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Frequency control over networks is done using the frequency droop control technique which has the simplicity advantage although it allows that, in certain situations, frequency control is not very efficient. Artificial intelligence techniques have been increasingly used, so it is justified to explore their viability in electrical networks. The present work analyzes the use of Artificial Intelligence in networks to improve the frequency droop control. In order to realize this, a deep reinforcement learning (DRL)-based agent is proposed to tune the controller parameters for voltage source converter (VSC) in this paper. The DRL-based agent is trained by numerous hybrid grid operation conditions to lean the optimal control policy, which make it achieve a good adaptability to variety of operation conditions. For the purpose of demonstrating this method, a time-domain simulation model of hybrid power system is built with MATLAB/Simulink to act as test system. The simulation results verify the effectiveness of the proposed method.

The injection of a significant amount of wind power tends to increase the difficulty of the grid frequency control. Therefore, wind farm (WF) has to have the ability to participate in system frequency support. However, during the frequency control process in a large WF, individual wind generators may prone to instability due to possible over-deceleration. In addition, the deeply-intertwined cyber physical power system (CPPS) has gradually replaced the traditional grid due to the development of information and communication technology. However, CPPS also introduces cyber uncertainties that have a significant negative impact on the real-time control of power system. To address above issues, this paper considering possible cyber uncertainty, and proposes a grouping-based hierarchical frequency support scheme for WF. In the proposed scheme, the variable speed wind turbines (VSWTs) are clustered into several groups, according to their wind profiles at first, by dispatching the same control commands to the VSWTs belonging to the same group. As a result, the number of the VSWTs and control variables reduces significantly. Then, a hierarchical control scheme based on virtual inertial control is proposed with consideration of possible cyber uncertainty. The top-layer controller determines whether cyber uncertainty exists, and calculates power increment based on frequency deviation and cyber uncertainty. The bottom-layer controller coordinates the power increment distribution on the basis of ensuring the safe operation of all VSWTs. By do this, it is expected to achieve not only optimal frequency response but also wind generator stability. The effectiveness of proposed scheme is verified on a simulation platform built using MATLAB.

The penetration of renewable energy sources on a large scale in conventional electric networks increases exponentially for environmental preservation. However, the increase in RESs leads to some technical problems that include (a) fluctuations in power production, (b) less or no generation units for power balancing, (c) reduced inertia due to RESs decoupling from the conventional grid using converters. Micro-hydro power plants (MHPPs) are emerging as a mature balancing technology and a great alternative to large hydropower plants as they encounter population displacement and many environmental problems. Modern pump storage MHPPs uses a power converter between the grid and machine to control the consumption of power during pumping mode. This power electronic interface loses the ability of the rotating mass of the machine to contribute to the grid inertia by making it independent of the grid frequency. This problem is solved through the control of power converters in such a way that the inertia effect is synthesized termed as synthetic inertia. MHPPs can also be operated in a standalone mode where they are not connected to the grid. This paper reviews control schemes applied in literature for the frequency, voltage, and inertia control, in both the grid-connected and standalone modes. This study starts with the standalone MHPPs, covering the literature review and control structure for voltage and frequency control of standalone MHPPs. Then it presents a detailed operation, control principle, synthetic inertia concepts, and architecture of grid-connected MHPPs. The mathematical formulation of the grid, permanent magnet synchronous generator, synthetic inertia inclusion, and control structures, including direct torque control, virtual synchronous machine, and model predictive control, is also presented. Finally, the paper presents the concluding remarks with the comparative analysis of various control structures to include synthetic inertia, its suitability, and future scope for MHPPs.

