689445 research-article2017
WIE0010.1177/0309524X16689445Wind EngineeringKim et al.Kim et al.
Original Article
Evaluation of wind resource potential in mountainous region considering morphometric terrain characteristics
Wind Engineering 2017, Vol. 41(2) 114–123 © The Author(s) 2017 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav https://doi.org/10.1177/0309524X16689445 DOI: 10.1177/0309524X16689445 journals.sagepub.com/home/wie
Hyun-Goo Kim, Yong-Heack Kang and Jin-Young Kim
Abstract In South Korea, where 64% of the national territory is mountainous, good wind resources in inland areas are mostly situated in high mountain regions. To develop a wind farm in a mountain region, the sloping of mountains and their shielding from wind by the surrounding topology should be considered. It is generally most advantageous to install wind turbines along a ridge that is open in all directions. This article presents a methodology for evaluating suitable sites for wind farm development by identifying suitable ridges using morphometric analysis, while excluding the geographical and social environmental exclusion factors and superposing them on the wind resource map to find the area having a specific level or higher wind power density. The result of the proposed suitable site analysis and existing wind farms was assessed to verify the feasibility of the method of analyzing suitable sites on ridges. The wind resource potential in Gangwon Province when calculated using a method based on conventionally suitable site and the method based on ridge analysis was 9 and 5 GW, respectively. The result confirmed that the conventional area-based potential calculation without consideration of the morphometric terrain characteristics overestimated wind resource potential by around 80% compared to the ridge analysis method of calculation presented in this article.
Keywords Wind resource potential, morphometric terrain characteristics, wind turbine spacing, wind turbine capacity density, South Korea
Introduction One of the most important factors to consider when designing an inland wind farm is the surrounding terrain. Among the geographic features that determine the feasibility of a wind farm are the shape and slope of the site. Installing wind turbines on a steep slope will increase the construction cost. It is unfeasible to install wind turbines in a valley or pit since wind is blocked or wind power density is low. As shown in Figure 1, a terrain suitable for the installation of wind turbines in mountainous regions generally consists of plains and ridges without a steep slope. Calculation of the existing wind resource potential only takes into consideration “slope” as the exclusion factor and does not consider any morphometric features (Grassi et al., 2012; Mari et al., 2011; Nguyen, 2007). According to the statistics issued by the Korea Wind Energy Association, the total capacity of wind power in South Korea is 853 MW including the new installation of 208 MW in 2015. Although this figure represents an increase of around 32% over the previous year, it is still 20% less than the target for 2015. The Korean Ministry of Trade, Industry, and Energy enacted the Renewable Portfolio Standard (RPS) in 2012 to promote the dissemination of wind energy; however, the plan for the construction of wind power plants is not progressing as planned due to stringent environmental regulation. The RPS mandates power companies to supply a specific portion of total power generation using renewable energy. The main cause of the delay in the adoption of wind energy in South Korea is strict environmental regulation. In South Korea, the Act on the Protection of Baekdudaegan was enacted to protect the Baekdudaegan Mountain Range, while the Management of Mountainous Districts Act was enacted to restrict the development in mountainous regions (Korea Forest
New and Renewable Energy Resource Center, Korea Institute of Energy Research, Daejeon, Korea Corresponding author: Hyun-Goo Kim, New and Renewable Energy Resource Center, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, South Korea. Email:
[email protected]
Kim et al.
115
Figure 1. Morphometric features suitable for the installation of wind turbines.
Service, 2011, 2012). The reason for this is that 64,139 km2 or 64% of South Korea’s territory consists of mountainous terrain. According to the draft of the Onshore Wind Energy Guideline announced by the Korean Ministry of Environment (MOE) at the end of 2013, Baekdudaegan and its ridges are supposed to have a buffer area where the wind turbines cannot be installed. Moreover, reviews are being conducted to protect the small mountain ranges (Korea Environment Institute, 2012). Upon completion of the “Study on the Development of a Mountain Range Connection Network System” initiated by the Korea Research Institute for Human Settlements in 2013, the regulation against damage to mountainous landscapes by such structures as power transmission towers is expected to be even more restrictive. To diagnose the poor dissemination of wind energy in South Korea and accurately study its potential, an analysis using the existing potential calculation would be insufficient. What is necessary is a more detailed review of the country’s mountainous regions, especially mountain ridges, where the environmental protection regulation is in conflict with the need to develop wind energy.
