Present spacecraft water recovery technologies are susceptible to fouling and failures due to biological and mineral scaling (Carter, 2010; Wong et al., 2010; Carter et al., 2013). Such failures require expensive and time-consuming maintenance and resupply and are thus limited to spacecraft where such resources are available (Pruitt et al., 2015). Long-duration spaceflight missions, such as extended stays at Lunar outposts or during Mars transit, will increasingly benefit from water recovery equipment that minimizes the impact of fouling. Such systems would also benefit from a minimization of moving parts, energy usage, and maintenance requirements. In this work, capillary solutions are used in conjunction with nanophotonic hyperlocalized heating to promote fouling-free fluid management at all gravity levels. Our target application concerns urine water recovery aboard spacecraft where we uniquely exploit Leidenfrost (Leidenfrost, 1756) droplet levitation due to the underside evaporation layer to achieve noncontact heat-driven distillation with expectations of >98% water recovery. Figure 1 presents a model of the system with passive phase separator (PPS), nanophotonic surfaces, and Leidenfrost distiller identified; and Figure 2 presents a block diagram representation of the system process.
Capillary fluid management systems have been exploited aboard spacecraft for decades. These systems typically use clean, single-component fluids, such as fuels, cryogens, and thermal fluids for which the wetting conditions are known and favorable, with contact angles
Nanophotonic heating may be exploited for the pasteurization of wastewater directly within the PPS. As an example, when spherical nanoparticles (i.e., gold or carbon) interact with a specific wavelength of light, the nanoparticles experience a harmonic shift in electron polarity, causing the particle to resonate, thus creating a localized heating effect over distances of approximately 10 μm (Richardson et al., 2009). This phenomenon is known as surface plasmon resonance (Agrawal et al., 2018). Hogan et al. (2014) demonstrate both localized heating and steam generation in a water bath without significant heating of the bulk solution by using light-induced localized heating of dispersed nanoparticles. In such a regime, light is both scattered and absorbed, concentrating electromagnetic energy in a small volume. Neumann et al. (2012) demonstrate this effect using solar light. Such light-induced heating of nanoparticles has gained significant attention in the past decade (Zhang et al., 2006; Pattani and Tunnell, 2012; Bora et al., 2016) for applications in localized drug delivery (Niikura et al., 2013), cancer treatment (Lee et al., 2010; Zhang et al., 2013), saltwater distillation (Dongare et al., 2017), and others (Govorov et al., 2006; de Aberasturi et al., 2015). Herein, we use infrared (IR) light-emitting diode (LED)-induced localized heating of carbon black nanoparticles. For batch operations, our system may require the temporary storage of wastewater, which must be maintained at elevated temperatures to discourage biofouling. Thus, our nanophotonic heating approach is designed for the pasteurization of wastewater. We demonstrate the approach with saltwater.
The enhanced microgravity Leidenfrost phenomenon can be exploited to promote heat-driven noncontact distillation of wastewater droplet streams. On earth, heat-driven distillation is typically used in the pooling boiling regime to perform distillation of liquid–liquid and liquid–solute solutions. Liquid droplets undergo pool boiling when deposited on substrates at or slightly above the liquid's boiling temperature. In this regime, wall-bound droplets undergo nucleate boiling and boil away in seconds. However, if the liquid droplets are deposited on a substrate with a temperature significantly higher than the liquid's boiling temperature, the droplets do not undergo nucleate boiling but will instead levitate on their own vapor layer. The droplets are insulated from the heated substrate by this vapor layer, reducing heat transfer to the droplets by orders of magnitude and eliminating nucleation sites for bubble formation. These combined effects significantly increase droplet lifetimes. For example, capillary-sized droplets of water (< 0.08 mL) deposited on an aluminum substrate held at the Leidenfrost point of water on aluminum (193°C) will require several minutes to evaporate completely.
Leidenfrost phenomenon is enhanced and occurs over significantly larger length scales in the near-weightless microgravity environment. As shown in Fig. 3 a series of images during a drop tower experiment demonstrate this point where ethanol droplets 2.6 mm in diameter are injected into a glass U-tube above the dynamic Leidenfrost point of ethanol on glass (>360°C). The noncontact nature of the phenomenon is attractive as a potentially fouling-free mechanism for distillation aboard spacecraft. Since Leidenfrost distillation is a heat-driven process, complete distillation of wastewater droplets to their salty constituents is possible. The streams of vapor-insulated Leidenfrost droplets are highly mobile and readily circulated using bulk air flows or the evaporating droplets’ own vapor pressure.
