Lead Institution: University of Edinburgh
Principal Investigator: Professor Markus Mueller
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.
Further details of the project can be found on the project website.