Combining hydro and variable wind power generation by means of pumped-storage under economically viable terms

Pumped hydro storage (PHS) systems which are located at isolated regions and are able to exploit the rejected wind energy amounts produced by local wind farms, seem to gain interest worldwide and to become essential in regard to higher shares of renewable-generated electricity. Despite the high wind potential encountered in many Greek island regions, the wind energy contribution to the electrification of these areas is significantly restricted due to imposed electrical grid limitations. In this context, the current work examines the economic viability of a wind-based PHS system (wind-hydro solution) which provides the local electrical grid of an Aegean Sea island, Lesbos, with guaranteed energy amounts during the peak load demand periods. Based on the maximization of the project’s net present value, the optimum system configuration is proposed while many other feasible solutions are revealed. According to the results obtained the implementation of this project demonstrates excellent technical and economic performance, while at the same time renewable energy sources (RES) contribution is doubled reaching almost 20% of the Lesbos island electrical energy consumption.     

Power limitations and energy yield evaluation for wind farms operating in island systems

Wind farms operating in island systems are subject to output power limitations, related with technical constraints of the conventional generating units, namely the minimum loading levels of the thermal units (technical minima) and a dynamic penetration limit, applied for stability purposes. Evaluation of the expected wind energy yield in isolated island systems requires, therefore, proper consideration, not only of the prevailing wind conditions at the installation site, but also of the power limitations imposed by the system, which ultimately depend on the total load demand. In the paper, a methodology is presented for the evaluation of the wind energy absorption capability of island systems and the expected energy yield of wind farms operating therein. This methodology is currently applied in Greece for the determination of the power restrictions applied to existing and new wind farms, to schedule new wind capacity tenders and for the evaluation of the expected energy yield of new investments for permitting and financing purposes.     

Analysis of a pumped storage system to increase the penetration level of renewable energy in isolated power systems. Gran Canaria: A case study

A significant number of islands have been forced to restrict the penetration level of renewable energy sources (RES) in their conventional electrical power systems. These limitations attempt to prevent problems that might affect the stability and security of the electrical system. Restrictions that may apply to the penetration of wind energy can also be an obstacle when meeting European Union renewable energy objectives. As a partial solution to the problem, this paper proposes the installation of a properly managed, wind-powered, pumped hydro energy storage system (PHES) on the island of Gran Canaria (Canary Islands). Results from a dynamic model of the island’s power system show that the installation of a pumped storage system is fully supported in all circumstances. They also show that the level of wind penetration in the network can be increased. These results have been obtained assuming that two of the largest existing reservoirs on the island (with a difference in altitude of 281 m and a capacity of aprox. 5,000,000 m³ each) are used as storage reservoirs with three 54 MW generators. Likewise, the ability of such facilities to contribute to the stability of the system is shown. This type of installation can reduce fossil fuel consumption, reducing CO₂ emissions. Moreover, not only can the PHES improve wind penetration level, but it also allows the number of wind farms installed to be increased. Regions with geographically suitable sites and energy problems similar to those on the Canary Islands are encouraged to analyze the technical and economic feasibility of installing similar power systems to the one in this paper. Such systems have an enormous, unexplored potential within the general guiding framework of policies promoting clean, renewable energy.     

Characterizing future large,rapid changes in aggregated wind power using Numerical Weather Prediction spatial fields

