Hossein Yousefi, Sachio Ehara
Department of Earth Resources Engineering, Kyushu University
819-0395, 744 Moto-oka, Nishi-ku, Fukuoka, Japan,
Email: [email protected]
In this study, a Geographic Information System (GIS) was used as a decision-making tool to target potential geothermal power plant sites in Sabalan geothermal field, Northwestern part of Iran. The aims of the study are to identify suitable areas to establish a geothermal power plant (GPP), as the base study for the future investigations and development.
After comprehensive study about available data in the area, and required data layers for site selection project, firstly, the available data layers for GPP siting in 1:25000 scale are categorized in three datasets; Physical dataset (slope, hydrology and faults), socioeconomical dataset (population centers, land use and access roads) and technical dataset (anomaly area, wells locations and hot springs). Secondly an integration model in GIS environment was programmed and run, and then the areas were marked as GPP suitable sites.
In this knowledge-driven GIS method, from evidence layers, some factor maps were generated and then the Boolean integration methods were used for the combination of the factor maps. ArcMap, consisting of geoprocessing and model builder tools were used for running the GIS Model for Geothermal Power Plant Siting (GM-GPP). Finally, 7 suitable sites, around 1% of the study area were selected.
The use of geothermal energy for electric power generation has become widespread because of several factors. Countries where geothermal resources are prevalent have desired to develop their own resources in contrast to importing fuel for power generation. In countries where many resource alternatives are available for power generation, including geothermal, geothermal has been a preferred resource because it cannot be transported for sale, and the use of geothermal energy enables fossil fuels to be used for better purposes. Also, geothermal steam has become an attractive power generation alternative because of environmental benefits and because the unit sizes are small (normally less than 100 MW). Moreover, geothermal plants can be built much more rapidly than plants using fossil fuel. But any GPPs have some requirements before construction to product electricity with minimum impacts on environment and maximum economic benefits for developers. Basically GPP siting programs use such requirements and other investigation techniques to identify the best sites for utilization structures.
In this study ArcGIS was used as an effective tool for the integral interpretation of geoscientific data using computerized approach. This approach has been used to determine GPP sites by combining various digital data layers in Sabalan geothermal field NW Iran.
After comprehensive study about available data in the area, and required data layer for site selection project, firstly, the available data layers for GPP site selection in 1:25000 scale are categorized in three datasets; Physical dataset (slope, hydrology and faults), socioeconomical dataset (population centers, land use and access roads) and technical dataset (anomaly area, wells locations and hot springs). Secondly an integration model in GIS environment was programmed and run, and then the areas were marked as GPP suitable sites. The GIS (ArcMap 9.1) was used as a decision support system tool for performing site selection.
In the regional scales the ability of GIS software allows to successfully site selecting for GPPs or any industries at low cost and with a high success ratio. The model builder tools in ArcGIS were used as a graphical environment to develop a diagram of the multiple steps required to complete complex geoprocessing tasks. When the model was run, the model builder processes the input data in the specified order and generates output data layers. In the made model for siting GPP, the input data layers and related parameters are variable and can be defined by the user when the model is applied to other areas.
Thus in this study 7 suitable areas were selected in the Sabalan area for construction of a GPP.
The Sabalan geothermal area is located in the Northwestern part of Iran, south of Meshkinshahr city. The field chosen for study is around 282 km2 and included the Khiav River watershed.
The field is located between 38° 12′ and 38° 22′ North and 47° 39′ and 47° 49′ East and includes the villages Moil, Valezir, and Dizo. These villages are located approximately the 16 km main road that connects Meshkinshahr city to Moil which is the biggest village in the study area.
Sabalan Mt. is the 29th highest mountain in the world and a Quaternary volcanic complex that rises to a height of 4811 m above sea level. An asphalt road provides access to the field from Meshkinshahr to the village of Moil, then to the valley south of the village by a paved road. The location of the study area is shown in Fig. 1.
GIS is used to carry out a suitability analysis and site selection process because it can handle a large amount of data and information, is a powerful tool to visualize new and existing data can help produce new maps and allows the effective management of the data (Yousefi et al, 2007). Boolean intersect analytical method was used for selection queries. This method is described briefly in the following section.
