventional vapor compression refrigeration systems dominate the AC market, due to their long history of development, reliable performance, and widespread availability. These systems, however, depend heavily on electricity and environmentally harmful refrigerants [17]. Alternative technologies, such as absorption and adsorption cooling systems, have been explored but often face challenges such as high costs, complexity, and low efficiency when compared to conventional vapor compressor systems [18].

2. Evaporative Cooling Technology

In this context, evaporative cooling (EC) technologies appear as energy-efficient, refrigerant-free alternatives to other mechanical cooling systems. By using the effect of just water evaporation, EC requires up to 80% less electricity than conventional HVAC systems [19].

2.1. Overview

As a principle, EC relies on the latent heat of vaporization. As water evaporates, it absorbs heat from the surrounding air, lowering the temperature. In practice, warm air is passed over a wetted or water-saturated surface, where sensible heat is used in the liquid-to-vapor phase change. This cooling effect is more pronounced under lower humidity conditions.

Consequently, EC systems are more sustainable than traditional vapor compression refrigeration, as they consume less energy, lower greenhouse gas emissions, and avoid high global warming potential refrigerants. Their simple design also reduces both initial capital and maintenance costs, making them suitable for sustainable development, especially in arid regions with access to water.

Integration with renewable energy sources further enhances the environmental benefits. For instance, coupling evaporative coolers with solar-powered fans can create self-sustaining cooling systems [20,21].

Evaporative cooling technologies have demonstrated significant potential for improving energy efficiency in cooling systems compared to traditional vapor compression systems. Studies have shown that evaporative cooling can enhance the energy efficiency ratio (EER) of chillers by up to 63% when using cooling towers and improve the EER of air handling units by 67% with indirect evaporative cooling (IEC) [22]. EC hybrid and multi-stage designs have shown the best performance, reaching coefficient of performance (COP) values as high as 35 [23]. Evaporative-cooled condensers can increase the COP by around 50% and decrease power consumption by up to 20% compared to air-cooled condensers [24]. IEC systems have demonstrated very high COPs in hot, dry, and humid climates, requiring energy only for fans and a small amount of water [25]. Experimental studies have shown that evaporative cooling can increase the COP by up to 44% and decrease power consumption by 20% in split air-conditioning systems [26].

Overall, EC technology is promising. However, it faces challenges in humid environments or where water access is restricted. Still, recent research and development aim to overcome these limitations, potentially increasing IEC’s market share in the coming decades and reducing energy consumption and carbon footprint in the building sector [27].

2.2. Fundamentals

Evaporative cooling has been used by people for a long time now [28,29,30], as the technology is simple and affordable [31]. The progression from direct evaporative cooling (DEC) to indirect evaporative cooling (IEC) is illustrated in Figure 2, highlighting advancements in cooling performance and control over humidity. While DEC reduces air temperature through direct contact between air and water, it results in an increased humidity ratio of the treated air. IEC improves upon this by preventing a humidity increase in the treated air but is constrained by the wet-bulb temperature as the cooling limit. DIEC overcomes these limitations by achieving air temperatures below the wet-bulb temperature, approaching the dewpoint. One of the latest development paths integrates an intermittent water spray system into DIEC to enhance energy and water efficiency, further advancing the sustainability of the technology. The evolution of EC technology with a short description is presented in Figure 2.

In order to clearly state the differences and the limitations that occur in EC, a schematic representation of the device and channel arrangement with a psychometric representation is presented in Figure 3. As a supplement,

Table 1. Performance and efficiency comparison of DEC, IEC, and DIEC.

Parameter	DEC	IEC	DIEC
Process	Untreated air passes over a water film, leading to direct evaporation and cooling.	Air passes through separate dry and wet channels; heat is transferred from the dry channel to the wet channel.	Builds on IEC with part of cooled air returned into wet channels.
Result	Air is cooled but gains humidity, following a constant wet-bulb temperature line.	Air is cooled without increasing humidity.	Achieves cooling below the wet-bulb temperature.
Thermodynamic Limitations	Performance is limited by increased humidity, making it less effective in humid climates.	Separates air streams for humidity control but still limited by wet-bulb temperature.	Partly returned cooled air canlead to approaching the dewpoint temperature limit.
Efficiency and Applicability	Energy-efficient but dependent on ambient humidity; less effective in humid regions [32,33].	More efficient than DEC and suitable for dry climates [34,35].	Highest efficiency; suitable for both dry and humid climates [36].
Typical Components	Fan, wetted surface, and water supply system.	Fan, heat exchanger, and water distribution system.	Fan(s), heat exchanger, and water distribution system.
Advantages	High energy savings, low greenhouse gas emissions, and simple design.	Maintains humidity control; more effective than DEC in dry climates.	Achieves the lowest temperatures; is highly energy-efficient and adaptable to various climates.
Limitations	Increase in treated air humidity ratio.	Cooling limited to wet-bulb temperature.	More complex design.

presents a comparison of the system in a condensed way, consisting of crucial aspects in three evaporative cooling technology types.

3. Research Gap and Scope

Recent review articles on the topic of EC have been presented by the authors in

Table 2. Recent research reviews on evaporative cooling.

