Description
SCORE (Sustainability of using Ria Formosa Currents On Renewable Energy Production) proposed to test for the first time a floatable tidal energy converter (TEC) on Portuguese waters, the Evopod 1:10th scale prototype from OceanFlow Energy (OE). Evopod device is at the technology readiness level (TRL) 7 and a 1:4th scale prototype was tested on Scottish waters on combined ocean-current environment, the requisite required for reaching TRL 8 (i.e. pre-commercial stage).
The innovative aspect of TEC testing in Portugal lies with the unique morphological characteristics associated with the device deployment site at Ria Formosa, a coastal lagoon protected by a multi-inlet barrier system located in southern Portugal (Algarve Region). Ria Formosa can be used as representative of the vast majority of shallow coastal areas where TECs can be used in the future. It is therefore ideal to analyse both the energy extraction efficiency and eventual impacts that extracting energy from the flowing currents will have on the ecological communities and physical settings.
Almost no information exists on how cumulative effects of multiple devices will impact the near and far-field flow and sediment transport patterns from array deployments. The main expected outcome of the proposed research is to construct an operational envelope which can be used by technology developers on design concepts of efficient TECs based on environmental and sustainability principles, contributing to the growth of the blue economy.
The measured energy extraction efficiency of the operational prototype will be compared against numerical models in order to validate these tools. Model simulations using different hydrodynamic settings and number of units can then be performed to (1) evaluate the impacts that energy extraction will have on the temporal and spatial changes of the flow, on sediment transport patterns and, ultimately, on the habitat; (2) develop cost benefit analysis balancing supply and demand using tidal energy.
Location
The experience with the prototype was performed at Faro-Olhão Inlet, the main inlet of Ria Formosa system (hereafter RF), a coastal lagoon located in the South of Portugal (main city Faro). The RF is a multi-inlet barrier system comprising five islands, two peninsulas separated by six tidal inlets, salt marshes, sand flats and a complex network of tidal channels. The tides in the area are semi-diurnal with typical average astronomical ranges of 2.8 m for spring tides and 1.3 m for neap tides. A maximum tidal range of 3.5 m is reached during equinoctial tides, possibly rising over 3.8 m during storm surges. The wind is on average moderate (3 ms−1) and predominantly from the west. Variance analysis of both tidal and non-tidal signals has shown that the meteorological and long-term water-level variability explains less than 1% of the total recorded variance.
The deployment site was selected nearby the navigation channel of Faro-Olhão Inlet, Faro Channel, the largest and most hydraulically efficient channel of RF. Faro-Olhão Inlet is the main inlet of the system, trapping 60% of the total spring-tidal prism of the RF system. The inlet is characterised by strong currents (depth average velocities over 2 ms-1 at the inlet throat). Due to the narrow inlet mouth and the strong tidal current, limited offshore wave energy is reaching the lagoon. Nevertheless, the mooring location could experience fetch dependant waves generated by wind blowing over the lagoon water from the NW or NE directions.
Energy from tides was harvested before at Ria Formosa with tidal mills (XII century) and recent tidal energy assessments determined a mean and maximum potential extractable power of 0.4 kWm-2 and 5.7 kWm-2, respectively. The RF has attracted research interest in all environmental aspects and hence there is a lot of background literature available about biology, morphodynamics and hydrodynamics. The system is particularly adequate for testing floatable TEC prototypes, and representative of the vast majority of transitional systems where these devices can be used to extract energy to power small local communities.
