Increasing the Life of Marine Turbines by Design and Innovation

Lead Research Organisation: University of Cambridge
Department Name: Engineering


Unsteady loads on tidal turbines are much larger than in wind turbines because of the high density of water and the high levels of unsteadiness in offshore marine environments. In addition the welded mild steel structures normally used to support marine turbines have a low fatigue life in salt water. This can lead to life of 30 months or less instead of the design requirement of 30 years. In addition fatigue life limits the number of locations where tidal turbine can be deployed, limiting the overall practical UK tidal resource. This project aims to develop innovative technologies which will reduce the unsteady loads which result from flow unsteadiness and thus increase the longevity of a marine turbine by an order of magnitude.

Two technologies will be developed. The first uses innovative hydraulic drive trains, developed in the UK, to reduce the unsteady loads created by large length unsteady 'gusts', those larger in size than the machine diameter. The hydraulic drive train allows the speed of the turbine to respond quickly to the 'gust' ensuring that the load on the machine remains constant. The second innovative technology is designed to reduce the unsteady loads created by short length unsteady 'gusts', those smaller than the machine diameter. The technology is similar to that used in aircraft to stop sudden changes in aircraft lift as the wing is hit by an air 'gust'. The technology uses 'spoilers', 'fluid ejection' or 'flaps' on the wing to automatically hold the blade lift constant as the 'gust' moves over it. This technology will be employed on tidal turbine blades to reduce unsteady loading due to short length scale unsteady 'gusts'. Once developed this second technology could also be used as an alternative to variable pitch mechanisms to avoid peak loads being exceeded.

The project approach will be to use experimental testing and computational modelling developed in the jet engine industry to understand how the unsteadiness in marine environment resulted in unsteady turbine loading. A simplified model describing the turbine response to unsteady flow will then be implemented in the tidal turbine design code of a number of UK tidal turbine companies. This will improve the UK tidal turbine industries capability to predict the effect of flow unsteadiness.

Experiments and computation will be used to develop the new automated 'spoiler' and 'flap' control system for gust loading control. This control system in conjunction with a model for the innovative hydraulic drive train will be implemented in the tidal turbine design codes. This will allow the success of the unsteady load reduction systems to be modelled.

Current published measurements of sea turbulence are limited in both their spatial and temporal resolution. It is important to this study that improved measurements are obtained. The project members will work in collaboration with the tidal turbines companies partnered in this project, and with the Supergen partners, to improve the resolution of the available measurements. A new method of using conventional acoustic doppler equipment has been developed and if necessary this will be used in the project to achieve improved measurement resolution.

Finally the unsteady load reduction systems and the hydraulic drive train will be incorporated into a scale model of the tidal turbine and this will be tested in a water flume. The tests will be undertaken with flow unsteadiness characteristic of the marine environment. These tests will determine the project's success in achieving an unsteady load reduction.

Successful application of the load reduction systems to full scale tidal turbines will provide benefits in terms of extended maintenance periods and increased service life. Furthermore as turbines become less sensitive to their environmental operating conditions a larger number of tidal sites will become available as viable for producing power thereby increasing the UK tidal resource.

Planned Impact

The essential aims of this project are to improve the prediction of unsteady marine loads (used for design) and to develop new technologies for improving the longevity of tidal turbines. The ultimate objective is a more reliable, longer lasting, more efficient tidal turbine which is more adaptable to rough conditions. The adaptability to rough conditions will, in particular, greatly extend the number of sites around the UK coast where tidal turbines can be installed.

The marine turbine industry is in its infancy. The durability and operability of existing tidal turbines, in marine environments, remains unclear. It is, however, clear that one of the major factors limiting tidal turbine durability and operability is unsteady loading generated by flow unsteadiness from turbulence/waves. The most significant impact of this work will be technologies to reduce turbine unsteady loading at its source. Such technology is vital in increasing the longevity of the rotating components and turbine support structure.

In most conventional tidal turbines variable pitch mechanisms are used to reduce loading in rough conditions or at high flow speeds. These mechanisms are costly and require frequent maintenance. The technologies in this project, though specifically designed to reduce unsteady loading, can also be used as a cheap, reliable alternative to a variable pitch mechanism. Switching from variable pitch control to these new technologies would significantly reduce turbine complexity and increase time between maintenance.

