Turbine Efficiency And Grid Integration Project Outline Engineering Essay
Cold climate sites will affect the design of a wind turbine in different ways: Ice and rime as well as high air density at low temperatures will cause severe effects on the aerodynamics and thus, on the loads and the power output of the turbine. Temperature effects and especially high masses of ice on the structure can change the natural frequencies of wind turbine components and change the dynamic behaviour of the whole turbine. Also, the control system can be affected. The stall of the rotor may occur earlier or later due to changed airfoil shape, the electrical or hydraulic pitch control can change their settings. Frozen or iced control instruments give faulty information to the supervisory system of the turbine. Extreme low temperatures will require special materials. For example normal steel will become brittle at those temperatures. The safety of the wind turbine as well as the vicinity at the site will be also affected by icing or in general by cold climate operation. Ice fragments thrown away or even large ice pieces falling down from the rotor can harm persons or animals or damage objects. The structural integrity of the turbine itself can be affected by heavy unbalance due to unsymmetrical icing, by resonances caused by changed natural frequencies of components exceeding the designed fatigue loads. Low air density can increase the loads and maximum power output. If the turbine does not automatically react, windings or transformers may burn, gearboxes can be overloaded and damaged. Also the overall economy of wind energy projects will be affected by cold climate operation, especially at ice endangered sites. The site prognosis has to include type and duration of icing events, the frequency distribution of the temperature has to be known in correlation to the wind situation in order to predict the energy production as well as down times due to icing. Possibly, a special class of cold climate turbine has to be defined, as the standard IEC classes 1 to 4 will not apply at those sites. This may require special equipment of the turbines such as heating elements for the blades, heating of gear boxes and electronic boards, use of special steel for extreme low temperatures, heated wind vanes and anemometers or special ice sensors. Special requirements for maintenance and repair at cold climate sites should be taken into account even during the planning phase and during the calculation of the economics of the project. Access to the site during the cold period may be very difficult or costly. The access to the site may be impossible if the roads are iced or full of snow for longer periods. In these conditions the erection, maintenance or repair of the turbine will be not possible or will produce long standstill periods without power production.
Aerodynamic performance of a wind turbine's icing conditions
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During the rotation of the rotor blades in icing weather conditions the leading edge of the rotor blade collects more and more ice around the stagnation point of the airfoils. Due to the increasing air velocity along the radius, the ice accretion builds up more at the outer part of the blade with an approximately linear increase. In principle, two types of icing during operation can occur, clear ice and rime ice. The right sketch in Figure 1 shows the situation of a cross section of a rotor blade during operation with ice accretion at the leading edge. The cross section area increases as the "chord length" of the airfoil grows. Aerodynamic forces act on the ice fragment and - if too large - breaks it off. New ice builts up and the leading edge will look like a saw blade after some time.
Figure 1: Icing during idling and operation
The left side of the Figure 1 shows the situation at low wind speed when the turbine is idling. Here the aerodynamic forces on the ice fragment are very small and no centrifugal forces will act on it as the rotor speed is close to zero.
2. Ice types in cold climates
Icing occurs at temperatures below 0Â°C when there is some liquid water in the air. The type, amount and density of ice depend on both meteorological conditions and on the water droplet dimensions and type of states, including moving and static. There also is different icing climates, such as in-cloud icing, when small water droplets in the cloud impact and freeze on the surface of structures under cold and extreme low temperature icing conditions. Two forms of atmospheric in-cloud icing are glaze and rime ice (including hard and soft rime ice) in icing events.
