Structural Design of Sandwich Towers for Wind Energy Converters

A new kind of tower construction, called a sandwich tower, has been developed for wind energy converters. The tower consists of two steel shells, which are bonded together with a core material. Unlike a standard steel tower, the plate thickness is split into an inner and outer steel face. The core between the faces increases the stability of the shells. It works together like a sandwich shell. Different composite shell theories have been used to estimate the stability of such double-skin shell constructions. A model-scale test series has been carried out to analyse the influence of different core materials. The test specimens are loaded by uniform axial force to observe the shell buckling. The experimental results are compared to numerical simulations including measured geometrical imperfections. Within a numerical pre-design, the use of high-strength steels for the inner and outer faces is also considered to compare the various types of tower configurations. The goal is to find the best combination of steel faces with a core material in the ultimate limit state.

By Peter Schaumann and Christian Keindorf, ForWind, Germany

Strength and Fatigue Experiments and Modelling

The wind turbine rotor is an important part of the connection between the moving air (containing the wind energy) and the generator (used to capture this energy). The rotor consists of (usually) three blades, which are almost entirely made of fibre-reinforced plastic. These blades experience a large number of load cycles during their economic life. The magnitude of the load cycles is not at all constant. The rotor materials and structure need to withstand all these loads, great and small. Extensive research is devoted to predicting strength and life for rotor blades. For this research, numerous tests are done, exposing small fibre-reinforced coupons (specimens) to realistic loading conditions. Based on this data, fatigue models can be made predicting blade life. But, of course, ‘the proof of the pudding is in the eating'. Full-scale blade tests are required to see if reality matches expectations.

By Rogier Nijssen, WMC, The Netherlands

Can this Concept Ever Leave the Cradle of Innovation?

Offshore wind turbine technology is being pushed into deeper offshore locations. This is sometimes because of pressure from environmental lobbying or sometimes because of geographical restrictions, such as in the case of Malta, a Mediterranean island with limited space and resources. A novel concept has been suggested where the wind turbine is suspended from a floating element instead of being supported from underneath as found in today's conventional offshore installations. Can this novel concept ever leave the cradle of innovation and solve the problem of having wind turbine technology wade into deeper waters?

By Richard Galea, Malta

Quantifying the Risk for Offshore Wind Farm Developments

The marine renewables energy sector is an emerging force in energy provision globally and recent announcements by the UK government have created ambitious targets for the domestic market. In the UK it is driven principally by the Energy White Paper (2003) and, more specifically, by the Renewables Obligation, which requires electricity suppliers to source 15% of their supply from renewable sources by 2015, increasing to 20% by 2020. There is more than 1GW of projects with planning consent waiting to be built, and a second round of offshore tenders from The Crown Estate, with a total of 7.2GW, is awaiting application - equivalent to 7% of UK electricity requirement. There are plans for a 2,000 turbine, 10GW wind farm in the North Sea, the largest in European waters, which would supply energy to 8 million homes.

By Kevin Black, Partrac Ltd, UK