The wind turbine transports the line in the complex external environment, realizes its energy conversion function, needs to withstand each kind of external load through the reliable durable structure, but also needs to defend the system to resist three kinds of external attack, although the summary is not comprehensive, but the editor uses four systems to describe for a while:

One is the Lightning protection system,

Second, the anti-corrosion system,

Third, air traffic protection system,

Four is the waterproof system (including the ice-proof);

These four systems carry other external attacks other than mechanical loads. It is noteworthy that the current wind turbine key components of the blade of the higher frequency ratio is the anti-corrosion system, leading edge protection of the durability of the main problem, usually 2-4 years need to carry out different degrees of maintenance.

This paper mainly discusses the surface protection of wind turbine blades.

Surface protective layer, can be used in plastic or surface coating, the material can be unsaturated polyester, epoxy or polyurethane, but also acrylic (acrylic plastic). Wind turbine blades are eroded by prolonged exposure to harsh environmental conditions such as extreme weather.

At present, through some test methods, can confirm the surface protection system to the external environment protection ability, from the guarantee surface protection system can help the blade to receive as small as possible damage.

The durability of the surface coating can be validated and ensured through the use of existing marine and helicopter protection standards, accelerated corrosion testing of the protective system, such as UV or chemical corrosion tests.

This issue mainly introduces the test method and evaluation means of the performance of the protection system.1. Blade Surface Protective Layer Performance test.

Blade surfaces are exposed to a variety of adverse environmental conditions, so it is certainly best to test the protective performance of blade protection systems in a variety of conditions in advance.

But in reality, external environmental conditions are changing rapidly, and it is difficult to predict which factors have changed and how to change, so it is impractical to verify the protective performance under all conditions. The problem that the manufacturer needs to grasp is which criteria to select as a Test object.

1.1 Issues to consider for performance testing

There are many different ways to test the surface protection system of blades.

Fan operators expect to be able to withstand the adverse effects of climate conditions by ensuring that the fans are able to serve the entire life cycle intact. Manufacturers expect rapid testing to verify the performance of the protection system and meet customer requirements.

The main difficulty of the manufacturer is how to meet the needs of the entire lifecycle to verify the performance of the protective layer in various environmental conditions through limited time testing.

The best way of course is to test in the natural environment, so that the performance degradation of the protection system can be observed.

In fact, however, this is not really a method of material life testing, especially the application environment of the test object is a variety of climatic conditions.

Therefore, the life test of the material is often carried out under man-made environment conditions.

Comparison of accelerated testing and laboratory environment testing is realistic, but compared to the man-made environment and the real environment is difficult to implement. In other words, the feasibility of accurately predicting the life of the surface protection system is poor.

A large testing plant has been designed to perform outdoor and UV radiation testing. The test project and the use of the artificial environment, depending on the test object and the location of the wind turbine planning machine.

For example, if a wind is installed in the desert, you need to enter the line UV degradation and sandblasting test. For offshore fans, humidity testing, UV radiation and salt spray tests are required.

Under test conditions, these factors are considered separately or at the same time, depending on the purpose of the test. Under the condition of natural environment, these factors will affect each other and aggravate the erosion of the protective system.

Under test conditions, these factors are considered separately or at the same time, depending on the purpose of the test. Under the condition of natural environment, these factors will affect each other and aggravate the erosion of the protective system.

It is important to understand the degradation of polymers prior to the testing of the surface protection layer.

Performance degradation of polymers

Degradation is the change in the performance of polymers that are not expected to occur. Change can be physical or chemical.

Changes include the degradation of polymer main chain structure, the change of side chain groups, transverse links, or the deletion of some elements.

The degradation or absence of fillers can also affect this system, resulting in degradation of the performance of the protective system.

1.2.1 In the process of aging, energy is transmitted from the sun to the blade surface through radiation. The energy transmitted by the radiation depends on the angle between the sun’s rays and the surface of the blades and the wavelength of the light.

The combined effects of oxygen and moisture in the atmosphere can lead to chemical degradation. Corrosion and sand erosion and other factors will also attack the protective system surface damage.

1.2.2 Solar radiation

The sun is the visible part of the sun’s electromagnetic radiation, including a variety of different wavelengths of light.

