Tidal stream energy exploitation trought marine spatial planning

Resumo

In the last years, environmental concern has increased. Protests against climate change have been massive in the main cities of the world. Deadlines for points of no return have been reached again and again. Now, the problem of climate change caused by humans is a reality, and adverse effects can only be mitigated. This problem is closely linked to emissions of greenhouse gases. One of the main contributors to emissions is power generation. Technology and research are developing more and more competitive green technologies. In some cases, like wind power, it is a high proportion of the power generation mix of developed countries. Following the example of wind power, marine energy might help to decrease the use of fossil fuels. Tidal energy is one of the marine energies with the highest development capacity. There are serveral ways to obtain energy from the tides. A dam in an estuary can be built, in order to harness the dammed tide every tidal cycle. Morphological features that create currents can also be used to harvest energy. To take advantage of morphological features, so far, most converters had to be installed in high draft places. So, potential locations of moderate depths could not be used. For example, estuaries, where depths are usually lower than 20m. In the northwestern coast of the Iberian Peninsule, estuaries have been studied for energetic use of tidal currents. New tidal energy converters are able to work at places with low depths. So, the chance to use the energy of tidal currents in estuaries is open. Currently, there is no commercial use yet, due mainly to three causes that keep the take off of this renewable technology on standby: 1. Technological difficulties arising from the installation of machinery on marine environment. 2. Preservation of natural areas can generate protests against these projects. 3. There is a real risk, hard to quantify, of economic failure, due to changes on boundary conditions. Thus, generating electricity from marine energy has lights and shadows that can only be tackled and solved by a thorough scientific analysis of all the points that can take the project to failure. A marine spatial planning is important, but so is assessing risks arising from the installation of these devices on places with other uses. The central axis of this thesis is focused on three basic objectives: studying accurately the morphodynamic processes, the uses of the estuaries and minimizing the risks to find the best locations. These objectives should be useful for decision-making when planning tidal stream farms, based on the example of the estuary of Ribadeo. So, the work is structured in three main chapters: In Chapter 1 a study of the sedimentary dynamics of the estuary was done, in order to know how it affects the traffic of the main ports. To this end, following the Spanish recommendations for construction of access channels to ports and docks, the dimensiones of the access channel to the estuary are defined taking into account the technical determinants of ships. Subsequently, a high resolution numerical model, which is able to model the sedimentary dynamics of the estuary, is defined. Once the numerical model is validated, it can be used for a high resolution modelling projecting four years into the future. This information is essential to check where increasing and erosion take place and, therefore, to determine how this movement of sediments affects the functionality of the access channel. Based on safety limits set for port operations, functionality values that cannot be exceeded are set. Finally, a timetable of maintenance dredgings is set. It will be vital to keep the port structures, and the local economy, in operation. When modelling an estuary as a potential location for energetic use, bathymetry is one of the most important boundary conditions. As seen in chapter 1, regular dredgings are needed to keep the access channels safe. So, the bathymetry of the studied zone is going to change regularly. Chapter 2 is focused on studying currents for energetic use, and investigating potential impacts of bathymetric changes on TEC performance. In addition, preliminary figures (operation hours, magnitude, and direction) are suggested to ensure economic viability of the installations. For this purpose, a 3D numerical model has been implemented, to calculate two different scenarios, before and after a maintenance dredging. Three possible zones, with appropriate dimensions to build tidal stream farms, were found. At each location, one point was analyzed for a medium tide time. At two places, after a periodical dredging, available velocity and energy decreased. At the third point, both increased. Based on these results, the effect of these changes on turbine production is studied. Fitting tidal stream farms in surroundings with multiple uses and activities is the third goal of this thesis. To this purpose, a classification of uses is proposed, based on conducted studies, EU, state and regional law, and technical criteria. This classification limits and excludes installing TECs at zones with existing uses. Thus, potential conflicts decrease, the environment is preserved, zones where investment is feasible are created and positive synergies between different uses arise. In parallel, the work performed in chapter 2 is resumed. A study of annual resource, before and after a dredging is performed by means of a concise numerical model. This model shows the annual energy in all the domain of potential energy use. So far, conducted studies always confined the analysis of energetic resource to a point in the map and a determined time. With this new projection, maps with information of all the annual resource can be created. These maps can be cross-checked with exclusion layers, so that an investor can quickly decide the most appropriate location (Chapter 3). Finally, a tool for location and quantification of the risk associated to periodical dredging (DAR) is created. Combined with the previous layers, it allows to end the work of locating an energetic use zone. Thus, the risk of increasing or decreasing stream velocities and less hours of operation of the devices is limited. This way, the uncertainty is reduced, and so are the zones where TECs can be installed In conclusion, this thesis addresses in a global way the difficulties of finding suitable locations for tidal stream exploitation, without clashing with existing uses. Risk and operation uncertainty are decreased. So, decision-making is made easier for investors. A multidisciplinary action protocol has been created. From the example of the estuary of Ribadeo, it can be easily extrapolated to other estuaries.