Downwind and high level benefits
There are several factors that drive choice of the rotor orientation. In the 2 bladed case the orientation of the rotor can contribute substantially to the stability or general behavior of the nacelle. Furthermore, orientation can impact structure choices and blade design, where upwind wind turbines are driven by tower clearance requirements for certification and design, driving both structure design choices, blade pre-bending and stiffness as well as control strategies to avoid extreme blade bending. Given the traditional tubular towers, downwind rotor have experienced more severe wakes due to tubular towers.
For the 2B6, the rotor configuration and choice of orientation was fundamental in defining an optimum between a variety of parameters achieving:
- Controlled behavior and yaw stability
- Lower loads generated by the nacelle – lower loads on structure
- Less blade clearance issues
- Simpler blade, no pre-bending
- Allowance of a full integrated jacket design, also providing low wake downwind.
Downwind vs Upwind
Basic principles and facts:
- Blade/tower clearance is a main driver for design
- 3 bladed airfoils are inherently softer – 2 blades stiffer
- Wind load impacts a blade with downwind thrust loading
- Full jacket very challenging – tubular tower best upwind option
- Less measures needed (carbon, pre-bending, cone/tilt)
- Upwind rotor – negative, downwind rotor – positive
The conceptual difference to upwind:
- Upwind rotor with upwind cone angle in operation forces the blades in down wind direction towards the tower due to centrifugal force
- A downwind rotor with downwind cone angle during operation is forcing the blade in upwind direction
- Thrust load and centrifugal out of plane loading work in the same direction and creates need for more compensation in blade or rotor design
- The loads compensate each other and reduces the total blade bending
The full jacket (truss) structure
The jacket design by 2-B Energy is a result of full modeling of wind turbine behavior resulting in both extreme and fatigue loads. As a part of the design integrations, the relationship between nacelle and structure loading has been carefully considered to reach a harmony between the components. The jacket design also integrates the components of traditional tower, transition piece and foundation in to one design. Being a fully integrated process and design from nacelle to seabed, hereby an optimized structure can be provided also eliminating the traditional interface risk.
The 2B6 jacket results in a simpler transition piece (from jacket legs to circular design) with lower weight, also host of elevator and other equipment.
The full jacket design provides for several benefits, of which few can be mentioned as:
- 3 legs – 15% weight saving to 4 legs and hereby lower weight
- 3-leg geometry has smaller footprint than 4 legs, allowing for more units / barge
- Internal nacelle crane reaching deck of ship due to smaller footprint
- Simpler installation with flanges and both foundation and tower in same sequence
- Deeper water design with competitive weights
- HV transformer section can be sandwiched-in with the standard design (1 turbine per 9). Please see illustration on the right and explore sections below.
Safer and better access system
One the most promising components of the 2B6 is the helicopter landing access. Today, using larger and larger ships and seeking more and more advanced methods to access wind turbines in higher sea results in a natural limits of only three crews per vessel to avoid increasing transit and waiting time across a workday. Even at three crews per ship, the efficient time left over to perform service work becomes less of half a twelve hour workday. The helicopter landing is fundamentally different than the current hoisting process and can fundamentally change the way we operate offshore wind turbines in the future. By landing the helicopter, the passive position of the craft makes it possible to safely exit and enter the helicopter without stress and in wind speeds up to 50 knots. A turbine can be reached and accesses within 10 minutes at up to 95% of the time, possibly also 24 hours, 7 days per week. This way we no longer need to consider the driving factors created by ship access restriction such as complex redundancy systems and strategies to avid longer downtime. In principle a fault that may not be defined within a week of high seas in a conventional project with ship access and possibly not be repaired for yet another week can be defined and repaired within the very same day. Hereby availability has the potential to be substantially enhanced and safety for access vastly enhanced.
