Time filter

Source Type

Fernandez-Peruchena C.M.,Renewable Energy National Center CENER | Bernardos A.,Renewable Energy National Center CENER
Solar Energy | Year: 2015

In this study, one-minute global horizontal solar irradiance distributions conditioned to the optical air mass, m, and hourly average of global horizontal solar irradiance were studied at sites in five different climate regions. For this purpose, the clearness index, kt, which accounts for the atmospheric transmittance, has been used. These distributions are fitted by functions based on the Boltzmann statistic. The one-minute distributions of kt conditioned to m found are either unimodal or bimodal, depending on the location and the value of m. These distributions are different for each of the locations analyzed. The one-minute distributions of kt conditioned to their hourly value (kth) are unimodal, and are in turn different at each of the locations analyzed. The one-minute kt distributions conditioned to both m and kth analyzed are also unimodal. These distributions were found to be the same (Kolmogorov-Smirnov test, p > 0.05) at different sites in 5% of the cases compared, the majority of which show very cloudy sky conditions and decrease monotonically at clearer-sky conditions. These results point to the importance of local distribution and type of clouds in one-minute solar irradiance distributions, and highlight the role of local atmospheric clear sky transparency in differentiating these distributions. © 2014 Elsevier Ltd.


Yang W.,Renewable Energy National Center CENER | Yang W.,Northwestern Polytechnical University | Tavner P.J.,Durham University | Court R.,Renewable Energy National Center CENER
Mechanical Systems and Signal Processing | Year: 2013

Induction generators have been successfully applied to a variety of industries. However, their operation and maintenance in renewable wind and marine energy industries still face challenges due to harsh environments, limited access to site and relevant reliability issues. Hence, further enhancing their condition monitoring is regarded as one of the essential measures for improving their availability. To date, much effort has been made to monitor induction motors, which can be equally applied to monitoring induction generators. However, the achieved techniques still have constrains in particular when dealing with the condition monitoring problems in wind and marine turbine generators. For example, physical measurements of partial discharge, noise and temperature have been widely applied to monitoring induction machinery. They are simple and cost-effective, but unable to be used for fault diagnosis. The spectral analysis of vibration and stator current signals is also a mature technique popularly used in motor/generator condition monitoring practice. However, it often requires sufficient expertise for data interpretation, and significant pre-knowledge about the machines and their components. In particular in renewable wind and marine industries, the condition monitoring results are usually coupled with load variations, which further increases the difficulty of obtaining a reliable condition monitoring result. In view of these issues, a new condition monitoring technique is developed in this paper dedicated for wind and marine turbine generators. It is simple, informative and less load-dependent thus more reliable to deal with the online motor/generator condition monitoring problems under varying loading conditions. The technique has been verified through both simulated and practical experiments. It has been shown that with the aid of the proposed technique, not only the electrical faults but also the shaft unbalance occurring in the generator become detectable despite the external loading conditions. Moreover, the rotor and stator winding faults can be readily discriminated through observing the variation tendencies of the proposed condition monitoring criteria. © 2012 Elsevier Ltd.


Llorente Iglesias R.,European Commission | Lacal Arantegui R.,European Commission | Aguado Alonso M.,Renewable Energy National Center CENER
Renewable and Sustainable Energy Reviews | Year: 2011

The aim of the paper is to analyse the evolution of wind turbine concepts or topologies with a specific focus on their power electronics content, and to demonstrate the tendency of wind turbine manufacturers towards the development of generators connected to the grid by means of power electronics converters. The paper provides a review of the power electronics converters used in wind turbines and a briefer description of the components that make up those converters. Then the research, supported by a market study based on 91% of the total installed wind capacity during the period 2000-2009, demonstrates the evolution of the wind turbine market towards the use of power electronics converters with their market share increasing from 38% in 2000 to 80% in 2009. In particular, the type D wind turbine configuration - containing a full power converter - appears set up to increase market share in the next five years. If projections of these findings are realised wind turbines without a power converter could be reduced to a niche market as soon as 2013. The paper refers these findings to, among other reasons, the increasingly more strict technical requirements of the grid operation codes. Finally, the paper details some of the current research and development trends plus a vision of the future by the industry. © 2011 Elsevier Ltd. All rights reserved.


