Wind energy - General aspects. Renewable energys are at our time a wasted of nature. They originate from the Sun, the Earth and the gravitational interactions of the Moon and the Sun with the oceans. There are renewable energys coming from wind, solar, hydro, geothermal coming from biomass.
Wind energy still carries the name of wind energy. This name comes from the mythology, Aeolus, God of wind. Wind energy has been used by humans for very long. In ancient times boats and models walked with the help of wind. Later people have built wind mills with which they grinded the grain. Now, after many years, people have built some special device that captures the energy of the wind and turned it into electricity.
Local effects of wind energy
Wind energy has an inevitable effect on the local environment, but it can be limited through careful design.
Birds:
Birds can collide with the turbine blades or to fall into the trap of turbulence behind the rotor. The estimated number of victims of the crash ' is relatively small, about 21,000 victims for an installed capacity of 1,000 MW per year (in the Netherlands). While it looks great, it is small compared with the number of birds that are killed each year due to traffic (2 million) or who die because of power lines (1 million).
Fish:
Offshore wind farms have also positive effects. Overfishing is a known problem and stocks of many species of fish are threatened. Considering that the navigation and fishing are prohibited in the vicinity of wind parks, marine biologists hope that these areas become breeding areas for many species of fish. Recent research in the vicinity of wind parks confirms these positive effects on fish stocks.
Noise:
Wind turbines produce noise. The rotor produces a background sound and a mechanical noise of the generator and the gearbox. A careful design of the impeller blades, limiting the speed of rotation and the sound isolation of the gearbox and the generator can limit noise. Maintaining a sufficient distance from the residential area or sensitive areas, you can eradicate noise pollution .
Shadow:
Rotating the turbine blades creates a poignant shadow which can cause unpleasant effects when, for example, the shade at sunset that falls on a window. A proper location relative to the housing can be sufficient to prevent this problem. If this problem is limited to a few hours per year, turbines can be paused during that time without losing significant productivity.
Harmonization with the landscape:
Wind turbines are visible structures in the landscape. They can be made so as to harmonize with the landscape, for example, by arranging them in along the line of structures such as dams or canals. Research has shown that the positioning of turbines in groups is much more accepted when it is clear to the citizens of the neighbourhood that you can make such a big production of electricity. If a turbine alignment is desirable or not, and always would be, is a matter of choise. Much more important is the relation of the height and diameter of the impeller shaft. Another important aspect is the size of the rotor as a rotor diameter is slower and therefore quieter.
The potential of wind energy on Earth
The wind power source available is assessed on the world scale at 57.000 TWh per annum. The contribution of offshore wind energy (offshore) is estimated at 25,000-30,000 TWh per year, being limited to locations that do not exceed the depth of 50 m. The world electricity production in 2000 was 15 000 TWh (which corresponds to a primary energy consumed 40,000 TWh), resulting in a yield of Thermo-mechanical cycle of 30-40%. Theoretically, the wind energy may cover the requirement of electricty of the world. At the same time, the main shortcoming of this source of energy is unstable. During periods of frost, as in the case of heat, cases where energy demand is fierce, the effect produced is practically non-existent, which resulted in the development of wind, from attaching to other renewable installations characterised by a better balance in life, or energy storage systems. However, should be taken into account in the case of electricity storage capacity, the high cost of these systems, which are now in the developing world.
Europe has only 9% of the available wind potential in the world, but it has 72% of the installed power in 2002. It produced 50 TWh of wind electricity in 2002, the world production beeing 70 TWh. Wind technical potential available in Europe is 5,000 TWh per year.
How it will look the global energy consumption in the future? We can be sure that the power consumption will grow worldwide. The International Energy Agency predicts an increase to nearly 3.6 million megawatti until 2020 from 3.3 million in 2000. However, the global fossil fuel reserves-the main source for the production of electrical energy-will be exhausted from 2020 to 2060, according to the best estimates of the oil industry. How will we fill the demand for electricity? The best answer would be green, renewable energy.
It is one of the oldest sources of energy UN-polluting. It began to be used on a large scale only in the 70-80 's, when the US has adopted several programs intended to encourage the use of them. In California, at the end of 1984, there were already over 8469 of wind turbines. Total capacity of these units is about 550 MW. They are built in places with high winds, grouped into so-called wind farms. Wind turbines can be used to produce electricity, individually or in groups, known as wind farms. Wind farms, which are now fully automated, ensuring, for example, 1% of the electricity requirements of California, that is necessary for 280 thousand homes.
Approximately 80% of the world's wind energy is produced now in California, but wind energy is found in the american Midwest, in Europe, and particular in Belgium and the Netherlands - but also in other regions. Turnover on the wind in the EU application was, in 2003, 6.9 billion euros, as was specified in the framework of the International Conference \"Clean-Energy funding and support in Central and Eastern Europe\" organized in the capital of Hungary. In terms of installed power, Europe growth is getting stronger in the field of wind energy position, the European market with an increase of 39 per cent per year during the period 1998-2003. Incidentally, the world wind energy market could be worth over 30 billion euros annually until 2010. The energy produced so equates to that obtained by burning 20 million tons of coal in a conventional system for the production of electricity. In the Czech Republic, for example, the share of wind energy will grow from 3.8 percent in 2000 to 8% in 2010, in Estonia from 0.2% to 5.1% in Hungary from 0.7% to 3.6%, in Poland from 1.6% to 7.5%. However, according to the report of the International Energy Agency, in Romania there are \"delays in the elaboration and implementation of programmes on the use of wind energy. European papers highlight a good adaptation to the requirements of the EU in terms of renewable sources of electricity by hydropower plants of great power.
