Net Power Density Research Articles

Optimal design of a high-performance heat exchanger for modular thermoelectric generator towards low-grade thermal energy recovery

Thermoelectric generator (TEG) has been identified as a promising method for low-grade thermal energy recovery owing to their lack of moving parts, scalability, and compatibility with other devices generating waste heat. Heat exchanger, as one of the most important components of the modular TEG, plays a crucial role in improving the overall performance of the TEG. Nevertheless, flow maldistribution inside the heat exchanger results in uneven surface temperature field of the heat exchanger, which will ultimately limit the output capacity of the TEG. Achieving a homogeneous flow distribution within the heat exchanger while minimizing flow resistance is essential. To address this, optimization of a plate-shaped heat exchanger for the modular TEG is conducted using CFD analysis and the Taguchi method to identify the optimal combination of parameters. The optimized heat exchanger demonstrates a flow maldistribution intensity (ζ) of only 4.75 % and a low flow resistance of 1.16 kPa. Furthermore, a unit of the modular TEG is constructed using two optimized heat exchangers and commercial thermoelectric modules (TEMs), and its performance is analyzed via an analytical model. The results indicate that the power per module, net power density, and conversion efficiency reached 1.2 W, 51.4 kW/m3, and 1.92 %, respectively, at a temperature difference of 70 °C. These findings suggest that the optimized heat exchanger could provide high output performance compared with other literature, offering significant potential for low-grade heat energy recovery.

Investigating the effect of temperature and concentration on the performance of reverse electrodialysis systems

Reverse electrodialysis (RED) is a membrane-based technology proposed for harvesting electricity from a salinity gradient. In this study, a detailed examination of the effect of temperature and concentration on RED membrane properties is undertaken. Modelling of the co-ion concentration as a function of temperature allows the Donnan potential to be estimated, while membrane resistance and ion diffusivity can be modelled by an Arrhenius relationship with temperature. Membrane resistance is best modelled using a reciprocal relationship with concentration, while permeability and diffusivity show a power law dependence upon concentration. Using these results, the effect of increasing the temperature of the entire RED system is compared to the case where only the temperature of the diluate solution is increased. The model is in good agreement with experimental results across temperatures from 10 to 40 °C. It shows that a comparable increase in gross power density can be achieved when only the temperature of the diluate solution is increased, relative to increasing the temperature of the entire system. However, increasing the temperature also reduces the pumping energy required for each stream. In the present case, this reduction in pumping energy with temperature is more significant than the changes within the stack itself. Thus, an increase in temperature of the entire system from 20 to 40 °C results in a 27 % increase in net power density as compared to an increase of the diluate solution alone. It is thus concluded that increasing the temperature of the entire system (rather than just the diluate stream) provides a pathway to increasing the power density of the system if waste heat is available, promoting its adoption for energy harvesting.

Performance of the multi-U-style structure based flow field for polymer electrolyte membrane fuel cell

The design of the reactant gas flow field structure in bipolar plates significantly influences the performance of proton exchange membrane fuel cells (PEMFCs). In this study, we introduced four innovative U-shaped flow field designs, namely: In-Out Multi-U, Out-In Multi-U, Distro In-Out Multi-U, and Distro Out-In Multi-U. To investigate the impact of these various flow fields on PEMFC performance, we conducted computational fluid dynamics (CFD) numerical simulations, validated through model experiments. Our results indicate that the Distro Out-In Multi-U flow field offers notable advantages compared to the conventional parallel flow field (CPFF) and conventional serpentine flow field (CSFF). These benefits include reduced inlet and outlet pressures, lower liquid water content, more uniform liquid water distribution, and a more even current density distribution. Furthermore, the Distro Out-In Multi-U design demonstrates improved efficiency, consuming less H2 (91.9%) than the CSFF while achieving a higher net power density output (10.1%). As a result, for the same power output, the Distro Out-In Multi-U utilizes only 83.5% of the H2 consumed by the CSFF. In summary, the U-shaped structured flow field exhibits superior output performance, enhanced energy efficiency, and improved resistance to flooding. These findings suggest that the U-shaped flow field design holds significant potential as a reactive flow field for PEMFCs.

