Design and structural testing of tall Hexcrete wind turbine towers

Wind energy continues to be an important renewable energy resource across the United States (U.S.) with the goal of the U.S. Department of Energy (DOE) to install wind capacity in all 50 states. In order to increase the amount of installed wind energy, technology innovation such as taller wind towers is needed. At taller hub heights, wind speed increases, there is often less turbulence, and power production significantly increases from a moderate increase in height. These benefits can allow wind energy in new regions, such as the southeast U.S., and also increase energy production in current wind rich regions such as Iowa and Texas. Current wind towers are 262 ft (80 m) tall and constructed of circular steel shells. As hub heights increase, the shells must become wider and are difficult to transport. Concrete shells can also be used but these towers require specialized curved formwork which can be expensive. In order to effectively increase tower hub heights, the Hexcrete tower system was developed at Iowa State University (ISU). The Hexcrete tower is a hexagon shaped, precast concrete tower system constructed out of six hexagon shaped columns, and six flat wall panels. All the tower members are precast from high strength concrete materials and are designed to fit on a standard flatbed semi-trailer. The members are assembled at the wind farm site and connected by unbonded steel post-tensioning tendons. As part of a DOE funded project, three full concrete Hexcrete towers and three hybrid Hexcrete towers (steel shells compose top third of tower) were designed for hub heights of 394 ft (120 m) and 459 ft (140 m) for Siemens SWT 2.3-108 and SWT 3.2-113 turbines. A strength and design validation experimental test was performed on a full scale section of one of the 120 m towers in a laboratory setting and it was found that the Hexcrete tower design adequately met the required operating and extreme load requirements and also had significant reserve capacity. Finite element and numerical models were created to aid in the design process and were verified with experimental test data. A numerical surface pressure analysis was conducted according to the American Society of Civil Engineers (ASCE) 7-10 guidelines and compared to Computational Fluid Dynamic (CFD) analysis to compare the Hexcrete wind interaction with that of a circular tower section. It was found that the Hexcrete section created larger drag forces and surface pressures than a circular tower but that the Hexcrete hybrid towers had the potential to reduce the Hexcrete tower drag and corresponding pressures. Based on test and analysis results, the Hexcrete wind turbine tower provides a cost-effective opportunity to employ precast concrete for hub heights above 262 ft (80 m) and enable economical wind power in all 50 states