EDRIVE-MEC: All Electric Drive Train for Marine Energy Converters

Conversion of energy from wave into electricity is ideally performed by a PTO and power conditioning system that can convert motion in multiple directions, react large forces or torques whilst operating at low velocity, variable voltage and frequency, with high reliability, availability and efficiency over a wide range of loads. All aspects of this demanding specification contribute directly to the Life Time Cost of Energy and hence economic feasibility of devices. At present no single PTO technology that has been demonstrated is able to meet this specification for wave energy. The two main options for the PTO used in a wave device: hydraulics and direct drive. Wave device developers have focussed on using hydraulics as the PTO, whether it be high pressure oil or water (Pelamis, Aquamarine). In discussions with our industrial partners we learnt that the only reason for using hydraulics was due its availability off the shelf, but all partners were concerned about the limitations including, low efficiency at part load; ability to control over a wide range of frequencies; and displacement leading to potential end-stop problems.

The alternative to hydraulics is direct drive, in which the mechanical interface is eliminated, but now the generator has to operate at low velocity and high force. Direct drive systems have been proven through lab tests at Durham and Edinburgh, and through sea trials by Uppsala in Sweden, Archimedes Wave Swing and Oregon State University. In each of these cases a permanent magnet synchronous machine has been used and the generator has been of a linear planar or tubular topology. Energy can only be taken out of the device from motion in one direction, principally heave, whereas devices surge and pitch as well as heave. The use of linear generators in their current form has constrained the functionality of direct drive power take off systems, as it has not allowed energy to be converted from more than one motion. No consideration has been given to speed enhancing techniques, such as magnetic gear boxes, as developed at Sheffield for rotary machines, or the use of springs, either internally produced through control, or external physical springs, such as air springs. Speed enhancing allows a more optimised machine design, resulting in a reduction in physical size and an increase in efficiency. Previous work in direct drive power take off has proved the concept will work, but solutions are not fully optimised, designed for reliability or matched to the characteristics of the wave device. As with the generator, developers have proved the concept of connecting direct drive systems to the grid, but making use of conventional power converter approaches. However, it is well known that there is a reliability issue with power converters in the wind industry, and in the tidal sector developers use an onshore power converter for easy access. The main cause of faults within the power converter is the continuous thermal cycling due to the variable nature of wind and wave. There is therefore an opportunity to investigate alternative power converter solutions, such as multi-level systems, where the stress on the power devices are now shared across a number of devices.

The main aim of the project has been formulated in discussions with our industrial partners: develop an integrated electrical power take off system with non-mechanical speed enhancement, integrated and reliable flexible power electronics, providing adaptive control over a range of operating regimes, taking into account nominal and extreme load conditions. E-DRIVE proposes to fulfil this aim through the development of novel integrated low speed generators with speed enhancement and power converter topologies with associated control to replace hydraulic systems. In doing so we will mirror developments in all/more electric systems in automotive and aerospace.

Potential Impact:
The UK has one of the best marine renewable resources in the world. According to the Energy Technologies Institute (ETI) wave could generate up to 14% of UK electricity demand. In the last 5-10 years in the region of £700m of private and public money has been invested in marine renewables, wave and tidal. The Carbon Trust estimate that UK companies could take a 22% of the global marine market by 2050, worth £26b and leading to 20,000 jobs. The UK research councils have invested £24m in the establishment of the Supergen Marine Hub, associated grand challenge projects, doctoral training centres and the establishment of the unique FloWave Ocean Energy Research Centre at University of Edinburgh. It is clear that these investments have had significant impact on UK in terms of society, capacity building, economy, scientific development, international development and inward investment.. The component in the wave device that has received the least in terms of R&D funding is the electrical power take off (PTO), which converts the capture mechanical energy to electricity, as it is believed that the PTO is an "off-the-shelf" component that is used in the offshore oil and gas industry. However, many of the problems in the wave sector have been on the PTO, which mirrors the experience in the offshore wind sector. In its marine technology roadmap, the ETI consider the electrical power take off to be of a high priority for industry.
The main impact of EDRIVE is to demonstrate a new concept and produce new design tools specifically orientated to electrical power take off in wave energy converters. In doing so additional impact will be delivered as follows:
1. Industrial Engagement - WP7 in EDRIVE is focussed on engaging with our named partners (Carnegie (AUS/UK), Albatern (UK), Columbia Power Technologies (US), Tecnalia (Spain) and Turbo Power Systems (UK)) as well as engaging with additional wave device developers and the electrical power supply chain, in particular the electrical machines & power electronics industry. To facilitate this engagement EDRIVE has established an Industrial Advisory Board, which will meet annually.
2. Investment and Wealth Creation - Wave energy has suffered major setbacks in the past 12 months with the closure of Pelamis and the scaling down of Aquamarine, thus investor confidence is low. EDRIVE will present at investor focussed conferences to demonstrate that the proposed research is contributing to the de-risking of wave technology. Investigators will participate in trade missions organised by UK and Scottish Government agencies to attract large international companies to the UK.
3. Scientific Advances - this represents the main academic impact, which will be achieved through dissemination at national and international conferences in electrical machines, power electronics and marine renewables, and in high impact international peer reviewed journals. EDRIVE will also report to relevant research hubs funded by EPSRC, namely UK Centre for Marine Energy (UKCMER) and the Centre for Underpinning Power Electronics widening access to the outcomes from the project. At international level we are working with TU Delft in the Netherlands and the University of Chile in Santiago.
4. Skills and People Pipeline - it is acknowledged that there is a skills gap in electrical machines and power electronics in the UK. EDRIVE's contribution to impact in this area will be through education activities such as Headstart to stimulate 6th formers, inclusion of material in undergraduate teaching, workshops for PhD students and postdocs, and finally tutorial sessions at conferences.
5. Policy - The Policy and Innovation Group at Edinburgh will inform national and international government policy through technology road-mapping, dissemination through the IEA Ocean Energy Group, the European Energy Research Alliance and the EU Ocean Energy Association, all of which Edinburgh is heavily involved in.