/library/oar/handle/123456789/9974 2025-11-06T12:38:36Z /library/oar/handle/123456789/101458 Title: Thermo-mechanical studies of large hadron collimators in accident scenarios Abstract: The performance of the Large Hadron Collider (LHC) strongly depends on the correct functionality of the LHC collimation system. With a nominal stored beam energy of 362 MJ and a beam momentum of 7 TeV / c, beam accident scenarios must be studied well to assess if the collimator design is robust against possible error scenarios in operation. One of the serious accident scenarios in the LHC is an asynchronous beam dump. While the primary and secondary collimators are designed to withstand such beam impacts, there may be machine conditions that expose the metal-based tertiary collimators (TCTs) and put them at risk of damage. A numerical finite element (FE) model is thus developed in this thesis and applied to investigate the thermo-mechanical response of a TCT in novel jaw error cases derived from an asynchronous beam dump accident. The effectiveness of operating with tilted collimator jaws is identified and it is found that a jaw inclination of -1 mrad can mitigate the damage caused by an accidental beam impact on a TCT. The thermo-mechanical response of TCTs to proton and 208Pb82+ ion beam impacts is also compared and it is concluded that heavy ion operation in the LHC poses no additional challenges on the structural integrity of TCTs during a beam impact. The developed FE model is complemented by the commissioning of thermal and structural characterisation campaigns of the TCT jaw insert material (INERMET® 180) under varying temperature and strain-rate conditions. Moreover, a dedicated beam experiment at the CERN High Radiation to Materials (HiRadMat) facility has successfully validated the reliability of the developed FE model, thus giving confidence in the prediction of damage by such numerical simulations. The work presented in this thesis provides a more thorough understanding of the thermo-mechanical behaviour of TCTs during beam impact in accident scenarios. This is achieved by taking into account all relevant collimator damage limits in a detailed study of various relevant scenarios for the operation of the LHC and its risk optimisation. Description: M.PHIL. 2015-01-01T00:00:00Z /library/oar/handle/123456789/101262 Title: An energy descent recovery advisor for large transport aircraft Abstract: As the journey of an aircraft approaches its final destination, the all'- craft needs to descent in a way that allows it to lower its energy in a timely manner in preparation for final approach and landing. This is routinely achieved through descent planning and executing the appropriate plan. However there are circumstances, such as but not restricted to those associated with air traffic constraints, in which the aircraft will remain high and fast to the extent that it will be unable to then dissipate sufficient energy in time to land. In such occurrences, aircraft tend to approach the destination airfield hot and high, potentially jeopardising the continued safety of the flight. This thesis focuses on the development of techniques and algorithms that detect exceedances in aircraft energy in the descent and provide the crews with assistance in recovering appropriate aircraft energy states when exceedances occur. A monitoring system was developed to trigger a recovery planner on the detection of an exceedance. The planner first generates a vertical profile that allows the aircraft to arrive at a specific waypoint at the right energy and contracted time and this vertical profile is then used by a plan path planner that generates a plan deviation from the flight plan as and when appropriate. The algorithms developed were evaluated using fast-time simulations to assess the capability of the methods in identifying valid recovery strategies and the expected accuracies. The results indicate that real-time crew support is possible and can contribute towards achieving improved safety in flight operations of large transport aircraft. Description: PhD 2015-01-01T00:00:00Z /library/oar/handle/123456789/101069 Title: Innovative approaches to solar distillation Abstract: It is estimated that by 2025, two-thirds of the world's population will be living under water-stressed conditions. In many countries, the supply of fresh water from natural resources is already scarce. In such instances energy driven desalination of sea or brackish water is required. People living in remote or disaster areas may lack fresh water as well as energy supplies. A possible solution for such circumstances is renewable desalination which uses renewable energy to desalinate seawater. This study focused on solar stills, the basic constituent of solar desalination technologies. The design of a conventional solar still was modified by replacing the insulated opaque sides with transparent glass. This increased the condensation capacity and when tested under natural weather conditions the productivity was improved by around 43% to 5.5 litres/m2 .day. The improved simple still was used as a benchmark to compare other modified solar still configurations. The sides of the latter stills were also made of glass. In an attempt to create a thin film of water to enhance the evaporation process, a titanium dioxide evaporator was used. When irradiated with ultraviolet radiation, Ti02 exhibits photo-induced hydrophilicity. Two solar stills comprising an inclined evaporator were tested under natural weather conditions simultaneously: one with a Ti02 coating and the other painted black. They both reached the same productivity, however the Ti02 unit operated at a lower temperature due to the lower solar absorptance of Ti02. The improved wettability of titanium dioxide compensated for the lower temperature. However scale formation was observed and it is believed that this attenuated the photo-induced hydrophilic properties. The inclined evaporator stills reached an evaporator efficiency 8 .6% lower than that of the improved simple solar still. A solar chimney and externally water cooled copper condensers were used to improve the convection and condensation processes of a solar still. The buoyant