Assessment Technology and Application of Wind Energy Resources in Complex Mountain Areas at High Altitude

Based on the meteorological stations and wind measurement data of a high-altitude region in Liangshan Prefecture, Sichuan Province, collation, analysis, and evaluation are carried out. The results show that: the prevailing wind direction in the region is stable and the wind energy resources have a certain development value, but the turbulence intensity is high and belongs to IEC Class B, which has a certain impact on the fatigue load of wind turbines, and the safety of local wind turbines needs to be reviewed.


Introduction
China is currently in a golden period of wind power development; the cumulative installed capacity and total power generation have increased for eight consecutive years.With the increasing development and utilization of wind resources, wind resources in flat terrain such as coastal areas are exhausted, and wind power is being extended to areas with complex terrain, many influencing factors, and difficult development, wind energy resource assessment is being paid more and more attention as an important preliminary work for micro-siting of wind farms.
In recent times, extensive scholarly research has been conducted concerning the challenge of assessing wind energy resources in intricate terrains [1] .A wind turbine wake model was developed, drawing from both the Lissaman and Jensen models.This comprehensive model accounted for wake shading across various wind speed ranges and heights [2] .The study delved into potential issues related to wind measurement towers, contours, and constraints during the micro-siting phase of wind farms in complex terrains, aiming to enhance the efficiency of wind resource assessment.Through flow field measurements, it became feasible to quantify surface winds and wake disturbances between multiple wind turbines, thereby optimizing turbine layout on intricate terrains.This optimization effort subsequently enhanced both the efficiency and durability of turbine power generation [3][4] .The research also entailed an analysis and synthesis of methodologies utilized in wind resource assessment [5] .Furthermore, the suitability of LiDAR in three distinct terrain types-complex mountainous regions, plains, and coastal areas-was examined and confirmed.Notably, this examination encompassed diverse measurement heights, thus establishing accuracy variations across different conditions.
In this paper, based on the meteorological and wind measurement data of a high-altitude region in Liangshan Prefecture, Sichuan Province, wind energy resources are evaluated to provide a reference for the development and utilization of wind energy resources in the region.

Overview of Wind Measurement Information
A wind farm is planned for this area, which is located in a highland mountainous region, consisting of a north-south ridge and its branches, with a total ridge length of about 13 km, an altitude of 3530 m -3950 m, and a total area of about 40 km 2 .The wind energy resources are relatively abundant due to the southwest airflow, and the ridge and windward slope are more obvious.Meteorological data from the weather station for the last 35 years (1981-2015) show that the average annual wind speed has been decreasing from 1981 to the present, mainly due to the replacement of wind measurement equipment and the increase in the number of tall buildings around the weather station due to urban development and global climate change.The windy months in the region are concentrated between January and May each year, and the less windy months are concentrated between July and September each year, i.e., the winter and spring winds are high and the summer winds are low.The prevailing wind direction in the region is SSE, and the annual prevailing wind direction is mainly south-southeast.
In addition to the data from weather stations, there are four wind towers in the wind farm area, and the wind measuring instruments are NRG, which makes observation records every 10 minutes.The wind measuring instruments are all calibrated, and the integrity and reliability of wind measuring data are well guaranteed.The basic situation of the anemometer tower is shown in Table 1, and the geographical location of the anemometer tower is shown in Figure 1.Among them, 5568# and 6119# anemometers are too far away from the fan position, which is too poor in mountain projects.7842# and 7844# anemometers are located on the southern ridge of the test site, far away from other anemometers, which are representative.Therefore, 7842# and 7844# anemometers are used as wind data sources, and the wind measurement period is selected from May 1, 2018, to April 30, 2019.

Completeness check
In the preliminary work of wind farm design, the integrity, reasonableness, and reliability of the original wind measurement data should be verified, and the interpolation and extension of missing data and the replacement of unreasonable data should be analysed.According to statistics, the valid data completeness rate of 7842# and 7844# wind measurement towers is 99%, which can meet the requirements of wind resource assessment.

Scope check
The original wind measurement data was tested to determine whether the values were within a reasonable range.For the wind speed and wind direction, the national standard was used; for the air pressure, the reasonable range of 94 kPa-106 kPa set in the national standard is not suitable for the actual situation of the wind farm in this area due to the high altitude and low air pressure of the site.Through the study of the distribution pattern of temperature and air pressure, combined with the analysis of wind tower data, the reasonable range of hourly average air pressure in the area of the wind farm is 60 kPa-90 kPa, and the reasonable range of hourly average air temperature is -20℃-30℃.The criteria for the range test are shown in Table 2 below.-20-30

Trend check
Conduct a trend test on the original wind data, test the continuous change of each measurement parameter, and judge whether the changing trend is reasonable.According to the corresponding national standards and meteorological industry standards, the criteria for the trend test are shown in Table 3.
Table 3 Criteria for identifying trends in wind measurement data.

Check items Judgment criteria
Wind speed values (m/s) If the wind speed does not change for 300 minutes, it is considered unreasonable.

