Can individual wave energy converters effectively share power take offs to reduce costs within arrays?

Lead Research Organisation: University of Plymouth
Department Name: Sch of Marine Science & Engineering

Abstract

Energy converted from ocean waves has the potential to make a significant contribution to the worlds energy requirements. However to do this wave energy converters will need to form part of wave energy farms, where arrays of devices are deployed in the same geographical vicinity. Current plans for arrays of point absorber wave energy converters generally have devices with individual PTO systems. These generate electricity locally, which is then delivered to a central hub before being transmitted ashore.

In this project an alternative to each device having an individual PTO is investigated, with the aim of significantly reducing the cost of arrays. Several individual point absorber devices will pump air to and from a single central common PTO, which will then generate electricity centrally. Such a configuration has the potential for significant cost savings, resulting from a reduction in the amount of electrical generation technology used within the array. There are also potential power smoothing effects resulting from several point absorbers, at different stages in their power capture cycle, feeding into the same PTO. However by sharing a common PTO control strategies, such as latching and variable damping, can no longer be applied to individual devices. There will also be losses brought about by the longer transmission distances of air between the power capture mechanism and the PTO. This may be significant. The use of a common PTO will also further complicate the intra array effects between individual point absorbers within the array.

The project will examine these potential advantages and disadvantages of using a common PTO, with the aim of providing design guidance to the wave energy industry and research community. The work is being conducted as a collaboration between Plymouth University and Dalian University of Technology. Physical experiments will be conducted at Plymouth University with arrays of up to 4 devices and at two different scales. Results from these measurements will then be used to validate both a frequency and a time domain model developed at Dalian University of Technology. The numerical models can be used to model larger arrays and different configurations, before finally both experimental and numerical results are used to provide design guidance on the use of common PTO.

Planned Impact

The research project proposed here will investigate the use of a common PTO within arrays of point absorber wave energy converters. Instead of individual devices each having a turbine and generator several devices within an array pump air through a common PTO. It is anticipated that this concept could provide significant savings in the cost of arrays of devices, as well as potential smoothing effects in the flow delivered to turbines.

There is clearly a long term benefit in research development that leads to improvements and cost reductions in the wave energy industry. The benefit is to society at large in contributing to the development of the wave energy industry, providing a renewable source of energy, reducing carbon emissions, proving greater energy security for the UK and China, in an environmentally sensitive manner.

Research developments made in this project will add to both UK and China expertise in the new wave energy industry, with the potential to develop into a significant export industry for both countries. Research training in the field of wave energy and wave tank testing for the PDRA will benefit the industry and academic community. The establishing of collaborative links between the research communities at Plymouth University and Dalian University of Technology will be of benefit to the industry in both countries, with the potential for collaborations in marine renewable energy envisioned.

The results of this research will be disseminated through the usual channels by presentations at international conferences such as EWTEC, AWTEC and the Marine Energy Conference of China, at national professional body conferences and special interest groups, such as ICE and RINA and by publication in peer reviewed journals. In addition the project findings will be disseminated to the academic and industry communities through the PRIMaRE network and through the principal investigators links to SuperGen UKCMER on other projects.

In order to make the impact happen, we propose to carry out the following activities:
1. Involve key stakeholders, academics and key industrialists through the PRIMaRE network collaboration and the South West Marine Energy Park.
2. Create a website to guide interested parties to the information produced.
3. Disseminate results within China via the Marine Energy Conference of China.