Data and method Analysis areas Gangwon, as the most mountainous province in South Korea, is already home to the Gangwon Wind Farm and the Daegwallyeong Wind Farm in Daegwallyeong (840 m above sea level); the Pyeongchang and Mt Maebong Wind Farms (1303 m above sea level) in Taebaek; and the Mt Taegi Wind Farm (1259 m above sea level) in Hoengseong. There are 49 wind turbines at the Gangwon Wind Farm, 6 at the Daegwallyeong Wind Farm, 9 at the Mt Maebong Wind Farm, and 20 at the Mt Taegi Wind Farm, all of which are situated along the ridges of these mountains or nearby. Together, they account for 18% of South Korea’s total wind turbines and wind power capacity. Gangwon Province is mostly mountainous as its 20,569 km2 area takes up 16.8% of South Korea’s total area, while its forest area occupies 21.5% of South Korea’s forest area. According to the longitude data issued by the Korea Forestry Service (KFS), Baekdudaegan is located in the eastern part, Hanbuk Mountain Range and Hangang Mountain Range in the western part, and Nakdong Mountain Range in the southern part of Gangwon. In addition, there are 18 smaller mountain ranges in the province (Figure 2(a)). Baekdudaegan is divided into core and buffer zones for its protection, with development prohibited in the core region by law.
Wind resource map The most important factor when assessing the feasibility of wind farm development is wind resources. This study uses the high-resolution wind resource map created by the Korea Institute of Energy Research (KIER). The first-phase wind resource map (Kim and Kang, 2010) is a 1 km × 1 km horizontal spatial resolution of the Korean Peninsula using Weather Research and Forecasting (WRF), which is a meso-scale numerical weather prediction (NWP) model, whereas the secondphase wind resource map is a 100 m × 100 m spatial resolution using WindSim which is a micro-scale computational fluid dynamics (CFD) model that has been dynamically down-scaled. The result of the first-phase analysis was used as the boundary condition of the second-phase analysis. According to the wind resource map produced by KIER, the
116
Wind Engineering 41(2)
Figure 2. Gangwon Province geographical data: (a) ridges and Baekdudaegan core zone, (b) wind power density distribution, and (c) geographical exclusion factors.
Figure 3. Morphometric categorization of terrain.
From the top left to bottom right: plain, channel, ridge, pass, peak, pit (Wood, 2009).
mountainous region in Gangwon has excellent wind power density because it consists largely of alpine districts. The area where the wind power density exceeds 250 W/m2 at 100 m above ground covers 4468 km2 or 27% of the total land area of Gangwon, while the area where the wind power density exceeds 350 W/m2 covers 2761 km2 or 17% of the total land area of Gangwon (Figure 2(b)).
Morphometric categorization of terrain Terrain can be morphometrically categorized into plains, channels, ridges, passes, peaks, and pits. From a construction point of view, it is easiest to build a wind farm on a plain. Since a ridge protrudes from its surrounding area, it has outstanding wind resources as it is not affected by the shape of the surrounding terrain. Although peaks offer the same geographical advantages, their surface areas are too limited for wind farms to be built on them (Figure 3). This study used the shuttle radar topography mission (SRTM) digital elevation model (DEM) 3 arcsecond for the assessment of geographical data and LandSerf v2.3 for the morphometric analysis of terrain (Wood, 2009). LandSerf can categorize mountainous terrain morphometrically by adjusting factors such as window size, distance decay exponent, curvature tolerance, and slope tolerance. The window size determines the kernel size for quadratic approximation. Since this study set window size to a minimum of 3 × 3, DEM 9 arcsecond × 9 arcsecond is considered as the kernel. The size of the kernel was calculated based on the assumption that the minimum area required for the installation of a wind turbine was DEM 1 arcsecond × 1 arcsecond. The distance decay exponent determined how to compare the center cell of a kernel with an outer boundary cell. “0” meant considering all cells the same, “1” represented the linear distance decay, and “2” represented the squared distance decay. This study used 1. The slope tolerance was the limit of the slope applied for morphometric categorization. As the value increased, the number of categorized features increased. This study assumed a maximum of 90°. The curvature tolerance is the standard value set to categorize a terrain as either convex or concave. Since a higher value meant more terrains being categorized as plains, this study used the minimum value of 0.1.
Kim et al.
117
Figure 4. Comparison of analyzed ridges of Mt Seorak.
To check the validity of the morphometric categorization of terrains using LandSerf v2.3, it was compared with the ridge map produced by the KFS. Figure 4 shows Mt Seorak in Gangwon (right) and indicates that the KFS’ ridge map (left) matches the analyzed ridges very well (center).