Glaze ice: A smooth, transparent and homogenous ice coating occurring when freezing rain or drizzle hit a surface Glaze is the type of precipitation ice having the highest density. Glaze is caused by freezing rain, freezing drizzle or wet in-cloud icing and normally causes smooth evenly distributed ice accretion. Glaze may result also in formation of icicles, and in this case the resulting shape can be rather asymmetric. The accretion rate for glaze mainly varies with:
Rate of precipitation
Droplet size distribution and water content
Figure 2:Glaze ice in icing tunnel: a) rotating propellers and b) a wind turbine blade model
Rime ice: A smooth-surfaced, usually transparent dense formation of ice. Its crystalline structure is rather irregular, surface uneven, and its form resembles glazed frost. Supercooled cloud droplets with some wind are needed to form rime. Rime ice can be defined with respect to density, hardness and appearance: Hard rime is a granular, usually white, ice formation. It forms ice granules among which there is trapped air which causes the white color. The density of hard rime ice ranges typically between 100 and 600 kg/m3. Hard rime ice adheres firmly on surfaces making it very difficult to remove it. Soft rime is a fragile, snow-like formation formed mainly of thin ice needles or flakes of ice, when the air temperature is well below 0Â°C. The growth of soft rime starts usually at a small point and grows triangularly into the windward direction. The density of soft rime is less than 100 kg/m3, and it can be easily removed. Rime is the most common type of in-cloud icing and often forms vanes on the windward side of linear, non-rotary objects, i.e. objects which will not rotate around the longitudinal axis due to eccentrically loading by ice. During significant icing on small, linear objects the cross section of the rime vane is nearby triangular with the top angle pointing windward, but as the width (diameter) of the object in-creases, the ice vane changes its form. The accretion rate for rime mainly varies with:
Dimensions of the object exposed
Liquid water content in the air
Droplet size distribution
Figure 3: Simulated icing events showing rime ice: airfoil (icing phase)
Wet snow: A high density snow with high liquid water content above about 3% or up to 15% created at temperatures very close to freezing can appear quite sticky and adheres to structures. Freezing wet snow develops a strong bond to structures. Wet snow is, because of the occurrence of free water in the partly melted snow crystals able to adhere to the surface of an object. Wet snow accretion therefore occurs when the air temperature is just above the freezing point. The snow will freeze when wet snow accretion is followed by a temperature decrease. The density and adhesive strength vary widely with, among other things, the fraction of melted water and the wind speed.
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Dry snow: Snow with a solid crystal structure which typically will not stick to structures and easily drifts in the wind.
Ice accumulation: The amount and rate at which ice accumulates on structures, specifically on wind turbine blades, towers, and guy wires. Accumulation depends on many factors and affects turbine performance, noise and safety aspects. Accumulation must be sufficiently well estimated or measured to correctly estimate the need for de-icing and anti-icing equipment.
3. Cold climate site effects on the design of a wind turbine
Table 1 Cold climate operation affecting the design, safety and economics of wind energy plants.
Cold climate sites will affect the design of a wind turbine in different ways, some most important of problems in cold climates sites mentioned below:
Icing and snow drifts can make vehicle access difficult or impossible without snowmobiles or other over snow transport. Turbines should be selected according to site accessibility, taking into account road and bridge limitations for heavy cranes and trucks. The logistics of turbine installation must be planned according to seasonal and climatic limitations, and special care may be required to avoid damage to equipment during transportation.
Temperature consideration is critical to project development, construction, operation, and decommissioning. A wind turbine contains components that often can be readily adapted to cold climates. The lowest operational temperature limit for the turbine is usually governed by qualities of steel and welding. Consequently, the local temperature distribution must be measured along with the wind speed and icing events during site investigation to enable a turbine to be selected with the correct CC modifications. Air density variations affect the power output of wind turbines. Based on the equation of state for an ideal gas, air at -30Â°C is 27% denser than at 35Â°C, resulting in a similar increase in power output at the same wind speed. This may cause the generators in passive stall controlled wind turbines to operate above its rated power, which could require the turbine to be shut down at low temperatures or risk causing damage to the generator or whole turbine system.
Icing on any exposed part of the turbine can occur in the form of wet snow, freezing rain or drizzle, or in-cloud icing. Icing can cause decreased performance of the turbine with ice accumulation on the turbine blades, and excess vibration problems from uneven blade icing or making control hardware, such as anemometers and wind direction sensors, to stop functioning. Icing is a key parameter for CC in project development, construction, operation, and decommissioning. The performance of an iced-up wind turbine will normally degrade rapidly as the ice accumulates. If the icing continues without proper anti-icing, the turbine will either stop because of excess vibrations or disconnect from the grid because of increased aerodynamic drag that slows the rotor down. The wind resource outside the operational icing limit of a wind turbine design cannot be harvested. Consequently, the local icing distribution must be measured along with the wind speed and temperature during the site investigation so that an optimal CC wind turbine selection can be made. Designers should consider the influence of fatigue caused by extended operation with iced blades. Icing might also cause surfaces to be unserviceable, which would prevent turbine access. Ice thrown from the blades or that falls from the tower or nacelle may pose a significant safety hazard.
Snow is quite easily suspended and transported by wind, it forms drifts wherever there is an interruption or discontinuity in the airflow . Wind turbine nacelles are generally not airtight compartments, and in fact usually incorporate many openings to provide cooling. Snow can accumulate inside the nacelle, damage equipment, and prove detrimental to the electrical machinery. It can also obstruct openings and prevent normal air circulation. Heated surfaces, for example on heated anemometers and ice detectors, have been shown to melt snow and, as a consequence, create artificial icing conditions during snow fall. Although not yet proven, de- and anti-icing systems based on heated blade surfaces are likely to act in a similar manner during snowfall
4. Ice Detection
There are three major methods for ice detection:
Ice detection by observation
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One of the possibilities to detect ice is the observation of already installed wind turbines, power lines, trees or high antenna towers in the neighbourhood of the planned site.
