In 2013 Airborne Marine delivered 7 turbine blades for the tidal energy turbine CoRMaT of Nautricity Ltd. These 4m blades, manufactured with a 1-shot RTM technology in order to make abundant the use of adhesives in the blades, included a metal embedded root insert which enabled a slender blade design for maximising energy- yield. The tidal blades went through a semi-fatigue testing cycle, after which a maximum bend test was performed to test both the composite blade and embedded root insert. Both the blade and insert performed as predicted.
The customer requirements
Our customer for this project, Nautricity ltd, has taken up the challenge of harvesting tidal energy,as a resource to provide commercially competitive electricity while having a positive impact on our environment.
In order to meet expected future demand for tidal energy as a sustainable resource, a future- focused approach to the engineering and manufacturing of tidal turbines that are suited for large scale manufacturing is essential. There are many other challenges related to the quest for efficient tidal energy production, such as:
- Maximizing energy yield (performance)
- Withstanding great forces subjected to the tidal blades by the water currents
- Maintenance & repair
- Cost-effective manufacture to meet cost demands of the energy market
In 2013, when Airborne manufactured the 7 tidal blades for the CoRMaT test turbine, the tidal energy market was in a preliminary phase of exploration, with only a limited amount of functional demonstrator projects deployed in open waters. As a result, Airborne was able to use experience in previous marine projects,to successfully define the requirements and provide a benchmark for production of the CoRMaT test turbine.
Maximising performance (Low Cost Of Energy )
The energy yield of tidal turbines is determined to a great extent by the design/geometry of the blades, and their ability to smoothly transfer loads. In general terms, slim tidal blades translate to high performance. The main challenge was in the root hub connection, where very high loads of water current are transferred to to the turbine, where limited space was reserved for assembly to the turbine.
For a 1 MW turbine on average 3 blades with a blade length of about 8 meter and a weight of about 1800 kg are required. Industrialised manufacturing of structures this size, requires commitment to finding smart solutions. Although the Nautricity CoRMaT turbine is smaller in size and output, achieving cost effective manufacturing of the 7 blades was at least equally challenging, as it required the manufacture of more than twice the number of blades. Consequently, enabling cost-effective series manufacturing had to be accounted for in the design of the blades.
Maintenance at sea
In the first prototypes, tidal turbine blades were manufactured much in the same way as wind turbine blades; two shells bonded together by adhesives. In contrast to wind turbine blades, tidal energy turbines must be able to withstand much harsher conditions, in terms of loads as well as direct environmental factors.
Additionally, offshore underwater maintenance is both dangerous and costly and must be limited as much as possible. To combat this, tidal turbines including the blades, are engineered to realise a lifetime of at least 20 years.
In the blades which Airborne delivered to the CoRMaT, Airborne applied an efficient root-hub connection design through an embedded insert. This robust solution enabled efficient transfer of loads from the blade to the turbine, while it provided us with the design freedom necessary to optimise the blade design based on hydrodynamic behaviour in order to maximise performance. We are currently working in close collaboration with institutes such as the technical University of Delft to further apply the principles of hydrodynamic behaviour in the design of our composite blades in order to further optimise the efficiency of the tidal blades.
To achieve cost-effective industrial scale manufacturing, Airborne is investing in the development of advanced one-shot Resin Transfer Moulding (RTM) injection technology in combination with pre-forming technology. At the core of these investments lies the Low Capex, Flexible and Integrated automation concept developed by our dedicated team at Airborne Composites Automation.
Maintenance at sea
In order to meet the robustness and reliability demands of our customer, our team of engineers took a radically different approach to the manufacture of the tidal turbine blades. Rather than approaching the blade design in a traditional manner, as two separate shells, the blades are manufactured as one part using one-shot Resin Transfer Moulding. The one-shot RTM manufacturing process eliminates the need for shell-bonding and thereby the risks caused by adhesives, of which the long-term reliability in seawater conditions is uncertain.