Renewable energy plays a key role in societal development, particularly given the increasing demand for energy coupled with pressing environmental and energy security issues. In January 2009, the European Commission announced that e500 m of the e5 bn economic recovery package will finance offshore wind energy projects. Additional support of e1.75 bn was pledged to energy infrastructure including strategic electric grid interconnectors, which are a vital component of expanding wind energy installations and integrating power from renewables into the European electricity network.

Targets are ambitious. The European Wind Energy Association (EWEA) estimates that of the total 180 GW wind energy capacity expected to be installed in the European Union (EU) in 2020, between 20 and 40 GW will be offshore. EWEA projections are that of the total 300 GW of wind energy to be installed in the EU in 2030, half (150 GW) will be offshore.

In 2007, installed wind energy capacity worldwide was 94 GW, with offshore capacity passing 1.0 GW. By the end of 2008, 1.5 GW of offshore wind was operating. Although the pace of developments offshore is not as fast as had been expected, due in part to financing issues, according to the EWEA, an additional 2.5 GW is under construction. Although not all plans may come to fruition, planned offshore wind farms in the EU by 2015 have a total capacity close to 30 GW. Plans for offshore developments are also well underway outside of the EU in Norway, the USA, China, Canada, India and Japan.

In the space of less than 20 years from the first installation of an offshore wind farm, enormous progress has been made. From the 5 MW Vindeby wind farm with two rows of 450 kW turbines in 1991, the average size of an offshore wind farm will increase to over 100 MW in 2009, and the standard size of turbines will be at least 3.6 MW. This is therefore a good time to take stock of what has been learnt from offshore wind farm development, installation and operation over the past 5 years, a period that marks the transition from smaller research wind farms into large-scale commercial developments.

Some of the challenges facing successful development of offshore wind energy are evident, focusing on the need for cost-effective and efficient solutions to resource and environment assessment, design of turbines and installation equipment, and creation of innovative safety-conscious operation and maintenance procedures. Other challenges relate to the desire of the market for an ever-increasing amount of low-carbon energy, but at costs that are difficult to match given the harsh physical environment in which the power of the wind must be harnessed.

One challenge that is less evident is how to optimize the way in which the wind energy industry transforms from research-based activity to full-scale commercial production without losing vital links between research and commercial partners. In this special issue of Wind Energy, the authors share their experience in developing techniques and methods for all areas of offshore wind energy from public perception, environmental issues, meteorology, power prediction on resource time scales to short-term forecasting, economics, grid stability, operation and maintenance and design. In the first part of the special issue, we focus mainly on meteorological, environmental and social aspects of offshore wind energy. Despite the obvious success of wind energy to date, it is evident that research is required in the future to design turbines and wind farms at the most cost-effective size and form, and for increased efficiency in planning, integration and operation. In the second part of this special issue to be published later in 2009, we focus mainly on technical and design aspects of offshore wind energy.

We hope this special issue will be a good reference collection for those working in the field of sustainable energy and encourage commercial, public and academic partners to continue to collaborate sharing ideas and results to promote the success of offshore wind energy.