Wind direction
If the wind direction does not change for 300 minutes, it is considered unreasonable.
Wind speed values (m/s) In the wind speed above 5 m/s, if the wind speed or direction has not changed for 6 consecutive hours, it is considered unreasonable.
Wind speed values (m/s) If the average difference between two adjacent hours is greater than a given value, it is considered unreasonable.

Temperatures (℃)
If the average difference between two adjacent hours is greater than a given value, it is considered unreasonable.

Pressure
If the average difference between two adjacent hours is greater than a given value, it is considered unreasonable.

Relationship check
The relationship test is mainly set for the rationality of the relationship between wind speed and wind direction at different heights.It refers to checking whether the difference in wind speed or wind direction at each height is within a given reasonable range.Based on the national standards and meteorological industry standards, combined with the characteristics of wind speed and direction in mountainous areas, the relationship test criterion is formulated, as shown in Table 4.When the cut-in wind speed is 5 m/s, the average wind speed or wind direction at different heights with a height difference in a certain range should meet the following criteria for discrimination.
Table 4 Criterion for relationship test of wind data.

Check items Judgment criteria
Hourly average wind speed (m/s) The difference in hourly average wind speed is less than 8 m/s for heights greater than 20 m apart.
Hourly average wind speed (m/s) The difference in hourly average wind speed is less than 4 m/s when the heights are not more than 20 m apart.

Hourly average wind direction difference
The hourly average wind direction difference between any two different heights is less than or equal to 45° and greater than or equal to 315°.

Correlative factor analysis
Correlation analysis of the data from the two wind measurement towers showed that the correlation coefficient between the wind speed of tower 7844# and the 80 m height of tower 7844# was 0.89, which was a good correlation.
Correlation analysis of the valid data in the average wind speed at each height (10 minutes) of the wind measurement tower, the correlation coefficients of different heights of the wind measurement tower 7842# are above 0.9165, with good correlation; the correlation coefficients of different heights of the wind measurement tower 7844# are above 0.5669, with good correlation at the upper levels.

Missing measurement and invalid data processing
To meet the requirements of the resource assessment, missing and invalid data from wind measurement towers are dealt with and data from towers that have been measured for less than one year are interpolated to extend to one year.In the same period, invalid data from different height levels of the same tower are interpolated by data from other adjacent height levels.If there is no invalid data at all heights of the tower within the same period, the data before and after the tower can be substituted if the invalid data has not been recorded for a long time; if the invalid data has been recorded for a long time, the data from nearby towers with good correlation can be interpolated.If all towers have missing or invalid data at the same time in a full year, the correlation analysis is carried out using the mesoscale data of the same period.Through the above process, the wind measurement data for each tower is 100% complete and the wind measurement data for each tower meets the requirement of a full year.Wind Energy Resource Assessment

Air density calculation
As the air density at the site is generally not equal to the standard air density of 1.225 kg/m 3 , an air density correction is required.The multi-year average air density is derived from the statistical values of meteorological elements measured by the wind measurement tower: In the above Equation, ρ is the annual average air density (kg/m 3 ), T is the annual average air Kelvin temperature (K), and R is the gas constant, 287 J/(kg•K).The average air density at 7842# and 844# wind towers is 0.81h and 0.811 respectively, based on the average air pressure and temperature at 10 m from the wind tower.The altitude of the wind farm area ranges from 3530 m to 3950 m, and the average altitude of the whole site is 3710 m.The average air density for the whole site is 0.81 kg/m 3 .The wind speed frequency and wind energy frequency distribution of each tower at 80 m height are shown in Figure 5, Figure 6.From the wind speed distribution, the effective wind speed of 7842# tower is 7927 h, accounting for 90% of the whole year, of which 1042 h is 11 m/s-20 m/s, accounting for 11% of the whole year.The number of hours is 7732 h, accounting for 88% of the whole year, of which 1846 h is 11 m/s-20 m/s, accounting for 21% of the whole year.From the above, it can be seen that each effective wind speed period is long and the ineffective wind speed period is short.

Wind frequency curve and Weibull parameters
The wind frequency curve uses the Weibull distribution and the probability distribution function is expressed in Equation ( 2): In the equation, V is the wind speed; A and k are the Weibull parameters.According to the curve fitting calculation, the Weibull parameters are A=7.795 and K=2.579 for wind tower 7842# and A=7.971 and K=2.441 for wind tower 7844#.The Weibull distribution of wind speed at 80 m for each wind tower is shown in Figure 7.
Figure 7 Velocity Weibull distribution at 80 m from the wind measurement tower 3.8.6.Turbulence intensity The 10 minutes turbulence intensity is calculated according to Equation (3), where σ is the standard deviation of the wind speed for 10 min and V is the average wind speed for 10 minutes.The calculated turbulence intensity at each height of the wind tower is shown in Table 5 below.We can see from Table 5 that turbulence is relatively low at all heights of tower 7842#; turbulence is low at the height of tower 7844# and high at the low end.The wind turbine class is classified by the wind speed and turbulence parameters in the wind farm siting and design.Comprehensive analysis shows that the turbulence level in this wind farm is of IEC Class B standard, which has a certain impact on the fatigue load of the wind turbine, and the safety of the local machine in this area needs to be reviewed by the whole machine manufacturer.

