By Michelle Froese, Senior Editor
Windpower Engineering & Development
While OEMs strive to build reliable machines, at times the effects of weather are no match for even the most well-engineered wind turbines. Winter icing conditions alone pose a serious challenge to turbine blades without a pre-installed ice-prevention or de-icing system. Ice left to accumulate on a blade will typically cause degradation of a turbine’s aerodynamic performance or halt power production altogether.
“Some wind farms report up to 20% annual energy production losses due to icing,” says Matthew Wadham-Gagnon, a Project Manager at TechnoCentre éolien (TCE). TCE is a Canadian organization that supports wind-industry development through research, technology transfer, and technical assistance for business. “In addition to production losses, ice accretion can affect the structural design load case of a blade, as well as other components in a wind turbine.”
For example, ice shedding from a blade can damage other blades or hit the roof of the nacelle, adds Wadham-Gagnon. But quantifying or predicting the exact effects icing may have on a wind farm — say, for a proposal or business case made to a financier — is challenging because weather conditions change. “Icing could have a significant effect on the annual production of a wind project. It depends on the frequency, duration, severity, and intensity of icing which varies from year to year, site to site, and turbine to turbine,” says Wadham-Gagnon.
At first, cold climates presented a small, niche market for wind-turbine OEMs. Only recently have choices in ice-protection systems (IPS) for turbines become available to wind-farm developers and operators. “There are a number of IPS proposed with some offered by OEMs and third parties,” says Wadham-Gagnon. “These include passive systems such as icephobic [or ice-resistant] coatings, and active ones such as hot air or electro-thermal systems.”
A passive system, usually a coating or spray, is applied to the surface of turbine blades to minimize icing and maximize heat absorption. “The concept of icephobic coatings is appealing because of their low cost and high efficiency for preventing ice build-up on blades,” Wadham-Gagnon says. “However, while some coatings show promise, most if not all are still a few years away from showing their full potential.” These systems also typically require some maintenance or re-application over time, or after a serious icing event. This means sending a wind tech up-tower, which is not ideal in winter conditions.
Active systems typically work to heat turbine blades using thermal devices, such as built-in electric foils or heated air. “These systems depend on the ice-detection method, power available for the IPS, severity of the icing event, and local health and safety requirements, they may be activated while the turbine is in operation or may require that a turbine come to a complete stop before activation,” he explains.
So, what system is ideal for preventing ice build-up on turbine blades in cold climates? The answer depends on a number of variables such as site location, turbine type, and severity of icing conditions — which vary each year. A better question may be: Are wind-farm owners maximizing the performance of their cold-climate assets by implementing ice-protection systems? Despite an increase in wind-farm development in colder regions and more choices in ice-protection turbine systems, it seems there is a lack of concern.
Just ask Fred Carrier, Founder and Co-President of Hélicarrier Helicopters, a company that operates helicopters for specialized work in extreme environments, such as the Canadian arctic. A helicopter can provide a safer option for turbine blade O&M in cold climates, particularly when iced blades, falling ice, or severe conditions prevent technicians from safely climbing up-tower.
Carrier equipped company’s Eurocopter AS350 B3 chopper with a Simplex Aerial Cleaning and De-Icing System. It is designed for maintaining turbine blades and power lines that experience icing events. Carrier says it is composed of a high-strength, low-weight composite water tank and high-pressure spray boom that can be sprayed over icy blades. “This product is used after an icing event has occurred,” he explains.
However, the chopper-ready de-icing system has been sitting idle for a year. “Initial project plans fell through so, for now the wind-turbine de-icing system sits unused for lack of interest,” explains Carrier. “It is unfortunate because I have no doubt there is a business case — and unnecessary downtime in the wind industry that comes from a lack of proper de-icing measures for blades.”
It is unfortunate, too, that there are no international standards or regulated methods for assessing icing on a turbine or energy loss because of icing conditions at a wind farm. “The wind industry has recently expressed a need for IPS standards or guidelines,” says Wadham-Gagnon. “Some type of guidelines would be especially beneficial for moving forward in IPS performance expectations and warranties.”
