Wind Energy: Technologies and Trends in Wind Power Plants Development
The development of technologically and cost effective solutions for the wind energy conversion into electrical energy has been one of the greatest technical challenges in the last decades. The idea of using wind energy for the generation of electricity could rightly be regarded as the logical continuation of its utilization for centuries as the driving power for various types of equipment, among which wind mills undoubtedly have greatest popularity. Despite the relatively short history of wind plants . the first modern wind turbine was set to work in the 80ties of the last century, today wind farms are a typical elements of the landscape of many European countries. An evidence of the wind plant technical development during the years is their output power increase by hundreds of times in comparison with the first turbine models. The designs of the 80-ties had power between 20 to 60 kW and a rotor diameter less than 20 m. Modern solutions achieve an output power of 5000 kW at a rotor diameter of 100 m.
Together with the other renewable energy sources (RES) . the sun, water and timber, the actuality of the wind energy has been growing in parallel with the increasing ecological requirements related to the harmful emissions to the atmosphere, generated by the conventional electrical power plants. It would not be wrong to say that the progressively increasing investment interest in RES around the world is also a political issue in the developed industrial countries. In this aspect, the share of the HPP produced energy in the country, including wind plants, ac- cording to the strategy for the development of the Bulgarian energy sector (prepared entirely in line with the European vision) must reach 8% by 2012.
Wind power energy specific features
A key factor for the increasing relative share of the electric power, generated by wind plants, is the effective integration of the constructed facilities into the electric power system. The construction of wind plants deals with various technical, economic and legal requirements, aimed at the provision of overall electric power system stability. For the purpose of the wind plants. successful integration with the respective electric power transfer and distribution systems, the following wind energy specific features must be taken into consideration:
• the wind farms. output power changes depending on the atmospheric conditions;
• a great part of the wind farms are built in regions, located in the periphery of the existing distribution networks. The electric power systems of most European countries are designed to transmit electric energy in the direction from the network center to its peripheral part;
• the technical characteristics of the wind power generation facilities are different from those of the conventional power plants;
• the liberalization of the national markets and the common European electric energy market, on one hand, and the high ecological requirements, on the other, increase the interest in smaller power generation facilities.
The main technical problem, related to the utilization of wind energy is the changing power output of the wind plants. The results from the executed analyses show that one turbine.s output power changes insignificantly for a period of several minutes while the output power of a large wind farm would remain relatively constant for hours. Today, a great deal of work is focused on the development of automated systems that will allow the preparation of comparatively accurate output power estimates for a future period, based on the weather forecasts and the information about the wind plant condition.
Economic situation on the wind power market
According to information from the European Wind Energy Association, known as EWEA, the installed wind power capacity in the end of 2003 amounted to 40 000 MW. Over 28 000 MW of them or about 70% have been built in countries from the European Union. The driving force in the development of the European market of wind power generation facilities is the continuous increase of the turbine efficiency and output power, in parallel with the decreasing investment costs for their construction. Today, the average price for a kW of installed electric power is within the range from Euro 900 to 1100.
Another element of the total price of the wind power energy is formed by the costs, related to the operation and maintenance (O&M costs) of the wind plants. EWEA.s estimate of this item amounts to 20 - 25% of the total production costs for a kWh of wind energy or 1.2 eurocents/kWh over the turbine lifetime. The total wind power production costs for 2003 are estimated at approximately 4 - 5 eurocents/kWh for applications where the wind speed is high and from 6 to 8 eurocents/kWh for farms, built in regions with a lower wind speed. The specified values are valid for wind turbines, having power within the range between 850 and 1500 kW, O&M costs of 1.2 eurocents/ kWh for the period of the turbine operation, investment costs from 900 to 1100 eurocents/kWh and a discount percentage of 7.5% per year. The EWEA experts. expectations are that the wind power production costs will fall with 9 to 17% in the following years
The prognoses for the development of the wind power energy market in Europe as a whole up to 2012 envisage an increase of 20.6% annually within the period 2003 - 2007 and 15% over the period between 2008 to 2012.
Wind turbine design
Despite the continuous design improvement and the substantial development of the technical characteristics of wind turbines (WT), their principal design has changed insignificantly over the recent decades. The wind turbines operation is based on à rotor, oriented against the wind and equipped with three blades. The turbine is activated through the rotation of the blades in the direction, coinciding with the wind direction. Through a low-speed large-diameter rolling bearing (in the case of powerful turbines the bearing connection dimension reaches several meters), the rotor is mounted on a shaft, connected to a reducer that optimizes the speed of rotation. The reducers used nowadays have experienced a significant technical development in comparison with the ones used in the 80-ies of the XX centuries. The application of directdrive generators, where the traditionally used gear reducers are not part of the turbine structure, has become more popular. The modern wind turbines are fitted with aerodynamic rotors that drive directly the generator. Different hybrid solutions are also used, including a single-stage reducers and multi-pole generators.
Most of the modern wind turbines are equipped with four-pole generators. At the largest wind plants, the step between the rotor blades is not fixed but has been changed continuously with the aim of using the largest possible amount of wind power. The specially designed automated systems carry out continuous control on step of the rotor blades which in practice ensures the turbine operation at the highest possible wind speed and thus leads to greatest output power. The construction of high-power wind turbines has been possible over the last years due to the development of large-scale lifting and transportation machines appropriate for the wind energy power facilities development trends.
The most common wind turbine support structure is a steel tube pile (called also a tower) having a cone end section. Other pile types are also used, including such having a grid structure. The pile height is determined according to the wind turbine size and the specific atmospheric conditions.
