Estimating Hydropower Potential of an Ungauged Stream

Transcription

1 Estimating Hydropower Potential of an Ungauged Stream Umaru Garba Wali Water Resources and Environmental Management Programme, Department of Civil Engineering, School of Engineering, College of Science and Technology, University of Rwanda-Huye Campus. P. O. Box 117, Huye. Abstract This paper present a simple method of assessing hydropower generation potential of an ungauged Stream/River. Sites on Rukarara and Mushishito in the southern province of Rwanda were selected as case studies. The approach uses flow data of an analogue site to generate the data of an ungauged river. During the study a reconnaissance field visits were conducted. The concerned catchments were delineated and characterized based on elevations, landuses, rainfall and soils types. GIS technology was used for characterization. Relative positions of the component of the hydropower plant and it net available head was proposed using topographic map and the contour map of the study area developed with the help of Digital Elevation Model in ArcGIS. Net head and design discharges were determined and the hydropower potentials for the two sites were estimated. The presented methodology could be applied during pre-feasibility study to facilitate preliminary technical and socio-economic analysis before conducting detail investigation. Keywords ArcGIS, DEM, Hydropower Potential, Mushishito, Rukarara, Rwanda, Ungauged Stream/River. I. INTRODUCTION Generation of electricity from hydropower has been and remains the first source of renewable energy. Development of energy from the renewable source is one of the important steps in reduction of CO 2 emission that could mitigate the effect of climate change [1]. Decision on whether or not to implement a hydropower project requires carrying out feasibility study [2]. One of the important components of feasibility study of a hydropower is the estimation of potential electricity energy generation [2, 3]. It is well known that amount of electrical energy a hydropower plant can generate is directly proportional to the product of flow (Q) and net available hydraulic head (H) [1, 2, 4]. While the gross available head can be determine at a site after propose layout of the hydropower plant, determining the design flow requires long-term record of River or Stream flow at the selected site which is not available in case of ungauge site. This paper present a simple method of determining the hydropower generation potential of an ungauged River site. The method include determination of gross and net available heads, stimulation of long-term River flow using flow record of an analogue River, development of a flow duration curve and selection of design flows. This was demonstrated with a case study of Rukarara and Mushishito Rivers in the southern province of Rwanda. Rukarara and Mushishito River sites are amongs the proposed potential sites for small hydropower development in Rwanda [5]. II. BRIEF DESCRIPTION OF THE CASE STUDY SITES The case study sites on Rukarara and Mushishito Rivers are located at Kibirizi sector, Nyamagabe district of the southern province of Rwanda. Mushishito is a subcatchment of Rukarara catchment. All the two case study catchments are located between 2.37º to 2.58º South and 29.27º to 29.53º East (see Figure1). The area of the two catchments is estimated to be km 2, with Rukarara catchment having about km 2 and Mushishito 61.5 km 2. The topography of the area is mountainous with steep slopes. The elevation ranges from about 1,823 to 2,774 above sea level. There is a river gauging station at Musabeya on the Rukarara River located about 800 m upstream to the proposed intake point (weir position) on the same river. III. MATERIAL AND METHODS Assessment of hydropower potential of an ungauge Stream or River requires the followings: (i) stimulation of long-term flows of the River for the selection of design flow, (ii) topographic assessment of the site for selection of appropriate locations and alignments of the components of the power plant such as diversion weir or dam, diversion canal (where applicable), forebay, penstock and the powerhouse as well as determining the net available head, (iii) selection of Turbine machine that marches the design discharge and net available head [1]. 592

2 Figure 1. Location of the study area in Rwanda. A) Drainage Area Delineation A Digital Elevation Model (DEM) 90 by 90 meters obtained from the USGS seamless website was used in the ArcGIS environment to delineate the case study catchment areas. The DEM was corrected for depression then used to generate the flow direction grids that determine the direction of movement of water from each individual grid within the catchment. The flow direction grids were then uses to generate flow accumulation grids which determine the nature of flow concentration within the catchment. The flow direction grids were used to generate the drainage network of the catchments. A GPS point of the catchment outlets together with the above generated grids were used to delineate the catchment areas contributing water to the proposed outlets using the hydrology tools of the ArcGIS 9.3. The delineated catchment areas are shown in Figure 1. B) Stimulation of River Flow Several methods are available for stimulation of flow of an ungauged river site. In this study the methods of scaling down stream flow values of an analogue gauged site of larger catchment with similar characteristics to the catchment of interest was considers. The characteristics taken into consideration includes: drainage area, topography (mountainous, slopes, valley etc), climatic patterns (rainfall pattern, etc.), soil characteristics (porous, impermeable, etc), landuse (settlement, forested, agriculture, etc.). Flow data (17 years) of Rukarara River at Musebeya location E and S was used. Digital Elevation Model was used to delineate the area of the catchments as mentioned in (A) above. ArcMap for ArcGIS 9.3 was used for the analysis of drainage area, elevations, rainfall pattern, soil characteristics and landuse that were used for comparison. 593