Year	Source	Summary
2025	[37]	Critical review of the thermodynamics and kinetic theories behind liquid–vapor transitions in evaporative cooling, comparing models like Kirchhoff-type and Hertz–Knudsen formulas to improve efficiency and design.
2024	[38]	Comprehensive overview of the current techniques used for evaporative cooling systems for photovoltaic (PV) modules, highlighting their potential to improve the overall performance of PV systems, especially in hot and dry climates.
2024	[19]	Review of advances made in evaporative cooling technologies and their potential as a promising alternative to conventional mechanical cooling systems for providing indoor thermal comfort.
2024	[36]	Comprehensive review of dewpoint evaporative cooling (DPEC) technology, including its principles, performance improvements, mathematical modeling, and integrated applications in both hot and dry as well as hot and humid regions.
2023	[39]	Conceptual review of evaporative cooling technologies, including their working principles, types, and advantages over conventional HVAC systems in terms of energy efficiency and environmental friendliness.
2023	[40]	Comprehensive review of the current literature on direct evaporative cooling (DEC) systems, focusing on the different approaches adopted to model and predict the performance of these systems.
2022	[41]	Review of the different types of evaporative cooling technologies and their applications in providing energy-efficient and environmentally friendly cooling.
2022	[42]	Comprehensive overview of dewpoint evaporative cooling (DPEC) techniques, including their current applications, limitations, and future research directions to support carbon-neutral development.

. Only recent reviews have been chosen from last three years. Recent advances in evaporative cooling technologies have been investigated across multiple research domains. Each paper is described with a short summary. Together, these works demonstrate the growing scientific interest in evaporative cooling in many different directions and applications. Hardly any of the recent reviews tackle the water management system or heat exchanger surface modification in a comprehensive way.

3.1. Research Gap

The authors identify the research gap in this section. As presented in Figure 2, the development of EC technology is moving toward IEC and even further to DIEC. As it is the most developed configuration, ideas on further development were presented by the researchers. One of the main methods was to reduce water pump energy consumption, adopting the idea of using porous materials that can store water within the heat exchanger. The results are visible in Table 3, which presents basic data on studies that evaluated intermittent water spraying (coefficient of performance (COP), wet-bulb effectiveness (ε_wb), and dewpoint effectiveness (ε_dp) are also presented). Only one work investigated intermittent water spraying in a non-porous heat exchanger. As a supplementary but crucial part, the spray time and pause time are presented in Figure 4.

As a principle, intermittent water spraying should be analyzed on regular, non-porous heat exchangers to evaluate the operation of such devices [43].

Figure 4. Water system operation intervals for publications: A [44], B [45], C [46], D [47], E [48], F [49], G [50], H [51], I [52], J [53], K [54], L [55], M [56], and N [57].

Figure 4. Water system operation intervals for publications: A [44], B [45], C [46], D [47], E [48], F [49], G [50], H [51], I [52], J [53], K [54], L [55], M [56], and N [57].

Table 3. Studies on intermittent water spraying.

Year	Source	Type *	Porous Heat Exchanger Surface	Surface Type	Inlet Air Conditions °C/%	COP	εwb	εdp
2017	Xu et al. [44]	E	Yes	Fabric	37.8/21.7	52.5	1.14	0.75
2017	Wang et al. [45]	E	Yes	Ceramics	36.5/21	34.9	0.40–0.42	-
2020	Sun et al. [46]	E	Yes	Ceramics	34–40/53	-	0.76–1.08	-
2021	Elahi et al. [47]	E	Yes	Wood fibers	28–41/30	45.4	0.80	-
2021	Chen et al. [48]	E	Yes	Plant fiber–polymer composite	30.4–38/21–48	-	1.24	0.98
2022	Shi et al. [49]	T/E	Yes	Stainless steel + sintered porous nickel	23, 32/70, 30	146.3	-	0.63
2022	Shi et al. [50]	E	Yes	Stainless steel + sintered porous nickel	26–32/41	17.3	0.68	-
2023	Shi et al. [51]	E	Yes	Stainless steel + sintered porous nickel	24–36/40	22.5	0.63	-
2023	Chen et al. [52]	E	Yes	Fiber fabric	35/40	-	-	-
2023	Chen et al. [53]	E	Yes	TiO2/SiO2 nano-coated polypropylene	22/60–64	9.0	-	-
2023	Jin et al. [54]	T/E	No	Fin-plate aluminum	35/45	-	-	-
2024	Chen et al. [55]	E	Yes	TiO2/SiO2 nano-coated polypropylene	35/45	-	0.81	-
2024	Khan et al. [56]	E	Yes	SuperKool cellulose cooling pad	31–34/55–75	35.2	0.85	0.80
2024	Shim et al. [57]	E	Yes	Aluminum-layered double hydroxides	30/60	-	-	-

* E—experimental; T—theoretical.

Table 3. Studies on intermittent water spraying.

Year	Source	Type *	Porous Heat Exchanger Surface	Surface Type	Inlet Air Conditions °C/%	COP	εwb	εdp
2017	Xu et al. [44]	E	Yes	Fabric	37.8/21.7	52.5	1.14	0.75
2017	Wang et al. [45]	E	Yes	Ceramics	36.5/21	34.9	0.40–0.42	-
2020	Sun et al. [46]	E	Yes	Ceramics	34–40/53	-	0.76–1.08	-
2021	Elahi et al. [47]	E	Yes	Wood fibers	28–41/30	45.4	0.80	-
2021	Chen et al. [48]	E	Yes	Plant fiber–polymer composite	30.4–38/21–48	-	1.24	0.98
2022	Shi et al. [49]	T/E	Yes	Stainless steel + sintered porous nickel	23, 32/70, 30	146.3	-	0.63
2022	Shi et al. [50]	E	Yes	Stainless steel + sintered porous nickel	26–32/41	17.3	0.68	-
2023	Shi et al. [51]	E	Yes	Stainless steel + sintered porous nickel	24–36/40	22.5	0.63	-
2023	Chen et al. [52]	E	Yes	Fiber fabric	35/40	-	-	-
2023	Chen et al. [53]	E	Yes	TiO2/SiO2 nano-coated polypropylene	22/60–64	9.0	-	-
2023	Jin et al. [54]	T/E	No	Fin-plate aluminum	35/45	-	-	-
2024	Chen et al. [55]	E	Yes	TiO2/SiO2 nano-coated polypropylene	35/45	-	0.81	-
2024	Khan et al. [56]	E	Yes	SuperKool cellulose cooling pad	31–34/55–75	35.2	0.85	0.80
2024	Shim et al. [57]	E	Yes	Aluminum-layered double hydroxides	30/60	-	-	-

