NOVEL NONWETTING SURFACES FOR ENHANCED CONDENSER PERFORMANCE IN THERMAL POWER PLANTS

Motivation

Thermal power plants are significant consumers of water with estimates of up to 49% of the total water use in the U.S. being attributed to them (Fleischli and Hayat, 2014). As water resources become more precious due to climate change and population growth, steam condensers in power generation must adapt and become more efficient (Murrant et al., 2015; Brown et al., 2019). Power plants warm nearby rivers and coastlines, contributing to anthropogenic climate change (Liu et al., 2020). Efficiency improvements to power plant condensers used worldwide, most of which are once-through heat exchangers, could provide substantial environmental and economic benefits by reducing water consumption and the levelized cost of condensers (Nithyanandam et al., 2021).

Two factors—steam condensation on the exterior surfaces of the tubes in a condenser and fouling of the interior surfaces of tubes through which the coolant water flows—affect the thermal resistance in the condensation process and, in turn, govern the performance of steam condensers. Condensing steam tends to form a thin film of water on a surface as it liquifies. Filmwise condensation (FWC) produces a continuous barrier to heat transfer from the vapor environment to the condensing surface. On the other hand, dropwise condensation (DWC) is characterized by small droplets that shed continuously from the surface, keeping a majority of the highly conductive condenser surface clear for additional condensation. DWC in pure steam can achieve three to five times the heat transfer rate of FWC, making DWC a preferred mode of condensation. On the inside of the tubes, condensers involve flow of coolant water containing trace amounts of mineral salts that show inverse solubility with temperature. Prolonged exposure of heat transfer surfaces to such salt solution results in gradual accretion of mineral salt particles leading to the formation of larger aggregates as time progresses. A direct consequence of mineral salt fouling of heat transfer surfaces is significant reduction in overall heat transfer performance leading to adverse effects on the efficiency of the condensers. In addition, the maintenance efforts required to clean the heat transfer surfaces off foulant deposits adds to the cost of system operation and leads to disruptions to continuous operation (Hatte and Pitchumani, 2022a–c; Hatte et al., 2022).

Improving condenser performance, therefore, requires promoting DWC while mitigating fouling. Fundamentally, both requirements call for reducing the wettability of the surface to water. With this motivation, nature-inspired nonwetting surfaces have been studied extensively over the past decades. Two commonly studied bioinspired surfaces are superhydrophobic surfaces (SHS) (Jeevahan et al., 2018) and lubricant-infused surfaces (LIS) (Wong et al., 2011). The air cushion in a textured SHS that is responsible for the Cassie state of wettability and superhydrophobicity is quickly lost in a condenser environment resulting in a fully or partially Wenzel state of wettability. The inevitable wettability transition that leads to droplet pinning and surface flooding degrades condensation heat transfer on SHS. SHS are, therefore, not practical in condensation heat transfer enhancement, despite their fantastic promise and excitement. LIS obviates droplet pinning and surface flooding by incorporating a lubricant layer in place of air in the textured surface. However, they face lubricant retention challenges (Villegas et al., 2019), that render the surfaces less durable in continuously operating vigorous condensation environment.

The Achilles' heel of conventional nonwetting SHS and LIS, therefore, remains their durability challenges of wettability transition (in SHS) and lubricant depletion (in LIS) that deteriorate their nonwetting characteristics and application potential. Moreover, most nonwetting surfaces studied in the literature are fabricated based on nanoscale texturing of small, flat plates achieved through lithography technique (Kong et al., 2019). Such methods are less suitable for cost-effectively upscaling to practical condensation systems that use tens of thousands of tubes or otherwise large condensing surfaces.

Solid Infused Nonwetting Surfaces

A novel class of nonwetting surfaces called solid-infused surfaces (SIS) was recently introduced by the authors to address the challenges (Pitchumani and Stoddard, 2022). A solid-infused surface is a hybrid interface created by texturing a metallic substrate by industrially well-established methods, such as chemical etching or electrodeposition, and infusing the resulting asperities with a nonwetting polymer that is cured in place. The resulting surface is a solid with nonwetting properties that is robust to the dynamic shear in application environments. Since the surface has the exposed metallic asperity tips in an otherwise nonwetting medium, the surface retains the excellent thermal conduction of the base metal, which is essential for surface condensation. Furthermore, by virtue of the polymer being infused in the asperity valleys, SIS is not subject to the challenges of conventional coatings such poor adhesion and peel-off. SIS, thus, offers much potential for applications as heat transfer surfaces for dropwise condensation and fouling mitigation (Hatte and Pitchumani, 2022a,b), among others.