One part of the light is so powerful that it interrupts the primary bond of the blade’s gum or coating, or covalent bond. After the primary key is destroyed, the surface coating degrades.

Sunlight can be divided into UV radiation, visible light and infra-red. Most of the radiation is visible. But the shorter the wavelength of the light contains the higher radiant energy.

UV radiation is also divided into UVA, UVB, UVC three levels, of which the UVA wavelength is the longest of the three, so the energy is the lowest among the three;

But this article only discusses UUA (wavelength 350–400 nm) because UVB and UVC are absorbed by the atmosphere before the Earth.

1.2.3 Oxygen and humidity

Polymers react with oxygen and water, so oxygen and humidity affect the surface of the polymer.

The material itself absorbs water, as in most cases polymers and coats/coatings absorb moisture, because the polymer contains oxygen and the hydrophilic bonds present in the nitrogen molecule.

Because of the influence of UV radiation on the leaf surface, the free Atom group exists in the polymer. Oxygen reacts with these free groups of atoms to form new chemical molecules.

If the structure is open, the water molecule will enter it, and if the polymer’s horizontal link is not much, the water molecule will invade the coat/coating or composite structure layer.

The risk of subsequent degradation increases, so the adhesion between the composite structure layer and the adhesive or coating is greatly reduced.

This process is often referred to as thermal oxidation degradation.

This is the starting point for chemical degradation of surface coatings, coatings and structural layers and the following structures.

The degradation of open space is usually thermal oxidative degradation.

1.2.4 Degradation mechanism

The degradation of the polymer system can be caused by various reasons such as polymer main link, transverse link rupture or change of covalent bond system or lateral link.

But the severity of the problem does not depend on the type of change that occurs, but on the type of polymer affected.

Horizontal links are visible in most thermosetting materials, as most surface protection layers are selected with thermosetting materials.

Horizontal links are formed through the energy of the functional groups. Therefore, there are functional groups in the thermosetting material system, and they react to each other.

In the unsaturated polymer system, there is a two-strand reaction, but this double chain will produce transverse links after absorbing solar radiation.

Pai Link energy is low, if the polymer surface has a double chain presence, it will cause horizontal link formation. Horizontal linking makes the polymer system stronger and more rigid, but lacks toughness, making the material brittle.

When the volume of the polymer shrinks, the surface of the polymer is prone to crack, and if the moisture invades the polymer system, the polymerization reaction occurs.

In most cases, the polymerization can lead to voids in the structure, which further aggravates the entry of the ⽓ gas and creates a vicious circle.

The bond in the polymer system usually depends on the covalent bond of the main link. Table 2 gives some key energy. The lower the bond energy of polymers, the higher the risk of corrosion degradation.

In fact, the keys shown in table 2 are not high, and are susceptible to UV radiation erosion.

The oxidation rate depends on the substructure.

Double bonds are more likely to be oxidized than lipids, because of their molecular structure, which can become more stable after being degraded by UV radiation, especially when the lipid base is used as a lateral link.

For example, MMA (methyl methacrylate) is used as a transverse link in a certain adhesive to consider this.

1.3 Degradation degree test of polymer system

Many different methods can be used to test the degradation of polymers affected by external environment.

One method is to measure the changes in color and smoothness produced by the degradation of polymers.

Most of the wind turbine blade surface protection color is white or silver-gray (in order to prevent icing, people have also used black surface protective color).

Once the color is changed it is easy to see. Color change is the first feature that occurs when it degenerates.

Measuring color changes does not know exactly what has changed inside, but it is a pointer to the degradation.

The internal degradation of polymers can be known by Fourier infrared spectroscopy.

If degradation occurs, the peaks in the spectra change.

Chromatography and Spectrophotometric method can be used to determine the molecular weight changes of polymerization system before and after radiation, but these two methods can only be used for soluble materials.

Another method is to determine the degradation by measuring the physical, mechanical, and chemical properties of the surface of the polymer.

When the system deteriorates, its strength and stiffness usually change.

If a horizontal link is generated, the test object becomes more brittle and stronger. Both mechanical properties and IR spectra can be used to determine which chemical changes affect the polymer.

In artificial environment, the life of polymer for surface protection can be judged by the change of physical, mechanical and chemical properties of the surface.

The method can also be used to calculate the life of the surface protective material under the natural environment by introducing an accelerating factor.

