Experimental anti-icing flux correlations.
Atmospheric icing effects is a critical issue for wind farms in Nordic regions; it is responsible for production losses, shortens the equipment’s lifetime, and increases safety risks. Electrothermal anti-icing is one of the existing techniques of ice mitigation, and its energy consumption for wind turbines has been numerically investigated over the years but never fully validated experimentally in the literature. In this work, we aimed to determine the energy consumption for anti-icing systems based solely on experimental investigations. Our methodology is to quantify the energy required to protect a custom-built NACA 0012 airfoil from ice buildup in a wind tunnel. The results are extrapolated to a full-scale wind turbine.
- wind turbines
- ice mitigation systems
- energy consumption
- electrothermal anti-icing systems
To perform anti-icing, knowing the energy required to prevent ice buildup on the wind turbine’s blade is the most important parameter for a wind farm operator. It can be used to select the most suitable ice protection system for a given site and to decide whether or not to operate wind turbines during icing events. There are a few softwares, primarily designed for aeronautics, such as LEWICE, TURBICE, and FENSAP-ICE based on computational fluid dynamics (CFD) methods that can be used for modeling anti-icing for wind turbine blades. They require a certain level of expertise to be used. However, more importantly, there are very few research results in the literature for anti-icing experiments on full-scale wind turbines. These results are essential for validation and improvement of existing numerical models.
Our approach is to experimentally investigate the power requirement for anti-icing on a NACA 0012 airfoil partially equipped with resistive heaters and instrumented with fluxmeters and thermocouples. Based on wind tunnel measurements, correlations are developed between the anti-icing energy flux and the airflow wind speed for a given temperature and liquid water content. Finally, under some assumptions, we use these correlations to evaluate the anti-icing power requirements for a full-scale wind turbine. Our experiments have been conducted in the icing wind tunnel of the Anti-icing Materials International Laboratory at the University of Québec at Chicoutimi.
2. Wind turbine Icing
Ice accretion on wind turbine blades can affect both the energy production and the lifetime of the wind turbine. Ice accumulation on blades reduces wind turbine power due to airfoil shape alteration and increased surface roughness . This disrupts the airflow, increases drag, and reduces lift [2, 3, 4]. In severe meteorological conditions, ice accretion on the blades can cause downtime for days or weeks at a time . An imbalanced ice load on the blades can increase their vibrations and reduce the wind turbine’s lifetime [6, 7]. Also, ice blocks fall represent a risk for both staff and facilities in the vicinity. In certain cases, the ice on the blade can delay the stall causing a power surge that can damage components and cause fires . In areas subjected to frequent icing events, an ice mitigation strategy is mandatory in order to reduce production losses [9, 10].
To overcome the effects of icing, wind farm operators have generally two solutions. The first is to stop the wind turbine during icing events. With this strategy, the turbines must remain stopped until there is no more ice on the blades; this can have a strong impact on the profitability of a wind plant if icing events are frequent because the recovery time is often very long. The second alternative is to install an ice protection device. Several more or less complex approaches and technologies have been implemented over the years in wind farms. Currently, wind turbines designed to be much better adapted to cold climate regions and equipped with ice protection systems are available in the market.
Ice protection systems can be regrouped in two categories: anti-icing and de-icing. Anti-icing systems prevent ice buildup at the surface of the blade while de-icing systems remove accumulated ice from the surface of the blade .
There are several types of ice protection systems using various technologies for mitigating ice accretion on wind-turbine blades. These technologies can be passive (ice-phobic/hydrophobic coatings, thermal coatings such as black paint, etc.) or active (antifreeze coolants, pneumatic and expulsive techniques, hot air injection, resistive heaters, microwaves or infrared heating, etc.) [12, 13]. Heating the wind turbine’s blade is currently the most efficient protection technique against ice accretion. It can be achieved by driving hot air inside the blade or by installing resistive heaters on or in the blade. Driving hot air along a high diameter rotor results in important energy loss. Besides, retrofit is not possible with the hot air system. Therefore, resistive heaters installed on the outer surface of the blade seems to be the most suitable technique for most applications.
