First published in Cleantech magazine, Volume 6, Issue 2. Copyright Cleantech Investor Ltd
By Elisabeth Jeffries
Anti-wind campaigners can go home; the wind farm has been abolished. Gone is the farm, with its faint suggestion of a 1980s cottage industry. In its place is the wind power plant, a massive offshore network preparing to slug it out with the foulest coal-fired power station. That, at least, is the indication coming from Vestas, whose latest annual report only once makes reference to a farm.
It is to fully equip these cohorts of turbines for service from remote areas offshore that many scientists supporting the work of Vestas and its competitors are now employed. Their aim is to ensure wind parks match onshore gas and coal plants by, for example, applying routine procedures that help stabilise the grid.
“Part of our work is to ensure the functionality of a plant is increased by providing a series of services a conventional fossil fuel plant finds normal,” explains Remus Teodorescu, Vestas professor in power electronics at the University of Aalborg, Denmark. His work is part of a major five year €40 million research programme kick-started by Vestas in 2008 in collaboration with a range of organisations.
Teodorescu’s programme, concerned with grid connection issues rather than power generation, assumes a step change in turbine power to over five megawatts in coming years. At that scale, wind park management and turbine performance is a whole different ball game.
“Because the turbine is bigger [more than 5MW] the power generated is greater. We’re looking at solutions to increase the voltage to medium voltage from low voltage...it’s difficult because it’s a new technology; there are a lot of issues to sort out regarding reliability when you do this which are very important,” Teodorescu says. Voltage and frequency control, a common capacity at fossil fuel plants, are among the new facilities the group wants to introduce into these larger wind power plants.
And if wind plants have been at the receiving end of a barrage of criticism for years concerning intermittency, Teodorescu says his team is finding answers to that, too. While most experts working in this area are addressing longer term energy storage, the Aalborg project aims to integrate shorter term energy storage on site, which would also help stabilise the grid. “We’ve started with short term storage of a maximum of 15 minutes using lithium-ion batteries which have been improved in recent years,” he says.
But the batteries in wind farms operate differently from those of an electric vehicle, where a battery is charged using a low current and discharged at a high current. “With wind generation you do this at the same rate, charging at high current the whole time. This will reduce the battery’s lifetime. But you need to be able to charge and discharge with the same power rate to provide or absorb energy, which is demanding for the battery technology,” explains Teodorescu. One of the challenges, he says, is to predict the lifetime of the battery - all the more difficult because few have ever been used in wind power.
Few members of the public are aware of the remarkable blueprint for North Sea wind power generation. Wind parks built over the coming decade are likely to be several kilometres in length, include hundreds of turbines and cover a wide area. At 100 miles offshore, they will be well out of sight of even the most determined conservationist. Several are planned across the whole sea in the various territorial waters of the countries surrounding it.
But putting lots of wind turbines - let alone several wind farms - next to each other can cause problems, as Dr Richard Barth, managing director of ForWind, a German wind energy research group, points out. “There are few regions with wind parks of several hundred megawatts onshore, but offshore this will be standard. It will create a wake situation, in which one turbine decreases the wind energy available for the next,” he says.
Depending on the layout chosen, one turbine could produce two thirds of the wind power of its neighbour. “The question is how to arrange and operate a wind farm optimally so that the overall efficiency of the wind farm improves, while reducing the mechanical loads,” explains Barth, who is also concerned about groups of wind farms.
“If you have a cluster of several gigawatts there’s a feedback effect or impact on the atmosphere which is not fully understood. If you plan a wind farm, what will be its efficiency? There are some risks,” he comments. The financial impacts, too, are not known. While Barth estimates the energy impacts of adjacent wind turbines and wind farms could amount to a few percentage points, he suggests the financial effects could be much greater. “Two percent more wind energy production can produce a lot more money,” he asserts. Modelling these issues and different layouts is one of the issues with which ForWind is concerned.
At Durham University in the UK, Dr Simon Hogg has a similar remit, this time funded by the UK Government to the tune of £4.8 million. “Our brief is to get offshore wind to operate as a power station,” says Hogg, outlining the programme of the Supergen wind consortium, which includes several universities such as Manchester and Strathclyde. But rather than considering layout, a large part of the team’s work covers maintenance. “If they’re 100 miles out to sea, the issue of accessing turbines is an order of magnitude higher. It’s a long boat trip and you can only get to an offshore turbine a third of the time anyway because conditions are very rough,” he states.
Nevertheless, as Hogg points out, in any wind farm there is usually a turbine that is out of order. While he and his team cannot do anything about access, they can improve the intelligence of signals emitted by the wind farm. “Diagnostics are not very sophisticated in wind. It’s bearable on land but not really appropriate for offshore,” says Hogg.
Hence the aim is to monitor conditions more effectively “We’re looking holistically at the information coming off the turbine so that a combination of signals indicates a particular fault...we want to detect, know when things are going wrong, diagnose what it is and also carry out a prognosis – which is by far the most difficult.” Thus, he indicates the work could eventually allow operators to predict a few months ahead when a fault might occur, so readying them for repair trips. It is an awkward problem. Unlike offshore oil rigs, most turbines do not have sufficient living space for more than a couple of engineers.
As in the case of Vestas, Hogg and his group have been grappling with the sheer scale of the new turbines and the impact that has on efficiency, loading and so on. “When you go over 5MW you get a blade diameter of 120 metres that would dwarf a jumbo jet. In that situation the impact of gusts of wind is not constant and varies across the span of the blade and also according to the different blades,” he says. To resolve these issues, scientists at Strathclyde University are looking at different ways to pitch each blade.
The increasing power generation of turbines is another problem under investigation. “If you get to beyond 5MW you are getting to the limit for direct drive [gearless] turbines,” says Hogg, indicating that at that scale they might need a gearbox. Experts are therefore considering different solutions. “You’d still have a gearbox and generator but it wouldn’t be under as much stress. We’re looking at the problems converting the energy of the rotor into electrical power. It’s not about the reliability of the gearbox, but about getting a solution that will produce more power.”
Siemens is among the companies that has been considering similar issues, in partnership with Sheffield University. According to a Siemens spokesman: “The aim of the collaboration...is to achieve a deeper understanding of the electrical generator for direct drive wind turbines, to achieve increased output power and reliability with the focus on providing cost effective wind power generation. Siemens also has an R&D centre at Keele University. The focus of the team at Keele is the development of new power converter technologies for use primarily in large offshore wind turbines.” He added: “work done at the Siemens-Sheffield and Keele Wind Power Research Centre is already finding its way into commercial products.”
Wind turbine design has been greatly influenced by aerospace technology, of course. At the National Composites Centre (NCC) at Bristol University, which has contributed to aircraft development for decades, experts are trying to solve problems relating to the increasing blade length. Materials used to make the blade have evolved from wood to fibreglass to carbon fibre, and the task is to make the bigger blades more efficient through lighter materials or other techniques. Paul Weaver, professor in lightweight structures at NCC, explains: “materials have changed but the structured configuration hasn’t over the last 15 years.”
Most of his group’s work, some funded by Vestas, focuses on changing that configuration to cope with growing demand for longer blades. “The blade has to be more reliable as it will be in service for 20 years. We’re looking at removing complexity and becoming more efficient aerodynamically, reducing stress on the blades due to the gust load,” Weaver says. One possibility includes creating a hingeless flap like the fin of a fish. They are also working with manufacturers to design out defects. “Composites are fibre-based and can wrinkle by a few millimetres, so the shape you want it to be is not the shape you get.”
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