Another major difference by landing helicopter instead of hoisting is that the helicopter capacity and efficiency is virtually doubled by landing. This is driven by the fuel requirement by hovering in active mode. Assuming site conditions in a North Sea location as Alpha Ventus and a stationed helicopter of same kind offshore, the comparison can be as follows:
As a hoisting helicopter working within a wind farm is required to hold enough fuel to reach a “secondary” landing deck (often onshore), it must carry so much fuel that only two passengers and only 50 kg of tools can be transported (assuming the use of an EC 135 t2) before it needs to refuel and pick up another team of two technicians. For three full service teams of twelve technicians this means six trips and six re-fuelling’s and a total of 300kg of tools.
For the landing craft the fuel restriction goes away due to the multiple “secondary” landing pads on each wind turbine. This means that a helicopter can use its full capacity and transfer four technicians and 275kg of tools each transfer. This would mean three trips of four technicians and 275kg of tools each, resulting in twelve technicians and 825kg of tool over three trips and thereafter refueling.
Furthermore, several systems and processes can be tailored for the use of helicopter making full integration possible and resulting in both higher efficiency and lower cost. For a 300 MW wind farm, a craft may use in rough assumption 500 -700 hours per year. Should problems arise requiring more technicians in the field, the helicopter can easily double its operating hours per year, where the alternative with ship use would require an additional ship and related infrastructure.
From 10 kV to HV in one transformation
The 2-B Energy strategy is to take integrated system views, which is also the case for the electrical generation system. The 2B6 offshore wind turbine is based on a doubly fed induction generator with small converter (DFIG-light) concept. A significant aspect where the electrical configuration of the 2B6 WTG distinguishes itself in comparison to turbines of other manufacturers, is with respect to the Stator Voltage.
It’s common that the generation voltage is set to 400 V or 690 V and is thereafter transformed by means of a step-up Transformer to a MV grid voltage, most often 33 kV. The 2B6 has a direct rotor output Voltage of 10 kV and is therefore able to directly connect into a MV grid, without the use of a step-up Transformer.
This aspect has far going consequences within the MV grid of the Wind park and gives rise to an unique park design, known as the “PowerTown grid”. Instead of a String topology, the PowerTown grid makes use of a Star topology, where 8 wind turbines connects in a star to the center wind turbine and a 3 x 3 pattern (individually called a “PowerBlock”). The multiple of PowerBlocks grids then constitutes the PowerTown grid. The PowerBlocks can be connected on HV level through string or loop. With loop there is a very high degree of redundancy in the system.
A benefit of this topology is that the cable lengths within the PowerBlock (10 kV) are kept small and identical with very simple equipment. But the real benefit lays in the fact that this design can step the grid voltage up to HV (110 – 220 kV) with an group transformer integrated in to the center turbine in a “sandwich” design, built in between the two jacket sections. Hereby there is no need or of a separate offshore transformation platform. See Illustration…
Hereby the power generated from the wind turbine benefit from less losses (no full converter, no full size transformer) and less equipment as a full transformation step is eliminated. Furthermore spare transformer on 60 MVA level is a fairly cost efficient and risk mitigating measure.
Transportation and installation process
One of the greater costs and project risk areas beside normal wind turbine components is the transportation and installation process. The high cost of the new generation of larger vessels in combination with multi-discipline processes requiring narrow weather conditions (wave height and wind speeds), results in more time on the sea with high risk for delays. Ripple effects through a full project can have substantial economical outcome. The benefit of the 2-bladed rotor benefits this process in multiple ways. The principle of having full rotors mounted on nacelle during shipping and installation increases efficiency and reduces installation time. The stacking and transportation of nacelle and rotors also maintains a narrow width of vessel that otherwise can be a hindrance on rivers or through narrow channels.
The installation process of a 2 bladed rotor brings the most benefits where full nacelle and rotor can be lifted and installed in only one lift with higher degree of control by help of taglines. Hereby installation can fully eliminate blade handling, rotor turning and tool changes as a result where no vertical flanges needs precision and alignment but only mating of horizontal flange where bolt alignments are easily made by tag line control. This enables installation in higher wind conditions in much quicker and less complex process.
By tailored installation process also including full jacket structure, the installation capacity is expected to reach more than double of that of a standard 3-bladed wind turbine of today.