The Impact of Wind Turbines on Atmospheric Boundary Layer under Different Topography

International Journal of Energy and Power (IJEP) Volume 4, 2015 www.ijep.org doi: 10.14355/ijep.2015.04.013

The Impact of Wind Turbines on Atmospheric Boundary Layer under Different Topography Zhengren Wu*1, Fei Li2,Yunlei Zhai3,Mei Liu4 School of Energy Power and Mechanical Engineering, 4school of Economic Management, North China Electric Power University, P O Box 29, Yonghuabei Street 619, Baoding 071003, China 1‐3

*1

zhengren_wu@163.com; 2lifihihi@126.com; 3yunlei_zhai@163.com; 4liumei_w@163.com

Abstract In order to explore the influence of different topography on the velocity and turbulent kinetic energy when the wind turbine operating, Fluent software was used to simulate the fluid flow at the bottom of the atmospheric boundary layer caused by the wind turbine. Gambit software was chosen to build the horizontal axis wind turbine and the whole flow field, the velocity distribution function of the boundary layer was loaded by the UDF as the inlet boundary condition. Compare simulation results of the four types of topography to explore the topography’s effect on the influence of the wind turbine to the atmosphere boundary layer. Results showed that the velocity at the same height gradually decreased along with the ground roughness coefficient increased under different topography and the turbulent kinetic energy increased with the elevate of the roughness.

So the wind turbine makes velocity and turbulent kinetic energy change obviously and the extent of the impact is related to the topography, and the change of velocity and turbulent kinetic energy will influence some parameters of the boundary layer, then may change the environment even the migration of pollutants around the wind farm.

Keywords Wind Turbine; Atmospheric Boundary Layer;Topography; Wind Speed; Turbulence Kinetic Energy

Introduction Wind power generation as a clean energy technology, it can replace a number of coal‐fired power plants to reduce the consumption of conventional energy and the emissions of pollutants. But wind farms will reduce the wind speed and increase the intensity of turbulence in the atmosphere boundary layer by capturing the wind to change the nature cycle. The exchange and transform of some physical quantity caused by the turbulence will change the local energy distribution and water exchange, make the wind speed and temperature change [1‐2], and the surface roughness as the parameters of the atmospheric boundary layer friction effect, directly affects the material energy exchange between the ground and atmosphere [3]. These may affect the spread and diffusion of the pollutant. So there exists important significance in the study of the effect of wind turbine to the atmosphere boundary layer.

Many researchers did the studies [4‐8] and proved that wind farms may have some impacts on the surrounding area environment climate. CFD (Computational Fluid Dynamics) always chosen to the study of wind turbine, usually used for the numerical aerodynamic performance and the analysis of the wake flow [9‐10], and it is also common to applying CFD to simulate atmospheric boundary layer, but there are few article to combine wind turbine and the atmospheric boundary layer by using CFD. In this paper, the flow field near the wind turbine is numerically simulated by CFD, to analyze the influence of the wind turbine to the atmospheric boundary layer, at the same time, it can provide certain guidance for the relationship between the energy and the environment.

The Compute Model and Boundary Condition Build the wind turbine model of 1.2MW in the GAMBIT software. The rotating flow field is a flat volume including blades. Considering the main focus on the downstream of the turbine, so ignore the tower, the whole flow field is the field including the flat volume. The calculate area is simplified to be a cube, stream moving along the Z axis, the turbine plane parallel to the XY plane. The height, width and depth of the cube are 1200m, 300m, 300m, that is 20d×5d×5d, and d is the diameter of the rotor. In order to ensure the coming flow has been fully developed when it reached the wind turbine, the turbine is located at 3d after the entrance. Comparing three kinds of grid distribution

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to verify the grid independence and finally choose the model which has 4.18 million grid to compute.

FIGURE. 1 THE SIZE OF THE WHOLE FLOW FIELD

In the inlet, using the user‐defined function (UDF) to compile the wind speed function. Atmospheric boundary layer is strongly affected by the ground, the topography was classified as A, B, C, D, as shown in table 1. Different topography has different roughness index.

Using the index of the boundary layer equations [11]: U Z  U 0 (Z / Z 0 )a

Note: U0 for Z0 height of wind speed, Z0 for reference height, UZ for the Z height of wind speed, Z for any height, a is the ground roughness coefficient.

The outlet was defined as outflow, assume the blade as rigid, using no slip boundary condition, set it as the rotate solid wall. The bottom was under no slip boundary condition and the surface roughness was along with the topography. The side and top of the flow field was under symmetry boundary condition. The flow field around the wind turbine was simulated by the rotating coordinate system, using MRF (Multiple Reference Frame) model, the area around the wind turbine together with the blade and hub rotate with 19.27r/min. Simplify the model to a three‐dimensional steady and incompressible flow without considering the energy equation. Using SST k‐w turbulence model, based on the continuous equations and momentum equations, the coupling between velocity and pressure using SIMPLIC algorithm, and the finite volume method was used for discrete equations, advection schemes using second‐order upwind scheme, the convergence criteria for the continuous equation of dimensionless residual and all variables fall below 10‐3.

TABLE 1. THE DIFFERENT SURFACE FEATURE OF DIFFERENT TOPOGRAPHY

Category

Topography

Roughness height(m)

Gradient wind height(m)

Roughness index

A

Open water, Lake

0.01

300

0.12

B

Grassland, Pasture

0.05

350

0.16

C

The fields, Village

0.30

400

0.22

D

Forest, City

1.00

450

0.30

The Result and Analysis The atmosphere boundary layer was located at the bottom of the troposphere, and it was the bridge for the material and energy exchange between the earth and atmosphere. The main features of the atmosphere boundary layer are the regularity of velocity distribution and turbulent characteristics of the movement, the process of the turbulent exchange determines the distribution of the variables of the boundary layer. So it is possible to study the influence of the wind turbine to the boundary layer from velocity and turbulence under different topography.