Effect of tapered porous ribs on the performance of Proton exchange membrane fuel cells

The conventional trench ridge structure suffers from the difficulty of removing liquid water and poor gas transport performance. The water removal and gas transport performance inside the flow field are enhanced by introducing a tapered porous rib flow field (TPRFF). In order to investigate the performance of TPRFF, a complex three-dimensional, multiphase, non-isothermal agglomeration model was developed using numerical techniques. The numerical results show that the TPRFF exhibits significant improvements in oxygen concentration distribution, oxygen distribution uniformity and water removal compared to a conventional parallel flow field (CPFF). In addition, the dissolved water content remained almost constant while the liquid water saturation was lower. The study also investigated the effect of different cathode exit heights on the performance of the fuel cell using TPRFF. The results showed that the net power density increased by 14.0 %, 11.9 %, 8.7 %, and 1.5 % for outlet heights of 1.3 mm, 1.0 mm, 0.7 mm, and 0.4 mm, respectively, while it decreased by 27 % for an outlet height of 0.1 mm. Finally, the effect of different stoichiometric ratios on cell performance was also investigated in the novel flow field.

Numerical Comparative Investigating of PEMFC with Novel Hybrid Zigzag Channels: Reaction, Transport and Drainage Enhancements

The impact of flow channel design on mass transport and drainage in proton exchange membrane fuel cells (PEMFCs) is significant, thereby influencing the reaction rate. Based on conventional wavy design, this study introduces two novel hybrid zigzag flow channels (asynchronous and synchronous) with both zigzag sidewalls and bottom wall, aiming in further improving mass and heat transfer, as well as drainage capacity to achieve better fuel cell performance. Numerical simulations demonstrate that the net power densities of both asynchronous and synchronous hybrid zigzag channels show a 28.7% and 44.4% improvement at low voltage, respectively. The implementation of the asynchronous hybrid zigzag flow channel has been observed to result in a notable reduction in pressure drop, amounting to 9.2%, while concurrently enhancing power output by 10.7% in comparison to a conventional zigzag channel. Additionally, the novel hybrid zigzag designs improve mass transfer efficiency at high current density and exhibits better temperature distribution uniformity. Moreover, the volume of fluid simulations illustrate that hybrid zigzag channels are highly effective in removing accumulated water, surpassing the straight channel with a drainage rate exceeding 54%, as well as a lower surface liquid coverage.

Airfoil cross flow field to enhance mass transfer capacity and performance for PEMFC

Proper design of the flow field in bipolar plates is important for improving proton exchange membrane fuel cells in water transport, gas distribution and net power density enhancement. Inspired by the fact that the streamlined design of an airplane airfoil makes the flow resistance reduced, a new airfoil cross flow field is proposed. Airfoil cross flow field is composed of a regular pattern of airfoil pins which are categorized in pin-type flow fields. The effects of different airfoil-pin shapes and arrangements on cell performance were explored. The results showed that airfoil cross flow field improved water management by finely modulating the airflow significantly increasing the gas flow rate in the channel. In addition, the airfoil cross flow field design achieves higher and more uniform oxygen distribution at the interface between the gas diffusion layer and the catalyst layer. The proper shape and arrangement of the airfoil-pins can effectively increase the net power density of the cell by 10.65 % and 2.49 % compared to the conventional parallel flow field and the square-pin flow field. Due to the streamlined design of the airfoil cross flow field, the voltage drop is approximately 60 % of that of the conventional parallel flow field and even slightly lower than that of the square-pin flow field, which demonstrates its high practicality. In summary, the airfoil cross flow field well coordinates the high current density and low voltage drop requirements of proton exchange membrane fuel cells, and some new points of view on flow field design are presented.