* E—experimental; T—theoretical.

3.2. Scope

Considering the identified research gap, the authors decided to focus on very narrow and yet not described topics. In terms of intermittent water spraying in IEC/DIEC-type devices, the scope must tackle only the heat exchanger material and water supply system. These are the only factors that may affect the possibility of intermittent water spraying applications in non-porous IEC/DIEC devices.

4. Heat Exchanger Surface Modification

An important goal of modifying the surface of wet-side exchanger walls is to ensure an even and thin layer of water film on the entire sprayed surface. In an operating device, a situation may occur when only part of the wall on the wet side is covered with a water film, which limits the surface area from which water evaporates [27,58]. The potential of the evaporative cooling process is thus limited. There cannot be too much water on the walls either. Due to its high specific heat, water becomes an additional insulator in the heat exchange path between the dry and wet channels [59].

A key challenge in the design of these heat exchangers arises when water supply is intermittent. Under such conditions, the ability of the heat exchanger material to absorb and store water becomes important. The selection of materials for constructing these heat exchangers is therefore driven by a range of properties (the most important factors are presented as a graph in Figure 5):

• Wettability—A measure of how easily a liquid spreads over a surface. High wettability ensures even water distribution, which is essential during short water spray cycles.
• Water Storage Capacity—The ability of a material to hold water—often a function of its porosity and capillary structure—which becomes crucial when water is supplied intermittently.
• Application Versatility—Materials may be used as standalone structures or as surface coatings on substrates. Their mechanical strength and adhesion characteristics determine their suitability in either role.
• Bacterial Risk—Extended water retention can lead to biofilm formation and microbial growth. Hence, antibacterial properties or the ability to undergo surface treatments is a significant factor.
• Thermal Performance—The efficiency of heat transfer, which is largely governed by the material’s thermal conductivity and surface area, directly impacts cooling performance.

4.1. Wettability

Wettability is an important factor of materials in evaporative cooling systems, with higher wettability generally leading to improved performance [60]. Studies have shown that hydrophilic surfaces, characterized by low contact angles, promote better water spreading and heat transfer [61,62]. Recently, polymeric foam materials have been investigated for evaporative cooling applications [63,64].

The study by Caruana et al. [60] found that surfaces with higher wettability, characterized by lower contact angles, enhance cooling efficiency by promoting uniform water distribution. Among the tested surfaces, the hydrophilic lacquer (HPHI) had the lowest static contact angle (average 69°), followed by the standard epoxy coating (STD) at 75°, and the uncoated aluminum (AL) at 89°.

Further experiments by Caruana et al. [62] showed that the hydrophilic lacquer (HPHI) surface demonstrated the lowest contact angle (median: 50°), confirming its great wettability, while aluminum (AL) and standard epoxy-coated (STD) surfaces had contact angles of 75° and 81°, respectively. The presence of limescale altered wettability, with HPHI and AL showing improved wetting behavior, while STD experienced reduced wettability.

Choi et al. [61] provide more universal and general results. Wettability significantly influences the heat transfer performance of evaporative cooling devices by affecting the contact angle and fluid distribution on surfaces. A lower contact angle, indicating higher wettability, enhances heat transfer by promoting uniform liquid spreading and reducing thermal resistance. Studies show that hydrophilic surfaces improve evaporation rates due to larger heat transfer areas and thinner liquid films, leading to higher cooling efficiency.

The study by Castillo-Gonzalez et al. [64] showed that foam materials produced at a low flow rate exhibit enhanced wettability, which is beneficial for evaporative cooling. Under optimized conditions, the foam demonstrated a static contact angle of about 72° ± 3°, alongside a 250% increase in water absorption and a capillary rise of 120 mm. In contrast, foams processed at higher flow rates had a higher contact angle (approximately 80° ± 4°), lower water uptake, and reduced capillary rise of 90 mm, indicating diminished cooling performance.

When water is supplied intermittently, the time to fully saturate the whole area of the wet channels is relatively short. Therefore, the better the wettability, the greater the chance to fully wet the channel surfaces within a shorter spray time.

4.2. Water Storage Capacity

Porous materials can become a reservoir for water within their microstructure. This stored water can continue to evaporate during the intervals between sprays, thereby maintaining cooling performance [65]. Therefore, it is a crucial factor in terms of a non-constant water supply to the system.

The high water storage capacity of a heat exchanger material examined by Shi et al. [49] shows that a porous indirect evaporative cooler can maintain cooling for a maximum non-spraying duration of 2410 s, reducing water pump operation time by 95.2% compared to continuous spraying. This ability is strictly connected with the water retention capacity of porous media, which allows for intermittent spraying strategies, significantly lowering energy consumption.