Zaversky F.,Renewable Energy National Center CENER | Sanchez M.,Renewable Energy National Center CENER | Astrain D.,University of Pamplona
Energy | Year: 2014

This work focuses on the transient numerical modeling of multi-pass shell-and-tube heat exchangers that apply single-phase fluids. A one-dimensional modeling approach is used for the heat exchanger ducts. The governing partial differential equations are solved numerically by applying the finite volume method. In particular, the commonly applied cell-method is used, which is presented in a flexible, intuitive and simulation-platform-independent way. Simulation results are checked for consistency by comparing them to theoretical as well as experimental data available in the literature. Subsequently, the presented modeling approach is used for a specific case study, showing the transient behavior of a typical heat exchanger train configuration currently used at active indirect thermal energy storage systems for CSP (concentrated solar power). Typical process parameters (process gain, dead time and time constant) are given for charging as well as for discharging mode at different heat exchanger loads. Furthermore, transient response simulation results are discussed in detail, providing all used boundary conditions and assumed heat exchanger specifications, thus enabling future model comparison studies. Finally, suitable degrees of discretization are discussed for transient CSP performance simulations on system level, offering a good trade-off between simulation speed and accuracy. Modelica is used as modeling language. © 2013 Elsevier Ltd.


Sallaberry F.,Renewable Energy National Center CENER | Pujol-Nadal R.,University of the Balearic Islands | Martinez-Moll V.,University of the Balearic Islands | Torres J.-L.,Public University of Navarra
Renewable Energy | Year: 2014

Concentrating solar collectors are mentioned in the International Standards, but the general testing methods for solar collectors mentioned cannot easily be applied to such unusual collector designs. In this study, the best optical and thermal model for a variable geometry solar concentrator has been investigated. In the particular case of a collector with a fixed mirror concentrator, the relative position of the receiver with respect to the reflector is not constant during the day, and this variable geometry is not taken into account in the current testing Standards. An optical characterization of the prototype using a ray-tracing program has been performed, and the results have been used as an initial hypothesis to define two thermal models adapted from the European Standard. Those two different models have been compared. The optical results obtained from experiments have been compared to ray-tracing simulation results, and they have been found to be quite similar, considering the measurement uncertainties. This validation procedure of the optical simulation could be an important point to be taken into account in a future Standard revision for variable geometry collector types for which the normal incidence is not easy to obtain. © 2014 Elsevier Ltd.


Yang W.,Northumbria University | Little C.,Renewable Energy National Center CENER | Court R.,Renewable Energy National Center CENER
Renewable Energy | Year: 2014

Condition monitoring (CM) has long been recognised as one of the best methods of reducing the operation and maintenance (O&M) costs of wind turbines (WTs). However, its potential in the wind industry has not been fully exploited. One of the major reasons is due to the lack of an efficient tool to properly process the WT CM signals, which are usually non-stationary in both time and frequency domains owing to the constantly varying operational and loading conditions experienced by WTs. For this reason, S-transform and its potential contribution to WT CM are researched in this paper. Following the discussion of the superiorities of S-transform to the Short-Time Fourier Transform (STFT) and Wavelet Transform, two S-transform based CM techniques are developed, dedicated for use on WTs. One is for tracking the energy variations of those fault-related characteristic frequencies under varying operational conditions (the energy rise of these frequencies usually indicates the presence of a fault); another is for assessing the health condition of WT gears and bearings, which have shown significant reliability issues in both onshore and offshore wind projects. In the paper, both proposed techniques have been verified experimentally, showing that they are valid for detecting both the mechanical and electrical faults occurring in the WT despite its varying operational and loading conditions. © 2013 Elsevier Ltd.


Yang W.,Renewable Energy National Center CENER | Court R.,Renewable Energy National Center CENER | Jiang J.,Northwestern Polytechnical University
Renewable Energy | Year: 2013

Wind turbines are being increasingly deployed in remote onshore and offshore areas due to the richer wind resource there and the advantages of mitigating the land use and visual impact issues. However, site accessing difficulties and the shortage of proper transportation and installation vehicles/vessels are challenging the operation and maintenance of the giants erected at these remote sites. In addition to the continual pressure on lowering the cost of energy of wind, condition monitoring is being regarded as one of the best solutions for the maintenance issues and therefore is attracting significant interest today. Much effort has been made in developing wind turbine condition monitoring systems and inventing dedicated condition monitoring technologies. However, the high cost and the various capability limitations of available achievements have delayed their extensive use. A cost-effective and reliable wind turbine condition monitoring technique is still sought for today. The purpose of this paper is to develop such a technique through interpreting the SCADA data collected from wind turbines, which have already been collected but have long been ignored due to lack of appropriate data interpretation tools. The major contributions of this paper include: (1) develop an effective method for processing raw SCADA data; (2) propose an alternative condition monitoring technique based on investigating the correlations among relevant SCADA data; and (3) realise the quantitative assessment of the health condition of a turbine under varying operational conditions. Both laboratory and site verification tests have been conducted. It has been shown that the proposed technique not only has a potential powerful capability in detecting incipient wind turbine blade and drive train faults, but also exhibits an amazing ability in tracing their further deterioration. © 2012 Elsevier Ltd.


