Additional services at Kihansi hydro plant16 July 2007
Leonard B Kassana investigates whether a mini-hydro plant could be added to the existing Lower Kihansi hydro power scheme in Tanzania
Tanzania’s hydro plants have been very unlucky as far as hydrological conditions are concerned, with large parts of the country experiencing low hydro storages over the last three to five years. The country's largest hydro power reservoir, Mtera, reached its minimum supply level of +690m asl recently, exhausting live storage that would normally operate Mtera and Kidatu power plants, which together have an installed capacity of 280MW. The loss of these power plants meant that 31% of the country's entire installed capacity was unavailable.
Although this situation was helped slightly thanks to supplies from the Ubungo/Songas and IPTL natural gas and Jet-A fuel fired power plants, there were still severe economical repercussions as the country had to invest in expensive emergency IPP projects. These emergency gas fired power plants included rental based plants totalling approximately 200MW capacity involving Aggreko, Alstrom and Dowans companies. In addition the government contracted Wartsila to install a 100MW natural gas fired power plant that will be fully owned by Tanzania Electric Supply Company Limited (TANESCO). All these plants began coming on-line gradually in 2005 and are expected to be fully available by the end of 2007.
One important difference between electricity produced using hydro and electricity obtained from burning fuel is that hydro energy is lost if not tapped while unburned fuel can be saved and used another day. Therefore it is wise to make use of any hydro potential available from wherever that may be. For example, the mandatory water discharges through the Lower Kihansi dam could be utilised more effectively with the development of a mini-hydro plant.
Thus, this paper looks at the possibility of utilising the bypass flow at Kihansi to generate usable energy without impeding environmental requirements for the mandatory bypass flow. It will further highlight benefits available if this scheme is built.
TANESCO's Lower Kihansi hydro power plant was commissioned in late 1999 on the Kihansi River Falls, a tributary of the Kilombero river. The hydro power plant, located in the Ududzungwa escarpment, utilises a head of 850m with an installed capacity of 180MW. It currently produces about 32% of Tanzania's hydro generation.
The existing system at Kihansi includes a 25m high concrete gravity dam, which impounds a small reservoir with total water storage of 1.6Mm3. The intake connects to the 2.2km inclined 1:7 headrace tunnel of 34-37.5m2 via a 25m2 circular unlined 500m vertical headrace shaft. The power house cavern (12.6m wide, 98m long and 32m high) connects to the 2km tailrace tunnel and 1.9km access tunnel. Total tunnel length is in the order of 10km.
Justification for the proposed plant
Bypass flows can have a serious impact on firm energy production because environmental requirements must first be satisfied before energy production can begin. A 1m3/sec bypass release reduces Kihansi's annual energy capability by 16% while 2m3/sec reduces it by 31%. The corresponding reduction in total system capability is 9% and 12% respectively. At 7m3/sec flow bypass, the plant's firm energy is reduced by 100%. In other words, in a dry year, the plant will only be able to operate during the rainy season.
Since we can't fully utilise the bypass flow to generate power for the entire 850m head without compromising the environmental requirements, we could utilise the bypass flow as shown in the figure above. In this design the bypass flows could be utilised to generate power without causing a detrimental effect on the ecosystem. After generating power the water is released back to its originally intended route to nurture the ecosystem.
From the National Energy Policy document (2003), Tanzania's government encourages energy projects by all stakeholders either through government departments and/or private stakeholders, and is spearheading energy sector reforms in the country.
Currently, electricity coverage in Tanzania is only about 10%, while in rural areas the access is even lower – just over 1%. These low access rates are major constraints to economic development and alleviation of poverty. The government is however committed to improving the situation and has set a goal to raise access to electricity to 25% of the population – estimated at 36M – by 2010.
To achieve such ambitious targets, several strategies have been conceived.
• ?Expansion of the present main grid to cover new areas.
• ?Promote mini grid/isolated grids to provide electricity to areas outside the main grid.
• ?Promote rural electrification through the establishment of a Rural Electrification Agency (REA).
• ?Promote renewable energy sources (solar, wind and small hydro plants).
• ?Promote private sector participation in the power sector.
Such small hydro schemes as the one proposed in this paper are in existence throughout the world. Penche (1994), an editor to the Layman's Guidebook on how to develop a hydro site says 'there is also the possibility of installing a power plant on an existing conventional multiple purposes dam, by either an existing conduit or outlet works or siphon intakes'.