Wind shear coefficient
The wind shear index is derived from Equation (4).According to the results, the maximum wind shear is 0.0528 for 7842# in the range of 10-80 m and 0.431 for 7844# in the range of 10-80 m.The data of the 7844# wind tower is mainly due to the complex topographic conditions where the wind tower is located.The wind shear will be raised rapidly when passing through, increasing the wind shear.
3.8.8.Calculation of maximum wind speed in a 50-year return period Depending on the actual conditions of the wind measurement tower, the maximum wind speed for a 50year event can be calculated according to Equation (5) or Equation (6).
50 max ave 1 where V50max is the 50-year maximum wind speed, Vave is the annual average wind speed, C1 is the empirical coefficient and C1 takes the value of 5.0.
where V50max is the 50-year maximum wind speed, u is the distribution location parameter and α is the distribution scale parameter.The formula uses a 7-day maximum 10 min average wind speed sampling method to estimate the maximum wind speed of the wind farm.30 samples of the 7-day maximum wind speed were obtained from the 80 m height of the wind measurement tower 7842#.These 30 samples were used to estimate the 50-year maximum wind speed at 80 m height using the extreme value type I probability distribution and the modified moment parameter estimation method.From Equation ( 5), the maximum wind speed at 80 m height of 7842# and 7844# wind measurement towers in 50 years is 36.1 m/s and 39.9 m/s respectively.From Equation (6), the maximum wind speed at 80 m height of 7842# and 7844# wind measurement towers in 50 years is 20.18 m/s and 32.6 m/s respectively.The maximum 50-year wind speed calculated by the above two methods is taken as the larger value of the 50-year maximum wind speed for each tower at a height of 80 m.The corresponding 50-year maximum wind speed for the standard air density (ρ=1.225kg/m 3 ) under equal wind pressure conditions is derived.The wind pressure is calculated from Equation (7).The maximum 50-year wind speed at standard air density is calculated to be 29.35m/s at the 80 m height of the 7842# tower and 29.61 m/s at the 100 m hub height; the maximum 50-year wind speed at the 80 m height of the 7844# tower is 32.44 m/s at the 100 m hub height.34.57m/s.
According to IEC 61400-1-2005, the maximum wind speed for a 50-year event in an IEC Class III wind farm is 37.5 m/s.According to the available wind measurement data, the maximum wind speed for a 50-year event at the hub height of the two wind measurement towers in the field meets this requirement.

Conclusions
The wind power density class of this wind farm is level 2. The main wind direction and main wind energy direction of this wind farm are the same, with the highest wind speed and wind energy in the west-southwest (WSW) direction and the highest frequency.The prevailing wind direction is stable, with westerly winds prevailing throughout the year.Due to the influence of the terrain, the wind direction differs significantly from that of the weather station.The wind speed frequency of each wind measurement tower is mainly concentrated in the range of 3.0 m/s to 15 m/s, with less invalid and destructive wind speeds below 3.0 m/s and above 20 m/s.The variation within the year is small, and the time available for power generation is relatively long throughout the year.
According to the wind measurement data from the wind tower, the maximum wind speed for a 50year event is 29.61 m/s at the 100 m hub height of the wind tower 7842#; the maximum wind speed for a 50-year event is 34.57m/s at the 100 m hub height of the wind tower 7844#.Both are less than 37.5 m/s; based on this resource analysis and the feasibility study data, it is judged that this Phase II wind farm is classified as an IEC II/III wind farm.
The turbulence intensity of wind measurement tower 7842# belongs to IEC category B and the turbulence intensity of wind measurement tower 7844# belongs to IEC category C. Based on the location and representativeness of the wind measurement towers, the turbulence level of this wind farm belongs to IEC category B. There is a certain influence on the fatigue load of wind turbines, and the safety of the local wind turbines needs to be reviewed.
In summary, the prevailing wind direction of this wind farm is stable and its wind energy resources have a certain development value.

3. 8 .
Wind energy calculation 3.8.1.Average wind speed and wind power density The average wind speed and wind power density of the wind measurement tower for each month are shown in Figure2, Figure3.The intra-year variation of wind speed and wind power density is the same, and the wind speed and wind power density are larger from November to March, and smaller from April to October.7.98 m/s wind power density is 403.4W/m 2 .

Figure 2
Figure 2 Variation of monthly average wind speed and wind power density at each height of the wind measurement tower 7842#.

Figure 3
Figure 3 Variation of monthly average wind speed and wind power density at each height of the wind measurement tower 7844#.

Figure 4
Figure 4 Daily variation of mean wind speed and wind power density at 80 m height of wind measurement tower

Figure 5 Figure 6
Figure 5 Wind speed and wind power density distribution at 80 m height of wind measurement tower 7842#

Table 1
The basic situation of the anemometer tower.

Table 2
Criteria for the range test of wind measurement data.

Table 5
Turbulence intensity for each height of the wind tower