No description available.

No description available.

An experimental investigation has been carried out into Liquid dielectrophoresis (LDEP), in which dielectric forces are used to actuate bulk liquid spreading and to imprint a deformation pattern at a liquid air surface. Our basic conguration uses a set of co-planar stripe interdigital electrodes to create a non-uniform A.C. electric field that exponentially decays in amplitude and penetrates above the structure into a dielectric liquid droplet. Dielectric forces drive a voltage-induced spreading of the droplet into a low contact angle droplet/film, an effect which will be referred to here as "dielectrowetting". Further application of the A.C. voltage to the electrodes results in a static sinusoidal wrinkle forming at the oil-air interface on the spread film. Three main areas of investigations are reported, the first of which is a study of the statics and dynamics of the voltage induced spreading of a stripe of the material 1,2 propylene glycol. In the limit of thick droplets, a theoretical prediction has been derived that the cosine of the static equilibrium contact angle is proportional to the square of the amplitude of the voltage applied to the inter-digital electrodes. This relationship is analogous to that found for electrowetting-on-dielectric (EWOD). Experimental observations confirm this predicted dielectrowetting behavior for the advancing static contact angle of the voltage induced spreading of a stripe of the material 1,2 propylene glycol. The predicted scaling relationship with the electrode dimensions has been confirmed for electrodes of linewidth 20 um, 40 um and 80 um. It is also shown that with an appropriate surface treatment the induced wetting is made reversible. The dynamic contact angle ( o) of the liquid droplet was also measured as a function of voltage during spreading and the results are found to agree with the predictions of the Hoffman-de-Gennes macroscopic viscous dissipation approach extended to account for the electric field. Three distinct regimes have been investigated experimentally and theoretically: partial wetting and exponential approach to equilibrium shape, spreading to complete wetting obeying a Tanner's law relationship, and super-spreading towards a complete wetting film. The second area is the fabrication of the solid phase transmission grating with a minimized zeroth order using LDEP and a UV curable liquid resin. A new electric-field assisted approach to fabricate solid diffractive optical devices is demonstrated in which the optical properties of the device can be tuned by adjusting the applied voltage whilst the optical medium is in its liquid phase. Continuous adjustment of the voltage, and hence the optical diffraction pattern, during UV curing can produce a solid grating with suppressed intensity of the transmitted zero order. The third area is a study of the dynamics of the formation of a periodic wrinkle deformation at the oil-air interface on a spread dielectric liquid film. An experimental study of how the switch on time ton and the switch off/relaxation time toff of the wrinkle scales as a function of the applied voltage V is presented. The amplitude A(V, t) of the wrinkle as a function of time was obtained from fitting the time dependent intensities of the positive zeroth, first and second diffracted orders of laser light transmitted through the oil film assuming that it acts as a thin sinusoidal phase grating. The results for relaxation have been compared with an analytical expression for toff that has been derived by considering the LaPlace pressure restoring force at the oil-air interface under a creeping flow approximation.