In India, according to Kumar (2007), there are approximately 36 small hydro power stations that operate at the dam toe, with a total installed capacity of 3259.89MW. These stations amount to approximately 6% of total small hydro installed capacity in India to date.
The power and energy benefit calculations are based on the fundamental theory of hydro energy and power formulae as depicted in the given two equations:
E = ?*[9.807*Q*t*H]/[3.6*106] = [? *Q*t*H]/[367.1] in kWh
P = [9.81*Q*H* ?] in kW
Where: E = energy, ? = overall efficiency of turbine-generator assumed to be in the range of 0.72- 0.87, Q = discharge in m3/sec, H = head in m, t = time in hours; P = power in kW
Based on the above formula and the parameters as shown in Table 1, the scheme is expected to generate about TZS 150M (US$150,000) annually as per rough calculations.
Since this is going to be done internally staff will enhance their technical capacity in hydro power skills.
This is towards the energy sufficiency efforts in line with national energy policy
Kyoto protocol compliance on climate change
Development of this scheme will cut CO2 emissions that would have been emitted by the equivalent fossil fuel generator. Furthermore, under Carbon Credits TANESCO could get money from GEF for partly or full funding of the scheme.
Costs for Implementation
The majority of costs are sunk cost as the following infrastructures are in place at the site: 33kV transmission line; dam and reservoir; telecommunication; penstock pipe; air strip; and all weather road. Technical human resources are also available within TANESCO which eliminates the need for foreign experts.
The following infrastructure and equipment would need to be procured for the scheme: a small turbine to be designed accordingly; generator; transformer; control equipment; switch gear panel and protection; penstock material of less than 50m; and other construction requirements.
At this very preliminary stage, precise cost figures can not be obtained. However, experience from similar projects around the world show that such installation costs can range from US$1200 to US$3000 per kW unit. Since we expect to generate 266kW, then preliminary installation cost for planning purposes and thus by assuming US$1200 per kW, the scheme will enjoy a lot more sunk cost than had been incurred during the construction of the Lower Kihansi plant system – costs would amount to US$319,200. Again this is still a conservative figure and we may have a lower figure after the pre-investment study that is currently underway in TANESCO.
For preliminary guidance, Table 2 highlights installed capacity for the Indian small hydro schemes with corresponding installation costs in rupees (x106).
At this very preliminary stage, the crude method that can be used to evaluate the investment is the Payback Period Method. The expected investment cost is US$319,200 and forecasted cash inflow annually is US$125,000 at US$1.00 = TZS1200 rate of exchange. This investment therefore has a payback period of about three years. However, more work is needed for precise financial calculations and refinements to ascertain this figure.
The system will consist of a short length of penstock, power house and a very short tailrace open channel. The selection of the turbine is based on empirical experience. Sayann, K.S (2003) suggests that Tubular turbines with the following parameters are suitable for small hydro schemes:
• Head – 2 to 15m
• Discharge – 1.5 to 40m3/sec
• Capacity –50 to 5000kW
For heads under 100m, the size of the power house and its concrete volume are the functions of the turbine size. However when heads exceed 100m, the size of the power house is governed by the diameter of the generator casing. Empirical equations permitting estimation of the turbine and generator casing sizes are available and can be used for a rough estimate of power house dimensions during the initial conceptual study. These equations are elaborated in Gordon, J.L (1983).
However in this case, the pre-investment study will take care of sizing of the power plant and all other pertinent auxiliary works.
The proposed power plant is envisaged to take about 6-8 man months of implementation from turbine-generator procurement to project commissioning. The pre-investment study underway will ascertain a definite time frame for implementation. However, implementation is likely to occur as follows:
• Turbine-generator unit procurement - Month one to two
• Power house construction - Months two to four
• Penstock and tailrace channel construction - Month four to five
• Switchgear facility - Month four to six
• Transmission Line connection - Month five to six
• Commissioning - Month six to eight
There are numerous dams in Tanzania and Africa as a whole that also mandatorily discharge environmental flows and should be reviewed as possible sites for small hydro schemes. This can add value to existing dam structures while preserving environmental integrity.
Typical arrangement for the proposed scheme Arrangement Existing bypass flow outlet Flow outlet Map of Tanzania showing the hydro project Map View showing the bypass flow Bypass View from the left bank Left Bank Downstream view of the Kihansi project Downstream Table 1 Table 2 TablesTable 1 Table 2