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Access to electricity in Indonesia is limited due to the remote area of the community. In the modern era, electricity is a primary necessity and indicator of welfare. To overcome this problem, using renewable energy resources in the remote areas of Indonesia is an appropriate alternative. Based on this condition, the pico hydro-type undershot waterwheel is suitable to be used in Indonesia, because undershot waterwheels are appropriate for low head conditions or low river flow. This study will examine the effect of different blade numbers on the performance of undershot waterwheels for straight blades. There are four blade number variations: 8, 12, 16, and 20. The computational fluid dynamic method was used because it can visualise the flow pattern with more detail than other methods. Activating six degrees of freedom is needed to predict the rotational speed of the turbine’s interaction with the water and blade. Based on the results, 20 blades produced higher efficiency than others. However, the 8 blades had a stable performance and produced higher rotational speed than the other. The empirical equation for determining the number of blades of undershot waterwheels adopted by the Pelton turbine is correct.

This study will analyze the impact of gap size in two different runners called runner A (five blades) and B (six blades) to provides recommendations in design and manufacture of open flume turbine runners so that maximize the conversion of kinetic and potential energy. There are three methods was used to investigate its: analytical method is used to design the turbine; experimental to determine the actual turbine performance; computational fluid dynamics (CFD) to study the physical phenomena and re-check the velocity triangle on the runner to validate the design and manufacturing process. Using the results obtained, gaps between the blades can alter the velocity vector on the outlet and unbalance the rotation of runner; this imbalance could cause cavitation. Then, the decreasing torque is assumed because water pressure in the draft tube is similar to atmospheric pressure. Two conditions must be satisfied to maximize the performance of the turbine: swirling flow is required after the water flows past the runner in order to minimize the radial velocity on the outlet so that the draft tube can function properly; the dimensions of the blade must be carefully selected to avoid the formation of gaps between the blades.