Evaluation of the Double Snap-Through Mechanism on the Wave Energy Converter’s Performance

Lower efficiencies induce higher energy costs and pose a barrier to wave energy devices'commercial applications.Therefore,the efficiency enhancement of wave energy converters has received much attention in recent decades.The reported research presents the double snap-through mechanism applied to a hemispheric point absorber type wave energy converter(WEC)to improve the energy absorption perfomance.The double snap-through mechanism comprises four oblique springs mounted in an X-configuration.This provides the WEC with different dynamic stability behaviors depending on the particular geometric and physical parameters employed.The efficiency of these different WEC behaviors(linear,bistable,and tristable)was initially evaluated under the action of regular waves.The results for bistable or tristable responses indicated significant improvements in the WEC's energy capture efficiency.Furthermore,the WEC frequency bandwidth was shown to be significantly enlarged when the tristable mode was in operation.However,the corresponding tristable trajectory showed intra-well behavior in the middle potential well,which induced a more severe low-energy absorption when a small wave amplitude acted on the WEC compared to when the bistable WEC was employed.Nevertheless,positive effects were observed when appropriate initial conditions were imposed.The results also showed that for bistable or tristable responses,a suitable spring stiffness may cause the buoy to oscillate in high energy modes.

Changes in the Urban Surface Thermal Environment of a Chinese Coastal City Revealed by Downscaling MODIS LST with Random Forest Algorithm

Analysis of the formation and evolution of urban surface thermal environment is crucial for urban planning and improving the environment of a settlement.Qingdao was selected in this study as a typical coastal city undergoing rapid urbanization,and the spatiotemporal dynamic characteristics of its urban surface thermal environment from2010 to 2019 were evaluated.The random forest(RF)algorithm was adopted to obtain its land surface temperature(LST)map with 30-m resolution by downscaling the Moderate Resolution Imaging Spectroradiometer(MODIS)LST product;the remote sensing indices emphasizing different land cover types,LST calculated by the radiative transfer equation,and elevation were used as input variables in the algorithm.The heat island intensity(HII),urban heat island(UHI)volume,and UHI grade were used to analyze the spatiotemporal dynamic characteristics of the urban surface thermal environment in Qingdao.The results show an increasing trend in average HII between 1.1 and 2.52°C in the study area over the past 10 years.The northern city appeared to have the highest UHI volume,while change of the UHI volume in Huangdao District of southwestern Qingdao was the most significant.The areas with high HII have gradually expanded during the last 10 years,and the areas with a 10-yr persistently high HII are distributed mainly in old urban areas with high building density and a dense population.Different factors may influence UHI,such as artificial heat sources,surface heat sources,and hybrid heat sources.Finally,adjusting the urban structure,increasing the vegetated area,and changing building colors are proposed to mitigate UHI in the areas with continuously high HII.