Wind power is an important and growing source of renewable energy in Europe. Due to the wind conditions, offshore wind power is a promising option. Hence, optimization and reducing disruption to production of any kind is of interest. One cause of interruption for wind power stations is the occurrence of fire within the wind turbine. It is estimated that 0.3-0.5 fire incidents of this nature occur per 1000 power stations (onshore and offshore) per year. SP Technical Research Institute of Sweden carried out a study providing a state of the art of fires in wind turbines and lifting a series of questions on the topic. As a consequence a research project is launched aiming at providing solutions and reducing the risk for fires in wind turbines.
There is a societal push to identify and optimise alternative sources of power generation. In 2013, wind power provided 1% of total energy consumption worldwide. Offshore wind power represents about 2% of global installed capacity, a total of 8759 MW. At present, more than 91% (8045 MW) of all offshore wind installations can be found in European waters, mainly in the North Sea (5094 MW: 63%), the Atlantic Ocean (1808 MW: 22%) and the Baltic Sea (1142 MW:14%).
It is of interest to optimise the output of this green and blue source of energy and to reduce losses. Fire in wind turbines has been identified as one possible cause of loss and an aspect of safety for workers. Hence, SP Safety at the Technical Research Institute of Sweden carried out a study on fire primarily in offshore wind turbines. The study revealed a series of challenges, which were discussed at a workshop in September 2016 and will be dealt with in a forthcoming research project.
Background on fire safety in wind turbines
Offshore wind power is a growing source of energy. In Sweden, the general trend in recent years is that wind power stations have been built in increasing numbers. In 2011, about 200,000 wind turbines existed worldwide. An increasing number are being built offshore but also at increasing distances from the shoreline. In 2013, the first turbines were erected at distances of more than 100 km from the coast. At the beginning of 2013, 1662 power stations were located offshore. Wind energy has grown from a global generation capacity of 6 GW in 1996 to 369 GW in 2014. Global offshore wind power production increased to 8.8 GW in 2014. To indicate the relativity generation capacity of wind energy, the total world electricity generation in 2013 was 23322 TWh.
There is a beneficial environmental impact from optimising this source of energy production and hence minimising all significant sources of disruption to production. The occurrence of fire in wind turbines is one type of disruption and the focus of a study conducted by SP Safety at the Technical Research Institute of Sweden on offshore wind turbines. Currently, limited statistical information is available in the public domain relating to wind power plants (both onshore and offshore) and their functionality.
The study shows that, in relation to both onshore and offshore wind turbines, 10-30% of all loss-of-power-generation incidents in wind power plants are due to fire. According to Caithness Windfarm Information Forum data, the annual number of incidents from 2007-2012 was steady at 120-160 worldwide. A study by G. Rein, Reader at Imperial College, London, supported by anonymous suggestion that this represents only about 10% of the actual fire incidents. Unfortunately, the validity of the data is uncertain, as there is currently no formal process for recording incidents.
In the study by the Technical Research Institute of Sweden, the number of wind power station fires varied depending on the source of information. The most reliable source is estimated to be DNV GL (Det Norske Veritas Germanischer Lloyd), which collects statistics and cites 0.5 fires per 1000 turbines a year.
With respect to the frequency of occurrence, the problem with fire may appear minor compared to general problems with quality and mechanical failures in offshore wind power stations. However, wind turbine fires tend to cause substantial losses (equal to or above the original cost of the wind power plant), especially offshore. Offshore wind power stations are more difficult to access in case of fire. With onshore wind power stations, however, fire may spread and cause wildfires. Causes of fire incidents in onshore and offshore installations appear to be similar. The fires in wind turbines not only lead to business discontinuation but also are mainly a safety issue. In 2013, a crew of four engineers died in Ooltgensplaat, Netherlands in a wind turbine fire.
Fire causes are currently only registered by insurance companies when the loss exceeds the deductible, i.e. when the insurer actually has to make a pay-out. Various sources of information indicate the following range in fire incidents. The highest frequency was fire after maintenance, followed by lightning strikes, which still lead to fires though the number has been reduced with design changes, electrical failure, hot surface ignition, hot work maintenance and cooking. Three out of six incidents involve a human presence in the nacelle; hence, a fire becomes a safety concern.
