Green hydropower design and evaluation case for Nepal

Abstract

Existing green sustainable hydropower concepts have mainly focused on environmental and management issues while the technical design improvement for the power optimization at maximum downstream release is booming and require further study. Despite the concepts of green hydropower have been proposed by the last century, but there are differences between countries and there is no specific method on how to improve the fish migration throughout the year in different fluctuating water level in Natural River. Consequently, the traditional design is only about releasing the sediments during high flow period. However, they do not have proper method to release the sediment continuously during dry period. So, a run-of-river small hydropower (diversion type hydropower station) project of Nepal was considered as a case study, and green hydropower design and evaluation was presented. Apparently, there seems to be enormous problem existed in small hydropower run of river (ROR) especially during dry season, in downstream-depleted reach between the diversion weir to power station, causing eco-environment and other socio-economical issues. Hence, there is a need to develop a proper sustainable design and method to solve these issues and improve the future small hydropower development in an effective way. Therefore, the objective of this research is to assure continuous release of downstream water and sediment with optimal power generation based on the green hydropower design concept. Furthermore, this research has also developed a fishway design for fish migration without being influenced by the year fluctuating river flow. Taking ROR small hydropower project of Nepal as a case, conceptual green hydropower design has been proposed by analyzing average monthly river hydrology, providing technical support for future green hydropower design concepts. This research was divided into following five sections to meet the research objectives. 1. In this research work, an experiment on effects of acute temperature difference in fish behavior was carried out, as it is very important parameter for the fishway designing. Consequently, it was obtained the new result on acute temperature change effect on fish critical swimming speed (Ucrit), oxygen consumption rate (MO2), tail beat frequency (TBF) and tail beat amplitude (TBA) at 5oC(acute low temperature), 15oC (normal temeperature) and 25oC (acute high temperature). The Ucrit value decreased significantly when acute temperature changes at 5°C and 25°C; Ucrit was highest near the optimal temperature. Oxygen consumption rate (MO2) increased with the swimming speed at 15°C; however, at 25°C and 5°C, the MO2 decreased with the swimming speed. Both TBA and TBF decreased at 5°C and 25°C compared with values at 15°C. The slopes of the regression lines (ΔTBF/ΔU) at 5°C and 25°C seemed lower compared with 15°C. 2. The fishway design was developed by considering fluctuating water level in Natural River during wet and dry periods. According to the Natural River flow direction, the positions of the fishways are selected to attract more fish naturally. Design was based on computer model and ANSYS Fluent 14.5 software was used to simulate the flow pattern and the velocity inside the fishways. Discharge, flow velocity, attraction velocity, water depth, volumetric power dissipations which are the key parameters in fishway designing were calculated following standards and formulas given in different literatures. The results of the fishway designing indicate that, fishways can work efficiently even during flood period when the overflow height rose to 0.5m above from the normal flow depth of 2.5m in upstream river at fish exit (weir crest) position. Consequently, when the river flow depth dropped from 2.5m to 0.5m at the bottom surface of upstream weir during dry period, the new design can work smoothly to migrate the fish continuously for the spawning at different periods and other related lifecycle activities. Environmental flow release through open channel situated between the sluice fishway and weir fishway was utilized to create additional attraction velocity during dry period when the river has a minimum flow. Open channel designed between two fishways can be used as a fish passage for the downstream migration throughout the year including the regular sediment release to the downstream river. 3. This research has further developed the additional evaluation standard as per the new research findings for the future sustainable green hydropower development. Accordingly, the standards were divided into three parts such as technical, environmental and socio-economical. The maximum value was given to the technical standard as environmental and socio-economical factors are depended on the technical values. 4. Hydraulic calculation for the hydropower project was carried out to find the optimize power generation at maximum downstream release. Current Nepalese hydropower policy requires only 10% release of the average monthly flow to maintain downstream environmental flow, which is not sufficient during the dry period of the monsoon cycle. Accordingly, the environmental flow specified in the proposed design is 1.151m3/sec, but current policy would allow a release rate of only 0.23m3/sec during the driest month of the year. The proposed design can minimize the negative downstream impacts with minimum release of 30% of the average monthly flow throughout the year with 63.9% during the driest month creating very minimum effect on regular power generation. Furthermore, the continuous environmental release to the downstream river will help to increase the river depth from existing 30cm to about 60cm even during the driest month, which in turn will help to maintain the normal temperature and Dissolved Oxygen (DO) to required level. Moreover, this new design has a new option to manage the extra water during flood season using two designed spillway, one at upstream of weir and the other after the intake section, which will help to maintain the normal velocity and discharge inside the fishway. 5. An economic analysis was conducted to evaluate the new developed green hydropower design concept comparing with the existing design from aspects of socio-economy, ecology and environment issues. The proposed new design has shown the good results compared with the existing design. Economic analysis result shows that new developed design has Benefit Cost ratio (B/C ratio) of 2.11, Internal Rate of Return (IRR) of 20.67% and a Net Present Value (NPV) of US $16,109,000 compared with the existing design having (B/C ratio)=2.08, (IRR)=20.21%, NPV=US $16,081,000.