The size of the acceleration factor depends on the cause of the degradation and the type of artificial environment introduced.

1.4 Accelerated Testing

1.4.1 UV Test and hernia lamp

Experiments in laboratory conditions, there are many different methods can be used for accelerated testing.

The prediction of the life expectancy of the protective materials using different wavelengths of UV or hernia lamps is two of the accelerated testing methods.

By increasing the temperature, the reaction time decreases, and the relationship between the two can be expressed as a Ahennis equation, as shown in formula 1.

The specimen can be fixed without moving and the factor can be measured by means of the measured factors, and the test pieces will be moved by a preset orbit in the various environmental factors.

The sequence and influence intensity of the measured factors can be simulated according to the natural environment that will be applied.

Mixed tests usually get a better result.

Actual blade surface protection layer accelerated test.

Given time and practicality, it is always hoped that a method of accelerated testing can be compared with the actual surface coating, and the life of the surface protection system can be predicted as accurately as possible.

2.1 Standard

At present, there is no ready to be used for wind turbine blade surface coating testing of the systematic standard, can only refer to the helicopter blade surface coating test standards.

But there’s a difference. It’s important to note that the life expectancy of a helicopter surface coating is much shorter than that of a wind turbine blade coating.

When setting test parameters, you can refer to the test criteria used for offshore structures and coatings.

Other standards can also be referenced,

such as “surface treatment and coating protection” in the Norsok standard M501 developed by the Norwegian Petroleum Industry Association;

The requirements of the marine and related structures of the European standard ISO20340 on the performance of coatings and paints;

In ISO4628, the “Evaluation of the degradation of coating and topcoat” and “the definition of the size and quantity of defects and the degree of surface variation”, especially the 4628-2,-4,-5, have a better applicability to fan blades.

When the standard of blade protection system is formulated, the relevant standards of coating protection for offshore steel structures are more referential than those of helicopter blades. This is because the wind turbine installation and offshore oil platforms have more similarities.

The offshore architecture of the petroleum platform is divided into 4 categories according to the degree of exposure to the environment (ISO 20340).

One of the categories is exposed to the atmosphere (C5-M),

The other three categories are divided according to the Im2, respectively.

Underwater area

Tidal zone

Splash Zone

All 4 categories are similar to fan towers, but only c5-m are similar to fan blades.

Although the standard is steel structure, many parameters can also be applied to the composite surface protection system.

Both C5-m and Im2 define corrosion by using the material thickness or mass loss of the standard specimen, and also give a description of the typical atmospheric environment.

2.2 Test Selection method

The test is to put the specimen in the circulation body of different atmospheric environmental factors.

The test environment is a simulation of different atmospheric environments, starting with a salt mist cabinet, then a UVA cabinet or a hernia cabinet.

The temperature of the UVA cabinet or the hernia cabinet can be elevated to speed up degradation.

The test is usually tested for 32 weeks at elevated temperatures, and then tested by standard iso4628,2,4,5 for bubble, cracking, and shedding of specimens, and then based on iso12944-2 to test the bonding properties of the protective layer and FRP structural layers.

This test is time-consuming, but the results are comparable with the degradation of the blade protection layer in the marine environment.

2.2.1 Abrasion resistance test

The Norsok test does not require the abrasion resistance of the protective system to be tested, nor is there an ISO standard for reference.

The abrasion resistance test of the blade surface is very important, especially the blade tip and the 1/3-length part near the tip of the blade, the surface of which is highly exposed to abrasion, and in extreme weather conditions, the particulate matter in the air is $number to the blade surface.

Hail and rain eclipses are not included in the Norsol test, but they all need to be tested.

By mixing the flow test with the abrasion test, it is possible to predict the degradation of the protective system due to rain erosion.

Hail can be tested by using ice cubes instead of sand or emery because the tip of the ice will erode the surface of the protective system.

Generally, the resistance of the flexible protection system to abrasion is better.

2.2.2 Moisture Environment Test

Norsok tests and other tests have taken into account the effects of moisture.

Once the specimen has inhaled enough water vapor, freezing and thawing alternately, simulate the actual cryogenic environment in this way.

After the test is completed, it is necessary to confirm that the specimen produces bubbles. When the water freezes or melts alternately, its volume changes, the volume change will cause the ⽓ bubble phenomenon to produce.

