Conclusions
This paper characterized the life cycle GHG emissions from wind electricity generation systems with respect to the development of a new categorization index that integrated the names from the three classification systems based on the axis of rotation of the wind turbine (HAWT, VAWT), the location of the installation (ON, OFF), and the power generation capacity (S, I, L). A total of 29 wind electricity generation system LCA studies that summarized 74 wind system cases were identified in the literature and reviewed in this study. Each of the reviewed wind electricity generation system LCA study was assigned an unique category code in accordance with the developed categorization index. The categorization of the reviewed 29 wind electricity generation system LCA studies yielded a total of five distinct categories, namely, HAWT-ON-S, HAWT-ON-I, HAWT-ON-L, HAWT-OFF-L, and VAWTON-S. While the VAWTs were noted to be used for only small scale electricity production, the offshore-based wind electricity generation systems were only used for large scale electricity generation. HAWTs were used for both onshore and offshore wind electricity generation. The mean life cycle GHG emissions from HAWT-ON-S, HAWT-ON-I, HAWT-ON-L, HAWT-OFF-L, and VAWTON-S wind electricity generation systems were computed to be 38.67, 11.75, 15.98, 12.9, and 46.4 gCO2e/kWh, respectively. The mean EPBT period was the highest for VAWTON-S (6.5 years), followed by HAWT-ON-S (1.1 years), HAWT-ON-L (0.58 years), and HAWT-ON-I/HAWT-OFFL (0.39 years). The mean life cycle GHG emissions from wind electricity generation systems were noted to decrease with an increase in the CF, the cycle time period considerations for the infrastructure, and the power rating of the wind electricity generation systems. The HAWT-ON-I wind electricity generation systems provided the best environmentalfriendly option with the lowest GHG emissions and EPBT period. The HAWT-OFF-L wind electricity generation systems performed better than the HAWT-ON-L wind electricity generation systems. All the four categories of HAWT-ON-S, HAWT-ON-I, HAWT-ON-L, and HAWT-OFF-L wind electricity generation systems had lower GHG emissions and EPBT periods than the VAWT-ON-S wind electricity generation system, thereby, indicating that HAWTs outperformed the VAWTs and are recommended for future use. There was only a single representative LCA study noted in the literature for VAWT-ON-S wind electricity generation system. More research efforts are needed to study the LCA of VAWT-ON-S wind electricity generation systems.
