High temperature evaporative cooler utilizing boiling suppression at water’s boiling point

Abstract

We experimentally demonstrate boiling suppression in a water film confined within superhydrophilic hierarchical nano/microstructured surfaces, enabling intense interfacial steam generation without bubble formation at the boiling point. Leveraging this phenomenon, we develop a high-temperature dew point evaporative cooler capable of reducing hot airflow from 437 °C to below ambient temperature. The cooler also performs effectively below the boiling point, lowering inlet air from 43 °C to 16.7 °C. These findings open pathways for potential practical applications of dew point evaporative cooling in power generation, internal combustion engines, and generative AI systems.

The authors demonstrate boiling suppression in a water film confined within nano/micro-structured surfaces, enabling steam generation at the boiling point without bubble formation. Leveraging this discovery, they develop a dew-point evaporative cooler capable of reducing hot air from 437 °C to 23 °C.

Introduction

Coal-fired power plants and transport powered by fossil fuels are the major sources of environmental pollutants (CO, CO₂, NO_x, SO₂, and O₃), which alter Earth’s climate^1–4. Thermal power plants (coal-fired and nuclear) use a huge amount of water for cooling and steam generation, leading to a serious aggravation of the global water scarcity, caused by population growth and climate change⁵. Energy, water, and climate constitute the basis for human society’s sustainability. Therefore, the interrelation between energy, water, and climate, known as the energy-water-climate nexus, is presently an active research area⁶. Recent studies show that the present-day skyrocketing rise in energy/water consumption by the generative AI industry also leads to serious energy/water/environment concerns^7–10, making the generative AI industry, in addition to the power generation and transportation industries, an important factor in the energy-water-climate nexus. The increase in the electricity generation efficiency of thermal power plants, the improvement in fuel efficiency in transportation, and the reduction of energy/water consumption in generative AI systems are critically important ways to address the global energy/water/environment challenge. Many existing and emerging technologies related to energy efficiency in these three fields are intrinsically based on water evaporation. In particular, dew point (DP) evaporative cooling systems, which cool down to the dew point temperature, offer significant advantages in cooling performance over traditional evaporative cooling devices capable of cooling down only to the wet-bulb temperature, which is fundamentally higher than the dew point temperature. For example, theoretical studies demonstrate that the application of DP evaporative cooling systems in steam turbine power generation essentially enhances the efficiency of coal-fired and nuclear power plants^11–13. Furthermore, the application of DP coolers in gas turbines considerably enhances electricity generation efficiency and reduces pollution emissions from gas turbine power plants^14–20. It has also been demonstrated that the integration of DP technology into a conventional combined cycle composed of a topping gas turbine (Brayton cycle) and a bottoming steam turbine (Rankine cycle) provides the enhancement of thermal efficiency by 6%²¹. The point of great importance is the applicability of the DP technologies to both future gas turbine power plants fueled by H₂ and future steam turbine power plants exploiting thermal energy from nuclear fission (thorium fuel cycle and fast neutron reactors), nuclear fusion (tokamak and stellarator), and laser nuclear fusion, providing both fuel and water savings. Another important environmental and economic problem is the energy efficiency in cars, trucks, buses, aircraft, and ships driven by fossil fuels. Currently, about 30% of fuel energy is exhausted from internal combustion engines as waste heat. Therefore, waste heat recovery is considered to be an important and effective method for reducing fuel consumption in internal combustion engines^22–26, where DP technologies offer essential advantages^26–28. A promising feature of DP evaporative cooling is its applicability in green hydrogen-fueled internal combustion engines for reducing fuel consumption. Concerning the generative AI systems, DP evaporative cooling can provide significant energy savings in the generative AI data centers, where about 30% to 45% of the total consumed energy is used for cooling^29–31.

Currently, the low-temperature (<100  °C) DP cooling technologies quickly expand in the air conditioning industry due to their essential (10-fold) energy savings compared with mechanical vapor compression cooling systems^{14,26,32–35}. However, the high-temperature (≥100  °C) DP cooling technologies remain at the stage of theoretical developments, and their practical implementations are not yet realized despite their very attractive opportunities^26,32. The absence of practical high-temperature DP cooling systems is caused by a fundamental scientific challenge of overcoming the boiling barrier in creating the wicking materials, the surface of which remains wet under extreme conditions of high-temperature (100–500  °C) airflows at which the nucleate boiling onset in the wicking materials results in unstable and inefficient evaporative performance due to vapor bubble bursts, which destroy the surface water film integrity and cause the ejection of tiny microdroplets, leading to the rapid formation of dry-out spots and failure of DP devices.

In this work, we experimentally find the effect of boiling suppression in a water film confined in superhydrophilic hierarchical surface nano/microstructures, which gives rise to intense non-boiling (bubbleless) steam generation at the water boiling point through the “thin film” evaporation mechanisms^36–42. Based on this finding, we developed an interfacially engineered nano/microstructured material for high-temperature DP heat and mass exchangers (DP-HMXs). Using this material, we built a pioneering high-temperature DP evaporative cooler that provides cooling of a hot airflow from 437  °C to a temperature even below room temperature. The fabricated DP evaporative cooler also demonstrates excellent cooling performance at T < 100  °C, decreasing the inlet airflow temperature of 43  °C to 16.7  °C at the cooler exit. This extends its applications to the air conditioning sector, where energy savings and reductions in environmental impact are critically needed⁴³.