This study was carried out in the1:25,000 scale and 8 important required data layers are employed. In every made factor map the study area was classified into two classless; suitable or non-suitable and binary maps were generated. These operations can be represented by the following simple equation (Noorollahi et al, 2007):
where the I is “AND” operations, Sa is suitable areas and F, Ri, S, PC, AR, A, W, HS are Faults, Rivers, Slope, Population Centers, Access Roads, Anomaly, Wells and Hot Springs, respectively. A diagram of the method that was used in the decision-making process is illustrated in Fig. 2
Fig.2.The schematic method of geothermal power plant siting
Boolean intersect method (AND)
The intersect tool in ArcInfo calculates the geometric intersection of any number of feature classes and data layers that are indicative of the suitable area. Features that are common to all input data layers were selected using this method (Bonham-Carter, 1994). This implies that the selected area is suitable for the purpose of the study based on all input data layers.
In this study, the suitable areas for GPP in NW Sabalan area were identified by using available digital datasets including physical, socioeconomic and technical. Each dataset includes some data layers (Fig.2). These data layers were used to make factor maps and factor maps were applied to the GIS Model for Geothermal Power Plant Siting (GM-GPP). The data layers introduced in the model are spatial distribution of slope, rivers, faults, population centers, access roads, anomaly zone, wells location and hot springs (Fig. 3).
Physical data set
Physical studies play an important role in all stages of GPP siting. In the initial stages of siting programs, the study areas were typically studied together, with one being chosen for detailed investigation (Rybach and Muffler, 1981). Physical studies also provide background information for interpreting the data obtained using other siting methods. Physical information can also be used in the production stage for other developments and management. The duration and cost of development can be minimized by physical siting program.
Fig.3. Physical, Socioeconomic and technical evidence layers
Slope refers to how steep the surface of the land is. Steep slopes are a limitation for GPP development, not only because of the cost and transportation but also water that can find pathway from the drain to flow on the surface. Basically the slope limitations for any development are slight if the slope is less than 8%, moderate if the slope is 8-15% and severe if the slope is greater than 15%.
In this study, topography counter map of the study area was used to create a slope binary factor map to use in the GM-GPP (Fig. 3). To identify suitable areas based on the slope the study area is divided into two features; less than 15% and more than 15%. The area with less than 15% slope is 149 km2 which in selected as suitable areas for GPP based on slope.
River limitations refer to the location of rivers and potential for flooding by streams or rivers around GPP in the study area. On the other words, the area without river, stream or big tributary drainage with their buffer can be assumed as a suitable area for GPP based on the river.
There are 82 km river in the study area. 47 km of these rivers which is called khiav chay, is a beautiful river, runs north from Mt. Kasra, between the two villages of Moil (to the east) and Dizo (to the west), to meshkinshahr city on the middle of wide Darreh Rud valley had a calciummagnesium hardness of only 80 mg/l and temperature 7.5°C (Fig. 3).
This valley has 120-280 meter depth that surrounded by steep slope therefore the selected area in the West side of the river were not selected because plumbing geothermal fluid from the wells which all located in the East side of the river is not economic.
In this study, 200 m buffer size was given around the rivers data layer to identify river limitation areas. The areas beyond this limitation are suitable area for GPP based on the river.
In geology, faults are discontinuities (cracks) in the earth’s crust that have been responsible for many destructive earthquakes.
Blewitt et al. (2003) indicated that at a regional scale, geothermal plumbing systems might be controlled by fault planes. Therefore fractures and faults play an important role in geothermal fields, as fluid mostly flows through fractures in the reservoir rocks.
In the current study for avoiding of risk-taking of faults, 200 meter buffer size was applied by using the ArcMap Buffer tool and a certain area is selected as potential hazard area based on faults and fractures. The made fault limited factor map was used in GM-GPP to identify suitable area by avoiding fault risks. In this scale there are 42 faults and fractures in the study area (Fig.3).
Socioeconomic study and conditions are usually hard to identify and investigate, as they are related to the human beings and their characteristics, which usually differ widely within the same community and from one community to another.
In the study area among the socioeconomic parameters, maybe population center, access road and land use can affect the GPP site selection project. Based on the land use data layer all around the study area is suitable for GPP construction. For this reason land use data layer did not appear in the model.
The location and distribution of Villages, single buildings, agro nomads camping, sheep farming, stadium and sport centers, burial grounds, mosques and etc considered as population center data layer. To avoid of selection or affect these areas, 500 meter buffer size applied around these features to make the population center limited data layer. The clip tool in ArcInfo between the population center limited map and study area map was applied to make the suitable area based on the population centers or factor map which was used in the GM-GPP.
