Net Power Density Research Articles

Channel to rib width ratio influence with various flow field designs on performance of PEM fuel cell

A three dimensional (CFD) model is applied to investigate the effect of geometry designs and dimensions of the flow field on the performance of PEM fuel cells with an area of 5.1 × 5.1 cm2 and composed of 25 channels. Three designs including serpentine, interdigitated and parallel with six channel to rib width ratios defined as (η = channel width/rib width) have been analyzed. In order to keep the area of the cell and number of channels constant, when the width of the channel increases the width of the ribs decreases and vice versa for a comparative purpose. At high operating voltage, the geometric designs and channel to rib width ratios have little influence on cell performance while at low operating voltage, it affects considerably the cell performance. The PEM fuel cell with serpentine designs has better performance. It increases by 4.6% and 39.1% compared to interdigitated and parallel, respectively. Moreover, decreasing channel width and increasing rib width improve cell performance. It increases up to 120% for serpentine, 45% for interdigitated and 23% for parallel when channel to rib width ratio decreases from 2.66 to 0.25. The local transport phenomena have been also analyzed. The pressure drop and under rib convection increase with decreasing channel to rib width ratio, which results in high reactants speed under the ribs and more homogenous local transport phenomena in the catalysts layer; enhancing the net power density.

Millimeter-scale lithium ion battery packaging for high-temperature sensing applications

Continuously growing and miniaturizing autonomous electronic devices and sensors for large temperature window is mostly depends on stability and performance of power-up systems. The net achievable power and energy densities of such miniatured rechargeable energy storage systems are largely dominated by internal hardware and external packaging materials. Similarly, temperature stability of electrochemically active materials and packaging components is also crucial to realize desired activity at few millimeter scale devices. Herein, we explore energy storage devices at tunable miniatured scale by selecting optimal electroactive materials, thermally stable packaging components, and their fabrication process. Further, an assembled device is subjected to evaluate its energy and power density per foot-print-area along with other key electrochemical parameters such as internal resistance, self-discharge, and thermal stability. Detailed studies reveal that the presently packaged millimeter size rechargeable batteries (2 mm–5 mm) have the ability to work up to 120 °C with minimal load loss and compatible with energy harvesting renewable source of the solar cell.

Optimization design of the cathode flow channel for proton exchange membrane fuel cells

Two novel cathode channel designs (multi-plates structure channel and integrated structure channel) are proposed and investigated by a three-dimensional multiphase numerical model. The numerical results indicate that the PEMFC with 30° angle, 0.5 mm width and 6.0 mm distance of air-guide plates exhibits the best performance, and the effect of plates angle on PEMFC performance is most significant. Compared with the conventional channel, the novel channel designs are found to be capable of forcing more oxygen towards cathode catalyst layer (CCL) to improve the electrochemical reaction rate. In addition, these two novel channel designs are able to remove more liquid water from PEMFC to effectively avoid from water flooding. According to the simulation results, the maximum improvement of PEMFC net power densities for the multi-plates structure and integrated structure are 4.7% and 7.5%, respectively.

Divalent Cation Removal by Donnan Dialysis for Improved Reverse Electrodialysis.

Divalent cations in feedwater can cause significant decreases in efficiencies for membrane processes, such as reverse electrodialysis (RED). In RED, power is harvested from the mixing of river and seawater, and the obtainable voltage is reduced and the resistance is increased if divalent cations are present. The power density of the RED process can be improved by removing divalent cations from the fresh water. Here, we study divalent cation removal from fresh water using seawater as draw solution in a Donnan dialysis (DD) process. In this way, a membrane system with neither chemicals nor electrodes but only natural salinity gradients can be used to exchange divalent cations. For DD, the permselectivity of the cation exchange membrane is found to be crucial as it determines the ability to block salt leakage (also referred to as co-ion transport). Operating DD using a membrane stack achieved a 76% reduction in the divalent cation content in natural fresh water with residence times of just a few seconds. DD pretreated fresh water was then used in a RED process, which showed improved gross and net power densities of 9.0 and 6.3%, respectively. This improvement is caused by a lower fresh water resistance (at similar open circuit voltages), due to exchange of divalent for monovalent cations.

