50 research outputs found
Microbial Fuel Cell Construction Features and Application for Sustainable Wastewater Treatment
No full textA microbial fuel cell (MFC) is a system that can generate electricity by harnessing microorganisms’ metabolic activity. MFCs can be used in wastewater treatment plants since they can convert the organic matter in wastewater into electricity while also removing pollutants. The microorganisms in the anode electrode oxidize the organic matter, breaking down pollutants and generating electrons that flow through an electrical circuit to the cathode compartment. This process also generates clean water as a byproduct, which can be reused or released back into the environment. MFCs offer a more energy-efficient alternative to traditional wastewater treatment plants, as they can generate electricity from the organic matter in wastewater, offsetting the energy needs of the treatment plants. The energy requirements of conventional wastewater treatment plants can add to the overall cost of the treatment process and contribute to greenhouse gas emissions. MFCs in wastewater treatment plants can increase sustainability in wastewater treatment processes by increasing energy efficiency and reducing operational cost and greenhouse gas emissions. However, the build-up to the commercial-scale still needs a lot of study, as MFC research is still in its early stages. This study thoroughly describes the principles underlying MFCs, including their fundamental structure and types, construction materials and membrane, working mechanism, and significant process elements influencing their effectiveness in the workplace. The application of this technology in sustainable wastewater treatment, as well as the challenges involved in its widespread adoption, are discussed in this study
Computational Modeling of Multiphysics Multidomain Multiphase Flow in Fracturing Porous Media: Leakage Hazards in CO<sub>2</sub> Geosequestration
No full textGeological CO2 sequestration, also known as CO2 geo-sequestration, is a process to mitigate CO2 emission into the earth atmosphere in an attempt to reduce the likely greenhouse effect. It involves injection of carbon dioxide, normally in a supercritical state, into a carefully selected underground formation. Selection of an appropriate geological formation for CO2 geo-sequestration requires a good knowledge of the involved processes and phenomena that occur at the subsurface, and in particular, an estimate of the amount of leakage that might take place in time. Modeling leakage of CO2 in a deformable porous medium constitutes the focal point of this thesis.To this aim, a computationally efficient multiphysics multidomain multiphase numerical modeling framework has been developed which accounts for all important physical processes, interacting domains, and different material phases. The computational efficiency is achieved via tailoring several state of the art numerical techniques in order to attain an accurate, geometry-independent, and mesh-independent model. Deriving such a model for thermo-hydrodynamic-mechanical behavior of a multiphase domain, exhibiting deformation and crack propagation requires a well-designed conceptual model, a descriptive mathematical formulation and an innovative numerical method. The conceptual model distinguishes different domains representing a porous matrix domain, an abandoned wellbore domain, a fracture domain and a fracture-matrix domain. The mathematical formulation adopts the representative elementary volume (REV) averaging based conservation equations for porous media, the drift-flux model averaging of Navier-Stokes equations for the wellbore and fracture domains, and equations of state and constitutive relationships for the involved brine, CO2, air, and solid phases. The numerical solution method adopts a mixed discretization scheme, in which, the standard Galerkin finite element method (SG), the partition of unity finite element method (PUM) within the framework of the extended finite element method (XFEM), and the level-set method (LS) are tailored together to obtain an accurate, geometry-independent, and mesh-independent solution. The thesis introduces four computational models. The first model deals with CO2 leakage via formation layer boundaries, which is capable of simulating multiphase flow in rigid heterogeneous layered porous media, with particular emphasis on the inter-layer leakage of CO2. This model is presented in Chapter 2. The second model deals with CO2 leakage via abandoned wellbores, which is capable of simulating all important physical phenomena and processes occurring along the wellbore path, including fluid dynamics, buoyancy, phase change, compressibility, thermal interaction, wall friction and slip between phases, together with a jump in density and enthalpy between the air and the CO2. This model is presented in Chapter 3. The third model introduces the integration of the first and second models to create an integrated wellbore-reservoir numerical tool for the simulation of sequestrated CO2 multi-path leakage through formation layers and abandoned wellbores. This model is presented in Chapter 4. Finally, the fourth model deals with fracturing and CO2 leakage through cracks. It presents a fully coupled thermo-hydrodynamic-mechanical computational model for multiphase flow in a deformable and fracturing porous media. This model is presented in Chapter 5. These four models cover all important CO2 sequestration processes and leakage mechanisms which might occur in a CO2 geo-sequestration site. The numerical examples show that the proposed computational model, despite the relatively large number of degrees of freedom of different physical nature per node, is computationally efficient. Physically, the numerical examples show that for the normal initial and boundary conditions encountered in CO2 