Ferreira H.L.,TU Eindhoven | Garde R.,Renewable Energy National Center CENER | Fulli G.,European Commission | Kling W.,TU Eindhoven | Lopes J.P.,University of Porto
Energy | Year: 2013

In the current situation with the unprecedented deployment of clean technologies for electricity generation, it is natural to expect that storage will play an important role in electricity networks. This paper provides a qualitative methodology to select the appropriate technology or mix of technologies for different applications. The multiple comparisons according to different characteristics distinguish this paper from others about energy storage systems.Firstly, the different technologies available for energy storage, as discussed in the literature, are described and compared. The characteristics of the technologies are explained, including their current availability. In order to gain a better perspective, availability is cross-compared with maturity level. Moreover, information such as ratings, energy density, durability and costs is provided in table and graphic format for a straightforward comparison. Additionally, the different electric grid applications of energy storage technologies are described and categorised. For each of the categories, we describe the available technologies, both mature and potential. Finally, methods for connecting storage technologies are discussed. © 2013 Elsevier Ltd.


Iribas-Latour M.,Renewable Energy National Center CENER | Landau I.-D.,CNRS GIPSA Laboratory
Wind Energy | Year: 2013

To achieve load reduction and power optimization, wind turbine controllers design requires the availability of reliable control-oriented linear models. These are needed for model-based controller design. Model identification of wind turbine while operating in closed loop is an appropriate solution that has recently shown its capabilities when linear time-invariant controllers and complicated control structures are present. However, the collective pitch control loop, one of the most important wind turbine loops, uses non-linear controllers. Typically, this non-linear controller is a combination of a linear controller and a gain scheduling. This paper presents a new algorithm for identification in closed-loop operation that allows the use of this kind of non-linear controllers. The algorithm is applied for identification the collective pitch demand to generator speed of a wind turbine at various operating points. The obtained models are presented and discussed from a control point of view. The validity of these models is illustrated by their use for the design of a linear fix robust controller. The performance based on simulation data of this linear controller is similar to that obtained with simulations based on a linear controller with gain scheduling, but its design and implementation is much simpler. Copyright © 2012 John Wiley & Sons, Ltd. Copyright © 2012 John Wiley & Sons, Ltd.


Yang W.,Northumbria University | Court R.,Renewable Energy National Center CENER
Measurement: Journal of the International Measurement Confederation | Year: 2013

Bearings are critical components in the products of many industries, and their failure can result in long downtime and costly maintenance. Prolonging a bearing's service in a safe manner is vital for operators of equipment, and condition monitoring is regarded as one of the best approaches to achieve this. In general practice, condition monitoring does successfully indicate the presence and growth of bearing faults. However, standard condition monitoring techniques do not usually have a proven method to determine the best time to conduct maintenance of a defective bearing. Traditionally, maintenance will be conducted when the measured vibration of a bearing is found to exceed an acceptable vibration level, as defined by an industry standard. However, the industry standard is only a general guideline to the design and operation of the kind of machines/components under certain conditions. The actual dynamic response of a bearing is reliant on a variety of factors, such as lubrication, loading, temperature, and operational and environmental conditions. As a consequence, although the empirical vibration levels suggested by the standard are helpful in ensuring the safe operation of a bearing, they cannot guarantee full utilization of the residual life of a defective bearing. The purpose of this paper is to try and find a feasible solution for this issue, and moreover validate it through a number of experiments conducted under various loading and operational conditions. In the research, four dimensionless condition monitoring criteria: normalized information entropy; J-Divergence; Kurtosis; and a composite criterion based on them, are employed to assess the actual health condition of the test bearings with different types and severity levels of failures. Experimental results have shown that in comparison of the industry standard, the proposed method provides an effective and feasible approach for predicting the optimum time to conduct bearing maintenance. It is deemed that the achievements of this work will help operators in further improving their management of assets. © 2013 Elsevier Ltd. All rights reserved.

Loading Renewable Energy National Center CENER collaborators
Loading Renewable Energy National Center CENER collaborators