A similar approach was presented by Duan et al. [66], where the results show that cellulose/PET fiber achieved the highest permeability-to-effective-pore-radius ratio, increasing its ability to retain and distribute water for prolonged cooling. Moreover, Coolmax and cellulose/PET fibers demonstrated significant water evaporation rates, with cellulose/PET achieving an average evaporation rate of 4.34 × 10⁻⁴ kg/(m²·s), contributing to improved cooling efficiency.

A study at a smaller scale is provided by Tang et al. [67]. The study highlights a bilayer structure (BLS) composed of a bottom hydrogel layer and an upper microporous aerogel layer, which significantly enhances water retention. With a 2 mm thick SiO₂ aerogel, the evaporative cooling time span is extended by 11 times compared to a single hydrogel layer, ensuring prolonged cooling even in arid conditions.

Materials with lower porosity may require the addition of surface coatings that improve water absorption. Polymer composites, for instance, can be engineered with embedded hydrophilic particles to enhance their overall water storage ability [68].

Water storage capacity is a crucial factor for intermittent water spraying implementation. The longer a material can store water for evaporation, the shorter the time required for water to be supplied; so, the pump operation time and energy consumption is lower.

4.3. Standalone Materials and Coatings

The design of evaporative cooling systems may call for materials that serve as either the primary structural element or as a functional coating applied to an underlying substrate [69]. Standalone materials such as monolithic porous ceramics are advantageous due to their water retention ability and durability. However, they may be heavier or more brittle compared to metals or composites [70,71]. Wicking metal plates, particularly aluminum, have been identified as suitable for indirect evaporative cooling due to their shape formation ability and durability [72]. Fibrous materials like cellulose/PET fiber and Coolmax fabric have shown promising wicking and evaporation performance, outperforming wood pulp paper [66].

Beside popular materials, new natural materials are investigated. Geopolymers, made from industrial and agricultural waste, have been proposed as green alternatives for evaporative cooling [73]. Those experimental studies on novel organic materials for direct evaporative cooling in hot–dry climates have revealed eucalyptus fibers and ceramic pipes as the most effective, with cooling effectiveness ranging from 72 to 33% and from 68 to 26%, respectively, at air velocities between 0.1 and 1.2 m/s. Yellow stone also showed competitive performance, while dry bulrush basket and Cyprus marble exhibited relatively poor results [74].

In contrast, coatings—for example, hydrophilic layers on aluminum fins—offer flexibility by combining the high thermal conductivity of metals with the favorable surface properties of advanced coatings [75]. The key challenges in coating applications are ensuring robust adhesion under cyclic wetting/drying conditions and maintaining long-term stability without degradation of wettability or water storage capability.

Hydrophilic coatings, such as TiO₂ and graphene, have been shown to improve the efficiency and coefficient of performance of indirect evaporative coolers. The study by You et al. [76] compared hydrophilic coatings applied to an IEC system. Their esults showed that the TiO₂-coated IEC improved enthalpy efficiency by 5.33%, wet-bulb efficiency by 9.91%, and the COP by 3.22 compared to uncoated surfaces. The graphene-coated IEC increased the performance even more, with enthalpy efficiency increasing by 14.75%, wet-bulb efficiency by 12.07%, and the COP by 7.69 compared to the uncoated IEC.

Another study provides results of more advanced coating applications. Fathi et al. [77] investigated the effects of a dual-scale hierarchically porous aluminum coating (AL-HPC), created by brazing aluminum powders of different particle sizes onto a flat aluminum plate. Their experimental results showed that coated surfaces exhibit superior wickability compared to plain aluminum, with larger particle sizes (114 µm) achieving a 60% higher wicking height than smaller ones (27 µm). The coating also significantly increases the dryout heat flux, with the 114 µm particle size achieving 8.1 kW/m²—3.4 times higher than that of the smallest particle size tested. Additionally, coating thickness plays a critical role, with the thickest coating (1200 µm) achieving a dryout heat flux of 10.6 kW/m² and a maximum heat transfer coefficient of 251 W/m²K, 13 times higher than that of a plain surface.

For intermittent water spraying, there is no significant difference regardless of whether the material is standalone or coated (in terms of heat exchanger construction). The important part is that it can store water for a certain amount of time. Thus, it opens the possibility for further development and investigation into materials for intermittent water spraying.

4.4. Microbial Risk

The topic of microbial risk in EC is not discussed widely and is mainly connected with Legionella [78,79] and rarely with, for example, fungi [80,81]. Thus, this section is mainly based on previous work by the authors that elaborated on all kinds of microbial risk in EC [82].

The selection of heat exchanger materials in evaporative cooling systems is crucial due to the risk of biofilm formation, which can significantly impact both efficiency and hygiene. Biofilms, composed of bacteria, fungi, and other microorganisms, can develop on heat exchanger surfaces when water is constantly present. These biofilms pose potential health risks, especially in systems where air and water interact, leading to indoor air contamination [83,84,85]. Despite the fact that porous and hydrophilic surfaces are beneficial for water retention and distribution, they can at the same time cause microbial colonization if proper maintenance is neglected.

The hydrophobic and superhydrophilic coatings discussed earlier can become potential solutions for controlling water behavior on heat exchanger surfaces. Superhydrophilic coatings allow water to spread evenly, forming a thin film that enhances evaporation and prevents localized stagnation, thereby reducing microbial growth [85]. On the other hand, hydrophobic surfaces repel water, limiting moisture retention and making it difficult for biofilms to establish.