Ice detection by sensors
However, ice detection by observation needs manned campaigns and is thus extremely expensive, especially a continuous observation also at night. Ice sensors seem to be a solution for an automatic and reliable ice detection. This can be performed either by special ice sensors directly or by recording of standard instruments indirectly. Recent measurements at the Tauernwindpark (left side and top of right side of Figure 4) compared with parallel observations and measurements of heated and unheated anemometers created some doubts about the trustworthiness of the sensors.
Figure 4: Direct and indirect ice sensors
Ice detection by aerodynamic noise
Another detection of even small amounts of ice accretion can be the increase of aerodynamic noise from the rotor blades. Figure 5 shows a measurement during the beginning of slight icing at the leading edge and the resulting increase of noise as well as the shift of the frequency to higher levels (small graph).
Figure 5: Acoustic noise measurement: Sound pressure level versus the normalised power output during beginning of slight icing conditions (LE = leading edge).
The disturbed aerodynamics result in fully turbulent boundary layer from the leading edge on and thus produces a higher noise and frequency level which can be heard clearly. Ice detection by detection of damages such as break down of meteorological masts or power lines due to buckling and possibly resonances of the structures caused by the high additional masses should be an exception, but can be an additional indicator at sites, where heavy icing is not expected.
5. Removing ice from the rotor blade
Two types of systems to prevent wings from icing are known in the field of aviation. De-icing systems and anti-icing systems, where the first one actively removes the ice from the wing and the second one prevents the wing from icing. Also in the wind energy these two concepts have been tested at prototypes and small serial production lines. As anti-icing systems so-called passive systems are used for example in painting the rotor blades black. The advantage is that at daylight the blade heats up and the ice melts earlier than with white painted blades. However, in summertime the temperature of the blade's surface may affect the material properties of the glassfibre reinforced plastics (GRP), which is sensitive to high temperatures. Also special coatings which shall reduce the shear forces between the ice and the blade's surface are put to the test as at one of the Tauernwind turbines. The advantages of coating the whole surface of a rotor blade are relatively low costs, no special lightning protection is required, the blades are easy to maintain and the whole surface is protected. Furthermore, these types of coating may reduce the sensitiveness against dirt and bugs during the warm periods, improving the aerodynamic performance of the rotor. Disadvantages are the ice throw during operation. It is expected that the ice fragments will break off regularly and will be thrown away from the rotor. At heavy icing conditions and low wind speeds due to low shear forces during idling, there will be also large ice accretion at the leading edge. Also unsymmetrical ice accretion can be possible, leading to unbalance. Figure 6 demonstrates the situation at a pitch controlled turbine during idling (left side) and operation (right side). It is assumed that in case of a small ice accretion at the leading edge the shear forces are relatively small and thus the ice will break off only if the aerodynamic and centrifugal forces on the fragment are strong enough.
Figure 6: pitch controlled turbine during idling (left side) and operation (right side)
There are also two principles to be discussed for the active systems: A de-icing system, which removes collected ice during operation or idling and an anti-icing system which avoids the accretion of ice on the rotor blades during operation or idling. Small airplanes often use mechanical de-icing systems by means of so called inflatable rubber boots on the leading edge of the wing and control surfaces. However, for wind turbine rotor blades with their high centrifugal loads at the outer radii a pneumatic system will inflate itself or has to be divided in short sections. Furthermore, it will disturb the aerodynamics and cause more noise. During the 20 years of service life of a wind turbine under harsh climatical conditions the rubber boots will require intensive maintenance which may not be economical.
Figure 7: Principle sketch of mechanical de-icing by "rubber boots".
In the past, active heating of the blade or parts of it have been tested or are under operation. Typical technical solutions are electrically heated foils at the leading edge (heating wires or carbon fibres) or blowing warm air into the rotor blade at standstill. Heating the rotor blade interior with warm air needs special tubes to pipe the hot air. The advantages are that the leading edge surface and thus the blade's aerodynamics is not affected. There is also no negative effect on the lightning protection system. At standstill the complete surface can be de-iced. On the other hand, GRP material is a good insulator. During high wind speeds or during rotation of the rotor at low temperatures the forced convection will require very high heating power. If this heating system is used at standstill at low wind speeds after icing events the high energy prices without production have to be paid by the operator. During operation - for pitch controlled turbines also at idling and standstill - it is sufficient to heat the area around the stagnation point of the airfoil only. In practice, heating elements at the blade's leading edge are mounted. The use of heating foils at the blade's leading edge surface as shown in Figure 8 has proved to be an effective anti-icing method during operation. Without any heating systems at these types of sites, the turbine would be full of ice over a long period, just at the time when the good winds are blowing.