A critical limiting factor to the successful deployment of a large proportion of wind power in power systems is its predictability. Power system operators play a vital role in maintaining system security, and this task is greatly aided by useful characterizations of future system operations. A wind farm power forecast generally relies on the forecast output from a Numerical Weather Prediction (NWP) model, typically at a single grid point in the model to represent the wind farm's physical location. A key limitation of this approach is the spatial misplacement of weather features often found in NWP forecasts. This paper presents a methodology to display wind forecast information from multiple grid points at hub height around the wind farm location. If the raw forecast wind speeds at hub height at multiple grid points were to be displayed directly, they would be misleading as the NWP outputs take account of the estimated local surface roughness and terrain at each grid point. Hence, the methodology includes a transformation of the wind speed at each grid point to an equivalent value that represents the surface roughness and terrain at the chosen single grid point for the wind farm site. The chosen‐grid‐point‐equivalent wind speeds for the wind farm can then be transformed to available wind farm power. The result is a visually‐based decision support tool which can help the forecast user to assess the possibilities of large, rapid changes in available wind power from wind farms. A number of methods for displaying the field for multiple wind farms are discussed. The chosen‐grid‐point‐equivalent transformation also has other potential applications in wind power forecasting such as assessing deterministic forecast uncertainty and improving downscaling results. Copyright © 2008 John Wiley & Sons, Ltd.     

The economics of large-scale wind power in the UK A model of an optimally mixed CEGB electricity grid

Previous calculations of the economics of large-scale wind power have been generally limited to the evaluation of the marginal cost of energy, assuming that the addition of a wind farm to an electricity grid does not change the mix of base, intermediate and peak load plant in that grid. Here a simple but powerful numerical generation planning model has been constructed for grids containing wind farms and three classes of thermal power station, but no storage. Electricity demand and available power are specified by empirically based probability distribution functions and the plant mix which minimizes the total annualized costs of the generating system is determined. Capacity credit of wind power is automatically taken into account in the optimization. Using the model, the breakeven costs of wind energy in a model British CEGB grid, containing coal, nuclear, oil and wind driven power plant, are evaluated under various conditions. For a wide range of parameter values, large-scale wind power is likely to be economically competitive in this grid.     

Development strategies for wind power industry in Jiangsu Province,China: Based on the evaluation of resource capacity

Jiangsu Province is one of the planned strategic areas for wind power development in China, but its current development of wind power industry is not so outstanding. Since, Jiangsu would encounter little market resistance, this paper focuses on the evaluation of resource capacity for wind power development in Jiangsu Province, including the wind energy capacity, land resources capacity and power grid capacity, in order to find their supportive or restrictive effects on the development of wind power industry. The results show that the wind energy resource in Jiangsu Province are sufficient to meet the needs of the development of wind farms; the extensive mudflat in the coastal areas also provides ideal site conditions for wind farms; but the power grid capacity is insufficient for the wind power development in Jiangsu. Therefore, from the aspect of enhancing the capacity of power grid for carrying wind power and from the other aspect of combining the non-grid-connected utilization and the large-scale storage of wind power, this paper suggests some strategies to overcome the constraints of grid capacity and promoting the wind power development in Jiangsu Province.     

Wind farms connected to weak grids in India

This article describes the mutual influence between the wind turbines and the power quality in the Indian power systems. The Indian power systems are weak in general and the wind energy development has been very fast and concentrated in a few rural areas where the existing transmission and distribution grids are very weak. Therefore the mutual influence between wind turbines and power quality is particularly strong in India. The wind farms influence power quality aspects such as steady state voltage, power factor, flicker and harmonic and interharmonic distortion. The power quality of the grid influences the power performance and safety of the wind turbines and the lifetime of mechanical and electrical components. The findings presented are general for wind farms connected to weak grids, but the Indian case is pronounced concerning weakness of grid as well as wind energy penetration level. Copyright © 2001 John Wiley & Sons, Ltd.     

Design of a cost-effective on-grid hybrid wind–hydrogen based CHP system using a modified heuristic approach

An economic model and optimization procedure is developed in this paper for grid-connected hybrid wind–hydrogen combined heat and power systems for residential applications in northeastern Iran. The model considers various significant factors: energy production cost, electrical trade with local grid, electrical power generation from the wind/hydrogen energy system, thermal recovery from the fuel cell, and maintenance. Also, various tariffs for purchasing and selling electrical energy from the local grid are considered for the hybrid system operation. The optimization objective is to minimize the system total cost subject to relevant constraints for residential applications. To achieve this aim, an efficient optimization method is proposed based on particle swarm optimization. The proposed algorithm performance is compared with that for the imperialist competition algorithm. The results show that the hybrid system is the most cost-effective for the residential load, and the results of the proposed algorithm are more promising than those for the alternative algorithm.     