There are two anti-icing systems using resistive coatings. The first one consists in heating the surface enough to evaporate the impinging droplets; it is the evaporative anti-icing. This technique requires a lot of energy, and the effects of the heat on the blade lifetime are unknown at this time. The second approach consists in heating the droplets enough to avoid accretion both on impact and during runback; it is the wet running anti-icing. This approach requires transferring an additional energy to the droplet in order to prevent the freezing when it streams along the blade. Our experimentation is based on the second approach.
For the experimental work, we use a rectangular NACA 0012 airfoil blade with a constant section of 0.254 m and length of 0.381 m. The airfoil is covered with resistive heaters and instrumented with fluxmeters and thermocouples. For various airflow wind speeds, temperatures, and liquid water contents, we measure the heating energy required to maintain the airfoil surface temperature at 5°C determined as a threshold for keeping the runback water in liquid form. The data are processed to find correlations that will help to extrapolate the anti-icing energy consumption of an operating full-scale wind turbine blade.
4. Fundamentals of power requirements
4.1. Sensible heating
The sensible heating is the energy required to heat up the impinging droplets in order to keep them in a liquid state. Part of this energy serves to heat up the runback water and avoid secondary icing. If we assume that the droplets are heated from their initial temperature to the surface temperature, the sensible heating can be expressed by
The impinging mass flow of droplets is the amount of water per unit of time and surface that can be caught by the airfoil. It is also known as the intensity of ice accretion and identified according to the formula :
The collection efficiency
4.2. Kinetic heating
Kinetic heating is the gain of energy due to the velocity of the droplets and is given by 
4.3. Convective energy loss
The convective heat loss is given by 
The convective heat transfer coefficient
4.4. Evaporative energy loss
Even if the anti-icing strategy does not consist in the evaporation of the impinging droplets, the heat at the airfoil surface is high enough to evaporate a fraction of the impinging water. The evaporated heat loss can be estimated with 
4.5. Aerodynamic heating
Aerodynamic heating is due to the friction between the droplet and the air :
4.6. Energy balance
The energy balance equation is given by
5. Experimental investigations
5.1. Icing wind tunnel
AMIL’s icing wind tunnel is a closed-loop, low-speed, refrigerated wind tunnel which is able to operate at negative temperatures. The refrigeration system is capable of varying the air temperature between −20 and 20°C by passing it through a 1.6 m × 1.6 m heat exchanger powered by a compressor and a glycol pump. A fan connected to a motor allows for an empty section to reach flow rates up to 31 kg/s at an air temperature of 22°C.
The icing wind tunnel has two test sections. The smaller one measure 0.5 m wide by 0.6 m high and can be used to perform experiments at wind speeds up to 37 m/s at ambient temperature. The second test section is 0.91 m wide by 0.76 m high and can reach wind speeds up to 86 m/s at ambient temperature. Experiments in this study have been done using the small section (Figure 1).
5.2. Experimental setup
The airfoil has been made of 2.54-mm-thick fiberglass resin. It is equipped with 10 resistive heaters pasted on the external surface of the upper surface (Figure 2).
A thermocouple is placed between the surface of the airfoil and each resistive heater. Another thermocouple is placed inside the airfoil right under the external thermocouple. In addition, a fluxmeter is pasted above each resistive heater; these fluxmeters give the outgoing energy flux and an additional indication of the surface temperature. Each heater is powered individually by an independent power source. The setup has a total of 20 thermocouples, 10 fluxmeters, 10 resistive heaters, and 10 power sources (Figure 3).
5.3. Energy consumption
We have conducted 30 experiments at five different wind speeds (10, 15, 20, 25, and 30 m/s), two different temperatures (−10 and −15°C), and three different values of liquid water content. The surface temperature in each experiment has been fixed to 5°C and the angle of attack is 0°.
For the first set of experiments, there was no precipitation (dry air):
These experiments quantified the convective effects. Figure 4 shows the variation of with the airflow wind speed.
In the second set of experiments, we fixed the airflow temperature to −10°C and added water droplets to the airflow (wet air):
The different values of liquid water contents experimented are 0.3 and 0.9 g/m3. Figure 5 shows the variation of with the airflow wind speed.
