1,721,042 research outputs found
A Study of Flow through Cellular Structures with Periodic Pore Geometry and Varying Unit Cell Sizes
No full textNumerical Investigation of Pressure Retarded Osmosis with Brines for Power Generation
No full textIn 2016 the global electric power consumption was 15567 TWh. A recent estimate claims that saline reservoirs contain salinity gradient power equivalent to 647 GW which amounts to 5668 TWh or 36.4% of the global consumption. For exploiting this energy reserve pressure retarded osmosis (PRO) has shown promise. PRO is a technology which can partially extract the energy from salinity gradients by separating a saline fluid from a pure/low-salinity fluid with a semi-permeable membrane. By implementing this membrane the trans-membrane gradients are converted into flow energy that can be extracted as mechanical work. The research presented in this report focuses on geothermal brines and shows that trans-membrane temperature differences between the geothermal and the low-salininty water similarly can be converted into mechanical work. Subsequently a phenomenological membrane model that can predict permeation rates based on the gradients is formulated and coupled with a computational fluid dynamics framework. The model can predict the effects of the important degradative phenomena of internal and external concentration polarization in terms of an energy flux loss. Performed simulations show that even a low solute mass fraction in the low-salinity stream of 1.39% yields an energy loss of 47%. Furthermore, it is concluded that trans-membrane gradients in temperature are not negligible and that the permeation rate exhibits a non-linear dependence on these. The model is the first to represent the effects of temperature gradients and the research has formed a basis for procuring a comprehensive framework which can predict membrane performance considering all fundamental driving forces. This will prove valuable for advancing the commercial implementation of the technology
Numerical Investigation of Manifold Flow Distribution for a Proton-Exchange Membrane Fuel Cell Using Computational Fluid Dynamics
No full textA computational analysis of heat transfer in an air-cooled proton exchange membrane fuel cell for unsteady flow conditions and feasibility of heated turbulence grid in subzero conditions
No full textA computational fluid dynamics study to evaluate the effect of an air-cooled fuel cell stack using a turbulence inducing grid under transient operating conditions
No full textCooling a Proton Exchange Membrane Fuel Cell (PEMFC) using only air flow is gaining popularity as a cooling method due to its low cost and simplicity. Fuel cells that use air cooling are already commercialized up to a power range of a few kWs. These fuel cells are limited in current density, and by extension power density, in the amount of heat that can be effectively dissipated by the airflow. By placing a grid in front of the cathode flow channel of the fuel cell stack, experiments have shown the power density could be increased by over 30\% as a direct consequence of increased convective cooling. This study uses Computational Fluid Dynamics (CFD) to verify and quantify the effect of a turbulence inducing grid placed at various distances from the cathode channel, as well as under transient operating conditions. At an electrode heat flux of 2587 W/m2 the study showed that the average temperature of the Gas Diffusion Layer (GDL) and bipolar plates could be reduced by 2.5°C with a turbulence inducing grid. A further temperature reduction of the GDL between 0.3-2°C was seen by continuously varying the speed of the cooling fan
Investigation of Overpotential Distributions in a Solid Oxide Electrolysis Cell using Experimental and Modeling Approaches
No full textThis thesis investigates the distribution of overpotentials in a solid oxide cell using both experimental and modeling methods. A 3D finite element model is employed in COMSOL Multiphysics to simulate electrochemical, thermal, and transport phenomena. Experimental validation includes \textit{I/U} curve measurements, electrochemical impedance spectroscopy, and gas composition analysis via mass spectrometry. The model shows good agreement with experiments and is able to capture variations in overpotentials along the cell. Electrolysis operation at thermoneutral voltage is studied in detail with the model to evaluate overpotentials and local heat sources and sinks. The results highlight the importance of spatially resolved modeling in understanding and optimizing solid oxide cell performance. The model enables detailed analysis of current density distribution, local overpotential behavior, and heat source contributions. Combined with experimental data, it provides a valuable tool for evaluating operating strategies and guiding design improvements in high-temperature electrolysis systems
CFD analysis to determine the local Nusselt number over a heated micro wire in multicomponent flow
No full textIn order to gain knowledge about the influence ofa hot wire anemometer to its surrounding and tovalidate experimental results from a laboratory, aCFD model is developed which contains a pipe andthe wire. The wire is placed in the center of the pipe.This model is capable of running with different gascompositions and humidifications, only dependenton temperature. To achieve a fast meshing andsimulating process, the model is two dimensional.This model showed that at low velocities heatdiffuses upstream and that the outlet temperatureis dependent on the flow velocity. A higher velocityresults in a lower temperature. Furthermore naturalconvection can be neglected in the presented cases.The coefficients for the Nusselt number correlationwere found similar to the coefficients from thelaboratory, but containing some deviations thatmight be caused by the way of evaluating.Furthermore a three dimensional model is build toestimate the deviation between the two dimensionaland three dimensional model. It was found that inthe two dimensional model more heat per wire lengthis transferred to the fluid
Modelling of stand alone micro-grid using hybrid energy storage Solar - Battery - Hydrogen
No full text
Improving the performance of an air-cooled fuel cell stack by turbulence inducing grid
No full textProtonudvekslingsmembranbrændselsceller (PEMFCs) vinder mere popularitet som en alternativ strømkilde for sin enkelhed og hurtig opstart. De kommercialiseres til et stort antal applikationer, der spænder fra bil til stationære, f.eks. Strømforsyningsenheder til telekommunikation. Et af problemerne vedrørende PEMFC er den termiske styring af den elektrokemiske reaktion, hvilket resulterer i en overophedning af den Ballard 1020 ACS luftkølet brændselscellestabel ved lave strømtætheder 0.4 A per kvadrat centimeter. Forskere i Energiavdelingen på Aalborg Universitet fandt ud af, at det største problem i termisk styring er varmeoverførslen i luften inden for katodekanalerne. Denne undersøgelse har til formål at løse problemet ved at placere firkantede og honeycomb turbulensgitter før katodeindgangen. Formålet med turbulensgitterne er at inducere turbulenser i katodekanalerne og derved øge blandeffekten for at forbedre varmeoverførslen inde i kanalerne. En computational fluid dynamic (CFD) model er bygget og assisteret af eksperimentelt arbejde. CFD-resultaterne viste en forbedring af blandeffekten i katodkanalerne, og der opnås en reduktion af kanalvægstemperaturen. Forsøgene har resulteret i en forøgelse af ydeevnen med henholdsvis 10,42 % og 2,75% for henholdsvis kvadratisk og honeycomb gridProton exchange membrane fuel cells (PEMFC’s) are gaining more popularity as an alternative power source for its simplicity and quick startup. They're commercialized for large number of applications ranging from automotive to stationary e.g powering telecom backup units. One of the problems regarding PEMFC's is the thermal management of the electrochemical reaction resulting in an overheat of the Ballard 1020 ACS air-cooled fuel cell stack at low current densities \SI{0.4}{A \per cm^2}. Researchers in Energy Department at Aalborg University found that the biggest problem in the thermal management is the heat transfer into the air inside the cathode channels. This study aims to solve the problem by placing square and honeycomb turbulence grids before the cathode inlet. The purpose of the turbulence grids is to induce turbulences in the cathode channels and thereby increase the mixing effect in order to improve heat transfer inside the channels. A computational fluid dynamic (CFD) model is build and assisted by experimental work. The CFD results showed an an improvement of the mixing effect in the cathode channels and a reduction of the channel wall temperature is obtained. The experiments have resulted in an increase of the performance by 10.42 %}and 2.75 %}for the square and honeycomb grid respectively
- …