Simulation of a combined wind and solar power plant

The combined generation of electricity by wind and solar energy is a very attractive solution for isolated regions with high levels of yearly wind energy and insolation. A computer model is developed for the simulation of the electricity system of a Mediterranean island, including a wind power plant, a photovoltaic power plant and a storage system. In order to obtain an overall view of the system performance and economic aspects, the model also incorporates a number of diesel generators. Daily simulations for the Greek island Kythnos show that such a combined system of moderate size can provide a large fraction of the electrical energy requirements. Various parameters calculated in the simulation can be used to improve the configuration of the system and to estimate the cost of the electrical energy unit.     

Economics of off-shore/on-shore wind energy systems in Qatar

This work presents an assessment of the potential and economical feasibility of adopting off-shore/on-shore wind energy as a renewable source of energy in Qatar. An analysis is presented for the long term measured on-shore wind speed (1976–2000) at Doha International Airport. A similar analysis is presented for the measured off-shore wind speed at the Qatari Haloul Island. For the on-shore measurements, the average annual wind speed (at 20 m height) was found to be about 5.1 m/s. On the other hand, for the off-shore measurements at Haloul, the average annual wind speed was found to be about 6.0 m/s. This result indicates the suitability of utilizing small to medium-size wind turbine generators, efficiently. Such generators can be implemented for water pumping and to produce sufficient electricity to meet vital, limited needs of remote locations, such as isolated farms, which do not have access to the national electricity grid. An economical assessment is presented which takes into consideration the interest recovery factor, the lifetime of the wind energy conversion system (WECS), the investment rate and operation and maintenance costs. The results indicate that the cost of electricity generation from the wind in Qatar compares favorably to that from fossil fuel resources. The feasibility of utilizing off-shore wind turbine systems to meet the power requirements of the island of Haloul and possibly provide additional power for nearby on-shore areas is discussed.     

Comparative study on the performance of control systems for doubly fed induction generator (DFIG) wind turbines operating with power regulation

As a result of the increasing wind power penetration on power systems, the wind farms are today required to participate actively in grid operation by an appropriate generation control. This paper presents a comparative study on the performance of three control strategies for DFIG wind turbines. The study focuses on the regulation of the active and reactive power to a set point ordered by the wind farm control system. Two of them (control systems 1 and 2) are based on existing strategies, whereas the third control system (control system 3) presents a novel control strategy, which is actually a variation of the control system 2. The control strategies are evaluated through simulations of DFIG wind turbines, under normal operating conditions, integrated in a wind farm with centralized control system controlling the wind farm generation at the connection point and computing the power reference for each wind turbine according to a proportional distribution of the available power. The three control systems present similar performance when they operate with power optimization and power limitation strategies. However, the control system 3 with down power regulation presents a better response with respect to the reactive power production, achieving a higher available reactive power as compared with the other two. This is a very important aspect to maintain an appropriate voltage control at the wind farm bus.     

Grid support of a wind farm with active stall wind turbines and AC grid connection

One of the main concerns in the grid integration of large wind farms is their ability to behave as active controllable components in the power system. This article presents the design of a new integrated power control system for a wind farm made up exclusively of active stall wind turbines with AC grid connection. The designed control system has the task of enabling such a wind farm to provide the best grid support. It is based on two control levels: a supervisory control level, which controls the power production of the whole farm by sending out reference signals to each individual wind turbine, and a local control level, which ensures that the reference power signals at the wind turbine level are reached. The ability of active stall wind farms with AC grid connection to control the power production to the reference power ordered by the operators is assessed and discussed by means of simulations. Copyright © 2005 John Wiley & Sons, Ltd.     

