Marine Energy Conversion

As the Earth's surface is mostly covered by water, the sea represents an enormous source of clean energy. This source is so far minimally exploited, but its potential is much larger than the entire global energy demand.

This energy can be converted from different forms as tidal, wave, marine current, temperature variation, and salinity, representing an effective resource for most of the planet. Conversion strategies and technologies developed so far have demonstrated the feasibility and the interest in such a large issue, showing the way forward to big challenges to match with. Recent advances in engineering and technology led to more efficient and cost-effective solutions.

Different governments have provided funding to research activities regarding the exploitation of marine energy, most notably UK, China, USA, Canada, Sweden, Portugal, Ireland, Spain, Australia, Korea, Nigeria, New Zealand, Mexico, and Japan.

As a result of this interest, a large number of devices, exploiting different marine energy components, have been proposed. Many marine energy converters have been patented, while some have already been considered mature for application. The extraordinary variety of the proposed technologies testifies both the vitality of the field and the ingenuity of the researchers in the field.

In this special issue authors submitted their original contributions and review articles that cover the main important aspects related to ocean technologies, mainly focusing on wave, ocean current, and tidal energy harvesting. Numerical simulations and experimental validation have been carried out. Several devices have already been built and performance is reported. Efficiency is a key point for this kind of application; thus, a strong effort has been made in order to develop control algorithms and mechanical solutions which improve the performance of the energy conversion.

It is now a common opinion that marine energy conversion will be one of the leading research fields at least in the next 15 years.

Overview of the Articles of the Special Issue. Marine energy conversion is a multidisciplinary topic, requiring the expertises of sectors ranging from ocean engineering to electrical engineering. The scope of this Special Session is accordingly wide, helping to build a global and complete overview of the topic.

This special session comprises the work of some of the leading institutions working in the field. The contributions are relevant to four main areas.

In the field of wave energy conversion, Z. Liu et al. investigate the operating performance of a designed impulse turbine for the pilot oscillating water column (OWC) system which is under the construction on Jeju Island, Korea; D. Li et al. focus on the performance of floating buoys of a wave power generating jack-up platform, called Haiyuan 1; Lei Huang et al. propose a novel hybrid excitation flux-switching generator for direct drive wave energy converters; Z. Shi et al. propose a wave energy extracting structure which can be completely sealed and works under the principle of double oscillation. L. Zhu et al. propose a new type of floating oscillating buoy wave power device, a buoy-rope-drum wave power system, and H. He et al. carried out simulations on the expected performance of the Pelamis wave energy converter in China.

Concerning water current harvesting, H. Chen et al. present a nonlinear simulation model of a switched reluctance (SR) ocean current generator system on MATLAB/simulink which described the structure of generator system, while M. Grabbe et al. studied the efficiency of a directly driven generator for hydrokinetic energy conversion.

The tidal energy field is explored by C. Yang et al. who present a bidirectional power performance of a tidal unit with unilateral and double guide vanes; A. F. Tao et al. studied a two-dimensional hydrodynamic model in order to predict the distribution of mean density of tidal current energy and X. Ge et al. introduce and study the optimization of a form of tidal power station, which can satisfy bidirectional power.

Finally, two review articles are presented in this special issue: Z. Chen et al. made a review of offshore wave energy extraction system in order to provide some useful guidelines for future studies in this field; Q. Guodong et al. carried out a review of development of ocean energy in China giving recommendations for the future development of ocean energy.

Fabrizio Marignetti Haitao Yu Luigi Cappelli