However, any material needs to be discussed in terms of microbial risk. The selection of materials for heat exchangers must balance factors such as water resistance, structural rigidity, and microbial safety. Many EC systems utilize plastics due to their cost-effectiveness and durability, though they must be carefully engineered to support effective water evaporation. Still, bacterial adhesion remains a concern, particularly for textile-based surfaces where the interaction between hydrophilic and hydrophobic fibers is still debated. Research suggests that both superhydrophilic and superhydrophobic surfaces reduce bacterial attachment, whereas moderately hydrophobic materials exhibit the highest microbial adhesion.

In terms of intermittent water spraying, it can be concluded that a lack of standing water and thick water films should improve safety. Still, proper maintenance is needed.

4.5. Discussion and Summary

Overall, the literature indicates that an integrated approach—where material selection is closely aligned with operational strategies (such as water spray timing and flow rates)—can lead to significant improvements in evaporative cooling performance. Future work should continue to refine these materials, focusing on advanced surface treatments and composite formulations that can meet the dual demands of high thermal performance and effective water management under intermittent supply conditions.

For ease of use, the above details are summarized and presented in

Table 4. Summary of material types.

Material Type	Wettability	Water Storage Capacity	Standalone/Coating	Bacterial Risk
Porous Ceramics	High wettability (contact angles typically < 20°)	High porosity (pore volume often in the 40–70% range)	Often used as standalone elements	Moderate risk; may require antibacterial treatments
Metal Foams/Perforated Metals	Moderate to high (may need surface modification for optimal wettability)	Moderate water storage due to engineered pore structure	Can serve as both standalone and substrate for coatings	Lower risk due to smoother surfaces; still requires monitoring
Polymer Composites	Variable; often improved via incorporation of hydrophilic fillers	Typically lower than ceramics; can be enhanced with additives	Commonly used as coatings or integrated in composite panels	Risk depends on formulation; additives can reduce biofilm formation
Hybrid/Coated Systems	Designed for high wettability; surface treatments lower contact angles significantly	Dependent on the coating’s porosity and structure; tailored for water retention	Primarily used as coatings on thermally conductive substrates	Formulated to minimize bacterial adhesion through biocidal additives

.

5. Water Supply System

In this paragraph, the authors focus on advancements in water spraying systems for indirect evaporative coolers (IECs), emphasizing design parameters that optimize cooling performance while balancing energy and water consumption. The goal of this section is to provide a comprehensive understanding of how water spraying systems can be optimized to enhance cooling performance while minimizing energy and water consumption. The basic water spraying system division and critical factors are presented in Figure 6.

5.1. Nozzle-Based Systems

Nozzle-based water spraying systems are widely used in IECs to enhance cooling efficiency. These systems rely on nozzles to atomize water into fine droplets, which are then distributed over the heat exchanger surface. The primary function of nozzles is to create a uniform water film on the heat exchanger surface, ensuring efficient heat and mass transfer.

Research has focused on optimizing nozzle types, arrangements, and spray strategies to improve water distribution and overall performance. Spiral nozzles have shown superior coverage and uniformity [46], while optimal nozzle arrangements can increase system COP by up to 16% [86]. CFD simulations have revealed that top-side nozzle configurations paired with air–water counter-flow can significantly improve water film coverage and temperature drop [87]. Additionally, hydrophilic and fiber coatings can further enhance water film coverage and the COP [87]. However, modeling IEC systems remains challenging due to the complexity of boundary conditions and operating parameters, such as nozzle orientations and water flow rates [88]. Despite these challenges, nozzle-based systems continue to be the most common water spraying method in IECs due to their effectiveness and potential for optimization.

As nozzles are an important factor affecting the overall performance of the device, they are described in depth in the following sections.

5.1.1. Nozzle Type

A study by Sun et al. [46] experimentally evaluated five common spray nozzles: spiral, conical, square, sector, and target impact types. The target impact nozzle provides the highest coverage (89.2%) and best uniformity but delivers too little water volume, making it unsuitable for IEC applications, though effective for spray chambers requiring fine droplets. The spiral nozzle offers the second-best coverage (78.4%) and uniformity (1.35), with a balanced water distribution, making it the most suitable choice for IEC applications. Conical and sector nozzles have the lowest coverage and uniformity, while the square nozzle performs moderately with a 68.4% coverage ratio. Yang et al. [89] provided a study on a gas–liquid two-phase swirling atomizing nozzle that uses bubble cutting technology to produce micron-sized, evenly distributed droplets with a stable spray at 0.2–0.6 MPa and a liquid flow rate of 2–8 kg/h. It features a simple, easy-to-clean threaded design, interchangeable nozzle styles, and low energy consumption (7.59 W/kg), making it highly efficient for spray cooling applications. Generalized nozzle-type characteristics are presented in

Table 5. Nozzle-type characteristics.

Nozzle Type	Coverage Ratio	Uniformity	Water Consumption	Remarks
Gas–Liquid Two-Phase Swirling Atomizing	-	-	Low	High atomization quality even with small water flow
Target Impact	High	High	High	Best performance, high water use
Spiral	Medium	Medium	Medium	Optimal balance
Square	Medium	Medium	Medium	Moderate performance
Conical	Low	Low	Low	Less efficient
Sector	Low	Low	Low	Least efficient

.

