Turbine technology gears up

As the UK ramps up its wind energy resources, Paul Fanning looks at the technological challenges facing the sector and the innovations that are overcoming them.

With the country committed to generating 15% of its energy from renewable sources by 2020, the challenge facing the UK's energy industry is, by any standards, enormous.

But that bare figure almost serves to obscure the scale of the task. The issue is put into clearer focus when one considers that the level of renewable energy currently being generated in the UK is just 2% of the total and that the 4GW capacity that now exists will have to rise to more than 30GW by 2020. Or, to put it another way, onshore wind farms will have to increase the number of homes they supply with electricity from 2million today to 7m by 2020.
Whether this will be achieved and how remain major questions. As things stand, wind energy is clearly seen as leading the way. According to renewable energy business association RenewableUK, there are 4GW of wind plants installed and a further 9GW are either being built or have planning permission and are going into construction. By the end of 2012, it is estimated that there should be 10GW of wind energy operating in the UK. In addition, there are currently 10GW of schemes awaiting planning permission, meaning that more than 23GW of wind plant is in development in the UK, not including the 32GW of offshore wind licences recently awarded by the Crown Estates in the Round 3 process.

These figures may make the 30GW target by 2020 seem more achievable, but success will ultimately depend upon the technology employed to achieve it. Over a relatively short time, wind energy technology has advanced considerably in response to the lessons learned. Some of these lessons have been bitter. Failed wind turbines generally make for serious potential risk, damaging headlines and (as even the briefest search on YouTube shows) spectacular and popular footage. For this reason, much of the innovation of the past 20 years has been in increasing the reliability of wind turbines.

Wind turbines are not only under extreme mechanical strain, they are also exposed to high risks in terms of storm, lightning, fire and ice. Because of the extreme strain on the materials, there are often high damages. The insurance industry demands more time for the development and test of new constructions, and higher standards for maintenance and repairs. Replacing a gearbox in a large wind turbine normally requires using a crane capable of lifting more than 100 tonnes to and from a height that can be more than 100m. As can be imagined, that is a costly process, both in terms of the replacement costs and the lost power generation time, particularly as such failures will usually take place in winter when the wind is at its strongest and cannot be replaced until the following spring or summer.

Gearbox bearings are a common source of turbine failure. The high dynamic loads to which wind turbine gearboxes are subject are one problem, but another is the variability in load due to changing wind condition. The variation in load spectrum from high peak to low loads places considerable – and often contradictory – demands on the bearings. The high static safety required for maximum load means that bearings with high load-carrying capacity are required, but when there is little wind and loads are low, this can lead to damage due to sliding of the rolling element set.

A consortium led by engineering consultancy Ricardo has developed a possible solution to this problem that could increase bearing life fivefold. Investigation by Ricardo showed classic fault categories, such as unequal load distribution applied to the bearings in epicyclic gears, or running at partial turbine power, when the rolling elements are prone to skid rather than roll and cause scuffing of the precision ground surfaces. Whatever the cause, however, wear on the inner bearing ring is invariably concentrated over a small arc of some 40°, while the remainder of the ring remains unworn. Says Dr Jonathan Wheals, chief engineer at Ricardo:"We thought if the inner ring only suffers damage over a 40° arc, what if we indexed it around so that the wear was distributed more evenly?"

The solution the consortium designed involved a conical clamp forced between the inner race of the bearing and its associated shaft by means of a large Belville spring. The clamp is actuated by an oscillating piston acting on a roller ramp using a single pulsed hydraulic supply from the bulk gearbox oil. The same mechanism is then used to rotate the race. The mechanism achieves fully failsafe operation because, if the oil pressure fails at any stage, then the device is spring-driven to a state in which the race is locked to the shaft.

An actuation ring drives the roller ramp and is itself held in place by a spring-loaded pawl. If pressure is lost at any stage, then the anti-reverse pawl disengages and the mechanism unwinds under the action force of the conical spring acting upon the roller-ramp mechanism.