Results

Fabrication and morphological characterization of superhydrophilic hierarchical surface nano/microstructure

In this work, we fabricate a long-term stable superhydrophilic hierarchical surface nano/microstructure on the surface of Ti/Al/V alloy using femtosecond laser processing^44–47. Specifically, we fabricate an array of microgrooves, the surface of which is textured with nanostructures and fine microstructures (see the Method section and Supplementary Note 1 for the laser fabrication procedure and technique for rendering long-term stable superhydrophilic properties). The optical image acquired with 3D laser scanning microscopy demonstrates the fabricated surface structure in Fig. 1a (see the Methods section for details on the surface characterization). The period and depth of the microgrooves are 100 and 62 μm, respectively. The ordered and disordered nano/microstructural textures covering the surface of the valleys, walls, and ridges of the microgrooves are illustrated with the images obtained by scanning electron microscopy (SEM) in Fig. 1b–k. The structural texture on the walls includes ordered/disordered fine microstructures in a range of about 1–20 μm (Fig. 1b) and nanostructures (Fig. 1c). The typical structural feature on the surface of valleys is funnel-like microcavities (Fig. 1d,h) formed due to the keyhole effect, which occurs in multipulse laser ablation⁴⁸. The surface of the funnel-like microcavities is extensively structured with both disordered fine microstructures/nanostructures and ordered laser-induced periodic surface structures (LIPSS) (Fig. 1h). The structural texture on the surface of the ridges consists of irregular micropillars of various dimensions in a range of about 1–15 μm (Fig. 1e). The surface texture of the micropillars includes both fine disordered structures (Fig. 1e) and periodic structures (Fig. 1f,g,j,k), which are covered with disordered fine nanostructures with dimensions down to about 7 nm (Fig. 1k). The areas between the ridge micropillars are also textured with both ordered and disordered nanostructures (Fig. 1i). Figure 1 shows that the fabricated capillary structure is extremely hierarchical with a large number of LIPSS-textured patches, where the LIPSS period ranges from 0.1 to 5 µm. The mechanisms of the formation of the disordered and ordered surface nano/microstructures following the laser ablation have been extensively studied in the past and reviewed in refs. ^{44–46,49–52}. Overall, our hierarchical structure includes structural features in a range between 7 nm and 100 µm, covering four orders of magnitude in length scales. Thus, the fabricated structure gives rise to both microwicking, which transports a large amount of water for a long distance, and nanowicking, which transports a small amount of water for a short distance but at a speed much higher than that of microwicking.

Fig. 1. Characterization of surface structure morphology.

a 3D optical image of the array of parallel microgrooves on the surface of the Ti/Al/V alloy. b, c SEM images of patches textured with laser-induced periodic surface structures on microgroove walls. d SEM images of funnel-like cavities textured with laser-induced periodic surface structures on the valley bottom. e Textured micropillars on the microgroove ridge. f, g Area textured with laser-induced periodic surface structures on the ridge edge. h Ordered/disordered texture on the surface of funnel-like cavities on the valley bottom. i Area textured with laser-induced periodic surface structures between the ridge micropillars. j, k Nanostructural texture of the area with laser-induced periodic surface structures on the microgroove ridge.