3 When close relation was confirmed between the two catchments, equation (1) was used to scale down the flow of Rukarara River to that of Mushishito River at the indicated site Figure 1. This was based on the fact that if the two catchments are similar in relations to the mentioned parameters then the ratio of flow to the drainage area for the smaller catchment can be considered approximately equal to that of the larger one [1, 2, 3, 4 ]. Where: A 1, & A 2 drainage area of the site of interest and an analogue site respectively, Q 1 & Q 2 - discharges at the site of interest and an analogue site respectively. C) Design Flow The discharges of the Rukarara River and the stimulated flow of Mushishito River, were used to generate the Flow Duration Curves (FDC) of the two proposed diversion sites. The discharges were arranged from maximum to minimum and ranked from R=1 to N. The percentage of the time (P) flow equals or exceeds a given value was estimated using equation (2) [6]. DEM was analyzed in ArcMap environment to generate contour map of the study area. The obtained contour map and the topographic map of Rwanda [9], was used during a field visit to proposed the most suitable location and alignment of diversion weirs, diversion canals, forebays, penstocks and the powerhouse Figure 5. This was used to estimate the available geometric head (H g ) for the two sites as difference in elevation between the proposed water elevation at the headrace and elevation of water at the tailrace. The net available head (H) was estimated as the difference between available geometric head and total head losses from headrace to tailrace, equation (3), resulting from simplification of energy equation [10]. Where - -sum of head losses (equation 4) from headrace to tailrace that could include head losses due to pipe friction h f (determine with equation 5) and local constrictions h L, (losses from trash racks, bends, valves, etc. determine with equation 6) [11]. The FDC is constructed with P in the abscissa and Q on the ordinate. The ordinate is constructed in logarithmic scale to allow data to be stretched out making it easier to read the FDC at all points [6]. The FDC were used for the selection of design flows. In hydropower design practices the design flow is selected base on the potential power generation classified as minimum, small, median or mean potential power with the exceedence provability corresponding to 100%, 95%, 50% on the FDC and the arithmetic mean of the mean annual discharges of the site for a period of 10 to 30 years for the mean potential power generation respectively [1, 7]. However, in selecting the design discharge it is important to take into consideration residual flow, which is the minimum flow that must be maintained in the river to sustain the ecology and the requirements of downstream consumers. Any flow above this value could be used for generation [8]. D) Net Available Head Gross head available is established based on elevation differences between headrace and tailrace for reactive turbines or the axis of the turbine for impulse turbines. Where: λ-darcy-weisbach friction factor (no unit), L- length of the pipe (m), V- mean velocity of flow (ms -1 ), d- diameter of the penstock (m), K- resistance coefficient based on the type of local constriction (no unit) and - gravitational acceleration ms -2 The theoretical potential power generation was estimated using equation 7 [11]. Where: P T theoretical power generation potentials, ρ- density of water ρ=1000 kg.m- 3, -gravitational acceleration ms -2, Q- design flow (discharge) m 3 s -1 and H- net available head. E) Diameter of Penstocks The main purpose of penstock is to convey water from the intake (forebay) to the powerhouse. The diameter of the penstock influences the velocity in it, which in turn affect the head loss due to friction h f (see equation 5). 594