Reverse electrodialysis: Modelling and performance analysis based on multi-objective optimization

In this paper, we proposed a refined model to describe the RED process by considering the variation of flow rates along the flow direction, and the concentration depended density and viscosity. The model was verified by good accordance between the calculated and experimental measured data. For evaluating the performance of a RED stack for some special applications, the net power density and the energy efficiency are two main criteria. However, they could not achieve their maximum values simultaneously. To achieve such a compromise, an optimization based on Non-dominated Sorting Genetic Algorithm II (NSGA-II) was conducted. Besides, the net power output and energy efficiency under single-objective optimization methods were calculated and compared. Results revealed that compared to the results under the maximum net power density, the net power density under the multi-objective optimization is slightly less than the maximum one, meanwhile the energy efficiency was much greater. The performance under the multi-objective optimization exhibited no obvious disadvantage against that under the maximum energy efficiency, considering the significant increase of the net power density.

Flexible Transparent Nanogenerators Utilizing Shape‐Modulated ZnO Nanorod Arrays on Graphene Electrodes

AbstractFlexible and transparent hybrid piezoelectric nanogenerators (PENGs) are prepared, based on vertical arrays of shape‐modulated ZnO nanorods (ZnO‐NRs) placed between two single‐layer graphene sheets. A simple transfer printing method using polydimethylsiloxane (PDMS) stamps in conjunction with polyethylene terephthalate (PET) enables the fabrication of flexible stacked structures of ZnO‐NRs/graphene/PDMS/PET, and the morphology of the ZnO‐NRs is controlled by metallic additives during their hydrothermal synthesis. The hybrid PENGs are transparent and generated electrical power upon bending; the power output performance is dependent on the nanorod morphology with a net volume power density of 0.0513 mW mm−3 for Cd‐doped ZnO‐NRs, which is comparable to that of today's energy storage devices. The variation in output power is ascribed to the differences in the deformation modes of ZnO‐NRs on bending because of the differences in their aspect ratios that are, in turn, controlled by the metallic additives; this suggests the possibility of customizing the power generation characteristics within a single material system.

Optimization on the operating conditions of PRO process: towards improved net power density

Optimization on the operating conditions of PRO process: towards improved net power density

Development of a Bi‐cell Proton Exchange Membrane Fuel Cell with Optimized Groove‐designed Piezoelectric Actuator

AbstractPrevious studies demonstrated a piezoelectric proton exchange membrane fuel cell (PZT‐PEMFC) stack design composed of three bi‐cells in series and a single bi‐cell, with a maximum net power density of 0.1608 W cm−2. The present study developed a modified bi‐cell design with lower volume and weight by using gold‐plated aluminum 6061. A groove‐designed PZT actuator enclosed with poly‐di‐methyl‐siloxane (PDMS) curing for 30 min can reduce uneven air feeding. Such an actuator can also improve the performance of both sides of a bi‐cell, with only a 0.7% difference in the open circuit voltage (OCV). The experimental results showed that the net power density of the new version of the single bi‐cell PZT‐PEMFC module was 0.1658 W cm−2.

A Fully Regenerable Thermal Silver Ammonia Battery to Convert Low-Grade Waste Heat to Electricity

A vast amount of low-grade thermal energy (temperature < 130 ⁰C) is available globally at industrial plants and from solar and geothermal sources. Converting these sources of energy to electricity has drawn increasing attention in recent years. Solid-state thermoelectrics (SSTs) and liquid-based thermoelectrochemical systems (TECs) have been widely investigated as means of converting low-grade waste heat to electrical energy. However, despite much progress in both the methods, these systems have failed to produce high power densities, and have not been cost-effective. A recently developed thermally regenerative ammonia batteries (TRAB) showed a significantly higher power production (~12 times more) than both SSTs and TECs. In a TRAB, electrical power is obtained from the formation of metal ammine complexes, which are produced by adding ammonia to the anolyte, but not to the catholyte. After the cell discharges, ammonia is separated from the anolyte using a conventional technology, such as distillation with low-grade waste heat, and then added to the other electrolyte for the next discharge cycle. The previous TRAB based on copper electrodes and a copper salt electrolyte showed a relatively high power production. However, unbalanced rates of anode dissolution and deposition of copper on the cathode limited the use of copper in closed-loop cycles. To address the reversibility issue, a silver-based TRAB was developed as an alternative to the copper-based TRAB. With silver, the cathodic and anodic coulombic efficiencies of the TRAB were the same (~100%), resulting in a reversible system for converting low-grade waste heat into electricity over many successive cycles. The developed silver system produced a net maximum power density of 30 W m−2-electrode area, with a net energy production of 490 Wh m−3-anolyte in a flow cell with an optimal hydraulic retention time (HRT) of 2 s. Successive deposition and dissolution cycling (i.e., the electrode reversibility) showed the system was stable over a hundred cycles. An initial economic analysis of the system showed that the price of electricity produced based on materials costs was 1.8 times more than the average electricity price is the U.S. ($ 120 MWh−1), due primarily to the cost of the membrane as well as sliver. However, this could be reduced to $ 120 MWh−1 if the cost of a membrane used in the system could be reduced to $10 m–2. Other potential benefits, such as elimination of air pollution, and beneficial issues related to health and climate change were not included in the comparison. Although the cost of building and operation relative to energy production of Ag-TRAB is currently higher than that of conventional technologies, this approach could generate a cleaner method of electrical power generation using a waste source of heat if the commercial cost of ion exchange membranes could be significantly reduced.

