Eize de Vries Why did Fraunhofer invest in a test rig for 90-metre blades when one US supplier has already announced a 10MW turbine with 190-metre rotor, and how does the facility rank worldwide?
Arnoldus van Wingerde Our previous test rig permits the static and dynamic testing of rotor blades up to 70 metres, which has been sufficient for the state-of-the-art 5–6MW+ class turbines.
With several new 6-10MW offshore turbine in development, the German government made available €8 – 9 million for our new facilities.
With regard to future size, a 90-metre blade length is already huge and can test blades for turbines with rotor diameters greater than 180 metres.
The test rig load-bearing capacity does allow for 100 metre-plus blades; test rig investment costs rise by about 30% when the blade testing size increases from 90 to 100 metres. Based on our information this 90-metre test rig is the world’s largest.
It also has a unique feature — a built-in ability to tilt the test rig.
EdV What are the main challenges in technical-physical terms and the overall economics linked to developing increasingly longer rotor blades?’
AvW Bending moments (in Newton-metres) typically increase with blade length (in metres) to the power of three.
This puts a high demand on building in sufficient stiffness while keeping mass within acceptable limits. Currently, the longest commercial wind turbine blade is 61.5 metres, and a leap to 90 metres requires different technologies and alternative design approaches.
The Square Cube Law or SQL dictates that when a given component geometrically doubles in size, mass increases eightfold.
These SQL-linked scaling factors force designers of very large rotor blades to adapt to new design methods that make full use of available materials.
That in turn requires advanced know-how and high-level experience. At the moment there seem to be no real technical limits to size, although there are major differences in aerodynamic design philosophy among leading suppliers, dictated by economics.
Interesting in this respect is the trend for more square metres rotor swept area per given megawatt power rating of the wind turbine resulting in substantially lower specific power ratings (MW per square metre of rotor swept area).
EdV What are key development challenges with regard to choice of materials, blade aerodynamics, structural design and manufacturing?’
AvW For rotor blades in the 40 – 50 metre range aerodynamic design is fairly straightforward and governed by the Betz law’s principles on aerodynamic efficiency.
However, the design of much longer blades should no longer be approached as if only perpendicular airflow, such as over an aircraft wing, has to be considered.
A secondary airflow, directed from the rotor’s inner part to the outer area becomes increasingly critical.
The overall challenge is to optimise airflow for maximum aerodynamic efficiency while minimising additional costs.
For the structural design of these long blades, manufacturers have to consider whether to incorporate carbon fibres into highly stressed load-carrying areas. Use of carbon makes stiffer, stronger lightweight blades compared to fibreglass-based blades.
On the other hand, the relatively fragile carbon fibres need to be integrated into the laminate’s matrix in a precisely determined, fully stretched manner.
I have come across a substantial number of blade failures attributable to mistakes with carbon fibre.
By contrast, a fibreglass-based composite matrix is much less liable to production errors due to a built-in fault tolerance capability.
A role for steel?
A relevant question is whether fibreglass-based composite blades will face size limits in the future. Equally interesting is a possible increased role for steel in blade design.
In the past, several kW-class wind turbine models were fitted with steel blades, but at the moment steel is not very common.
The inner section of Enercon E-126 segmented steel-plastics composite blades is one of the exceptions. Steel is certainly more sensitive to fatigue damage compared to reinforced plastic composite and results in heavier blades.
On the other hand, if load levels with steel are being reduced below a certain critical minimum, typically by applying more steel and making the blade heavier, there will be no more fatigue damage.
Regarding manufacturing technology, several dedicated concept solutions are in use, each closely linked with a specific structural design philosophy.
Vestas, for instance, uses a central spar concept integrated with upper and lower shells into a single assembly.
Several others apply a shear-web-type inner reinforcement structure combined with separate upper and lower shells and adhesive seams at the leading and trailing edges.
Siemens by contrast applies a one-shot manufacturing technology — constructing the blade before applying epoxy resin over the whole product, which eliminates seams on the edges.
One overall manufacturing trend is the pre-fabrication and pre-assembly of specific structural parts, including cylindrical sections for the blade foot area and long individual internal reinforcement sections.
A key challenge is to ensure high-quality, fault-free bonding between component sections even when these are very long and lack sufficient built-in structural stiffness of their own.
But independent of design and manufacturing, experience and track record are crucial to minimise the risk of costly issues.
EdV Can advanced blade rig testing reliably and accurately predict component performance during a common 20-year operational lifetime?’
AvW On our test rigs such a period can indeed be simulated with a substantial degree of reliability.
During a 20-year operational time the wind does not blow very hard for 90% of the time; so we apply a slightly higher dynamic loads level compared to actual wind loads measured in the field — but simultaneously peak impact loads are removed.
The outcome is a rather regular load pattern, which complies with common international blade-testing standards.
We have also introduced a refined testing method with two or more load levels. One test is bi-axial blade testing, where a blade is loaded simultaneously in two directions rather than in one direction at a time.
Another is a genuine fatigue test where moving hydraulic cylinders hit a blade with, for example, one million test cycles at a modest load level, followed by 100,000 cycles with higher load levels.
This is not yet an international standard, but early adoption helps Fraunhofer stay at the forefront of global R&D and testing.
BLADE MATERIALS FROM POLYESTER TO ALUMINIUM
Plastic composite and fibreglass Today, nearly all rotor blades consist of plastic composite, predominantly composed of a fibreglass matrix embedded into either epoxy or polyester resin.
Initially, from the late 1970s, polyester-based composite was the main material, using know-how from boat-building industry. The method of placing fibreglass sheets in a mould and applying the resin by brush or broom also came from boat-building.
Epoxy resin The use of epoxy resin originates from about the mid 1990s. Former Dutch blade maker Aerpac was one of the pioneers, introducing vacuum infusion manufacturing technology. Epoxy resin is known to cause allergic skin reactions, which may have led to manufacturing with fully enclosed moulds. Product quality is also enhanced by this method and today it is used for nearly all plastic composite blades.
Carbon fibre The incorporation of carbon is most popular for larger rotor blades and the fibres are typically integrated in highly stressed areas including the load-carrying structure. Carbon is expensive and known to increase manufacturing complexity, but enables building relatively lightweight, strong, stiff and yet slender blades.
Wood-reinforced epoxy Wood-reinforced epoxy technology has for a number of years enjoyed a minority wind market share, with Aerolaminates — now Vestas — a prominent pioneer. One of the largest commercial wood and carbon-reinforced epoxy (W&CR) blades is 40-metre long and was developed for a 1.65MW NEG Micon turbine, (now Vestas V82). Plans for a 4.5MW Vestas turbine with 120-metre rotor diameter and W&CRE blades were dropped.
Aluminium In 2008 Enercon introduced an aluminium blade for its new 100kW E-20 turbine. The company said aluminium blades enhance turbine recycling and, in smaller machines, improve running characteristics, adding that aluminium casting also offers greater size and dimensioning accuracy, giving greater efficiency.
RECYCLING RECYCLING OF COMPOSITE BLADES IS A BIG, LARGELY UNRESOLVED ISSUE THAT REQUIRES SUSTAINABLE, COST-EFFECTIVE SOLUTIONS.