The adoption of smart energy management systems at demand and distribution levels of the grid is mandated by the ever-increasing presence of smart grids. In this dissertation, a smart Home Energy Management System (HEMS) that reduces prosumers' daily energy costs and offers the grid ancillary services is proposed. Photovoltaic (PV) systems, Electric Vehicles (EV), Energy Storage Systems (ESS), flexible and critical loads, and a Multi-Port Converter (MPC) that connects the aforementioned modules with the power grid make up the residential prosumer residence. Optimal power management problem is solved using the Particle Swarm Optimization (PSO) method. The HEMS also features the ability to function independently when the residential microgrid is disconnected from the main grid. Additionally, the designed system can support grid frequency while taking into account the operational flexibility of the controlled power sources and loads. The use of k-means clustering and feed-forward neural networks in a load demand forecasting system is also proposed. The results obtained by the proposed system demonstrate that the HEMS minimizes the residential prosumer's operating costs, supports system frequency optimally, satisfies all operational requirements of the controlled system, and maintains the operational flexibility of all cooperating power components at the highest level., Η ραγδαία ανάπτυξη και διάχυση της τεχνολογίας Smart Grid στο ηλεκτρικό δίκτυο επέβαλε και την ανάγκη για πιο έξυπνα συστήματα διαχείρισης ενέργειας, τόσο στο επίπεδο μεταφοράς όσο και στο επίπεδο διανομής. Σε αυτή τη διατριβή μελετήθηκε, σχεδιάστηκε και αναπτύχθηκε ένα έξυπνο Οικιακό Σύστημα Διαχείρισης Ενέργειας (ΟΣΔΕ) το οποίο μειώνει το ημερήσιο ενεργειακό κόστος της οικίας και επιπρόσθετα προσφέρει βοηθητικές υπηρεσίες στο δίκτυο. Η έξυπνη οικία του καταναλωτή/παραγωγού θεωρείται ότι εμπεριέχει φωτοβολταϊκά πάνελ, ηλεκτρικό όχημα, συστήματα αποθήκευσης ενέργειας, ευέλικτα και κρίσιμα φορτία, καθώς και ένα μετατροπέα πολλαπλών θυρών ο οποίος διασυνδέει όλα τα παραπάνω με το κεντρικό ηλεκτρικό δίκτυο. Η βέλτιστη διαχείριση ενέργειας επιτυγχάνεται με τη χρήση του αλγορίθμου Particle Swarm Optimization - Βελτιστοποίηση Σμήνους Σωματιδίων. Το ΟΣΔΕ είναι επίσης σχεδιασμένο να λειτουργεί και αυτόνομα, δηλαδή χωρίς να είναι συνδεδεμένο με το κεντρικό δίκτυο. Η στήριξη συχνότητας του κεντρικού δικτύου εξασφαλίζεται βέλτιστα, λαμβάνοντας ταυτόχρονα υπόψη την ευελιξία ισχύος της κάθε μονάδας του συστήματος. Προτείνεται επίσης ο συνδυασμός των μεθόδων k-means clustering και feed-forward νευρωνικών δικτύων σε ένα σύστημα πρόβλεψης της κατανάλωσης ενέργειας από τα οικιακά φορτία. Τα αποτελέσματα που προέκυψαν από το προτεινόμενο ΟΣΔΕ αποδεικνύουν την αποτελεσματικότητα του συστήματος στην μείωση του ημερήσιου κόστους ηλεκτρικής ενέργειας, ενώ ταυτόχρονα τηρούνται και όλοι οι περιορισμοί που είναι εγγενείς στο παραπάνω σύστημα ισχύος.

This paper proposes an innovative inertial response imitation (IRI) and rotor speed recovery (RSR) control scheme of a doubly-fed induction generator (DFIG, Type 3 wind turbine generator) to provide better frequency support response and RSR services for a high wind power penetrated electric power grid. To achieve the first benefit, the coupling relationship between the control coefficient of DFIGs and the frequency deviation was established by using the exponential function so that the control coefficient becomes large with the increasing frequency deviations and sizes of disturbance. After supporting the system frequency, the exponential function was employed to schedule the dynamic control coefficient to alleviate the negative effects of RSR on the instantaneous system frequency. The benefits of the proposed IIR and RSR strategy were investigated in a test system under various scenarios of sizes of disturbance and wind speed conditions. Test results clearly demonstrate that the proposed IIR and RSR strategy is capable of boosting the maximum system frequency excursion and reducing the negative influences on the system frequency during the speed recovery period.

Europe’s initiative to reduce the emissions of harmful gases has significantly increased the integration of renewable sources into power networks, particularly wind power. Variable renewable sources pose challenges to sustain the balance between generation and demand. Thus, the need for ancillary services to cope with this problem has increased. In this regard, the integration of larger shares of wind generation would have a clear system benefit when wind generators are able to provide these ancillary services. This would also have implications for electricity markets, enabling these services from wind power plants. This article gives an overview of several European markets for frequency support (FS) services, also referred to as FS markets. It identifies the changes in national regulations of 10 European countries to standardize these services based on the ENTSO-E guidelines. However, most of the countries still use their national service definitions, which presents a problem for researchers to understand the national regulations in relation to the ENTSO-E guidelines. This article provides a classification of the national FS services under the definitions of the ENTSO-E guidelines to facilitate research on this topic. Furthermore, it highlights the main requirements for the market practices that would encourage the participation of wind power generation in the provision of these services. An estimation of the economic benefits for wind producers from the provision of FS services is provided as well to show a possible outcome if changes are not made in national policies.