Solid-infused surfaces are fabricated on the exterior and interior surfaces of copper tubes by creating rough multiscale features on the metallic surfaces via scalable methods of chemical etching and electrodeposition, as the first step. In the second step, a two-part hydrophobic polymer such as Gentoo is infused into the multiscale asperities. For comparison nonwetting SHS and Krytox-infused LIS were also fabricated. All fabrication steps are detailed in Hatte and Pitchumani (2022a–c) and Hatte et al. (2022, 2023). Figure 1 shows the scanning electron microscope (SEM) images of SHS, LIS, and SIS, each fabricated using etching and electrodeposition methods. SHS fabricated by the two fabrication methods, Figs. 1(a) and 1(d), each shows the presence of roughness features of multiple length scales with a high static apparent contact angle, CA $=$ 161 $±$ 2° (etched) and 163 $±$ 2° (electrodeposited), and a low contact angle hysteresis, CAH $=$ 2 $±$ 1°. For LIS, Figs. 1(b) and 1(e) show the slightly protruding asperities of darker color in a sea of infused lubricant (Krytox 104), rendering the surface to be hydrophobic with CA $=$ 122 $±$ 3° and CAH $=$ 9 $±$ 2° (etched) and 124 $±$ 3° and CAH $=$ 8 $±$ 2° (electrodeposited). Like LIS, SIS exhibits filled asperity interstices with the infused cured polymer instead of the lubricant, as evident from Figs. 1(c) and 1(f), where the solid copper asperity tips are seen protruding slightly out of the cured Gentoo polymer. The composite surface comprising the hydrophobic solid infusion material (Gentoo) and the solid asperity tips shows a contact angle value, CA $=$ 132 $±$ 3°, and contact angle hysteresis, CAH $=$ 10 $±$ 2°.

Between an etched and an electrodeposited surface, the latter shows the presence of deeper asperities, as seen from a comparison of Figs. 1(a)–(c) with the corresponding images in Figs. 1(d)–(f). As a result, the immiscible lubricant or the nonwetting polymer is seen to be filled closer to the level of the asperity tips on the etched surface [Figs. 1(b) and 1(c)], and somewhat below the asperity tips for a greater exposure of the asperity peaks above the lubricant or polymer layer in the case of the electrodeposited surface [Figs. 1(e) and 1(f)]. Although the morphological distinction does not show any significant difference in the wettability measures of CA and CAH obtained by placing a sessile droplet on the surfaces at ambient conditions, the surface characteristic has profound impact on condensation dynamics, as discussed below.

Condensation Heat Transfer and Fouling Studies

Tubes with the three different types of nonwetting surfaces on the outside were used in a condensation chamber for the measurement of the condensation heat transfer coefficient, $h_c$, following the procedure and the steps described in Hatte et al. (2023) and Stoddard et al. (2021). As detailed in Hatte et al. (2023), for LIS and SIS fabricated by chemical etching exhibited heat transfer coefficients ranging from about 110 kW/m²K (subcooling $\Delta T =$ 2.5 K) to about 40 kW/m²K ($\Delta T =$ 20 K) representing a four-fold to about six-fold enhancement compared to the Nusselt correlation for FWC. Electrodeposited LIS and SIS were characterized by $h_c$ values ranging from about 75 kW/m²K ($\Delta T =$ 2.5 K) to about 35 kW/m²K ($\Delta T =$ 20 K) representing enhancements of about 4.3-fold ($\Delta T =$ 2.5 K) to about 3.5-fold ($\Delta T =$ 20 K) compared to the Nusselt correlation. For SHS, the heat transfer coefficients improved by 40% to about 15% compared to $h_c$ for FWC based on the Nusselt correlation for all subcooling values, regardless of the method of texturing.