Effect of non-boiling interfacial steam generation at the boiling point

The non-boiling interfacial steam generation at the boiling point of water was observed in our experiments on the capillary flow dynamics on a hot (>100  °C) surface of the fabricated surface structure, utilizing a similar experimental method as in works^53,54. In this method, we track the water flow dynamics by high-speed [1000 frames per second (fps)] video recording of the water spreading following the supply of a 15-μl water droplet to the sample edge. From the obtained video recording, we derive the water spreading distance h and the spreading velocity v as a function of time t. The water spreading velocity is obtained as a numerical derivative Δh/Δt, where Δh is the difference of spreading distance between two consecutive video frames and Δt = 10⁻³s is the time between two consecutive video frames. The spatiotemporal behavior of the temperature in wet and dry areas of the sample surface is captured using an infrared (IR) camera with an operating spectral range of 7.7–10 μm and a speed of 100 fps (see Supplementary Note 2 and Supplementary Figs. S1,S2 for more details on the experimental setup for high-speed optical and IR imaging of capillary flow and calibration procedure of IR camera temperature measurements). Figure 2 demonstrates the experimental results acquired at the sample temperature of 230  °C. Figure 2a reveals that the overall water film behavior includes two stages, namely the spreading stage, where h increases with time due to the dominance of the capillary effect, and the receding stage, where h decreases owing to the dominance of the evaporation effect. The plot of the spreading distance as a function of time in the spreading regime (Fig. 2a) shows that the fabricated surface structure provides good high-temperature wicking properties characterized by a very large maximum water spreading distance of 10.3 mm at t = 600 ms, which is larger than that previously reported in^53,54 at high temperatures. At a low temperature (23  °C), the spreading distance of our sample is 22 mm at t = 600 ms (see the inset i1 in Fig. S3a in Supplementary Note 3). For comparison, the low-temperature spreading distance at t = 600 ms of the best wicking materials reported in the literature is between 17 and 24 mm^55–58. The detailed h(t) and v(t) dependences in the time domain of 0–100 ms (Fig. 2b,c) along with optical Snapshots Opt.1 in Fig. 2a,b demonstrate that both the initial water front acceleration on the sample surface and the water boiling onset occur during the first millisecond after the water drop touches the sample. The boiling onset is identified by blurring of the water film image due to bubble bursts (see Snapshot Opt.1 in Fig. 2b). At 2 <t < 44 ms (Fig. 2b), the water spreading follows the inertial regime similar to that observed in⁴⁷, where the water spreading distance is a quasilinear function of time (h ∝ t) with three linear spreading substages 1,2,3 marked by black, green, and blue lines. The maximum water spreading velocity reaches 248.6 mm/s (Fig. 2c), which is very large despite the high dry surface temperature of 230  °C. Snapshots Opt. 2–4 in Fig. 2b and Opt.1 in Fig. 2c illustrate the water dynamics in the inertial regime. At t = 20 ms, the entire water droplet on the sample surface boils (see Snapshot Opt. 2 in Fig. 2b) and its temperature is in a range of 97.6–100.1  °C (see infrared Snapshot IR1 in Fig. 2c). Figure 2d with its insets i1 and i2 demonstrates the overall v(t) dependence in the spreading and receding stages. The v(t) dependences in Fig. 2c,d show that after the inertial regime, which ends at 44 ms, the velocity undergoes a rapid drop in an exponential manner, which lasts up to about t = 250 ms (see Fig. 2d). Previously, it has been demonstrated that this rapid drop of the velocity is associated with the visco-inertial flow regime, which follows the inertial one⁵⁹. Thus, the lifetime of the visco-inertial flow regime in our experiment is between 45 and 250 ms. Infrared Snapshots IR2,3 captured in the visco-inertial regime shows that the water temperature is between 100.2 and 103.1  °C. Optical Snapshots Opt. 2 in Fig. 2c and Opt.1 in Fig. 2d illustrate the water spreading and boiling in the visco-inertial flow regime, where the entire bulk of water undergoes very intensive boiling with ejection of microdroplets. Infrared Snapshots IR2 and IR3 in Fig. 2a,c captured in the visco-inertial regime shows that the water temperature is between 100.2 and 103.1  °C (color temperature scale is shown in Fig. 2e). Snapshots Opt. 2 and Opt. 3 in Fig. 2d along with IR4 and IR5 demonstrate the final phase of boiling, which terminates at t ≈ 750 ms when the air-water interface (AWI) descends below the microgroove ridges and becomes curved by menisci formed between the microgrooves. The water temperature at this moment is about 103  °C. At t > 750 ms, the water behavior is completely governed by non-boiling steam generation (see optical Snapshots Opt.4–6 in Fig. 2d) despite the water temperature being at the boiling point (see infrared Snapshots IR6-IR8 in Fig. 2d and color temperature scale in Fig. 2e). The time domains of boiling and non-boiling steam generations are indicated in Fig. 2d. The water film boiling mainly occurs in the water-spreading stage and partially in the beginning of the receding stage (see Fig. 2a). The onset of the receding stage occurs at 596 ms (Fig. 2a). Initially, the receding velocity increases slowly with time (see the overall plot of the spreading/receding velocity of the water front as a function of time in Fig. 2d). At t > 1266 ms, the receding velocity begins to increase quickly (see the inset i2 and Snapshots Opt. 5 and Opt. 6 in Fig. 2d), achieving about 80 mm/s at the end of the evaporation. Overall, the results obtained at 230  °C demonstrate very strong capillary action of the created material in terms of the large maximum spreading distance (10.3 mm) and very high maximum spreading velocity (about 250 mm/s), even under conditions of intensive boiling. Our most important finding is the suppression of boiling in a water film confined in the hierarchical surface nano/microstructure that gives rise to the non-boiling steam generation at the boiling point.

Fig. 2. Dynamics of spreading, boiling, and evaporation of the water drop at 230 °C.

a The overall spreading distance h as a function of time t along with optical (Opt.) and infrared (IR) snapshots. b Detailed h(t) dependence at 0 <t < 100 ms along with optical (Opt.) and infrared (IR) snapshots, where three linear spreading substages 1,2,3 are marked by black, green, and blue lines. c The spreading velocity v as a function of time at 0 <t < 100 ms, along with optical (Opt.) and infrared (IR) snapshots. d The overall v(t) dependence along with optical (Opt.) and infrared (IR) snapshots. e Color temperature scale of infrared images IR1-IR8. Note: The white dashed line in Snapshots IR1-IR8 shows the location of the wet area.