Publications

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Description This research has investigated the use of a common PTO within an array of point absorbing wave energy converters. This has the potential to reduce capital and maintenance costs within device arrays. The use of such a system has been demonstrated in scale model tests using an array of SQ3 devices, a research outcome from EPSRC project EP/K012177/1. Valuable experimental data has been obtained into the additional inter-array device interactions which occur due to the sharing of a single PTO system. This data is still being analysed to fully understand these interactions. The data is also being used by our project partners at Dalian University of Technology to enhance the numerical modelling of arrays of wave energy converters and to establish techniques to simulate the additional inter-array interactions resulting from sharing a PTO system.
Exploitation Route One of the stated aims of the Newton Fund is to establish collaborative links between UK university's and those outside the EU. The collaboration built between Plymouth and Dalian in this project are continuing, with Dalian contributing to a EPSRC research bid currently in production.
The developments of array testing methods and equipment at Plymouth University as part of this project are continuing to be used for array tests on various ongoing and future research projects. This includes the array testing of 5 interconnected point absorber wave energy devices as part of the EU Horizon 2020 project WETFEET (641334), the results of which have been accepted for 2 conference proceedings at the 2017 EWTEC (European Wave and Tidal Energy conference).
The project has demonstrated that the use of a common PTO within an array has the potential to improve the economics of an array. However how a common PTO would be deployed at full scale, and what the associated losses with its use would be, have not yet been considered. These issues need to be fully considered before the approach developed in this project can be put to use. This would be an obvious route for these findings to be taken forward.
The collaborative links formed with Chinese Universities through this project have resulted in two research proposals being submitted as part of the Joint UK-China Offshore Renewable Energy (ORE) call between EPSRC and China's NSFC. One of these in particular proposes to carry on the work of this project, experimentally and numerically modelling an array of flexible point absorber wave energy devices, both for performance and extreme loads. This is designed to develop the modelling tools available to the UK and China renewable energy industry.
Sectors Aerospace, Defence and Marine,Energy

 
Title Development of array testing infrastructure for COAST laboratory 
Description The experimental tests involved in this project were the first on an array of wave energy devices conducted in Plymouth University COAST laboratory. As such a number of developments of the facility were made, including the addition of a second gantry and improved data logging equipment. Mapping of the wave field over the whole basin was also carried out, to extend the region over which array testing can be conducted. 
Type Of Material Improvements to research infrastructure 
Year Produced 2015 
Provided To Others? Yes  
Impact The facility development resulting from these experiments are being used for several other research projects as experimental array testing of wave energy converters becomes increasingly required. These projects including the European Horizon 2020 WETFEET project and a Swedish research project examining the SeaBased wave energy device. A EPSRC proposal currently in development also plans to conduct array tests in the Plymouth University COAST laboratory using the tools developed for this project. 
 
Description Collaboration between Plymouth University and Dalian University of Technology (DUT) 
Organisation Dalian University of Technology
Department State Key Laboratory of Coastal and Offshore Engineering
Country China 
Sector Academic/University 
PI Contribution This project was designed to foster a collaboration between the Plymouth University team and the Dalian University of Technology team. The Plymouth University team has provided a series of measurements from the experimental campaign which are being used to develop Dalian's numerical modelling tools for array's of wave energy converters. The Plymouth University research team visited Dalian University to facilitate the data exchange, discuss the project modelling and also to provide a seminar regarding Plymouth's experience in Marine Renewable Energy development.
Collaborator Contribution Dalian University of Technology's contribution in-kind included providing high-performance computing resource and 3 months works with 640 hours of Dr Ning and Dr Gou and 1920 hours for 2 postgraduate students. They are working to develop numerical models of the array tests conducted at Plymouth University. Dr Ning and Dr Gou also visited Plymouth University to discuss and share their experience in regards to numerical modelling of marine renewable energy devices.
Impact Publications resulting from this collaboration are still in preparation. These examine the influence that sharing a PTO within an array of wave energy devices has on array performance and demonstrate the changes that can be made to numerical modelling techniques to capture the additional inter-array interactions.
Start Year 2015
 
Title Common PTO for Wave Energy Devices in Arrays 
Description This project examined the use of a common PTO (Power Take Off) which several devices within an array of wave energy converters could share to reduce the cost of electrical generating technology. A model PTO was developed to test the concept of PTO sharing during scaled model tests. This was a novel set-up where 4 individual point absorbing wave energy devices pumped air through a single system representing an electrical generator. This is a unique configuration which could potentially have significant cost saving benefits in wave energy device arrays. 
Type Of Technology New/Improved Technique/Technology 
Year Produced 2015 
Impact The results from this research have not been published yet so the impact of this development has yet to be felt. However there is the potential for cost saving (both capital and maintenance) in commercial arrays of wave energy devices by the reduction in the required amount of PTO equipment. 
 
Description School Outreach Program 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact Workshops run for 5 groups (so far) of approximately 30 secondary school students over 2 hours to introduce them to marine renewable energy concepts. These workshops aim to teach the students a little about the research being conducted, and to encourage them to develop an interest in Engineering.
Year(s) Of Engagement Activity 2015