Design and optimization of onshore wave energy conversion systems

Energy is an essential element for human life. Increasing population and associated industrial development have raised energy consumption. There are two type of energy sources: non-renewable and renewable. Non-renewable resources are those resources that can't be replaced once consumed. Fossil fuels and nuclear material are good examples of non-renewable energy resources. As opposed to these, renewable energy resources are those resources that can be replenished through natural processes once they are used. Most of these resources are affected by the sun in one way or another. For example, thermal, photochemical and photoelectric energies are directly derived for the sun; wind, hydropower and photosynthetic energy stored in biomass are indirectly derived from the sun, whereas geothermal and tidal energies are derived from other natural movements and mechanisms of the environment. Abundant thermal energy from the sun's heat, mechanical energy from the tides and waves of water could be produced from the ocean as the oceans cover more than 70% of Earth's surface, making them the world's largest solar collectors. The ocean thermal energy is created because of temperature deference in surface water and deep ocean water. Ocean thermal energy can be used for generating electricity and many other applications. The conversion of mechanical energy into electrical energy can be achieved by making use of conversion systems such as: closed-cycle, open-cycle, and hybrid. In the closed cycle systems, working fluids that have low boiling point (for example ammonia) are used to vaporize water from the warm ocean water surface, which is then used to turn turbines. In open-cycle systems, low pressure is used to boil the ocean surface water and the resulting steam is used to turn turbines. Hybrid systems are those system that work by combining the principle of both closed-cycle and open-cycle systems. There are differences between ocean mechanical and thermal energies. In ocean mechanical energy, gravitational pull of the moon is the primary force that drives tides and the winds are the primary forces that drive waves, even though all the ocean activities are affected by the sun. As a result, the ocean mechanical energy is variable, whereas the ocean thermal energies remain fairly constant. In addition, the energy conversion system of the ocean mechanical energy usually involves the use of mechanical devices as opposed to ocean thermal energy. Recently, wave energy has attracted significant attention because of its non-polluting nature, environment friendliness, low operational cost and simple maintenance procedures compared to other clean energy sources. There is a considerable amount of wave energy that can be extracted from the oceans. When one compares the renewable energy types, it can be seen that wave energy in a square meter is 10 times denser than solar energy and 5 times than wind energy. By considering these ratios, it can be inferred that a small efficiency increase in wave energy conversion will result in energy generation increase, which is 5-10 times more than other renewable energy forms. Wave energy conversion involves three basic systems: channel systems that funnel the waves into reservoirs; float systems that drive hydraulic pumps; and oscillating water column systems that use the waves to compress air within a container. This systems create mechanical power that either directly activates a generator or transfers power to a working fluid, water or air that is then used to drive a turbine/generator. This thesis deals with an Oscillating Water Column (OWC) Wave Energy Converter system. OWC is a mechanical system that utilizes fluctuating water level from sea waves to drive an air turbine which, in turn, provides electricity when transmitted to a generator. In OWC systems, a low-pressure Wells turbine is often used to remove the need to rectify the airflow as it rotates in the same direction irrespective of the flow direction. OWC systems are known to have less environmental impact than other devices installed in the ocean to generate renewable energy. The main advantage of the OWC systems over most other WECs is its robustness and ease of construction and installation. Other advantages of OWC system include: moving parts are housed outside of the water for a greater lifetime of the material, can be built near shore for easy access to the power grid and for maintenance and has less maintenance cost. In addition OWC devices can be installed at onshore or near-shore as a fixed structure and offshore in a floating moored-structure configuration. In this thesis, it was attempted to optimize an Oscillating Water Column (OWC) wave energy converter, which is constructed on a shoreline. In this thesis, five sets of modelling, each involving a numerical modelling and a physical experimental modelling, were conducted in a wave flume to optimize OWC systems. First, numerical models from the five different wave energy conversion systems were designed. Numerical models of the OWC system were performed by a software called Flow 3D software by changing the shape of the OWC structure, water depth and wave parameters. Second, physical models of the OWC structure were conducted in a wave flume to optimize the OWC systems. Physical experimental models were carried out by a piston-type wave maker in order to investigate the influence of wave parameters, water depth and geometry of coastal structures on the efficiency of the system. The data used for designing the optimal geometry of the chamber that may yield the maximum conversion of wave energy to useful energy were provided from the interpretation of the measurements of these parameters. In this thesis, the five the numerical and the five physical model based experimental results were compared with each other by taking in to consideration the mean squared error (MSE), coefficient of determination (R2) and Nash-Sutcliffe efficiency (NSE) as performance evaluation criteria. Considering the results of the research, when the air volume ratio in the two structures is constant, the efficiency of the structure with sloppy front plate (Structure No. 1) was found to be higher than the structure with vertical plate (Structure No. 3). This shows the importance of shape and front plate slop on the efficiency of OWC structure. When the results of the first, fourth and fifth structures are compared, the efficiency of the fourth structure is higher than the first structure. In the modified structure (Structure No. 4), the outlet part of the OWC structure is better adapted to the streamlines and losses are less. For this reason, the efficiency of the modified structure is more than the classical structure. According to the results of this research, an optimum efficiency was achieved for the multicellular structure. The highest efficiency of the OWC chamber for all structures was obtained at an approximate value of (y≈e+H/2). This result was verified by equalizing the opening height of the chamber (e) to the value of (y-H/2).