There are three villages in the study area located in the southern, northern and eastern parts respectively. The Valezir village is located in the northern part and comprises about 50 families. The second village is Dizo in the north-western part of the area comprising about 30 families and the third and largest village is Moil that is located in the south-eastern part of the area with more than 400 families whose dominant occupation is sheep keeping and cultivation (Yousefi, 2004) (Fig. 3).
One of the important parts of every socioeconomic study is the condition of road network. In the study area 16 km asphalt road provides access to the field from Meshkinshahr city to the village of Moil, then to the geothermal site south of the village by 14 km paved road (Fig.3).
In the GPP site selection project 100 meter buffer size was applied around the road features to make the restricted road map. By using clip tool, factor map without this restricted area was made to use in the siting model.
Technical data set
Like all forms of electric generation, both renewable and non-renewable, geothermal power generation has some technical requirements. In this study the most important requirements including anomaly zone, well locations and hot springs categorized in to the technical data set in GM-GPP model.
Geothermal fluids can be transported economically by pipeline on the Earth’s surface only a few tens of kilometers, and thus any generating or direct-use facility must be located at or near the geothermal anomaly zone.
Anomaly zone in the study area is around 7 km2 (Fig.3). To find the GPP suitable area based on the anomaly zone in NW Sabalan 3km buffer size was applied which it surrounded anomaly zone feature.
By the means of using clip tool in ArcMap only the buffer of anomaly zone around 75 km2 selected as a factor map and suitable area for GPP siting model.
Until now there are 3 exploration and 2 injection wells at the study area (Fig.3). To avoid of selecting area so near to well pads 200 m buffer size was given to the wells features to make the restricted wells map.
The area without this limitation was selected to make the suitable area for GPP based on the wells location which was used as factor map in the siting GIS model.
Hot springs are evidence of a subsurface heat source and the temperature of springs has correlation with amount of heat flow. Those locations where hot springs rise to the surface are geothermal potential prospected areas because it is assumed that the probability of the occurrence of a geothermal resource is higher than that in the surrounding area.
There are 7 hot springs (Fig. 3) including hottest one in the country which is called Geynarjeh with 86°C located in the study area. With regard to the chemical and physical characteristics of the thermal waters, they have been traditionally used for recreational and balneological purposes in the form of swimming and bathing pools as a fundamental version of direct-heat utilization of geothermal energy in the region (Saffarzadeh and Noorollahi, 2005).
The clip tool was applied between the study area and limited hot springs map with 100 meter buffer size to make the hot spring factor map to use in the model.
DATA INTEGRATION METHOD
Boolean integration model which was used in the current study involves the logical combination of binary maps resulting from the application of conditional “AND” (Intersect) operator. For performing Boolean logic model the study area based on each evidence layer was classified into two different areas. The area which assumed suitable area assigned the value of 1 and the others value of 0. Fig.2 shows the conceptual model of the Boolean integration method which was applied for data integration in the site selection process.
Physical suitability was determined by integrating the selected suitable area based on slope, hydrology (river) and faults factor map. This three evidence layers were overlain by Boolean “AND” operator and the selected areas were combined (union) to identify physical suitable areas. Socioeconomical suitability was identified by integrating selected areas on the base of population centers and access roads factor maps. These two layers were overlain and the selected areas were combined (union) to identify the socioeconomical suitable area.
Technical suitable area was determined by overlapping of the anomaly zone, well locations and hot springs factor maps by using the Boolean “AND” method. The selected areas were merged to identify the technical suitable area for GPP.
Table 1 shows the employed evidence layers and criteria which were used in geothermal Power plant site selection process.
Tab.1.Iintegration criteria of employed evidence layers
Finally the Physical, Socioeconomical and Technical suitable area overlain and intersected using Boolean “AND” operator to identify the suitable geothermal power plant sites. Fig.4. shows the location and extend of 7 suitable sites.
Fig.4. Defined sites for geothermal power plant installation
In the current study the geothermal power generating site selection in NW Sabalan geothermal area were investigated and identified by using available physical data including slope, river and faults, socioeconomical data such as population centers, access roads and land use and technical data consisting anomaly zone, wells locations and hot springs. All of the involved digital maps provided in the 1: 25,000 scale with the precision of 10 meter.
Boolean integration method by using “AND” (Intersect) operators was applied to combine the evidence layers in GIS environment. Finally 7 suitable sites, around 1% of the study area were identified.
Table 2 shows suitable areas for constructing the geothermal power plant in the study area. The designed model in GIS environment is a dynamic model and can be improve by adding new data layers or changing the criteria.
Tab.2. Location and the area of defined suitable sites
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