Coming up with wave energy concepts has not been a problem in this nascent industry. Creating power take off (PTO) systems for wave energy appears to have been less easy.
As a result, the University of Edinburgh is leading a project to develop an all electrical drive train for marine energy converters, as part of a project dubbed Edrive-Mec.
Meanwhile, Wave Energy Scotland, set up in the wake of key Scottish wave energy developers Pelamis and Aquamarine Power going out of business, has also been seeking new PTO solutions.
Wave Energy Scotland’s work saw it run two PTO programs last year, run in stages, from early stage feasibility to prototype demonstration. Ten feasibility studies were looked at, six concept developments and one prototype demonstration.
The early feasibility studies included hybrid electro-hydraulic power PTO systems, a power electronic controlled magnet gear, from Ecosse Subsea, a direct drive contra-rotating generator, and a direct contact dielectric elastomer PTO from the University of Bologna, Tim Hurst, CEO, Wave Energy Scotland, told All Energy in Glasgow.
Phase 2 projects include a hybrid digital displacement hydraulic PTO, using a digital displacement pump-motor and a reciprocating ball screw generator. Another concept, HiDrive, which is a direct drive PTO, is also being considered.
The difficulty around wave energy PTO is that generally a PTO and power conditioning system is needed to convert motion in multiple directions, as well as react to large forces or torques, while operating at low velocity, variable voltage and frequency, and with high reliability, availability and efficiency over a wide range of loads.
The two main options are hydraulics and direct drive and because of the difficulty outlined above, many have opted for hydraulics, either through high pressure oil or water, aided by the fact it’s also available off the shelf, despite limitations around low efficiency at part load, ability to control it over a wide range of frequencies and possible displacement leading to end-stop problems.
The Edrive-Mec project was launched in April 2016 and is due to run through to March 2019.