5.1.2. Nozzle Position

Nozzle placement can be categorized based on the mounting capability as upper, middle, or lower configurations. The upper configuration, suitable for most IEC setups, generates counter-flow water streams against secondary airflow (when air is blown upward). The middle configuration, though challenging to install, provides effective water distribution across large exchanger blocks. The lower configuration, ideal for space-constrained installations, produces concurrent water streams aligned with secondary airflow [28]. Most studies spray water into wet channels using top-mounted nozzles [43,90]. Experiments with top-mounted nozzles and downward airflow demonstrated higher wet-bulb effectiveness in counter-flow configurations [91]. Other researchers confirm that secondary airflow and water spray should generally oppose each other [92]. A general division of nozzle position is presented in Figure 7.

A study by Al-Zubaydi and Hong [93] evaluated the impact of three different water spraying modes—external, internal, and mixed—on the performance of an IEC system. The mixed spraying mode was better than the other two, achieving a maximum wet-bulb efficiency of 76%, compared to 72.9% for the internal mode and 70% for the external mode. The cooling capacity was also highest in the mixed mode, reaching 1.8 kW at an inlet temperature of 37.2 °C, while the internal and external modes achieved 1.59 kW and 1.4 kW, respectively. The coefficient of performance (COP) followed a similar trend, with the mixed mode reaching a peak COP of 17.9, which is higher than the internal (15.5) and external (14.25) modes. Additionally, at lower primary air velocities, the temperature drop was more significant, improving cooling effectiveness. Increasing secondary air velocity enhanced the system’s overall performance, leading to an 11% increase in COP in the mixed mode. The study concluded that optimizing water spraying configurations, particularly using a mixed mode, can significantly enhance IEC performance.

The study by Yang et al. [89] analyzed the effect of a spray evaporative cooling system on the performance of an air-cooled chiller, comparing horizontal and vertical spray modes. The results showed that applying the spray system increased the coefficient of performance (COP) by 3.78–8.33%, with the highest improvement observed at a water flow rate of 20 kg/h. Vertical spraying was more effective than horizontal spraying, with a COP increase of 0.28–1.94 due to better evaporation and greater air cooling efficiency. At an ambient temperature of 40 °C, the COP increase ranged from 6% to 9%, while at 25 °C, the increase was only around 2%, indicating that the system is more beneficial in hotter climates. Total electricity consumption was reduced by 2.37–13.53%, demonstrating significant energy savings. The optimized spray system, using gas–liquid two-phase swirl nozzles, achieved high atomization quality with minimal power consumption of 7.59 W/kg. The study suggests that adjusting spray water flow rates based on cooling capacity can further enhance performance and prevent water waste.

Ma et al. [87] analyzed the effect of different nozzle configurations on the performance of an indirect evaporative cooler (IEC) using CFD modeling. The best performance was observed with the top-side nozzle arrangement and counter-flow air–water configuration, achieving a 59.2% increase in water film coverage and a 27.4% temperature drop compared to the bottom configuration at a water supply rate of 65 L/s. The study also found that further increasing the water flow rate beyond 65 L/s did not provide any gains, primarily increasing pump power consumption.

Nozzle placement plays a crucial role in the performance of evaporative cooling systems, with top-mounted and counter-flow configurations generally providing the best efficiency. Studies indicate that mixed spraying modes and vertical spray patterns enhance cooling effectiveness, energy savings, and overall system performance. Optimizing nozzle arrangements improves water distribution and heat transfer.

5.1.3. Nozzle Arrangement

Ma et al. [86] examined an IEC with a 400 × 400 mm spray area and 330 mm nozzle height. For single-nozzle setups, a 45° spray cone angle and 30° inclination angle were optimal. The best performance was achieved with a centerline nozzle arrangement spaced 160 mm apart, which improved spray uniformity to a coefficient of 0.74 and increased the coverage ratio to 0.72. Compared to the original single-line nozzle setup, this optimized arrangement enhanced surface wettability, raising the wettability factor from 0.48 to 0.89. As a result, the optimized IEC design achieved a 16% increase in the coefficient of performance (COP) and improved cooling efficiency. The temperature drop of the primary air was greater in the optimized setup, reaching 26.1 °C compared to 27.4 °C in the original design. Additionally, energy efficiency improved due to better water distribution, leading to reduced overall power consumption.

Another study by De Antonellis [91] found that a four-nozzle configuration slightly outperformed an eight-nozzle setup, as it reduced water droplet collisions with exchanger walls. The conclusion drawn from the experiments indicate that it is the water flow that has the strongest impact and not the type and number of nozzles.

Still, there are a limited number of experiments that evaluate nozzle arrangement in evaporative cooling. Those which are presented differ in results. Thus, it is difficult to state what impact nozzle arrangement has on performance.

5.2. Nozzle-Free Systems

Systems without nozzles rely on passive mechanisms such as capillary action or gravity to distribute water over the heat exchanger surface. These systems are gaining popularity due to their simplicity and lower energy requirements. In nozzle-free systems, water is distributed through porous materials or gravity-fed channels, eliminating the need for pumps and nozzles. Nozzle-free systems are highly efficient in water use, making them suitable for arid regions; also, no pumps or nozzles are required, reducing energy use. These systems are simpler to design and maintain.

