Sun & Wind Energy 2/201432

Just 3 % of wind turbines have fire extinguishing systems installed. Photos: Police department Stade (3), dpa (3)

Fire-Fighting Wind energy

Wind energy opponents often use it as a counter argument:

you cannot extinguish a burning wind turbine and it

therefore damages its surroundings. Unfortunately this

allegation is correct – far too few wind turbines have a

fire-fighting system in place.

A neglected risk

New Year’s Eve 2010 is a night that fire crews in the small town of Norden, in the German region of Frisia, will not soon forget. For the

firefighters, celebrations came to an abrupt halt just after 11pm. A burning wind turbine lit up the area like a torch. The nacelle was ablaze, showering burning debris onto the ground from a height of some 70 m. However, the numerous members of the fire and emergency services who arrived at the scene could do practically nothing, apart from what firefighters refer to in their jargon as a “controlled burn.”

In the case of such advanced fires, direct fire-fighting is no longer an option. Even if the fire had just gotten started, the fire crews could not have done anything, since standard turntable ladders are only 30 m long. When it comes to dealing with a fire in the nacelle, all firemen can do is prevent injuries and material damage caused by burning debris – nothing more. Helpers become bystanders.

Particularly tragic are those cases where people are trapped in the nacelle by flames. In October 2013, a wind turbine in Oolgensplaat in the Netherlands caught fire, preventing the two technicians from reaching the abseil hatch and thus blocking their descent from the tower. While one of the men fell from the tower, his colleague burned in the nacelle.

In view of such cases, the small number of wind turbines with fire-fighting systems in place is a total mystery. Fire protection expert Axel Wörner manages the Wind & Special Hazards competence centre at

Minimax GmbH & Co KG. His estimate is that, “Just 3 % of wind turbines have fire extinguishing systems installed.”

Locating the fire

Reliable and technologically advanced detection sys-tems increase the chances of preserving valuable assets or even life and limb by means of a fire- fighting system adapted to its location.

State-of-the-art fire-fighting technology is distin-guished by four concepts. Heat detectors measure the temperature difference within a period of time – based on the consideration of temperature rise. How-ever, fires unfold in a specific chronology. First, smoke appears, and then flammable material ignites. The temperature in the area surrounding the source of the fire rises much more slowly than it takes for smoke and flames to develop.

A flame detector – the second sensor concept – will therefore trigger an alarm before a heat detector will. “A flame detector reacts to the opacity of the air between the transmitter and the receiver,” explains Wörner. Additional smoke aspiration, as is the case with wind turbines, increases the reliability of these detectors.

The third class of detectors reacts to combustion gases such as carbon monoxide. Ionisation detectors are triggered when the normal air is displaced by combustion gases and conducts electricity between

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Wind energy Fire-Fighting

Sun & Wind Energy 2/201434

two poles. Flame detectors as the fourth method react to the infrared and ultraviolet radiation from flames – they can see the fire.

A fire in a wind turbine typically conforms to one of two scenarios. If the turbine is in operation, the fire is usually fought by means of automatic extin-guishing equipment in the nacelle. This can be either a low-pressure system with an operating pressure of 10 bar at the nozzle, or a high-pressure system with 100 bar. Around 90 % of wind turbines fitted with fire prevention equipment rely on a high-pressure system.

Stopping the fire

In the event of a fire, a heat detector or combination of detectors activates the high-pressure atomised spray system, which releases a very fine mist of demineralised water. The drops are just a few dozen micrometres large. The enormous size of the mist’s reactive surface thus draws the heat out of the fire that it requires for combustion. The mist also has a suffocating effect, as it displaces oxygen. The water

tank is heated to ensure it is ready for operation, even in periods of frost. The system runs on a battery, and a 230 V charger fed either by electricity from the tur-bine itself or from the grid ensures the extinguishing system is always ready for action.

Fine-spray technology is another effective method and is available from Minimax competitor Protec Fire GmbH, among others. These systems uses larger drops, because fine nebulisation of water, which is also used, is not suitable for every location. If there is movement in the internal air, the kinetic energy of the fine droplets of water is not always sufficient to ensure that they reach the source of the fire.

In a Protec Fire system, the extinguishing func-tions are also triggered by a heat detector. The Protec Fire fire protection fluid is a pre-mixed extinguishing agent which can be used down to minus 30 °C, and in a special formulation, down to minus 40 °C. Like water, the extinguishing agent cools and smothers the fire. It also forms a film on flammable hydrocarbons, such as lubricating greases and oils, which covers the flammable surfaces.

Protec Fire uses several heat detectors which can be individually adjusted according to the trigger tem-perature and each control a separate section of the wind turbine. These heat detectors are usually set to a trigger at temperature of around 30 °C above the maximum operating temperature of the monitored sections of the turbine. They also control the extin-guishing function. “The extinguishing equipment is only activated where there is a fire,” says Sales Manager Michael Zölck.

The equipment of an extinguishing system in a 2 to 3 MW class wind turbine contains 800 litres of extinguishing agent (water or foam). There is no way to rule out the fact that switch cabinets may also be covered by water or foam. “The operators accept it may be a total write-off,” says Wörner. The economic decision is relatively simple: sacrifice the switch cabinet to save the turbine as a whole.

