**Preface to "Wave and Tidal Energy"**

The development of renewable energy around the world has been given increased importance due to concerns regarding energy supply and climate change. Wave and tidal energy are very important components of the renewable energies that can be extracted from the oceans. Different locations around the world have good natural conditions for one of these forms of energy, with some locations even being suitable for both. One of the important aspects related to the industrial development of the wave and tidal energy is the identification of the locations where the energy resource has levels that are high enough to justify commercial exploitation. This book contains three papers dealing with the evaluation of the wave energy resources around the world, covering areas like USA, Taiwan and Chile where important developments are being considered. Such energy resource predictions have been questioned in regard to their long-term consistency, in view of the climate changes that are being predicted. One of the papers addresses this topic specifically, presenting a methodology that is applied to the Black Sea but can also be applied to other areas. Wave energy farms not only absorb energy, but also change the wave fields in their vicinity; one of the papers addresses this issue by modelling the impact of offshore wave farms on coastal process and water users, with a special emphasis on beach morphology. Other impacts on the environment are also of concern and need to be addressed for the proper development of wave and tidal energy technologies. The potential environmental risks associated with wave energy converter deployments are addressed in a paper that describes the development of a spatial environmental assessment tool. Another paper deals with the efficiency of multi-type floating bodies for a novel heaving point absorber that can be applied in low-power unmanned ocean devices. This is one of the many devices that are under development, although in this case it covers a very specific type of application. The most common class of energy conversion devices are those that aim at producing large amounts of energy for supply to the electric grid, a problem that is treated in another paper that considers the energy produced by an oscillating water column device. Finally, the economic viability of this form of energy depends on economic considerations; this topic is covered in another paper that develops a method to assess the economic feasibility of wave energy farms, in the present case applied to coastal regions around Portugal. Two papers address the developments in tidal energy. One of them deals with a marine current energy converter for profiling floats, where the energy converter aims again at supplying very low levels of energy to an autonomous device, which is important to make it autonomous. The final paper addresses the issue of control of the devices, considering the effect of control strategy on tidal stream turbine performance in this particular case. Control is an important issue both for tidal energy and for wave energy devices, and it is essential for optimising their performance. Overall, this book covers a set of problems that cover different aspects of interest for wave and tidal energy conversion, and we hope it may be of interest to readers.

> **Carlos Guedes Soares, Matthew Lewis** *Special Issue Editors*