The current status of wind energy
The production of commercial wind turbines began in the 1980s, with Denmark, the leader in this technology. From units of 40-60 kW with rotor diameter of about 10 m, wind turbines have grown in strength up to over 5 MW and rotor diameters of more than 120 m. Continuous improvement increased the ability of the turbine to capture more energy from the wind. The result was that the use of wind energy has grown rapidly in Europe. In Denmark, for example, the number of persons involved directly or indirectly in the wind turbine industry has grown from about 2900 in 1991 to 21,000 in 2002. Estimates based on the screenplay ' Wind Force 12 ' shows that the number of people employed in this sector in Europe will reach 200,000 in 2020.
Other facts about wind energy in the world and in Europe:
by the end of 2005 installed capacity has reached 60 000 MW;
in recent years, the annual global growth was approximately 25%. during the year 2004 was 7,500 MW and in 2005 was 11600 MW
for the most part, 60-70% of the total power installed in Europe, 5,800 MW in 2004 and 6,200 MW in 2005.
It is estimated that in the year 2006 will be installed in the world, 15,000 MW.
outside of Europe, most wind installations are in the US, China and India are booming.
wind plants have developed consistent in Europe, with a capacity multiplied 27 times in the Decade 1992-2002.
developed countries in this field are Germany, Spain, Denmark and the Netherlands, who have 84 percent of the total power installed. The new markets include Austria, Italy, Portugal, Sweden and the United Kingdom. The ten new Member States that joined the EU in May 2004 have also adopted targets on renewables.
în anul trecut, în Germania, rulajul de capital în industria eoliană a fost de 4,2 bilioane EUR
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The potential wind energy in Romania
The need for the use of alternative resources in Romania
The oil crisis of the 1970s put the industrialized states, in energy supply as a vital issue for them. Programs have been initiated for the construction of expensive nuclear power plants and important subsidies are allocated for alternative energies. Planned interventions of those states have not shown but the results, such as, only ten years later, responsibility for investment in the energy sector began to be moved to the private sector. The enthusiasm to identify the new sources of energy has fallen significantly, then the investments being extremely high, but also due to the fact that new deposits have been identified. After 45 years after the first great oil crisis, the world once again find out that it's vulnerable in terms of energy security. The oil era is coming to an end, appreciate the specialists, but the sources of renewable energy is still far to make their presence felt, given that the European Union is increasingly dependent on imported energy resources. Meanwhile, a new challenge appeared worldwide: pollution. Under pressure from the commitments of the Kyoto Protocol, the debates on the 'green energy' also got a special scale. European Directive 2001/77 states that 'the promotion of electricity from renewable resources, on the unique market, aims at increasing the share of renewable sources of energy (SRE) from 14 to 22% (till 2010) of gross consumption of electricity in the European Union. The directive also brings a number of measures of encouragement and facilities for those who invest in RENEWABLES. The strategies are followed by deeds only now, when Petroleum Announces a new possible energy crisis.
According to the european model Romania has worked out a strategy in the field of 'green energy', by which he established as the target, until 2010, a 4% share of energy obtained from renewable sources: small hydro power facilities, wind energy, solar energy, bioenergy (primarily biomass). In the power installed, this share represents about 750 megawati. From the start it must be said though that the energy obtained from renewables will have a price by almost 50% higher than current rates, financial mechanism adopted since beeing pretty complicated. The desire to use renewable energy resources as well as more intensive existed in Romania and before 1989, when stressed, especially on solar energy. After 15 years, the need to align the European requirements, we start from the end. The facilities created make increasingly more Romanian and foreign companies to want to get involved in the production of electricity from renewable sources.
So far, Romania has one group of 0.66 MW wind park in Ploiesti, industrial and investment were made in the central works with sawdust in five cities. Producers of energy from wind sources, solar, biomass, fermentation, gas energy waves, as well as those who operate small hydro power stations are upgraded for each megawatt-hour network provided a certificate. This document can then be traded on a specific market, where it will be purchased by the electric power providers, who are obliged to buy these certificates in a limit. For 2005, the mandatory quota was established at 0.7%. Thus, electricity providers are obliged to purchase a number of green certificates equal to the product of the value of the required share and the amount of electricity supplied to final customers annually. For example, if a vendor delivers 100 MW per year, he has to buy the equivalent of seven certificates at a price set between 24 and 42. The Ministry of economy proposes raising the odds the 2,22% in 2006, 3.74% in 2007, reaching 8.3% for the period 2010-2012.
In the table below is presented by types of synthetic sources, the energy potential of renewable energy sources in Romania:
THE RESOURCE
YEARLY POTENTIAL
ENERGY PRODUCTS
SOLAR ENERGY
1.433 mii tep / 1.200 GWh
Electric power/ Thermal energy
WIND ENERGY
8.000 GWh
Electric power
HYDRO ENERGY
34.000 GWh
Electric power
HYDRO ENERGY FROM MHC
6.000 GWh
Electric power
BIOMASS
7.597 mii tep
Electric power/ Thermal energy
GEOTHERMAL ENERGY
167 mii tep
Electric power
Table 1. Meteorological parameter characteristics 'wind' in Romania
Average annual speed
It is directly influenced by orografie and thermal stratification of the air, which can intensify or attenuate. In the mountain area are characteristic annual average speeds that decrease with altitude from 8-10 m/s on the Carpathian heights (2000-2500 m) up to 6 m/s in areas with altitudes of 1800-2000 m, on sheltered slopes annual fall speeds 2-3 m/s, and in the ideeper valleys between mountains they are 1-2 m/s. Inside the arc of the Carpathians, average annual speed varies between 2-3 m/s, and in the Carpathians, in Moldova, they are 4-5 m/s, the highest annual averages remarked in the eastern part of the country, in the plain below the siret River (5-6 m/s), on the Black Sea (6-7 m/s), in Dobruja and Clement (4-5 m/s).
The lowest annual average values (1-2 m/s) stands out in intracarpatice closed depressions.