Escaping the fire

The second scenario involves a wind turbine that is out of operation for maintenance or repair purposes. The service engineers are on the ground or in the na-celle. In the – somewhat less common – event that the engineers are on the ground, the rule is simple: it goes without saying that climbing the wind turbine to extinguish the fire with fire extinguishers, blankets or other means is futile. Moreover, it is unnecessary, says Wörner.

However, in the far more common case of the idle-turbine scenario, the engineers are in the nacelle when the fire breaks out. In general, the nacelle should be evacuated immediately in the event of a fire. If, that is, initial attempts at extinguishing the fire do not work. Every large fire starts out as a small fire and should be fought in the initial stages with simple fire extinguishers. These offer the maximum effect with a minimum of effort – if used correctly. “You have to extinguish the fire before it is

A burning converter in the tower base can usually be deleted. Photo: BFK Mödling/Herbert Wimmer

Wind Edition

self-sustaining. But that assumes that the main-tenance engineer has been trained in how to fight fires,” says Wörner. The importance of this is illustrat-ed by the case of a fire in a wind turbine in Böbs, in the German Federal State of Schleswig-Holstein. “Two people were injured because they didn’t know how to use the fire extinguishers,” says Wörner. “You have to have done it once,” is the guiding principle of fire-fighting training courses and instructions on using fire extinguishers .

But even that does not guarantee success. If at-tempts to extinguish the fire prove fruitless, there is only one thing left to do: evacuate the nacelle imme-diately. Wind turbine manufacturers and the safety industry offer various technical solutions for evacuat-ing the nacelle. One of the most well-known rescue devices is the Milan 2.0 from Skylotec. The device is an unofficial standard, and competitors offer similar products.

• A centrifugally controlled brake keeps the speed constant during the descent.

• The system has an aluminium housing and twin-bearing brake shaft.

• It is certified according to international standards by established certification bodies such as DNV GL.

• Extensions to the basic version come with a hoist-ing function. C

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In the event of a fire in a wind turbine, there is usually only one option: let the fire burn in a controlled manner. Photo: Police department Stade

Wind energy Fire-Fighting

Apart from that, offshore wind turbines present the same scenarios as onshore.

Assuming that no personnel are involved, a fire on an offshore wind turbine is not a question of sur-vival for the operator. However, an extensive fire on a transformer platform would be a catastrophe. It would put a wind farm out of action for months and result in massive financial losses.

Using the fire extinguisher properly, even the fire of a wind turbine can be put out in the early stage. Graphic: Firefighters

Tackle the fire from

downwind.

Fight the trouble spot, not the

flames.

Use several extinguishers at once,

not successively.

Be aware of re-ignition.

Extinguishing the fire in the initial stage

The more the better

Fire on an offshore wind turbine is a particularly pre-carious situation. A fire on board a ship is one of the most feared scenarios among mariners. Fleeing the fire – if it is possible at all – involves entering an other threatening environment, the sea. If the evacuation goes wrong, another struggle for survival begins.

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The enormous losses that would accrue in such a case prompt operators to have a tailor-made fire pro-tection system designed for the transformer platform. And such designs make use of all the available tech-nology. “One system isn’t enough,” says the Head of Wagner Group GmbH, Werner Wagner, “We received the contract for planning a comprehensive protection scheme. Instead of relying on a single system, the planning team used a combination of different systems and technologies. The result is a complex system consisting of very early smoke detection, fire prevention, gas extinguishing technology, sprinklers, foam extinguishing systems and a fire risk management system.”

No oxygen, no fire

The ultimate in automatic fire-fighting systems is extin-guishing with gas. Nitrogen robs the fire of oxygen. In addition to flammable material and heat, oxygen is the third essential prerequisite for a fire – no oxygen, no fire. Such a system requires six to eight gas cylinders, which are usually located at the foot of the tower.

Minimax expert Wörner explains how the fittings are designed according to temperature parameters, as different pressures are used in the cylinders for different climatic zones around the globe. In a normal climate, at an average temperature of 15 °C, the gas pressure is 200 bar, but in tropical regions it reaches

208 bar, and in arctic locations it falls below 200 bar. “This requires different fittings,” states Wörner, iden-tifying this adaptation of the pipes and valves as a unique selling point of the Minimax system.

Wörner also lists the low running costs of using gas as one of the advantages of gas extinguishing systems. The nearly € 100,000 investment in the system is some 2.5 times the cost of a water mist sys-tem. But the system requires practically no clean-up after use. In contrast, Wörner quotes a case of clean-ing up the aftermath of a fire extinguished by a water mist system costing € 170,000.

However, compared to the costs of total failure which can stretch over several months, that is peanuts. The experts therefore agree on one thing: in the long run, fire-fighting systems are here to stay. One interest-ing aspect of this issue is that the damage which can occur to the wind turbine is not subject of much of the attention, but rather the risk to the surrounding area.

The insurance industry provides material assis-tance. Insurance company Axa offers a discount of up to 10 % on annual premiums for turbine operators that install fire protection systems. This should also sensitise the sector to the issue. There are no statu-tory regulations and no standard for fire protection systems. However, manufacturers must guarantee state-of-the-art technology. But this requirement only becomes an issue in the event of damage.

Jörn Iken