Enhancement of mass transfer through under-rib convection in polymer electrolyte fuel cell with dimensionless moduli analysis

The significance of under-rib convection in the flow field development of polymer electrolyte fuel cells (PEFCs) is increasingly recognized. This study quantitatively evaluates the interplay between mass transfer and reaction processes in adjacent channels, affected by under-rib convection. A comprehensive analysis identifies that oxygen mole fraction and current density profiles in the channel-rib direction are governed by five dimensionless moduli. The effectiveness factor, defined as the ratio of the integral average under-rib current density to that without mass transfer resistance, elucidates the potential for current density enhancement due to under-rib convection. Experimental results demonstrate a significant increase in the under-rib current density from 0.15 to 0.32 A cm⁻2 as the pressure difference between adjacent channels rises from 0 to 929 Pa at 0.60 V and 0.52 relative humidity. This corresponds to an increase in the effectiveness factor from 0.37 to 0.77 and a Péclet number increase from 0 to 34.8. Model predictions indicate that the maximum net power density is achieved at a Péclet number of 60, balancing under-rib convection benefits and pumping pressure. Validated by experimental results, the model aids in reaction kinetics analysis and polarization prediction, offering significant insights and practical guidelines for electrochemical reactor optimization.

Parametric analysis of reverse electrodialysis process for improved power generation: Influence of spacer thickness, feed solution flow rate, membrane, and concentration

Reverse electrodialysis (RED) technology produces electrical energy from a salinity gradient by transporting ions through ion-exchange membranes (IEM). Although research into this technology has recently seen considerable growth, there is still interest in improving its efficiency. As a result, a mathematical model to study the RED process was developed using Aspen Custom Modeler software, focusing on three membranes (Fumasep (FAD/FKD), Fujifilm Type 10, and Selemion (AMV/CMV)). A parametric analysis was conducted, considering spacer thickness, salinity gradient, and feed-solution flow rates. Evaluation of internal resistance, gross power density, and net power density was also performed. Results showed that using a thicker spacer (270 μm) in the high compartment and a thinner spacer (150 μm) with a high Reynolds number in the low compartment led to higher net power densities of 2.17 W·mcp−2 and 6.74 W·mcp−2 for Fumasep (FAD/FKD) membrane with (brackish/seawater) and (brackish/brine), respectively. Among the membranes tested, Fumasep (FAD/FKD) and Fujifilm Type 10 had similar results, with the former slightly exceeding the latter. However, the Selemion membrane (AMV/CMV) performed poorly in all tests.

Modeling of Lithium-Ion Convection Cell Effects on Reaction Distribution and Different Cathode Active Materials

Li-ion batteries (LIBs) could play a substantial role in mitigating climate change if they become more widely adopted in transportation and stationary energy storage applications. Increasing this adoption requires improvement in both power density and energy density. At the cell level, this dual requirement calls for designs and active materials that overcome challenging trade-offs in nominal cell voltage, rate capability, and loaded capacity.In our previous research,1,2 we simulated that a convection cell with liquid electrolyte flow, using LCO, can overcome the trade-offs by enhancing accessed capacity under high-rate operation. Electrolyte flow achieves this enhancement by effectively addressing both bulk electrolyte mass transfer and thermal limitations. The flow of electrolyte raises cell voltage by promoting a uniform Li+ salt concentration and regulates temperature by removing heat and lowering heat-producing overpotentials.In our current work, we extend our macrohomogeneous modeling analysis of a Li-ion convection cell to explore the impact of concentration uniformity on charge-transfer reaction distribution and the flexibility of this approach to work with different active materials. Concentration uniformity evens the reaction distribution, facilitating spatially consistent and full utilization of the active material in the electrodes. Additionally, the use of flow enables the cell to maintain rate capability across various electrode thicknesses and areal capacity loadings. At high C rates, a convection cell with flow exhibits significantly higher output capacity than a closed cell without flow. The degree of increase in running voltage and delivered capacity depends on the choice of positive electrode active material—LFP, LCO, NCA, NMC, or LMO.Finally, simulations of net power density and net energy density account for pumping energy loss across the cell. The improvement in net energy density with flow becomes substantial at high power density. W. Gao, M.J. Orella, T.J. Carney, Y. Román-Leshkov, J. Drake, F.R. Brushett, J. Electrochem. Soc., 167, 140551 (2020).W. Gao, J. Drake, F.R. Brushett, J. Electrochem. Soc. 170, 090508 (2023).

