Els cultius cel·lulars

Offshore wind turbines have evolved rapidly from small turbines closely related to their onshore counterparts, to large, 6-8 MW turbines specifically developed for offshore installations. The ongoing growth in turbine size, continued innovation in drive trains, blade design, wind turbine concepts

40_{The European Union WetMate project is at the engineering} study phase and focused on wave and tidal arrays, but serial production could make this a viable technology for offshore wind underwater connection. The Energy Technologies Institute (United Kingdom) provided funds to MacArtney (Denmark) to develop a prototype 11-kilovolt (kV) quick-connect cable termination as part of its collaborative RD&D programme.

and assembly all offer opportunities to reduce installed costs and raise electricity yields.

For offshore wind turbines, the main areas for innovation are improvements that focus on reducing installed costs; improving reliability, which will reduce O&M costs and improve electricity yields; and improving efficiency to increase capacity factors.

The electricity yield and reliability improvements discussed here are also related to installed cost reduction opportunities. The main turbine improvements will relate to (IRENA, 2016b):

» Blade tip speed: Increased blade tip speeds

can be envisaged as noise constraints are less pronounced than onshore. Higher speeds can improve electricity yields and, by reducing drivetrain torque loading, also reduce installed costs. This reduction will be modest, however, because greater foundation costs occur from higher loads.41

» Blade design and manufacturing improvements:

These could include enhancement of existing designs, including new aerofoil concepts and improved passive aerodynamics unlocked by new, advanced tools and modelling techniques. This also includes novel materials and manufacturing processes that give stiffer, lighter, lower cost and higher quality blades.

» The introduction of innovative drive trains: This

includes the introduction of direct drive and mid-speed drive trains, continuously variable drive trains, and superconducting generators. The last two technological developments can contribute to lower installed costs.

» The introduction of new power take-off

systems: This can help reduce installed costs and improve electricity yields. New AC take- off systems use advanced materials (e.g., silicon carbide or diamond) to achieve greater reliability in smaller, more efficient and faster

41_{Another issue is that this will require improved repair processes,} as even today, offshore blades experience higher leading edge wear than onshore systems, and this will be exacerbated by an increase in blade tip speed.

switching power conditioning units. A DC take-off system would eliminate the need for a power converter, which converts the DC current from the generator back to grid frequency AC. This would save on capital costs and increase reliability. DC collection also reduces the number of array cable cores from three to two and material needs by 20-30%. The first commercialisation of this is likely to be in wind farms commissioned by 2025.

» Improved hub assembly components: These

include improved bearing concepts and lubrication, hydraulic and electric systems, back- up energy sources for emergency response and grid fault ride-through, and hub design methods and material properties. These will reduce installed costs and improve reliability, reducing unplanned maintenance costs and outages.

The ongoing trend towards increased turbine ratings – the commercialisation of 10 MW turbines is likely to take place in the early 2020s – will help support the introduction of many of these innovations in

blade and drive train technology. Yet this trend may also necessitate new innovations as rotor diameter and tower heights increase. These innovations will include those in modular blade technology, where different materials can be incorporated into blade components that are eventually assembled together. Assembly could also be closer to the wind farm site than it is currently. These larger turbines will slightly reduce the savings in installed cost per kilowatt that can be achieved through some of the innovations, however, as higher hub heights and longer blades are, per kilowatt, typically more expensive without light-weighting. Yet, this factor has already been taken into account and all cost reductions presented in this report are net.

Other potential innovations include the development of downwind and/or two-bladed turbines that have lower rigidity requirements due to decreased issues with tower clearance, and therefore can be built cheaper and lighter given that the flex tolerances are larger. The drawback is that two-blade turbines result in increased noise, from faster blade tip speeds, and greater visual intrusion. These issues are, however, less relevant offshore.

BOX 4

Drive train innovations to 2025

In 2015, a range of drive train innovations were under development. The first is a continuously variable transmission using a hydraulic or mechanical device to provide a variable ratio of input to output speed between the rotor and a synchronous generator. This removes the need for a power converter, as the variable transmission device provides compliance and generator speed control. This reduces installed nacelle costs, as it allows the use of less expensive generators and avoids the need for power convertors. Electricity yield is also increased, as this configuration should have improved reliability. This innovation has the potential to be used in some of the next generation of offshore turbines that will be commercialised in the early 2020s.

Superconducting generators are another option for future wind turbines, as they use wires that have zero electrical resistance when cooled below a critical temperature. This reduces installed costs by avoiding the use of expensive rare-earth metals that are used in permanent magnet generators (today’s standard) and improves electricity yields as the generator is more efficient due to lower internal losses. Technical advances in recent years have increased the critical temperature of wires to more than 77° Kelvin, so that cooling can be achieved with liquid nitrogen, making their use much more feasible. Further innovations are anticipated in the efficiency of the cooling system and its insulation. This has the potential to be used in some of the next generation of offshore turbines that will be commercialised in the early 2020s (IRENA, 2016b)

4 oFFshorewind

Wind farm development, foundations,