Abstract
Growing public awareness of utilizing renewable energy has led to popularity to adopt wind energy in building environments[1].Distributed wind systems,which are located near the consumption center,can provide clean,renewable power for on-site use and have gained rising attention in the world recently[2].There is an excellent potential for wind energy utilization on tall buildings rooftop for micro-wind turbines installation.Nevertheless,the installation of micro-wind turbines in urban areas is limited because the lack of accurate estimation of wind potential.Some researchers have used Computational Fluid Dynamic(CFD)method to address wind potential of different roof shapes on the same cubic building,including flat,domed,and pyramidal shapes[3].Roof mounting site analysis for micro-wind turbines has found that flat roofs are likely to yield higher and more consistent power[4].The study about wind energy on a flat roof with staircase is limited.
This paper presented the CFD simulation of the wind flow over a tall building's roof with two staircase out of the roof.The optimum mounting sites of turbines were researched by evaluating the vertical distribution of wind speed and turbulence intensity.In addition,the influence of non-staircase on wind flow characteristics was compared.
The cross-section of target building is shown in Figure 1.The cuboid domain is used with the size settings refer to reference[5].The distance from the windward side of the building to the inlet is 5H,from both sides of the calculation domain to the building's lateral is 5H,from domain's top to the building's roof is 5H,and from the outlet to the back of the building is 15H.Where H is the height of the building.The block rate of the calculation domain is less than 5%,which meets the requirement of numerical calculation.
Figure 1 Potential locations layout
Unstructured prism grids are divided in ANSYS ICEM.Boundary layer grids are set around building with 0.1 meters initial height and 1.15 times growth rate.The grid has a total amount of about 120 000 cells.Fluid dynamics simulation was carried in ANSYS FLUENT.The boundary conditions are presented in Table 1.The velocity inlet satisfied Eqs.(1)to(3),which were stream wise velocity profile,turbulent kinetic energy(TKE),and turbulent dissipation rate(TDR)respectively.
Table 1 Boundary conditions for thesimulation domain
Whereκis the von Karman constant,and taken as 0.42.u*is the friction velocity,z0 is the aerodynamic roughness length.And Cμ=0.028,C1=1.5,C2=1.92.The bottom boundary is modeled as a rough wall and standard wall functions are used.
A critical precondition affecting the consistency of the CFD simulation is the horizontal homogeneity of domain.The vertical profiles of wind speed,turbulent dissipation rate(TDR)and turbulent kinetic energy(TKE)at the inlet,1/4 stream wise length,1/2 stream wise length,3/4 stream wise length,outlet were shown in Figure 2.
Figure 2 Comparison of velocity,TDR and TKE at different location of stream wise
Turbulence intensity affects the normal operation and service life of the micro-turbines,and service life would be shortened if turbulence intensity exceeds the threshold of 16%～18%.Wind speed influences the efficiency of wind power generation.The variations of turbulence intensity and wind speed with height were shown in Figure 3.
Figure 3 The variations of turbulence intensity and wind speed with height
The turbulence intensity increases sharply with height at first and reaches its peak value following with slowly decrease,and wind speed increases sharply at first with the increase of height ratio,which indicates that a small rise in height will bring a significant increase in wind energy yielding,and then the wind speed increases flatly.The minimum installation height ratio of turbines was shown in Table 2.
Table 2 Height ratio limits for installation
Results showed that normal operation of micro-wind turbines can be satisfied without affecting its service life as long as the installation height is higher than 1.15H.Figure 4 showed the distribution of turbulence intensity and wind speed at Z=1.15H.
Figure 4 The distribution of turbulence intensity and wind speed at Z=1.15H
High-rise buildings generally have staircases out of the roof for the necessity of vertical traffic inside,which would cause differences of wind flow on the roof compared with the ordinary flat roof of the low-rise and multi-story buildings.To compare the influence of roof staircase on the utilization of wind energy,two staircases out of the roof of the target building are removed and re-simulated.Take the cross section Z=71.6 m through the stairwell of the roof as an example.The distribution of wind speed of this section was shown in Figure 5.
Figure 5 The distribution of wind speed of section Z=71.6 m
It can be seen that the presence of staircases outside the roof has a great impact on the distribution of wind speed.Specifically,the area of the low wind speed area of the roof is enlarged and the wind speed is lower,which means obvious blocking ef fect and makes against to the wind energy utilization.The windward turbulence intensity of the staircase increases and the area of the high turbulence area decreases slightly.
The simulation results show that the suitable mounting site for wind turbines is on the top of the staircase with higher wind speed and lower turbulence intensity.The optimum installation height ratio is 1.15H with the acceleration factor of 1.5,which means good potential for wind energy application.