Maximum wind speeds
The highest values of over 40 m/s, register in all areas of high mountain, in the Moldavian Plateau, Northeast Baraganului and in Dobruja as a result of intense traffic from the North and North-East, on the Black Sea, as a result of low roughness, as well as in the southern-central part of the Campiei, between the Jiu Valley and See. Areas with maximum wind speeds of between 30-40 m/s surround all the areas with speeds in excess of 40 m/s, occupying parts of the lower parts of the Moldavian Plateau, North-East of Campiei Romaniei, Northern Dobruja and pericarpatice areas. The West and East of the Campia Romaniei, central and southern Dobruja and the greater part of the Danube Delta is characterized by maximum annual wind speeds between 20 and 30 m/s. Same-speed register are in the highest part of the Transylvanian Plateau, in Central and Northern and Western Campiei and on The Mures Coridor. The lowest values of maximum annual rates of less than 20 m/s, island records (very small area) in Podisul Mehedinti, Depresiunea Petrosani and defileul Jiului, areas situated at Jiu shelter massifs. At a more careful examination of the distribution of these values can be taken out and some bonuses rules. Thus, the absolute maximum speeds are significantly higher than on dominant directions specific to each filling.
Considerations on wind energy in Romania
Regarding wind energy in Romania, there were identified five wind zones, depending on the environmental conditions and surveying, taking into account the potential of resources of this type at the height of the average of 50 meters and over. The results of the measurement result recorded, result that Romania is a temperate continental climate, with a high energy potential, in particular in the area of the coastline and the mild, as well as in areas with plateaus and alpine peaks (severe climate). Based on the evaluation and interpretation of results as in Romania, the potential wind energy is the most favourably on the Black Sea, in mountain areas and plateaus of Moldova or Romania. Also, there were identified sites in regions with relatively good wind potential, if it follows the energy exploitation of the effect of flow over the peaks of Hill, the effect of sewage of air currents and so on. Preliminary evaluation on the Black Sea coast, including in the area of off-shore wind potential that amenajabil on the short and medium term is high, with possibilities of obtaining an amount of energy of the thousands of GWh/year. While worldwide wind energy was at an advanced stage of technological maturity, we can appreciate that in Romania the share of wind energy sources in the energy balancein the short term, is under the real possibilities of economic recovery.
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Wind Energy General concept
Wind plants can have a major contribution to the use of renewable energy. The oil crisis of the 1970s has stimulated in Europe the development and production of the turbines to generate electricity. The development of wind energy use has improved continuously and, in the last decade, the electrical energy produced from wind has witnessed a considerable development. The turbines have become bigger, efficiency and improved availability and wind farms have become more important. World electricity consumption continues to grow. Many European Governments have set targets for the reduction of the emission of carbon dioxide in order to decrease global warming. The widely accepted opinion is that these goals will be achieved, on the one hand, using incentives for energy saving and, on the other hand, the widespread use of renewable energy. The use of wind power is a serious option for achieving these objectives. Several European countries have impressive plans relating to the installation of a large number of wind turbines in the future. Some Governments support these actions with the help of tax incentives. North-Western Europe, with windy shores and a power supply strongly branched offers exciting possibilities for investment and the development of wind parks.
Basic principle
Wind turbines extract the wind energy transferring the air passing through the turbine rotor to the rotor blades. The rotor blades have a profile of the wing, as shown in the cross section of Figure 1.
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Figure 1. Transversal section of the impeller blades with speeds and directions
The plane of rotation of the rotor is controlled so as to be perpendicular to the wind direction. The resulting air flow on the rotor above (i.e. the vector sum of wind speed with the speed of the impeller) produces a difference in pressure between the wind and the blade exposed. (The air that flows over the upwind run at a higher speed and therefore a lower density and pressure). This difference in pressure produces a thrust force perpendicular to the resultant airflow. A component of this force produces a mechanical rotation that rotates the rotor and shaft. The power to the axle can be used in many ways. Hundreds of years it has been used for milling grain or pumping water, today large modern facilities with integrated generators convert it into electricity.
Figure 8.11 illustrates a cylinder of air approaching a rotating horizontal axis wind turbine.
The cylinder of air shown in has the volume:
, (8.8)
where A is the cross-sectional area and ΔL is the length of the cylinder of air. Let us assume that the density of air Ïair is approximately constant. The air mass of the cylinder is:
. (8.9)
Suppose the cylinder of air is moving with speed vair directly at the turbine. The air speed vair is the speed of the wind. The kinetic energy of the moving air is:
. (8.10)
The length ΔL of the air cylinder that reaches the wind turbine in a time interval Δt is:
. (8.11)
Substituting eq. (8.11) into eq. (8.10) gives:
. (8.12)
The rate of arrival of air is the wind power, or
(8.13)
Wind power is proportional to the cube of wind speed. The area A is the surface area of the circle formed by the rotating tip of the rotor blade. If the rotor blade has radius R, the area is:
(8.14)
We can use eq. (8.14) to write wind power in the form:
. (8.15)
Equation (8.15) shows that wind power is proportional to the square of the radius of the fan created by the rotating rotor blade. We have assumed that the wind direction is perpendicular to the plane of rotation of the rotor blade, and that the wind speed is constant. Wind power is at maximum if the wind direction is perpendicular to the plane of rotation of the rotor blade, otherwise the wind power will be less than maximum. If the wind direction is parallel to the plane of rotation of the rotor blade for an infinitesimally thin rotor blade, the wind turbine will not provide any wind power.
A change in wind direction can put stress on wind turbines because of the gyroscope effect. A gyroscope is a symmetrical rigid body that is free to turn about a fixed point and is subjected to an external torque. The wind turbine is a symmetrical rigid body that is free to turn about the fixed post. The change in wind direction subjects the wind turbine to a torque that causes the wind turbine to behave like a gyroscope. In addition, wind speed is seldom constant; it can vary from still to tornado or hurricane speed. The speed of rotation of the tip of the wind turbine is:
, (8.16)
where ω is the angular frequency of the turbine. If vtip is sufficiently large, it can be lethal to animals entering the fan area of the rotor blade. This creates an environmental hazard that must be considered when selecting locations for wind turbines. Electrical power output from a wind turbine is a product of the efficiency ηwind times the input wind power.
