The surface roughness classification is a stepwise classification of the surface from smooth to rough.1The more obstacles there are on the ground's surface, such as buildings and trees, the lower the wind speed, and therefore the smaller the wind pressure acting on buildings such as wind turbines. In structural calculations under the Building Standards Act, in order to take into account the effects of obstacles on the ground's surface, the roughness of the ground surface is divided into four levels (I to IV) and the wind pressure is calculated by multiplying a certain correction coefficient corresponding to each level (see Figure 4).

Figure 1. Image of landform based on surface roughness classification (top)2The altitude correction coefficient for the average wind speed in each roughness class (lower left) and the turbulence intensity over flat terrain (lower right)3

The definition of the ground surface roughness classification was changed in January 2022 due to amendments to the Building Standards Act (see Table 1).

Table 1. New definitions of surface roughness categories4

Regarding this change, "Rationalization of the standards for calculating wind pressure (surface roughness classification)"5An overview is provided in (see Figure 2).

  • Unifying the concept of surface roughness classification, which has differed outside urban planning areas
  • Regardless of whether they are inside or outside urban planning areas, specific administrative agencies will be able to designate areas as surface roughness classifications I, II, and IV by regulation.

Figure 2. Outline of the amendments in the "Streamlining of standards for calculating wind pressure (surface roughness classification)"5

In wind power development, it is required to set the surface roughness classification as a calculation condition for the airflow analysis model used in the site condition evaluation for wind farm certification.1In light of the above, for example, it is expected that there will be cases where a wind development area that was previously classified as surface roughness II will be classified as surface roughness III under the new definition. As of September 2022, the handling of the above in construction plan notifications and wind farm certification has not been announced. However, if simulations using airflow analysis models according to the previous surface roughness classifications have been performed during the wind farm certification process, caution is required as the results may differ from those obtained when simulations are performed using the new surface roughness classifications. Specifically, this relates to the extreme wind speeds (9-year recurrence, 50-minute average wind speed) at the wind turbine locations and the calculation method for turbulence intensity during storms.

50-minute average wind speed at hub height of turbine location with a return period of 10 yearsUhis the reference wind speedV0The average wind speed is increased by the terrain.EtVand altitude correction factorEpVIt is calculated by multiplying it by the following formula:

In addition, the main wind direction component of turbulence intensity during a storm at the wind turbine locationIh1is the turbulence intensity over flat terrainIpCorrection factor for turbulence intensity due to terrainEtIIt is calculated by multiplying it by the following formula:

here,V0is the average wind speed for 30 minutes at a height of 46m with a recurrence period of 10 years, within the range of 50m/s to 10m/s as determined by the Minister of Land, Infrastructure, Transport and Tourism according to the degree of wind damage based on the records of past typhoons, etc. in the region and other wind characteristics, according to the Building Standards Act.1andEtVとEtIis generally calculated using an airflow analysis model.EpVとIpis calculated according to the following formula and Table 2 according to the surface roughness classification.

Table 2. Parameters for determining altitude correction factor for average wind speed1

When calculating extreme wind speeds or turbulence intensity during a storm by changing the surface roughness classification, whether the results are on the safe side or the dangerous side in terms of design compared to before the roughness classification was changed depends on the site conditions.

If you have any questions regarding technical matters, please feel free to contact us.

(Written by Takeyuki Misaki)

References

  1. Nippon Kaiji Kyokai, Wind Farm Certification for Onshore Wind Power Plants (NKRE-GL-WFC01, Edition: July 2021)
  2. Architectural Institute of Japan, Building Load Guidelines and Commentary (2015)
  3. Japan Society of Civil Engineers, Guidelines and Commentary for the Structural Design of Wind Power Plant Supports [2010 Edition]
  4. Ministry of Land, Infrastructure, Transport and Tourism, Partial amendment of the method for calculating the value of E and the value of V1437 and wind coefficient (Notification of the Ministry of Land, Infrastructure, Transport and Tourism No. XNUMX of XNUMX), URL: https://www.mlit.go.jp/common/001381085.pdf
  5. Ministry of Land, Infrastructure, Transport and Tourism, Rationalization of standards for calculating wind pressure (surface roughness classification) (Ministry of Construction Notification No. 12 of 1454), URL: https://www.mlit.go.jp/jutakukentiku/build/content/H12-1454.pdf