Multi-objective optimization of comprehensive performance enhancement for proton exchange membrane fuel cell based on machine learning

The comprehensive performance of proton exchange membrane fuel cells depends on operating conditions. This paper innovatively uses the Pearson correlation coefficient to screen the optimization objectives (uniformity index of oxygen, standard deviation of temperature, net power density), and obtains the optimal operating conditions of the proton exchange membrane fuel cell through a multi-objective optimization method. The optimized dataset comes from the simulation results of the three-dimensional numerical model, and the regression model is established through the response surface method. Moreover, the non-dominated sorting genetic algorithm-II is used for processing to obtain the Pareto front solution set, and the optimal operating conditions are obtained from it through the Technique for order preference by similarity to an ideal solution. The analysis of variance result shows that the influence of cathode operating conditions on the comprehensive performance is greater than that of anode, especially the influence of cathode stoichiometry ratio is the most significant. The optimal solution obtained 1.0981 %, 10.5845 %, and 1.0376 % enhancement compared to the optimal values in the simulation results. The differences between the three optimization objectives are only 0.8190 %, 1.0315 %, and 0.8789 % as verified by numerical simulation, thus the machine learning results are reliable and accurate.

AI optimization framework using digital layouts of array structures: A case study for fuel cells

It is challenging for common artificial intelligence (AI) frameworks to optimize layouts of array structures, because AI is unable to recognize and adjust layouts. To address this challenge, a novel AI optimization framework has been proposed, where layouts of array structures are represented in digital codes to enable AI in layout optimizations. The optimizations of baffle layout and modular channel layout in proton exchange membrane fuel cells are used to validate the methodology. For the multi-objective optimization of baffle layout, although the channel with optimal baffle layout (OC) exhibits a little higher pressure drop than the conventional channel, it increases the net power density and oxygen uniformity by 2.4% and 1.5%, respectively. Consistently, for the multi-objective optimization of channel layout, the optimal flow field with an optimal channel layout (OFF-OC) shows a small increase of pressure drop that can be compensated by the much-enhanced output. So, the net power density of OFF-OC has been elevated by 7.0% in comparison with that of the conventional parallel flow field. Meanwhile, the oxygen uniformity of OFF-OC has also been improved by 4.0%. The simulation results have been verified experimentally. These examples have demonstrated the effectiveness of the AI optimization framework using digital layouts.

Nitrogen and Sulfur Co-doped Biomass-Derived Porous Carbon Electrodes for Ultra-High-Performance All-Aqueous Thermally Regenerative Flow Batteries.

Developing high-performance electrodes for the all-aqueous thermally regenerative ammonia battery (ATRB) system, serving as superior substitutes for commercial carbon cloth electrodes, is anticipated to enhance performance, yet it lacks effective guidance and research. In this work, theoretical analysis is initially used to evaluate the effective conversion and adsorption capacity of nitrogen and sulfur co-doped carbon with respect to copper ion by density functional theory calculation. On the basis of this concept, the nitrogen and sulfur co-doped biomass-derived porous carbon electrode (DGC) is prepared using natural porous carbon materials and thiourea. Compared with commercial carbon cloth electrodes, ATRB with DGC achieves a significant improvement in maximum power density of 49.2%. Via optimization of the doping conditions, the active sites can be effectively regulated to boost charge transfer at the reaction interface. Furthermore, the rapid charge transfer can match the excellent mass transfer performance, generating an impressive net power density of 847.5 W/m2.

Microscale structure optimization of catalyst layer for comprehensive performance enhancement in proton exchange membrane fuel cell

The catalyst layer plays a significant role in promoting the redox reaction of proton exchange membrane fuel cells (PEMFC). Especially at high current densities, as the oxygen reduction rate increases and more water is produced, the pores of the catalyst layer are easily clogged, thus preventing the oxygen from reaching the reactive sites. In this work, a cathode catalyst layer with micro-grooved structure is proposed to optimize the water-gas transport and output performance of PEMFC. Three-dimensional two-phase numerical simulation results show that the catalyst layer with grooves significantly improves the water-gas transport performance of PEMFC, with a maximum increase of 4.76 % in output performance. Combining the response surface methodology and non-dominated sorting genetic algorithm-II for multi-objective optimization (MOO), the optimal solution corresponding to anode/cathode relative humidity and ionomer volume fraction is confirmed to be 86.73 %, 80.10 %, and 0.2592, respectively. The optimization objectives (oxygen concentration, liquid saturation, and net power density) are improved by 3.88 %, 1.95 %, and 1.79 %, respectively. The differences between the three optimization objectives obtained by numerical simulation verification are only 0.61 %, 0.32 %, and 0.55 %, indicating that the MOO results are reliable and accurate.

