The scarcity of habitable land, growing energy consumption and environmental repercussions of fossil fuels are fostering the development of renewable energy projects in the marine environment. The oceans receive 70% of the global primary energy resource: radiation from the sun [1
]. Intensive research is devoted to developing technologies in offshore wind, wave and tidal energy as the main forms of marine renewable energy [2
Regarding wave energy, there exists a vast resource with a high energy density and good predictability, two properties of major interest for electricity generation [3
]. For this reason, many wave energy conversion concepts have been proposed during the past decades. Most of the wave energy converters can be grouped into one of the following categories: oscillating systems (e.g., CECO [4
]), overtopping devices (e.g., Wavecat [5
]) and oscillating water columns (e.g., [6
]). However, despite the large research effort and number of available concepts, no technology appears to be mature enough at this point for commercial projects [7
Tidal energy is another well-known marine renewable energy, which can be harvested by means of either tidal barrages or tidal stream turbines. Tidal barrages are a well-proven technology but have two major downsides—the large capital expenditure required, and the environmental impact [8
]. As for tidal stream turbines, the locations of interest, i.e., where a project can be economically viable, are limited [9
As for wind energy, offshore farms have been in operation and connected to the grid since the 1990’s, and have experienced a substantial growth in the last decade, especially in Europe [10
]. These commercial farms consist of wind turbines fixed to the sea bottom in water depths below 50 m by means of foundations, such as monopiles, gravity structures, jackets, tripods, and tripiles ([11
], Figure 1
). Nonetheless, because of the limited amount of shallow waters for deploying fixed foundations, most of the future offshore wind farms will be installed in deeper water [12
]. For this purpose, wind turbines with floating foundations are required, with some concepts already tested under real conditions in the recent years (e.g., WindFloat [13
In addition to these marine renewable energy resources, there is an alternative that has been little explored in the marine environment: solar energy [14
]. To harvest this resource in the oceans and seas, floating photovoltaic (FPV) systems are required (Figure 1
). Although applying this technology in the marine environment is new, FPV farms have been deployed worldwide in freshwater, including lakes and reservoirs [15
]. The main advantage of FPV systems against land-based ones is the water cooling on the solar cells [16
]. This effect results in a higher energy conversion efficiency of the floating panels, which can generate up to 10% more electricity [17
]. Other advantages of offshore FPV systems include: the availability of abundant water for cleaning the panels, the scalability of the systems from microwatt to megawatt, and the reduction in the growth of algae by the shading effect of the panels [14
FPV systems in the marine environment can be more economical than wind farms at latitudes between 45° South and 45° North [18
]. On these grounds, China and the Netherlands have started to deploy FPV systems in their maritime areas [19
]. Notwithstanding, much work remains to be done in assessing the offshore photovoltaic potential (only a handful of areas investigated so far, e.g., India [20
] and the Maltese islands [21
]), and in developing reliable structures to resist the accelerated rusting in saltwater and the extreme dynamics of the marine actions (mainly winds, waves and tides) [22
On another note, the combination of marine renewable energies is a promising solution that is supported by many synergies, such as the increase in the energy production and the reduction in the operation and maintenance cost [23
]. Previous studies have shed light on the synergies between wind and wave energy, including topics such as the optimal array design (e.g., [24
]) and the reduction in operation and maintenance costs (e.g., [25
]). Nonetheless, wind–solar farm synergies remain unexplored, and only their combined use with aquaculture has been proposed [26
]. Bearing in mind this scenario, the potential of combined wind–solar farms should be evaluated. A basic arrangement would be filling with FPV panels the free-surface amidst the offshore wind turbines, which avoids interferences in the production of both renewables (Figure 2
Asturias, a coastal region in Northern Spain with more than 300 km of coastline, is keen to develop its marine renewable energy potential. The wave energy resource in Asturias has been assessed [27
]. Recently, Abanades et al. [28
] proposed using wave energy converters in this region with a dual-purpose: the production of carbon-free energy and the mitigation of coastal erosion—a severe issue in the context of climate change. Regarding tidal energy, the available locations are scarce along the Asturian coastline. Some examples are the Ria of Ribadeo [29
] and the Port of Aviles within the Nalon River estuary [9
] (Figure 3
). As for the offshore wind and solar energy resource, a detailed assessment is lacking. Although the narrow continental shelf compounds the deployment of fixed wind turbines, floating solutions may be used (Figure 1
In this work, the potential for developing floating wind and solar energy off the coast of Asturias is assessed and the benefits of their combination are examined. For this purpose, the gross resource of each renewable across the area of study is estimated by means of the available data, mainly from observational and numerical (hindcasting) meteorological databases. Then, the expected energy production of offshore solar and wind farms—separately and combined—is estimated considering the specifications of commercial technology. In the discussion, the potentials of offshore solar and wind energy are compared, taking into account the rated power of each technology, the marine space occupation, and the variability of their power output. From the results it is clear that combined offshore wind-solar farms present a production synergy that should be considered in future marine renewable energy projects.
The remainder of this paper is structured as follows: Section 2
describes the study area and presents the data used to assess the resources, the technology specifications used to calculate the energy production, and the methodology. In Section 3
, results are presented and discussed. Finally, conclusions are drawn and further research lines suggested in Section 4
The first research into combining offshore wind and solar power was conducted in this work through a case study off Asturias (North Spain). Floating technology would be required virtually anywhere in the study area, given that water depths exceed 50 m except in a narrow coastal fringe. The performance of several technologies—three wind turbines and three solar panels, separately and in combination—was examined by considering the energy output, the specific yield, the power output variability, and the capacity factor. Bearing in mind the results, a combined offshore wind–solar farm is proposed and its performance examined.
The wind energy resource across the study area decreases towards the coast, from ~400 W/m2 in the deeper parts to ~100 W/m2 near shore. The wind turbine with the highest rated power would yield 15 GWh/year in the North West section of the study area, which corresponds approximately to the limit of the continental shelf—where water depths increase abruptly from 200 m to over 1000 m.
As for the solar resource, the horizontal irradiance on a horizontal surface varies only weakly across the study area, with values of approx. 150 W/m2, while the expected energy output for a commercial PV panel would vary between 300 to 400 kWh/year, depending on the model and the location. Unlike wind energy, the expected solar energy output varies little across the study area. Therefore, power plants in deep waters would not be required to maximize the solar energy output—an advantage of offshore PV farms versus offshore wind energy farms.
When the performance of a commercial 6.2 MW wind turbine and a 5 MW FPV farm are compared, the former presents far higher values of the energy output and the capacity factor. This first approximation could be considered favourable to wind energy, but the FPV farm presents a much lower variability of the production, and its specific yield is some seven times higher. These properties justify combining both with a view to smoothing the power output and realising economies of scale.
On this basis, a hybrid system was proposed and the production synergies were investigated. A combined offshore wind–solar farm can reach 57.5 MW/km2 of capacity density and 61.2 GWh/(km2·year) of specific yield—10 and 7 times the typical values for stand-alone offshore wind turbines, respectively. Furthermore, the power output of offshore wind turbines and FPV systems is significantly smoothed when both are combined, with a 68% reduction in the power output variability relative to a stand-alone wind farm.
In summary, a hybrid marine renewable energy farm with offshore wind turbines and FPV would not only increase the power output per unit surface area of marine space, but also improve the quality of the power output by reducing its temporal variability.