AbstractFragmentation of marine snow affects the downward flux of organic matter, and other aggregate‐associated compounds such as oil. Using phytoplankton aggregates, we demonstrate that marine snow with oil, termed marine oil snow, had a higher resistance to fragmentation compared to marine snow without oil when exposed to turbulence ex situ. At moderate shear levels, typical of the ocean mixed layer, 17% of marine snow without oil broke, whereas 63% of marine snow fragmented at intermediate shear. In contrast, only 17% and 33% of marine oil snow fragmented at the intermediate and highest shear levels, respectively. Our results suggest that oil increases the cohesion and stability of aggregates making them less susceptible to breaking. This work contributes toward explaining the exceptional oil sedimentation event following the 2010 spill in Gulf of Mexico. It also enhances our understanding of the factors that determine the probability of sinking aggregates to fragment.

Many natural and industrial processes involve the sedimentation of nonspherical particles. Many of these particles do not have uniform mass distributions; two cases of interest are aggregates (which may be composed of multiple materials) and microplastic pollutants (which may experience localized fouling or degradation). For such particles, the centers of mass and buoyancy are not colocated. This leads to interesting settling dynamics, particularly at transition points near the onset of wake instabilities. We investigated the orientation and terminal velocity of initially horizontal, freely falling cylinders, in which the mass distribution was either constant (uniform density) or bipartite, undergoing a step change halfway along the length (compound density). Cylinders had low aspect ratios (1 AR 4) and fell at intermediate Reynolds numbers (around 200). We recorded the position and orientation of each cylinder as it fell through still water, as well as the distribution of landing sites. We also performed planar particle image velocimetry to visualize wake dynamics. Results showed significant differences in the settling characteristics of uniform- vs compound-density cylinders, and revealed three distinct settling modes: rectilinear, oscillatory, and oblique. Each of the three modes displayed distinctly different wakes and vortex shedding patterns. All compound density cylinders, regardless of aspect ratio, were biased to land on the side of the tank, where the more dense end of the cylinder was initially oriented. Our results show that the interplay between buoyant torques and wake instability strongly impacts particle motion in the context of still-water settling, and is likely to play a role in more complex flows. This is true even for particles with an extremely small offset between the center of mass and the center of buoyancy, carrying strong implications for the dispersion of relevant particle classes such as microplastics.

There is a significant amount of low-temperature heat (< 100 °C) available globally from various sources such as geothermal energy, industrial processes, and thermal power plants. Electrochemical and membrane-based methods for harnessing this heat are becoming of greater interest due to their ability to utilize the low temperature energy sources and because they can be sized modularly to fit the power, energy, and spatial requirements of the application. The all-aqueous thermally regenerative ammonia battery (Cu aq -TRAB) is a leading technology in the waste heat to electrical power production space due to recent results showing operational power densities of 25 mW cm -2 with an estimated thermal energy efficiency of 7%, outperforming most technologies in this space. Previous research on the Cu aq -TRAB has shown that the battery suffers from the crossover of ammonia through the membrane which adversely impacts battery performance. To investigate these impacts, we developed a simple numerical model that simulates discharge curves using one fitting parameter to account for parasitic crossover instead of modeling species transport through the membrane. The model was able to replicate experimental data from Cu aq -TRABs with different membrane materials and for multiple applied current densities with < 5% error for average power and energy capacity. Results showed that there is a 20% loss in Cu aq -TRAB energy capacity due to ammonia crossover when compared to a discharge with no parasitic crossover losses. The model was then extended to demonstrate the impact of different changes to the electrolyte chemistry on the resultant power and energy capacities for Cu aq -TRABs. Using this model, we estimate that if chloride was the ligand for the positive electrolyte instead of bromide, it would decrease the power and energy capacity of the battery by 30%. However, considering that an economic analysis showed that chloride is 80% cheaper than bromide, it may be beneficial to consider the use of chloride ligands in this technology despite its worse electrochemical performance.