Energy storage is widely considered to be an important component of a decarbonised power system if large amounts of renewable generation are to provide reliable electricity. However, storage is a highly capital intensive asset and clear business cases are needed before storage can be widely deployed. A proposed business case is using storage to prevent overvoltage in low voltage (LV) distribution networks to enable residential photovoltaic systems. Despite storage being widely considered for use in LV networks, there is little work comparing where storage might be installed in LV networks from the perspective of the owners of distribution networks (DNOs). This work addresses this in two ways. Firstly, a tool is developed to examine whether DNOs should support a free market for energy storage in which customers with PV purchase storage (e.g. battery systems) to improve their self-consumption. This reflects a recent policy in Germany. Secondly, a new (published) method is developed which considers how DNOs should purchase and locate storage to prevent overvoltage. Both tools use a snapshot approach by modelling the highest and lowest LV voltages. On their own, these tools enable a DNO to determine the cost of energy storage for a particular LV network with a particular set of loads and with PV installed by a given set of customers. However, in order to predict and understand the future viability of energy storage it is valuable to apply the tools to a large number of LV networks under realistic future scenarios for growth of photovoltaics in the UK power system. Therefore, the work extracts over 9,000 LV network models containing over 40,000 LV feeders from a GIS map of cables provided by one of the UK’s electricity distribution networks- Electricity North West. Applying the proposed tools to these 9,000 network models, the work is able to provide projections for how much LV energy storage would be installed under different scenarios. The cost of doing so is compared to the existing method of preventing reinforcement- LV network reconductoring. This is a novel way of assessing the viability of LV energy storage against traditional approaches and allows the work to draw the following conclusions about the market for energy storage in LV distribution networks in the UK: - Overvoltage as a result of PV could begin to occur in the next few years unless UK regulations for voltage levels are relaxed. There could be a large cost (hundreds of millions of pounds) to prevent this if the traditional approach of reconductoring is used. - If overvoltage begins to occur, a free market for energy storage (randomly purchased by electricity consumers) cannot offer large benefits to DNOs in reducing the reinforcement cost unless this is properly controlled, located and/or widely installed by customers. - Optimally located storage by the DNO can reduce overall reinforcement costs to mitigate overvoltage. This would enable more energy storage to balance renewable generation and present large savings to the power system. The exact topology of storage and the storage rating in each LV network could be determined using the tool proposed in this work.

Increased reliance upon renewable energy sources, chiefly wind, places a growing emphasis on the reliability of the technology used in Wind Turbines. The current Wind Turbine fleet is dominated by the Doubly Fed Induction Machine WT, which utilises a partially rated power electronic converter to vary the speed of the rotor and thus ensure the maximum energy capture available from the wind. This converter is associated with a significant percentage of WT failures. This thesis examines the low frequency temperature cycling occurring in one half of the back to back converter which results in a high failure rate of the rotor side converter as compared to the grid side converter. To this end a MATLAB/PLECS model was constructed to demonstrate the temperature cycling occurring in a 2.5MW DFIG WT. Lifetime of the semiconductor devices was extrapolated. An adaptation to the standard Maximum Power Point Tracking control method was suggested in which the lowest operating frequencies (less than 2.33Hz) were avoided. In doing so, lifetime was observed to increase at a minor cost to energy yield from the WT.

In a switched reluctance (SR) drive, the transfer of the de-fluxing energy in stator windings back to the dc-link results in a large dc-link capacitance. This limits its applications where weight and size of the drive are restricted. This thesis describes a control technique for the dc-link capacitance minimisation in an SR drive. The proposed control technique maintains the constant power transfer between the de supply and the H-bridge converter. The average dc-link current over a switching period is kept constant. When the output of the integrator, i.e., the average dc-link current, reaches a predefined value proportional to the torque demand, appropriate switching takes place. This is achieved by integrating the dc-link current in each switching period. This technique is called dc-link current integration control (DLCIC). The de-fluxing current from the outgoing phase is not fed back to the dc-link capacitor. Instead, it is transferred to the incoming phase to prevent a negative dc-link current, which causes a fluctuation in the capacitor voltage. Extensive simulation studies of the DLCIC and other techniques reported in literature have been performed and the simulation results from DLCIC are compared with those from other techniques such as Hysteresis Current Control (HCC) and Pulse Width Modulation Current Control (PWMCC). It has been shown that the peak-to-peak voltages across the dc-link capacitor from DLCIC are the lowest amongst other techniques. The operational speed range of the DLCIC is determined and the optimal turn-on and turn-off angles are proposed. Filter components under the DLCIC operation has been designed and compared with the filter for HCC. It is shown that the weight of the filter for DLCIC is far lower than that for HCC. The proposed control technique have been validated by experiments. The experimental results show that at the dc-link voltage ripple which results from DLCIC is much lower than that from HCC. This demonstrates that DLCIC can minimise the dc-link capacitance in an SR machine drive.