For a fire to start, a source of ignition and combustible material is needed. Possible sources of ignition are lightning, overheating, surface ignition and electrical fault. The presence of ignition sources and combustible material creates the possibility of ignition and fire development. Combustible materials inside a wind turbine can include composite materials found in the blades and the walls of the nacelle and the low speed shaft. Furthermore, highly combustible liquids such as gearbox, hydraulic and lubricating oils are used in various parts of the turbine inside the nacelle, the gearbox, the yaw, the hydraulic system, the blade pitch change mechanism, oil pumps, the mechanical brake and the oil-filled transformer. However, details of the design will influence the particular outcome in the event of a fire occurring.
Fire safety engineering provides two types of fire protection systems, namely active fire protection and passive fire protection. Both active and passive fire protection systems play an important role in fire safety in wind turbines. These systems help to achieve the desired fire safety objectives, whether it is the safety of people, property protection, environmental impact or business continuity. In the context of wind turbines, the basic principles of fire protection systems apply whether the location is onshore or offshore.
The active fire protection systems applicable in relation to wind turbines comprise detection (flame, heat, gas and smoke), warning of staff and rescue service, as well as the activation of systems for fire control or extinguishing. However, active systems may lead to the application of control or extinguishing systems and the release of gas or aerosols. These are intrusive and can in some cases lead to non-thermal damage of the electric devices in the nacelle.
Passive fire protection achieves safety by design and includes material selection, compartmentalization and other measures to minimise the fire spread. Containment is achieved by the use of fire- and smoke-rated elements of construction such as walls, floors, doors, fire-stopping, protective coatings, etc. Reaction-to-fire performance characteristics are achieved by selecting materials that have low combustibility and smoke production properties, which are either inherent or due to modification of the materials’ fire performance. Fire resistance is generally achieved by protecting vulnerable elements of construction and/or designing for residual performance. There are various sources in the international literature of guidance and recommendations on how passive fire protection systems can improve fire safety in wind turbines.
With respect to human safety, national health and safety rules apply. Depending on the location and ownership of the turbine, the development of an emergency response plan can be one of the requirements. It is recommended that the operator of an offshore wind facility provide a detailed fire safety plan that contains details such as an Emergency Response Plan and an Emergency Evacuation Plan. This documentation should be developed in collaboration with the relevant rescue services. Furthermore, it is recommended to carry out relevant training for workers as well as possible firefighters.
The results from the study by SP are an incitement to look deeper in to the topic of fire safety of wind turbines. Combustible materials as well as ignition mechanisms have been identified in power plant designs. Although the blades are constructed from combustible materials, potential ignition sources are mainly inside the nacelle, where there are hot surfaces in the gearbox, generator, brake system, pumps and transformer. In combination with the possible presence of combustible hydraulic and lubricant oil and solid combustible material in the nacelle, a fire can ignite and develop, leading to the possible complete destruction of the power station.
Appropriate fire safety engineering in the design phase is important with respect to safety and business continuation.
Further work: A research project on fires in wind turbines is launched
The SP Technical Research Institute of Sweden arranged a conference and workshop in September 2016 in Copenhagen, where recognised researchers in the field of fire safety discussed the topic of wind turbine fires with turbine manufacturers, energy suppliers and delegates representing the industry of active and passive fire protection systems. A research project was kicked off. The project shall deal with questions that arose at the workshop. Funding is currently being sought. The project will be led by Dr Anne Dederichs, a senior research scientist at the SP Technical Research Institute of Sweden and Associate Professor at the Technical University of Denmark, and Dr Guillermo Rein, Reader at Imperial College, London.
- How can a reliable collection of data on fires in wind turbines be achieved?
- How can the impact of fires in wind turbines be mitigated by using active and passive fire protection measures?
- What design criteria with respect to active and passive fire protection technology are needed to minimise the risk of wind turbine fires?
- Can we achieve agreement on guidelines and possibly standards for fire-safe wind turbine design?
- Can we achieve agreement on testing standards for wind turbines with respect to fires?
- How can numerical tools be applied in wind turbine design to increase the fire safety of the stations?