Assessment of Power Compeers Prospective of Gobecho Micro Hydropower Plant on Ganga River, Genale Dawa River Basin, Ethiopia

Micro hydropower helps the rural areas to supply power with off grid electrification. This study deals with the design of micro hydropower scheme in Gobecho kebele, Genale Dawa river basin, Ethiopia. The site for the construction of diversion weir is located upstream of the Ganga river in the kebele. The design were implemented based on the available hydrological data collected from 1991 to 2020 river discharge at different stations by selecting the one, which is near to the Gobecho catchment. Ererti is a station geographically near to Gobecho catchment by using GIS 10.7 application. The Gobecho peak probable discharge of 3.503m/s is computed using log normal distribution after L-moment analysis. Based on this peak discharge, vertical drop weir and components of headwork structure designed and provided relevant dimensions. The proposed vertical drop weir has a height of 0.703m, crest length 10m and the design flow for a hydropower scheme is 0.132m/s estimated by FDC. The water passes through trapezoidal power canal of a 0.35m width and a 0.43 height to a forebay tank. A forebay of size 9 × 1.3 × 1.7 is provided at the end of the power canal. Moreover, the water leads from forebay to a 0.24m diameter of penstock running one unit turbine and capacity of 25kw. The selected type of turbine for proposed plant is cross flow turbine. Finally, the quantity of calculated estimate cost is 50,000 USD and the B/C ratio of 1.48 provided for this micro hydropower project.


Introduction
In Ethiopia, the electricity generation from water came to existence in the beginning of 1930s, when Aba Samuel hydropower scheme which was commissioned in 1932. This station is capable of generating 6MW of electricity [1]. According to Ministry of Mines and Energy, in 1990 the total energy requirement in Ethiopia was estimated 177.6Twh per year, out of which 76.1% from wood, 16.1% from agricultural by product, 5.3% from fuel oil, 1.1% from electricity, 0.8% from charcoal, and 0.6% from other energy sources [2,3]. Ethiopia has got substantial hydropower potential estimated as 45,000MW, out of this, less than 10% has been utilized and the remaining should be developed at small to large scale so that the source of energy for various uses can be replaced by this more environmentally friendly, highly efficient and perpetual alternative energy source [10]. When the micro hydropower plant that develops on Ganga River implemented, it will play its own role in solving the electric scarcity problem in rural areas of Bona zuria woreda and Gobecho kebele even though it does not have any contribution to the national grid.

Description of the Study
The proposed micro hydropower project is located on Ganga River that found in Sidama zone Gobecho

Filling Missing Data
Data with year or month missed because of different reasons. Some of these are failure of instrument, carelessness of the observer etc. In this project, graphical correlation used to obtain missing mean flow, maximum flow and minimum flow value [2,9]. We have selected this method due to the following reasons: -It is the most widely used method when compared to other Method. It has applied by creating a correlation with the given run-off and select best r value with in power, exponential, linear etc. And the correlation coefficient, r in all cases is in between 0.6 and 1.

Transferring of Stream Flow Data
The hydrological data of different stations are given in the original data. After filling the missing data, we try to find the station, which is near to the weir site (gobecho) from GIS 9.3 application we use the data of ererti that is geographically near to the weir site. Then from [4] and [8] ( /) b *Qg AU =54km 2 , Ag= 58.9km 2 Since the ratio of / = 0.9168 is b/n 0.8 to 1 then use b=1.

Consistency of Stream Flow
Before using stream flow data, it should be check for consistency [6]. And 30 years of flow data is used from 1991-2020. If the conditions relevant to the recording of gauge station have undergone a significant change during the period of record, inconsistency would arise.

Flow Duration Curve
Flow duration curves are plot using the average monthly values of the flow. Depending on the flow duration curve, the conventional discharge values tabulated as follows:-

Potential Power Available at the Selected Site
The actual use of the above equation however is made difficult because the discharge of any river various over the wide range of the year [11,12,14]. We adopt the 95% of the time available discharge. In the case of Hydropower plant without storage reservoir (i.e. in this case diversion canal plants) technically available power calculated by taking the overall efficiency as 55 to 85%. Therefore, P=gρηQHgross P=0.55 to 0.85*(gQHgross). The head available to generate power =the gross head is estimated as 25m (from topography map after contour generation) P=0.77*9.81*0.132*25=25kw

Flood Frequency Analysis
The frequency analysis in hydrology is a statistical method used to show that events of certain magnitudes may on average expected once every N year. There are different methods of flood frequency analysis. The most commonly known methods [5]

Method of Moments
Sample estimator of L-moment are linear combinations of the ranked observation and thus do not involve squaring or cubing the observations. That is why it said L moments. In a wide range of hydrological applications, L-moments provide simple and reasonably efficient estimator of the characteristics of hydrologic data and distribution parameters [6].