Durability is one of the major issues for the successful commercialisation of polymer electrolyte membrane fuel cells (PEMFCs) and it mainly depends on the stability of the individual cell components. In order to minimise the durability issues, the development of new materials or modification to replace the existing fuel cell components is required. The typically used proton exchange membrane (PEM) is the perfluorosulfonated polymer such as Nafion® and electrocatalysts for PEMFC is high surface area carbon supported platinum electrocatalyst (Pt/C). A higher temperature of operation (>80 oC) of PEMFC would boost their performance by enhancing the electrochemical kinetics and also improve the carbon monoxide tolerance of platinum catalysts. A Nafion® type membrane is not suitable for higher temperature operation as its proton conductivity mainly depends on the hydration level. An approach to improve the proton conductivity of Nafion® based membranes is the incorporation of hydrophilic inorganic oxide materials into the Nafion® polymer matrix. A composite membrane based on graphite oxide (GO) has been developed and demonstrated as an alternative PEM for high temperature operation up to 120 oC. GO is an insulator and hydrophilic in nature. GO exhibits proton conductivity due to the presence of acidic functional groups like, carboxylic acid, hydroxyl groups and epoxy groups. Further functionalisation of GO with sulfonic acid (called SGO) improves the proton transport properties of GO which in turn improves the composite membrane proton conductivity. Free standing GO and SGO papers were fabricated and evaluated to understand their proton transport mechanism. The in-plane and through-plane proton conductivities of GO paper were 0.008 and 0.004 S.cm-1 at 30 oC and 25% RH respectively. The in-plane and through-plane proton conductivities of SGO paper were 0.04 and 0.012 S.cm-1 at 30 oC and 25% RH respectively. The fuel cell performance of a membrane electrode assembly made with SGO paper gave a maximum power density of 113 mW cm-2. GO/Nafion composite membranes were fabricated with different GO content. The composite membranes with an optimum of 4 wt% GO showed better mechanical strength (tensile strength of 8.17 MPa) and water uptake (37.2%) compared to recast Nafion. A GO (4 wt%) /Nafion composite membrane gave a high ion exchange capacity (IEC) value of 1.38 meq g-1. The proton conductivity of GO (4 wt%) /Nafion was 0.026 S.cm-1 at 120 oC. SGO/Nafion composite membrane showed improved proton ii conductivity (0.029 S.cm-1). The SGO/Nafion composite membrane gave peak power density of 240 mW cm-2, whereas GO/Nafion composite membrane gave a power density of 200 mW cm-2 at 120 oC and 25% RH. The stability and durability of GO and SGO/Nafion composite membranes was investigated under fuel cell operating conditions and compared with recast Nafion. A non fluorinated proton exchange membrane based sulfonated poly ether-ether ketone (SPEEK) was used to develop a composite membrane with SGO. SGO (4 wt%) /SPEEK composite membrane showed high IEC of 2.3 meq g-1 and proton conductivity of 0.055 S.cm-1 at 80 oC and 30% RH. SGO (4 wt%) /SPEEK composite membrane gave a power density of 378 mW cm-2 at 80 oC and 30% RH, which was higher than that of recast SPEEK (254 mW cm-2). Transition metal nitride based electrocatalyst support such as titanium nitride (TiN), has been used to replace carbon to support Pt and Pt-Co alloy for PEMFC cathode. Nafion® stabilised Pt nanoparticles supported on TiN (Pt/TiN) were prepared and evaluated as cathode electrocatalyst for PEMFC. Pt/TiN showed better electrocatalytic activity, stability and durability under fuel cell operating conditions compared to commercial Pt/C. Pt/TiN retained 66% of electrochemical active surface area (ECSA) after 1000 potential cycles (cycled between the potential range of +0.6 to +1.20 V vs. RHE) under fuel cell operating conditions. The ECSA of the Pt/C catalyst fell by 75%. Pt/TiN was also evaluated for its suitability in phosphoric acid based PEMFCs. Pt/TiN showed better durability than Pt/C under fuel cell operating conditions. Pt/TiN showed a two-fold increase in mass and specific activities than Pt/C as calculated from oxygen reduction reaction data at 0.9 V. An improved durability of Pt/TiN resulted from a Nafion® layer surrounding the Pt protecting from phosphate ion adsorption. Alloying of Pt with 3d transition metals changes the electronic structure of Pt (Pt becomes e- deficient) and enhances the electrocatalytic activity of PtM alloy compared to Pt. 3d transition metals such as Fe, Co and Ni are reported to be more active than other metals. Pt-Co alloy supported on TiN was prepared and evaluated. Pt-Co/TiN showed about +21 and +32 mV positive shifts in half-wave potential compare to Pt/TiN and conventional Pt/C respectively. After 5000 potential cycles, the ECSA of Pt-Co/TiN had decayed by about 55%, whereas Pt/TiN and Pt/C showed a greater loss in ECSA of 70%.

