ORE Open Research Exeter TITLE The influence of channel geometry on tidal energy extraction in estuaries AUTHORS Garcia-Oliva, M; Djordjević, S; Tabor, GR JOURNAL Renewable Energy DEPOSITED IN ORE 20 October 2016 This version available at http://hdl.handle.net/10871/23988 COPYRIGHT AND REUSE Open Research Exeter makes this work available in accordance with publisher policies. A NOTE ON VERSIONS The version presented here may differ from the published version. If citing, you are advised to consult the published version for pagination, volume/issue and date of publication The impact of tidal energy extraction is estuaries: analysis of the influence of channel geometry. Miriam Garcia-OlivaI , Slobodan Djordjevic, Gavin R. Tabor College of Engineering, Mathematics and Physical Sciences University of Exeter, North Park Road, Exeter, EX4 4QF, United kingdom Abstract Macro-tidal estuaries in the UK not only have a high tidal range but also present strong currents in some cases, where tidal farms could be used for energy extraction but not forgetting about the associated environmental impacts beforehand. The purpose of this study is to delimit the influence that the geometry of the channel could have over the impact of a tidal farm deployed in the estuary. A hydrodynamic model (Mike21) has been used to create several cases of idealised estuaries with dimensions based on a group of locations suitable for tidal energy extraction in the UK. The maximum changes in low and high water levels with the tidal farm have been identified for each case and located within the estuary. The changes in the time for the low and high tides in a point inside the estuary have also been analysed. The effect of the drag coefficient of the turbines over the changes in water levels has been addressed as well. As a conclusion, it can be noted that the maximum changes in water levels due to the farm range from the order of mm to a few dm and the locations of these changes are strongly dependent on the geometry of each case. In addition, the effect is generally more noticeable in the increase of low water levels and the decrease of high water levels than vice-versa. This would be associated with the loss of intertidal areas and the reduction of flood risk levels. In terms of the changes in the time of low and high water levels, the effects of the farm are negligible in all cases. Finally, the use of a higher drag coefficient seems to increase the changes on water levels. Keywords: Tidal farm, Hydrodynamic impact, Estuaries, Idealised model, Sensitivity analysis, Finite volume method 1. Introduction in offshore locations while the highest tidal ranges are mostly observed in estuarine areas, as can be seen from Tides are a constant and predictable source of en- [2]. Nevertheless, it should be mentioned that some ergy. This fact represents an advantage over other renew- estuaries also show tidal currents over 1 m/s, reaching able energy sources which are intermittent and involve spring values over 2 m/s at specific locations like the a higher level of uncertainty in the estimation of the Severn estuary [3]. Thus, the use of tidal schemes in available resource. Therefore, tidal technologies could such estuaries could benefit from the combination of be used in a complementary way with other renewable tidal stream and ranges. schemes in order to cover the electricity demand in a On the other hand, environmental aspects related to more efficient way. Additionally, tidal energy can be the use of tidal technologies must be considered care- considered as an autonomous solution for less accessible fully in estuaries, which are typically protected areas locations. As indicated in the report by [1], the use of for the conservation of nature. An effect of tidal energy marine renewables can provide lower costs and reliability extraction that can be analysed more immediately is the in the supply of far-peripheral areas and islands. change in the hydrodynamic behaviour of the estuary, There is a high availability of tidal energy resource which will affect other environmental aspects, such as in the UK. The strongest tidal currents usually happen sediment transport, water quality, flood risk, intertidal habitats, etc. ICorresponding author. Email address: [email protected] Previous studies have been carried out in relation to (Miriam Garcia) the assessment of environmental impacts of tidal energy Preprint submitted to Renewable Energy August 3, 2015 schemes in estuaries. Some of them are related to the de- tidal fence. ployment of tidal barrages and lagoons [4], while others This paper is organised at follows. Initially, the analysed the use of tidal stream turbines [5], [6]. Most methodology used in order to select the estuaries covered of these studies apply numerical models to describe the in this study, classify their geometries and create the ide- hydrodynamics of a certain location with the level of alised models is presented. The next section describes detail given by the use of the real bathymetry and bound- the numerical model used, the parameters involved and ary conditions. Analytical models have also been used the design of the tidal farms included in the models. Fi- extensively to analyse the effects that energy extraction nally, the results are shown and discussed in order to could have on the hydro-environmental conditions and extract the main conclusions about the effect that geome- the remaining available power in channels. Some exam- try has on the impact of tidal farms in estuaries. ples can be found in ([7], [8], [9]). These models offer a reduced computational demand and can give an estima- 2. Materials and Methods tion of the desired results when a high degree of accuracy is not required. However, it must be highlighted that they 2.1. Estuaries Classification are only valid under certain specific circumstances. For the purpose of this study, the main estuaries in Accordingly, the results from the aforementioned mod- the UK with a high resource of tidal energy were first els depend on intrinsic parameters, such as the geometry identified based on the report carried out by [2]. The or bathymetry of the basin, the boundary conditions and report differentiates between the locations with a high the bed roughness, amongst others. In this sense, the tidal range or tidal stream energy resource (please see Fig. study by [10] stated that the sensitivity of a simple chan- 1 ). It can be seen that most of the tidal stream resources nel to energy extraction is related to water depth, length, are generally located in offshore areas, except for the width and the nature of boundary roughness. Based on Bristol Channel, while the tidal range resources appear this idea, the aim of this study is to delimit the effect to be only related to estuaries. It should be mentioned of the geometry over the impact that tidal farms have that schemes such as barrages or lagoons are not the only when they are installed in estuaries. This impact is here options for some of the locations with high tidal range. mainly related to the water levels because they affect According to the information provided by the Atlas of aspects such as flood risk levels and changes in intertidal UK Marine Renewable Energy [17], velocities frequently habitats, which are important issues in estuaries. show values over 1m/s in some locations, reaching peak This study considers the deployment of block tidal flows up to 2 m/s in a few cases . This fact allows them farms, which occupy partially the width of the estuary, to be considered as potential locations for the installation leaving space for other activities. It has also been consid- of tidal in-stream turbines, which are the focus of this ered the use of turbines supported by floating structures, project. which adapt to the tidal levels and can be moved to dif- In order to complete the list of estuaries suitable for ferent positions within the estuary if necessary. tidal energy extraction, other macro-tidal estuaries were For the purpose of this research, idealised models identified through the information provided by the estu- have been used to represent different geometries based ary data base [18] and the peak flows in these estuaries on existing estuaries in the UK suitable for tidal energy were analysed by means of the Atlas of UK Marine Re- extraction. The same tidal farm and conditions have been newables [17]. In some estuaries where the Atlas did used in all cases and a sensitivity analysis has been per- not have coverage, some of these details were obtained formed over the variation of the geometrical parameters from the existing literature ([19], [20], [21], [22], [23], in the idealised models. Idealised models benefit from [24]). Only the estuaries with peak flows above 1 m/s a reduction in computational demand, which allows to have been considered for this study (Please see Table 1) test numerous scenarios, and an easier identification of physical trends [11]. Regarding previous studies about 2.2. Geometrical Characterisation of the Estuaries the application of idealised models to estuaries two main Following, the main dimensions of the estuaries se- groups can be found: the projects related to the estuarine lected for this study were obtained from different sources, morphodynamics ([11], [12], [13]), and the more recent such as the estuary guide [18] and [25], [26], [27], [28], research about the effects of tidal energy extraction in [29]. These values can be seen in Table 2. estuaries ([14], [15]). Regarding the second group, an The study by [29] presented a formula