Planning for development in the Blue Nile basin10 February 1999
New software tools are making it possible to assess potential hydro projects in Ethiopia quickly and with a high degree of confidence
Ethiopia’s population of 44.2M (1989 estimate) is spread over 1,145,000km2. The country has huge water resources potential, and in planning for development the Ministry of Water Resources has decided to implement Integrated Development Master Plan, with projects organised according to the main river basins.
The Abbay river (Blue Nile) is the most important river basin in Ethiopia, with:
•20% of Ethiopia’s land area.
•50% of the country total average runoff.
•25% of its population.
•40% of its agricultural production.
In 1995, the Ministry of Water Resources entrusted a French Consortium of BCEOM, BRGM and ISL to carry out the Abbay River Basin Integrated Development Master Plan Project. ISL was specifically in charge of large dams for irrigation and hydro power development.
The study had three phases:
•Investigation and data collection.
•Master plan preparation and pre-feasibility studies of selected projects.
The study required that some 200,000km2 be investigated in a limited time, so the identification and preliminary analysis of projects required the mobilisation of the most modern engineering techniques. The potential of more than 20 rivers (including the Abbay river itself) were analysed with several possible alternative development schemes and layouts.
The current situation
Electric power supply in Ethiopia is under the jurisdiction of the Ethiopian Electric Power Corporation (EEPCO), which is responsible for the overall planning, implementation and operation of the country’s electric generation and transmission systems.
The Ministry of Water Resources, being responsible for natural water courses, also participates in the planning and studies of hydro power projects.
At present, around 80% of the electrical energy in Ethiopia is produced by six hydroelectric stations feeding the Interconnected System (ICS). The remainder is produced by numerous diesel generators, feeding local self-contained systems (SCSs). The total hydro installed capacity is 371.50MW.
As regards new capacity, the 194MW Gilgel Gibe scheme and a 73MW extension of the Tis Abbay scheme are under way. There is a commitment to implement two additional stations soon. These are the 203MW Tekeze station and the 102MW Gojeb station.
Due to its topographical and hydrological features, the Abbay basin has without doubt the highest hydro power potential of the country. As mentioned above, the Abbay basin covers 20% of the country area but yields 50% of the total runoff. At the Sudanese border, the mean discharge is 1570m3/sec — an average yearly inflow of 49,400M m3. The total potential capacity is in excess of 10,000MW, while only some 110MW is presently installed in the basin. Besides satisfying the country’s future power need, the Abbay basin also offers an opportunity to meet the needs of neighbouring countries through power export.
This potential has been acknowledged for many years and the first comprehensive study was carried out in the early sixties by the US Bureau of Reclamation. During the Master Plan study, the US study was extended and completed by using additional information (topography, hydrology, geology, environment, etc) and adopting updated techniques. After a preliminary screening of a large number of potential schemes, about 30 were selected for study at reconnaissance or pre-feasibility level. The schemes are shown on the map below, along with schemes previously studied at pre-feasibility or feasibility levels. The total represents a potential of 7000MW.
All the schemes were studied using the software package described below. The main difference between reconnaissance and pre-feasibility levels lies on the available data: 1/50,000 maps and general geological settings were included at reconnaissance level, while more detailed topographical maps and geotechnical investigations were considered at pre-feasibility levels.
The Eva and Peach software have been developed by ISL in the last 10 years to assist water resource and hydro power engineers at the master plan or pre-feasibility stage. The Peach software was initially developed for small hydro power, but the new version can be used for both small and large schemes and was indeed utilised for all the schemes in the study. Both pieces of software aim for a maximum flexibility in use.
Eva is used for preliminary quantity and cost estimates of dams. The data used were:
•Valley topography (taken from any available map).
•Reservoir topography (elevations and surface).
•Dam definition, to be selected among six types of dam with figures introduced by the user considering the site parameters (depth of excavation, slope of dam, depth of grout curtain, etc).
•Unit prices for selected items (fill material, rip-rap, transition, concrete, upstream facing, etc).
The software results are then:
•Topology characteristics of the site.
•Unit value of a stored m3.
•Project definition and cost for selected full supply levels.
This program is particularly useful for preliminary analysis where many sites have to be compared and analysed.
Peach is used for preliminary quantity, cost estimates and economic analysis of hydro power projects. Compared to Eva, this software may include many additional items such as the intake, headrace canal or tunnel, penstock or powerhouse (including equipment selection). The data are basically similar to Eva, with some additional information:
•Topological data for all project components.