Thanks to an extensive effort by the global research community, the electronic technology has significantly matured over the last decade. This technology has enabled certain operations which humans could not otherwise easily perform. For instance, electronic systems can be used to perform sensing, monitoring and even control operations in environments such as outer space, underground, under the sea or even inside the human body. The main difficulty for electronics operating in these environments is access to a reliable and permanent source of energy. Using batteries as the immediate solution for this problem has helped to provide energy for limited periods of time; however, regular maintenance and replacement are required. Consequently, battery solutions fail wherever replacing them is not possible or operation for long periods is needed. For such cases, researchers have proposed harvesting ambient energy and converting it into an electrical form. An important issue with energy harvesters is that their operation and output power depend critically on the amount of energy they receive and because ambient energy often tends to be sporadic in nature, energy harvesters cannot produce stable or fixed levels of power all of the time. Therefore, electronic devices powered in this way must be capable of adapting their operation to the energy status of the harvester. To achieve this, information on the energy available for use is needed. This can be provided by a sensor capable of measuring voltage. However, stable and fixed voltage and time references are a prerequisite of most traditional voltage measurement devices, but these generally do not exist in energy harvesting environments. A further challenge is that such a sensor also needs to be powered by the energy harvester’s unstable voltage. In this thesis, the design of a reference-free voltage sensor, which can operate with a varying voltage source, is provided based on the capture of a portion of the total energy which is directly related to II the energy being sensed. This energy is then used to power a computation which quantifies captured energy over time, with the information directly generated as digital code. The sensor was fabricated in the 180 nm technology node and successfully tested by performing voltage measurements over the range 1.8 V to 0.8 V.

No description available.

Knowledge of the parameter values of the dynamic generator models is of paramount importance for creating accurate models for power system dynamics studies. Traditionally, power systems consists of a relatively limited numbers of large power stations and the values of generator parameters were provided by manufacturers and validated by utilities. Recently however, with the increasing penetration of distributed generation, the accuracy of these models and parameters cannot be guaranteed. This thesis addresses the above concerns by developing a methodology to estimate the parameter values of a power system dynamic model online, employing dynamic system modes, i.e. modal frequencies and damping. The dynamic modes are extracted from real-time measurements. The aim of the proposed methodology is to minimise the differences between the observed and modelled modes of oscillation. It should be emphasised that the proposed methodology does not aim to develop the dynamic model itself but rather modify its parameter using WAMS measurements. The developed methodology is general and can be used to identify any generator parameters., However, thesis concentrates on the estimation of generator inertia constants. The results suggest that the proposed methodology can estimate inertias and replicate the dynamic behaviour of the power system accurately, through the inclusion of pseudo-measurements in the optimisation process. The pseudo-measurements not only improves the accuracy of the parameter estimation but also the robustness of it. Observability, a problem when there are fewer numbers of measurements than the numbers of parameters to be estimated, has also been successfully tackled. It has been shown that the damping measurements do not influence the accuracy and robustness of generator inertia estimation significantly.