geo-sequestration, leakage via abandoned wellbores and leakage via formation layers can be equally important. Deformation and fracturing, together with leakage via the fractures seem, following the studied cases, a secondary concern. Although the leakage via abandoned wellbores and the leakage via formation layers appear to be equally important in terms of the quantity of leaked CO2, the leakage through the wellbore comes with a greater risk because it can rapidly reach the ground surface. The results of leakage via the fractures show that, in case of having a relatively less permeable cap-rock, the risk of leakage via the fractures increases.The proposed computational models presented in this thesis can be utilized as a framework for the development of efficient and comprehensive numerical software, in such a way that engineers can carry out realistic simulations on relatively limited hardware resources and CPU time. This is due to the computational efficiency of the proposed mixed discretization scheme. Further extensions of this work include: tailoring to other applications, improvement of the constitutive relationships of the solid phase, adding crack initiation and velocity, and adding dynamic forces effects to the solid medium in order to account for the seismic forces.<br/
A Spectral Element Model for Ground Source Heat Pump Systems: Forward and Inverse Calculations
No full textThe ground source heat pump (GSHP) system is a well-established technology that utilizes a renewable energy source for heating and cooling of buildings. This technology is attractive because it relies on energy gain from shallow depths which are available nearly everywhere. Furthermore, it produces minimal CO2 emissions into the atmosphere. Accordingly, this technology is thriving, and currently adopted in many countries all over the world. Nevertheless, due to the lack of accurate and efficient computational models, the design of GSHP systems is not yet optimal and requires further development, which constitutes the main goal of this thesis.Applied Mechanic
On the accuracy and convergence of the finite element method for truss and frame structures
No full textDuring applied mechanics courses for undergraduates, the words `finite element' will be mentioned if a structural problem is so complex that it cannot be solved analytically or simply because it would require too much time to solve it by hand. The lecturer refers to a software package and explains how to work with it. Unfortunately, during these courses, no time is spend on the inner workings of this software. How does the program know, after virtually assembling the structural members and specifying the load, how the structure mechanically behaves? Another term that is mentioned in conjunction with the finite element method is `approximation'; the method is not an exact method, only approximated values are obtained. The accuracy depends on the amount of elements and the type of element. In this thesis the following question will be answered: what is the influence of the mesh size and the element type used in a finite element protocol for trusses and frames, on the accuracy of the displacements and forces and how does it affect the computation time? The goal of this thesis is also to get a better comprehension of the inner workings of a finite element program with a structural application. All this can be achieved by developing, analyzing and using a finite element program created in the MATLAB environment. To do this, prior literary study is required. Use is made of [1] and sub-paragraph 5.1.1 of [6]. A total of four MATLAB programs are written, one for trusses in 2D space, one for trusses in 3D space and two for frames in 2D space. Frames can namely be described by so-called linear and quadratic elements. What that means will be explained in paragraph 2.2. The structural models will be discretized and solved by MATLAB. The created finite element programs will be described and explained in the second chapter. After reading that part, it will be clear how the programs work and how to use them. In the chapter that follows, three elementary frame structures will be analyzed by modelling them with a varying number of elements and different element types. A comparison will be made with the analytical solution. Other aspects of the programs and how they compare to commercial finite element software is the focus of chapter 4. A geometrically complex structure, like an arch, will be modelled by beam elements and analyzed in the fifth chapter. The findings will be summarized and the central question will be answered in chapter 6.Civil Engineerin
Freezing-thawing of porous media: An extended finite element approach for soil freezing and thawing
No full textThis paper introduces a thermo-hydro-mechanical computational model for freezing and thawing in porous media domains, with focus on freezing and thawing in soil. The model is formulated based on the averaging theory and discretized using a mixed discretization scheme, where the standard and extended finite element methods are simultaneously employed. It is capable of capturing the strong coupling between all important phenomena and processes occurring during relatively high freezing-thawing rates in porous media. Solid and fluid compressibility, buoyancy, phase change, thermomechanical behavior, water volume change, pores expansion, cryogenic suction, melting point depression and water migration to the freezing zone are all considered in the model. The cryogenic suction, in particular, is central to the occurrence of many of these phenomena and processes, and thus treated as a primary state variable, and discretized using the partition of unity method to make sure that it can be captured accurately. The paper presents detailed formulation of the governing equations and the numerical discretization. Verification and numerical examples are given to demonstrate the accuracy and computational capability of the model in describing the behavior of a soil mass subjected to boundary conditions resembling those occurring in the vicinity of an energy pile. The numerical examples show that the model is effectively mesh-independent and can simulate all important phenomena using relatively coarse meshes.Applied Mechanic