Stamey and Mihara (1980) present the experimental results for bacterial growth in sterile urine from human subjects, finding that exponential bacterial growth occurs after 2 h for many types of bacteria. Leidenfrost distillation rates are comparable to human urine delivery rates (minutes), which permits essentially real-time distillation of streams of droplets at elevated temperatures before bacterial contamination can occur. Such rapid distillation of urine at temperatures >70°C retards microbial growth, thus reducing or eliminating the need for using urine pretreatment chemicals to prevent bacterial growth in urine in current spacecraft water recovery equipment.
The Leidenfrost effect has been studied extensively for its relevance to numerous applications, including metal manufacturing, fuel combustion, jet and rocket engine propulsion, spray cooling, nuclear reactor cooling, and others (Itaru and Kunihide, 1978; Avedisian and Koplik, 1987; Bernardin et al., 1997; Rein, 2002; Abramzon and Sazhin, 2005; Tarozzi et al., 2007; Sazhin et al., 2010; Gradeck et al., 2011; Wu and Sirignano, 2011). The progression of a particularly applicable line of research has led to the extreme enhancement of Leidenfrost phenomena in microgravity using superhydrophobic (SH) substrates (Biance et al., 2003; Vakarelski et al., 2012; Orzechowski and Wciślik, 2014; Maquet et al., 2015; Attari et al., 2016; Wollman et al., 2016; Rasheed and Weislogel, 2019a; Rasheed and Weislogel, 2019b; Rasheed, 2019). Though our intended application requires a reduced-gravity environment, our reported research is limited to terrestrial demonstrations.
The omni-gravity nanophotonic heating and Leidenfrost-driven fouling-free water recovery system (WRS), pictured in Figure 1, is a two-loop system capable of continuous processing of wastewater at a rate of 500 mL/h. The processing rate is selected to accommodate the production rates of a crew of four (assuming 1.5 L/d per crew member; Chiaramonte and Joshi, 2004). The block diagram of Figure 2 remains the same for the WRS in all gravity environments, though specific unit geometries may vary. Wastewater streams aboard spacecraft typically comprise humid cabin air condensate (respiration and perspiration), urine, and gray water sources (hygiene, sink, and laundry). Humidity condensates are typically collected by condensing heat exchangers and are sufficiently sterile to be directly distilled. For this approach, humidity condensates may be sent directly to the Leidenfrost distiller, as shown in Figure 2. Urine and gray water are delivered to the distiller from the PPS. The distiller is designed to process streams at a rate of up to 500 mL/h. The PPS has a capacity of 1 L for separation and storage. The distiller outlet stream is checked for quality and reprocessed until adequate.
The PPS device geometry was previously developed and tested in a low-
In gravity-dominated environments, the separation of liquid–gas streams in the PPS is accomplished predominantly by buoyancy. The interior core of the PPS is designed with a 5° tilt with respect to the gravitation vector. The tilt does not affect the microgravity performance but provides for gravity draining of the liquid from the PPS inlet to the finned PPS reservoir, as shown in Figure 5. For the same purpose, the vaned reservoir is sloped at 30° to promote fluid containment in a gravitational environment. Figure 5b presents a computer-aided design (CAD) rendering of the assembled PPS. The multicomponent device consists of a helical coiled teardrop sectioned core, which expands into a vaned capillary collection and containment/reservoir section. As demonstrated by Weislogel et al. (2009), and as depicted in Figure 4a, for an intermittent jet and droplet-laden airflow, the helical entrance region provides for the passive low-
The triangular aluminum vanes (shown in Figures 4b and 5) are made of 2.5-mm-thick aluminum and coated with carbon black nanoparticles deposited by a plasmonic nanoparticle solution. Activated carbon (CEP21K; surface area 2040 m2/g) was purchased from Power Carbon Technology Co., Ltd. Gold nanoparticles (AuNPs) of 15-nm outer diameter (OD) suspended in 0.1 mM phosphate-buffered saline (PBS) was purchased from Sigma-Aldrich. Fully hydrolyzed poly(vinyl alcohol) (PVA; molecular weight = 89,000–98,000 g/mol) and 25 wt% aqueous solution of glutaraldehyde (GA) were purchased from Sigma-Aldrich.
