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| Gearbox being changed at a wind farm in Wexford with 2.5MW Nordex turbines. This wind farm was installed in 2013. (Source:www.windnoise.info) |
A new study by Irish engineer, John Dooley, based on information from Denmark, casts doubt on the lifetime of larger wind turbines. It has been assumed in most business plans and by most financiers that the lifetime of these turbines are 20 years, the same as the smaller older models. If this is not the case, there is trouble ahead for newer wind farms and their investors - only time will tell.
The report can be accessed here:
http://docs.wind-watch.org/Danish-Wind-Turbines-from-1977-to-2014.pdf
A two and a half mw turbine broke down after 3 years service in Wexford and there are numerous examples of others. German Turbine maker Siemens have placed a provision in their financial accounts to plan for the expense of repairing turbines under warranty. So what could be the problem?. Step down gearboxes (as in a car) are easy of themselves and their housing and bearings. Step up gearboxes are the opposite. Simply, when a very big cog wheel drives a small one under heavy torque resistance , tremendous strain is placed on the bearings, shafts and housings.Also the main bearings in the main shaft seems to be failing, While designs vary, many have tapered roller bearings and one roller alone could weight 30 kg. Keeping these moving uses a lot of the turbines energy. If proper hardened and ground steels is used, the weight uses too much energy, so they use lighter allows which are not as long lasting. Another problem is that normally gear trains are driven in only one direction, but turbines can be driven from the grid in calm weather. Also there can be enormous braking forces in strong winds to prevent the turbine running away. Information is patchy, but there are design problems with big turbines which are hard to get over. The blades must be kept below about 20 revvs per minute in order to keep tip speed down and centripetal forces down. The alternator must run at about 1750 rpm or more to give a decent output. That's a step up of 87.5 to 1. If the output is 2 mw, that's enough to power 1,000 x 2kw vacuum cleaners. A fossil fuel generator is direct drive. Tractor driven generators were a virtual failure for the same reason. With engine @ 5,000 rpm, the power take off is only 750 rpm, (step down) this must be stepped up to c 2,500 rpm before entering the generator. They are not as good as direct drive ones. This could have enormous problems if an investor bought a wind farm on commissioning, It may need expensive repairs in a few tears. I heard a figure of 600,000 Euros in one case.
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