Focus on Angola15 December 2005
António Pereira da Silva, Carlos Lopes Gonçalves, and José Cruz Morais discuss the hydro power potential in Angola, and investigate recent developments in the country
Angola covers an area of 1,250,000km2, and has an estimated population of 13.1M. The population is young (50% less than 15 years old) and the growth potential is huge (current growth rate 2.9%/year).
Angola is rich in natural resources (oil, diamonds, water, forests, farmland, fisheries), but a long history of civil war has left its economy in a chaotic state, and has significantly affected the social fabric.
Since the end of the civil war in 2002, the Angolan people have been trying to rebuild their country, which has a large potential for development. The reconstruction of Angola implies the rebuilding of its infrastructure (roads, water, power production and distribution, etc.), which could be financed by the country’s oil wealth, which accounts for more than half of its current GDP.
Geographically, Angola comprises a narrow coastal strip, less than 100km wide, with an average 400m altitude, and a large inland plateau, with an average 1500m altitude. The drop between the plateau and the coastal strip is abrupt, forming the so called ‘serra’.
Rainfall is scarce in the coastal strip (150mm/year) but increases markedly in the plateau, varying from 900mm/year in the south to 1600mm in the northern provinces and central highlands.
Rivers in Angola originate in the central plateau and radiate in all directions: the northern areas drain to the Zaire river, the eastern areas area drained by the Zambezi basin, and the Cuando and Cubango rivers flow south, into Namibia. Finally, a series of rivers, of which the Cuanza and the Cunene are the most important, drain west, to the Atlantic.
Given the generous rainfall and huge area, Angola possesses ample water resources, the third largest in Africa. Average water flow in Angola is estimated to be about 180km3/year, all of which is generated in the country: Angola receives hardly any water from neighbouring countries.
Rivers on the west slope rise and develop in the central plateau and then fall abruptly to the coastal plain (a total drop of about 1000m). So, although they are not the largest, they have a very significant power potential. The theoretical hydro power potential of Angola is estimated at 150TWh/year, of which 65TWh/year is economically exploitable (corresponding to 50,000MW of installed capacity).
Hydroelectric facilities generate more than two-thirds of Angola’s electricity. Current hydro power generating capacity is about 500MW, of which the largest part is concentrated in two schemes on the Kwanza river basin (Capanda and Cambambe). The main characteristics of existing hydro power plants are indicated in the table below.
The long civil war has very much affected existing electric infrastructure, and has impeded the development of new projects. Capanda dam, for instance, began construction in 1986, but the first unit only come online in 2005.
Despite the large hydro power potential of the country, only 15% of Angola’s population has access to electric power, and blackouts occur frequently for those who do have access to electricity.
Angola’s electricity is supplied through three separate systems. The Northern System supplies the provinces of Luanda, Bengo, Kuanza-Norte, Malange and Kuanza-Sul, mainly with hydro power generated on the Cuanza river. The Central System supplies the provinces of Benguela, Huambo and parts of Bie using the Catumbela river. Finally, the Southern System supplies Huíla and Namibe using the Matala power station in the Cunene river.
A significant rebuilding effort has taken place in the last few years, after peace returned to the country. Capanda power plant has been completed and other hydro power plants have been rehabilitated. The first design of Capanda hydroelectric scheme and the heightening of Cambambe dam and its second power plant were undertaken by COBA in the 80’s, alone or in association, in the case of the Cambambe studies.
The rehabilitation of Gove dam, on the Cunene river, and the installation of a new 60MW power plant is underway. Further hydro plant developments on the Cunene basin are planned, namely the Jamba Ia Mina and Jamba Ia Oma projects. These Cunene hydroelectric schemes were studied by COBA – Gove in 2004/05, and Jambas in the 1980’s.
Construction of a new dam near Epupa Falls on the international stretch of the Cunene river has also been proposed jointly with Namibia, but is currently on hold due to environmental problems.
A number of high capacity power schemes are planned for the larger Angolan rivers, namely the Cuanza, Queve, Catumbela, Cunene and Cubango rivers.
The electrical grids’ capacity and extension are limited and requires significant investments. The government plans to create a national grid, linking the three regional electricity sectors, and establishing linkages with neighbouring countries. The additional generating capacity from new projects, together with the national grid system, could enable Angola to become an exporter of electricity to neighbouring countries.
