UC Irvine

Wind power is a quickly growing renewable energy resource within the continental United States and is expected to continue increasing as more wind farms are installed on and off shore. Large wind turbines benefit from economy of scale from larger components such as taller towers. However, onshore turbine development is hindered by conventional transportation restrictions which limit the diameter and weight of the tower segments. The average height of conventional wind turbine towers installed in the U.S. is slightly over 80 m tall. An ultra-tall 140 m tower would increase the amount of energy produced by more than 21% at a site with moderate wind shear, but is a challenge to construct based on conventional transportation size limits. One proposed solution to this problem is to employ concrete additive manufacturing technology to build ultra-tall wind turbine towers on site. To gauge the potential environmental impacts of this approach, this study performed a life cycle assessment (LCA) comparing the environmental impacts of for four prototype 7.5 MW wind turbine towers with hub heights of 140 m: a conventional tubular steel tower assembled using bolted connections, a concrete tower with segments prefabricated with high-strength (78 MPa) 3D printed shells and precast off-site with normal-strength (35 MPa) concrete with final assembly on-site, a concrete tower additively manufactured on-site using normal-strength (35 MPa) concrete, and a concrete tower additively manufacture on-site using high-strength (78 MPa) concrete. The steel tower was designed similar to National Renewable Energy Laboratory’s (NREL) adapted 5 MW Big Adaptive Rotor (BAR) project, with a power rating scaled from 150 W/m2 up to 229 W/m2 to better match California’s moderate wind speeds. The concrete towers were designed using the ASCE-7-16 direct winds and dynamic turbine wind loads, in combination with other important loading such as dead load and seismic load. For all four towers, five stages of life cycle assessment were considered: material production, transportation, construction, use, and end of life. A life cycle inventory was developed to catalog the inputs (e.g. raw materials and energy) and outputs (e.g. emissions to air) associated with each tower’s life cycle. The input variables for the LCA incorporated the differences in the tower materials, structural designs, and manufacturing methods. The results of this study indicate that compared with the steel tower, the concrete tower additively manufactured on-site with 35 MPa concrete will have 24% lower CO2 emissions and 26% higher energy consumption; however, the concrete tower additively manufactured on-site with 78 MPa concrete will have 15% higher CO2 emissions and 62% higher energy consumption than the steel tower. The difference is due to the significantly higher cement content in the 78 MPa concrete than the 35 MPa concrete. Cement is the most energy-intensive ingredient in concrete and is responsible for most of the greenhouse gas emissions of concrete. The results also show that compared with a concrete tower with sections prefabricated off-site and assembled on-site, a concrete tower that is additively manufactured on-site has a 25% reduction in CO2 emissions attributed to both the transportation and materials phases. Furthermore, among the five stages of the life cycle, the material production stage dominated and was found to contribute over 92% of the total CO2 emissions and 67-93% energy consumptions of each tower. This is due to the large volume of materials used for the ultra-tall towers, and relatively low need for repairs and maintenance during the tower’s life cycle. A parametric study was conducted for the on-site additively manufactured towers and revealed the strong effect of cement percentage in the concrete on the CO2 emissions of the tower. Additional parametric studies were conducted for the other four life cycle stages to examine the effects of variables including the distance from the concrete plant to tower construction site, the size and number of tower segments, rated tower life, and tower end-of-life recycling rate. The results highlight the need for future development of environmentally friendly 3D printing concrete materials for ultra-tall tower applications. Low-energy and low-CO2 concrete that incorporates waste or recycled products and can be additively manufactured will significantly reduce the environmental impacts of ultra-tall turbine towers. If the 3D printing concrete can possess high strength, it will also lead to more efficient structural designs that use less concrete, further reducing the environmental impacts. The results reveal an opportunity for further research and development of concrete additive manufacturing technology for the wind energy applications including towers, foundations and energy storage.