No need of fixed and impacting structures onshore or on the seabed: we believe that floating wave energy converters (WECs), characterised by dimensions and mooring system like those of a boat, could be the best option in the case of small islands of great natural interest. The device we are developing at the University of Bologna is small, modular and wholly made of recycled and eco-friendly materials. It can be classified as a point-absorber, which means a WEC, symmetric in shape, that produces energy regardless of the direction of waves – a characteristic that is particularly indicated in the case of multidirectional sea states, like those of an island. Our compact floating WEC could be placed in an area of low seabed outside Giglio Porto, paying particular attention to the underwater flora when positioning the anchors, and could become a symbol of the possible sustainable use of the sea, figuratively in opposition to the past tragedy of Costa Concordia which took place in those shallow waters.
Based on our experience, we propose the installation of a floating device which would have many advantages in the case of a small touristic island: no need to rebuilt harbour structures, therefore avoiding any visual impact and major structural work; no large fixed structure onshore or on the seabed and therefore reduced visual and environmental impact; greater social acceptance since the installation would only require anchors on the seabed. Moreover, we propose a WEC of the type “point absorber”, which, compared to other devices, has the additional benefit of harvesting energy regardless of wave direction, therefore maximizing the production in the case of an island. Our device, in particular, is small, with a minor part emerging from the water, and ensures a minimum occupation of space in case of multiple installation. It will be realized with only recycled and eco-friendly materials in the context of circular economy. The device has already been successfully tested through numerical simulations with different wave conditions, while the experimental test campaign is planned for the next months in the new wave basin that has recently been inaugurated at the University of Bologna. The experimental tests will allow the assessment of the producibility with the Power-take-Off (PTO) system supplied by Umbra Group, a leading company in this field. In the case of Giglio Island, the device could be placed off the coast of Giglio Porto and could supply energy to the desalination plant. Moreover, the wave farm could be integrated with other renewable energy sources in order to cover the variable energy demand of the Island during the whole year, minimizing the need of fossil-fuels.
Context
Giglio Island is the second largest island of Tuscan Archipelago, an unspoiled place of great natural interest. At the same time, like the majority of small island of the Mediterranean region, it depends on polluting and expensive imported fossil fuels for its electricity needs, despite the large availability of renewable sources. It should also be considered that islanders consume on average more electricity than the inhabitants of the coastland, since, on islands like Giglio, potable water is produced through desalination, which is a highly energy demanding process. Thus, renewable energies, and specifically wave and tidal energy, represent a great opportunity for small islands, since they could ensure energy self-sufficiency.
Objective
In this context, the Municipality of Giglio Island is looking for Blue Energy devices that could adapt to the island landscape, with low visual and environmental impact, allowing for greater social acceptance. In particular, they are questioning whether fixed onshore devices (integrated with existing structures) or offshore underwater devices could be feasible solutions. Our opinion is that small floating offshore devices might be a better option. In fact, although fixed onshore devices that could be integrated in the structures of the port are very promising (see for example the OBREC in Naples), their installation would require the removal of the rubble-mound breakwater, replaced by a concrete dam, which could have a low social acceptance in the case of a small touristic port, due to the visual impact. Moreover, these WECs can harvest energy for only a few wave directions (front waves). On the other hand, underwater devices require the occupation of a vast area of the seabed, which does not seem feasible in the case of Giglio Island, an area of great biological interest. Those considerations done, our team proposes the installation of a floating device, in particular a so-called point absorber, that compared to other types of devices have smaller dimensions and can harvest energy regardless of waves direction. The main advantage of a floating device would be the absence of impacting fixed structures, since these devices are anchored on the seabed in a similar way to that of boats.