Because it is critical to know the exact degree of bearing rotation, a 'percussion bell' is used to amplify the sound made by the pawl as the actuation ring is rotated. By analysing these sounds using existing condition monitoring systems, it is possible to determine the position of the bearing accurately. While the incremental costs of these bearings are estimated as being around 35% more than existing solutions, the cost savings possible in terms of O&M would more than compensate for this. The system is currently being prototyped in conjunction with a large bearings manufacturer, while testing will take place at Ricardo's Leamington facility.

As increasing numbers of wind turbines move out of their manufacturers' warranty period, third party suppliers are increasingly in a position to apply new technologies to increase the longevity and reliability of existing turbines. As Dr Wheals puts it: "Retrofit is very attractive because many turbines have ailments and are coming out of their warranty periods."

Clearly, the wear and tear on gearboxes means that monitoring is another key technology for avoiding failures. According to Matt Fielder, wind energy manager of Parker Hannifin Europe's Hydraulic Filter Division, the use of the latest particle counting technology and telemetry systems are being used to extend the life of these systems.

Says Fielder: "Offshore wind turbines in particular take an absolute pounding and if you look at the mean time between failure rates of some of these things, they're not actually that great. One of the most common failure areas is obviously in the gearbox, and an offshore gearbox is not an easy thing to replace. The ability to be able to determine or monitor when you are going to have a catastrophic failure as a result of wear metal debris build up in the oil, is something that's not really been looked at.

"They use these things called wear metal sensors, which are monitoring the large pieces of debris coming from gear teeth and bearings in the turbine, but what the hydraulics industry has been doing for many years is using laser particle counting to monitor the much, much finer particles of contamination that are building up because of wear metals failure."

One of the solutions is to undertake particle counting, but, says Fielder: "One of the problems we face with the gearbox industry is that we're using very high-viscosity oils and also we have 'splash feed', which means we have a lot of air bubbles in the oil. One of the problems with particle counting is that you have to make sure there are no air bubbles in the oil or that gives spurious readings. So what we're doing is developing a system to extract oil from a gearbox, pressurise it to in excess of 100bar to suppress any air bubbles and that enables us to give a proper count."

Another innovation being employed by Parker to extend gearbox life is to employ larger filter housings to prevent bypass of oil on cold start-up. Says Fielder: "If you imagine you have a turbine with 300cts oil in the North Sea and the oil is at 2 or 3°C. The oil will be very, very thick – in excess of 1000cts. With a lot of filters, if they're not correctly sized then they will go into bypass straight away, which allows dirty, contaminated oil to go past the filters and straight into the gearbox. So we're trying to develop systems that prevent bypass on start-up."

Also a threat to bearings is the fluting or pitting caused by a build-up of electrostatic energy resulting from the current that runs down the shaft between the generator and the gearbox being insufficiently grounded. While Morgan Carbon's European electrical director Rob Threapleton concedes that this may not be the largest cause of gearbox failure, he nonetheless believes it to be a significant factor. "I do think there is a universal problem with grounding of shafts."

With this problem in mind, Morgan Carbon is currenty developing a product based on its existing 'Aegis Ring' (a grounding ring shown to be highly effective in other applications). Says Threapleton: "We are developing this technology alongside another company to be suited robustly and effectively for the wind market. It potentially can guard against generator and gearbox problems, which are the biggest threat in terms of O&M costs and will potentially save the industry hundreds of thousands of pounds." The solution is currently under test.

Of course, one of the best ways to avoid gearbox problems is to dispense with the gearbox altogether. Increasingly, direct drive systems have been developed. The last couple of years has seen the emergence of commercial-scale, direct drive permanent magnet generator [PMG] systems, with the hub directly connected to the generator. In order to achieve this, however, a much larger diameter generator is required, to accommodate the required increase in the number of magnetic poles on the rotor.