A liquid-gas flow across a bluff body can result in non-uniform distribution of void fraction due to the interaction of the bubbles with the bluff-body wake. This experimental investigation was conducted to characterize a bubble-trapping region and the clustering dynamics of two distinct bubble sizes in an upward water channel with a rectangular cross-section. Bubble trapping in the near wake of a cylinder is shown as a function of bubble size, liquid flow rate, and gas flow rate through patterns of local void fraction. Particle tracking velocimetry and particle image velocimetry were used to calculate time-averaged trajectories of 3 mm diameter and 0.5 mm diameter bubbles as they flowed around a cylinder. The liquid Reynolds number (Re), based on the cylinder diameter of 9.5 mm, was varied from Re = 100 to Re = 3,000. In addition, the injection of liquid flow tracers and application of particle shadow image velocimetry allowed measurements of the time-averaged liquid velocities in the wake. The phase-resolved tracking results were evaluated to determine the effects of the added mass, pressure gradient, lift, drag, and buoyancy forces acting on the air bubbles. Trapping of both bubble sizes was observed across a range of operating conditions; however, this behavior was not explained solely by the Reynolds number of the flow. The force balance analysis revealed that inertial forces and lift forces acting on the bubbles both contributed to the clustering, which occurred when either the inertial forces or the lift forces acting on the bubble were sufficiently high compared to the bubble drag forces. Both the inertia- and lift-to-drag ratios were necessary to predict the bubble clustering dynamics of the distinct bubble sizes investigated.

Thermally regenerative ammonia batteries (TRABs) are an emerging technology that use low temperature heat (T < 150 °C) to recharge a flow battery that produces electrical power on demand. The all-aqueous copper TRAB can provide high power densities and thermal energy efficiencies relative to other devices that harvest energy from waste heat, but its performance is adversely impacted by the crossover of undesired species through the membrane and lower cell voltages compared to conventional batteries. In this work, we developed a numerical model to simulate discharge curves while accounting for crossover inefficiencies without tracking all electrolyte species through the membrane. The model was able to successfully reproduce discharge curves across a diverse range of battery conditions using a single fitting parameter to account for decay of electrode standard potential due to species crossover with minimal error (< 5%). The model was then used to simulate different design scenarios to estimate changes in energy output from alterations to the aspects of the battery electrolyte chemistry. Results from this study are used to identify pathways for improving future TRAB designs with respect to energy capacity and cost-effectiveness of the technology.

Low-temperature heat (T<130 degrees C) can be utilized by thermally regenerative batteries (TRBs) for power production, allowing the thermal energy to be converted to storable chemical potential energy. However, TRBs suffer from high ohmic losses and ammonia crossover, which has slowed their development. In this study, we examined how the use of six different membranes influenced TRB performance, determined the most influential membrane parameters, and identified promising membrane candidates that cost-effectively increase TRB performance. Of the six membranes examined, an inexpensive, hydrocarbon CEM (Selemion CMVN) had low ammonia crossover without compromising resistance, resulting in good performance across all metrics studied. A thin anion exchange membrane (Sustainion, 50 microns) showed a high peak power density of 82 mW cm(-2) due to low resistance, but the average power density and energy density were low due to high ammonia flux. Full discharge curves using Selemion CMVN provided an average power density of 26 +/- 7 mW cm(-2) with an energy density of 2.9 Wh L-1, which were large improvements on previous TRBs. A techno-economic analysis showed that Selemion CMVN had the lowest levelized cost of storage ($410 per MWh) at an applied current density of 50 mA cm(-2).

Two-phase wakes generated from a cylinder in a crossflow were experimentally studied. A water-air mixture traveled through a vertical water channel with a rectangular cross section, in which a cylinder was installed horizontally. Liquid Reynolds numbers, based on a cylinder diameter of 9.5 mm, were varied from Re = 100 to 3,000; the air superficial velocities were varied from j(g) = 0.06 m/s to 0.60 m/s; and mean bubble diameters were varied from 0.48 mm to 3.5 mm. Void fraction distribution in the wake of the cylinder was determined from high-speed visualizations, where a correlation was applied to the shadow fraction measurements to account for overlapping bubble images. It divided the wakes into a liquid-phase region with a low void fraction relative to its freestream condition (a/a(8)<1/2) and a bubble-trapping region with a relatively high void fraction (a/a(8)>2). The liquid-phase region occurred in all flow conditions, but its length decreased with increasing Reynolds number. In contrast, the bubble-trapping region occurred only at relatively high Reynolds numbers depending on the bubble size and air superficial velocity. Transitional bubble-trapping behavior was identified at Re = 1,200 for the 3.5 mm bubbles, where bubble trapping only occurred at low air superficial velocities. Once the bubble-trapping region developed sufficiently, the location of the maximum void fraction was consistently located at y/D = 1.3-1.5 downstream from the center of the cylinder.

