Physical modelling of wave energy converters

Alcorn

2014

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This is an investigation on the development of a numerical assessment method for the hydrodynamic performance of an oscillating water column (OWC) wave energy converter. In the research work, a systematic study has been carried out on how the hydrodynamic problem can be solved and represented reliably, focusing on the phenomena of the interactions of the wave-structure and the wave-internal water surface. These phenomena are extensively examined numerically to show how the hydrodynamic parameters can be reliably obtained and used for the OWC performance assessment. In studying the dynamic system, a two-body system is used for the OWC wave energy converter. The first body is the device itself, and the second body is an imaginary “piston,” which replaces part of the water at the internal water surface in the water column. One advantage of the two-body system for an OWC wave energy converter is its physical representations, and therefore, the relevant mathematical expressions and the numerical simulation can be straightforward. That is, the main hydrodynamic parameters can be assessed using the boundary element method of the potential flow in frequency domain, and the relevant parameters are transformed directly from frequency domain to time domain for the two-body system. However, as it is shown in the research, an appropriate representation of the “imaginary” piston is very important, especially when the relevant parameters have to be transformed from frequency-domain to time domain for a further analysis. The examples given in the research have shown that the correct parameters transformed from frequency domain to time domain can be a vital factor for a successful numerical simulation

mentioning

confidence: 99%

Assessment of primary energy conversions of oscillating water columns. I. Hydrodynamic analysis

Alcorn

2014

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“…The importance of studying air compressibility using numerical modelling is because air compressibility is not scalable using the conventional Froude similarity, as shown by Weber, 31 Sheng et al, 32 and Falcao and Henriques. 33 For modelling air compressibility, the volume of the air chamber must be scaled using the square of the scale ratio, rather than that of the cube of the scale ratio as required by the Froude similarity.…”

Section: Introductionmentioning

confidence: 99%

Wave energy conversion of oscillating water column devices including air compressibility

2016

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Wave power calculations for a wave energy conversion device connected to a drogue J. Renewable Sustainable Energy 6, 013117 (2014) This paper presents an investigation on air compressibility in the air chamber and its effects on the power conversion of oscillating water column (OWC) devices. As it is well known that for practical OWC plants, their air chambers may be large enough for accommodating significant air compressibility, the "spring effect," an effect that is frequently and simply regarded to store and release energy during the reciprocating process of a wave cycle. Its insight effects on the device's performance and power conversion, however, have not been studied in detail. This research will investigate the phenomena with a special focus on the effects of air compressibility on wave energy conversion. Air compressibility itself is a complicated nonlinear process in nature, but it can be linearised for numerical simulations under certain assumptions for frequency domain analysis. In this research work, air compressibility in the OWC devices is first linearised and further coupled with the hydrodynamics of the OWC. It is able to show mathematically that in frequency-domain, air compressibility can increase the spring coefficients of both the water body motion and the device motion (if it is a floating device), and enhance the coupling effects between the water body and the structure. Corresponding to these changes, the OWC performance, the capture power, and the optimised Power Take-off (PTO) damping coefficient in the wave energy conversion can be all modified due to air compressibility. To validate the frequency-domain results and understand the problems better, the more accurate timedomain simulations with fewer assumptions have been used for comparison. It is shown that air compressibility may significantly change the dynamic responses and the capacity of converting wave energy of the OWC devices if the air chamber is very large. Published by AIP Publishing. [http://dx

“…However, in a model scale, the volume of and the chamber pressure in the air chamber are both scaled down, and they both may be too small, so that the air compressibility is not correctly scaled in the model test of the OWC wave energy converters, especially when the small test models at the early development stages of the wave energy conversion are used, because as shown by Weber 2 and Sheng et al, 3 the air compressibility is not fully scalable according to the conventional Froude similitude. To approximately represent the air compressibility in the model scale, the air chamber must be designed to be much larger than that from the Froude similarity, and Weber 2 suggested that the length of the air chamber must kept same regardless of the scale ratio used in the model test.…”

Section: Introductionmentioning

confidence: 99%

“…To approximately represent the air compressibility in the model scale, the air chamber must be designed to be much larger than that from the Froude similarity, and Weber 2 suggested that the length of the air chamber must kept same regardless of the scale ratio used in the model test. Sheng et al 3 pointed out that keeping the same air chamber height is one of many choices, and different approaches can be attainable if an appropriate air chamber volume is produced. Nonetheless, this has highlighted the difficulties in representing the air compressibility in the model test.…”

Section: Introductionmentioning

confidence: 99%

Assessment of primary energy conversions of oscillating water columns. II. Power take-off and validations

Alcorn

2014

Self Cite

This is the second part of the assessment of primary energy conversions of oscillating water columns (OWCs) wave energy converters. In the first part of the research work, the hydrodynamic performance of OWC wave energy converter has been extensively examined, targeting on a reliable numerical assessment method. In this part of the research work, the application of the air turbine power take-off (PTO) to the OWC device leads to a coupled model of the hydrodynamics and thermodynamics of the OWC wave energy converters, in a manner that under the wave excitation, the varying air volume due to the internal water surface motion creates a reciprocating chamber pressure (alternative positive and negative chamber pressure), whilst the chamber pressure, in turn, modifies the motions of the device and the internal water surface. To do this, the thermodynamics of the air chamber is first examined and applied by including the air compressibility in the oscillating water columns for different types of the air turbine PTOs. The developed thermodynamics is then coupled with the hydrodynamics of the OWC wave energy converters. This proposed assessment method is then applied to two generic OWC wave energy converters (one bottom fixed and another floating), and the numerical results are compared to the experimental results. From the comparison to the model test data, it can be seen that this numerical method is capable of assessing the primary energy conversion for the oscillating water column wave energy converters.