The solution for treatment of plasmonic nanoparticle surfaces was composed of 70 wt% of high-surface-area powdered activated carbon (PAC), 20 wt% of 15-nm AuNPs in PBS, and 10 wt% of polymeric binder (linear polymer PVA and cross-linker GA) based on total mass. First, 6 wt% PVA solution was prepared by mixing PVA in deionized (DI) water at 90°C for 4 h. Next, 5 mol% (relative to PVA repeat units) of GA solution was added and mixed for 1 h. PAC was added slowly along with additional DI water, giving a solid content of the solution of approximately 30 wt%. The resulting mixture was stirred for 12 h to ensure solution homogeneity. Lastly, 20 wt% of AuNPs was added to the solution and allowed to mix for an additional hour. In total, 6 mL of nanoparticle solution was manually flow-coated onto each aluminum fin. A pipette was used to deposit the solution across the fin surface and then spread into a thin film using an acrylic planar surface. The vanes were allowed to dry at room temperature and then placed in an oven at 130°C for 1 h to cross-link. The aluminum vanes coated with carbon black nanoparticles are shown in Figure 4b. Partially wetting states with water are achieved on such surfaces, the contact angles in this case being
The Leidenfrost distiller is a gravity-sensitive device that must account for adequate droplet residence time to complete the desired distillation. If residence times are too long, complete droplet distillation is achieved before exiting the distiller, potentially resulting in fouling of the distiller surfaces with precipitates. If too short, the droplets may be insufficiently distilled and must be either remixed and recirculated through the device or must be postprocessed. For our highly aqueous solutions, ≈ 90% droplet distillation in the Leidenfrost distiller is chosen, which significantly distills the droplets while protecting the distiller surfaces from potential fouling due to the precipitation of solutes in wastewater solutions. Passive, postprocess evaporation in a heated, disposable catch basin performs the remainder of the distillation. Figure 6 shows a series of images of a urine droplet distilled on a SH surface held at 140°C, which is above the Leidenfrost point for this wetting condition. After approximately 92% droplet distillation, the solutes in urine begin to precipitate, changing the urine composition from predominantly water-based to oil-based. Oily solutions have drastically lower surface tension values than water, resulting in dramatically elevated Leidenfrost point temperatures relative to water (Chen et al., 2018). This transition is shown in Figure 6 at
Droplet evaporation rates are predicted from a simple mathematical model adopted to size the system, which characterizes the heat transfer from a hot surface to a droplet on a surface above the Leidenfrost temperature. Figure 7 provides a schematic of a droplet with time-varying radius
Order of magnitude comparisons show that thin film convection is the dominant mode of heat transfer to the droplet, while the magnitudes of the outer region's forced convection, free convection, and radiation are small. Forced outer region convection over the spherical drops may be considered (Rasheed and Weislogel, 2019b), but for the operating conditions of the Leidenfrost distiller device here, the magnitude of spherical drop forced convection is also small. The rate of change of mass for an evaporating spherical droplet can be expressed as
Integration of Eq. (4) with
The Leidenfrost distiller uses a gravity feed for wastewater droplet delivery. Such droplets are typically capillary-sized droplets. The capillary length scale
Sample values used to compute residence time.
As shown in Eq. (5), the distillation rates in the Leidenfrost state have a weak power law dependence on gravity, but changing the gravitational constant from Earth (1-
Other system components include Leidenfrost distiller heaters, heater controls, thermal insulation, a vacuum pump, condenser, and plumbing elements. Figure 9 provides an image of the full system build with labeled components. The distiller heaters consist of two Nichrome resistance heater wires stapled to K23 firebricks. Each heater consists of a wire with 8.4 W nominal resistance capable of dissipating 1440 W per heater wire at 110 V, for a total of approximately 2900 W. This high level of power is required to quickly establish steady state operating temperatures. Once steady conditions are established, the heater power is greatly reduced to approximately 36 W to balance the evaporation rates (≈ 21 W) and losses to the environment (≈ 15 W). A temperature control circuit uses controller, thermocouples, and high-power relays for heater power and temperature set points. The distiller is wrapped in 10-cm-thick high-temperature ceramic fiber insulation to limit losses to the environment.