Thermally regenerative ammonia batteries (TRABs) use low-temperature (T < 100 degrees C) heat to provide stationary energy and power with higher power densities and efficiency relative to other waste heat devices. TRABs are an active area of research in waste heat devices, but currently there is little consensus on what aspect of the system is limiting TRAB performance and what maximum efficiencies are possible. Experiments and numerical models were used here to examine the sensitivity of the battery and distillation column in a TRAB system to key operating variables, thereby establishing practical limits and identifying focus areas for improving performance. Battery power was eight times more sensitive to ohmic losses than kinetic and mass transfer losses, regardless of the operating temperature, and the peak power density was simulated to be 18.8 mW cm-2 at 75 degrees C. Theoretical energy efficiency limits were defined for a series of ammonia concentrations and operating pressures, ranging from 5 -12%, which is 2-3 times higher than previous experimental estimations. Atmospheric pressure column operation used a larger amount of waste heat compared to sub-atmospheric pressure. It is estimated that the volume of the battery would take up 9.2 m3 for every 1% of the power output of a natural gas turbine, but with realistic improvements to cell conductivity, the size would reduce to 2.5 m3. The results presented in this work will help streamline future development by focusing on minimizing ohmic losses and provide specific data for full system evaluation of future TRABs.

Understanding the effect of hydrodynamics on aggregate size and structure is key to predicting mass transport in the aquatic environment. Aggregation theory of particles is well established but our knowledge of deformation processes, biological bonding forces, and their effects on fragmentation of aquatic aggregates is still limited. To better comprehend fragmentation processes and adhesion forces we implemented breakup experiments with diatom and microplastic aggregates made in the laboratory. We captured a substantial number of events showing deformation and subsequent fragmentation of these aggregates in an oscillatory shear flow. Polystyrene and polyethylene aggregates showed distinct fragmentation strengths and provided comparative upper and lower limits to the biological bonding strength of the diatom aggregates. Additionally, we employed a force balance model to evaluate attractive interactions within clusters of particles using the Lagrangian stress history and morphology. We found that the fractal structures of aggregates led to a power law of breakup strength with size and that time-integrated stress governed the overall fragmentation process. We also found that the weakening of the aggregates through deformation with shear exposure enabled their disaggregation at very low shear rates typical of the ocean environment.

Redox flow batteries are emerging as a promising method to provide grid-scale power and long-duration energy storage safely and economically. The thermally regenerative ammonia battery (TRAB) is a new redox flow battery category that can be recharged using low-grade waste heat rather than electric energy, adding further flexibility to the applicability of flow battery systems. Recently, a new TRAB with all-aqueous electroactive species (referred to as the Cuaq-TRAB), as opposed to deposition-dissolution reactions, was found to have superior energy and power densities relative to competing TRABs. The use of bromide and ammonia stabilizes for both Cu(I) and Cu(II) oxidation states, while also creating a cell potential of up to 1.0 V. The potential can be recovered by thermally separating ammonia from the electrolyte and adding it back to alternate electrolyte chambers in successive cycles. Potential improvements in overall cell performance are possible by reducing ohmic losses associated with membrane selection. However, the ideal membrane for the Cuaq-TRAB is not obvious and raises interesting transport questions relative to the dominant redox-active copper species being negatively charged in the catholyte but positively charged in the anolyte. Furthermore, membrane crossover of ammonia, a small, uncharged molecule, was previously shown to be a primary source of parasitic losses for TRABs. Therefore, we investigated how different membrane types (cation, anion, and non-selective) affected ion transport and TRAB performance. A batch symmetry cell was used to determine membrane conductivity in the Cuaq-TRAB environment and membrane diffusion coefficients of each species in the electrolytes. Flow cell experiments were also conducted to find peak power, energy density, average power during discharge, and capacity fade over successive electric charge/discharge cycles. Tradeoffs between membrane conductivity and permeability observed in the symmetry cell are manifested in flow cell results as either high peak and average power or high energy density and low capacity fade. These results expand on potential methods for controlling transport in redox flow batteries and demonstrate the potential for non-selective membranes for electrochemical energy technologies.

