After an initial stage in which the pre-feasibility of the plant was positively assessed, Frosio Next was commissioned to draw up the feasibility project, based on an in-depth hydrological studyconcerning the exploitable flow rates, and on a detailed topographic survey aimed at defining the available heads.
Subsequently, 4 possible plant schemes were identified and the different alternatives were subjected to a technical-economic analysis to identify the most attractive one, also taking into account potential hydrogeological risks.
The selected scheme foresees the installation of a Banki turbine able to produce about 2,000,000 kWh/year. The energy is partly consumed by the surrounding villages and by the carpentry laboratory of the nearby E&E Professional School of Njombe, and partly sold to the national grid through the 11 kV line to which the school is already connected.
The plant layout includes a diversion weir made up of reinforced concrete, 6,80 m high (in highest section), which allows the maximum plant flow rate of 3.0 m3/s to enter a long supply canal (220 m length). Before entering the supply canal, the diverted water will be conveyed into a little desilting basin.
The supply canal ends with a forebay, where the penstock starts. The penstock is about 46 m long and it is characterised by a nominal diameter of 1,200 mm.
An interesting chance to save money is to build a wooden penstock instead of a steel one, taking advantage of the nearby presence of the E&E Professional School, equipped with wood working machines. Usually there are many leakages when the pipeline is filled for the first time, but in a few days the wood swells and the pipes become completely watertight. The hydraulic efficiency of this kind of penstock is very high, thanks to the smooth surface of wood. Other advantages of the wooden penstock is its flexibility to conform to ground settlement, ease of laying on the ground with almost no grade preparation, no require-ment for expansion joints, no necessity for concrete supports or corrosion protection.
Last but not least, this experience paves the way for knowledge transfer for further developments.