The optimum power output is approximately:
. (8.17)
The efficiency ηwind depends on several factors. One factor that affects the efficiency of a wind turbine is the efficiency of converting mechanical energy of the rotor blade into electrical energy. Another factor is the reliability of the wind turbine. The rate of rotation of the rotor blade depends on wind speed. If the wind speed is too large, the rotor blade can turn too fast and damage the system. To avoid this problem, wind turbines may have to be taken off-line in high wind conditions.
The analysis of wind power above presented assumes that all of the kinetic energy of the wind incident on the turbine is converted to rotational energy of the rotor blades. In reality, the wind speed vupwind upwind of the turbine is reduced to a wind speed vdownwind downwind of the turbine. According of Shepherd analysis (1998), we can estimate wind power for a more realistic system. In this aim, we again consider the cylinder of air in Fig. 8.11 and the eq. 8.10. The kinetic energy of the moving air that is extracted for power production is:
. (8.18)
The mass of the air that is needed to move the rotor blade in a time interval Δt, is:
, (8.19)
where vactuate is the wind velocity that actuates the rotor and A is the cross sectional area shown in Fig.8.11. If we assume that the reduction in kinetic energy in the upwind air stream is transferred to the wind turbine, conservation of energy and the continuity of the wind flow, vactuate results as a mean value, given by:
. (8.20)
Substituting eqs. (8.19) and (8.20) into eq. (8.18), gives
. (8.21)
The extracted wind power is:
. (8.22)
This last equation, it can be written in a simplified form, as follows:
(8.23)
where βwind = vdownwind/vupwind is the ratio of downwind velocity to upwind velocity, and Cp is the dimensionless power coefficient
. (8.24)
The power coefficient is typically in the range 0≤ Cp ≤ 0.4 for actual wind turbines. If we compare eq. (8.23) to eq. (8.17), we see that the power coefficient Cp serves as an efficiency factor to convert the input wind power to output rotor blade power. A theoretical maximum power coefficient is obtained by solving the extremum condition:
(8.25)
for βwind. The physically meaningful solution to Equation (8.25) is
. (8.26)
Substituting eq. (8.26) into eq. (8.24) gives the power coefficient
. (8.27)
Equation (8.27) is the theoretical maximum power coefficient and is called Betz' limit, after Albert Betz, the person who first made the calculation in 1928. Betz' limit says that approximately 59.3% of the wind power is the maximum percentage that can be extracted of it. It is obtained when the downwind velocity is one third of the upwind velocity, as shown in eq. (8.26).
Power and efficiency rates
The movement has a certain energy. This energy varies according to the product of mass and the square of the speed. Reported at the time, it represents power. The kinetic energy per second is:
where:
P is power (Nm/s sau W);
m is the mass per second - flow rate by mass (kg/s);
v is the speed of the wind (m/s).
This physical law is also applicable to the air in motion. The mass of the air passing through the rotor is considered to be a cylinder. The volume of the cylinder is dependent on the surface area of the rotor and wind speed, i.e. the length of the cylinder passing through the rotor in the unit of time.
The mass of the air passing through the rotor of the turbine in a second is:
m=
where:
is the air density (kg/);
A is the surface area of the rotor ();
v is the wind speed (m/s).
This leads to an important feature: the resulting energy depends on the wind speed to the power of three.
As an example, the wind speed of 6 m/s energy is 132 W/. When wind speed of 12 m/s power increase at 1053 W/. Doubling of wind speed leads to multiplication with the power of eight. Cannot be converted into useful energy the whole wind energy from the rotor shaft. Using physical principles, it can be shown that the maximum theoretical efficiency of wind power is limited to about 59%. This limit is also known as the coefficient of power or the . Accordingly, the above equation becomes:
Where
is the mechanical power (the slow axis)
( at the 'slow axis' is defined as the efficiency of converting energy from wind power (unperturbed) at the mechanical rotation in relation to the main shaft of the turbine, which is located downstream from the rotor and gearbox. The speed of rotation of the shaft is equal to that of the turbine rotor and, in this case refers to the ' slow axis ' shaft. can be also defined as the gearbox, the spindle speed of the generator, called a ' fast ' or shaft out of the processor, the of the ' network ').
The output power of a turbine, the , which take account of the mechanical and electrical efficiency is given by:
Where
is the rate of electric efficiency (electricity)(%)
Wind turbine technology
TechnologyC:\Users\SpeedY\Desktop\licenta\raw files\Section in a wind turbine.jpg
Modern wind turbine technology has developed rapidly in the past two decades. The basic principle of turbine remained almost unchanged and consists of two conversion processes carried out by the main components:
the rotor which extract kinetic energy of the wind and converts torque generator;
generator that converts this couple into electricity and delivers it to the network.
Although it seems simple, a wind turbine is a complex system which leverages knowledge in the field of aerodynamics, mecanics, electrical engineering and automation.
Rotor and blades
A modern wind turbine has two, preferably three, blades or wings. Blades are made of polyester reinforced with glass fiber or carbon. For commercial reasons, the blades have lengths from 1 m to 100 m and even more. Blades are mounted on a steel structure called a block. As mentioned, some blades are adjustable by controlling the angle of inclination (pitch control). C:\Users\SpeedY\Desktop\licenta\raw files\worlds-largest-wind-turbine-660.jpg
The NacelleC:\Users\SpeedY\Desktop\licenta\raw files\nacelle_diag.jpg
The nacelle can be considered the turbine machinery room. This space is designed so that you can rotate the Tower (steel) to allow the rotor orientation perpendicular to the wind direction.