The increasing level of wind power penetration is seriously threatening the frequency stability of the power system. In this article, we suggest an enhanced frequency response strategy of a doubly fed induction generator (DFIG) based on over-speed de-loaded curve using a novel power function to boost the frequency nadir and settling frequency and reduce the maximum rate of change of frequency (ROCOF) with more efficiency. To achieve this objective, the reference power increases to the torque limit at the de-load operating point and then decreases with the rotor speed toward the maximum power point tracking operating conditions. The simulation results on various wind power penetrations clearly demonstrated that the enhanced frequency response strategy is beneficial to boosting the frequency nadir and settling frequency and reduce the ROCOF.

With the fast growth in the penetration of wind power, doubly fed induction generators (DFIGs) are recommended for their ability to enforce grid codes that provide inertial control services by releasing rotational energy. However, after supporting the system frequency, a second frequency drop (SFD) is prone to occurring to regain the rotor speed caused by the sudden reduction in output. In this article, we propose a torque limit-based fast stepwise inertial control scheme of a DFIG using a piecewise reference function for reducing the SFD while preserving the frequency nadir (FN) with less rotor energy released. To achieve the first objective, the power reference increases to the torque limit and then decays with the rotor speed toward the preset operating point. To achieve the second objective, the power reference smoothly lessens over time based on the exponential function. The performance of the proposed stepwise inertial control strategy was studied under various scenarios, including constant wind speed and ramp down wind speed conditions. The test results demonstrated that the frequency stability is preserved during the frequency support phase, while the second frequency drop and mechanical stress on the wind turbine reduce during the rotor speed restoration phase when the DFIG implements the proposed stepwise inertial control scheme.

As wind penetration level into power systems increases, provision of frequency support from wind plant is anticipated to become a requirement in grid codes. In addition to the virtual inertial response, two different approaches to delivering droop response from wind plant are presented and compared in this paper. The combination of droop and inertial response from wind plant has been assessed for a frequency time series. Simulation results show that the proposed approaches to delivering combined droop and inertial response can provide superior performance to either inertial or droop response alone.

The focus of the work presented here is to raise awareness of how ancillary services within the NordPool area could be of value in supporting the future grid, and who could be the provider of these services. The ancillary services considered here are not limited to the current market, but also services for future market solutions as well as services for fulfilment of grid codes. The goal is to promote the development of existing and novel solutions to increase the utilisation and thus the value of equipment within the power system. The paper includes a techno-economical categorisation of ancillary services, from a provider’s perspective, presenting opportunities and competition. Furthermore, procurers of services could utilise this kind of categorisation to identify possible providers or partners. The analysis of the categorisation shows a broad range of possible providers for each service and a broad range of possible services from each provider.

This study proposes a dual-loop supplementary frequency control (SFC) scheme for back-to-back voltage source converter high-voltage DC (BTB-VSC-HVDC) interconnecting asynchronous AC grids, to provide frequency support for each other after a large disturbance. The proposed dual-loop SFC consists of the frequency-active power (f–P) loop and the frequency-reactive power (f–Q) loop. The former deployed on the P-loop of VSC-HVDC can provide frequency support for the disturbed grid during the primary frequency regulation and improve the steady-state characteristics of the frequency response, while the latter attached to the Q-loop of VSC-HVDC supports virtual inertia to ameliorate the transient characteristics of the frequency response. Simulation studies are conducted based on the equivalently simplified model of Southwest Power Grid and Hubei Power Grid interconnected by Chongqing-Hubei BTB-VSC-HVDC. Simulation results show that the proposed SFC scheme can effectively improve the transient and steady-state characteristics of the system frequency response under a wide range of operating condition.