Licensing Information
Authorization for deployment was obtained from local maritime authorities following a fast and simple administrative procedure. The device was tethered to the seabed using a four-line catenary spread mooring system. The flow speeds, wave and wind characteristics at the deployment site were used for the design of the mooring system. The moorings consists of chain and galvanised wire mooring lines attached to 4 concrete anchors weighting approximately 1 ton each. The device is a simple fixed pitch downstream turbine, which aligns freely with the predominant current direction. A load cell was placed for the two south and north lines, respectively, measuring the tension while the device was extracting energy. Since the prototype has been deployed for three months, it was not connected to the grid and therefore the excess generated power was dissipated as heat into the sea. The prototype was installed in collaboration with a local marine services company, which was subcontracted to provide a barge boat equipped with a winch, essential to lower the anchoring weights at their exact planned location, using RTK-DGPS positioning. The operation was performed at slack tide and involved a staff of ten people, including skippers, researchers, divers and technical operators, supervised by the maritime authorities. The prototype operated at site for three months when it was towed back to the harbour and removed from the water. All the anchoring system was removed except the anchoring weights that remained on site.
Project Progress
The general objective of SCORE was to examine a small-scale tidal current turbine (Evopod E1) to be deployed in a shallow-water estuarine environment, looking at both the impacts of the turbine on its environment and the effects of the flow conditions on the turbine. We intended to demonstrate the viability for community scale projects along with design verification for upscaling to megawatt sized devices for multiple deployments. The project embodied an extensive pre and post installation environmental monitoring program coupled with numerical modelling tools in order to better add to the body of environmental impact knowledge and de-risk future projects. Cost benefit analysis were downscaled to a regional level in different scenarios. Regional demand and supply analyses facing site specificities allowed driving down the uncertainty of this renewable energy and improved the state of research and the ability of industrialisation of the supply chain. Those analyses will focused on: (1) analyse the viability of the Ria Formosa site to produce green energy from tides; (2) analyse the viability of installing small tidal devices to power small communities or coastal facilities; (3) analyse the cumulative impacts of energy extraction.
Key Environmental Issues
Prior to the deployment a baseline marine geophysical, hydrodynamic and ecological database for the pilot site was created. Several underwater data acquisition methods were tested to identify their viability of use in a high current condition. Tested sampling methods included: (1) collection of sediment using a "Van Veen" type grab – intended for the quantification and identification of invertebrate species of infauna and also of epibenthic species that are buried in the sand; (2) bottom trawling with a beam-trawl, following the Water Framework Directive’s standards, which allowed the quantification of epibenthic species (fish and macroinvertebrates) on mobile substrates; (3) underwater visual censuses (UVC) through transects with SCUBA diving, for the identification and quantification of epibenthic fish and invertebrate on mobile substrates; and (4) video transects with Remote Operating Vehicle (ROV, SEABOTIX L200 equipped with two forward-facing cameras), which was used to identify/quantify epibenthic habitats and species through the analysis of videos collected during each immersion of the underwater vehicle. Moreover, the interactions between marine mammals, marine turtles, seabird and fish with the turbine were evaluated through visual census and the colonization of the mooring system was assessed by visual inspection of 3 fixed quadrats (10x10 cm) in two opposite anchoring weights (3x3 photoquadrats).
Prior to the installation of E1, a baseline measurement of noise level was performed in January 2017. The acoustic data was collected with an autonomous hydrophone, the digitalHyd SR-1, installed on a tripod structure at a water depth of approximately 11 m, for 13 full days. A similar acquisition procedure was repeated during the device operation for an interval of 19 full days in August 2017. In both occasions, the equipment was set to record 90 s of acoustic data every 10 minutes, over a frequency band from 0 to approximately 24 kHz. The data analysis consists in obtaining estimates of sound pressure levels (SPL) over the entire acquisition interval, mainly based on statistical indicators both for broadband sound pressure level (SPL) and frequency levels. In November 2017, a complementary data recording was carried out during half of a tide cycle from a boat, by displacing the boat from a flow line passing the rotor.
The data collected able identifying all possible environmental impacts and interactions from TEC operation and array devices on different time scales, allowing to propose mitigation procedures for optimising the extraction capacity of TECs, while assessing the potential local/specific impacts for different extraction quantities and for implementation/operation/decommissioning.