8,803 research outputs found
Adaptive graphene electronics and plasmonics
No full textGraphene, the world’s first two dimensional material, has attracted great interest in the scientific community due to its unique behaviours such as the dramatic tunability of its electronic and optical properties, strong lightmatter interactions, and high values of carrier mobility which render graphene as a fascinating material for photodetection, plasmonic devices, and modulation of terahertz frequency radiation. To realise the next generation of graphene devices we must find new ways to adaptively control the electronic and optical properties of graphene and other two dimensional materials. In this thesis I propose a method to optically control the electronic properties of graphene in a spatially resolved, non-volatile, yet reversible manner via the use of photorefractive lithium niobate. The method I propose relies on the ability of lithium niobate to sustain optically defined charge distributions which result in large electrostatic fields at the surface of the crystal. By transferring graphene onto lithium niobate crystals I show that the optically defined electrostatic surface charges in the substrate are capable of tuning the DC electrical conductivity of graphene in a behaviour which is non-volatile yet reversible under thermal annealing. Further, I utilise this effect in a plasmonic device consisting of a hybrid graphene-metal metasurface on lithium niobate where the optical tuning effect is capable of altering the transmissive properties of the device at terahertz frequencies. Finally, I show through simulations that by spatially patterning charge distributions it is possible to create optically defined plasmonic devices on graphene on lithium niobate. Such devices would not need permanent lithographic patterning of metasurfaces, yet instead would rely on optically defined regions of high and low conductivity graphene to sustain a plasmonic resonance. If such devices can be experimentally achieved this would allow for rewritable yet non-volatile plasmonic structures in graphene and open the doors to a truly reconfigurable plasmonic platform for two dimensional materials
A Code of Practice for Grocery Goods Undertakings and an Ombudsman: How to Do a Lot of Harm by Trying to Do a Little Good
No full textThe Department of Enterprise, Trade and Employment in its August 2009 Consultation Paper, Code of Practice for Grocery Goods Undertakings, argues that a Code governing grocery supplier/retailer relations, enforced by an Ombudsman, should be introduced. The Code constrains the behaviour of the retailer with respect to certain practices that, for example, shift risk from the retailer to the supplier as well as those that result in unexpected costs to suppliers. The rationale for the Code appears to be that due to the devaluation of sterling, combined with the recession, retailers are able to put increased pressure on local suppliers for lower prices, which in turn squeezes suppliers’ margins. The paper argues that the Consultation Paper does not present a sound rationale for the Code, in reality the Code is a form of protectionism occasioned by the inflow of lower priced imports. Local suppliers should adapt through developing better products and becoming more efficient, rather than seeking shelter from market forces. The impact of the Code will likely be to lead to: higher consumer prices lowering consumer welfare and thus inconsistent with the declared aim of the Code; increased costs of doing business with local suppliers thus leading to an incentive for retailers to use more imports; and, perhaps, a less competitive grocery sector. It is argued that the Consultation Paper should be withdrawn and reissued, but in a manner consistent with the government’s better regulation agenda which is currently ignored. To the extent that the issue of concern is excessive buyer power of retailers then that should be addressed directly: by liberalising the Retail Planning Guidelines as the Competition Authority has been arguing for sometime; and/or sponsoring entry of new retailers; and/or amending competition law, if a problem exists and can be demonstrated to exist, but retain the competition test. The answer, based on the evidence presented in the Consultation Paper, is not the Code.
Common Mode Currents in DC Power Routers
No full textThe grid reinforcement and energy redirection needs have led to the emergence of Back-To-Back Voltage Source Converter (BTB-VSC) based dc power routers. This paper investigates the low frequency Common Mode Currents (CMCs) that arise in the system if the employed BTB-VSCs have an un-isolated ac path connected in parallel to their output ports. Simulation results are presented to show a sensitivity analysis of lower order harmonics in CMC with respect to the operating active and reactive power of the dc router, dc link voltage, link resistance, modulation method and pole capacitance. Experimental results are shared to show existance of lower order CMC in 3-wire ac link operating in parallel with the dc power router and these are mitigated using zero sequence controller
Modeling, Control, and Operation of an M-DAB DC-DC Converter for Interconnection of HVDC Grids
No full textFuture high-voltage direct-current (HVDC) networks based on voltage source converters (VSCs) will have different structures (asymmetric monopolar, bipolar, or symmetric monopolar), voltage levels, control, and protection schemes. Therefore, dc-dc converters are needed to interconnect those VSC-HVDC grids and several technical issues on their control and operational systems must be adequately addressed. A dc-dc converter based on a modular-dual active bridge (M-DAB) converter is suggested to reach a desirable interconnection of the HVDC grids and regulate power flow (PF) between them. A dynamic averaged model is proposed for the M-DAB converter and its stability is analyzed using the Lyapunov function. Moreover, a new local controller based on nonlinear control theory is proposed for the M-DAB. The new M-DAB local controller is integrated with the energy management system (EMS), by updating the PF equations, to create a complete control structure. Considering the CIGRE DCS3 HVDC test system and the studied M-DAB, static, dynamic simulation, and experimental studies are conducted and the dc-dc converter and the performance of the designed controllers and the EMS are examined and validated.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Intelligent Electrical Power Grid
