VINCENZO NAVA
PABLO RUIZ-MINGUELA
Source: ©ewg3D - istockphoto.com
The FP7-funded DTOcean [1] project produced a first generation of freely available, open-source design tools for wave and tidal energy arrays. These tools have been used on leading tidal and wave energy projects, including the recently installed four-turbine 6 MW MeyGen tidal array in the UK, and a wave energy application by Sandia National Laboratories in the USA. The software tools enable the user to design the balance of plant (i.e. array layouts, electrical infrastructure, moorings and foundations, installation procedures and operations and maintenance plans) required to put an array of ocean energy converters into operation.
The Horizon 2020-funded DTOceanPlus [2] project, which began recently, is extending the functionality of this integrated suite of design tools for ocean energy technologies, including sub-systems, energy capture devices and arrays. This second generation of design tools will support the entire technology innovation process from concept through to development and deployment. More specifically, the farm layout will model the hydrodynamic interactions between the resource (waves, tides) and the device(s), achieving a workable compromise between computational speed and accuracy.
Figure 1: Photograph of shared mooring for aquaculture in Korea and illustration of application to wave energy in the OPERA project. Source: OPERA
On the other hand, the Horizon 2020funded OPERA [3] project has collected more than 2 years of operating data and experience at sea, to validate and de-risk four industrial innovations for wave energy, paving the way for long-term cost-reduction of ocean energy technologies. One of these innovations is a shared mooring configuration which provides a clustered array layout of wave energy converters, as demonstrated in aquaculture. This integrated mooring system can make better use of ocean space, while at the same time reducing mooring costs.
'The three projects DTOcean, DTOceanPlus and Opera show that the array layout of wave and tidal arrays is an area in which new developments must take place'
The three projects above show that the layout of wave and tidal arrays is an area in which new developments must take place. While individual devices are progressing through the stages of commissioning and at-sea testing, the hydrodynamic interaction between individual devices within an array as well as the relationship between the overall resource and neighbouring devices are not yet fully understood. Moreover, the experience of DTOcean shows that while hydrodynamic interactions certainly impact upon the final revenue of the ocean energy array, only a holistic view of all the subsystems (i.e. energy capture and delivery, station-keeping, lifecycle logistics and operations) can lead to the optimisation of lifetime costs for the final array layout.
In DTOcean, the hydrodynamic modelling of array interactions for ocean energy converters (OEC) has been solved through an exact algebraic method (for wave) and parametric modelling (for tidal). An optimal farm layout in terms of captured energy is obtained using different evolutionary optimisation algorithms. The cost function maximises the annual energy production (AEP) of the farm, while holding the average q-factor of the array above a user specified threshold. The q-factor is an energy loss index due to the modification of the absorbed energy caused by the hydrodynamic interaction between bodies. It is calculated as the AEP of the array (AEParray) over the AEP of the same array without considering the interaction. Where AEPOC is the energy production of the OEC at the same location, but without interaction with other devices, and NOEC is the number of devices installed in the array.
Figure 2: Optimised farm layout. Source: DTOcean v2.0 tools
The optimisation parameter space is defined by the array parameters, such as device inter-distance and row and column angles. The optimum search is constrained by the following variables: the no-go areas, the minimum distance between devices, the maximum number of devices and the q-factor.
A no-go area identifies a zone of the lease area where the installation of the device is not possible due to reasons such as unfeasible water depth or environmental impact. No-go areas are specified by the user, if known a priori, or internally calculated based on site and machine specification.
'[...] only a holistic view of all the subsystems (i.e. energy capture and delivery, station-keeping, lifecycle logistics and operations) can lead to the optimisation of lifetime costs for the final array layout.'
The minimum distance between devices is constrained by physical limitations, such as vessel operation, or by theoretical limitations of the numerical model. For example, for an array of wave energy converters, the minimum distance between devices should be at least the diameter of the inscribing cylinder, while devices placed at distances greater than 8-10 times the device diameter have not shown significant hydrodynamic interactions.
Besides, DTOcean offers the possibility of carrying out multi-variable sensitivity analyses to explore further options for layout optimisation, not only from a hydrodynamic perspective, but also accounting for other variables which directly or indirectly impact lifetime costs, such as design constraints, exclusion zones, length of export cable, shared components in mooring systems, internal cable routes and vessel routes.
Lastly, experience in OPERA has revealed that a shared mooring system configuration of two rows and up to four columns can be feasible. Cell distances around 40-50 m for a heaving floating point absorber of 5 m diameter, have a compatible q-factor whilst facilitating vessel access for installation and maintenance.