The condensation heat transfer coefficient, $h_c$, is a measure of the thermal resistance on the condensing surface and is one of the resistances contributing to the overall heat flow during condensation. It is instructive to examine the influence of this condensation heat transfer resistance ($R_c$) and, in turn, the effect of the enhancement in the condensation heat transfer coefficient in the context of the overall thermal resistance in the system that additionally includes tube side convection heat transfer ($R_i$) and conduction within the tube material ($R_m$). A thermal resistance circuit considering all the resistances in series is shown in Fig. 2(a), from which the actual heat flux, $q''$, can be calculated. For a fixed coolant flow rate and condenser material, the theoretical maximum condensation heat flux ($q''_{\max}$) through the system is obtained by assuming an ideal condensing surface ($h_c→∞$; $R_c =$ 0). The role of the condensation heat transfer coefficient enhancement may then be understood by considering a condensation effectiveness, $ε$, that is commonly used in heat exchanger design and optimization and is defined as the ratio of the actual (measured) value of heat flux ($q''$) to the theoretical maximum heat flux, $q''_{\max}$ (Hatte et al., 2023; Stoddard et al., 2021):

$$\varepsilon = \frac{q''}{q_{\max}''} = \frac{R_{i} + R_{m}}{R_{i} + R_{m} + R_{c}}$$

(1)

Figure 2(b) summarizes the variation of heat transfer effectiveness $ε$ with condensation heat transfer coefficient for SIS, LIS, and SHS surfaces at low (2.5 K), medium (10 K), and high (20 K) surface subcooling conditions, for both fabrication methods of etching and electrodeposition. It is seen that the heat transfer effectiveness $ε$ approaches 1 asymptotically as the outer surface condensation heat transfer coefficient $h_c→∞$. Further, it is evident that a heat transfer coefficient of about 60 kW/m²K is sufficient to achieve an effectiveness of 0.90, and that for $h_c≈$ 80 kW/m²K, the effectiveness is about 0.93. Further increase in heat transfer coefficient only results in a marginal increase in effectiveness.

Over the range of surface subcooling values considered, $h_c$ ≲ 15 kW/m²K for FWC based on the Nusselt correlation; correspondingly, Fig. 2(b) shows that $ε$ ≲ 0.70 for progressively filmwise condensation (FWC), as denoted by the dashed line. The solid line in Fig. 2(b) may be regarded as the line of increasingly dropwise condensation (DWC) as $h_c$ and $ε$ increase along this curve. SHS fabricated using electrodeposition or etching, denoted by the half-filled circular or square markers, show FWC (for higher $\Delta T$) or marginal improvement over FWC (for the lower $\Delta T$) with 0.67  ≲ $ε$ ≲ 0.81. Etched SIS and LIS (filled and unfilled square markers) as well as electrodeposited SIS and LIS (filled and unfilled circular markers), on the other hand, undergo distinct DWC at all subcooling values studied, and are characterized by heat transfer effectiveness, $ε$ ≳ 0.85 with values of 0.85–0.88 for $\Delta T =$ 20 K, 0.89–0.91 for $\Delta T =$ 10 K, and 0.92–0.95 for $\Delta T =$ 2.5 K. As noted in Fig. 1, the lubricant or polymer infusion in electrodeposited surfaces is recessed into the asperity valleys [Figs. 1(e) and 1(f)] whereas the infusion is nearly at the level of the asperity peaks in etched surfaces [Figs. 1(b) and 1(c)]. Etched SIS and LIS, therefore, are seen to have relatively higher effectiveness compared to electrodeposited SIS and LIS, suggesting the ability to design the condensation enhancement through tailoring the surface characteristic.

The effectiveness plot, therefore, provides a convenient means of assessing the impact of condensation heat transfer enhancement of the various nonwetting surfaces. It is evident that SHS improves effectiveness by about 7–10% in absolute terms with respect to the effectiveness corresponding to the Nusselt correlation (0.60–0.70) at the respective subcooling. In comparison, SIS and LIS both offer significant increase in the condensation heat transfer with 25% improved effectiveness compared to that for FWC, for each subcooling.

Similarly, tubes with SHS, LIS, and SIS on the inner surface were used in an accelerated flow fouling set up for the measurement of the asymptotic fouling resistance, $R_{f∞}$, as presented in our previous work (Hatte and Pitchumani, 2022a–c; Hatte et al., 2022). The variation of the asymptotic fouling resistance with temperature is presented in a semi-log plot in Fig. 3 for SIS and LIS in comparison to a smooth, conventional surface, for a representative flow Reynolds number and a CaSO₄ supersaturation of 2. Among the three different surfaces, the fouling resistance of the two nonwetting surfaces is seen to be consistently lower than that for the conventional smooth surface. Owing to the hydrophobic polymer in the asperity interstices, SIS offers lower adhesion of the foulant and its easy removal by the shear of the flowing fluid, leading to lower asymptotic fouling resistance when compared to smooth surfaces. Similarly, the Krytox 104 lubricant-filled asperities in LIS present far fewer nucleation sites for foulant deposition compared to SIS and smooth surface and show the lowest asymptotic fouling resistance values. It is evident that $R_{f∞}$ increases with increase in temperature, closely following a linearly decreasing trend with the inverse of the absolute temperature on the semi-log plot, suggesting an Arrhenius dependence of the asymptotic fouling resistance with temperature, as shown by dashed lines in Fig. 3.