IR LED (LEDLightsWorld; SMD3528-300-IR) strips of 940-nm nominal wavelength, shown in Figure 9, are used for nanoparticle irradiation. A 200-mm-long jacketed glass coiled reflux condenser is assembled with a Koolance HX-240XC computer cooler capable of ~1700 W cooling (see radiator, cooling fans, and pump; Figure 9). We note that the expeditiously chosen condenser does not account for low-
The nanoparticle-coated aluminum vanes were directly exposed to an IR LED source at a peak wavelength of 940 nm. Under the presence of the photon-emitting field, electrons inside the nanoparticles are mobilized as energy carriers through plasmon resonance phenomenon. Excitons are created as heat dissipated through an interband absorption process, both scattering and adsorbing light. The nanoparticles concentrate the light energy into mesoscale volumes near the illuminated fin surface, creating intense localized heating and providing an elevated temperature source for the surrounding mediums. Three case scenarios are tested for localized heating using a SEEK thermal imaging camera. For the baseline scenario, an aluminum coupon was coated with 3 mL of activated carbon CEP21K-H2O solution, and room temperature readings were recorded. For the second scenario, the coupon was exposed to a 940-nm IR LED strip with a density of 60 LEDs/m placed in a fixed position 70 mm parallel from the plate surface. Temperature readings were recorded at 1-min intervals until incremental changes were no longer observed after 4 min. For the third scenario, 20 wt% of 15-nm AuNP constituent was added to activated carbon CEP21K-H2O solution and 3 mL was surface-coated onto a fresh aluminum coupon. The coupon was placed in the same test configuration as Scenario 2.
Figure 10 shows the images highlighting the thermal imaging results for the three test cases. Test coupon 2 (Figure 10b) coated with activated carbon-H2O solution reached a maximum surface temperature of 28.9°C after 4 min of LED exposure. Test coupon 3 (Figure 10c), which was coated with activated carbon-H2O and AuNP constituent, reached a maximum surface temperature of 32.8°C after 4 min of exposure. These elevated temperatures, in combination with the biocidal nature of AuNPs (Ravishankar Rai and Jamuna Bai, 2011; Lima et al., 2013) due to the oligodynamic effect, are capable of rendering microbes inactive in both wastewater and recovered distillate streams.
The Leidenfrost Distiller of Figure 9 was tested at 230°C with saltwater as a urine ersatz fluid. A saltwater salinity of 40 milliSiemens (mS) was chosen to approximate seawater, with similar dissolved solids as urine. The saltwater was supplied to the distiller as a stream of drops, as shown in Figure 11. Distilled salty brine drops were captured in the catch basin (Figure 9). Product steam was pumped to and condensed in the condenser via vacuum pump. All operations were recorded by a high-speed high-density video camera. Deposition of salts on the distiller surfaces was not observed during all the 20 hours of the tests.
The volume and salinity of the distillates and captured brine were measured at different intervals. Figure 12a shows a bar graph of the percent distillate by volume collected. The amount of distillate collected (“water recovery”) was on average ≈ 40% of the total initial influent volume—less than half of the target 90% water recovery rate, for reasons to be discussed shortly. Figure 12b provides a plot of the experimentally measured evaporation rates of individual droplets measured from videos of distiller experiments at operating temperatures of 230°C and 310°C. Also plotted in Figure 12b are the theoretically determined droplet evaporation rates from Eq. (5), to assess the accuracy of the developed droplet lifetime model. The comparisons of droplet evaporation rates at 230°C indicate good agreement, with an average underprediction of 35%. At 310°C, the underpredictions increase to 54%. We expect that the increased underprediction with increased temperature is due to the increased heating of the droplets from the sidewalls of the channel, which are assumed to be flat for the predictions of Eq. (5).
The model of Eq. (5) provides a conservative estimate for the droplet evaporation rates. The distiller was designed to operate at 230°C for 2.7-mm-diameter droplets with residence time of ≈ 75 s to achieve ≈ 90% distillation. Unfortunately, the residence time achieved was found to be ≈ 30 s, due to the inadvertent enhancement in air/vapor flow caused by the vacuum pump, which could not be avoided. We note that for a constant evaporation rate, nearly 100% (≈ (75/30)*40%) distillation is expected without the enhanced vacuum pump flow. Despite the shortened residence time, the Leidenfrost Distiller performed the distillation function. Figure 13 provides a plot of the measured salinity values for the saltwater influent, captured distillate, and salty brine. The influent was fed to the distiller at a salinity of 40 mS. For all experimental runs, the distillate salinity was measured within instrument error to 0 mS. The average brine salinity was measured to be approximately 58 mS for all experiments, an increase of 45%.