Analysis of wind power generation and prediction using ANN: A case study

Many developing nations, such as India have embarked upon wind energy programs for areas experiencing high average wind speeds throughout the year. One of the states in India that is actively pursuing wind power generation programs is Tamil Nadu. Within this state, Muppandal area is one of the identified regions where wind farm concentration is high. Wind energy engineers are interested in studies that aim at assessing the output of wind farms, for which, artificial intelligence techniques can be usefully adapted. The present paper attempts to apply this concept for assessment of the wind energy output of wind farms in Muppandal, Tamil Nadu (India). Field data are collected from seven wind farms at this site over a period of 3 years from April 2002 to March 2005 and used for the analysis and prediction of power generation from wind farms. The model has been developed with the help of neural network methodology. It involves three input variables—wind speed, relative humidity and generation hours and one output variable-energy output of wind farms. The modeling is done using MATLAB toolbox. The model accuracy is evaluated by comparing the simulated results with the actual measured values at the wind farms and is found to be in good agreement.     

Aggregated dynamic model for wind farms with doubly fed induction generator wind turbines

As a result of increasing wind farms penetration in power systems, the wind farms begin to influence power system, and thus the modelling of wind farms has become an interesting research topic. Nowadays, doubly fed induction generator based on wind turbine is the most widely used technology for wind farms due to its main advantages such as high-energy efficiency and controllability, and improved power quality. When the impact of a wind farm on power systems is studied, the behavior of the wind farm at the point common coupling to grid can be represented by an equivalent model derived from the aggregation of wind turbines into an equivalent wind turbine, instead of the complete model including the modelling of all the wind turbines. In this paper, a new equivalent model of wind farms with doubly fed induction generator wind turbines is proposed to represent the collective response of the wind farm by one single equivalent wind turbine, even although the aggregated wind turbines operate receiving different incoming winds. The effectiveness of the equivalent model to represent the collective response of the wind farm is demonstrated by comparing the simulation results of equivalent and complete models both during normal operation and grid disturbances.     

The world's largest all-vanadium redox flow battery energy storage system for a wind farm

风力发电具有明显的随机性,间歇性,不可控性和反调峰特性,风力发电的大规模并网给电网调峰和稳定,安全运行带来了巨大压力,造成弃风限电现象愈加严重,严重影响了风力资源的有效利用和经济效益.全钒液流电池储能电站在能量管理系统的调度下,对风力发电输出功率进行平滑,配合风电场功率预报系统,提高风电场跟踪计划发电能力,改善了风电场并网电能质量,降低了对电网的冲击与影响,同时也提高了风电场输出功率可控性,有利于提高电网对风电的接纳能力.国电龙源卧牛石风电场配套的5 MW/10 MW∙h全钒液流电池储能系统为目前世界上最大规模的全钒液流电池储能系统.本文介绍了该全钒液流电池技术特点和储能系统的设计,成组方案及功能,并对储能技术在可再生能源发展中的作用进行了展望.     

Simulation and size optimization of a pumped–storage power plant for the recovery of wind-farms rejected energy

The hybrid wind–hydro power generation appears to be an attractive solution for isolated, autonomous electric grids in order to increase the wind energy penetration and cost-effectiveness. This work presents a numerical methodology for optimum sizing of the various components of a reversible hydraulic system designed to recover the electric energy that is rejected from wind farms due to imposed grid limitations. The algorithm is applied to study a practical case using time variation data of rejected power from a number of wind farms installed in the island of Crete, Greece. The free design parameters of the system include the turbine size, the size and the number of the pumps, the penstock diameter and thickness, and the reservoirs’ capacity, whereas some critical financial parameters are also considered. The numerical procedure combines an evaluation algorithm that simulates in detail the plant operation during a 12-month period, and an automated optimization software based on evolutionary algorithms. The economic analysis uses dynamic evaluation methods and the attainability of various objectives is examined using single or multi-objective optimizations. In addition, the developed numerical tool is used to perform several parametric studies and sensitivity tests in order to analyse in depth the influence of the most important parameters on the plant operation and economic behaviour. The results showed that a well optimized design may be crucial for the technical and economic viability of the examined system.     