Durability of LIS and SIS

It is evident from Figs. 2 and 3 that both SIS and LIS offer superior performance in enhancing condensation heat transfer while mitigating fouling. However, the ability of the two surfaces to sustain the enhanced performance is an important consideration in prolonged usage of the surfaces in application environment. The robustness of two surface types, LIS and SIS, was quantified by subjecting them to continual condensate droplet impingement and shedding, that mimics the environment in a steam condenser. Similarly, tubes with SIS and LIS on the inside were subject to continuous flow of coolant water for long durations. In both scenarios, the surfaces were subject to a continuous shear condition to assess its impact on the surface durability.

Two copper tube sections, one with etched lubricant-infused outer surface and the other with etched solid-infused outer surface, were fabricated for the testing. Each tube was initially weighed and the contact angle and contact angle hysteresis values on the outer surface were measured using a goniometer. The tubes were then exposed to two streams of water droplets each dripping at a frequency of 3 s⁻¹ onto and sliding off the surfaces along the tube length, as illustrated schematically in Fig. 4(a) for the LIS.

At nominally 24 h intervals, the two tube samples were removed from the durability test setup and dried at atmospheric pressure and room temperature to remove any residual water on the surfaces before conducting weight and wettability (CA and CAH) measurements. Following the measurements, the samples were returned to the durability experimental setup and placed under the dripping water streams. This procedure was repeated for 21 days, for a total of 25 sets of measurements. The effects of shear exhibited by the shedding condensate droplets, as mimicked in the durability experimental setup, are quantified in terms of retained volume fraction of the infusion material (derived from the weight measurements during and prior to the experiments) and surface wettability characteristics measured in terms of CA and CAH.

Figure 4(b) shows the variation of retained infusion material volume fraction, $V/V_0$, with time for LIS and SIS. It is evident that over the course of the 21 days, LIS shows a gradual decrease in retained volume fraction with time clearly indicating drainage of infused lubricant from the asperity interstices, and progressively increasing exposure of the solid asperities to condensation; by the end of 21 days, the LIS is seen to have a total of 50% drainage of the infused lubricant. On the other hand, as seen from Fig. 4(b), the SIS exhibits no loss of the infused cured Gentoo polymer, and the volume fraction remains 1 within experimental variation.

In addition to the retained lubricant volume fraction, the effects of the shear flow over LIS and SIS can be seen from the changes in the structural morphology of the surfaces, as elucidated in Fig. 5. Figure 5(a) shows that the as-fabricated lubricant-infused surface, before the exposure to shear flow, with the lubricant filled flush to the top of the solid asperities. However, post shear exposure, the SEM image [Fig. 5(b)] of LIS shows clear evidence of the loss of the lubricant with the lubricant having receded to a lower level in the asperity valleys. The higher magnification inset image in Fig. 5(b) further shows the exposed lateral sides of the solid asperities which were previously completely covered within the lubricant layer in Fig. 5(a). A comparison of Figs. 5(a) and 5(b) presents clear visual proof of the drainage of lubricant from the LIS.

Unlike LIS, the images of solid-infused surfaces in Figs. 5(c) and 5(d) reveal a different characteristic. Figure 5(c) shows a solid-infused surface at two different magnifications before the shear flow exposure, where the infused polymer is filled in the asperity valleys to about the same height as the solid asperities, as marked in the image. After the shear experiment, the SIS surface in Fig. 5(d) shows no apparent change in the structural morphology of the composite solid-polymer surface. Unlike LIS, there is no degradation to the surface; the higher magnification inset image in Fig. 5(d) shows that the polymer fills the asperity valleys to the height of the asperities, just as it was prior to being subject to the prolonged flow, offering proof of the durability of the solid-infused surface to prolonged flow exposure.