Technological trends in the WT development
The exact description of the technological trends in the wind turbine development would provide the necessary information basis for the precise diagnostics of the facilities . operation, as well as for the objective prediction of their future development. Presently, an active work is scheduled for the formation of a detailed statistic database about the design and main technical characteristics of wind turbines, starting from the 80-ties of the XX century until now.
A main trend in the development of the wind turbines is the continuously increasing rotor diameter. The statistical data analysis shows that the turbine output power grows in parallel with the increasing turbine diameter. It was found that the diameter, or more specifically its second stage, is a factor that determines the amount of generated electric energy.
On the other hand, the maximum output power is a main factor in the calculation of the loads taken by the turbine. The optimization of the turbine design is directed at using a greater part of the wind energy in applications where a low speed of the air masses is typical.
For the wind plants from the beginning of 1990, the turbine output power was determined through a relation in which the rotor diameter D was raised to the power of 2.4. The D2.4 parameter is important because the turbine height also becomes greater in parallel with the rotor diameter. In the designs from 2003, the exponent was decreased from 2.4 to about 2. The rotor diameter and the turbine output power are also main indicators in determining the price of a wind plant.
Circular velocity of the top crosssection of the rotor blade
It depends on the rotor rotation rate and the blade radius. Following the increase of the circular velocity of the top rotor blade cross-section, the level of the turbine generated noise rises sharply. Therefore in the selection of a wind plant, it must be taken into consideration that the turbines with a high circular velocity of the top rotor blade crosssection are much noisier than the low-speed models. For a specific output power, at high rotor rotation rate, the generated torque is smaller than that of the low-speed turbines. This fact explains the higher price of the high-speed turbine driving mechanism. This fact leads to the conclusion that the optimum wind turbine selection is made at a compromise between the noise generated and the price of the drive mechanism. For the wind power plants constructed on a solid surface (the so-called onshore turbines), the noise is an important factor in the turbine selection to a much higher degree. Presently, the most powerful wind plants are especially developed for the so-called sea applications (editor.s note: offshore turbines). The results from the completed studies, covering the technical characteristics of the wind turbines of different manufacturers, built on land and in the ocean show that the latter have a circular velocity of the crosssection of the rotor blade which is 10 to 30% higher.
Sequence of the rotor blades
One of the questions discussed in the wind electric power industry is the question about finding the optimum solution . controlling the sequence of the rotor blades or the separation of the air flow from them. Until the middle of the 90-ties of the XX century, most wind turbines controlled the flow separation from the blades. Today, controlling the sequence of the rotor blades is considered better in view of design and functioning. Among the main reasons is the better quality of the electric power in the case of control on the sequence of the rotor blades. An additional factor, contributing to the wider use of the sequence controller instead of the blade flow separation controller is the approximately equivalent cost for the implementation of both techniques. Models have been developed in which the sequence (interval) between every two rotor blades is controlled independently.
Operation at alternating speed
The capability of wind turbines to operate with changed speed offers various advantages, among which their improved compliance with the electric power system, load reduction, energy saving, etc. Today only an insignificant part of the produced wind turbines operate with a continuous rotation speed. For the plants, having power of 1 MW, it is required to maintain a different rotation speed, although in a narrow operating range.
Practically, the wind turbine operation with alternating rotor speed can be implemented through a wide range of technical solutions. The direct-drive systems, for example, offer an option for operation in a wide frequency operating range. In the traditionally used concept of turbine operation at alternating speed, a gear drive is used, by which the generator is connected to the electrical network through an electrical converter. The electric energy, produced by the wind plant, has an alternating frequency, depending on the rotor speed of rotation. Prior to transmitting it to the network, the frequency of the generated electric energy is converted to the network frequency. Several configurations based on synchronous or induction generators are used.
The solutions that are most widely applied in the modern wind turbines are based on asynchronous generators with a wound rotor induction generator. They provide almost all the advantages of the speed-control driving mechanisms. In comparison with the conventional speed-regulation driving mechanisms, the electrical converters are almost three times smaller and cheaper. For the wound rotor induction generators, the stator is directly connected to the network and the rotor to the electrical converter. A disadvantage of this technique is the lower speed range in comparison with the conventional speed-control drive mechanisms. On the other hand, reaching the operating speed range of a wound-rotor induction generator with an electrical converter is not very probable. Nevertheless it should be mentioned that the price of the converters is continuously falling, while their efficiency is rising.
Another new configuration of generators under development especially for wind plants, are the non-collector induction generators. A special feature in their design is the absence of the electro-contact rings that are typical for the wound-rotor induction generators.
Two entirely opposite concepts must be considered for the selection of the optimum turbine height. According to the first one, the higher the pile, the greater the wind speed at which the turbine operates, hence the greater the generated output power. The second concept supports the idea that the construction of higher wind plants requires greater investment which distributed over time makes up for the achieved turbine output power. The trend at the largest turbines is the pile height to be approximately equal to the rotor diameter.
The rotor cost is calculated as about 20% of the price of the entire wind turbine. One of the trends in the wind turbine development is the development of rotor designs taking up less space and having smaller weight which is especially valid for the greatest large-scale wind plants. If the blade internal stress is kept constant with the increased dimensions, the loads acting on the propeller and the required strength will be increased with value equal to D3. Therefore in the case of operating blades of similar geometry, it is possible to increase the blade weight with D3 as well. During the rotation of the rotor blades, they are also subjected to the gravitation forces which can lead to their overbending if the used material is not suitable for the respective application.
Also, in the case of a higher circular velocity of the top cross-section of the rotor blades at the large-sized ocean wind turbines, it is necessary to increase the blade flexibility. Generally this is achieved by reducing their thickness. It must be taken into account that the reduction of a blade overall area may result into its mass reduction in the case of high-strength used materials.
A technical challenge is to preserve the existing trend towards the reduction of the rotor weight if the rotor overall dimensions continue to increase.
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