Sustainability of small-scale hydropower with case evidence from Rwanda

Abstract

Climate change is increasingly affecting run-of-river power plants especially rivers in tropical regions with high variable flows and this raises their sustainability concerns. This study examines the sustainability factors of small hydropower plants using Rwanda as case study. It addresses the following specific research questions. First, the challenges and opportunities around small-scale hydropower plants. Secondary, which factors affecting sustainability of small-scale hydropower plants. Third, how to design the best size of a powerplant fed by run-of-river systems in tropical highlands. The study employs a nested methodological framework starting with descriptive analysis about opportunities and challenges around small-scale hydropower plants in Rwanda. A field survey was conducted to examine sustainability factors affecting small hydropower on 24 powerplants in Rwanda using structured questionnaire and field visit observations. A case study was applied at Sebeya river, and machine learning was used to predict the river flow, whereby energy generation of different turbines and levelized cost of energy at different sizes were estimated using a stochastic modeling. Lastly, experimental analysis using simulation for different sizes of a hydropower plants for different flow conditions was developed as a reference for stainability. The study finds indicated that the major positive factors for sustainability of small hydropower power plants in Rwanda are; selling electricity to the national grid, adequate yearly rainfall and appropriate topography for run-of-river hydropower plants structure. The major negative factors affecting the sustainability of small hydropower plants are; (a) reduction in river discharge during the dry season due to decrease in rainfall intensity, (b) high volume of sediments in rainy season resulting from soil erosion which increases maintenance requirements and shortens turbine lifetime, (c) unplanned outages caused by grid problems. Furthermore, the study finds that plants designed substantially above the ‘knee’ in the flow exceedance curve shown (Figure 16) produced a significantly higher LCOE. Investors are advised to design Run-of River plants at, or just below, the ‘knee’ in the flow exceedance curve in similar conditions with Sebeya river. Additionally, some turbines such as propeller performs poorly in Sebeya flow conditions and similar climatic conditions. The findings and methods used can guide future investments in small hydropower plants designed on rivers in tropical conditions