Concept and innovation
In particular, we propose an innovative device which is currently under development at the University of Bologna. It is square-shaped and its dimensions can be specifically designed basing on the wave climate: at the considered location, it could have a side length of approximately 2.5 m and a height at its highest point of about 1.5 m, only the half of which would be visible above the surface, as a maximum. Following the principles of the circular economy, it is essential for us to realize our WEC with only recycled and eco-friendly materials, which are currently under evaluation with internationally renowned companies. The colours of the WECs, as for markers buoys, have to be bright for safety reasons, however, a great deal of attention will be paid to the design of the device in order to make it easier to the eyes. The PTO system will be provided by Umbra Group, an Italian company of great success in this field: their innovative generator can be tuned on different wave states and has already proven its effectiveness with several point-absorber WECs chrachterized by different operating principles. Our device has been tested through numerical simulations in Ansys-AQWA with different mooring system configurations and different wave conditions typical of the Adriatic Sea and it has proven to be stable with a return time up to 10 years. The experimental test campaign is planned for the next months in the brand-new wave basin (https://site.unibo.it/vasca-marittima/it) that has recently been inaugurated at the University of Bologna and will hopefully lead to a patent. Moreover, compared to other point absorbers, which have a great footprint if installed in farms because of the necessary distance between one and the other, our device could be installed in a modular way, with a checkerboard pattern (sharing its sides with the adjacent devices), thus optimising the use of space. For example, a farm of 32 devices (4×8) would occupy an area of approximately 9x18m, almost like a medium size boat at anchor off the coast.
Landscape and environmental framework
The possible locations for the installation, only considering the surroundings of Giglio Porto, could be the area between the rocks of “Le Scole” and Giglio Porto, in particular the area off the red pier, or the area off cape “la Gabbianara”, where the wreck of Costa Concordia lied for years until its removal. In these areas, the depth of the seabed varies from 15 to 40 m, that is the ideal depth for the installation of a floating device. The underwater flora is flourishing again on the seabed after the removal of the wreck, thus a particular attention should be paid to the placement of the anchors, which could be located in the most convenient positions, following the recommendations of biologists and ecologists. The selected areas, although relatively close to shore, are out of the ferry route and pretty far from the most interesting locations for snorkelling and diving. Being the device not so far from the coast and from the urban centre, it would be quite easy to bring the energy onshore. The submerged cable would have an impact comparable to, or lower than, the submerged cable that the electricity supplier Terna is planning to install to bring the energy from the mainland.
Possible advantages and production
The number of devices to be installed depends on the energy requirements to be satisfied. For example, considering the favourable location of the WEC, it could easily power the desalination plant of Giglio Island, located in the vicinity of Giglio Porto. Being the project in a development phase, the estimation of produced energy is still uncertain. Anyway, our previous experience and international literature provide some general indication on point absorbers producibility. For example, an AquaBuoy device conveniently scaled based on the Adriatic climate could produce from 3 to 5 MWh/y depending on the location (personal elaborations within the PON project PlaCe, not yet published) while, in the Thyrrenian Sea, it could produce 10 MWh/y in Mazara del Vallo, Sicily, and up to 14 MWh/y in Alghero, Sardinia (Bozzi et al., Wave electricity production in Italian offshore: A preliminary investigation, Renweable Energy, 2014). If we suppose an energy production of 7.5 MWh/y for our device placed off the coast of Giglio Island, considering an average water consumption of 0.16 m3/day per capita and an energy consumption of 3 kWh/m3 for a reverse osmosis desalination plant, the energy needs of the desalination plant in order to meet the annual water demand of the resident population (1371 inhabitants) would be covered by a farm of 32 devices. Thirty-two is also the number of lives lost in the Costa Concordia shipwreck: in the specific case of Giglio Island, the installation could therefore represent a kind of monument to the victims bringing the island to new life and could be associated with a place of information onshore in Giglio Porto, where commemorating the disaster while sensitizing locals and tourists on the importance of blue growth and sustainable use of the sea. Together with the low aesthetic and environmental impact and the absence of huge fixed infrastructures, we believe that this floating farm could lead to a major public understanding and social acceptance than other kind of solutions.
Future development
Moreover, thinking ahead, a wider farm with higher producibility could be installed farther away from shore in deeper waters or, more easily, wave energy could be integrated with other renewable energy sources onshore, like small wind turbines or solar plants, in order to make the island energy independent, as in the case of the island of El Hierro (Canary Islands), Samso (Denmark) and many others. The University of Bologna has a wide experience in this field and has proposed a criterion for the optimal energy mixing and proved the feasibility of the combination of different renewable sources to supply a desalination plant in Tenerife Island (Dallavalle et al., Towards green transition of touristic islands through hybrid renewable energy systems. A case study in Tenerife, Canary Islands, Renewable Energy, 2021). A hybrid installation could take advantage of the seasonal variability of various renewable energy sources (sun, wind, waves) combining them in order to minimize the need of fossil-based back-up system. This kind of installations are likely to be economically advantageous in case of government incentives on renewable energy.
