Thermodynamic processes involved in wave energy extraction

Abstract

Wave energy is one of the most promising renewable energy sources for future exploitation. This thesis focuses on thermodynamic effects on Oscillating Water Column (OWC) devices equipped with Wells turbines, particularly humidity effects. Previous theoretical studies of the operation of OWCs have resulted in expressions for the oscillation of the water surface in the chamber of an OWC based on linear wave theory, and the air expansion-compression cycle inside the air chamber based on ideal gas theory.

Although in practice high humidity levels occur in OWC devices open to the sea, the influence of atmospheric conditions such as temperature and moisture on the performance of Wells turbines has not yet been studied in the field of ocean energy. Researchers have reported substantial differences between predicted and measured power output, and performance rates of OWCs presently coming into operation. The effect of moisture in the air chamber of the OWC causes variations on the atmospheric conditions near the turbine, modifying its performance and efficiency. The discrepancies in available power to the turbine are believed to be due to the humid air conditions, which had not been modelled previously.

This thesis presents a study of the influence of humid air on the performance of an idealised Wells turbine in the chamber of an OWC using a real gas model. A new formulation is presented, including a modified adiabatic index, and subsequent modified thermodynamic state variables such as enthalpy, entropy and specific heat. The formulation is validated against experimental data, and found to exhibit better agreement than the ideal approach. The analysis indicates that the real gas behaviour can be explained by a non-dimensional number which depends on the local pressure and temperature in the OWC chamber. A first approach to the OWC formulation through the calculation of real air ow in the OWC is given, which predicts a 6% decrease in efficiency with respect to the ideal case when it is tested with a hypothetical pulse of pressure. This is important because accurate prediction of efficiency is essential for the optimal design and management of OWC converters.