Thermodynamics and Morphodynamics in Wave Energy

Ocean waves offer a field worth of energetic use. Power from waves impinging the coast worldwide can be estimated in 106 MW, reaching 107 MW if that power is farmed off-shore, (Cruz in Ocean Wave Energy. Springer, 2008, [6]), (Falnes in Mar Estruct 20:185–201, 2007, [13]). Up to now, a long road has been travelled regarding the essentials in the start up of basic concepts and technologies, leading to an open field for a competitive use of the ocean energy resource. Once the world wide ground has been set ready for the urgent need of a better environmental use of natural resources, and once the main technical limitations, gaps and barriers and main economical, social and political issues have been identified, the time is now to take a step further and start to advance in the enhancement of technologies that make the concept of ocean energy usage become a sustainable reality. This book provides with a contribution along a path open by others, in which some advances and improvements in the field of wind wave energy resource conversion are presented, as a contribution aimed to a more efficient usage of clean and sustainable resources in general and to OWC more specifically.

Oscillating Water Column (OWC) are devices for wave energy extraction equipped with turbines for energy conversion. The purpose of the present chapter is to study the thermodynamic of a real gas flow through the turbine and its differences with respect to the ideal gas hypothesis, with the final goal to be applied to OWC systems. The effect of moisture in the air chamber of the OWC entails variations on the atmospheric conditions near the turbine, modifying its performance and efficiency. In this chapter the influence of humid air in the performance of the turbine is studied. Experimental work is carried out and a real gas model is asserted, in order to take a first approach to quantify the extent of influence of the air-water vapour mixture in the turbine performance. The application of a real gas model and the experimental study confirmed the deviations of the turbine performance from the expected values depending on flow rate, moisture and temperature.

Oscillating Water Column (OWC) devices are usually modelled as simple systems containing ideal, dry air. However, high humidity levels are likely to occur in a prototype device open to the sea, particularly in warm climates such as prevail in the lower latitudes. In this chapter, a real gas model is implemented to take into account humidity variations inside an OWC chamber. Using a modified adiabatic index, theoretical expressions are derived for the thermodynamic state variables including enthalpy, entropy and specific heat. The model is validated against experimental data, and shown to provide better agreement than obtained using the ideal gas assumption. By calculating real air flow in an OWC it is shown that the mechanical efficiency reduces and the flow phase alters with respect to the ideal gas case. Accurate prediction of efficiency is essential for the optimal design and management of OWC wave energy converters.

Air turbines are commonly used in Oscillating Water Column (OWC) devices for wave energy conversion. This chapter presents a proposed methodology to simulate the performance of an OWC turbine through the implementation of an Actuator Disk Model (ADM) in Fluent\(^{\circledR }\). A set of different regular wave tests are developed in a 2D numerical wave flume. The model is tested using the information analysed from experimental tests on a Wells type turbine, carried out in wind tunnel. Linear response is achieved in terms of pressure drop and air flow in all cases, proving effectively the actuator disk model applicability to OWC devices.

This chapter presents a numerical model to analyse the effects of changes in the bedforms morphology on Oscillating Water Column (OWC) wave energy devices. The model was developed in FLUENT\(^{\circledR }\) and based on the Actuator Disk Model theory to simulate the turbine performance. The seabed forms were reproduced with the morphodynamic model XBeach-G for a series of characteristic sea states in Playa Granada (southern Spain). These bedforms were used as input bed geometries in FLUENT\(^{\circledR }\) and compared with a hypothetical flat seabed to analyse the effects of changes in bed level on the OWC performance. Results of the simulated sea states reveal the influence of the seabed morphology in the power take–off performance, affecting the relationship between pressure drop and air flow rate through the turbine. Energy dissipation was found to be directly dependent on the bedforms unit volume. This lead to lower mean efficiencies for the cases with evolved morphologies (up to \(15\%\)) compared to those obtained for the hypothetical flat cases (\(19\%\)). The effects of seabed formations on the power take–off performance presented in this chapter can be of interest in planning control strategies for OWC devices.

Many worldwide coasts are under erosion with climate projections indicating that damages will rise in future decades. Specifically, deltaic coasts are highly vulnerable systems due to their low-lying characteristics. This chapter investigates the role of wave energy converter (WEC) farms on the protection of an eroding gravel-dominated deltaic coast (Guadalfeo, southern Spain). Eight scenarios with different alongshore locations of the wave farm were defined and results were compared with the present (no farm) configuration of the coast. Assuming that storm conditions drive the main destruction to the coast, we analysed the impact of the most energetic storm conditions and quantified the effects of the location of the farm. Significant wave heights in the lee of the farm were calculated by means of a calibrated wave propagation model (Delft3D-Wave); whereas wave run-up and morphological changes in eight beach profiles were quantified by means of a calibrated morphodynamic model (XBeach-G). The farm induces average reductions in significant wave heights at 10 m water depth and wave run-up on the coast down to 18.3% and 10.6%, respectively, in the stretch of beach most affected by erosion problems (Playa Granada). Furthermore, the erosion of the beach reduces by 44.5% in Playa Granada and 23.3% in the entire deltaic coast. Combining these results with previous works at the study site allowed selecting the best alternative of wave farm location based not only on coastal protection but also on energetic performance criteria. This chapter, whose methodology is feasibly extensible to other coasts worldwide, provides insights into the role of the alongshore location of WEC farms on wave propagation, run-up and morphological storm response of deltaic coasts.