5.2.1. Wicking Systems

Wicking systems use porous membranes to transport water via capillary pressure. For example, Zhou et al. [94] examined the effect of using a porous membrane for water transport via capillary pressure in a dewpoint evaporative cooling device. The membrane’s automatic wicking capability ensured uniform water distribution, eliminating the need for pumps and reducing energy consumption. The system achieved a maximum temperature drop of 14.8 °C, with cooling capacities ranging from 13 to 150 W/m² under optimal conditions. Higher air velocities (0.5–2.5 m/s) led to a 3.1 °C rise in product air temperature, reducing wet-bulb and dewpoint efficiencies by 25.1% and 17.5%, respectively. Therefore, it can be stated that a system without nozzles reacts similarly at higher air velocities as a system with nozzles. The study found that the best performance occurred in high-temperature, low-humidity environments, with an optimal air-to-water volume ratio of 0.5. Long-term tests confirmed the system’s stability, with product air temperature fluctuations limited to 0.6 °C. This design reduces water consumption by 50% compared to nozzle-based systems, making it ideal for water-scarce regions. These findings suggest that automatic wicking membranes can enhance cooling efficiency while minimizing energy use.

The study by Duan et al. [66] provides data on different materials used in wicking and evaporation performance testing. Among the tested materials, cellulose/PET fiber exhibited the highest permeability and capillary rise, ensuring efficient water distribution. Coolmax fabric demonstrated similar high evaporation rates, outperforming wood pulp paper in water transport efficiency. The study found that wicking performance is primarily influenced by the permeability-to-effective pore radius ratio, with cellulose/PET fiber showing the best results. Experimental validation confirmed that water evaporation rates increase with surface temperature and airflow velocity, improving cooling efficiency. The optimized wicking system maintained stable water distribution, reducing evaporation resistance and enhancing mass transfer.

Guo et al. [95] investigated the impact of wicking systems on water transport and evaporation performance in thin porous media using Dutch Twill Weave (DTW) screens. Experimental results showed that the evaporative wicking process significantly influences heat and mass transfer, with the temperature drop caused by evaporation improving cooling efficiency. The study found that evaporative cooling reduces liquid viscosity and alters evaporation rates, affecting wicking height and thermal performance. Ignoring the evaporative cooling effect led to an overestimation of wicking height by up to 12% and an underestimation of evaporation rate by 14%. The results emphasize the need to consider local thermal non-equilibrium effects when designing efficient wicking-based cooling systems.

Another investigation by Abada et al. [96] evaluated the impact of wicking systems on water transport and evaporative cooling performance in different fabric materials. Fabrics with high capillary rise, such as fiber-based textiles, demonstrated superior moisture-wicking ability, improving water distribution and evaporation efficiency. Compared to Kraft paper, the best performing fabrics showed 160% to 355% higher absorbency, leading to enhanced cooling effects. The vertical wicking height of certain fabrics reached up to 27.8 cm within 120 min, ensuring continuous moisture supply to the evaporative surface. Faster moisture diffusion rates prevented dry spots, while optimized evaporation properties enhanced cooling capacity. The study concluded that fiber fabrics with high capillary action and diffusion properties are ideal for evaporative cooling systems.

5.2.2. Gravity-Fed Systems

Gravity-fed systems are based on water stored in an upper reservoir and dripped into distributors which evenly spread the water across the heat exchanger surface. The study by Duan et al. [97] examined the performance of a counter-flow regenerative evaporative cooler using a gravity-based water transport system. The results showed that wet-bulb effectiveness ranged from 0.55 to 1.06, while the energy efficiency ratio (EER) varied between 2.8 and 15.5. Water evaporation rate was found to accelerate with higher inlet air velocity, improving overall cooling efficiency. Feed water temperature had a negligible impact, with effectiveness decreasing by only 5% when the temperature increased from 18.9 °C to 23.1 °C. As a water pump in this system is used just to transport water to the upper reservoir, energy consumption is significantly lowered.

5.3. Water Flow Rate

During water system design, the water flow rate is the first parameter to consider, as IEC performance depends primarily on flow rate and only marginally on nozzle number and size [91]. A higher water flow increases the wetted heat exchanger area. This leads to a larger amount of evaporated water and to a higher cooling capacity [98]. Similar results were presented by Sun et al. [46], where the coverage ratio improved with increased water flow, regardless of the types of nozzles.

The relation between coverage ratio and water flow is more important at lower flow rates [46]. IEC performance is particularly sensitive to water flow rate variations at low flow rates [98]. For example, heat exchangers coated with a novel hydrophilic lacquer demonstrated higher wet-bulb effectiveness compared to those with standard epoxy coatings, especially at low flow rates [99].

These results suggest that increasing water flow improves cooling performance up to a point. Beyond, further increases do not significantly enhance cooling capacity and may lead to unnecessary energy consumption by the water pump (if present in the system).

In terms of non-constant water spraying, the water flow must also be adjusted to the inlet air parameters, device construction, and the desired result.

5.4. Spraying Control Methods

Water can be supplied to the system in a continuous, intermittent, or adaptive way. Continuous spraying is the most popular as it does not require any control system. The water is supplied during operation with the set flow rate. In the case of intermittent water spraying, the topic is presented in Figure 4. The operation of the water system is cyclic and there is a set time for spraying and pausing. In the other case, the next spraying cycle can be triggered by a predefined temperature threshold (e.g., +0.5 °C) [49,51]. This can become an adaptive method for spraying the heat exchanger using real-time feedback from the device. A general division is described in

Table 6. Division of spraying control methods.

Method	Description	Advantages	Limitations
Continuous	Uninterrupted water spray.	Stable cooling output.	High water/pump energy use.
Intermittent	Cyclic operation (spraying time and pause time).	Reduces water use.	Complex control systems required.
Adaptive	Control based on real-time humidity/temperature feedback.	Optimizes water–energy trade-offs.	Requires advanced sensors/software.

.

In summary, non-constant water spraying should be further investigated as a promising way to reduce energy and water consumption and to increase the overall performance of evaporative cooling devices.