CO2 saturated water as two-phase flow for fouling control in reverse electrodialysis

When natural feed waters are used in the operation of a reverse electrodialysis (RED) stack, severe fouling on the ion exchange membranes and spacers occurs. Fouling of the RED stack has a strong influence on the gross power density output; which can decrease up to 50%. Moreover, an increase in the pressure loss occurs between the feed water inlet and outlet, increasing the pumping energy and thus decreasing the net power density that can be obtained. In this work, we extensively investigated the use of CO2 saturated water as two-phase flow cleaning for fouling mitigation in RED using natural feed waters. Experiments were performed in the REDstack research facility located at the Afsluitdijk (the Netherlands) using natural feed waters for a period of 60 days. Two different gas combinations were experimentally investigated, water/air sparging and water/CO2 (saturated) injection. Air is an inert gas mixture and induces air sparging in the stack. In the case of CO2, nucleation, i.e. the spontaneous formation of bubbles, occurs at the spacer filaments due to depressurization of CO2 saturated water, inducing cleaning.Results showed that stacks equipped with CO2 saturated water can produce an average net power density of 0.18 W/m2 under real fouling conditions with minimal pre-treatment and at a low outside temperature of only 8 °C, whereas the stacks equipped with air sparging could only produce an average net power density of 0.04 W/m2. Electrochemical impedance spectroscopy measurements showed that the stacks equipped with air sparging increased in stack resistance due to the presence of stagnant bubbles remaining in the stack after every air injection. Furthermore, the introduction of CO2 gas in the feed water introduces a pH decrease in the system (carbonated solution) adding an additional cleaning effect in the system, thus avoiding the use of environmentally unwanted cleaning chemicals.

Design and optimization of a reverse electrodialysis stack for energy generation through salinity gradients

Se presenta y resuelve un modelo para diseño de pilas de Electrodiálisis inversa. Se propuso una nueva función de optimización para RED, que tienen en cuenta el consumo de agua de río, la densidad de potencia neta y la eficiencia termodinámica del proceso. Los parámetros de tiempos de residencia y espesor de compartimientos fueron optimizados correctamente usando la nueva función propuesta. Los resultados sugieren pequeños tiempos de residencia, compartimientos de espesores delgados, poca área transversal y relaciones bajas de Longitud/ Ancho del compartimiento para máxima generación de energía.

Comparison of spacer-less and spacer-filled reverse electrodialysis

Reverse electrodialysis (RED) is a renewable energy technology used to recover dissipated chemical energy in river estuaries globally. This technology has recently attracted significant attention owing to its great reliability and scalability. In this study, we propose the use of a spacer-less RED (i.e., a system in which a woven mesh is excluded from the flow channel). The performance of spacer-less RED, including its gross power density, internal resistance, and hydraulic loss, is compared with that of the spacer-filled RED, in relation to the variation in the inlet flow rate. The mixing enhancement is more important than the spacer shadow effect when considering power generation. The spacer-filled RED has uniform internal resistance over the whole range of flow rates, while the spacer-less RED shows a dramatic decrease in resistance with the increasing flow rate. The hydraulic loss is much lower in the spacer-less RED. The maximal net power, accordingly, is generated at the flow rate of 3 ml/min (for spacer-filled RED) and 12.5 ml/min (for spacer-less RED). In the end, a maximal net power density of 0.62 W/m2 was obtained in both structures.

