Built-in fire brigade: water vs nitrogen

Wind turbines are essentially unmanned power stations, with access only intended for maintenance and inspection. But as with any such machinery of this sort – containing mineral oils, lubricants, plastics and electronic components – fire is an issue and over the years there have been a number of incidents around the world, eg in February at the Crockahenny wind farm in Ireland. It is therefore essential that early warning systems and, if the worst comes to the worst, fire extinguishing units are able to operate autonomously.

A burning nacelle at a height of 100 m – unreachable by any turntable ladder and water jet – poses something of a challenge. According to Walter Köhne of GL Wind, the answer lies in a combination of passive, preventive fire protection and intelligent systems for the rapid detection and suppression of an incipient fire.

Modern multi megawatt wind turbine nacelles have reached the dimensions of a single-family home. But inside they are anything but spacious: machinery components weighing tonnes rotate in oil baths surrounded by bulky switchgear cabinets and cables several centimetres thick. If a fire were to break out as a result of overheating, electrical malfunctions or a lightning strike, the flames would find plenty of fuel.

Although fire in a wind turbine has serious consequences, active firefighting systems are not very widespread. Worldwide, Köhne estimates, such systems have only been installed in about twenty plants. Up until recently, fire protection was not much of a topic for discussion. But that has changed. The increase in hub heights, to the 80-160 m mark, has made effective firefighting from the outside all but impossible. There is a danger of total loss, no small sum in the case of multi-megawatt units. And with the rising number of offshore installations, Köhne sees the topic of fire moving up the agenda – clearly the follow-on costs of fire damage at sea are much higher than onshore.

Active fire protection is attracting increasing interest from the underwriting industry. Various insurance companies have already announced that they will offer reduced fire risk premiums for turbines that have an automatic fire protection system and GL Wind is currently working on guidelines defining the minimum requirements for fire protection in wind turbines.

The prerequisites for the fire protection systems are being developed and established in co-operation with the Allianz Centre for Technology (AZT). In addition, the insurance industry is preparing a guide to fire protection in wind turbines.

Nitrogen vs water mist

There are basically two systems currently available for combating an incipient fire, one using nitrogen gas as the extinguishing agent the other water mist. Gas is an ideal extinguishant for electrical systems, because it displaces the oxygen needed for combustion, thereby smothering the flames. However, the site of the incipient fire must be isolated from the outside air, to reduce the oxygen concentration from 21 to 13 per cent or less. The advantage of firefighting with nitrogen is that, unlike powder or water, it leaves no residues. As a result, the damage due to the firefighting is usually less than the actual fire damage. Gas extinguishing systems are ideal for suppressing incipient fires – but only in combination with sophisticated sensors that “sniff out” the fire at an early stage. A drawback of the gas-based system is the relatively large space required. For each 15 cubic metres of space to be protected, a 30 kg gas cylinder weighing a total of 150 kg is needed. For a megawatt class wind turbine nacelle, ten cylinders or more are required – often, these can only be kept in the tower base. Also, malfunctions of the system are hazardous for anyone who might be in the nacelle at the time: the concentration of nitrogen involved is fatal.

The principle of the water mist option is simple yet effective: nozzles are used to produce a fine water mist, which immediately and radically reduces the fire temperature. At the same time, evaporation increases the water volume by a factor of 1640, displacing the oxygen. There is hardly any fire that can withstand the effect of the water mist, which is ejected at a pressure of 120 bar.

According to information from the manufacturers, people survive water mist unscathed. Another major advantage of water mist is that it is not necessary to seal the nacelle interior off from the outside air. In addition, the water mist system acts much more rapidly – the fire should be extinguished 20 to 30 seconds after triggering of the alarm.

Disadvantages of water mist? Critics say the use of water in electrical systems is problematic owing to the high level of consequential damage. But suppliers of the systems argue that high pressure water mist is so finely atomised that electrical installations suffer little damage. Köhne can cite test results that confirm this. He also points out that by the time the water mist is triggered, the electrical systems will have already been disconnected or have failed anyway – so will not be energised.

The best of both, at sea

Combined systems aim to have the best of both worlds: nitrogen for the switch cabinets, water mist for the rest of the nacelle. The water required for the nacelle of a 1.5 MW plant is about 200 litres. This is roughly the volume of a domestic hot water tank, so is not likely to create space problems. The small reservoir of nitrogen for the switch cabinets – about three to five cylinders – can also easily be accommodated in the nacelle.

This dual approach, implemented in the form of a prefabricated unit, is particularly well suited to offshore installations. It “will probably become standard equipment offshore,” predicts Walter Köhne.

Based on article in Germanischer Lloyd Nonstop magazine, 1/2007. For further information contact Walter Köhne, Germanischer Lloyd Industrial Services GmbH, Competence Centre Wind Energy, Machinery Components and Safety, +49 40 36149-7186, walter.koehne@gl-group.com