pressure generated by the solar chimney was used to induce a convective current and carry the water vapour from the evaporation chamber to the condensers. This improved the evaporator efficiency by 8.9%. However, since the evaporator area was reduced by 14% to make space for ventilation grilles, the overall production rate was 6.3% lower than that of the improved simple still. The improved simple solar still reached a productivity which compares and exceeds many improvements described in literature. Further improving this gain without making use of latent heat recovery, proved to be difficult. A double effect solar still, incorporating the vertical glass sides, reached a productivity 20% higher than that of the improved simple still. This study can serve as a basis for future research projects attempting to further investigate the use of titanium dioxide and a solar chimney in solar distillation. Description: PhD 2015-01-01T00:00:00Z /library/oar/handle/123456789/100866 Title: Control and management of distributed generation and energy storage systems in low voltage microgrids Abstract: (DG); such as renewable energy sources (RES) into the grid, the present generation and distribution model poses limits to the penetration of DG since large penetrations can effectively compromise the stability of the electrical grid. The microgrid concept was developed to provide a means for the coordinated integration of DG into the grid. Microgrids are self-contained low-voltage (LV) or medium-voltage (MV) power networks that contain RES, energy storage systems (ESS) and local loads working cooperatively as a single local system. The DG units must be interfaced to the microgrid via inverters which must be controlled to obtain a reliable local supply during grid-connected and islanded operation. Droop control is now widely accepted as the best solution to date that achieves autonomous decentralized control of the microgrid inverters but there are also performance limitations that are associated with this algorithm. During this research work, a laboratory based single phase LV microgrid was set up in the labs of the Dept. of IEPC at the University of Malta. Algorithms that enable both islanded and grid-connected operation for the microgrid were designed and implemented. Research that was performed in this thesis was then focused on the limitations of the decentralized operation due to the parallel operation of inverters using conventional droop control. Solutions to overcome its inherent limitations and optimize the performance of the microgrid were then proposed, modeled and experimentally verified. These algorithms address the following limitations in islanded operation: reactive power sharing; voltage and frequency restoration; and voltage harmonic compensation. Additional algorithms were also developed for seamless transitions from islanded to grid-connected operation and vice-versa; and current harmonic compensation in grid-connected operation. A hierarchical control architecture consisting of the primary and secondary control layers, was developed for the single phase microgrid. The integrated primary control loops which were developed enable the microgrid to work in both islanded and grid-connected operation. Results have shown that the droop control algorithm has operational limitations due to its inherent voltage and frequency deviations. In addition, reactive power sharing between the voltage-controlled voltage source inverters (VCVSIs) in the microgrid cannot occur due to mismatches in the output impedance of the inverters and the line impedances. Secondary control algorithms were proposed to remove these voltage and frequency deviations while sharing the reactive power demand between the inverters. The considered hierarchical structure avoids any critical communications among the microsources while employing low bandwidth communications between the microgrid central controller (MGCC) and the microsources so as not to compromise the reliability of the islanded microgrid. The effectiveness of these algorithms was verified by simulations and confirmed by experimental results. The operational robustness of the secondary control loops due to packet loss, delays and loss of communication with any node in the network were also verified experimentally. The seamless transitions of the microgrid as a single entity from islanded to grid-connected operation and vice-versa were also considered. The primary and secondary control loops were modified to achieve the required islanding protection and grid synchronization criteria described in grid interconnection standards. The effectiveness in achieving seamless transitions between the modes of operation were verified by simulations and confirmed by experimental results. The operational robustness of the secondary control loops due to packet loss, delays and loss of communication with any node in the network were also verified experimentally. The results show that the hierarchical architecture described in this research work can achieve seamless transitions between the two modes of operation without any disconnection times for the energy sources and local loads. Power quality aspects which arise due to harmonic currents during islanded and grid-connected operation were finally considered. Selective harmonic compensation algorithms were implemented in the primary control loops of the inverters to improve the power quality issues related to current harmonics in both modes of operation. A capacitive virtual impedance loop was proposed to reduce the harmonic distortion of the PCC voltage and improve the harmonic current sharing when non-linear loads are connected to the microgrid. Simulations and experimental results have confirmed that the capacitive virtual impedance loop developed during this research work achieved both of these two objectives simultaneously. A virtual admittance loop was also developed to reduce the harmonic current injected by the inverters during grid-connected operation of the microgrid. A detailed analysis has shown that the proposed strategy improves significantly the total harmonic distortion (THD) of the inverter output current. Description: PhD 2015-01-01T00:00:00Z