Mini Hydroelectric Power
Mawuli Tse
Originally appeared in African Technology Forum
February/March 1992
Vol. 5, №1
The world is always looking for cheap energy. In Africa, the type of power source promoted by development projects has been dictated by external factors with insufficient consideration of the specific needs and resources available locally. There may be several reasons for this. With much of the research and development energy systems taking place outside Africa, developing systems for the particular demographics and energy needs of the continent has not been the concern of the researchers. It is now accepted that large scale energy projects are notoriously difficult to manage and often lead to large scale environmental problems (see following article on hydro power). This awareness has opened up new possibilities for small scale power generation which had previously been ignored.
Small scale hydro is defined as a hydroelectric power plant with an output put of less than 5 MW. This article presents the main issues involved in executing a small scale hydroelectric project. It is intended to introduce organizations and agencies examining energy options to small hydro technology and economic feasibility. Critical factors such as hydrology and geology that vary widely dependent on local conditions cannot be examined in this treatment.
Endowed with a high proportion of the world’s hydroelectric potential, Africa can adopt small scale generating plants which would be best suited to the needs of disparate communities. Many rural communities are served by their national grids because of the high cost of transmission. Small scale hydro, whether initiated by government, private organizations, or community groups, offers many benefits:
• Greater control by users over the management of electricity supply
• Flexibility in meeting specific energy demand profiles
• Lower capital costs
• Use of local construction materials
• Savings from reduced fuel purchases
• Opportunities for skill acquisition and employment
• Irrigation and water supply
• Environmental damage control
Needs Assessment
Any energy project must consider the necessary technological, economic, and environmental factors during the planning stage before major funds have been committed to any one option. Some questions should be addressed here. How expensive would it be to utilize existing power sources such as the national electricity grid? Which activities in the community are to be supported by the generation of new electricity, and what is the distribution of load intensity over the course of a day? Which parties stand to gain from the new power source? Which parties stand to lose? What are the foreseeable social and environmental impacts of the project? Answering these and other questions early on in the project will help avoid costly mistakes.
Next, available resources should be assessed. The cost of fossil fuel, intensity of sunlight (insolation), proximity to suitable water sources, and local climatic patterns, are some of the parameters which help determine the optimum type of energy source to be harnessed.
Hydro
If hydroelectric power is a feasible option for the community, a few simple calculations can be performed to determine the potential electrical output of the planned system.
Available Power
The ‘potential energy,’ or head of a water source is measured by the distance through which the water flow will fall to drive the turbines. The head is usually determined by the size of the dam to be constructed.
The existing flow rate of the water can be measured using a flow meter such as a weir. Weirs are instruments which provide a constant cross-section channel for the flow of water. An energy balance through the channel yields the flow rate in terms of the channel dimensions. The available power from a given watercourse is calculated from the equation below:
P = g*ρ*Q*H
where P is the power available (watts), g is the acceleration due to gravity (9.81 m/s2), ρ is the water density (kg/m3), Q is the volume flow rate (m3/s), and H is the head (m).
The efficiency of a plant is defined by the fraction of its energy input which is converted to electricity. An ideal plant has an efficiency of 100%. The efficiency of hydroelectric plants usually lies between 50% and 90% depending on geological and climatic factors, and the design of the plant. For a plant with an efficiency of η, the electrical energy produced in watt-hours, W, is given by
W = P*t*η*f
The operating time is t (hours), and a coefficient, f, is used to account for seasonal variations in the stream.
Components of a Mini-hydro System
The essential elements of a small scale hydroelectric power plant are:
Dam: This is a solid barrier erected in the waterway to create a reservoir of water for driving the plant.
Turbine: A rotating wheel with paddles or vanes driven by the flow of water under gravity.
Intake system: A channel along the side of the dam for the flow of water into the turbine space. A grill is usually fitted to prevent aquatic life and debris from entering the turbine.
Head race: Gravity pipes leading down from the intake system to the turbine house.
Regulator: A mechanism which controls the speed of the turbine and ensures a steady output of electricity.
Generator: The actual electricity producing unit. It is coupled to the turbine.
Transport and distribution system: Over distances of 1 km or less (local consumption), electricity is transmitted from the plant at the same voltage as it is generated; usually 230 or 380 volts. For transmission over longer distances, it is more efficient to step the voltage up to 20 kV and step it back down near the destination. Transmission power losses are proportional to the line voltage, and to the square of the current. By transmitting at high voltage and low current, minimizing power lost as heat through the lines.
The turbine, generator sets, and distribution systems are the cost centers of the generating plant. They are also the components most likely to be imported. A classification of dam types by head, and the recommended turbine manufacturers is given in the box below.
Economic Factors
Most materials for the construction of a mini hydro plant can be obtained locally. Companies with civil engineering expertise can assess the terrain and undertake the necessary excavation and concrete work for the dam and intake systems. Some of the major manufacturers of turbine machinery have global affiliations and can assist in obtaining estimates for the particular plant under construction.
Cost estimates for hydroelectric plants vary by location, but range from $3,000/kW to $10,000/kW in most African, Asian and South American countries.
Civil engineering costs in mini hydro plants can account for up to 80% of the total project costs
Maximizing the use of local material in the construction, will reduce civil engineering costs.
Environmental Factors
Care should be taken during the construction and operation of hydroelectric plants not to disturb the natural habitats of fish and other life forms. Possible measures to protect fish include the provision of spillways along the dam that allow the fish to traverse the waterways at all times preserving breeding patterns and migratory trends. The natural habits of most animals are linked within the ecosystem, making it even more important to consider the broad impacts of construction on the environment.
Concentrating on a few visible species can shift the ecological balance, lead to the depletion of or an over-abundance other species. All plants and animals in the local environment should be identified, and an assessment made on how the dam will effect their existence.
Erosion is another major problem resulting from improperly planned dams. It is important to conduct an extensive survey of the flow patterns of the river or stream over a considerable time (at least 12 months). Structures must be incorporated to control any secondary currents, flow patterns, confluences which could erode the surroundings. Attention should also be paid to the possible effects of soil nutrient depletion. In some area downstream farming communities depend on annual flooding of rivers to replenish the nutrients in their soils. Building a dam will smooth the flooding patterns and reduce the amount of silting, and hence re-fertilization, that normally occurs.
The construction of dams may indirectly affect the health of the local populations. The incidences of many water borne diseases such as river blindness, malaria, and bilharzia have been linked to the construction of dams in the past. These diseases are a product of all hydroelectric dams, but the smaller bodies of water created by mini-hydro plants could reduce their range. Where possible measures should be taken during new construction to prevent the vectors carrying these disease from breeding and propagating.
It is still too early to tell whether small scale hydroelectric power will prove more viable in Africa than the larger variety. Placing ten small dams on a single river may be just as harmful as placing one large dam ten times the size. But the experience of other parts of the world, including China and Latin America, suggests that mini hydro power deserves a second look in Africa as well.
