MODELLING OF NALGAD DAM AND RESERVOIR OPERATION FOR HYDROPOWER GENERATION IN NEPAL USING HEC-RESSIM

Abstract

Theoretical power generation potential of Nepal was estimated as 83,500 MW in 1966, out of which 42,000 MW is technically and economically feasible to be produced. However, so far the country has managed to generate only 914.6 MW, which is about 2% of economically feasible power generation potential, and 2% of the total energy consumption in the country. Despite harbouring a huge hydropower potential, Nepal has not been able to meet its own domestic demand for electricity. Recently, Nepal is focusing highly on development of hydropower to fulfil its ever increasing demand of energy and economic development of the country. This Study aims at to build a simulation model of the proposed Nalgad dam reservoir operation for hydropower generation in Nepal using HEC-ResSim. The historical daily discharge data for the period 1966 to 2016 have been used for analysis with consideration to seasonal (dry and wet season) and hourly energy demand. Six months from December to May are categorized as dry period and remaining six months June to November are categorized as wet period. Five operation scenarios with various turbine units and generation patterns are analysed. Four turbine units in operation for ten-hour in dry season and one-hour in wet season, maintaining reservoir level between 1498 masl and 1580 masl found to be optimum. The dry season energy generation is 735.04GWh/year with 90.19% reliability and total annual energy generation is 1247.72 GWh/year. In addition to simulation, the monthly reservoir operation policy is also developed solving the formulated optimization problem using LINGO model for 90% dependable year. It is observed that the energy generation, reservoir storage and reservoir elevation, spill etc. obtained from both the methods are almost similar. However, the simulation model is considered to be more effective tool for analyzing reservoir operation simulation as it reflects real system behavior. Sensitivity of change in inflow, environmental release and full supply level (FSL) on of energy generation are analysed. It is observed that the effect of change in inflow on dry season energy generation is very less in comparision to wet season energy generation. The reduction in dry season energy is about 6% for a 20% decrease in inflow. The average annual dry season energy generation decreases in the range of 22-29 GWh/year with total annual opportunity cost of Nrs. 338-396 Million per year by increasing environmental release by 1 m3/s. The energy generation and gross return from energy linearly increases from FSL in the range of 1550 to 1580 masl and after this level iii there is negligible increment in energy generation. Hence the selected FSL at breakeven point i.e. 1580 masl is justified. The empirical area reduction method was used to develop Elevation-Area-Capacity curve after 25, 50, 75 and 100 years from reservoir operation date due to sedimentation. It is estimated that total storage volume is reduced by 14.23 MCM, 28.36 MCM, 42.46 MCM and 56.58 MCM after 25, 50, 75 and 100 years respectively. There is very less or negligible decrease in annual average total and dry season energy generation due to sedimentation even after 100 years as only about 2% loss in live storage capacity is observed. Each 1 Mm3 loss of live storage will reduce dry season energy by about 0.57 - 0.9 GWh/year after 25 years to 100 years. Moreover, due to sedimentation zero deficit years decreases from 22 years in present conditions to 14 years due to sedimentation after 100 years. Moreover, the 5-10% energy deficit years are almost doubled after sedimentation of 50 years. The future drinking water demand from the reservoir to downstream municipality such as Nalgad, Athbiskot, Bheri and Chaurjhari is estimated to be about 2.5 m3/s. To meet this demand and maintaining reliability of energy generation, the hydro power generation hour need to be reduced to nine-hour in dry season. In such situation there will be reduction of about 73.138 GWh/year in dry season energy.