On thermodynamics in the primary power conversion of oscillating water column wave energy converters

Abstract

Type of publicationArticle (peer-reviewed) The paper presents an investigation to the thermodynamics of the air flow in the air chamber for the oscillating water column wave energy converters, in which the oscillating water surface in the water column pressurizes or de-pressurises the air in the chamber. To study the thermodynamics and the compressibility of the air in the chamber, a method is developed in this research: the power take-off is replaced with an accepted semi-empirical relationship between the air flow rate and the oscillating water column chamber pressure, and the thermodynamic process is simplified as an isentropic process. This facilitates the use of a direct expression for the work done on the power take-off by the flowing air and the generation of a single differential equation that defines the thermodynamic process occurring inside the air chamber. Solving the differential equation, the chamber pressure can be obtained if the interior water surface motion is known or the chamber volume (thus the interior water surface motion) if the chamber pressure is known. As a result, the effects of the air compressibility can be studied. Examples given in the paper have shown the compressibility, and its effects on the power losses for large oscillating water column devices. V C 2013 American Institute of Physics.

Thermodynamics of an OWC containing real gas

“…It is found that the adiabatic index depends linearly on temperature and pressure. The real gas theory has also been applied to the hypothetical OWC device 405 proposed by Sheng et al (2013), obtaining modified expressions for inhalation and exhalation of the air flow. It is shown that differences between the ideal and real gas models can reach 6%.…”

“…Even when the assumption that the process is adiabatic appears totally justified (see e.g. Falcão & Justino (1999) and Sheng et al (2013)), local temperature and 45 moisture conditions might cause deviation from the polytropic ideal gas model. In fact, air confined inside the chamber and surrounding the OWC is a mixture of dry air and water vapour under ambient sea conditions.…”

“…Equations (33) and (32) are essentially extended versions of the equations presented by Sheng et al (2013), which take into account the effect on n of the real gas.…”

“…Using a similar methodology to that of Sheng et al (2013), who considered an ideal gas, the thermodynamic theory developed herein for real gas is applied to the basic OWC chamber. The subscript "g" is assigned to variables inside the chamber.…”

“…Air flow in the OWC is calculated for exhalation and inhalation processes driven by harmonic pressure change using equations (32) and (33) Figure 4: Temporal variations of efficiency of an OWC covering the inhalation (first half of the cycle) and exhalation (second half of the cycle) stages over one oscillation period T , for a sinusoidal pressure signal of amplitude 100kPa. Sheng et al (2013) for an ideal gas flow. Figure 5 shows the time-dependent phase difference (∆ψ) obtained between the real and ideal air flow estimates over a complete pressure cycle.…”