To gain insight into the effect of the boiling suppression at t > 750 ms, we present the more detailed experimental data for the time domain of the transition from the boiling to the non-boiling steam generation in Fig. 3, where the optical snapshots taken with a standard lens are complemented with snapshots captured using a microscope lens. Figure 3a shows that at t = 610 ms and a water temperature of 102.6–103.4 °C, the boiling occurs in a thick water film (TkWF) when the air-water interface (AWI) is above the microgroove ridges. However, the boiling doesn’t occur at the water temperature of 103.5–103.9 °C in a thin water film (TnWF) when the AWI is below the microgroove ridges and is curved by menisci (Fig. 3b), giving rise to a non-boiling steam generation at the boiling point. Due to evaporation, the AWI descends with time and almost reaches the microgroove bottom at t = 850 ms (Fig. 3c). The bubble formation is also not seen at this time, despite the water temperature of 103.4  °C, and the non-boiling steam generation at the boiling point continues to occur. At t = 950 ms, the water becomes confined in the structural texture on the surface of microgrooves (Fig. 3d), forming an ultrathin water film (UTnWF) at a temperature of 103.5 °C. The UTnWF does not support boiling as well, providing a continuation of the non-boiling steam generation at the boiling point until the end of the evaporation (Fig. 3e). Figure 3f demonstrates the color temperature scale of the infrared images in Fig. 3a–e. Thus, the data presented in Figs. 2 and 3 show that the boiling occurs only in sufficiently thick water films. A recent theoretical model of nucleate boiling in microporous wicking structures has shown that the critical liquid film thickness ${\delta }_{f,c}$ for supporting the boiling is given by⁶⁰

$${\delta }_{f,c}=\frac{4\sigma T_v\left(1+\cos \theta \right)}{{\rho }_v{h}_{fg}\left(T_w-T_v\right)}\left(1+\sqrt{1+\frac{k_p{T}_w{\rho }_v{h}_{fg}\left(T_w-T_v\right)}{2\left(1+\cos \theta \right)\sigma h_i{T}_v^2}}\right)$$ 1

where $\sigma$ is the surface tension, $T_v$ is the temperature of the bulk vapor, θ is the contact angle, ρ_v is the vapor density, $h_{fg}$ is the latent heat, $T_w$ is the temperature of the wall, $k_p$ is the effective thermal conductivity of the microporous wick filled with a liquid, and $h_i$ is the heat transfer coefficient dependent on vaporization at the liquid-vapor interface. For a mesh porous wicking structure, Eq. (1) predicts that the minimum liquid film thickness to support the boiling is about 100 µm. To gain insight into the effect of boiling suppression in our superhydrophilic structure, we calculate ${\delta }_{\mathrm{f},\mathrm{c}}$ as a function of the superheat (T_w – T_v) using Eq. (1) (see Supplementary Note 4 for details). The calculated plot of ${\delta }_{\mathrm{f},\mathrm{c}}$ as a function of the superheat (see Supplementary Fig. S4) shows that for the critical water film thickness of 62 µm observed in our experiment, the superheat is 30  °C. At this superheat value, the nucleate boiling is commonly observed⁶¹, but it is suppressed in the water film with a thickness of ≤62 µm confined in our superhydrophilic structure. Therefore, the transition from the boiling regime to the non-boiling one observed in our study can be explained by a small water film thickness (≤62 μm) at t > 750 ms.

Fig. 3. Transition from boiling to non-boiling steam generation at the boiling point.

a The boiling in a thick water film with an air-water interface above the microgroove ridges at a water temperature of 102.6 °C. b The non-boiling steam generation in a thin water film confined between the microgrooves at a water temperature of 103.5 °C. c The non-boiling steam generation in a final stage of the thin water film confined between the microgrooves at a water temperature of 103.4 °C. d The non-boiling steam generation in an ultrathin water film confined in microgrooves’ surface nano/microtexture at a water temperature of 103.5 °C. e The dry surface of the microgrooves after complete water evaporation. The IR snapshot reveals a significant evaporative cooling effect. f The color temperature scale of infrared images presented in Fig. 3a–e.