The advent of renewable energy has posed difficulties in the operation of power systems whose net inertia is becoming critically low. To face such challenges, grid-forming power has been one of the potential solutions pursued by the industry and research community. Although promising, grid-forming power converters are still immature for mass deployment in power systems. In the meanwhile, an enormous amount of grid-following power converters has been underexploited when it comes to grid-supporting functionalities. Therefore, this article proposes an external inertia emulation controller (eIEC) for grid-following power converter to provide frequency support to the grid. For the purpose of minimizing installation efforts and resources, the controller is designed in such a way that it can be implemented in an external controller communicating with the grid-following power converter via an industrial communication link. This article also investigates the effect of communication delay on the stability performance of the proposed controller. In addition to the detailed analysis, hardware-in-the-loop experiments are also carried out to validate the proposed eIEC. This work was supported by the European Commission under Project FLEXITRANSTORE-H2020-LCE-2016-2017-SGS-774407

Increasing penetration of power electronics interfaced generation decreases the stability of the system, due to the absence of the rotational inertia in their operation. Emulation of the inertia using converter controls in combination with storages can address this issue. However, this method relies on the use of large quantities of storage to compensate power during a transient power unbalance. Instead of increasing the supply, the smart transformer (ST), with a fast response, offers the possibility to dynamically regulate the demand. This paper investigates the use of an ST to dynamically control reactive power and demand to support voltage and frequency respectively in the grid. The demand is controlled dynamically to emulate inertia. From an analysis based on a 250 kVA, 10kV/400V LV distribution network, it is shown that a demand variation in the range of 6-10% can be achieved. These results are extended to a case study based on the entire all-Island Irish Transmission system which shows that widespread use of STs with these controls could potentially facilitate a 10% increase in wind penetration without the inclusion of any other storage.

Frequency control over networks is done using the frequency droop control technique which has the simplicity advantage although it allows that, in certain situations, frequency control is not very efficient. Artificial intelligence techniques have been increasingly used, so it is justified to explore their viability in electrical networks. The present work analyzes the use of Artificial Intelligence in networks to improve the frequency droop control. In order to realize this, a deep reinforcement learning (DRL)-based agent is proposed to tune the controller parameters for voltage source converter (VSC) in this paper. The DRL-based agent is trained by numerous hybrid grid operation conditions to lean the optimal control policy, which make it achieve a good adaptability to variety of operation conditions. For the purpose of demonstrating this method, a time-domain simulation model of hybrid power system is built with MATLAB/Simulink to act as test system. The simulation results verify the effectiveness of the proposed method.

Due to the rising shares of renewable energy sources, future power systems are facing significant changes in control complexity and system inertia, thus making frequency regulation in power systems more challenging. This paper proposes a novel control scheme based on model predictive control for grid-forming Voltage Source Converters (VSCs), with the goal of exploiting their fast response capabilities and available DC-side energy storage to provide fast frequency control service to the system. An observer based on support vector machine regression detects and estimates system disturbances using only locally available measurements at each VSC. Frequency evolution is then anticipated through state-space predictions and the VSC power output is adjusted to compensate the disturbance and prevent frequency threshold violations. The proposed control scheme is evaluated and its effectiveness demonstrated through detailed time-domain simulations of the IEEE 39-bus test system.

This paper provides the result of a techno-economic study of potential energy storage technologies deployable at wind farms to provide short-term ancillary services such as inertia response and frequency support. Two different scenarios are considered including a single energy storage system for the whole wind farm and individual energy storage for each wind turbine (located at either the dc or the ac side of its grid-side converter). Simulations are introduced to check the technical viability of the proposal with different control strategies. Power and energy capability requirements demanded by both specific services are defined for each studied case based on present and future grid code needs. Based on these requirements, the study compares a wide range of energy storage technologies in terms of present-day technical readiness and properties and identifies potential candidate solutions. These are flywheels, supercapacitors, and three chemistries out of the Li-ion battery family. Finally, the results of a techno-economic assessment (mainly based on weight, volume, lifetime, and industry-confirmed costings) detail the advantages and disadvantages of the proposed solutions for the different scenarios under consideration. The main conclusion is that none of the candidates are found to be clearly superior to the others over the whole range of scenarios. Commercially available solutions have to be tailored to the different requirements depending on the amount of inertia, maximum Rate of Change of Frequency and maximum frequency deviation to be allowed.