Although large power schemes are possible and are being envisaged, the huge dimensions of the country make it difficult to supply all the areas. Until an interconnected national grid can cover most of the country, the supply of smaller cities has been done using small, diesel-powered, thermal plants. Maintenance and fuel supply of these plants is a significant problem, given the bad condition of the road infrastructure.
It is no surprise then that a large number of small to medium sized hydro power schemes have been built or are under consideration for local supply, using local hydro resources. Many of these schemes are located in the northeast part of the country, and are intended to supply the mining operations located in that area.
In 2004 and 2005, COBA concluded the design and tender documents for the Chiumbe-Dala and M’bridge schemes. The former will supply the city of Luena, capital of Mochico’s Province, while the latter will supply M’Banza Congo, capital of the Province of Zaire.
These two schemes, together with the recently concluded Gove power station, are briefly presented in the next section. The studies were undertaken with the collaboration of Angolan consultancy firms PROGEST (Gove) and SOAPPRO (Chiumbe-Dala and M’bridge)
M’Bridge hydro power development
The M’Bridge hydro development will supply M’Banza Congo, the capital of the Zaire Province in the north of Angola. The hydro power scheme will be located on the M’Bridge river, 60km from M’Banza Congo, exploiting the resources provided by the high water falls.
The scheme will use the water resources of two rivers: the M’Bridge (78km2) and the Passassa (58km2), located just upstream of the natural waterfalls, with a total height over 300m.
The catchment areas have an average altitude of about 1050m and abundant rainfall (1440mm/year). Total average flow is estimated at 33.8hm3/year (1.1m3/sec) for the two river sections, with an important minimum flow during the dry season. The hydroelectric potential will be exploited by a run-of-river development. Storage capacity is provided only to allow for daily or weekly variations of flow.
The scheme includes two diversion dams, a hydraulic conveyance system comprising intakes, sediment traps, low pressure pipelines, head tank and penstock, the power house, adjacent switchyard and access roads. Both the M’Bridge and Passassa diversion dams are concrete structures, including a central uncontrolled spillway with 36 and 24m total crest length and 5m in height above the riverbed. Bottom outlet works and intakes are located at the abutments. Total storage capacity is about 1Mm3.
The hydraulic conveyance works have a total length of 4.9km, and include:
• Intakes at both dams, designed for a maximum unit diversion flow of 1.5m3/sec.
• Sediment traps, 4m wide and 20m long, located immediately downstream of each dam, to retain sand and protect the low pressure pipelines.
• Low pressure pipelines connecting the diversion dams with the head tank, HDPE pipes with 1.2m diameter and 3.5km total length.
• Head tank, 60m long and 14m wide, for sediment retention and flow regulation, with total a storage capacity of 1400m3.
• A 1200m long steel penstock, 1m diameter.
The installed capacity of the power plant will be 6.825MW, for a total flow of 2.4m3/sec and a rated net head of 324m. The power house will contain three horizontal twin-injector Pelton turbines, with 2.275MW unit capacity. The 6.0kV generators are rated at 2500kVA each. The building has 33.6 x 18.2m2 in plan, and a height of 9.5m above access platform level.
The 60kV switchyard will be located next to the power house, and will include an 8.0MVA 6/60kV. The 60kV transmission line from the switchyard to M’Banza Congo will have a total length of 60km. A second (6kV) transmission line will supply a local village.
The project also includes about 5.1km of access roads to the dams and to the power house.
Energy production potential will be about 23.8GWh/year. For an IRR of 6% the cost of produced energy will be about US$0.096/kWh. Installed capacity cost is evaluated at US$4,755/kW.
This development will replace and complement Diesel units that are currently used for electricity production, and are unreliable and have high operation costs. At the start of operation the new scheme will operate as the main source of an isolated network, consisting of several thermal units and a smaller hydro power station. It is envisaged that later it will be connected to the national grid, namely to a transmission line originating at the Inga power plant.
The Chiumbe-Dala hydro power development is located near Dala, in the Lunda Sul Province, and will supply Luena, capital city of the Moxico Province. Luena is currently supplied, very unreliably, by Diesel units, which will be replaced, with capacity to spare, with the new scheme. The hydro power station is located at Chiumbe river, at the downstream end of a rapids section. The construction of this development was started in 1981, but was interrupted by war, leaving only the river weir and power canal completed. Turbines where also acquired but were destroyed or lost.