The Influence of Topography on the Distribution of Velocity Due to the different topography, the inlet velocity distribution function is also different, so different distance

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downstream speed will be accordingly different. Correspond to the four kinds of topography at different distances from the turbine downstream, speed varies with the height as shown in Figure 2.

As can be seen from Figure 2, at different heights, the turbine downstream velocity distribution was similar under different topography, the trend of speed change was same. Considering the effect of topography, from A topography to D topography, surface roughness increases, surface becomes more and more rough, so in the same height, the speed is gradually reduced from A topography to D topography, and the export field velocity did not reach the income velocity. In different topography, the speed recovery rate is also different, as shown in Figure 3.

300

300 A B C D

250

250

200

Height/m

150

150

150

100

100

100

50

50

50

0

0 0

2

4

6

8

10

12

14

16

A B C D

250

200

Height/(m)

200

Height/(m)

300 A B C D

0 0

2

4

Velocity/(m/s)

6

8

10

12

14

16

0

2

Velocity/(m/s)

4

6

8

10

12

14

16

Velocity/(m/s)

(a) (b) (c) FIGURE. 2 THE DIFFERENT VELOCITY PROFILE ALONG THE HEIGHT FOR DIFFERENT TOPOGRAPHY (A) PLANE OF Z=5D; (B) PLANE OF Z=12D; (C) PLANE OF Z=16D

Speed recovery rate

1.0 0.8

A B C D

0.6 0.4 0.2 0.0

0

200

400 600 Position/(m)

800

1000

FIGURE. 3 THE RATIO BETWEEN THE VELOCITY OF EXPORT AND THE VELOCITY OF THE INCOME OF DIFFERENT TOPOGRAPHY

As can be seen from Figure 3, the air velocity attenuation at wind turbine hub corresponding to the D topography is maximum. From A topography to D topography, velocity ratio decreases gradually, in the same height of the downstream, the trend of the speed ratio accordance with A to D is decreasing. At 17D after the turbine, which is the exit, from A topography to D topography, speed recovers the entrance velocity of 98%, 98%, 94%, 92%.

Although the recovery rate of speed of A and B topography at the exit is approximately equal, but speed changes still follow certain rules. The surface more rough, the speed is more difficult to recover to the initial velocity.

Effect of Topography on the Turbulent Kinetic Energy Distribution Because the operation of wind turbine stirred airflow, it would produce turbulence in the wake, and it was bound to affect the turbulent kinetic energy in the turbine downstream. Coupled with the different rough surface caused by the different topography, turbulent kinetic energy after the turbine at different distances would be different accordingly. Correspond to different topography, the turbulent kinetic energy of different distances from the turbine changes with the height of the specific situation as shown in Figure 4.

As can be seen from Figure 4, from the general trend, with the downstream development, the turbulent kinetic

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energy of four kinds of topography corresponds with the downstream extension are gradually reduced, the turbulent kinetic energy by the downstream two diameters of 0.3m2/s2 gradually reduced to 0.1m2/s2 at the exit.

Near the wake, airflow is heavily influenced by wind turbine rotation, and then gradually spread to the surrounding, and the turbulent kinetic energy decreased gradually. Considering the factor of the topography, it can be seen that in the near wake, due to the rotation of the wind turbine, airflow is cluttered, turbulent degree is high, and the turbulent kinetic energy distribution regularity is not obvious. But in the field of the far wake, especially the distance ten times diameter after wind turbine, effect of topography on the turbulent kinetic energy is more and more obvious, the greater the topography roughness, the bigger the turbulent kinetic energy. So in a long distance, it can be seen from A topography to D topography, the increase trend of turbulent kinetic energy at the same height has become obvious.

300

300 A B C D

250

250

200

Height/m

150

150

150

100

100

100

50

50

50

0 0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

A B C D

250

200

Height/(m)

Height/(m)

200

300

A B C D

0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

2 2 TKE/(m /s )

0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

2 2 TKE/(m /s )

2 2 TKE/(m /s )

(a) (b) (c) FIGURE. 4 THE DIFFERENT TKE PROFILE ALONG THE HEIGHT OF DIFFERENT TOPOGRAPHY (A) PLANE OF Z=5D; (B) PLANE OF Z=12D; (C) PLANE OF Z=16D

Conclusions By simulating the influence of a single rotating wind turbine to the ABL under different topography, we receive the following conclusions: Due to the effect of the turbine perturbation, the velocity decays and the TKE changes in the ABL. Considering the effect of topography, with the increase of roughness, the wind speed that at the same height near ground is gradually reducing from A topography to D topography, the speed of the hub height recovers to the entrance velocity of 98%, 98%, 94%, 92%. Near the wake, because of the effect of wind turbine rotation, the turbulent kinetic energy distribution is in chaos, but in the far wake region, topography effect is more and more obvious, the turbulent kinetic energy at the same height increases gradually with the increase of roughness, but the turbulent kinetic energy is still follow the trend of decreasing, by the near wake of 0.3m2/s2 reduced to 0.1m2/s2 at the exit.

ACKNOWLEDGMENT

This work was supported by the fundamental research funds for the central universities (grant no. 2015MS114).

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