A novel design of inlet/outlet header structure for improving flow uniformity and performance in planar solid oxide fuel cell

The poor flow uniformity of parallel flow field in Solid oxide fuel cell limits its performance. Based on computational fluid dynamics, a three-dimensional numerical model of a solid oxide fuel cell was established to improve performance by improving flow uniformity. The influence of different number, shape and arrangement of obstacles on fuel cell is studied, and optimization is also made according to flow distribution. Results show that flow uniformity can be improved by using rectangular obstacles and arranging them at the rear of inlet header. Compared with structure without obstacles, the structure with four rectangular obstacles in inlet header can reduce flow non-uniformity coefficient by 55.8%. Optimized structure uses obstacles with different heights, and two obstacles are also set in outlet header. Compared with basic structure, the flow non-uniformity coefficient of optimized structure is reduced by 87%, while current density and net power density are increased by 10.1% and 24.2% respectively.

Polymer electrolyte fuel cell operating with nickel foam-based gas diffusion layers: A numerical investigation

Due to their outstanding structural, transport and electrical characteristics, nickel foams serve as excellent candidate materials for gas diffusion layers (GDLs) in polymer electrolyte fuel cells (PEFCs). In this work, a new three-dimensional PEFC model was developed to explore the local and global fuel cell performance with nickel foam-based GDLs. The fuel cell operating with nickel foam GDLs was shown to have, due to its superior mass and charge transport properties, higher oxygen and water concentration and current density compared to that operating with the conventional carbon fibre-based GDLs. The results show that the pumping power should be taken into account when optimising the dimensions of the flow channels and as such the net power density must be the criterion for optimisation. The optimal dimensions of the flow channels for the fuel cell operating with nickel foam based GDLs were found to be 0.25 mm for the channel height and 1 mm for the channel width; the maximum net power density with these dimensions was around 0.95 W/cm2 which is two times higher than that operating with carbon fibre based GDLs. All the results have been presented and critically discussed.

Effects of three-dimensional type flow fields on mass transfer and performance of proton exchange membrane fuel cell

Developing state-of-the-art bipolar plate structures, to optimize fluid distribution, is essential to achieve better cell performance. In this paper, a three-dimensional model of PEMFC is developed and two cathode-side flow field structures are designed: two-dimensional and three-dimensional types. The two-dimensional type is the traditional “bipolar plate + gas diffusion layer” flow field structure, easily resulting in uneven distribution of reactant gas and other problems. This study describes three novel three-dimensional type flow fields (metal foam, fine-mesh, and wire-mesh). Through comprehensive performance analysis using numerical simulations, it is found that the three-dimensional type flow fields significantly improve mass transfer and output performance compared to two-dimensional type flow fields. The proposed three-dimensional type flow fields create forced convective fluid flow. It further enhances the diffusion dispersion of reactant gas, thus making fuller use of the active region. Among them, the wire-mesh flow field shows the best performance in terms of oxygen distribution, water distribution and electrical properties. The net output power density produced is 0.75068 W cm−2, higher than parallel flow field by 32.78%.