Experimental investigation on PEM fuel cell cold start behavior containing porous metal foam as cathode flow distributor

Metal foam has been regarded as one of the most important replacement for the conventional flow distributor of commercial fuel cells in recent years. One critical issue for the commercialization of proton exchange membrane (PEM) fuel cell is the successful startup from subzero temperatures. In this study, experimental tests on a PEM fuel cell using nickel metal foam as the cathode flow distributor are carried out to investigate the cold start performance. The cold start performance is also compared to a PEM fuel cell with parallel flow channels. Both galvanostatic and potentiostatic control are considered. The results show that under normal operating conditions the metal foam PEM fuel cell exhibits higher maximum net power density than the cell with parallel flow channels, whereas the parallel channel case exhibits slightly better performance at lower current densities. For cold start tests, metal foam is superior to the conventional parallel flow channel under galvanostatic control, due to its extremely porous structure, uniform mass and heat distribution. It is more difficult for PEM fuel cell under potentiostatic control to successfully start up due to possible ice blockage at the outlet.

Mass transport enhancement in redox flow batteries with corrugated fluidic networks

Abstract We propose a facile, novel concept of mass transfer enhancement in flow batteries based on electrolyte guidance in rationally designed corrugated channel systems. The proposed fluidic networks employ periodic throttling of the flow to optimally deflect the electrolytes into the porous electrode, targeting enhancement of the electrolyte-electrode interaction. Theoretical analysis is conducted with channels in the form of trapezoidal waves, confirming and detailing the mass transport enhancement mechanism. In dilute concentration experiments with an alkaline quinone redox chemistry, a scaling of the limiting current with Re0.74 is identified, which compares favourably against the Re0.33 scaling typical of diffusion-limited laminar processes. Experimental IR-corrected polarization curves are presented for high concentration conditions, and a significant performance improvement is observed with the narrowing of the nozzles. The adverse effects of periodic throttling on the pumping power are compared with the benefits in terms of power density, and an improvement of up to 102% in net power density is obtained in comparison with the flow-by case employing straight parallel channels. The proposed novel concept of corrugated fluidic networks comes with facile fabrication and contributes to the improvement of the transport characteristics and overall performance of redox flow battery systems.

Improved fluid mixing and power density in reverse electrodialysis stacks with chevron-profiled membranes

Spacer-less RED stacks using membranes with integrated spacer profiles have been investigated during the last years to eliminate the spacer shadow effect. The presence of spacers partially blocks the membrane surface and creates a tortuous and thus longer path for ions in the channel, meaning higher ohmic resistance. Consequently, power outputs are reduced. Profiled membranes may solve this problem as they provide flow channels for the feed streams, while the relief formed on their surfaces keeps the membranes separated. Although the geometry and arrangement of so far used profiles led to lower ohmic resistance, it did not grant an efficient fluid mixing. Recently, so-called chevron profiles, with enhanced mixing, were proposed based on computational fluid dynamics (CFD) simulations. In the present study, the performance of such chevron-profiled membranes, prepared by thermal pressing, was experimentally validated in a reverse electrodialysis (RED) stack. According to the obtained experimental values of non-ohmic resistance and total pressure drop across the RED stack, the chevron-profiled membranes assure efficient fluid mixing at comparatively low hydraulic losses. The net power density obtained with chevron-profiled membranes was the highest obtained for the present stack design. It outperformed the alternative RED stack configurations investigated in this study, such as channels with optimized spacers and channels formed by pillar-profiled membranes. To allow for an even more straightforward and efficient RED stack assembling with chevron-profiled membranes, recommendations for a further simplified design, consisting of diagonal ridges that are assembled perpendicularly, are provided.

Investigation of the performance improvement in decreasing aspect ratio interdigitated flow field PEMFCs

The goal of this study was to determine the main cause of performance improvement in decreasing aspect ratio interdigitated flow fields, where aspect ratio is the channel length to width ratio. An experimental test cell with an interdigitated flow field operated under various aspect ratios found increasing net power densities with decreasing aspect ratio, after accounting for parasitic pump losses. In-situ neutron radiography found there was more water present in high aspect ratio flow field designs than in low aspect ratio designs. It also found more water in the 1.5/2 stoichiometry conditions than in the 2/4 stoichiometry conditions further indicating liquid water is a probable cause of performance changes. A single phase model used to determine the difference in power from the changing distribution of cross flow found decreasing aspect ratio resulted in higher overall performance. The experiment had significantly greater losses in power density with increasing aspect ratio, indicating liquid water removal was the major contributor to the improvement in net power density rather than the distribution of cross flow. Designers of PEMFCs should use a low aspect ratio design for interdigitated flow fields, or include a water removal mechanism for situations where high aspect ratio is required.