The sinking of marine aggregates impacts the global carbon cycle as it is one of the primary sources of carbon export from the surface ocean. Aggregation and fragmentation alter the size of aggregates, which governs their sinking speed. Although aggregation theory is well established, aggregate fragmentation strength and breakup characteristics are less well understood. This study developed a cylindrical tank that formed and then exposed diatom aggregates to calibrated laminar shear through a combined rotating and oscillating motion. The shear rate was predictable using a developed analytical solution for the fluid motion within the tank. Under proper operating conditions, this facility provided fluid shear with a similar magnitude to that in multiple ocean environments. We also developed a unique image processing method that enabled continuous tracking of particles' position, size, and morphology as well as determination of the individual aggregate breakup events. The method has great potential to capture breakup events of large marine snow particles, quantify the aggregate morphological changes leading up to and at breakup, and provide data sufficient for statistical analysis of laboratory aggregate populations. We tested the method using laboratory-cultured Odontella aurita and captured 79 breakup events of the resulting aggregates. These fragmented aggregates ranged from 1 to 5 mm in major axis length and underwent substantial morphological evolutions prior to the fragmentation.

A significant amount of the potential energy that is generated during energy harvesting worldwide is discarded as waste heat because of inefficient power generation cycles. Much of this wasted power source goes unused because it is trapped as low-grade thermal energy (< 100 °C), which traditional power cycles cannot viably harness. With the advent of electrochemical power systems such as redox flow batteries and fuel cells, researchers are investigating new methods of providing usable electric power from these unused low-grade thermal energy sources. The thermally regenerative ammonia battery (TRAB) is one promising technology in this space, as it operates with the same principles as a flow battery except that TRABs can be recharged using low-grade waste heat instead of electric power. Of the TRAB chemistries proposed, the all-aqueous copper TRAB chemistry is capable of both large power densities and energy densities, and high coulombic efficiencies. In this presentation, we discuss how key performance metrics relevant to power generation from low grade thermal energy sources are strongly influenced by electrolyte composition and battery operating parameters. Manipulating the ammonia to copper ratio demonstrated clear tradeoffs between achievable energy and power capacities. Increasing ligand concentration had a large impact on electroactive species solubility and cell potential differences, which increased the theoretical energy density limit of the battery. Increasing the amount of ammonia relative to dissolved copper raised peak power density, but adversely affected energy density. Moderate increases in discharge current density did not decrease the energy density due to reduced impact of ammonia crossover which appears to be a dominant source of energy loss in TRABs.

Thermally regenerative ammonia batteries (TRABs) can provide energy storage and produce electrical power from low-grade waste heat instead of electricity. The use of all-aqueous copper-based electrolytes has recently produced higher power densities than those achieved using previous TRAB approaches based on reversible metal deposition and dissolution processes, but further gains are possible in power and energy density. We investigated the limitations of power and energy density and how they are impacted by the electrolyte composition and discharge currents. By increasing the ammonia concentration from 1 to 5 M, the power density of the battery increased from 11.2 to 28.5 mW cm-2, but the energy density decreased from 0.56 to 0.31 Wh L-1. Increasing discharge current densities from 4 to 12.5 mA cm-2 increased the average power density during discharge from 2.4 to 5.9 mW cm-2 without appreciable losses in energy density. Increasing the copper concentration from 0.1 to 0.5 M increased both energy density to 2.15 Wh L-1 and energy efficiency to 2.2% but did not substantially impact the power density. These results represent the highest performance metrics achieved for a low-grade waste heat to electricity system.

Single-phase flow past a circular cylinder strongly correlates to Reynolds number, where the wake behind the cylinder becomes unstable and more turbulent as the Reynolds number increases [1]. In two-phase bubbly flow, the Reynolds number alone cannot describe the non-uniform distribution of void fraction that occurs in the wakes of circular cylinders [2,3]. This experimental investigation visualized bubble behavior in wakes generated from an upward liquid-gas flow around a cylinder. It focused on the bubble trapping phenomenon where bubbles were held in the wakes momentarily before flowing downstream.