Log normal distribution
In this method, the flow values in the record converted to the logarithmic form of base ten. In this distribution, logarithmic values of sample data assumed to follow normal distribution. The distribution is same as log Pearson type-3 when Cs= 0.     no tension is developed. Max stress δmax =ΣV/T (1+6*e/T) = 10.19KN/m 2 . Hence, the maximum stress is not much large which is safe against incoming stress.

Design of Impervious Floor
According to Bligh's theory, the percolating water flow the outline of the base of the foundation of the hydraulic structures. It assumed in this theory, that the loss of head is proportional to the length of the creep. For design of hydraulic structure like wears on previous foundation, Khosla's has evolved a simple, quick and an accurate approach [8].

Intake Design
For small-scale hydropower, it is better to select roughly finished masonry orifice type of intake with allowable velocity ranges 1-1.5m/s (small-scale hydropower lecture note). Hence, we assume V=1.2m/s and from our previous calculation Qd= 0.132 m 3 /s. Area, A=Q/V=0.132/1.2=0.11m 2 Assume orifice intake height, H=0.2 and we calculate the width. W=A/H=0.11m 2 /0.2m=0.55m Therefore, the size of the intake = 0.55*0.2. Check the flow through the orifice using submerged orifice equation. Qsub= A*C*√2 (Where for roughly finished masonry C=0.6, hh is the depth of water in the headrace canal=0.28 (for normal flow condition) hr=height of normal water level in the river u/s of the weir=hweir+ hovertop For normal flow condition hovertop=0 then hr=0.703+0=0.703m. Qsub=A*Cd*√2 =0.19m 3 /s>0.132m 3 /s, it is submerged. Therefore, our assumption is correct.

Design of Penstock
In hydropower scheme, the cost of penstock is very high and if the number of penstock is many, the total weight of steel required and construction cost is expensive [4,15,19]. On the other hand, large diameter, for a given discharge, will result small head loss and greater available net head. As the diameter increases the velocity decreases and the capital investment will get higher, therefore a size, which gives least cost selected.

Thickness of Penstock
The appropriate wall thickness for a penstock is generally a function of penstock material selected that is steel tensile strength, the diameter of the penstock and the operating pressure it will experience during its use [7,20].

Selection of Turbine
The applicable range of each type of turbine according to [16,17,18] .By using net head of 25m , required power of 25kw and discharge of 0.132m 3 /s. For this value of head and discharge cross flow turbine was selected from Figure 12. Figure 12. Turbine selection chart [21] 30 Abebe Temesgen Ayalew: Assessment of Power Compeers Prospective of Gobecho Micro Hydropower Plant on Ganga River, Genale Dawa River Basin, Ethiopia From the above ranges, we select cross flow turbine. Cross flow, turbines have the following advantage in micro hydropower.

Specific Speed and Rotational Speed
1. Simple for construction 2. It is locally manufactured and cheap 3. Easy for maintenance

Number of Units Installed
Normally, it is most cost effective to have a minimum no of units at a given installation.

Powerhouse Dimension
The length of the machine hall depends on the number of units and the size of machine. Using the standard layout set by [10] between ⁄ of two units is-L = [(4.5 to 5)× + (2-3m) for minimum clearance] +minimum one unit erection bay L = (4.5× 0.1 + 2.5) + 1=4m Width: The width of machine hall is determined by the size and the clearance from the halls needed as gantry way [9,10].
Width =2Lr +2.5 2 *0.10+2.5=2.7m Height: -The height of the machine hall fixed by the headroom requirement. The hall must have a height that will enable the crane to lift the rotor of the generator or the runner of the turbine clear of the floor without any abstraction. For this project, height lets taken to be 3m

Conclusion
The Gobecho micro hydropower project is low capacity low head plant due to the availability of the low head and low power production. Since the main purpose of the project is fulfilling the light scarcity in the Gobecho kebele in addition to considering the downstream ecology. That is why providing small discharge with low capacity to drive the generator accompanied by turbine rotation. The discharge provided in the hydrological data results in the provision of ogee weir height 0.703m in order to regulate the flow for maximum possible power production. Regarding the conveyance of the structure of masonry trapezoidal canal provided to minimize the cost that consumed due to usage of full concrete canal instead it is able to implement by locally available stones and a penstock of length 70m is suited to convey water under pressure to the turbine after removing from the fore bay. From the result of economic and financial analysis, the total cost estimated to be 50,000 USD with a benefit cost ratio of 1.48 and thus the project is economically feasible. Generally, taking all these points in to consideration the implementation of the project is feasible and crucial in order to satisfy a sustainable electric energy demand for the country.

Statement on Conflicts of Interest
There is no conflict of interest.