Figure 8: Wind turbine suited for icing conditions: Heating elements at the leading edge.
Heating foils can be applied at most of the rotor blades even after manufacturing them. However, the blade' s surface at the leading edge, where the air flow is most sensitive, is disturbed. Depending on the attachment of the foils, the aerodynamic performance of the airfoil might change during the un-iced conditions. With stall and active stall controlled turbines at standstill, e.g. during icing conditions combined with low wind speeds, the trailing edge might head towards the wind and thus collect the ice. Leading edge heating elements will not help de-icing this blade as Figure 9 demonstrates. The right side shows the rotor blade of a stall controlled turbine yawed out of the wind during a period of in-cloud icing on the top of a mountain in southern Germany.
Figure 9: Stall-controlled wind turbine in icing conditions at standstill: Possible
configuration if only the leading edge is heated. Observation of trailing edge icing (right side) of a stall-controlled turbine at stand still. The left side of Figure 10 shows a stall controlled turbine at standstill catching an icing period at low wind speeds with the rotor headed towards the prevailing wind direction. Even with heating elements at the leading edge an ice-free start with increasing wind speed will hardly be possible.
Figure 10: Stall- and pitch-controlled wind turbine with heating elements at the leading edge
during icing conditions at standstill and idling. The right side of the Figure shows the same situation for a pitch-controlled turbine in standstill or idling position. The activation of the heating system will de-ice the leading edge and enables the rotor to start energy production.
Figure 11: Loading on leading edge heating elements during operation.
The position of the heating elements at the leading edge involves additional problems. The rotor rotation in the gravity field causes typical high deterministic loads on the blade's structure as shown in the top of Figure 11. Aerodynamic driving forces and superposed socalled edgewise vibrations - caused by low damping of the natural frequency in this direction - are added to the gravity loads. Consequently, high strains in the GRP-load carrying girder as shown in Figure11 will cause even higher strain in the wires or fibres, respectively, of the heating elements. This will be especially true if the heating elements are carbon fibre made. Their Young's modulus is much higher compared to glass fibres of today's rotor blade structures. In other words the "heating fibres" take over the loads. Special technical solutions are required in order to avoid these effects and to avoid cracks in the heating elements.
Antifreeze coatings for rotor blades
Anti-freez proteins inhibit crystal growth and ice formation Synthetically prepared polymers can mimic the effect of the anti-freeze ings of such polymers could prevent icing. Various polymers were investigated in order to explore their freezing point depression properties. Polymers were coated on glass and the resulting coating was subjected to varying air humidity and cooling ramps in a cold chamber. The formation of ice on the coating was compared with the formation of ice on the glass. It was observed that ice forms on some of the coatings at lower temperatures than on the glass. Two effects can be distinguished: 1) Freezing point depression. Water freezes at lower temperatures on the coating. 2) Delay of condensate lower temperatures on the coating
Hydrophobic molecules in water often cluster together. Water on hydrophobic surfaces will exhibit a high contact angle. Examples of hydrophobic molecules include the alkanes, oils, fats, and greasy substances in general. Hydrophobic materials are used for oil removal from water, the management of oil spills, and chemical separation processes to remove non-polar from polar compounds. Hydrophobic is often used interchangeably with "lipophilic". However, the two terms are not synonymous. While hydrophobic substances are usually lipophilic, there are exceptions - the silicones, for instance.
According to thermodynamics, matter seeks to be in a low-energy state, and bonding reduces chemical energy. Water is electrically polarized, and is able to form hydrogen bonds internally, which gives it many of its unique physical properties. But, since hydrophobes are not electrically polarized, and because they are unable to form hydrogen bonds, water repels hydrophobes, in favor of bonding with itself. It is this effect that causes the hydrophobic interaction - which in itself is incorrectly named as the energetic force comes from the hydrophilic molecules. Thus the two immiscible phases (hydrophilic vs. hydrophobic) will change so that their corresponding interfacial area will be minimal. This effect can be visualized in the phenomenon called phase separation.
