David Latimer discusses the role of highly efficient electric machines in the offshore wind sector

In offshore wind, decisions are made based on the levelised cost of electricity (LCOE). Therefore, when bids for large offshore wind farms are made, they are based upon the £/MWh. In recent years this has seen the price drop substantially, to around £40/MWh. The result is a drive in good behaviour in terms of selecting the best all-round technology to reduce the LCOE based upon the full life of the wind farm.

This backdrop has set in motion the push for the greatest efficiency, with companies such as GE Renewable and Siemens Gamesa switching from geared to direct-drive generators. The cost-down drive has also led to the development of even bigger generators with all the major wind turbine suppliers announcing the launch of 14-15MW generators.

Back in November 2017 the InnWind project, funded by the EU, examined many different future technologies for large offshore wind. This largely desk-based research project examined the likely cost based upon 10MW and 20MW turbines. The generator technologies considered were geared permanent magnet (as used by Vestas), direct-drive permanent magnet (GE and Siemens Gamesa), superconducting and a novel new technology called the pseudo direct drive (PDD).

The InnWind project concluded that Magnomatics’ PDD offered the route to the lowest LCOE. However, this was all theoretical as at that time the largest PDD built was around 20,000Nm. On the back of the InnWind conclusion, Magnomatics was able to secure a further grant from DemoWind. In this project, the company designed and built a 500kW, 200,000Nm direct-drive generator incorporating its magnetic gear. The generator was tested at the Offshore Renewable Energy Catapult in Blyth. The results confirmed that the PDD could indeed achieve a very high efficiency, leading to a lower LCOE. This in turn has resulted in a large global engineering company taking an option over the technology for large offshore wind.

What’s the next step for the offshore wind sector?

The next challenge is to scale up the PDD to the 10-15MW required to meet future wind farm requirements. The key element of the PDD is the pole piece rotor (PPR), which sits at the heart of the magnetic gear. The PPR effectively modulates the magnetic field, which in turn creates the gear. To design a 10m PPR it is essential to fully understand the complex and dynamic loads. To achieve this, instrumentation has been added to the PPR of the 500kW generator and it now back on test at the ORE Catapult as part of the WINDER project co-funded by Driving the Electric Revolution.

The WINDER project aims to de-risk technology required for the manufacture of large generators for offshore wind to the UK. The testing aims to create a better understanding of the behaviour of the PPR under wind turbine loads and cycles.

The PPR is a cylindrical structure comprising multiple axial steel pole pieces within a non-magnetic composite structure. In operation these pole pieces are subject to massive forces and complex cyclical loads. The ORE Catapult testing will capitalise on the previous testing carried out during the Demo Wind project.

On completion of the testing, Magnomatics will develop sophisticated computer-based modelling software to aid the design of robust PPRs including dynamic modelling of the pole piece loads to predict wear and possible erosion of the composite structure. These methods will be validated using the test data from ORE Catapult. The data produced will enable Magnomatics to develop design methods and complete virtual life calculations for the large PPR.

The PDD has established itself as a credible and very efficient electric machine. Unfortunately, electric motor and generator operators are often not the people who purchase the machines. This is where there has always been a disconnect. The motor purchasers will often focus on the cost of acquisition and not the operating costs. As with much ground-breaking technology, cheaper does not mean better and more efficient; and this very true for electric machines. For high-use applications the cost of electric motors can be less than 5% of the total cost of ownership.

Over 10 years ago, in 2011, the IEA Report, Walking the Torque, laid out quite clearly the issues present with regards to motors, generators and even whole drive systems, where efficiency was not optimised. An example of this is where synchronous motors are used to drive constant speed pumps or fans controlled via a throttle or valve.

Reducing energy consumption in the offshore wind sector

Research by Anibal De Almeida has made this common knowledge for some time and has concluded that if all elements of these drive systems could be converted to the optimal efficiency, there would be an overall 10% reduction in electricity consumption. This is a significant saving; especially as electric motors are estimated to use 45% of the electricity generated.

So where are all these motors being used? Well, as you may expect, mostly in industrial applications. Surprisingly though, transport is one of the smallest sectors right now. However, this is expected to grow.

The focus must be on energy costs

The latest IEA Report recommends governments should set minimum standards in terms of motor efficiency, and whilst many motor manufacturers have responded by developing a more efficient range, it is the authors’ experience that many motor purchase decisions are still made on price, not the operating cost. Considering that the energy cost will be 95% of the cost of ownership, it does not seem to be logical.

Climate change has been the dramatic force that has made influencers sit up and take notice. We are now seeing a drive towards positive change and many governments are supporting the idea of innovation grants, and better still, linking them to the green agenda. Countries are declaring they will reach ‘net zero’ by 2050 or 2060. However, at the operating level the simplest route for many high energy using companies to achieve net zero will be to switch to renewable energy and retain their existing inefficient drives. Effective but perhaps not aligned with the more altruistic push to reduce energy consumption.