Finally, we repeat the wet experiments for an airflow temperature of −15°C. Figure 6 shows the variation of with the airflow wind speed.
The graphs of and , between the ranges of 10 and 30 m/s, can be approximated by linear functions with a very good precision. Thus, under those conditions, we can deduce the following correlations for required anti-icing energy flux.
|LWC = 0 g/m3||LWC = 0.3 g/m3||LWC = 0.9 g/m3|
6. Extrapolation to a full-scale wind turbines
To extrapolate the results presented in the previous section to a full-scale wind turbine, we do the following assumptions:
Linear approximations of and are good for wind speeds between 30 and 80 m/s.
The turbulence effects around the experimental setup and the wind turbine on site are similar.
The wind turbine is composed of NACA 0012 blades (3) at an angle of attack of 0°.
The efficiency of the experimental setup is the same as the efficiency of the anti-icing device installed on the wind turbine.
6.1. Chord distribution
The chord distribution is based on the NREL 5MW reference wind turbine for offshore development . We defined the chord distribution used in our study as follows:
|Zone 1 (hub)||0.02||0.02|
The graphical normalized chord distribution is shown in Figure 7.
6.2. Wind speed distribution
The relative airflow wind speed
|Tair = −10°C|
|Radius (m)||Tip speed ratio||Output (KW)||Anti-icing power (KW)|
For each zone of the blade determined in Section 6.1, we compute the average value of by integrating its expression (Table 1) with respect to the radius
|Tair = −15°C|
|Radius (m)||Tip speed ratio||Output (KW)||Anti-icing power (KW)|
At first glance, we note that the anti-icing power consumption is sometimes higher than the nominal output power of the wind turbine (Figure 8). In such conditions, stopping the wind turbines is a better alternative. Plotting the anti-icing energy in terms of the percentage of the nominal output power (Figure 9) showed that the anti-icing operation consumes more than 50% of the nominal output power.
Figure 10 displays the fraction of
In this chapter, we have introduced the effects of the icing on wind turbines and investigated the anti-icing power requirements of a full-scale wind turbine. Our methodology was to build a section of NACA 0012 airfoil covered externally by resistive heaters and instrumented with thermocouples and fluxmeters. We assembled the airfoil and conducted anti-icing tests in the Anti-icing Materials International Laboratory wind tunnel. The measured data have been used to establish correlations between the anti-icing energy and the airflow wind speed around the airfoil. Under some reasonable assumptions, we used these correlations to estimate the anti-icing power consumption for full-scale wind turbines.
The extrapolated power consumption showed that the anti-icing consumed a considerable amount of energy that can exceed the nominal output of the wind turbine itself. In reality, the anti-icing power consumption should be higher than the values that we extrapolated because
The NACA 0012 is slimmer than the wind turbines airfoil.
The wind turbine angle of attack is above 0°.
Thus, the blades collect more water droplets and consume more energy. Nevertheless, the work presented in this chapter proves that anti-icing for wind turbines is not a viable solution to mitigate icing effects. The correlation presented in this chapter can also be used for anti-icing on heated flat surfaces.
Given that wind turbines are designed to be more robust and to be able to withstand significant ice loads, then the de-icing seems to be a more promising strategy. This work is the first part of a project on the estimation and control of mass and heat transfers during the anti-icing and de-icing of wind turbines. The project, motivated by the need of the wind industry, answers some issues related to anti-icing strategy and its feasibility. The results are useful for the choice, design, and control of de-icing/anti-icing systems.
8. Abbreviations and acronyms
|AMIL: Anti-Icing Materials International Laboratory|
CFD: computational fluid dynamics
: impingement water, kg
LWC: liquid water content, kg/m3
: experimental convective heating, W/m2
: experimental water heating, W/m2
: experimental anti-icing energy, W/m2
|NACA: National Advisory Committee for Aeronautics|
: heating flux, W/m2
: anti-icing energy, W/m2
: sensible heating, W/m2
: impinging droplet heating, W/m2
: kinetic heating, W/m2
: convective energy, W/m2
: evaporative energy, W/m2
: aerodynamic heating, W/m2
: atmospheric pressure, Pa
: anti-icing energy (W/m2)
NREL: National Renewable Energy Laboratory