This is accomplished by an automatic control system linked to girueta which is mounted. The machine room is accessible from the tower and contains all major components such as shaft (spindle) with its historic centre, main gearbox, generator, brake system and the system of rotation (orientation). The main rotor transfers the shaft torque to the gearbox.
The Gearbox
A gearbox is required for the transition from the relatively low speed of the impeller (approximately 20 revolutions per minute to a diameter of 52 m) from the generator (1500 revolutions per minute).
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The Generator
Currently, there are three types of wind turbines. The main difference between these concepts refer to the generator and the manner in which the aerodynamic efficiency of the impeller is limited when the wind speed is greater than the nominal, in order to avoid overload. As in the case of the generator, almost all the turbines are installed using one of the following systems (see Figure 3):
squirrel cage asynchronous generator;
asynchronous generator with dual power supply (motor winding);
synchronous generator;
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Wind turbines of the first generation have used squirrel cage asynchronous generator. Due to the large difference between the speed of rotation of the turbine and the generator, it is necessary a gearbox. The rotor winding is connected to the network. This concept is called ' constant speed of the turbine' although the squirrel cage induction generator allows for slight variations of rotor speed (about 1%).
Because the squirrel cage asynchronous generator consumes reactive power, which is required, in particulary in the weak networks, it is necessary to connect the capacitors for compensation.
The other two build systems allow a 2 factor between maximum and minimum speed of the impeller. These different levels are adjusted by means of the power electronics that releases the frequency of the impeller.
The asynchronous generator with dual power supply uses power electronics to power the rotor windings of the generator. The frequency of the impeller is varied so that the frequency of the current in the stator winding generated is appropriate with the network to which it is directly conected. A gearbox is needed to match the speed of the rotor and the generator.
The synchronous generator is directly connected and it doesn't need a gearbox. The generator and electric network are decoupled completely by power electronics. The speed of the generator is much smaller than the indirect systems so that the generator can be used at small speeds; they are easily recognizable because of the large diameter and the proximity to the rotor of the turbine.
Locking system
Wind turbines are equipped with a robust safety system including an aerodynamic locking system. In cases of danger or for stopping is required a comprehensive disk lock. C:\Users\SpeedY\Desktop\licenta\raw files\loking system.jpg
Control/command system
Wind turbines have control/command systems that use computers and can also provide detailed information on the State of the turbine. Often this information can be retrieved and certain control functions are carried out through a communication path.
General classification of wind turbines
For reasons of stability and high torque, today's wind turbine engineers avoid build- ing large machines with an even number of rotor blades. A rotor with any number of blades (and with at least three blades) can be considered approximately like a circular plate when calculating the dynamic properties of the machine [10-12]. A rotor with an even number of blades will cause stability problems in a machine with a stiff structure; at the very moment when the uppermost blade bends backward (because it gets maximum power from the wind), the lowermost blade passes into the wind shade in front of the tower. With an odd number of blades, this phe- nomenon is minimized. Although one-blade wind turbines exist, their commercial use is not widespread, because the problems noted for the two-blade design apply even more strongly to one-blade machines. In addition to higher rotational speed, noise, and visual intru- sion problems, they require a counterweight to be placed on the other side of the hub from the rotor blade to balance the rotor. Compared to a two-blade design this feature adds weight to the generating system without generating additional power. Two-blade wind turbine designs save the cost of one rotor blade and its weight, but they increase the blade base traction by 3 2. They require higher rotational speed (with respect to the larger number of blades) to yield the same energy output. This makes its market acceptance difï¬cult and causes more noise and visual intrusion. In recent years, several traditional manufacturers of two-blade machines have switched to three-blade designs. The rotor of two- and one-blade machines must be sufï¬ciently flexible to tilt, to avoid too-heavy shocks in the no-wind position of the turbine when the blades pass the tower. The rotor is therefore ï¬tted on an axis perpendicular to the main shaft that rotates along with it. This arrangement may require additional shock absorbers to prevent the rotor blade from hitting the tower. Most modern wind turbines are three-blade designs with the rotor position main- tained on the wind side of the tower using electrical motors in their yaw mechan- ism. This design, usually called the classical Danish concept, tends to be a standard when other concepts are judged. The vast majority of turbines sold in world markets are of this design. It was introduced with the renowned Gedser wind turbine. Another characteristic is its use of an induction generator. For very simple types of wind power battery charger [5], the wind vanes are just a set of blades coupled directly to a ï¬xed-inertia flywheel and to a dynamo (or gen- erator) shaft with permanent ceramic magnets on the rotor. On the other hand, high- speed wind power turbines are better for bulky generation of electricity and are relatively less costly. However, most turbines are quite a bit more complex. In general, wind turbines can be divided into two groups: horizontal shaft and vertical shaft (with or without accessories). Horizontal-shaft turbines include:
Blade type with one, two, or three blades
Multiple-blade, farm, or spiked type, which can work with wind coming from the front or back (many variations exist, much as the multirotor type)
Double opposite blade type, which can use sails in place of blades
Vertical-shaft turbines are subdivided based on the following working principle: some use drags or friction, and some use lifting (as in an airplane wing). Some turbines utilize both principles.
Drag turbines are machines whose surface executes movements in the wind direction; in other words, they work with the force of the wind drag acting on them. The following types stand o ut:
Savonius: single or multiple blades, with or without eccentricity (when the rotation shaft is or is not shifted with respect to the shaft that contains its gravity center)
Blade, paddles, or oars
Cup
Lifting turbines are machines whose rotor movement is perpendicular to the wind direction, and they are moved by the lifting action of the wind. Among the lifting turbines are the triangular Darrieus, the Darrieus Giromill, and the Darrieus-Troposkien.