Dynamic model of wind energy conversion systems with variable speed synchronous generator and full-size power converter for large-scale power system stability studies

This paper presents a dynamic model for variable speed wind energy conversion systems, equipped with a variable pitch wind turbine, a synchronous electrical generator, and a full power converter, specially developed for its use in power system stability studies involving large networks, with a high number of buses and a high level of wind generation penetration. The validity of the necessary simplifications has been contrasted against a detailed model that allows a thorough insight into the mechanical and electrical behavior of the system, and its interaction with the grid. The developed dynamic model has been implemented in a widely used power system dynamics simulation software, PSS/E, and its performance has been tested in a well-documented test power network.     

Reactive power control of wind farms for voltage control applications

In the last few years, there is a strong trend towards decentralised production and supply, leading to a situation where a growing number of small and medium size producers will be connected to energy networks. But at the same time, the power quality of the generation must be ensured and this means that the electrical parameters of the distribution network have to be maintained within their upper and lower limits. Therefore, new problems related to the management and operation of energy transfer and distribution and to the efficient distribution of renewable energy in the grids are actually arising. Hence, it is reasonable to think that dispersed generation (wind energy generation in this paper) should start to take part in the control of electric variables, and in particular, in reactive power control which is directly related to the voltage level control of distribution networks. This paper presents a control strategy developed for the reactive power regulation of wind farms made up with double fed induction generators, in order to contribute to the voltage regulation of the electrical grid to which farms are connected.     

A review on reliability assessment for wind power

The application of wind energy in electric power systems is growing rapidly due to enhanced public concerns to adverse environmental impacts and escalation in energy costs associated with the use of conventional energy sources. Electric power from wind energy is quite different from that of conventional resources. The fundamental difference is that the wind power is intermittent and uncertain. Therefore, it affects the reliability of power system in a different manner from that of the conventional generators. This paper, from available literatures, presents the model of wind farms and the methods of wind speed parameters assessment. Two main categories of methods for evaluating the wind power reliability contribution, i.e., the analytical method and the Monte Carlo simulation method have been reviewed. This paper also summarizes factors affecting the reliability of wind power system, such as wake effect, correlation of output power for different windturbines, effect of windturbine parameters, penetration and environment. An example has been used to illustrate how these factors affect the reliability of wind power system. Finally, mainstream reliability indices for evaluating reliability are introduced. Among these reliability indices, some are recently developed, such as wind generation interrupted energy benefit (WGIEB), wind generation interruption cost benefit (WGICB), Equivalent Capacity Rate (ECR), load carrying capacity benefit ratio (LCCBR).     

Economic analysis of reactive power compensation in a wind farm: Influence of Spanish energy policy

Presently, renewable energies and especially wind energy are gaining a special relevance in the electrical market worldwide. This current rate of growth brings with it the need for the various wind farms to not limit themselves to producing energy but also provide stability to the network within its capabilities. So, the actual objective is to adapt the installations that produce wind energy in such a way that they give a maximum amount of support in any given moment to the electrical network. For this purpose, there are governing techno-economic parameters that influence the economic behavior of commercial wind farms. A complete cost-benefit analysis model is developed, focused on incorporating automatic capacitor banks into wind farms for the compensation of reactive power. This economic analysis is about doubly fed induction generator (DFIG) wind turbines. Although this kind of wind turbines have a certain capability in terms of modulating reactive power, this capacity is not enough to achieve the new requirements of reactive power regulation in Spain and it is necessary to invest in systems of external compensation.In this paper, we have studied the case of DFIG wind turbine and capacitor banks, although the used methodology can be applied to other technologies as well by simply amplifying the algorithms according to the specific characteristics of the option elected. Following this premise, a detailed analysis of the specific needs of a wind farm has been carried out, as well as a search for the optimum performance for the compensation of reactive power.