Restructuring the existing medium voltage distribution grids using DC systems
No full textGreen Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.DC systems, Energy conversion & Storag
Cold cracking in DC-cast high strength aluminum alloy ingots: An intrinsic problem intensified by casting process parameters
No full textFor almost half a century the catastrophic failure of direct chill (DC) cast high strength aluminum alloys has been challenging the production of sound ingots. To overcome this problem, a criterion is required that can assist the researchers in predicting the critical conditions which facilitate the catastrophic failure of the ingots. This could be achieved at first glance by application of computer simulations to assess the level and distribution of residual thermal stresses. However, the simulation results are only able to show the critical locations and conditions where and when high stresses may appear in the ingots. The prediction of critical void/crack size requires simultaneous application of fracture mechanics. In this paper, we present the thermo-mechanical simulation results that indicate the critical crack size distribution in several DC-cast billets cast at various casting conditions. The simulation results were validated upon experimental DC-casting trials and revealed that the existence of voids/cracks with a considerable size is required for cold cracking to occur.Materials Science & EngineeringMechanical, Maritime and Materials Engineerin
A Dynamically Reconfigurable Recursive Switched-Capacitor DC-DC Converter with Adaptive Load Ability Enhancement
No full textMultiple voltage conversion ratio (VCR) recursive switched-capacitor (SC) dc-dc converters, based on several basic 2:1 converters, are widely used for on-chip power supplies due to their flexible VCRs for higher energy efficiency. However, conventional multiple VCR SC converters usually have one or more 2:1 converters unused for some VCRs, which results in lower power density and chip area wastage. This article presents a new recursive dc-dc converter system, which can dynamically reconfigure the connection of all on-chip 2:1 converter cells so that the unused converters in the conventional designs can be reused in this new architecture for increasing the load-driving capacity, power density, and power efficiency. To validate the design, a 4-bit-input 15-ratio system was designed and fabricated in a 180-nm BCD process, which can support a maximum load current of \text{0.71}\,\text{mA} and achieves a peak power efficiency of 93.1% with 105.3\,\mu \text{A/mm} {2} chip power density from a 2-V input power supply. The measurement results show that the load-driving capacity can become 6.826×, 2.236×, and 2.175× larger than the conventional topology when the VCR is 1/2, 1/4, and 3/4, respectively. In addition, the power efficiency under these specific VCRs can also be improved considerably.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Electronic Instrumentatio
From DC nano- and microgrids towards the universal DC distribution system - a plea to think further into the future
No full textThe traditional ac power system is challenged by the increased amount of distributed energy resources. Enormous changes and investments are necessary in order to achieve an ac smart grid capable of coping with the challenges introduced. In dc, solutions seem to be more straightforward since most of the distributed energy sources and most of the loads connected to the low voltage grid operate with dc. For this reason, dc distribution systems should be considered as an alternative with significant potential. Nevertheless, nowadays research is mainly focused on local dc nano- and microgrids. This paper introduces relevant aspects related to dc distribution networks. A wide field of opportunities and challenges are briefly exposed.Accepted Author ManuscriptOld - EWI-ESE-DC&S DC systems & StorageElectrical Power Processin
Power Disequilibrium Suppression in Bipolar DC Distribution Grids By Using A Series-Parallel Power Flow Controller
No full textThe unbalanced power between positive and negative poles in a bipolar DC distribution network (DC-DN) generates an unbalanced current at the neutral line, which enlarges the power losses of the system and the voltage deviation of DC loads. An unbalanced power suppression strategy based on a series-parallel power flow controller (SP-PFC) is proposed in this paper. The SP-PFC is adopted as the interconnection between two different DC-DNs. The topology and operating modes of SP-PFC are analyzed. Subsequently, SP-PFC output voltage and line current expressions under constant power control are derived. The nonlinear relationship between the output voltage and line current is linearized at the operating point. On this basis, the influences of unbalanced load and receiving-end voltage on the SP-PFC are investigated. A small-signal model of bipolar DC-DN containing an SP-PFC is established, and the system stability is analyzed. A simulation model of the bipolar DC distribution network containing an SP-PFC is built up in MATLAB/Simulink, and the effectiveness of the SP-PFC in the suppression of unbalanced power is verified.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.DC systems, Energy conversion & Storag
DC-DC Converters
No full textDC-DC converters take a DC voltage as input and generate a different DC voltage (i.e., a DC voltage that is bigger or smaller than input) as output. This chapter studies some of the important DC-DC converter circuits. © 2024, The Author(s), under exclusive license to Springer Nature Switzerland AG
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