2.2.3 Chemical Corrosion

Wind turbine blades may be exposed to different chemicals and may be contaminated by dust, guano, insect carcasses, oil spills, salt mist deposits or acid rain.

No hybrid test can simulate these conditions.

The actual test is this: The test chemicals include carbon black (simulated salt mist deposition), diluted sulfuric acid (simulated acid rain), oil (analogue dust or other animal fats), artificial seawater.

None of these chemicals can cause fatal damage to the surface protection system of the blades, but the mixture could have a lethal effect.

2.2.4 Test Results

Because the test does not resemble the mechanical performance test, can not give the direct data result, but still need according to ISO4628 to ⾏ row to decide.

Bubbles, crazing and spalling may occur after testing, and these can be used to determine the properties of a coating or a rubber coat.

In addition, the adhesion to the structural layer also needs to be tested, and in order to perform this test, special preparation is required for the test, such as the fixture required to design the stripping and stretching test, or the need to introduce a notch (for the stripping test) on the specimen’s coating or adhesive.

3. Summarize future challenges and trends

3.1 Advantages and limitations of different protection strategies

Various protective materials have their unique advantages, but they are also subject to certain limitations, in the advantages and limitations, we need to find a balance, including the choice of flexible protection or rigid protection, selection of plastic clothing or coating.

Rigid coatings can be obtained by using a higher hardness of epoxy or unsaturated polyester.

For thermosetting materials, their hardness is high, and this protection system requires a high level of horizontal links, because horizontal links lead to low flexibility and greater shrinkage.

For flexible systems, you can achieve better protection by using a softer polyurethane adhesive or coating, or by sticking a polyurethane belt (protective film) in the leading edge or 1/3-length tip area.

Different materials can reduce the damage caused by some factors. In order to reduce the water absorption of the surface protective layer, the coating should be epoxy rather than polyester.

In order to obtain a lower rate of hydrolysis, vinyl ester rather than unsaturated polyester is needed.

If the transverse-linked polymers of isobutene are used, the UV protection performance will be improved.

In addition, if the protection system surface tension is low, it is conducive to deicing, but such a system to withstand erosion and erosion ability is very poor.

In weighing the strengths and limitations of different material systems, it is also necessary to consider the adhesion between the material system and the structure layer, because the lack of adhesion can lead to the spalling of the material, which can not protect the structure layer.

3.2 Future challenges and trends

The blade length will grow longer, and in extremely harsh environmental installations also become a trend, such as sea, desert, low temperature environment or peak.

These regional wind resource conditions are good, the fan performance is high, the noise limit is less. However, the problem of abrasion and chemical corrosion is even more serious.

At the same time, because it is difficult to reach, people’s demand for operational dimension is also getting higher.

The durability and strength of the surface protection system becomes very important.

3.2.1 New Materials

In order to meet these challenges, new materials need to be developed.

New coating materials provide better surface protection and can withstand even worse environmental conditions.

For example, a product based on imide and a surface protective material containing fluorocarbon compounds can be better ice-proof.

3.2.2 Environmental Protection

In the future, the legal provisions on marine and onshore environmental protection will be more stringent. The problems facing the environmental impact of wind farms are increasing, and many residents have a wind farm near their homes.

This has led to new requirements for blade surface protection, such as refractive index, aviation protection and lightning avoidance systems. Some of these problems have been solved.

At present, people have introduced the aircraft logo, the surface refractive index low paint and other solutions.

Because many factors are related to the aerodynamic noise of blades, the solution to this problem will become a problem for a long time.

People are now exploring the measures of reducing noise, such as shark fin, but the effective commercial application is still some time.

3.2.3 Manufacturing environment of Blade surface protection

In the process of surface protection to the blade surface, although people have adopted protective clothing, gloves, masks and other protective supplies, but these measures can only reduce the damage from the surface protective material harmful substances to the manufacturing workers, and can not be eliminated, and the air pollution is also difficult to eliminate.

3.2.4 test method

At present, some test methods and models have been established, such as lightning protection system or aerodynamic noise, but the model is not established.

It is worth mentioning that, with the elevation of the tower height, the blade operating environment in-depth cloud, cloud-induced icing problem is causing the blade icing problem beyond the low temperature of the scope of the environment has gradually been widely concerned.