Development of Efficient Wind Turbine Blades

David Cripps, Senior Technical Manager at Blade Dynamics Ltd. talks to AZoCleantech about the development of the world’s longest and more efficient wind turbine blades.

Please can you provide the audience with a brief introduction to Blade Dynamics and the key capabilities to this organisation?

Blade Dynamics is a British business focussed on designing and building wind turbine blades that are lighter and more accurately built than those made by conventional blade manufacturing processes. This is achieved through a unique design and manufacturing approach where blades are assembled from smaller, more accurate and higher quality pieces using an assortment of technologies that the company has developed and patented.

The company was founded by people with many years of experience in the composites and wind blade industry, and has particular capabilities in the cost-effective use of the high performance composite materials of which the blades are built, as well as in composite structural design and blade aerodynamics. The company is based in Cowes, in the UK, and has a subsidiary in the USA, in New Orleans.

A recent press release talks about the development of the world’s longest wind turbine blades. Can you discuss the inspiration behind the development of this technology?

In many highly populated countries, including the UK, there is limited scope for further expansion of the onshore wind sector due to space and visual impact constraints. Coastal countries that have strong offshore wind resources, such as the UK, are therefore increasingly developing wind farms in offshore waters. This is currently being done using large numbers of medium sized machines – in the order of 3MW (though some newer machines are being deployed in the 5-6MW range). However, due to very high installation and access costs, the cost of wind generated from these offshore farms is usually substantially higher than that generated from onshore wind farms and other energy sources, despite the wind resources generally being stronger and more reliable offshore.

Most people in the offshore wind industry recognize that one of the ways to bring down the cost of offshore wind is through the installation of a smaller number of much larger turbines - those in the order of 10MW per machine. However, up to now one of the limits in the development of these larger machines has been the lack of appropriate blades, since conventional blade manufacturing technology does not scale well, or cost-effectively to these very large sizes.

The ETI recognized this barrier to the development of the UK’s offshore wind industry and so put out a tender for a project to design and build a blade for this new generation of turbines. ETI recognized that the very different design and manufacturing approach of Blade Dynamics would be the most appropriate way of developing and building such a blade. This led to contract negotiations during 2012 and to the award of this contract at the end of the year.

Why have Blade Dynamics chosen to use carbon fibre rather than conventional fibre glass to construct this wind turbine and how does this compare with blades deployed offshore?

Most wind turbine blades, including many of those deployed offshore, are made using glass fibre as the primary structural material. Whilst this is a relatively cost-effective material, it tends to lead to blades that are considerably heavier than those built using carbon fibre. For example, the Blade Dynamics 49m blade weighs around 6t, compared to a similar blade made in glass fibre of 9-10t. This weight difference is particularly important when blades get to the kinds of length being proposed offshore, since the blade mass is largely linked to the volume of the blade, which is therefore proportional to the blade length cubed.

In addition, to get the necessary stiffness from a glass fibre blade at these lengths, it is usually necessary to increase the thickness of the blade’s cross section. Using carbon fibre, thinner and more aerodynamically efficient aerofoils can be used.

By using carbon as the primary structural material, we can therefore build blades that are lighter for a given length or longer for a given mass, and that are more aerodynamically efficient than those made from glass. For a given turbine, a longer blade can capture more energy due to the rotor’s larger swept area.  

A lighter weight rotor can also reduce some of the loads elsewhere in the turbine. If these are factored into the turbine design, substantial cost savings in turbine parts such as hubs, drive shafts, gearboxes, towers and foundations can be achieved.

How will the design of this wind turbine help reduce the cost of the energy produced?

By enabling longer, thinner blades, more energy can be captured from the wind in given wind conditions. This directly benefits the economics of a wind turbine. In addition, as explained above, if other parts of the turbine can be designed to take advantage of the lower rotor mass, turbine capital cost and installation savings can be achieved. Lower loads from the rotor can also reduce wear in parts of the turbine and so reduce on-going maintenance costs.

Besides the use of carbon fibre, Blade Dynamics’ blades are also built in a very different way:

Conventionally, blades are moulded from pieces that are the full length of the blade. When blades get very large it is hard to maintain the accuracy and quality of such very large pieces, and so there is a risk of in-built defects. In addition, the specialized tooling required to make such large parts becomes very expensive indeed.