Exclusion analysis The exclusion factors can be divided into geographical exclusion factors, social environmental exclusion factors, and technical exclusion factors. The geographical exclusion factors are those geographical features, such as steep slopes, urban areas, rivers, and streets, which make it impossible to install wind turbines. In the case of slopes, the US National Renewable Energy Laboratory (NREL) and others considered a slope with a gradient of 20% (11.3°) or higher to be a steep slope and excluded them. In South Korea however, slopes with a 20° angle; that is, the standard presented in the draft of the Onshore Wind Energy Guideline issued by MOE, are excluded, as are urban areas, rivers, and streets, including their buffer areas. Other environmental factors such as national parks are also included among the geographical exclusion factors and are thus excluded (Kim et al., 2014). Figure 2(c) shows a map with all the geographical exclusion factors superimposed accordingly. The technical exclusion factors evaluate the wind power density criteria to enable the economic operation of MW-class wind turbines, which are currently the average size of wind turbine. In general, the economic factor of wind resources refers to the wind class proposed by the US NREL. In other words, the minimum economic efficiency is only assured by at least wind class 3 or higher, which corresponds to a wind power density of 350 W/m2 (average wind speed of 6.8–7.5 m/s) or more at 100 m above ground. It should be noted that in South Korea, several wind farms are constructed in areas with wind class 2 (average wind speed of 6.0–6.8 m/s), but these are mostly situated on plains where it is easier to connect to the power grid and where the construction costs are low.
Calculation of wind resource potential To calculate the wind resource potential, areas (km2) where it might be possible to install wind turbines were first calculated after conducting an exclusion analysis, and the results were then converted to a capacity potential (MW) by multiplying a turbine capacity density (typically MW/km2). A wind turbine capacity density is a measure representing a wind farm land use and defined as a nameplate capacity per unit area. Theoretical potential was calculated without any exclusion factors, while geographical potential reflected the geographical exclusion factors, and technical potential additionally reflected the technical exclusion factors. The suitable area-based calculation of capacity potential used the wind turbine capacity density of 8 MW/km2 which was derived from the statistical analysis on South Korea’s wind farms. For a reference, Hoogwijk et al. (2004) suggested the wind turbine capacity density of 4 MW/km2, but 8 MW/km2 would be more practical considering a double increase in wind turbine size and capacity for the last decade. Moreover, it is inevitable for South Korea to maximize a land use efficiency of wind farm considering difficulties to secure suitable sites and to acquire a development permission avoiding strong environmental protection regulations. According to the geographical analysis of total 481 wind turbines in 67 wind farms in South Korea, the average turbine capacity density was evaluated as 7.7 MW/km2, and the average wind turbine spacing along mountain ridge line was 3.5D. Since 2004, the average capacity of wind turbine (302 wind turbines have been installed in South Korea) was 1.9 MW.
118
Wind Engineering 41(2)
Table 1. Morphometric categorization of terrain in Gangwon Province. Area Exclusion factor not applied Geographical exclusion factor applied Technical exclusion factor applied
Area (km2) Portion (%) Area (km2) Portion (%) Area (km2) Portion (%)
Plane
Peak
Ridge
Pass
Channel
Pit
Total
8246 49.3 2656 42.1 365 33.0
138 0.8 68 1.1 17 1.5
4050 24.2 1791 28.4 424 38.3
177 1.1 79 1.3 19 1.7
4106 24.5 1706 27.0 281 25.4
25 0.1 6 0.1 0.9 0.1
16,742 100 6306 100 1107 100
The suitable ridge length–based capacity potential was calculated with a number of wind turbines installed along the ridge line. The Unison U93, a leading wind turbine manufactured in Korea, was used for the calculation. The U93 has a nameplate capacity of 2 MW, a hub height of 80 m, and a rotor diameter of 93 m. As such, the rotor diameter was assumed to be 100 m, and the spacing between the wind turbines was set at 3.5D (350 m). If the length of a ridge was less than 3.5D or if the ridge was too isolated from other ridges, it was assumed to be unsuited to the installation of wind turbines.
Results and discussion Morphometric categorization of terrain Gangwon Province has 13,786 km2 of mountainous terrain, accounting for 82% of the total land area of the province. Therefore, the province is expected to have a large number of ridges, channels, and peaks. Table 1 shows the morphometric categorization of the terrain of Gangwon, indicating that plains occupy one-half of the total land area of Gangwon while ridges and channels account for about one quarter each. After excluding the geographical exclusion factors from the morphometric categorization of terrain in Gangwon, the area occupied by plains decreased at a maximum of one-third compared to other types of terrain, primarily because urban areas, rivers, and streets are located mostly on low-lying flatlands. After excluding the geographical exclusion factors, the technical exclusion factors were applied. Where wind power density was less than 350 W/m2, the economic feasibility of wind farm development was considered to be too low and the areas concerned were excluded. After excluding the technical exclusion factors, the potential wind farm areas occupied by plains, ridges, and channels were reduced to 13.7%, 23.7%, and 16.5%, respectively. The suitable areas of ridges, channels, and peaks were reduced less than plains after excluding the technical exclusion factors because these types of terrain are situated at a higher altitude and hence have superior wind resources. Figure 5 shows the peaks and passes located along the ridge line. Therefore, this study considers all peaks and passes to be parts of ridges.