5.5. Other Considerations

Techniques like rolling dot matrix twills, groove lines, or supplementary water distribution grids enhance water retention [28]. In order to supply water of required quality, water softeners are proposed to prevent scale buildup in long-term spray systems [89]. Water sources used for spraying can be connected to tap water [89] but also to some alternatives like condensed water from existing air handling units, which can result in additional water savings [100]. Khalid et al. [101] tested ice-cooled feed water and found that wet-bulb and dewpoint effectiveness decreased by less than 5% as water temperature rose from 20 °C to 24 °C. Consequently, it can be stated that feed water temperature minimally impacts wet-bulb or dewpoint effectiveness [97]. While evaporative cooling relies on significant amounts of water [102], regional water scarcity and local legislation may limit its adoption [28], especially in terms of the impact of climate change on water supply systems [103,104]. However, there is a promising way to gather rainwater [105,106] that can be used in EC systems [107,108].

Table 7. Comparison of IEC systems and traditional HVAC.

Factor	IEC Systems	Traditional HVAC
Water Use	Medium to High	Low
Energy Use	Low	High
Carbon Footprint	Low	High
Water Scarcity Impact	High in Arid Regions	Low

compares factors like water use, energy use, carbon footprint, and water scarcity impact between IEC systems and traditional HVAC [28,100,102].

6. Future Directions

• Optimization of intermittent spraying in non-porous heat exchangers. Although research has started exploring intermittent water spraying, most investigations have focused on porous heat exchangers (Figure 4). Future work should (a) evaluate the dynamic behavior of intermittent spraying on non-porous surfaces, (b) optimize spray–pause cycles under various ambient and operational conditions, and (c) compare performance metrics—such as COP and wet-bulb effectiveness—with those in porous configurations. This will help validate if minor modifications in the water supply strategy can yield significant energy savings without replacement of the heat exchanger.
• Integration with advanced material and coating technologies. The earlier sections highlighted the importance of surface wettability and water storage capacity in maintaining an effective water film (as discussed in Section 4). Further research is needed to (a) develop and test novel materials or coatings that combine high wettability with prolonged water retention, (b) ensure long-term stability under intermittent spray conditions, and (c) mitigate microbial risks through surface treatments. These studies should provide evidence and data to create heat exchangers that can sustain efficient cooling during the spray-off intervals.
• Adaptive control strategies based on real-time feedback. As discussed in Section 5.4, adaptive water spraying modes offer promising avenues for optimizing energy and water consumption. Future research should (a) refine sensor-based control systems that dynamically adjust spraying cycles in response to real-time temperature and humidity changes, (b) simulate the relation between adaptive strategies and different heat exchanger geometries, and (c) validate these models in full-scale systems. This approach will integrate the operational strategies with material performance, ensuring that the cooling system responds to varying environmental conditions.
• Utilization of alternative water sources and sustainable energy integration. In light of the discussions in Section 5.5 about the environmental impacts and challenges related to water scarcity, future research should (a) assess the feasibility and performance implications of using alternative water sources such as rainwater or reclaimed water, and (b) explore the integration of renewable energy—such as solar power—to sustainably run water pumps and control systems. This research direction will align the technological advancements with broader sustainability goals.

In summary, researchers have made good progress in making evaporative cooling systems more efficient and environmentally friendly, but there is still a lot of potential for further improvement. By focusing on smarter water spraying, better materials, and renewable energy, the next generation of cooling systems can save even more energy and water. This will not only help meet the growing need for effective cooling solutions but will also support wider efforts to reduce energy use and protect our water resources.

7. Conclusions

This review examines indirect evaporative cooling systems, especially when they use intermittent water spraying instead of a constant flow. The findings show that using water in short timed intervals can save both energy and water while keeping the cooling effective. This method helps reduce the amount of time the water pump needs to run, which cuts down on power use. For instance, studies have shown that intermittent spraying can reduce water pump operation time by up to 95.2% compared to continuous spraying while maintaining cooling efficiency.

The heat exchanger material has a great impact on overall performance. Materials that easily spread water and hold onto it help form a thin, even layer of water on the surface. This thin film is key to making the cooling process work well. Studies indicate that hydrophilic coatings, such as TiO₂ and graphene, improve wet-bulb efficiency by 9.91% and 12.07%, respectively, while enhancing the coefficient of performance (COP) by up to 7.69%. If these materials can keep water for longer period of time, the cooling effect lasts even during the off periods when the spray stops. For example, porous heat exchanger materials allow for extended non-spraying durations of up to 2410 s, further optimizing cooling performance.

Another important point is the need to balance good cooling with keeping the system clean. While a very wet surface can improve cooling, it can also encourage the growth of bacteria if the system is not properly maintained. Coatings that help stop bacteria from sticking to the surface could be a solution.

The way water is sprayed also has a big impact. Whether the system uses traditional nozzles, gravity-fed water, or materials that pull water along by themselves, each method affects how evenly the water is spread and how well the system cools. Well-designed spray patterns can help drop the air temperature faster and make the system work more efficiently, while some alternative methods may use even less energy. Studies have shown that using optimized nozzle configurations can increase system COP by up to 16%.

The most important finding considers water use. Implementing intermittent water spraying can lower water consumption. This is a promising affordable cooling technology that will not use excess water in regions with water scarcity.

Overall, this review shows that although current evaporative cooling systems are already more eco-friendly alternatives to traditional air conditioning, there is still great potential for improvement. By combining smart, timed water spraying with improved heat exchanger materials, these systems can become even more efficient and cost-effective.