Rotor Turbines
Blade turbines have high rotation with high efï¬ciency. They have automatic regulation of turn speed through the attack angle as a function of the wind speed. The blades present variable sections and great strength against mechanical stresses. This type of turbine needs some orientation mechanism with direct action on turbines at low loads and with indirect action on turbines at higher loads [2,3]. Blade turbines are those most used for electricity generation, feeding batteries or injecting energy directly into the grid. They can be used to pump water as they allow high-rotation driving. They possess the disadvantage of needing towers of great height for installation, on which will be generators, control equipment, and the transmission system. Blade turbines can vary with respect to the conventional model, but such varia- tions do not indicate notable differences in performance.
One is the multirotor, with several turbines mounted on the same tower and interlinked by shafts. Another is the sail turbine, which follows the conventional model with sails rather than blades. These are made of special fabric (sails of embarkation), usually in four or six pieces. They are low in cost, but their efï¬ciency is also lower. They operate at low rotation and high torque.
Multiple-Blade Turbines
Turbines with multiple blades compensate for their low operational rotation. They are simple to manufacture and have high torque. Such a turbine is usually mounted on a horizontal shaft for low power uses. They have lower efï¬ciency than that of blade turbines and are generally used to pump water. They demand very high towers. Multiple-blade turbines of the farm type are manufactured with metallic foils of uniform curved proï¬le. In addition to the usual multiple-blade models, there is a cup type used in the construction of anemometers and toys.
Figure A Cross section of a Savonius turbine.C:\Users\SpeedY\Desktop\licenta\raw files\Cross section of a Savonius turbine..jpg
Drag Turbines (Savonius)
Drag turbines operate on the principle of the friction caused by wind on the turbine blades. A Savonius turbine represents this design. This turbine model is of simple construction and consists of two parts of a barrel cut in the middle and fastened opposite each other by one of their opposed longitudinal edges (Figure A). Savo- nius turbines are in wide use in rural areas for water pumping, attic ventilation, and agitation of water to prevent freeze-up during the winter. They are used little in electricity generation. The best conï¬guration for the half-barrels is given by the relationships:
R = D - 0.5S
S= 0.1D
The torque of a Savonius turbine is due to the difference in pressure between the concave and convex surfaces of the blades and by the recirculation of wind coming from behind the convex surface. Its efï¬ciency reaches 31%, but it presents disad- vantages with respect to the weight per unit of power, because its constructional area is totally occupied by material. A Savonius rotor needs 30 times more material than is needed by a rotor of the conventional type. For the rudimentary installation of a Savonius turbine, metallic barrels of 200 L capacity with an H-shaped wood structure are used. The useful power can be deter- mined for several wind speeds through the dimensions of the barrel:
D=0.60m (diameter of each half-barrel)
H=0.85m (height of the barrel)
R=0.57m (radius projection exposed to the wind)
To calculate the area exposed to the wind:
A=2Rh=2(0.57)(0.85)=0.96
TABLE B Output Power (Watts) of a Savonius Turbine According to the Wind Speed
Number of Barrels, n
1
2
3
4
Wind Speed, V(m/s)
2
4
6
8
Total Height of Barrels, h(m)
0.85
1.70
2.55
3.40
0.70
1.40
2.10
2.80
5.52
11.04
16.56
22.08
18.66
37.32
55.98
74.64
44.24
88.48
132.72
176.96
Lifting Turbines
Lifting turbines operate through the lifting effect produced by wind. The most com- mon models are the triangular (delta) Darrieus turbine, the Giromill turbine, and the Darrieus-Troposkien turbine. The triangular Darrieus turbine has straight blades and variable geometry. The straight blades present disadvantages due to their nat- ural bending; however, the variable geometry alters the power coefï¬cient. There- fore, the rotation tends to be constant.
The Giromill turbine has straight, vertical blades and a rotating movement around its shaft. It is used for large-scale systems because it is considered a low-load system-those operating with only a few hundreds of watts. It has an automatic mechanism that maintains the attack angle position that supplies the best working conditions. Among vertical shaft turbines, the Darrieus-Troposkien turbine is best suited to wind power plants. Studies of this turbine have proved its economic beneï¬ts and constructive simplicity. It is the most used turbine for electric power generation and it is discussed below in more details.
Darrieus-Troposkien Turbine
The Darrieus-Troposkien turbine consists of blades, a shaft, a tower, and guy wires. The blades are curved with glide sections. The rotor is vertical and connects the top to the bottom of the blade. The tower is ï¬xed on the ground by solid foundations that sustain the shaft. The lower part of the shaft is attached to the tower by rollers. In the upper part, the rollers are ï¬xed to guy wires to keep the turbine in its upstraight position. The other extremities of the guy wires are ï¬xed on the ground.
Rotor
The rotor has curved blades with section glides of aerodynamic proï¬le ï¬xed on the shaft extremity of the rotor. The blades can be made of aluminum, ï¬berglass, steel, or wood. The rotor shaft can be tubular or latticed with an external cover to improve the aerodynamics.
Lifting
The rotor support is made of one tower and guy wires. The tower sustains the rotor through bearings and rollers. It can still shelter parts of the system, such as a gearbox, generators, or pumps. The guy wires are ï¬xed through bearings with rollers at the upper part of the rotor shaft in one of the extremities; in the other, they are fastened to the ground. Guy wires are essential for turbines driving small loads.
Speed Multipliers
The system of speed multiplication used in turbines to reach generating speed is carried out through gearboxes of parallel or perpendicular (smaller losses) shafts. A system of belts can also be a good solution. The cost of the multiplier depends on the multiplication rate. The cost of the generator increases with reduced rotation, as the number of poles or turns per coil must then be increased. Thus, the optimum value of the multiplication rate is a trade-off between the speed multiplication rate and the number of poles. Speed multiplication also causes a representative percentage of the total losses of a wind energy system. In extremely small systems, this loss may represent about 20% of the total loss.