Spectral Analysis of Heat Flow in U-tube and Coaxial Shallow Geothermal Systems
No full textCivil Engineering | Structural Engineering | Structural Mechanic
Numerical modelling of an experimental energy pile
No full textGeothermal energy is a way of reducing the cost of energy. Deep geothermal energy systems extract heat from very deep soil layers where the temperatures are very high. Shallow geothermal energy systems are about 150 metres deep and they are used to store heat in the soil, to extract it later and use it for space heating. These shallow geothermal systems are generally embedded in a borehole, but they can also be cast into structures, which are called thermo-active foundations. An example of such a foundation is the energy pile, a foundation pile with a heat exchanger embedded in it, connected to a heat pump. There is no need to drill an extra hole in the ground, but the downside is that it is not well known how the bearing capacity of the pile is affected by the heating/cooling cycles. An energy pile experiment is planned to investigate the thermo-mechanical behaviours of the pile and the goal of this thesis is a numerical investigation of the pile. It serves as an estimate of the pile and soil behaviour prior to installation and the results will be used to confidently design the experiment. The model was built with DIANA FEA software, that is capable of coupling thermo-mechanical behaviour. Firstly an experiment in London clay was recreated in order to verify and validate the model and modelling approach. The experiment in Delft was modelled using site investigation that was done at the location where it will be built. Along with old Cone Penetration Test (CPT) data the subsurface was mapped and soil parameters used in the material model were chosen. The thermal cycle that was imposed on the pile was chosen on the basis of preliminary modelling done at different temperature increments. The temperatures were chosen such that the pile was affected to a significant degree of its capacity. It was cooled for three weeks to 0 °C and then heated to 24 °C for three weeks, this was repeated for 6 years. The research focussed on finding which thermo-mechanical effects can be expected and what the scale of those effects could be. The effect directly linked to an increase in temperature is thermal strain. Materials tend to expand and contract with the temperature at different rates and so do the pile and the soil. The gradient of the heat flow is also an important factor as the pile is subjected to the temperature before the soil is. The pile will expand first and this will be resisted by the soil, the strain that is resisted by the soil is called the restrained strain and that is responsible for the change in stress in the pile. A pile that is heated will have more stress than with just a mechanical load and a pile that is cooled will see a reduction in stress. The pile will expand vertically around a null-point somewhere along the pile, this is the point that does not move. In principle, an unrestrained pile will have a null-point in the centre of the pile, but because some soil layers resist the pile movement more than others the null-point is closer to the stronger layers. In the Delft experiment model the null-point was halfway down the pile at first, but as the amount of cycles progressed it moved down. This is due to a decrease in resistance from the weaker layers and an increase in resistance in the strong sand layer on which the pile is based. The amount of stress that is generated is also less in these later cycles as the resistance of the soil became less. With that reduced resistance an increase in settlement is also seen. This can lead to differential settlements of structures that are built on such a pile, possibly damaging them. The results of the modelling are used to give an advice on the experiment details, such as geometry, thermal cycle and pile layout. An advice to the layout of the sensors is included as well as a prediction of the results
A compressible two-fluid multiphase model for CO2 leakage through a wellbore
No full textharvest published online: 15-01-2015Applied Mechanic
Parameter identification algorithm for ground source heat pump systems
No full textThis paper presents a new parameter identification (PI) algorithm for estimating effective and detailed thermal parameters of ground source heat pump systems using data obtained from the well-known thermal response test. The PI comprises an iterative scheme coupling a semi-analytical forward model to an inverse model. The forward model is formulated based on the spectral element method to simulate transient 3D heat flow in ground source heat pump (GSHP) systems, and the inverse model is formulated based on the interior-point optimization method to minimize the system objective function. Compared to existing interpretation tools for the thermal response test, the proposed PI algorithm has several advanced features, including: it can handle fluctuating heat pump power and inlet temperatures; interpret data obtained from multiple heat injection or extraction signals; produce accurate backcalculation for short and long duration experiments; and handle multilayer systems. The PI algorithm is tested against synthesized data, using a wide range of random noise, and versus an available laboratory experiment. The computational results show that the PI algorithm is accurate, stable and exhibiting relatively high convergence rate.Applied Mechanic