3D-printed fluidic networks for high-power-density heat-managing miniaturized redox flow batteries

The miniaturization of redox flow cells (RFCs) paves the way to novel energy conversion concepts combining power delivery and heat regulation. Envisioning the integration of high-power-density RFCs into electronic devices such as microprocessors, lasers, or light-emitting diodes for the purpose of providing power and heat management simultaneously, we introduce and investigate interdigitated, tapered multiple-pass microfluidic networks in miniaturized flow cells. Employing 3D-printing for the facile and inexpensive fabrication of these networks, we demonstrate RFCs with maximum power densities of up to 1.4 W cm−2 at room temperature and net power densities of up to 0.99 W cm−2 after subtracting pumping power losses. The electrolytes employed modest concentrations of 0.4 M K4Fe(CN)6 and 0.2 M 2,6-dihydroxyanthraquinone in alkaline electrolyte. We thereby show that rational tailoring of fluidic networks in RFCs is key for the development of devices effectively combining power delivery and thermal management.

Different Solar Potential Co-Ordinates of Pakistan

MIT, Stanford University and University of California have recently made a research on atmosphere/energy program. The research group has made a plan and concluded that a renewable energy powered world indeed a possibility by 2050. They have analysed the future energy demand of 139 countries and compared the solar, wind and hydroelectric potential of the countries. The research team made an interactive global map which shows the projected energy mix on each of the analysed countries. The report of the map study shows that Pakistan has potential of producing 92% of its electricity requirements from solar energy, at a rate that’s amongst the highest in the world. Solar PV plants can produce 58.1% and concentrated solar plants (CSP) can produce 15% of electricity production. Study of the different solar potential co-ordinates of Pakistan is worth for forth coming planning to utilize solar potential properly and meet power demand of the country. This research is aimed to study the irradiation net power density (in MJ/m2/day and MWh/m2/year) of different co-ordinate of Pakistan. In parallel to study the install capacity of solar thermal and solar PV on district and provincial levels, study solar maps of Pakistan and different seasons on annual basis.

Modeling of power generation with thermolytic reverse electrodialysis for low-grade waste heat recovery

Significant attention has been paid to closed-loop reverse electrodialysis (RED) systems using a thermolytic solution for low-grade waste heat energy recovery. They have several cost benefits when compared with open-loop RED with seawater and river water, such as no need of repetitive pretreatment and removal of locational constraints. This study presents the model of RED using ammonium bicarbonate (NH4HCO3), one of the promising solutes for the closed-loop RED, whose ionization has not been clarified. Because of the unclarified electrochemical information of NH4HCO3 electrolyte, the Planck-Henderson equation was used to approximate the membrane potential based on conductivity measurements, and the solution resistance was experimentally computed. Furthermore, the experimentally obtained permselectivity of the membrane was applied for a more precise estimate of the membrane potential. We found that the developed NH4HCO3-RED model was in good agreement with the experimental results under various operating conditions. We also characterized the net power density, which considers the pumping loss, by using our model. In our system, the maximum net power density of 0.84W/m2 was obtained with an intermembrane distance of 0.1mm, a flow rate of 3mL/min, and a concentration ratio of 200 (2M/0.01M) as optimum conditions. We expect that this study will improve our understanding of the NH4HCO3-RED system and contribute to relevant modeling studies, using NH4HCO3 or some other compounds, for generating higher energy densities.

The Breathing Cell: Cyclic Intermembrane Distance Variation in Reverse Electrodialysis.

The breathing cell is a new concept design that operates a reverse electrodialysis stack by varying in time the intermembrane distance. Reverse electrodialysis is used to harvest salinity gradient energy; a rather unknown renewable energy source from controlled mixing of river water and seawater. Traditionally, both river water and seawater compartments have a fixed intermembrane distance. Especially the river water compartment thickness contributes to a large extent to the resistance of the stack due to its low conductivity. In our cyclic approach, two stages define the principle of the breathing concept; the initial stage, where both compartments (seawater and river water) have the same thickness and the compressed stage, where river water compartments are compressed by expanding the seawater compartments. This movement at a tunable frequency allows reducing stack resistance by decreasing the thickness of the river water compartment without increasing permanently the pumping losses. The breathing stacks clearly benefit from the lower resistance values and low pumping power required, obtaining high net power densities over a much broader flow rate range. The high frequency breathing stack (15 cycles/min) shows a maximum net power density of 1.3 W/m2. Although the maximum gross and net power density ever registered (2.9 W/m2 and 1.5 W/m2, respectively) is achieved for a fixed 120 μm intermembrane distance stack (without movement of the membranes), it is only obtained at a very narrow flow rate range due to the high pressure drops at small intermembrane distance. The breathing cell concept offers a unique feature, namely physical movement of the membranes, and thus the ability to adapt to the operational conditions and water quality.