Braking System
Braking systems have both safety and maintenance purposes since a turbine must have some mechanical speed limitation. Sizing will determine the best system to be adopted (hydraulic, electromagnetic, or mechanic).
Starting System
The Darrieus-Troposkien turbine type needs a special starting system, for which it may adapt an electric motor connected to the network, a dc motor fed by batteries recharged by a generator connected to the turbine itself or to an auxiliary turbine. For instance, a Savonius turbine can be coupled to the shaft of the Darrieus itself, primarily for small-load turbines.
Generation System
The electricity generated can feed the grid directly or be stored in batteries. Due to the large variations in rotation, the use of induction gen- erators is recommended for stand-alone mode up to 10 kW (e.g., for places difï¬cult to access). In these cases, self-excitation capacitors will be needed (see Chapter 10). Larger generators can be used to feed the grid directly.
System TARP-WARP
The trend in present wind power technology is to increase the diameter of the rotor shaft as much as possible to have a larger sweeping area for higher generated output power. Contradicting this tendency, new technologies of energy generation from the wind are being proposed, such as the Toroidal Accelerator Rotor Platform (TARP), which combines generation and transmission. This concept of wind capture is based on the Wind Ampliï¬ed Rotor Platform (WARP). Small wind turbines are grouped and installed in modules, to conform to the needs of distributed generation. This entire platform consists of a determined number of TARPs piled up one on top of another.
Figure C. Effect of wind direction on the direction of wind-powered rotors.C:\Users\SpeedY\Desktop\licenta\raw files\Effect of wind direction on the direction of wind-powered rotors.jpg
Some toroidal form of aerodynamic turbine shelter characterizes the working principle of the TARP. Wind accelerates around the shelter, amplifying the density of wind power energy available. Each TARP structure supplies a ï¬eld of increased outlying flow in all directions, impelling two wind power rotors of small diameter disposed about 180 from one another around the channel of toroidal flow so formed on the shelter. TARP rotors have a typical diameter of 3 m or less and are coupled directly to the generator through a system of brakes but without a speed multiplication gear- box. As shown in Figure C, if the wind changes from direction (a) to direction (b), a torque will form on the rotating wheel shelter that moves until it is balanced in the new wind direction. The wind power structure also serves as a support, as protection, and as housing for the turbine controls and other internal subsystems. This conï¬guration differs dramatically from the traditional single rotor with a mounted horizontal shaft in a tower, and it is claimed to be of high efï¬ciency. The design just described overcomes the traditional conï¬guration of wind tur- bines through an odd combination of distribution/transmission, superior perfor- mance, easy operation, easy maintenance, high readiness, and reliability. It needs little land area, has a better appearance, has less interference and less electromag- netic noise in TV transmission, and reduces the mortality rate of birds. The esti- mated cost of a kilowatthour is from 2 to 5 cents, depending on the wind power resources. Such systems still have high installation costs (due to the need for higher tower heights) and are not economically feasible for small power applications. Figure D shows the technological evolution of wind power turbines.
Auxiliary Equipments
Auxiliary equipments are devices used to improve the efï¬ciency of wind power tur- bines and are most common for blade turbines. They include solar wind generators, conï¬ned vortexes, diffusers, wind concentrators, plane guides, deflectors, Venturi tubes, heating towers, and rotor accelerating shelters.
C:\Users\SpeedY\Desktop\licenta\raw files\Technological generations of wind turbines..jpg
Figure D. Technological generations of wind turbines.
C:\Users\SpeedY\Desktop\licenta\raw files\Principle of solar power towers.jpg
Figure E. Principle of solar power towers
Solar wind generators are special auxiliary equipment which are able to store the heat irradiated by the sun on black surfaces protected against convection effects. Warm-air circulation is guided by the chimney effect and ends up crossing a turbine when in its ascending movement. The conï¬ned vortex consists of a tower where, in its interior, the effects of a tornado are reproduced through the orientation of free wind heating. In a Spanish prototype, the tower sits in the center of a 7-km (4-mile)-radius circular glass building, as shown in Figure 4.12. Under the glass, the sun warms the air. As the warm air rises, it is drawn through turbines at the base of the tower, thus generating renewable electricity.
Future developments
Currently wind turbines with a proven technology are available in the range of 1.5 to 3 MW. In Western Europe the attention is directed mainly towards the range of wind turbines of 2-3 MW. All enterprises of the top have one or more turbines MW + a segment of the market.
In some regions, such as southern Europe, Asia and Latin America with a less developed infrastructure or where mountainous areas dominate, physically smaller turbines are more appropriate. For these reasons, wind turbines in the range 0.8-1.3 MW are more sought after all over the world.
Prototypes of some turbines of 5 MW and 6 will become commercial as of 2006. These turbines are characterized by the fact that the centerline is at the height of 120 m or more and have the rotor diameter often higher than 100 m apart from the high costs per MW installed on these 5 + MW turbines, the main problem is the weight and size of the components which are difficult to transport on road structure of Western Europe. Some manufacturers offering these turbines solves the problem only for location ' off-shore ' or to locations accessible by water. Other logistics solves this problem, at least in part, by building and installing the towers made by in-site installation of precast concrete elements in the tubular steel segments.
In wind technology, the following events are in progress or are anticipated:
the proportion of rotor technology with variable speed, including modern power electronics will increase;
in the segment above 1 MW, the gearbox is one of the weakest links requiring frequent maintenance operations or costs of repair or replacement. Some manufacturers now offer wind turbines without gear that use synchronous generators multi-polar (with diameter up to 5 m). It gets a hybrid project that has a floor with the gearbox followed by a multipolar synchronous generator less massive. It is considered that, in the next 5-10 years, these various concepts will develop.
the development of wind turbines of 1 MW will focus on weight reduction and limiting dimensions in order to simplify the transport on the road and need of construction cranes at the location. Ways of achieving these objectives are to optimize control strategies leading to a smaller load and thus the use of the less massive component. Another strategy is to increase the level of integration of components and systems, leading to fewer or more compact components.