Stationary non-boiling steam generation at the boiling point

The non-boiling steam generation regimes shown in Fig. 3b–d are transient due to the water supply from a droplet and can be used in evaporative cooling systems with a spray water supply. In this section, we experimentally study a stationary non-boiling steam generation regime, which we realize using the water supply from a reservoir, as shown in Fig. 4a. In this experiment, the back side of the sample is heated with a system of a heater and a temperature controller to provide stability of the preset heater temperature during the experiment. The temperature of the wet surface is measured with a thermocouple and an infrared camera. The temperature of the dry sample surface is measured with a thermocouple, as shown in Fig. 4a. The AWI is visualized using an optical video camera with a microscope objective. To measure the evaporation rate, we place the water container on a balance and record its mass change with time. The projected wet surface area is derived from the optical camera video recording of the sample surface. More details about the experimental setup are given in the Method section. Figure 4b–j demonstrates the optical images of the sample and AWI along with the IR snapshots at various fixed temperatures of the dry sample surface in a range between 23 and 250  °C. These optical images do not detect boiling in the wet area at the dry surface temperatures ≥100  °C. The optical images in Fig. 4b–j show the formation of TnWFs, which suppress the boiling when the dry surface temperature is ≥100  °C, as illustrated in Fig. 3b,c. As seen in Fig. 4b–j, as the dry surface temperature increases, the TnWF thickness decreases. Also, the TnWF thickness decreases with increasing distance from the water surface in the container; the larger the distance, the thinner the TnWF. As seen in the infrared camera snapshots in Fig. 4b–j, the wet surface temperature is always smaller than the dry surface one, due to the cooling effect caused by evaporation. The dependences of the wet surface temperature as a function of the dry surface temperature obtained at the same point of the wet area using a thermocouple and IR camera (see Fig. 4k) show that as the dry surface temperature rises to 250 °C, the wet surface temperature increases only to 100  °C. To gain more insight into the non-boiling steam generation, we measure the evaporation rate as a function of the dry surface temperature (see Fig. 4l), which shows that the evaporation rate begins to rise steeply as the dry surface temperature reaches 100 °C, indicating a behavior similar to the boiling phenomenon, where the increase in heat supplied to water at the boiling point does not increase the water temperature but enhances the liquid-vapor phase change. As seen in Fig. 4l, the evaporation rate of the TnWF confined in the hierarchical surface nano/microstructure is very high, reaching about 7000 µg cm⁻² s⁻¹ at the dry surface temperature of 250 °C. The same evaporation rate measurements performed 7 months later show that the evaporative performance degrades a little with time (see Fig. 4l), demonstrating long-term stable evaporative performance of the created material. Furthermore, to study the repeatability of the evaporative properties of the created material, we fabricated a new sample using the same laser processing parameters and measured its evaporation rate one week after fabrication. As seen in Fig. 4l, the evaporative properties of both samples are close to each other, demonstrating good repeatability. A serious problem in designing metallic wicking materials is the rapid deterioration of their hydrophilic properties due to the adsorption of volatile hydrophobic compounds from the surrounding air. For instance, the metals treated with a femtosecond laser are typically superhydrophilic directly after laser processing, but they turn superhydrophobic after being exposed to ambient air for a few days to several months^62–65. To prevent the rapid degradation of the superhydrophilic properties and ensure long-term superhydrophilicity stability, we heat the laser-treated Ti-6Al-4V sample to 150  °C and spray water droplets on its hot surface for 30 min to form a hydrophilic aluminum oxide hydroxide [γ–AlO(OH)] film known for its long-term stable hydrophilic properties⁴⁸ (see Supplementary Note 1 for details). In our study, to test the long-term stability of the superhydrophilic properties, we use an experimental technique where both the water contact angle θ and the time t₀ to achieve a static θ ≈ 0 ° are measured after a water drop deposition on the sample surface (see Supplementary Note 1 for details). Figure 4m shows that after 4 hours following the sample treatment, θ rapidly approaches zero (t₀ ≈ 140 ms). After 6.5 months, the sample remains superhydrophilic (θ ≈ 0 °), albeit t₀ extends to about 440 ms, indicating a small decline in the superhydrophilic/wicking performance. Thus, our material retains its superhydrophilic properties for a long time, in contrast to other laser-textured Ti-6Al-4V materials, which typically become hydrophobic or even superhydrophobic in 1–6 weeks^64,65. Thus, the created material demonstrates the long-term stable superhydrophilicity and the long-term stable stationary suppression of water boiling at the boiling point over a large surface area, providing a very large steam generation rate. In conclusion, it is worth noting that the superhydrophilicity is an important property of our wicking material, but not a key one as in other wicks of the DP-HMX. In contrast to other wicks, the key property of our material is the boiling suppression.

Fig. 4. Stationary non-boiling steam generation in a temperature range of 23–250 °C.

a Experimental setup. b–j Optical images of the sample, microscopic optical images of the air-water interfaces, and associated infrared (IR) snapshots at various fixed temperatures of the dry sample surface. k Plots of the wet sample surface temperature as a function of the dry sample surface temperature measured with the thermocouple and infrared camera. l Plots of evaporation rate as a function of the dry sample surface temperature. m The water contact angle θ as a function of time and the time t₀ to achieve a static θ ≈ 0°, measured at 4 h and 6.5 months after sample treatment. Note: The error bars represent the standard deviation. b–j Scale bar is 100 μm.

Performance of the created material at high-temperature airflows

The DP evaporative cooling systems for enhancing energy efficiency in power plants, internal combustion engines, and generative AI systems operate under conditions of a hot airflow. To gain insight into the potential of the created material for applications in the DP cooling technologies, we study its evaporative functionality using a wind tunnel setup (See Fig. 5a), where a video camera with a microscope objective and an infrared camera are utilized for tracking the AWI shape and the wet surface temperature, respectively. To find the evaporation rate of the sample, we measure the mass change of the water in the container with a balance (see the “Method” section for more experimental details). The plots of the evaporation rate as a function of airflow velocity at various airflow temperatures (140, 170, and 200  °C) show a significant enhancement in the evaporation rate with increasing the airflow velocity from 0.5 to 13 m/s (Fig. 5b). The photos of the sample in Fig. 5c–e demonstrate the dry-out spots formed in the upper right corner of the sample. The microscopic optical images of the AWI in Fig. 5c–e demonstrate the formation of a stable TnWF confined in the surface structure even at the airflows with high both temperature and velocity. An analysis of these images shows some reduction in the TnWF thickness with increasing temperature and velocity of the airflow. At high temperatures and velocities of the airflow, a dry-out spot forms in the upper right corner of the sample (Fig. 5c–e), revealing the area with insufficient capillary water supply. The infrared images in Fig. 5c–e show that the temperature of the wet surface is considerably smaller than that of the airflow. For example, at the airflow temperature of 200  °C and velocity of 8 m/s, the temperature of the wet surface is only 70–75  °C (see Fig. 5e). In overall, the wind tunnel experiments show excellent evaporative and wicking performances of the created material under conditions of the high-temperature/high-velocity airflows. An important finding is the high stability of the material performance under these conditions. The obtained results indicate a great potential of the created material in developing the DP-HMX for the efficiency enhancement in the power generation technologies and fuel saving in the internal combustion engines.