Before diode rectifier (DR) technology for connecting offshore wind farms (OWFs) to HVdc is deployed, in-depth studies are needed to assess the actual capabilities of DR-connected OWFs to contribute to the secure operation of the networks linked to them. This study assesses the capability of such an OWF to provide communication-less frequency support (CLFS) to an onshore ac network. It is shown that the HVdc link’s offshore terminal direct voltage can be estimated from measurements at the OWF’s point of connection with the DR platform. Two different methods are proposed for implementing CLFS in the OWF active power controls. In Method 1, the estimated offshore terminal direct voltage is used for estimating the onshore frequency deviation. In Method 2, the actual offshore terminal direct voltage measurement is used instead. Unique features of the provision of CLFS from OWFs connected to HVdc via DRs are highlighted, and the dynamic and static performance of the CLFS control scheme is compared to that of the communication-based frequency support scheme. To assess the impact of parameter estimation errors on the provision of CLFS, a parametric sensitivity study is presented as well, and recommendations are given to increase accuracy.The manuscript has been accepted for publication in IEEE Transactions on Sustainable Energy.

A torque limit-based (TLB) method was proposed in literature in order to emulate inertial response of variable speed wind turbines (VSWTs). In this paper, this conventional TLB scheme is firstly modified by considering a finite ramp rate for inertial power of the VSWT. It is exposed that the maximum values of the VSWT's inertial power and kinetic energy released by its rotor have a non-linear relationship with its operation point. Then, a linear TLB scheme is proposed to make the inertia emulation more flexible by customizing its key parameters based on the VSWT's operating point. Accordingly, the released kinetic energy and power ramp rate can be selected in proportion of the VSWT's power, rotor speed and/or its reserved kinetic energy. The derived scheme offers a significant reduction of the mechanical tensions on the turbine compared to the conventional one. In addition, when the parameters of the proposed strategy is designed according to the VSWT's power, the inertial response of the corresponding wind farm can be exactly estimated only by deploying its total generation, regardless of its wind turbines' installed capacities and operating points. Furthermore, a new approach is projected to estimate the VSWT's inertial response during the deceleration period using an analytical closed-form function. This facilitates large scale system studies. Finally, the efficiency of derived linear inertia emulation is evaluated through a typical grid with various levels of the wind power penetration.

A rapid deployment of renewable generation has led to significant reduction in the rotational system inertia and damping, thus making frequency control in power systems more challenging. This paper proposes a novel control scheme based on Model Predictive Control (MPC) for converter-interfaced generators operating in a grid-forming mode, with the goal of exploiting their fast response capabilities to provide fast frequency control service to the system. The controller manipulates converter power injections to limit the frequency nadir and rate-of-change-of-frequency after a disturbance. Both centralized and decentralized MPC approaches are considered and compared in terms of performance and practical implementation. Special attention is given to the decentralized controller by generating an explicit MPC solution to enhance computational efficiency and reduce hardware requirements. Simulation results obtained from a high-fidelity differential-algebraic equation model of the IEEE 39-bus system demonstrate the effectiveness of the proposed control schemes.

Orientador: Prof. Dr. José Luis Azcue Puma Dissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-Graduação em Engenharia Elétrica, Santo André, 2020. Com o aumento significativo da inserção de turbinas eólicas no sistema elétrico, a inércia global do sistema é reduzida resultando em perda da capacidade do mesmo em oferecer suporte de frequência. Isso ocorre pois é comum o emprego de turbinas eólicas de velocidade variável, baseados no gerador de indução duplamente alimentado (DFIG), que se acoplam à rede elétrica por meio de conversores eletrônicos, que não possuem as mesmas características dos geradores síncronos. Assim, neste trabalho é proposto o emprego do Sistema de Armazenamento de energia à Baterias (BESS) associado ao DFIG com o controle de descarga (De-loading) de maneira a fornecer suporte à frequência. Além disso, foi desenvolvida uma estratégia de controle, sendo considerados fatores como limitações da corrente de operação dentro dos limites do BESS. Simulações no domínio do tempo foram propostas para estudar uma microrrede contendo uma turbina eólica baseado no DFIG, mostrando as vantagens do BESS e o sistema de controle de descarga no DFIG, frente a distúrbios de frequência With the significant increase in the insertion of wind turbines in the electrical system, the overall inertia of the system is reduced resulting in a loss of its ability to support frequency. This is because it is common to use variable speed wind turbines, based on the double fed induction generator (DFIG), which generators are coupled to the power grid through electronic converters, and does not have the same characteristics as synchronous generators. Thus, this thesis proposes the use of the DFIG with De-loading control and Battery Energy Storage System (BESS) to support frequency. A control strategy was developed, and important factors such as charging and discharging current limitations and operation within BESS limits were considered. Time domain simulations have been proposed to study a Microgrid containing a wind turbine (DFIG), showing the advantages of BESS and De-loading control system of the DFIG, over frequency disturbances