4 ESHA guide [1] demonstrated that dividing a diameter by two could raise the head loss by 40 times. The same guide stated that it is acceptable to select the diameter (D) of the penstock by limiting the head loss to a certain percentage that will lead to about 4% loss in power generation. In that case the diameter is estimated using equation 8 resulting from transformation of Manning equation. [ ] Where: n-manning roughness coefficient (no unit), Q- design discharge, L-Length of the penstock and H-available geometric head. However, other approach is to conduct rigorous economic analysis for the selection of optimum diameter. This is done by selecting several possible diameters, estimating power and annual energy production and plot the graph of energy loss over the life of the plant for each diameter. Also the cost of the pipe for each diameter is also calculated and plotted on the same graph. The two curves are added graphically and the optimum diameter would be that closest to the theoretical optimum [1]. The velocity of flow in penstocks depends largely upon turbine regulations but is seldom lower than 1.8 ms -1. It ranges from about ms -1 for medium head plants at maximum discharge. In high head plants velocity as high as 9 ms -1 have been used [12]. F) Selection of Turbines The power generated by a turbine is proportional to the potential energy lost by the falling water and is given by equation 9. Where: P A actual power generation potentials, ρ-density of water ρ=1000 kg.m- 3, -gravitational acceleration ms -2, Q- design flow (discharge) m 3 s -1, H- net available head, m and η o - overall efficiency of the plant (no unit). Energy is then estimate using equation 10 [13]. Where: T-operating time in hours, f-coefficient for seasonal flow variation for run off river installation. Turbines are selected based on the design discharge, net available head and efficiency. Turbines are constructed to operate between two extreme a minimum and a maximum working discharges. The minimum and maximum working discharges can be express as follows, equation 11 and equation 12 respectively [14]. Where: Qmin and Qmax lower and upper working discharges of turbine respectively, q min and q max turbine characteristic parameters representing the fraction of its nominal flow rate Q r, corresponding the lower and upper extreme working flow rates, respectively. For the three common commercial turbines Voros [14] found the following values of q min and q max, Table TABLE I VALUES OF FRACTION OF TURBINES NOMINAL FLOW RATE Turbine Francis Pelton Axial q min q max IV. RESULTS AND DISCUSSION To achieve the required results the following activities were conducted: (i) delineation of contributing drainage area, (ii) drainage area characterization (elevations, landuse, and rainfall) leading to determining of location and alignment the basic structures that include weir and diversion intakes, diversion canals, forebays, penstocks and power house, (iii) comparison of the characteristics of drainage areas for assessment of their similarity (iv) stimulation of flow and construction of Flow Duration Curves and (v) selection of appropriate turbines followed by estimation of power generation potential. The drainage area and its location in Rwanda are shown in Figure 1. After some technical and economic considerations it was proposed that a single powerhouse could be used for the two sites. A) Drainage area characterization Topographic characteristics: The topography of the area is mountainous with localized steep slopes. The elevation ranges from about 1,823 to 2,774 above sea level. The nature of topography and elevation distribution is presented in Figure 2. These Figures shows that there is close similarity between the topography of Rukarara catchment and of its sub-catchment Mushishito. The elevations and slopes characteristic of two drainage areas are presented in Table

5 TABLE I TOPOGRAPHIC CHARACTERISTICS OF RUKARARA AND MUSHISHITO CATCHMENTS Elevation, m Rukarara Mushishito Minimum: Maximum: Mean: Standard deviation Slope, % Minimum: 0 0 Maximum: Mean: 18 9 Standard deviation Flow length, km Minimum: 0 0 Maximum: Mean: Standard deviation Landuse characteristics: There are two major land uses in the catchments agriculture about 60%, forest about 35% and the others range land, wetland, water body and settlements constitute about only 5%. The landuses in the catchments are identical and uniformly distributed, Figure 3. The catchment is mainly composed of forest altitude soil formation. Figure 2. Elevation distribution in the case study area 596

6 Figure 3. Land use distribution in the case study area Rainfall distribution: Metrological stations with longterm rainfall record were not found within or very close the study area. However, there are network of metrological stations in the country that surround the study area. Therefore a mean annual isohyetal map of Rwanda was generated using the long-term rainfall records of the country and the rainfall distribution of the study area was extracted from it Figure 4. The rainfall characteristics of the two catchments are shown in Table II. TABLE II RAINFALL CHARACTERISTICS OF THE PROJECT AREA Rainfall, mm Rukarara Mushishito Minimum: Maximum: Mean: Standard deviation B) Construction of flow duration curves, select the design flows and estimate the expected power generation Mushishito River is ungauged, therefore have no flow records. To get an idea on its flow, flow measurement were conducted during field reconnaissance visits once each month in April, May and July, 2010, there results were 1.7, 1.5, and 0.5 m 3 s -1 respectively. The methodology outline in section III.C was implemented for construction of the FDC of Rukara and Mushishito Rivers that is presented in Figure 6, and the Tables II and III of monthly exceedence probability for the two Rivers were also developed. For this case study the design flow was selected to be a flow with 90% exceedence probability. The results obtained from stimulation were closely related to the real values of the measured flow. The measured flows for April and May correspond with the flow of which equal or exceed 50% of the time, while for July 100% of the time (see Table III). In that case the design flow Q for Rukarara would be 3 m 3 s -1. Flow Q, in m 3 s Mushishito % of time flow equals or exceed Rukarara Figure 6. Daily Flow Duration Curve for Rukarara River at the intake point C) Proposed location of the components of the hydropower plant The generated contour map, topographic map of Rwanda and the results of the topographic survey, conducted with a total station, on the proposed alignment of the diversion channel and penstock at the study area was used to propose locations and determines the design elevations for different components of the hydropower. The proposed locations of the various components of hydropower station are shown in Figure 5. The estimated geometric heads H for Rukarara and Mushishito are 45 m and 145 m respectively. 597