CFD codes, experimental methods and analysis methodologies, commonly used in the gas turbine community will be introduced to the tidal turbine community. These are of special importance to the modelling of unsteady flows within the machine. This area of work will especially benefit from collaboration between the Whittle Laboratory, the world's premier gas turbine research lab, and Rolls Royce. Knowledge generated from this work will be used to improve the Blade Element Momentum codes used by the tidal turbine community to improve unsteady load prediction. This work will benefit the whole tidal turbine industry. Specific interest has already been expressed by supporting partners TGL, TEL and Green Tide.

The development of new transmission systems is also a key component of the current project. The aim is to replace existing direct drive gearboxes with a coupling system which will be self-limiting in terms of power transmitted to the generator. Improvements in duty cycle and durability will give real financial benefits (i.e. increased power generation and reduced maintenance costs - marine maintenance is very expensive). The type of technology required to achieve this objective is already in use in fixed speed gearboxes for aero-engine generators. The hydraulic transmission company, Artemis, a subsidiary of MHI, are keen to be involved in this potentially expanding field.

Another innovative aspect of this project is research into fast lift-shedding on turbine blades. Years of experience at the Whittle Laboratory in active control of aerodynamic instabilities will be turned around to actively destabilise the lift on a turbine blade (fluidic techniques will be used to induce momentary flow separation on the blades). The intention is to have a fast-response system which will limit sudden load changes caused by gusts/turbulence. This approach add to the longevity of the drivechain and turbine support structure - welded structures in sea water are very sensitive to high-cycle fatigue. Lift-shedding would also be an advantage during storm surge conditions and help to reduce the cumulative stress damage due to blade/pylon interaction.

This project draws together the three most important technologies necessary to make marine power generation viable - improved turbine design, improved drivechain design and improved load control - all of which contribute to more durable, efficient and widely-deployed turbines.
Description All three of the high level project objectives have been achieved: 1. We have understood how sea turbulence effects the unsteady loads experienced by tidal turbines. 2. We have successfully developed methods of reducing this unsteady loading at source. 3. We have demonstrated these technologies on a model turbine in large scale water tunnel trials.
A number of specific discoveries have been made: 1. Unsteady torque and thrust on the turbine can be reduced using blade-mounted 'load shedding' devices. These were tested in a wind tunnel and then in a scale model of a tidal turbine at IFREMER, France. A side-benefit from these tests has been the development of techniques for underwater communication using wireless telemetry. 2. The turbine control system and drivetrain can also be altered to reduce fatigue loads. A scale model turbine with constant torque and constant rpm controllers was used to test the impact of different levels of inertia in the drivetrain and reductions in loading were achieved. 3. The state-of-the-art methods of measuring gusts at sea have been found to be inadequate and an alternative device (based on aerospace technology) has been developed. 4. A novel turbulence model for use in LES simulations has been developed and tested.
Exploitation Route 1. The load shedding devices which have been developed can be used by the tidal turbine industry to reduce unsteady loading. 2. The turbulence model can be used by others in their simulations of tidal turbines under unsteady conditions 3. Commercial exploitation of the novel measurement device is being explored, and a grant application for sea trials is in progress. This would enable the industry to buy cheap devices which are capable of measuring sea turbulence. Currently available methods are expensive and can't measure the lower length scale turbulence. 4. Methods for the use of wireless telemetry for underwater communication have been published so that others can use these techniques in a wide range of applications.
Sectors Aerospace, Defence and Marine,Energy,Environment

Description The findings from the research have been transferred to GE (formerly Alstom Ocean Power) via an EPSRC Knowledge Transfer Fellowship. During the Fellowship, the work on unsteady load alleviation has been integrated into their design system. Further work on the industrial design process is planned. Work is underway to find a route to commercialising the novel turbulence probe.
First Year Of Impact 2016
Sector Aerospace, Defence and Marine,Energy
Impact Types Societal,Economic

Description EPSRC Impact Acceleration Account Knowledge Transfer Fellowship
Amount £59,849 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 03/2016 
End 03/2017
Description Sixth Form Masterclass 
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 Masterclass for sixth form students on renewable energy as part of a 2-day event at Pembroke College, Cambridge to encourage state school pupils to consider studying STEM subjects at University.
Year(s) Of Engagement Activity 2013,2014
Description martlet article 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Wrote an article describing the work of our research group for the Pembroke College Alumni magazine, detailing the key technical challenges with tidal power generation and what we are doing to solve them. Received letters and emails from interested readers in response.
Year(s) Of Engagement Activity 2013