The increasing cost of steel has resulted in renewed interest among wind turbine manufacturers in the use of concrete for support towers. Concrete, especially for tall towers over 100 metres in height, has the added advantage over steel of providing a firmer support for the nacelle, which may reduce the dynamic loads through the entire structure, enabling the use of lighter and thus cheaper components. This, in turn, could lead to cost savings. German Enercon has been using precast concrete towers with specially developed prestressed steel reinforcement for some time. The individual tower sections and foundation are fastened together to form an inseparable unit with stay cables running through jacket tubes in the core of the concrete tower wall. The tower sections are manufactured entirely in the precasting plant and specially constructed steel moulds assure manufacturing precision for each individual concrete section. The design has been used for Enercon's 6 MW E-126 machine at Emden, which has a rotor spanning 126 metres and a hub height of 131 metres. It consists of 35 concrete sections. More recently, Dutch firm Advanced Tower Systems (ATS) has taken a different approach with concrete, developing a two part tower consisting of a concrete section and a steel section to support a Siemens 2.3 MW machine in a research project going up at the German wind turbine test station at Grevenbroich, west of Cologne. The overall height of the structure is 133 metres, just taller than the E-126. The lower 76.5 metre section of the hybrid tower is constructed from long, narrow pre-cast concrete components made by ATS, with the shorter upper section using conventional steel. ATS argues that the additional expense associated with such a tall tower is repaid within about four years: the winds at that height are stronger and 20% extra energy yield is derived. ATS also points out that the various components of the hybrid tower are designed to be easy to transport, especially in complex terrain. Installation times are quick and the company suggests that a number of different turbine types can easily be installed on the tower. Participating in the project as equal partners alongside ATS are Siemens Project Ventures, a subsidiary of Siemens Financial Services, Germany's Juwi Netzwerk and two further Dutch companies working with ATS: engineering firm Mecal and the Hurks Groep, a large general contracting firm.
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