The nanophotonic heating and Leidenfrost-driven fouling-free WRS was evaluated using the equivalent system mass (ESM) metric as developed by Levri et al. (2000) and compared to the current state-of-the-art water recovery technology ESM metrics aboard the International Space Station (ISS). The ISS WRS is a batch process system, whereas the system developed herein is a continuous process system. Both systems, however, are sized to process wastewater streams for a crew of four. A valid ESM comparison can therefore be made to compare the performance of both systems. The ESM metric is an equivalent mass value reflective of a system's mass, volume, and energy requirements and is used when comparing technologies regarding launch and operating costs of a system. The ESM value is computed using the following relationship:
Mass and volume values for the present WRS are obtained by summation of the individual component mass measurements and the overall footprint measurements of the system prototype shown in Figure 9. Table 2 lists the mass, power, and volume requirements for the current state-of-the-art WRS aboard the ISS and for the presented Nanophotonic Heating and Leidenfrost-Driven WRS. These metrics are converted to an ESM value and also listed in Table 2. All values for the current state-of-the-art WRS are obtained from the study by Anderson et al. (2018). The Nanophotonic Heating and Leidenfrost-Driven WRS posits an ESM value of 392 kg, which is an attractive value compared to the current state-of-the-art WRS with ESM of 1506 kg. The present WRS fares favorably in terms of mass and volume, with total mass of 245 kg and volume of 0.5 m3, compared to the current state-of-the-art WRS of 1380 kg and 3.14 m3, respectively.
Mass, power, volume, and ESM for ISS WRS and Nanophotonic Heating and Leidenfrost-Driven WRS.
Power requirements for the present WRS are computed from the individual system components operating at peak power levels. The 12 VDC flexible IR LED strips consist of 60 LED/m at 940-nm peak wavelength and require 4.8 W/m of power (LEDLightsWorld SMD3528-300-IR). A 1.5-m length of IR LEDs is used, resulting in 7.2 W of power consumption. The vacuum pump requires 208 W, and the condenser cooling loop fans and pump require a total of 60 W. Power consumption of the Leidenfrost distiller is estimated using a thermal resistance model. Assuming a quiescent environment at 20°C, 10 cm of ceramic fiber insulation (
Distillation of water-based solutions includes the thermal load of sensible heat to heat urine from 20°C to 100°C, and latent heat
The first omni-gravity WRS utilizing noncontact Leidenfrost distillation and nanophotonic heating as methods to promote fouling-free water recovery from waste streams is demonstrated herein. An omni-gravity passive liquid–gas phase separator was developed utilizing a helical entrance, expansion, and vaned reservoir to promote the passive separation and containment of wastewater streams despite widely varying or unknown wetting conditions. The device may be vector-oriented to provide this separation and storage function for any gravity environment, especially microgravity. The phase separator surfaces were coated with carbon black nanoparticles that were irradiated with IR LED strips to induce hyperlocal nanophotonic heating to 32.8°C, which promotes and maintains pasteurization of wastewater streams, forestalling microbial growth.
An approximate mathematical model was developed describing droplet evaporation in the Leidenfrost state. The model was then used to design the Leidenfrost distiller. The aluminum distiller was fabricated as a helical hemicircular track ~ 8.5 m in length with 0.25° pitch to establish gravity-driven droplet drainage in a terrestrial environment. The distiller was heated above the Leidenfrost point of water on aluminum via resistance heaters. Tests were performed with 40 mS saltwater as a urine ersatz fluid. Throughout the ~ 20-h operations, no significant salt deposits or other forms of fouling were observed on the distiller surfaces. Volume and salinity measurements over time revealed an average water recovery rate of 40% with virtually no salt content in the recovered distillate, and an average salinity of 58 mS was noted in the captured residual brine. Thus, changes in salinity were consistent with percent distillation recovery. Recovery rates above 90% were predicted for the current design based on droplet residence times of 75 s. However, reduced residence times of only 30 s were achieved in practice due to droplet accelerations arising from an unforeseen vacuum pump assist. We expect that ≈ 90% recovery is readily achievable by reducing the pitch of the helical track.
The present WRS trades favorably with the current state of the art aboard the ISS, with ESM of 392 kg compared to 1506 kg—a nearly four-fold reduction. The nanophotonic Leidenfrost WRS demonstrates the feasibility of an on-demand fouling-free wastewater distillation system with significant reductions in mass, volume, moving parts, caustic consumables, and maintenance requirements. The heat-driven Leidenfrost distillation process boasts the advantage of achieving up to 100% water recovery from wastewater streams. Further development of the approach is recommended.
Mass, power, volume, and ESM for ISS WRS and Nanophotonic Heating and Leidenfrost-Driven WRS.
Sample values used to compute residence time.