Fig. 5. Performance of the created material at high-temperature airflows.

a Wind tunnel setup. b Plots of the evaporation rate as a function of the airflow velocity at various temperatures. c–e Optical images of the sample, microscopic optical images of the air-water interfaces, and associated infrared (IR) snapshots at various temperatures and velocities of the airflow. Note: The error bars represent the standard deviation. c–e Scale bar is 100 μm.

Design, fabrication, and performance of high-temperature DP cooling prototype

The design of a high-temperature DP cooling device is shown in Fig. 6a–c. We use a counterflow DP-HMX configuration^35,66,67, which is schematically illustrated in Fig. 6b for a single pair of dry/wet channels. The fabricated DP cooler consists of an inlet air chamber, an HMX, an outlet air chamber, and an exhaust humid air chamber. The vertical sliding plate of the outlet air chamber regulates the airflow fraction redirected into the wet channels (Fig. 6a). The fraction of the airflow redirected into the wet channels can be adjusted by the height H of the outlet air chamber opening shown in Fig. 6a. For example, when this sliding plate completely closes the exit of the outlet cold air chamber, 100% of the air from the dry channels will be redirected into the wet channels. In our DP-HMX, the optimal ratio of the outlet airflow to the wet channel airflow is 0.28. The horizontal sliding plate in Fig. 6d adjusts the opening of the wet channels for optimal cooling performance. In contrast to the low-temperature counterpart reported in ref. ³⁵, our high-temperature HMX consists of two sections with 20 wicking plates each. Each of these sections of the wicking plates pumps water into the wet channels from an individual water reservoir. The two-sectional configuration of HMX is used to reduce the potential thermal deformations of the wicking plates at high temperatures. More details of the HMX design are presented in Supplementary Note 5. To characterize the performance of the fabricated DP cooling device, we measure the temperature, velocity, and humidity of the air at characteristic points of the device (Fig. 6d). The temperature T₁, velocity v_i, and relative humidity RH_i of the inlet air are measured with a thermocouple TC1, an anemometer probe, and a humidity probe, respectively, as shown in Fig. 6d. To track the decrease in the temperature of the air as it passes through a dry channel, we use the thermocouples TC2, TC3, and TC4, which measure temperatures T₂, T₃, and T₄ at their locations (see Fig. 6d), respectively. The thermocouples TC5 and TC6 measure the temperatures of the outlet cold air T₅ and exhaust humid air T₆, respectively. The photo of the experimental setup is demonstrated in Fig. 6e.

Fig. 6. High-temperature dew point (DP) cooling device.

a Overall design. b Configuration of counterflow dew-point heat and mass exchanger. c Design of the dew-point heat and mass exchanger. d Locations of thermocouples (TC1–TC6) and probes (anemometer and humidity). e A photo of the experimental setup. f Outlet air temperature (T₅) versus inlet air temperature (T₁) at various airflow velocities v_i.

The cooling performance of the fabricated device is investigated in a range of the inlet air temperatures of 24–437  °C at fixed airflow velocities of 0.9, 1.2, 1.6, and 2.0 m/s. The obtained plots of the outlet air temperature as a function of the inlet air temperature at various airflow velocities (Fig. 6f) show that, overall, the best cooling performance is observed at v_i = 0.9 m/s, although somewhat better cooling performance is achieved at v_i = 1.2 m/s in an inlet air temperature range below 100 °C. For example, at the inlet temperature of 26.4 °C, the outlet air temperature is 18.3 and 15.4 °C for the airflow velocity of 0.9 and 1.2 m/s, respectively. The increase in the airflow velocity above 1.2 m/s essentially degrades the cooling performance, especially at the inlet air temperature above 100 °C. Thus, the optimal airflow velocity of our DP-cooler is found to be 0.9 m/s, which is used in our further experiments.