The result is a system with significantly increased reliability and reduced maintenance costs. Reduced downtime for maintenance also means less non-producing time offline. The elimination of associated mechanical losses that are inevitable with gearboxes, also leads to improved efficiencies in the power conversion process. The generator itself is also more robust than conventional systems, and gives greater efficiencies when wind speeds are not at full rating, compared to the earlier designs. Stephan Ritter, GE's general manager Renewable Energies in Europe, revealed that the company acquired the Norwegian company Scanwind because it makes direct drive wind turbines that do not require gearboxes. There are currently 14 of its 3.5MW 90m turbines operating in the Hundhammerfjellet wind farm on the West coast of Norway.

Ritter says the company used permanent magnet generators 'as a big as a Dutch house' and the whole nacelle weighed 'around 250 tonnes'. The plan was to make the design more commercial, although Ritter remarked: "We are going to change it as little as possible. Reliability is what we are going to focus on as well as a bigger rotor, taking capacity to 4MW. This year," he says, "we will be onshore, but the next step is wet feet, then scale production up slowly to 50 to 100 units, with first commercial products in the water in 2013."

The advantages of direct drive wind turbines are clear: to simplify the nacelle systems, increase reliability, increase efficiency and avoid gearbox issues. A general trend towards direct drive systems has been evident for some years, although there are considerable challenges in producing technology that is lighter or more cost-effective than the conventional geared drive trains. Although these developments continue, direct drive turbines have not, as yet, achieved a sizeable market share. The problem with direct drive and PMGs is that they are expensive and demand huge quantities of permanent magnet materials. Scanwind's 3500 DL wind turbine, rated to 3.5 MW of power, uses more than 2000 kg [4400 lb] of high energy neodymium-based [Nd-Fe-B] permanent magnet material. This equates to approximately 0.6 kg [1.3 lb]/kW produced.

Dr Wheals of Ricardo Engineering is sceptical about the long-term viability of the permanent magnet-based solution precisely because of the size of magnets necessary. "Such has been the gearbox failure rate with wind turbines," he says, "that it has given an impetus to people to find other solutions rather than improving the gearboxes. Many suppliers have taken fright and moved to direct drive, but in terms of mast top mass reduction, it's precisely the wrong solution." He also points out that China's intention to reduce its quota of rare earth materials (from which permanent magnets are made and of which China produces 95% of the world's supply) to just 35,000 tons per year by 2015 may well mean that 'there won't be enough neodymium, even if we want it'.

Dr Gary Taylor of Brunel University's Institute of Power Systems acknowledges these problems, but is less pessimistic. "There are issues with permanent magnet generators in terms of size, materials and decommissioning, but it is being taken seriously by a lot of OEMs." However, he does concede that the notable exception is Vestas, the world's largest manufacturer of wind turbines, which has stuck with geared machines.

An alternative to the direct drive system can be seen in the 'hybrid' drive train employed by Clipper Wind, which manufactures 2.5MW wind turbines. After initial research into systems with multiple induction generators, Clipper developed a system with an innovative gearbox with outputs to four permanent magnet generators. As with other hybrids, this again leads to a very compact drive train. Clipper Windpower Marine, a subsidiary of Clipper Windpower, has obtained £4.4 million DECC (UK Department for Energy and Climate Change) funding for development of blades in the 'Britannia Project', a 10 MW offshore wind turbine scheduled for deployment in 2011 and being built in Tyneside.

The question of how to generate these higher levels of energy from wind turbines is a vexed one. There are strong suggestions that the answer will be found in the use of high temperature superconducting [HTS] materials in the generator systems. Such materials allow significant increase in power density compared to wound copper or permanent magnet machines, as well as potentially offering significant benefits in terms of size and weight. One of the companies involved in this research, AML Energy, is working to incorporate its proprietary Double-Helix technology into a 10 MW direct drive HTS generator system for wind turbines. It claims that this technology, once proven, will be scalable to 20 to 30 MW power outputs. It reports that the design will be 75% lighter and 50% smaller turbines than the best turbines available today, with greater efficiency and reliability of operation.

Author
Paul Fanning

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