A direct consequence of lubricant loss from the LIS is the associated changes in the surface wettability characteristics that are instrumental in the enhanced condensation performance of LIS. Figure 6 presents the effects of durability tests on the wettability of LIS and SIS, in terms of the variation of CA [Fig. 6(a)] and CAH [Fig. 6(b)] with time. With the loss of infused lubricant, more of the functionalized solid asperity surfaces are exposed and the LIS gradually tends to be a SHS. This wettability change is evident in Fig. 6(a) where the CA of LIS increases with time corresponding to the depletion of the infused lubricant. The exposed asperities and generation of air cavities resulting from the removal of infusion oil collectively results in an increase in static contact angle. With time, the continuous depletion of infusion oil from the interstitial space of solid asperities results in contact angle values approaching and exceeding 150°, as seen in Fig. 6(a), at which point the LIS transitions to a SHS. Figure 6(a) shows that this transition occurs around 15 days in the durability experiments, and at the end of the 21 days of durability test, the CA is seen to be around 155°. A similar trend is observed in the variation of the CAH values measured on the LIS samples throughout the durability tests, presented in Fig. 6(b). Corresponding to lubricant depletion and increase in its CA, LIS also shows a steady decrease in its CAH. Beginning about 15 days into the test, the CAH reduces to as low as 3°, characteristic of pristine SHS [Figs. 1(a) and 1(d)].

Figures 6(a) and 6(b) demonstrate that the tube with a solid infused surface shows no deterioration of contact angle or contact angle hysteresis values throughout the durability experiment. SIS steadfastly maintains a contact angle of about 132° and contact angle hysteresis of about 11°. Collectively, from Figs. 6(a) and 6(b), it is evident that whereas the wettability characteristic of LIS that sustains DWC is degraded with prolonged droplet interaction, SIS retains its underlying wettability characteristic durably. It follows from the durability results, therefore, that lubricant loss is inevitable on LIS under prolonged exposure to droplets in a condensation environment, which results in the surface transitioning to being superhydrophobic. As seen from Fig. 2(b), the condensation effectiveness and, correspondingly, the condensation heat transfer coefficient of SHS are significantly lower than that of SIS and are, in fact, only insignificantly better than the values for FWC heat transfer. As a result, LIS will eventually have condensation heat transfer coefficient values comparable to conventional FWC, whereas SIS will maintain its four- to five-fold enhanced condensation heat transfer and 25% improved condensation effectiveness durably. Thus, although LIS offers an improved condensation heat transfer compared to FWC initially, its advantage diminishes with time in a condensation environment, whereas SIS presents a robust alternative for sustained superior convection heat transfer enhancement in the long run.

Figure 6(c) compares the asymptotic fouling resistance values of LIS and SIS for a range of coolant flow Reynolds number values, as obtained before and after the durability tests. For comparison, the asymptotic fouling resistance of smooth tube surface is also presented in Fig. 6(c). It is evident that SIS surfaces owing to their robustness to the shear flow show insignificant change in the asymptotic fouling resistance values. Krytox-104 infused LIS on the other hand shows a dramatic increase in the asymptotic fouling resistance by as much as 50%, after the prolonged exposure to flow, as seen from Fig. 6(c). With the increase in the asymptotic fouling resistance LIS has fouling values comparable to smooth surfaces, whereas SIS maintains a 20% improvement in fouling relative to smooth surface for all Reynolds numbers. Thus, although LIS has a greater fouling reduction compared to smooth surface initially, its advantage diminishes with time in a flow environment, whereas SIS presents a robust alternative for superior fouling mitigation in the long run over a range of operational temperature.

Summary

Advancing condenser surface designs is imperative for improving condenser effectiveness and efficiency of thermal power plants. Nonwetting SHS and LIS have long been explored for their promise of improving condensation heat transfer. However, their initial promise has fallen short in the face of practical requirements of scalability and durability and, indeed, nonwetting surfaces have not been adopted in practice to date. Here we introduced solid infused surfaces (SIS) as a novel class of nonwetting surfaces that are fabricated using scalable manufacturing processes and exhibit excellent durability under conditions expected in condenser environments. SISs yield four- to five-fold enhancement in heat transfer coefficient, a 25% direct increase in condensation effectiveness, and a 20% reduction in fouling resistance compared to conventional smooth surface. The performance improvements afforded by SIS translate to 10–50% reduction in the levelized cost of condenser depending on the condenser design, as examined in a separate technoeconomic study (Nithyanandam et al., 2021).

Acknowledgments

The material reported in this publication is based upon work supported by the U.S. Department of Energy under Award Number DE-FE0031556. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.