The recordings of temperatures T₁(t)–T₆(t) at various preset maximum inlet temperatures T₁ and fixed v_i = 0.9 m/s are presented in Fig. 7a–f, where the dependences T₁(t)–T₅(t) relate to a dry channel while T₆(t) relates to a wet channel at its exit. The exact locations of the thermocouples used to measure T₁(t)–T₆(t) are shown in Fig. 7g. The measurements of T₁(t) and T₅(t) demonstrate an extraordinarily strong cooling performance of the fabricated DP cooler in an extremely broad range of the inlet air temperatures, ranging from 26 to 437  °C. Figure 7c–d shows the cooling performance of the device in a range of the inlet air temperatures above 100 °C. As seen in Fig. 7c, the device cools the inlet air temperature of 150  °C down to 19.5  °C, which is below the room temperature (23.4  °C). Furthermore, even a very high inlet air temperature of 437  °C is reduced to 23  °C. Thus, our DP-cooler, which is built using the created wicking material capable of suppressing the boiling, successfully breaks the 100 °C barrier in the DP cooling technologies^26,32. It is important to note that the highest inlet air temperature of 437 °C achieved in our study was limited by thermal deformations of the PTFE thermoplastic components used in the DP device fabrication, but not by a failure of the wicking material performance. Therefore, the upper operational inlet air temperature can be further increased by using a better heat-resistant plastic or ceramic material. The overall length of the fabricated DP-HMX is only 150 mm (see Fig. 7g); therefore, by increasing the DP-HMX length, one can further reduce the outlet air temperature. The T₂(t)–T₄(t) recordings show the decrease of the air temperature as the air passes through a dry channel. The plots of the air temperature decrease in the dry channel as a function of the distance from the dry channel entrance (Fig. 7h) reveal a significant drop in the inlet air temperature even at a distance of only 25 mm from the HMX entrance. Another important finding is that our DP cooler demonstrates excellent cooling performance in a range of common air conditioning temperatures between 26 and 45  °C (see Fig. 7a,b), especially at v_i = 1.2 m/s (Fig. 6f). This finding extends the range of the DP-cooler applications to the air conditioning of buildings and data/big data/generative AI data centers³¹. We note that due to using the renewable evaporative energy of the atmospheric air, the DP air conditioners consume up to ten times less electric power compared with their traditional compressor-based counterparts¹⁴.

Fig. 7. Cooling performance of dew point (DP) cooler.

a–f Recordings of temperatures T₁–T₆ as a function of time at various preset inlet air temperatures for fixed airflow velocity v_i = 0.9 m/s. g Locations of thermocouples in the dew-point heat and mass exchanger to measure temperatures T₁-T₆, h Air temperature in the dry channel as a function of distance from the dry channel entrance.

Besides the very strong cooling, the outlet air temperature recordings T₅(t) in Fig. 7a-f show a very high stability of the outlet air temperature. To study the stability of T₅ in detail, we perform an experiment on the effect of large fluctuations in the inlet air temperature on the outlet air temperature. The data of this experiment (Fig. 8a,b) show that very large fluctuations of the inlet air temperature, ranging from 24.5 to 375 °C, cause very small fluctuations of the outlet air temperature in a range of only 16.5–20 °C. An important parameter in the DP cooler performance is the water evaporation rate R_e. The measurements of R_e as a function of the inlet air temperature at various fixed v_i (Fig. 8c) show a significant increase in R_e with increasing inlet air temperature and v_i. Another important parameter in the DP cooler performance is the exhausted humid air temperature T₆, which is plotted as a function of the inlet air temperature in Fig. 8d. In an ideal DP cooling device, the temperature of the humid air exhausted from the wet channels is close to the inlet air temperature of the dry channels. To estimate the DP cooler performance through the exhaust humid air temperature, we plotted the T₆/T₁ ratio as a function of T₁ in Fig. 8e. The large T₆/T₁ ratios at inlet air temperatures of 26 and 43 °C are explained by the fact that we optimized the overall DP cooler performance by varying the inlet airflow velocity at the fixed inlet air temperature of 26  °C. A significant decrease in the T₆/T₁ ratio at T ≥ 43 °C indicates that to maintain efficient performance, the airflow velocity should be adjusted in case of an operational inlet air temperature change. The observed increase in the T₆/T₁ ratio at the inlet air temperatures of 300 and 437  °C can be explained by the formation of dry-out spots on the walls of the wet channels, causing the rise of air temperature in the wet channels and, as a consequence, an enhancement of the evaporation rate due to an exponential increase in the saturated vapor pressure with the ambient temperature^68,69. As seen in Fig. 8e, the T₆/T₁ ratio is in the range of 0.43–0.54 at T ≥ 150 °C, indicating that the cooling performance of our device at high temperatures can be further improved by a proper choice of the inlet air temperature for the optimization procedure, varying the fraction of airflow directed to the wet channels, and varying the length, height, and width of the dry/wet channels.

Fig. 8. Performance of dew point (DP) cooler.

a, b Stability performance: recordings of the outlet air temperature T₅(t) and fluctuating inlet air temperature T₁(t). c Evaporation rate as a function of the inlet air temperature at various airflow velocities. d Exhaust humid air temperature T₆ as a function of the inlet air temperature. e The T₆/T₁ ratio as a function of the inlet air temperature T₁ at v_i = 0.9 m/s.

Discussion

In this study, we have shown that the boiling of water can be completely suppressed in a thin water film confined in the superhydrophilic hierarchical surface nano/microstructures, giving rise to intense non-boiling steam generation at the boiling point. Using this finding, we elaborated an advanced wicking material that provides highly efficient non-boiling steam generation. The DP evaporative cooling prototype, fabricated utilizing this wicking material, demonstrates unprecedented cooling of a hot airflow from 437 °C to a temperature even below room temperature. The successful overcoming of the existing boiling barrier in the development of high-temperature DP cooling technologies provides a platform to engineer practical DP cooling systems for enhancing energy efficiencies in the power generation, transportation, and generative AI industries.