The integration of renewable resources is quickly growing in the Nordic power system (NPS), and it has led to increasing challenges for the operation and control of the NPS. Nordic countries require that the first-generation power plants have a more flexible operation regime to overcomes power imbalances coming from fluctuations of the demand and supply. This paper assesses optimal frequency support of variable-speed hydropower plants installed in Telemark and Vestfold, Norway, considering future scenarios of NPS. The total kinetic energy of the NPS is expected to be significantly reduced in the future. This paper looks into the implementation of hydropower units with a variable-speed operation regime and battery energy storage systems (BESS), equipped with fast-active power controller (FAPC) technology, to provide fast frequency response after a system frequency disturbance. The frequency support was formulated as an optimization process; therefore, the parameter of the FAPC was optimally calculated for future scenarios of low inertia in NPS. Three main futures scenarios were developed for technology penetration in the Vestfold and Telemark area in Norway. The simulation results showed that the integration variable-speed hydropower units and BESS technologies improved the frequency response even in low-kinetic energy scenarios.

Emulating inertia using power converter has been an effective way to provide additional inertia to power electronic dominated power systems. Currently, the control algorithms for emulating inertia have been proposed considering embedded implementation meaning that control algorithms are implemented in digital controller of power converter. However, it is not always feasible or cost-effective to upgrade control firmware of power converters. On the other hand, most of commercial power converter are equipped with at least one fieldbus interface, which can be employed for control purposes. This paper explores the possibility of emulating inertia by means of an external controller, which communicates with power converter via a communication link. Detailed analyses, especially regarding the influence of communication delays on the system stability and on performance indices such as RoCoF and frequency nadir, are given to provide insight into such an external implementation. The proposed implementation approach are also validated through simulations and hardware-in-the-loop (HIL) experiments. This work was supported by the European Commission under project INTERRFACE - H2020-LC-SC3-2018-ES-SCC– 824330 and by the Spanish Ministry of Science under project ENE2017-88889-C2-1-R.

This paper proposes a frequency support control scheme for large-scale wind farm (WF) by considering distributed layout of wind turbines (WT) and wake effects inside the WF. The frequency support of WF is formulated as optimization problems. A novel clustering-based optimization method is proposed to coordinate all the WTs so that the whole WF can work at the optimal operating states. In large-scale WF, number of WTs is numerous. With the proposed method, the WTs are first clustered into groups according to their wind profiles. By dispatching the same control commands to the WTs belonging to the same group, each group can be further seen as one single WT. As a result, the number of the WTs and control variables reduces significantly and hence the optimization problems are highly simplified. A case study is carried out to verify the effectiveness of the proposed method.

The 2050 targets established by the EU will foster both larger penetration of renewable energy, especially wind power, and more cross-border interconnections. Moreover, this new framework requires the non-conventional power sources and power converter-based systems to be responsible for the duties traditionally carried out by conventional synchronous generators as frequency support. This paper presents how different power-electronic based technologies can provide frequency support individually and in a coordinated manner (with different priority given by the deadbands) ensuring a stable operation.The implemented scenarios examine challenging conditions, where the primary reserve of the interconnected conventional, renewable, and storage generation is fully utilized to tackle frequency incidents. This demonstrates how the joint regulation of the power electronic-based technologies enhances the frequency stability of the AC synchronous areas. The different control schemes and their interaction are investigated in Cigré DC grid benchmark adapted for frequency stability studies and implemented in Matlab/Simulink simulation tool. This modified grid includes 5-terminal HVDC grid with two offshore wind farms and three AC networks including battery and onshore wind farms. Fil: Attya, Ayman B.. University of Strathclyde; Reino Unido Fil: Domínguez García, José Luis. Catalonia Institute for Energy Research; España Fil: Bianchi, Fernando Daniel. Catalonia Institute for Energy Research; España. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Anaya Lara, Olimpo. Catalonia Institute for Energy Research; España