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Online ISSN 1752-1424 Print ISSN 1752-1416

IET Renewable Power Generation

Volume 5, Issue 3, May 2011

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Volume 5, Issue 3

May 2011

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    • Modelling of solar cells and modules using piecewise linear parallel branches
      • Author(s): Y.-J. Wang  and  P.-C. Hsu
      • Source: IET Renewable Power Generation, Volume 5, Issue 3, p. 215 –222
      • DOI:  10.1049/iet-rpg.2010.0134
      • Type: Article
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      • p. 215 –222 (8)
        This study proposes a novel equivalent circuit model for solar cells and modules. The proposed model approximates the non-linear I–V characteristic of a photovoltaic cell or module by a number of linear circuit branches connected in parallel. The number of these linear circuit branches is adjustable according to the required accuracy. An algorithm based on an equal-area criterion for determining the parameters of the equivalent circuit is also proposed. This algorithm, if incorporated into the software that simulates the proposed model, allows variations of solar irradiance and cell temperature to be taken into account during the simulation. Two numerical examples simulated by electromagnetic transient programme software are presented to demonstrate the performance of the model.
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      DC voltage control and power dispatch of a multi-terminal HVDC system for integrating large offshore wind farms
      • Author(s): L. Xu  and  L. Yao
      • Source: IET Renewable Power Generation, Volume 5, Issue 3, p. 223 –233
      • DOI:  10.1049/iet-rpg.2010.0118
      • Type: Article
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      • p. 223 –233 (11)
        Multi-terminal HVDC transmission technology using voltage source converters is proposed for integrating large offshore wind farms with transmission grids. Different DC voltage control and power dispatch strategies are proposed to demonstrate the flexibility and capability of such a transmission system in integrating large-scale variable wind generation. Various options for ensuring satisfactory ride through onshore grid faults are discussed. PSCAD/EMTDC simulations on a four-terminal high voltage direct current (HVDC) system with two offshore and two onshore converter stations during wind speed and power variations, and control mode switching are presented to show the robust performance and ability of the proposed system. Further studies during severe fault on one of the connected onshore AC networks are provided to validate the fault ride through capability of the multi-terminal HVDC system.
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      Control strategies for a wave energy converter connected to a hydraulic power take-off
      • Author(s): P. Ricci ; J. Lopez ; M. Santos ; P. Ruiz-Minguela ; J.L. Villate ; F. Salcedo ; A.F.deO. Falcão
      • Source: IET Renewable Power Generation, Volume 5, Issue 3, p. 234 –244
      • DOI:  10.1049/iet-rpg.2009.0197
      • Type: Article
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      • p. 234 –244 (11)
        Among the various types of wave energy converters currently being developed, heaving point absorbers are one of the simplest and most promising concepts. A typical efficient energy conversion system for point absorbers is based on hydraulic power take-off (PTO) systems, consisting in a double-acting cylinder, a hydraulic motor and two or more accumulators. This paper presents a simple model of a heaving oscillating buoy extracting power by means of a hydraulic system. The hydrodynamic behaviour of the absorber is modelled through application of the linear water wave theory. Apart from the basic elements listed above, the model of the hydraulic system includes leakages and pressure losses and takes into account the compressibility of the fluid. Also, possible control accumulators are considered in order to improve the performance of the hydraulic system by means of properly controlled valves. Different control variables are analysed depending on the wave inputs considered in order to improve the power extraction of the converter. The results prove that it is possible to achieve a great enhancement of the power extraction with the implementation of these control strategies and that a possible combination of some of them might be beneficial for improved efficiency of the components.
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      Operation analysis of a wave energy converter under different load conditions
      • Author(s): C. Boström  and  M. Leijon
      • Source: IET Renewable Power Generation, Volume 5, Issue 3, p. 245 –250
      • DOI:  10.1049/iet-rpg.2010.0153
      • Type: Article
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      • p. 245 –250 (6)
        This study analyses the electrical behaviour of a direct-driven linear generator under different load conditions. The studied generator is used in a wave energy converter (WEC) that converts the energy in ocean waves into electric energy. To enable a grid connection of a WEC, the voltage must be converted, and thereby, the generator will be subjected to a non-linear damping. Depending on how the conversion system is designed, the damping will be different. In the case studied, the voltage is first rectified, and on the dc-side of the rectifier the voltage is kept constant by controlling the power through a converter. In order to study the electrical behaviour of the generator in this operation mode, a simulation model was made in MATLAB Simulink. The model of the generator was verified with experimental data from an offshore operating WEC. The result of the study shows that the model of the generator agrees with the real generator and can be used for analysing the electrical behaviour of the WEC. Moreover, the results show that the operation with a non-linear load will be different compared to a linear load case.
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      Hybrid intelligent approach for short-term wind power forecasting in Portugal
      • Author(s): J.P.S. Catalão ; H.M.I. Pousinho ; V.M.F. Mendes
      • Source: IET Renewable Power Generation, Volume 5, Issue 3, p. 251 –257
      • DOI:  10.1049/iet-rpg.2009.0155
      • Type: Article
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      • p. 251 –257 (7)
        The increased integration of wind power into the electric grid, as nowadays occurs in Portugal, poses new challenges because of its intermittency and volatility. Hence, good forecasting tools play a key role in tackling these challenges. In this study, a hybrid intelligent approach is proposed for short-term wind power forecasting in Portugal. The proposed approach is based on the wavelet transform and a hybrid of neural networks and fuzzy logic. Results from a real-world case study are presented. A thorough comparison is carried out, taking into account the results obtained with other approaches. Conclusions are duly drawn.
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      Smart energy management system for optimal microgrid economic operation
      • Author(s): C. Chen ; S. Duan ; T. Cai ; B. Liu ; G. Hu
      • Source: IET Renewable Power Generation, Volume 5, Issue 3, p. 258 –267
      • DOI:  10.1049/iet-rpg.2010.0052
      • Type: Article
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      • p. 258 –267 (10)
        This study presents a smart energy management system (SEMS) to optimise the operation of the microgrid. The SEMS consists of power forecasting module, energy storage system (ESS) management module and optimisation module. The characteristic of the photovoltaics (PV) output in different weather conditions has been studied and then a 1-day-ahead power forecasting module is presented. As energy storage needs to be optimised across multiple-time steps, considering the influence of energy price structures, their economics are particularly complex. Therefore the ESS module is applied to determine the optimal operation strategies. Accordingly, multiple-time set points of the storage device, and economic performance of ESS are also evaluated. Smart management of ESS, economic load dispatch and operation optimisation of distributed generation (DG) are simplified into a single-object optimisation problem in the SEMS. Finally, a matrix real-coded genetic algorithm (MRC-GA) optimisation module is described to achieve a practical method for load management, including three different operation policies and produces diagrams of the distributed generators and ESS.
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