Superhydrophobic materials have surfaces that are extremely difficult to wet with water contact angles in excess of 150Â°. Many of these very hydrophobic materials found in nature rely on Cassie's law and are biphasic on the submicrometer level with one component air. The Lotus effect is based on this principle. An example of a biomimetic superhydrophobic material in nanotechnology is nanopin film. In one study a vanadium pentoxide surface is presented that can switch reversibly between superhydrophobicity and superhydrophilicity under the influence of UV radiation. According to the study any surface can be modified to this effect by application of an suspension of rose-like V2O5 particles for instance with an inkjet printer. Once again hydrophobicity is induced by interlaminar air pockets (separated by 2.1 nm distances). The UV effect is also explained. UV light creates electron-hole pairs, with the holes reacting with lattice oxygen creating surface oxygen vacancies while the electrons reduce V5+ to V3+. The oxygen vacancies are met by water and this water absorbency by the vanadium surface makes it hydrophilic. By extended storage in the dark, water is replaced by oxygen and hydrophilicity is once again lost.
From the Greek (hydros) "water" and (philia) "friendship," refers to a physical property of a molecule that can transiently bond with water (H2O) through hydrogen bonding. This is thermodynamically favorable, and makes these molecules soluble not only in water, but also in other polar solvents. There are hydrophillic and hydrophobic parts of the cell membrane.
A hydrophilic molecule or portion of a molecule is one that is typically charge-polarized and capable of hydrogen bonding, enabling it to dissolve more readily in water than in oil or other hydrophobic solvents. Hydrophilic and hydrophobic molecules are also known as polar molecules and nonpolar molecules, respectively.
Soap has a hydrophilic head and a hydrophobic tail which allows it to dissolve in both waters and oils, therefore allowing the soap to clean a surface.
Liquid and/or solid anti-icing fillers and/or oils are combined with erosion resistant silicone and/or fluorocarbon elastomeric materials to create erosion resistant anti-icing coatings. These coatings may be utilized to prevent ice build-up on various gas turbine engine components, aircraft components, watercrafts (i.e., boats and ships), power lines, telecommunication lines, etc.
Figure 12: Pictures taken by WEB camera from the hub of one of the Tauernwind turbines
Until today there are no standard solutions available on the market to keep the rotor blades icefree or at least solutions for reliable ice detection as an information for the turbines' supervisory system. Consequently, today's rotor blades should be designed for the operation with ice accretion if the turbine is situated at sites with the risk of icing. The changed aerodynamic loads as well as the changed mass loads shall then be taken into account in the load assumptions. Provided that a reliable ice detector is available, the turbine can safely be set to standstill if icing events occur and put in operation again after automatical sensing of ice-free conditions. A rather good instrument for detecting ice at the rotor blades seems to be a web-cam in the hub, as shown in Figure 12, where the pressure side of an iced rotor blade can be seen. As an ice detector the camera via the internet cannot be used efficiently, as it requires a manned campaign and good visibility also at night. For checking the blades' surface in order to compare ice detection with other instruments or to check for ice accretion before a manned restart of the turbine after icing events, the web cam seems to be an appropriate means at the moment. Some types of de-icing and anti-icing systems described above have been tested on prototypes or small serial production lines or are still under development. Thus, only little experience with anti-icing and de-icing systems is available compared to the large number of turbines being erected world wide. The size of the turbines is still growing and reaches easily 150 to 200 m with the blade in the upright position. These rotor blades can scratch low clouds and may collect ice even at coastal or offshore sites. But also the market for inland turbines, especially those with large towers, increases and requires standard solutions for operation during icing conditions. However, since finishing the research work, much more wind turbines of bigger size have been installed. Documentation of icing and its effect on the power production as well as on the ultimate and fatigue loads of the structures have to be carried out at certain research and demonstration projects on a pre-competitive basis in order to improve the theoretical background. This knowledge has to be used to improve the national and international Standards concerning cold climate operation. As icing is a common external condition for the aviation, the wind energy can take profit from the experience and adopt it to their special needs. Reliable prediction of energy production and the fatigue loads on the turbine's components at inland sites can only be done if ice detectors deliver exact information about icing. Also the control system of the turbines has to rely on sound information on icing situations in order to shut the turbine down or react in another way to prevent the surrounding or the turbine itself from harm and damage. The reliable detection of ice is an indispensable requirement for the operation of wind turbines in cold climates. These ice detectors and ice free wind sensors need standardised conditions according to which they can be designed and calibrated. These standard conditions are not available yet and have to be defined. In order to fit the turbine economically to the site reliable information about possible icing is necessary. An adequate instrumentation is therefore of fundamental importance.
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