Bio-inspired sinusoidally-waved flow fields with exchange channels to enhance PEMFC performance

Wave-shaped structures inspired by cuttlefish fins have been applied to the design of flow fields for proton exchange membrane fuel cells (PEMFCs). However, the wave structures often cause high parasitic power. Herein, we intend to address this issue by introducing exchange channels (ECs) between neighboring wave channels. First, the performances of bio-inspired sinusoidally-waved flow fields (BSWFFs) of different sinusoidal parameters are compared to identify the optimal sinusoidal structure. Subsequently, ECs are introduced into BSWFFs with the optimal sinusoidal structure to obtain EC-BSWFFs, and EC-BSWFFs with different numbers of ECs are compared. The introduction of ECs improves the distribution of reactant gases, pressure distribution, water management and reduces parasitic power density. Specifically, the maximum net power density of the single-cell PEMFC with EC-12 is enhanced by 10.26% compared to that of the conventional parallel flow field. Moreover, as the number of ECs in EC-BSWFFs increases, the uniformity of pressure distribution gradually increases and the pressure drop becomes smaller, suggesting a balance between performance and durability. Therefore, this work affords valuable insights to solve the difficulties of excessive pressure drop and inhomogeneous reactant distribution for waved flow fields.

The Effect of Obstacle Geometric Feature in Parallel Flow Field on PEMFC Output Performance

Abstract The performance of proton exchange membrane fuel cells (PEMFCs) can be enhanced by introducing barriers in the parallel flow field, which improves reactant transport and induces adequate reaction. In this study, five different shapes of obstacles where introduced in the research which were dispersed and placed in a parallel flow field. The effects of these different shaped obstacles on PEMFC output performance were compared by simulation. When reactants pass through the obstruction, the velocity increases, leading to higher concentration of reactants in the catalytic layer. This results in more a complete reaction and improved performance. The study demonstrates that incorporating 16 uniformly placed obstacles in the sub-flow channel of parallel flow field, with each obstacle being right-angled trapezoid shape and cut-in angle of 30°, measuring 1 mm in length, 0.3 mm in width, and 1 mm in height, resulted in net power density is 0.57 W/m2, an increase of 43% and 16% improvement in water removal capacity compared with the standard technique.

Design of a flow channel with vortex generators for performance improvement of flow-electrode capacitive mixing under low pressure drop conditions

Salinity gradient power generation is a novel renewable energy technology that generates electricity by exploiting the disparity in salt concentration between bodies of water with high and low salt content, using techniques such as capacitive mixing with flow electrodes (F-CapMix). In this research, we conducted a comprehensive numerical analysis to examine the fluid dynamics and particle behavior of saline water and activated carbon within a flow electrode system equipped with vortex generators on the inner walls of the flow channels. The maximum and net power densities of the F-CapMix for a flow channel with vortex generators were evaluated experimentally. On average, the introduction of vortex generators into the flow channel resulted in a 35.6 % increase in the maximum power density of the F-CapMix compared to a plain channel. Furthermore, when a vortex generator was incorporated, the net power density, considering the pumping power, showed an average enhancement of 26.3 % at flow rates of 15 ml/min or less. Therefore, we anticipate that the flow channel with integrated vortex generators can be effectively utilized in the F-CapMix electrode system with a negligible impact on the pressure drop.

Optimal design of locally improved structure for enhancing mass transfer in PEMFC cathode flow field

At high current density, the mass transfer in proton exchange membrane fuel cells (PEMFC) is significantly influenced by the flow field structure. In this paper, a bilateral track flow field (BTFF) is proposed to improve the transverse and longitudinal mass transfer as well as the water removal capacity of PEMFC. The geometry of the BTFF is optimized by orthogonal tests to obtain the optimal combination of geometric parameters and levels. Then using numerical modeling, the impact of the optimized BTFF (BT6) on the efficiency of the PEMFC was examined. The results show that BT6 significantly improves the utilization of the under-rib region by shortening the reactive gas diffusion path length to obtain a more uniform oxygen distribution, and the average oxygen concentration in the midline of the under-rib of the BT6 is increased by 102.60%, furthermore, the current density is increased by 16.32% compared with that of the parallel flow field (case0). In addition, by reducing the cross-sectional area of the gas flow channel, the BT6 has a higher gas flow velocity, which accelerates the removal of cathode liquid water, and the average liquid water saturation in the midline of the under-rib of the BT6 is reduced by 15.26% compared with case0. Finally, by analyzing the net power density of PEMFC, it can be found that the net power density of BT6 is increased by 15.68% compared with case0, which has good practical application value.