Example of floating point-absorber WEC with mooring lines.
Newly inaugurated wave basin at the Laboratory of hydraulic Engineering, University of Bologna.
Visual impact of a wave energy converted (e.g. OBREC) integrated in the harbour structure.
Realistically, many units like this should be installed in order to produce a sufficient amount of energy.
Giglio Porto and the possible locations for the wave farm. To give an order of magnitude, the wreck was 300 metres long, while the floating wave farm would be approximately 20 meters.
The possible advantages and positive effects of the project would be:
– – Absence of large fixed structures onshore or on the seabed.
– – No need to alter or rebuilt the existing harbour structures.
– – Reduced visual impact, almost no impact on nature and small space requirement.
– – Greater social acceptance on the grounds that the installation does not require permanent structural works but only anchors on the seabed.
– – Possibility to directly supply the desalination plant and/or the village of Giglio Porto.
– – Possibility of integration with other renewable energy sources in order to cover the variable energy demand during the whole year, minimizing the need of a fossil-based back-up system.
The WEC has been conceived and designed by:
Barbara Zanuttigh, Associate Professor at the Department of Civil, Chemical, Environmental, and Materials Engineering (https://www.unibo.it/sitoweb/barbara.zanuttigh/research). Research topics: wave energy converters – design optimisation for combined energy production and coastal protection purposes or installation in off-shore platforms; analysis and development of cost-efficient and eco-compatible interventions for beach defence planning, through interdisciplinary works; wave-structure interaction with the development of new formulae, neural networks, conceptual, physical and numerical models; coastal flooding and erosion risk, with the support of numerical and conceptual models.
Elisa Dallavalle, PhD Student at the Department of Civil, Chemical, Environmental, and Materials Engineering (https://www.unibo.it/sitoweb/elisa.dallavalle3/en). Research topics: wave energy converters; mooring systems; optimal mixing of renewable energy sources; green transition of isolated communities; offshore platforms decommissioning.
The PTO system will be provided by:
Umbra Group (https://www.umbragroup.com), founded in 1972 in Foligno, Italy. The company proposes innovative solutions in several market sectors: automation, automotive, biomedical, deformation, energy, machine tool, plastic, railway.
The following Professors could eventually contribute to the project with their expertise, ensuring an interdisciplinary approach:
Fabio Zagonari, Associate Professor at the Department for Life Quality Studies (https://www.unibo.it/sitoweb/fabio.zagonari/research). Research topics: Environmental Economics, Ethics and Sustainability, Decision Support Systems.
Luca Pietrantoni, Full Professor at the Department of Psychology “Renzo Canestrari” (https://www.unibo.it/sitoweb/luca.pietrantoni/research). Research topics: Human-technology and human-automation interaction; Human and organisational factors in safety critical organisations; Crisis and disaster psychology; Accident analysis, safety and risk management; Behaviour change programs.
Carlo Alberto Nucci, Full Professor at the Department of Electrical, Electronic, and Information Engineering “Guglielmo Marconi” (https://www.unibo.it/sitoweb/carloalberto.nucci/cv-en). Research topics: Dynamics of electric power plants and of power systems, with particular reference to power system restoration after blackouts and to the influence of load modelling on voltage collapse simulations; Electromagnetic transients of power systems, with particular reference to lightning originated ones; Smart Grids and Distribution networks operation in presence of small-scale generating plants; Fault location in distribution networks; Smart Cities and Local Energy Communities.
Marina Antonia Colangelo, Assistant Professor at the Department of Biological, Geological, and Environmental Sciences (https://www.unibo.it/sitoweb/marina.colangelo/cv-en). Research topics: Meiobenthos; Experimental designs and data analysis; marine communities; Macrobenthos; Coastal management; Sandy shores; Complexity; Disturbance.
