**1. Introduction**

The large-scale exploitation of fossil fuels has had important environmental repercussions such as climate change or the rise in sea level amongst others [1,2]. This means that there is currently a need to reduce the dependence on fossil fuels and place greater emphasis on renewable energy sources in order to fulfil future sustainable energy needs [3,4]. In 2009, the European Union (EU) established that 20% of final energy consumption should originate from renewable sources by 2020 [5], and additionally set 2050 as the target year by which emissions will have been reduced by 80% [6,7]. In this context, and with the aim of achieving a sustainable development, in 2015, the United Nations Framework Convention on Climate Change was established in order to reduce the causes of climate change as regards food production and limit the increase in temperature (increases of up to 1.5 ◦C) [8]. A number

of viable renewable energies could, therefore, be exploited to achieve this goal, among which marine renewable energy (MRE) is attracting increased attention [9].

MRE is currently recognized as being an abundant, geographically diverse energy resource that has both the public's acceptance and positive associated externalities (economic growth, job creation or the mitigation of the negative impacts of climate change, etc.) [10]. It can be exploited from offshore wind, waves, tides, tidal currents, thermal gradients or salinity gradients. This paper is focused on the exploitation of the tidal current resource, which it is hoped will play a major role in meeting future energy needs with regard to other renewable energy sources thanks to its high predictability, stability and high load factor [11]. If we wish to employ technologies to harness the energy obtained from tidal currents in order to attain sustainable development, it is necessary to use natural resources in an efficient manner, i.e., we must optimize their exploitation [12]. Devices that can be utilized to harness tidal current power where the depth is no greater than 40 m have been developed by various technology manufacturers [13,14]. These devices, which are denominated as first generation tidal energy converters (TECs), are normally supported on bases that are fastened to the seabed by means of various types of anchoring systems (monopole, piloted or gravity). It is undoubtedly technically feasible to employ these devices to harness energy from tidal currents, although very few tidal stream projects are currently operating at a commercial stage [15]. This is principally owing to the fact that it typically costs more to generate energy from tidal currents than it does when using other renewable technologies [16]. It is consequently vital to understand the parameters that may affect the cost structure in order to provide a framework containing areas in which these costs can be reduced [17].

A detailed analysis of the life cycle costs (LCC) for first generation tidal energy farms (TEFs) shows that the installation and maintenance procedures are of the utmost importance and must be optimized in order to increase their current potential, help their acceleration and sustainability and help attract investment in these technologies [18,19]. These procedures include the transportation of each of the TECs from the base port to its installation site, the preparation of the seabed, the placement and installation of anchor systems and/or the deployment of a mooring system, and the positioning, connection and disconnection of the main units of the devices [20,21]. These tasks necessitate the use of special high-performance ships that are equipped with dynamic positioning, large cranes, etc., and this implies high installation and maintenance costs [22,23]. The installation and maintenance costs are greatly dependent upon the accessibility and reliability of the device [16]. It would, however, be possible to increase system performance and decrease the aforementioned costs by automating the performance of the immersion and emersion maneuvers [24,25]. This can be done by controlling the ballast water inside the devices, which consequently permits the implementation of a closed loop depth and/or orientation control that makes(s) it possible to: (i) raise the generation unit from the seabed to the surface of the sea and (ii) carry out the same operation but in reverse. We can perform these automatic maneuvers by using small guide wires and by controlling the ballast water inside the device. The achievement of this objective will consequently influence: (i) less human intervention, (ii) the possibility of using the cheapest general purpose ships rather than high cost special vessels for maintenance purposes, (iii) a reduction in the number and duration of installation and maintenance operations or (iv) an increase in the competitiveness of these technologies, among others. The potential benefits of these systems are very important, but, as they are in an early stage of development, studies that address the economic feasibility of these systems have not yet been developed. Several authors have produced interesting papers comprising feasibility studies concerning other types of offshore projects. These include: wind energy [26], wave energy [27], co-located projects (wind and wave energy) [28] and hybrid projects (wind and wave energy) [29]. No economic-financial studies focusing on the automation of installation and maintenance maneuvers have, however, been produced to date.

The main contributions of this research are the following: (i) we discuss the merits of automated installation and maintenance maneuvers with regard to manual maneuvers for an idealized gravity (a substantial mass is used to support the structure on which the TEC is placed.) -based first generation TEC designed by our research group (Grupo de Investigación Tecnológico en Energías Renovables

Marinas, *GIT-ERM*); (ii) we provide interesting information about manual and automated installation and operation maneuvers for these tidal energy technologies, which is not usually found in scientific literature as these technologies are at an initial (pre-commercial) stage of development, and (iii) we carry out a comparative economic-financial feasibility study for these maneuvers, which illustrates that the development of advanced automation systems for these maneuvers may be a very interesting approach by which to increase the competitiveness of this source of renewable energy in the near future.

The remainder of the paper is organized as follows: Section 2 describes the procedures used to carry out installation and maintenance maneuvers for first generation TECs in both a manual and an automated fashion. The procedure used to evaluate the economic-financial feasibility of tidal energy projects using manual or automated maneuvers is briefly explained in Section 3. Section 4 shows the results attained after carrying out a numerical case study of a 50 MW TEF in the cases of both manual and automated maneuvers. Finally, Section 5 is devoted to our conclusions and proposals for future works.