•Hydrological data (flow-duration curves).
•Project definition data for all project components (such as maximum water velocity in headrace canal or tunnel, concrete thickness).
•Economic data (unit cost for power alternative, economical and financial data, etc).
For a selected installed discharge, Peach computes the cost of all project components and the energy produced. The economic analysis and financial analysis is the final stage where the hydro power project is compared to another power alternative (for instance thermal) and the project cash-flow is obtained.
In 1997-1998, Peach was improved under the aegis of the European Union’s ALTENER programme. The most recent version includes several new features, such as:
•Organisation as a database.
•Two distinct levels for designing the project components: a standard level and a more advanced level requiring additional engineering input.
•Production of drawings for each project component.
•Automatic report production.
•Six language possibilities.
The Chagni project
The Chagni scheme, studied at pre-feasibility level, illustrates how the software package has been used. The scheme uses many of the components of both Eva and Peach.
The scheme harnesses a 200m high fall. Two alternative layouts were considered, a reservoir and a run of river.
The reservoir scheme comprises the following structures:
•Dam, creating a regulating reservoir.
•Intake and canal, diverting a nearby river towards the main reservoir.
•Low pressure conduit.
The run-of-river scheme comprises a:
•Weir immediately upstream of the fall.
•Low pressure conduit.
Studies are based on 1/10,000 maps for the reservoir and 1/2,000 scale maps for the works. The mean annual rainfall over the catchment basins is 1600mm. The mean discharge at the dam site, including the flow diverted from the adjacent basin, is 9.30m3/sec.
During pre-feasibility studies, geological and geotechnical investigations, including seismic surveys and boreholes, were carried out.The methodology followed during the Master Plan studies is based on the following steps:
•Calculation of output and firm power for a large range of rated discharges and reservoir capacities.
•Costing of the scheme for the same parameters.
•Identification of the optimum size of the scheme.
•Design refinement for the optimum scheme.
The steps for the reservoir option are as follows:
Calculation of output and firm power. The basic data uses the flow duration curve at the site, corresponding to the natural flow without reservoir regulation. The maximum reservoir capacity, which
is restricted by topographic features, is about 40M m3 and represents only 7% of the mean annual inflow. The regulation
is thus seasonal and was taken into account by modifying the natural flow duration curve by evenly releasing the volume stored during the high flow season.
A Peach option automatically modifies the curve for a given reservoir capacity. An option is also available for peak hour operation. On this basis, output energy, installed capacity and firm capacity were computed for the following sets of parameters:
•Reservoir capacity varying from 5M m3 to 40M m3.
•Rated discharge varying from 2.50m3/sec to 20m3/sec. A total of 35 combinations were considered.
Costing the dam. The dam is an earthfill structure comprising an impervious core and random fill shells. The spillway is a gated concrete block located in the river bed. It was costed for various reservoir capacities.
Costing the power components. The power components were sized and costed using Peach software. Among the available structure modules the following were used: intake, sand trap, canal, penstock, forebay and power station. Typical outputs are given for the penstock and power station components.
The penstock is dimensioned on the basis of topographical and head data including transients. As an option, optimisation of diameter is possible, provided the value of energy is given.
Penstock diameter optimisation
The Peach power station module starts with equipment selection. Turbines can be selected among 12 different types. For a given configuration, ie gross head and number of units, the appropriate turbines are proposed, along with recommendations on setting (cavitation), possible speed increase characteristics, generator size, efficiency curves, etc. Once the equipment is selected, the powerhouse dimensions are given and the civil works estimated.
When the scheme is costed for a wide range of parameters, optimisation is possible. The optimum size depends on the selected criteria, which differ according to the project and the owner’s requirements. It can be the cheapest bulk generated kWh, the cheapest dependable kWh, the cheapest firm capacity, etc.
With the experience gained on many projects and lastly on the Abbay river basin, the Eva and Peach software packages have proven effective for surveying the hydro power potential of very large areas. They can also be used to optimise the development of specific river stretches or study a given project at the pre-feasibility level.
Such tools have become indispensable, considering their attractive features:
•The study time can be reduced, so that the dam and hydro power engineers can concentrate on the main project issues.
•Sensitivity analyses (dry year occurrence, extension of construction time, project overcost, etc) are easy to perform, so project selection and scaling can be made with a high level of confidence.
•User-friendly software allows the project owner or client representative to be closely involved in the project analysis.
•Reports and drawings can be easily issued.