The Chiumbe scheme dominates a catchment with an area of 2100km2. Rainfall and flow data are available at the site. The basin has an average altitude above 1250m and the average annual precipitation is 1216mm. Average annual flow is estimated at 257.7hm3 (17.5m3/sec), with a minimum flow about 8m3/sec even during the dry season. The total available head is 60m. Given the regularity of the flow, the hydroelectric potential of the site will be exploited by a run-of-river development. Storage capacity is provided only to allow for daily or weekly variations of flow.
The scheme includes a diversion dam, a hydraulic conveyance system comprising the intake, head tank/power canal and penstock, the power house, adjacent switchyard and access roads.
The diversion dam is a low concrete weir with 143.50m total crest length and is 1.65m high above the riverbed. Bottom outlet works are located in the right abutment and the power intake in the left abutment. The conveyance works, with a nominal capacity of 24.0m3/sec total length of 342m, include an intake with a total width of 44.8m, divided in nine sections, a canal / head tank, with a 18.0m bottom width, 25.0m top width and about 109m length, and two steel penstocks, with 213m total length, and 2.2m diameter.
The power house will contain four horizontal Francis turbines: two larger units with a unit capacity of 4.14MW and two smaller units with 2.07MW unit capacity. The 6.0kV generators are rated at 5000kVA for the larger units and 2500kVA for the others.
The power house building has maximum dimensions of 53.4 x 18.4m2, and an height of 7.5m above the access platform.
The 60kV switchyard is located next to the power house, in the opposite river bank, and includes a 10.0MVA 6/60kV transformer. The 60kV transmission line from the switchyard to Luena will have a total length of 90km. The designed works also include 1.4km of access road to the power house and a bridge over the Chiumbe river.
Annual production capacity is estimated at an average of 55,1GWh in the first phase and 66.3GWh in the second phase. Assuming an IRR of 8% the actual cost of the produced energy is estimated to be about US$0.047/kWh. Installed capacity cost is evaluated at US$1468/kW.
Gove hydro power station
The Gove hydro power station will be located at the toe of the existing Gove dam, located in the upper Cunene river, about 70km south of the city of Huambo.
The Gove dam was built in the mid 1970’s for regulation of the Cunene river flow, to increase firm energy production at existing further downstream run-of-river plants (Matala, Calueque and Ruacaná), and to guarantee water for irrigation. The dam and intake works were sabotaged with explosives during the 1990’s and are currently being repaired. The dam rehabilitation project was carried out by COBA in 2003-2005.
Gove dam dominates a catchment area of 4610km2. Average flow is estimated at 21,600hm3/year (49.8m3/sec).
The existing dam is an embankment dam with a total crest length of 1112m and 58m maximum height. The total reservoir capacity is 2547hm3, with a usable capacity of 2364hm3, capable of interannual regularisation of flow.
The hydraulic structures were built taking into account the needs of the hydro power station, to be built at a second stage. The intake, located in the left abutment, includes an intake tower, a 6m diameter tunnel and two 2.5m hollow jet valves. The intake tunnel has a derivation just upstream from the jet valves, prepared for the future extension to the power house.
The total capacity to be installed at Gove will be 60MW, for a total flow of 180m3/sec and a rated net head of 45m.
The power house will contain three identical vertical Francis turbines, with a unit capacity of 20MW. The 11kV generators are rated at 24,000kVA. The power house building has 53.4 x 18.4m2 in plan, and a maximum height of 11.2m.
The 220kV switchyard will be located next to the power house, and will include three 24.0MVA 11/220kV transformers.
Energy production capacity is estimated to an average of 149GWh/year at the start of operation, decreasing to 135GWh/year in the long term, when operation will be restricted by other uses. Considering an IRR of 8%, the cost of produced energy is US$0.020/kWh and the cost of installed power is evaluated at US$570/kW.
The station will be connected to the developing national grid, with connections to the Huambo-Lobito-Bengela axis (Lomaum and Biópio hydro power stations) and to the south along Lubango-Namibe axis (Matala hydro power station).
The authors are: António Pereira da Silva, Director Natural Resources and Equipment, Carlos Lopes Gonçalves, Director External Activities and José Cruz Morais, Director Studies and Development, COBATablesTable 1 Table 2