This study presents a multi-objective design optimization for the shape of rectangular two-phase microchannels to improve heat transfer performance. Along with simplified two-phase flow and heat transfer models, a multi-objective genetic algorithm is used to minimize the objectives, microchannel pressure loss and thermal resistivity, with constraints on microchannel footprint, mass flow rate, and vapor generation. It was found that microchannels with variable cross-sections exhibited superior performance in comparison to their constant cross-section counterparts. Furthermore, this performance enhancement was greater at higher heat inputs and flow qualities. At low heat inputs, it was found that microchannel aspect ratio had the strongest impact on performance, while at high heat inputs, hydraulic diameter was the most important design parameter.

The silver-based thermally regenerative ammonia battery is a new technology for converting low-grade waste heat ( < 100 degrees C) to electrical power. These hybrid flow batteries are charged using energy derived from waste heat, rather than electrical power, and can be cycled hundreds of times without a decrease in performance. Design factors that impact operational conditions and performance are needed to im-prove reliability and increase power densities. For example, the structure of the porous electrodes could alter silver deposition rates and porosities of the electrodes during operation, leading to preferential flow paths from clogging of pores that would impact cycle longevity and power. A 2D numerical model was therefore used to study the relationship between fluid flow and electrodeposition in the porous elec-trodes using cylinders in cross-flow to represent the internal structure of a porous carbon fiber electrode. Lower void fractions increased peak power by 2.5% to 7.1% but resulted in pore clogging 3-5 times faster due to nonuniform deposition. It was found that staggered fiber arrangements maintained higher sur-face concentrations and 4.9% to 8.9% higher peak power compared to in-line fiber arrangements with the same void fraction while also displaying less dependence on fluid velocity due to efficient advection of reactants. An electrode with variable void fraction was designed to increase peak power by 7.5% but the pores clogged 23% faster compared to a similar electrode with a homogeneous void fraction. (c) 2021 Elsevier Ltd. All rights reserved.

As the need for a sharp reduction in carbon emissions continues to grow, many researchers are looking to electrochemical technologies to provide sustainable power to the grid. One promising technology is the silver thermally regenerative ammonia battery (Ag-TRAB), which is a new approach for converting low-grade waste heat (<100 °C) to electrical power. These hybrid flow batteries are charged using energy derived from waste heat, rather than electrical power, and can be cycled hundreds of times without a decrease in performance. The Ag-TRAB uses silver as the active metal ion for deposition and dissolution on each half of the battery and porous carbon as the electrode material, driven by ammonia added into only one chamber. Following a discharge cycle, ammonia is removed using waste heat, and a new cycle is initiated using the ammonia added to the opposite electrolyte chamber. Design factors that impact operational conditions and performance are needed to improve reliability and electrode utilization. For example, the structure of the porous electrodes could alter silver deposition rates and porosities of the electrodes during operation, leading to preferential flow paths from clogging of pores that would impact cycle longevity and power. A 2D numerical model was created using COMSOL Multiphysics to study the relationship between fluid flow, electrode structure, and electrodeposition in the porous electrodes using cylinders in cross-flow to represent the internal structure of a porous carbon fiber electrode. Lower void fractions resulted in 2.5% higher power density, but less uniform deposition and the pores clogging 3-5 times faster due to nonuniform deposition normal to the membrane boundary. It was found that the electric field resultant from the void fraction and fiber arrangement was critical to the performance of the electrode. The electrode microstructure can be modified to better distribute the deposition rate relative to the pore size to allow for higher power output for a longer discharge time before the pores begin to clog. An electrode with variable void fraction increased the peak power by 7.5% but the pores clogged 23% faster compared to homogenous void fraction.

Deep cuts to global carbon emissions are only possible with a dramatic increase in energy storage capacity. However, the cost of the storage options available hinders the timely deployment of the necessary energy storage. Promising supplements to conventional storage are thermally regenerative ammonia batteries (TRABs), which have an additional advantage of being able to capture low-grade waste heat into dispatchable electricity for the grid. This new battery technology can potentially provide power on demand using low-cost chemistries and waste energy sources relative to their purely electric battery counterparts. Most of the research to date for TRABs has been proof-of-concept in nature. As such, little is known about how the electrolytes used in these devices impact their theoretical energy storage capacity. Here we present the general principles of TRABs from an electrochemical and aqueous chemistry perspective. Energy storage density values and Eh-pH diagrams are presented to highlight key factors that govern these complex but versatile energy storage devices. Ag-based and Cu-based TRABs are discussed, and some experimental data confirming the conclusions of the Eh-pH diagrams are presented.