Methods

Fabrication of long-term stable superhydrophilic surface nano/microstructure

A detailed description of fabricating the long-term stable superhydrophilic surface nano/microstructure is presented in Supplementary Note 1. Briefly, the array of parallel microgrooves is produced using a Ti:Sapphire femtosecond laser operating at a pulse length of 86 fs, a central wavelength of 800 nm, a laser pulse energy of 7.1 mJ, and a pulse repetition rate of 1 kHz. The laser beam is focused with an achromat lens onto a substrate mounted on a computer-controlled X-Y translation stage. The array of parallel microgrooves is produced by raster scanning the substrate across the laser beam at an optimized step between the adjacent scanning lines of 100 μm and a scanning speed of 1 mm/s. The substrate material is Ti90/Al6/V4 alloy, the thermal conductivity of which at room temperature is 6.7 W/(m·K)⁷⁰.

Surface morphology characterization

The 3D image and depth profile of the fabricated microgrooves were acquired using a 3D scanning violet laser confocal microscope (Keyence VK-X1100). The morphology of the hierarchical surface nano/microstructures on the surface of the microgrooves was characterized utilizing a scanning electron microscope, Sigma 300 from Zeiss.

Stationary non-boiling steam generation experiments

To visualize the AWI shape curved by menisci between microgrooves, we use an optical camera (Phantom VEO 710 L) with a microscope lens. The temperature measurements are performed utilizing an infrared camera (ImageIR 8855 from InfraTec) with a measurement accuracy of ±4% and thermocouple probes Omega TJC36-CASS-020G-6 connected to a TC-08 data acquisition module with a measurement uncertainty of ±0.5  °C. The IR camera temperature measurements were calibrated using thermocouples (see Supplementary Note 2 for a detailed IR camera calibration procedure). The temperature of the water film is measured at the same point on the sample using the thermocouple and the infrared camera. In these measurements, we first measure the temperature with the thermocouple being in contact with the sample at the sample-water interface. Then, we remove the thermocouple and measure the temperature at the point of the thermocouple location using the IR camera. The operating spectral range of our IR camera is 7.7–10 μm. Water is a highly absorptive liquid in this spectral range, and a water film with a thickness of about 65 μm almost completely absorbs the IR radiation in this spectral wavelength range, enabling the temperature measurements of thin water films. In the evaporation rate experiment, the mass change of the water container is recorded using a balance (AP135W from Shimadzu, Japan) with an accuracy of 0.01 mg. The sampling interval in the mass change recording is 10 s. The projected wet surface area is derived from the optical camera video recording of the sample surface. The ambient air temperature and relative humidity (RH) during the experiments were 23 °C and 51 ± 2%, respectively.

Wind tunnel experiments

To blow the air through the wind tunnel, we use a fan equipped with a built-in heater. The temperature and velocity of the airflow can be preset with controllers. The temperature and velocity of the airflow are measured utilizing a thermocouple probe Omega TJC36-CASS-020G-6 connected to a TC-08 data acquisition module with a measurement uncertainty of ±0.5  °C and a hot-film anemometer AR866A (SmartSensor) with an accuracy of ±1%, respectively. A humidity sensor HMP7 (Vaisala) with a measurement accuracy of 0.8%, located inside the wind tunnel, is used to monitor the relative humidity. The dimensions of the wet sample surface exposed to airflow are 20 × 24 mm². An optical camera (Phantom VEO 710 L) with a microscope lens is used to visualize the AWI shape in the top, middle, and bottom regions of the sample wet area. To find the evaporation rate of the sample, we measure the mass change of the water in the container using a balance AP135W (Shimadzu) with an accuracy of 0.01 mg. The projected wet sample area inside the wind tunnel is measured using video recording of the sample surface.

DP cooling prototype experiments

A fan equipped with a built-in heater is used to blow the air through the DP-HMX. The temperatures at various locations inside the DP-HMX are measured with thermocouple probes Omega TJC36-CASS-020G-6 and recorded utilizing an Omega 8-channel USB thermocouple data acquisition module TC-08 connected to a computer. A preset airflow temperature in a range of 24–500 °C and a preset airflow velocity in a range of 0.5–2 m/s can be varied with controllers. To measure the airflow velocity, we use a hot-film anemometer AR866A (SmartSensor), the maximum operational temperature of which is 45  °C. In experiments at T > 45 °C, the airflow velocity is first set at blowing the room-temperature air, and then the air heater is turned on. The airflow humidity is monitored with a Vaisala HMP7 probe (see Supplementary Note 6 and Supplementary Fig. S5).

Author contributions

R.F. and A.Y.V. conceived the ideas and organized the work. R.F., J.Z., and A.Y.V. designed the experimental setups. Q.C. performed experiments on the boiling suppression. Q.C. and Y.W. performed experiments on the dew point cooling prototype. Y.D., Y.L., Z.S., and Z.L. fabricated the samples and characterized structures with a 3D laser scanning microscope and SEM. Q.C., Y.L., and Z.S. performed optical and infrared imaging experiments, Y.D. performed EDS examination and analysis. Q.C. and Y.L. performed wind tunnel experiments. R.F., A.Y.V., J.Z., Z.Y., and N.P. analyzed the data. R.F. and A.Y.V. wrote the manuscript. All authors discussed the results and commented on the paper.

Peer review

Peer review information

Nature Communications thanks Alekos Ioannis Garivalis and the other anonymous reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Data availability

All data generated and analyzed during this study are included in the published article and its Supplementary Information. Source data are provided with this paper.

Code availability

The codes used in this study are available from the corresponding author upon request.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-025-67717-1.