According to Wadham-Gagnon, a wind-farm developer or owner should ask three key questions before choosing an IPS for a site.
1. What are the anticipated energy losses at the site due to icing? A site icing assessment may be required.
2. Of the estimated losses, how much power can be recovered with the IPS? This may require some comparisons between systems.
3. What is the cost and durability of the IPS?
Answers to these questions may lead to more cost-effective options. “For example, the project developer may then chose a lower-cost IPS, which may be slightly less efficient but a decent choice for a moderate icing site. For severe icing sites, however, the developer may opt for a high-quality system regardless of cost to maximize turbine up-time and production,” says Wadham-Gagnon.
Additionally, identifying the need for an IPS early in the project’s development typically means more choices and likely at a lower cost than a ‘wait and see’ approach. “If icing is only identified after a project is in operation, and no IPS has been integrated or applied to the turbines, there are fewer options. Certainly, retrofit systems are available but they are limited and generally more expensive compared to an integrated solution,” he says.
Although there is no “one size fits all” system for wind turbines situated in cold climates, Wadham-Gagnon says a well-organized O&M plan is a must. “Iced blades equate to lost production time at a wind farm whether that’s because a turbine has stopped working or because technicians cannot safely access it for maintenance due to the risk of shredded and falling ice from the blades. This can lead to unnecessary and extended downtime, so consider your options and plan ahead.”
Optimizing turbines in icing conditions
TechnoCentre éolien (TCE) is working on a three-year project to optimize the control function of Senvion MM wind turbines in icing conditions at several wind farms in Quebec, Canada.
The aim is to maximize energy production while minimizing risk to the structural integrity of the turbines and its components. The turbines will be fitted with meteorological instruments and icing sensors, and the meteorological, operational, and structural load data will be analyzed and correlated.
Based on the results, TCE intends to adjust the turbines’ control function for optimal production in cold climates.
“Over the past few years, we developed an expertise in icing characterization and video image analysis. Today, this expertise lets us undertake this large and important research project with Senvion Canada,” said Frédéric Côté, General Manager of TCE. “TechnoCentre éolien intends to help improve icing detection, validate icing loads, and optimize the turbine’s control parameter during icing events.”
Ice-prevention systems at work
The International Energy Agency (IEA Wind Task 19) published a 2016 report on “Available Technologies for Wind Energy in Cold Climate,” which states there are at least eight OEMs that now offer ice-protection systems for turbine blades, and four independent suppliers. Here are some of those products.
- Gamesa’s Bladeshield consists of an anti-icing “paint” formulated to prevent the formation of ice on turbine blades and boost the product’s resistance to erosion. “Most anti-icing solutions on the markets reduce blade paint´s resistance to erosion,” explains José Antonio Malumbres, Gamesa’s Chief Technology Officer. “Gamesa has attempted to remain one step ahead, using nanomaterials to create a system that not only prevents ice formation but also improves anti-erosion performance.”
- Vestas’ De-icing System (VDS) was developed to detect and efficiently remove ice formed on wind-turbine blades, while letting turbines operate at full power. VDS is an active de-icing system that consists of an ice-detection system and a hot airflow unit built into the blades. “The hot air targets a blade’s most critical parts to efficiently melt ice build-up without negative impact on the noise level or overall turbine performance,” said Chief Technology Officer Anders Vedel. Vestas also offers an integrated ice-detection (VID) option that stops turbine operation when ice has build-up and certain condition are met. This is to prevent the risk of ice throw.
- Nordex’s Anti-Icing System consists of an ice sensor mounted on a nacelle and heating devices built into turbine blades. The sensor continuously monitors conditions and, if an icing event appears likely, the blade’s heating elements are automatically activated. There is no requirement to stop or reduce turbine operation while the anti-icing system is at work.