Blade Dynamics builds blades by assembling them from a series of mouldings that are much smaller than the full blade length – typically less than about 25m. These smaller mouldings can be made much more accurately and at higher quality levels than can be achieved in very large mouldings. The tooling required by this approach is also much less expensive, so it becomes viable to start series blade production for a smaller number of blades than would be required in a conventional blade plant.

Furthermore, by using smaller mouldings, interesting supply chain options open up. For example, we can subcontract the manufacture of many of the mouldings to existing moulding plants who might not be able to mould an 80m part but who can certainly do parts of the size we require. This gives us the ability to scale up production quickly and flexibly, and to also manage logistics in the most cost-effective way. For example, we can bring in mouldings from different parts of the country and just assemble them near to where the blades will be used. In the case of this project, the assembly is likely to be at a port.

Of course, this manufacturing approach would not suit most companies as it requires the knowledge of how to put these multiple mouldings together in a cost-effective and reliable way. This is at the heart of Blade Dynamics’ capabilities and IP.

What is the intended end use of this technology?

The particular ETI project is about developing blades for the next generation of very large (8-10MW) offshore turbines, with a particularly interest in those to be deployed in the coastal waters of the UK. However, the technology is just as relevant to large blades for future onshore turbines. Indeed, the supply chain benefits of our manufacturing approach can be even greater onshore since the logistics of shipping extremely large blades by road to their destination can be challenging and expensive, whereas we can assemble our blades nearer to where they will be deployed.

How will this project test design and manufacturing technologies and how will this improve the manufacturing process?

The unique manufacturing approach of Blade Dynamics has been demonstrated on the D49 blade that is designed for a 2MW onshore turbine. The ETI project will enable some of the technologies developed for this blade to be further tested, refined and scaled up to the size of the offshore blade planned. Fundamentally, our approach of building up a very large structure from smaller pieces, rather than creating it all in one go, is a very scalable process. Bigger blades just require more pieces to be assembled.

The project also allows us to evaluate different materials and manufacturing processes for the different parts of the blade. With our approach we can use whatever process or material is the most appropriate for the particular part of the blade being built. For example, blade tips have different requirements to blade roots, so we can optimize materials and processes for those very different parts. With conventional blade manufacturing, the materials and process needs to be more or less the same along the whole blade length.

How do you see the development of this novel wind turbine technology transform a nation to a low carbon economy?

The UK is placing considerable emphasis on offshore wind for its future supply of renewable/low carbon energy. Currently offshore wind is an expensive option so any approach that helps to reduce the cost of offshore wind energy will accelerate the move to a low carbon economy. These economics also apply in other countries where offshore wind is being implemented, though where the alternative is largely coal-generated electricity (e.g., China) the impacts on carbon emissions are potentially much greater, than where natural gas is the primary generation source (e.g., the UK).

What are the major challenges ahead for this technology and how do you think the industry will aim to overcome these hurdles to achieve the ultimate goal of a low carbon economy?

The offshore wind industry is extremely conservative, as a result of the high costs of any failures offshore. This risk aversion provides a strong headwind to the introduction of any new technology, not just blades, yet new technologies are vital if the costs of offshore wind energy are ever going to be brought down to economic levels. Putting it more bluntly, we are never going to have a cost-effective offshore wind industry if we simply apply today’s technology to it.

The ETI project will involve extensive testing of blades and blade parts, including long-term fatigue testing, and this should help to demonstrate the reliability of this blade technology and boost confidence. On-going testing after the project, particularly on full-scale turbines, ideally using coastal demonstration locations or at high-wind onshore locations (which are both more accessible than offshore), will also help to establish the kind of track record needed for major deployment of this technology offshore.

About David Cripps

David Cripps is Senior Technical Manager at Blade Dynamics, a company based on the Isle of Wight, UK specialising in the design and manufacture of large, lightweight wind turbine blades. Prior to joining Blade Dynamics, David worked at the composite materials company Gurit (formerly known as SP Systems) for 25 years, in multiple technical and commercial roles, with a focus on the application of composite materials to the wind turbine industry. David holds a degree in aeronautics and astronautics from the University of Southampton.