7 Figure 4. Isohyetal mean annual rainfall distribution of the project catchments TABLE III MONTHLY FLOW DURATION CURVE FOR RUKARARA RIVER AT THE INTAKE POINT Month Exceedence probability in % January February March April May June July August September October November December TABLE IV. MONTHLY FLOW DURATION CURVE FOR MUSHISHIRO RIVER AT THE INTAKE POINT Month Exceedence probability in % January February March April May June July August September October November December

8 Wasserkraft chart [15] was used for preselection of turbine. Four types of turbines were considered i.e. Crossflow, Francis Spiral Casing, Pelton and Turgo Impulse Turbines. All the four look satisfactory for the situation at Rukarara and Mushishito except Pelton Turbine which does not satisfy head equipment for Rukarara. However, of all Francis Spiral Casing Turbine provides better range of flow rate between minimum and maximum working discharges within the limit of design net head and discharge. The obtained parameters of the proposed hydropower plant are shown in Table 5. TABLE 5. TECHNICAL INFORMATION FOR THE HYDROPOWER PLANT. Figure 5. Location of the various components of the hydropower station Parameter Mushishiro Rukarara Penstock diameter, m 1 2 Geometric head Hg, m Net head H, m Design flow (90%), m 3 s Theoretical power potential, kw Actual power potential (η o=80%) Annual Energy production, GWh V. CONCLUSION For technical and economic reasons it is recommended that three Francis Spiral Casing Turbine with nominal flow rate of 1.5 m 3 s -1 be consider for power generation project at the case study site. All the three turbines should be housed in one powerhouse. The flow from Rukarara River should conveyed with a 2 m diameter pipe and be divided into two on reaching the powerhouse each half directed to one Turbine. This design has both technical and economical advantages. There is cost saving because of construction of only one powerhouse and reduces the number of technical staffs required for operation and maintenance. It is recommended that detail feasibility study be conducted to aid decision making on whether to implement or not this propose hydropower project. 599 The details feasibility study should consider technical, socio-economic and environmental aspect of hydropower project at the site. REFERENCES [1] European Small Hydropower Association ESHA, Guide on How to Develop a Small Hydropower Plant. [2] Jones I.D. 1988, Assessment and Design of Small-Scale Hydro- Electric Power Plants. PhD Thesis, The University of Salfold, Department of Civil Engineering [3] Sarkar S. and Gundekar H.G Geomorphological Parameters: Are they Indicators for Installation of Hydropower Site? International Conference on Small Hydropower Hydro Sri Lanka. [4] Copestake P. and Young A.R., How much water can a river give? Uncertainty and the flow duration curve. BHS 10th National Hydrology Symposium, Exeter. [5] SHEARS, Hydropower Atlas of Rwanda. [6] Chow V.T., Maidment D.R., and Mays L. W., Applied Hydrology. McGraw-Hill, pp-572. [7] Atil Bulu. Hydroelectric Power Plants. Lecture Notes. Istanbul Technical University, College of Civil Engineering, Civil Engineering Department, Hydraulics Division. [8] Wilson E.M., Assessment Methods for Small-hydro Projects. IEA Technical Report. The International Energy Agency- Implementing Agreement for Hydropower Technologies and Programmes. [9] RoR, topographic map of Rwanda [10] Wali, U.G Kinetic Energy and Momentum Correction Coefficients for a Small Irrigation Channel. International Journal of Emerging Technology and Advanced Engineering, Vol. 3, Issue 9. ISSN , ISO 9001:2008 Certified Journal.

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