OPTIMAL DESIGN OF HYDROPOWER TUNNELS FOR COST EFFICIENCY IN ALBANIA

Albania, with abundant water resources but currently exploiting only about 30% of its potential, plans the construction of 43 hydroelectric power plants (HPPs) with dam heights of 20-160,m, capacities of 2-170,MW, and design flows of 2-200,m3/s. These projects will involve approximately 100,km of tunnels with diameters of 5-6m. Determining the optimal tunnel cross-section is critical to minimize total costs, which include both hydraulic friction losses and tunnel construction expenses. Hydraulic losses are influenced by flow, tunnel length, and friction factors, while construction costs depend on excavation, lining,and reinforcement requirements. The economic cross-section is achieved when the marginal cost of friction losses equals the marginal cost of tunnel construction. Sensitivity analysis shows that tunnel design is most affected by internal pressure, rock characteristics, energy price, and economic parameters such as bank interest and inflation. The study estimates that the economic tunnel diameter for Albanias HPPs ranges between 1.6 and 12.0,m, with corresponding water velocities of 1.0-1.8,m/s. The methodology presented offers a general and practical approach for the coste ffective design of hydropower tunnels, applicable to various types of tunnels and pipelines.