Routinely, the wind turbines ' offshore ' are similar to or derived from those used on land, but in the near future - each type will be developed so that they are better adapted to the environment in which it works. For machines that work ' offshore ' we put reliability issues, remote control and power on the unit (up to or over 10 MW). For machines that work on the land (' onshore ') we put the problems of the low level of messiness and acceptable (i.e., noise) to nearby areas, high efficiency, low cost and easy to install, with the aid of cranes available and installed power limited (up to 6-8 MW).
Comparison with conventional electricity production and the benefits of wind energy
There are several reasons that explain the recent success of wind energy. When compared with the conventional production of electric energy it is found that wind turbines produce ' clean ' energy without the emission of carbon dioxide and other air pollutants, or ground water during operation. Other advantages are that the wind is a ' fuel ', abundant and inexhaustible, independently of political sensitivities. The turbines are installed easy, fast and reliable with an availability of 98%. (This is the availability of the turbine. However, the wind is not always available so that the functional availability is much smaller).
A disadvantage of wind power is the unpredictability. Storms and fronts, in particular, can produce a sudden increase in wind power. In addition, low wind periods give less wind energy. Introduction to the network of energy produced by wind turbines is not so simple as it seems. In order to maintain stability, a certain percentage of energy produced should be still provided by conventional, centralized plants, controlled "stable". This percentage depends on the structure and stability of the network. If it is likely the network instability, it can be prevented by using an intelligent control system that makes the interface between different types of production units, consumers and intermediate network. In many EU countries, companies, associations, networks (independent) and academical institutions conduct researches in this area.
Wind energy applications
Description of typical situations where the use of wind energy can be/is recommended:
The amount of electricity produced by a wind plant depends on the type and size of the turbine and the location of the plant. Figure 2 shows the characteristic curve that represents typical output power in relation to the wind speed. At low speeds it produces electricity. From Beaufort 2 (approximately 3 m/s) up the turbine works even at Beaufort 6 (about 12-13 m/s) the turbine provides maximum power. At a wind speed of 25 m/s the turbines were designed to hang in a controlled manner in order to avoid overloading and damaging the turbine installation or construction. Past achievements are equipped with tilting angle control that changes the angle of the rotor blade from adverse weather condiţiide. The result is that the power can be generated even in bad weather conditions.
During strong storms, it is still necessary to lock the turbine.
Figure 2. Typical characteristics of turbine; power output depending on the wind speed.C:\Users\SpeedY\Desktop\licenta\raw files\Typical characteristics of turbine; power output depending on the wind speed..jpg
Project risks
The main risk is that, in the long run, the wind location is different from that anticipated in the feasibility study. Due to the cubic law dependence of the speed of the wind, a relatively small loss of long-term wind speed has a significant effect on the output energy. A significant reduction in the energy delivered, for example more than 10-15%, may make the cost recovery time of 10-15 years instead of 10 years, the usual value. The result is a loss of the design.
It is therefore advisable that, in financial and economic calculations, to use an average slower wind speed. Instead use the wind speed with 50% probability of being exceeded, it is better to consider a slower speed, with probability of 80% or 90% to be exceeded. By doing so, in 8 or 9 years-less than 10 years, will get a speed and therefore a greater energy output than expected.
The following aspects must be considered when constructing a wind power plant::
There must be enough space and plenty of wind. The deflections due to, for example the hills or nearby obstacles may affect the output power;
The area must have a permit for the operation of the following wind. In practice this means that you have explored several areas with industrial destination. Otherwise, it should be considered the possibilities to change the destination of the land.
The location must be accessible. During the construction of the wind turbines it is necessary to used huge cranes;
Connecting the equipment to the electrical network must be simple and economical. The power supply can be from 10 to 30 kV when connecting to the local network distribution. It is necessary to connect to the network when the wind power generated is much higher.
Turbine efficiency increased
The use of wind power installations, some improved components and better location has led to increased efficiency by 2-3% annually over the past 15 years.
In addition to the trends mentioned, it should be noted that the wind farms ' off-shore ' have increased in size and number. At first, the turbines ' off-shore ' were adjusted variations of the technology used on land, filled with a salt water marine protection. The current generation includes substantial changes as, for example, a peripheral speed of the impeller and embedded equipment for maintenance. The turbines must be positioned on the seabed, fixed on the basis of a project. You need to install many kilometres of cables that connect the turbines between them and the entire Assembly to the network. In order to ensure a high reliability turbines, it is necessary to perform effective maintenance of the turbines. For these requirements,vessels are needed to carry the maintenance team to the platforms in extreme weather conditions.
At the end of 2003, about 600 MW were installed in wind parks ' off-shore ' built in the coastal waters around Europe from Denmark, Sweden, Netherlands and United Kingdom.
Bibliografie
[1] Ackermann, T, (editor), 'Wind Power in Power Systems', John Wiley & Sons, Ltd, 2005, ISBN 0-470-85508-8.
[2] Troen, I, and Petersem, E L, European Wind Atlas, Risø National Laboratory, Roskilde, Denmark, ISBN 87-550-1482-8.
[3] WAsP (Wind Atlas Analysis and Application Program), Version 8, Risø National Laboratory, Roskilde, Denmark.
[4] Beurskens, J, and van Kuik, G, 'Alles in de wind', Questions and answers concerning wind power, October 2004.
[5] 'Wind Power Technology', Operation, commercial developments, projects, grid distribution, EWEA, December 2004.
[6] 'Wind Power Economics', Wind energy costs,investment factors, EWEA, December 2004.
[7] 'The Current Status of the Wind Industry', Industry overview, market data, employment, policy, EWEA, December 2004.
[8]www.lpelectric.ro
[9] http://en.wikipedia.org/wiki/Wind_power