Analysis of suitable terrain for wind farms To verify the validity of the suitable wind farm site identification method using the morphometric terrain categorization, the three representative wind farms in Gangwon Province were analyzed. The Gangwon Wind Farm is South Korea’s largest inland wind farm. It has a total power capacity of 98 MW derived from forty-nine 2-MW wind turbines installed in mountainous areas ranging from 983 to 1142 m in elevation (Figure 6(a)). This region has outstanding wind resources as the wind power density exceeds 550 W/m2 at 100 m above ground. Out of a total of 49 wind turbines, 37 are installed on ridges, while the remaining 12 are installed on plains (Figure 6(b)). The west side of the main ridge, where most of the wind turbines are installed, has a gentle slope and larger plains while the east side has a steep slope (Figure 6(c)). As such, the majority of the turbines are installed on the plains on the west side. The Taegisan Wind Farm has a total power capacity of 40 MW originating from twenty 2-MW wind turbines. Although its average elevation of 792 m is lower than that of Gangwon Farm, its wind resource potential is still outstanding due to its wind power density of 450 W/m2. The 20 wind turbines of Taegisan Wind Farm were installed exactly along the ridges (Figure 7(b)). The Maebongsan Wind Farm is located in the vicinity of Mt Maebong in Taebaek City. It has a total power capacity of 8.8 MW originating from nine wind turbines composed of one 2-MW turbine and eight 850-kW turbines. The wind turbines are installed in high mountainous areas with an average elevation of 1234 m (Figure 8(a)). The wind resource potential here is outstanding due to a wind power density of 550 W/m2 or more. All the wind turbines of Maebongsan Wind Farm are installed along the main ridge, with the southernmost wind turbine installed near the peak. The Maebongsan
Kim et al.
119
Figure 5. Contiguity of ridges, passes, and peaks.
Figure 6. Gangwon Wind Farm’s wind turbine layout: (a) terrain elevation, (b) morphometric classes, and (c) terrain slope.
Wind Farm has many areas with a slope of 20° or more, and much of the region consists of the urban buffer region near a railway station, elementary school, and so on, which are excluded from calculation (white areas in Figure 8(b)). The wind turbine spacing of Maebongsan Wind Farm is about 90 m (1.7D), which is much smaller than the 280 m (3.5D) spacing at the Gangwon Wind Farm and the 220 m (2.7D) spacing at the Taegisan Wind Farm. In fact, the wind turbine spacing at wind farms in mountainous Gangwon is much smaller than those at wind farms on the plains of Europe because the ridges mainly run from north to south while the main wind direction is westerly (Figure 9). In other words, when the wind turbines are laid out along ridges from north to south, they form a line running perpendicularly to the main wind direction, so wake loss can largely be avoided even when the wind turbine spacing is set to the minimum under such an arrangement (Kim et al., 2016). Superimposition of the locations of the wind turbines at Gangwon, Taegisan, and Maebongsan Wind Farms on the ridges analyzed using LandSerf v2.3 confirms that most of the wind turbines—with the exception of those installed on plains—are located along ridges (Figures 6(b), 7(b) and 8(b)). Moreover, the fact that the wind turbines are all located
120
Wind Engineering 41(2)
Figure 7. Taegisan Wind Farm’s wind turbine layout: (a) terrain elevation, (b) morphometric classes, and (c) terrain slope.
Figure 8. Maebongsan Wind Farm’s wind turbine layout: (a) terrain elevation, (b) morphometric classes, and (c) terrain slope.
in suitable areas, that is, none of them sit in geographically or technically excluded areas, confirms that the exclusion analysis is valid.
Calculation of wind resource potential The suitable types of terrain for wind farm development were categorized using the morphometric terrain analysis method. Plains, ridges, passes, and peaks were considered the suitable types of terrain for the installation of wind turbines. Thus, since ridge, pass, and peak are generally situated along ridge, they are assumed to be “ridge.” The wind resource potential can be calculated using either area or length of the potential site. To calculate it using the area, suitable areas were first identified and multiplied by 8 MW/km2, which is the wind turbine capacity density per unit area of South Korea.
121
Kim et al.
Figure 9. Wind roses at the wind farms in Gangwon Province: (a) Gangwon Wind Farm, (b) Taegisan Wind Farm, and (c) Maebongsan Wind Farm. Table 2. Wind resource potential based on area by wind power density range. Wind power density (W/m2)
Wind class
