Energy production from tidal currents in an estuary: A comparative study of floating and bottom-fixed turbines M. S  anchez a, * , R. Carballo a , V. Ramos a , G. Iglesias b a University of Santiago de Compostela, Hydraulic Engineering, Campus Univ. s/n, 27002 Lugo, Spain b University of Plymouth, School of Marine Science and Engineering, Marine Building, Drakes Circus, Plymouth PL4 8AA, United Kingdom article info Article history: Received 12 June 2014 Received in revised form 18 September 2014 Accepted 20 September 2014 Available online 12 October 2014 Keywords: Tidal stream energy Tidal turbine Ria de Ortigueira Estuary Numerical modelling abstract In a tidal stream project the selection of the most appropriate device is of major importance. The aim of this work is to investigate the difference between two tidal farms, one with floating TSTs (Tidal Stream Turbines), the other with bottom-fixed TSTs, in terms of annual performance and its monthly variability. This investigation is carried out considering real operational conditions in a case study: Ria de Ortigueira (NW Spain), a drowned river valleys which is one of the most promising sites for tidal stream energy exploitation in the Iberian Peninsula. A 3D, high-resolution, numerical model is applied to simulate the hydrodynamics of the ria during an entire year with either the floating or bottom-fixed TSTs, and, on these grounds, determine the most representative performance parameters. Significant differences emerge in the performance of both plants; these are due to a great extent to the vertical variation in the flow velocity, which is relevant at many sites of interest for tidal stream energy exploitation such as Ria de Ortigueira. Finally, relevant variations were identified in the intra-annual performance which must be borne in mind in dimensioning the plant. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction In the late 90's social pressure emerged claiming for a new, sustainable energy production and utilisation model [1]. This, together with a number of policies aimed at curbing greenhouse gas emissions and the realisation that the current energy model, with fossil fuels at its core, is necessarily limited in time, has resulted in companies and governments around the world being currently working intensively on the development and efficient utilisation of new and renewable energy sources [2]. Tidal stream energy is one of the most attractive and promising renewable energy sources owing to its advantages relative to other renewables: no land occupation, a high load factor (water density is ~800 times higher than air density), flow predictability and consequently power production predictability, inexistence of extreme flows (which might otherwise damage the conversion devices), etc. are some of the advantages of tidal stream energy exploitation [3e6]. However, due to the relatively recent interest in this type of energy (in comparison with other renewables such as wind or solar energy), only a few TECs (Tidal Energy Converters) have achieved at this point a commercial or pre-commercial stage [7,8]. The conversion principle varies between the different TECs developed or in a development stage. Generally speaking, there exist two types of TECs, based on either the reciprocating or rotating principle. The latter, also known as TSTs (Tidal Stream Turbines), is the most popular. TSTs can be: (i) floating beneath the surface and anchored to the bottom by means of chains or cables (floating TSTs), or (ii) rigidly attached to the bottom by means of a structure (bottom-fixed TSTs) [9,10]. As a result of their different configuration, despite them being installed at the same coastal site, their performance may differ due to the varying hydrodynamic conditions throughout the water column (each type of TST is located at a different position within the water column). However, the implications for the power performance of opting for either floating or bottom-fixed devices have not been investigated as far as the authors are aware. The aim of this study is to examine the differences in the power performance in real conditions of operation of two tidal stream plants, composed by either floating or bottom-fixed TSTs, proposed in a previous work [11], where their impacts on the estuarine hy- drodynamics were analysed. For this purpose, a 3D high-resolution numerical model is implemented in the Ria de Ortigueira, a drowned river valleys which is a promising site for tidal stream energy exploitation located in Galicia (NW Spain) (Fig. 1). On the basis of the numerical results a detailed intra-annual power per- formance assessment of floating and bottom-fixed tidal plants have * Corresponding author. Tel.: þ34 982823650; fax: þ34 982285926. E-mail address: marcos.sanchez@usc.es (M. S anchez). Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy http://dx.doi.org/10.1016/j.energy.2014.09.053 0360-5442/© 2014 Elsevier Ltd. All rights reserved. Energy 77 (2014) 802e811