Over the last 10 years, the offshore wind energy industry has faced its fair share of criticism. Public opinion has varied and industry pundits have in some cases labelled the large scale rollout premature, saying it is neither economical nor the most favourable clean energy source to harness.
Engineers on the ground – or rather on the sea – have, of course, battled through and have been keen to prove critics wrong. The early decision by the sector to develop larger turbines has played a substantial part in reducing costs – seen by many as validation that wind energy does indeed have a key part to play in meeting our future energy needs.
The cost reduction is impressive, with estimates currently in the region of just £120/MWh with the aim of getting to £100/MWh by 2020. By comparison new nuclear is expected to produce energy at a cost of £80 - £105/MWh and natural gas with carbon capture £60 - £130/MWh. It seems, as the industry matures, it is getting increasingly competitive.
The current generation of turbine blades are positive behemoths, spanning 80m with plans in place to get even longer. The question is - how can the materials cope? This is part of the development work being undertaken by the Offshore Renewable Energy (ORE) Catapult, which operates a world-class testing and research facility in Blyth, Northumberland.
As you’d expect, the facility is huge. Since opening in 2012 the 100m main test hall is the largest of its type in the world. It represents the ambition of the UK wind energy sector, as it has essentially future proofed any move to even longer blades.
The wind industry has steadily been moving to carbon fibres, from glass, over the last few years to enable larger blades to be manufactured, many of which have passed through Blyth for operational certification.
“We carry out full scale testing of blades in order to achieve certification, but it is also critical to blade development,” says Raul Prieto, a senior engineering specialist working at the Blyth facility. “During full scale testing, we force the blade to the maximum loads we expect it will see in its 20-25 years of operation. So that is the perfect storm, extreme gusts and changes in the direction of the wind.”
Structural testing is carried out in what is referred to as the flap- and edge-wise orientation of the blades. The load profile is extreme, which is applied using a number of saddles placed over the blade, that are in turn attached to lines that pull horizontally.
“We pull a very large deflection in the tip of the blade, more than 10m,” says Prieto. “You do it little by little, and track the measurements on the strain gauges that are positioned in the blade. We measure how the blade performs, and how it deflects. These test loads are then compared to a structural model of the blade to see how the two compare.”
Once the structural test is complete, the dynamic tests begin, looking at fatigue properties. This is essentially qualifying the blades for the daily battering they will experience in the offshore environment. The blades are vibrated at a frequency very close to their natural resonance. These tests can last weeks, and even months, as between 1 million and 5 million cycles need to be notched up.
Here, however, longer blades are throwing up another challenge as resonant frequency reduces with length, and as a result pushes existing test equipment beyond its practical limit.
The tests historically used a Compact Resonant Mass (CRM) system, a common test methodology used throughout the industry, to excite blades to the desired natural frequency. However, the current CRMs were meeting an operational limit as they could no longer exert enough force through the entire length of the blades at the desired low frequency. This led to a switch to the Resonant Actuator Test (RAT), which has one single floor mounted actuation point that is coupled to the underside of the blade.
This allows the blade to be driven in a sinusoidal motion to impart the forces and bending moments on the blade as needed. Despite the ‘faster’ test procedure, blades are typically put through their paces for six to eight months.
“Sometimes, manufacturers request the blades be tested to failure,” says Prieto. “Here, it is a one off test opportunity where we can learn what the failure modes of the blades really are.”
It is a chance to see how optimal a blade design actually is, not just how optimal it is thought to be. Engineers gain valuable information about whether the blade is over or under engineered – or indeed just right – and whether the margins used can be tweaked to improve future operations. In addition, it gives valuable information about failure modes: will it buckle in some regions initially, or have delamination in others?
The problem with single-axis testing
Aerodynamic loads mainly act in the flap-wise direction, and gravity loads mainly act in the edge-wise direction. Single-axis tests are designed to represent these two load sources, with equivalent test loads calculated for each direction, and then applied to the blades in separate flap-wise and edge-wise tests.
In service, however, these two loads occur at the same time, so testing in this way doesn’t truly represent how fatigue accumulates on the blade.
“We acknowledge it is not enough to test the blades separately in four directions,” says Prieto. “So we are developing new testing methods that are more accurate and more representative of the operational life of the wind turbine. We have to progressively improve, and we can see a change to a bi-axial testing procedure sometime in the future.”
A bi-axial test, or dual-axis test, would see the blades forced through an elliptical motion at the tips to more accurately assess the fatigue likely to occur in the blades’ material during service. The blades are loaded by the wind, which varies as the rotor spins because of turbulence and wind shear. Gravity, however, is also a major source of fatigue loading for the blades, particularly as they grow in length and mass. An optimised dual-axis test is potentially the jewel in the crown test for long blades as it offers significant benefit in terms of more representative testing.
“We are in the process of qualifying these testing methods and are trying to gain acceptance for these methods from the manufacturers and the certifying bodies,” says Prieto. “Here you can trigger different [and more unusual] failure modes like compressive buckling of the trailing edge, for example. Little by little manufacturers are more interested in testing these design conditions that are not yet required by the certifying bodies but are progressively getting more attention.”
Exasperating the erosion dilemma
Though larger blades are inevitable, there is a downside: an operational problem that was initially underestimated, but has since become a thorn in the side of the wind industry... leading edge erosion.
It is a monumental problem and one that is made worse by larger blades, as tip speeds are generally higher. It limits blades from turning at faster speeds and operating in the most efficient envelope. Solving this issue would help improve aerodynamics and overall efficiency.
“The ocean is more trouble than everybody initially acknowledged,” says Prieto. “You commission a wind farm and then inspect it in two or three years time and you see a large portion of erosion.
“The standard has been protective tape and high solids paint. But we haven’t yet found a solution that can cope with the problem as the blades need repainting three times or more in their life time.”
This has seen the formation of the blade leading edge erosion programme (BLEEP), which aims to reduce its effects by assessing the impact on aerodynamic performance and structural integrity. The programme personifies the ORE Catapult’s broad view of the industry, as it’s able to bring together major stake holders with academia, to solve wider operational issues.
“We have a privileged perspective,” says Prieto, “and it is one of the key parts of our raison d’être to try and be that hub that is able to facilitate the sharing of data to solve these kinds of challenges.
“As a test centre we work on behalf of clients and will always maintain their IP and respect confidentiality. But we also recognise the industry needs to collaborate to a certain extent. We want the industry to work together to solve the problems everyone suffers from."