Performance Evaluation of a Savonius Hydro Turbine
Kapil Khoja
2022ME11316
MCL346
Objective
The aim of this experiment is to experimentally evaluate the torque, power developed, and
efficiency of a Savonius hydro turbine by using a rope brake dynamometer setup.
Apparatus Required
The following equipment is necessary for the experimental setup:
• Savonius turbine experimental rig
• Flow measurement device (flow meter)
• Rope brake dynamometer arrangement
• Spring balance (attached at one end of the rope)
• Digital/analog tachometer for RPM measurement
• Set of detachable weights for applying load
• Measuring tape/scale
• Water reservoir and nozzle-based supply system
Theory
The Savonius turbine is a drag-based vertical axis hydro turbine that extracts energy from
moving water. Its performance can be studied using a rope brake dynamometer, which
provides a way to calculate the torque developed on the turbine shaft through controlled
braking resistance.
Principle of Rope Brake Dynamometer:
A rope is wound around the turbine shaft drum. One end is connected to a spring balance,
and the other to a weight hanger. When the shaft rotates, friction between the drum and
rope resists motion, and this resistance can be used to calculate torque.
The governing equations are:
• Torque (T): T = (W - S) gR
• Power Output (Pout): Pout = T × ω
• Power Input (Pin): Pin = 1/2 ρ A V³
Here, W = applied weight (kg), S = spring balance reading (kg), R = drum radius (m), g =
acceleration due to gravity, ρ = fluid density, A = flow cross-sectional area, V = velocity of
water.
Procedure
1. Install the Savonius turbine test rig in the hydraulic laboratory.
2. Wind the rope securely around the brake drum attached to the turbine shaft.
3. Fix one end of the rope to a spring balance and the other to a load hanger.
4. Start the water supply and allow the turbine to attain steady rotation.
5. Record the spring balance reading (S) and progressively add weights (W).
6. Use a tachometer to note the shaft speed (N) at each condition.
7. Measure the discharge rate with the flow meter.
8. Repeat the process for different flow rates and applied loads.
Given Values and Calculations
• Diameter of brake drum (D): 42.18 mm
• Radius (R): 21.09 mm
• Average water velocity (V): 0.35 m/s (based on multiple readings)
• Density of water (ρ): 1000 kg/m³
• Cross-sectional area (A): 0.60 m²
• Power Input: Pin = 0.5 × ρ × A × V³ = 12.86 W
• Power Coefficient: Cp = Pout / Pin
• Tip Speed Ratio (TSR): TSR = ωD / (2V)
Observations
Velocity Spring Weight Torque RPM Power TSR Cp
(m/s) Reading (Kg) (Nm) (W)
(Kg)
0.35 0 0.3 0.0621 144 0.936 0.909 0.073
0.35 0 0.5 0.1034 150 1.625 0.947 0.126
0.35 0 0.6 0.1241 123 1.599 0.776 0.124
0.35 0.02 0.7 0.1407 123 1.809 0.775 0.141
0.35 0.03 0.9 0.1800 108.5 2.045 0.685 0.159
0.35 0.13 1.3 0.2421 110 2.793 0.695 0.217
0.35 0.13 1.8 0.3455 98.4 3.560 0.621 0.277
0.35 0.22 2.0 0.3683 85 3.278 0.536 0.255
Results
• Maximum Torque achieved: 0.368 Nm
• Peak Power Output: 3.56 W
• Overall Efficiency: 27.7 %
• The Cp–TSR relationship is obtained from the experimental dataset.
Viva Questions and Answers
1. What advantages does a Savonius turbine offer in low-head sites?
A Savonius turbine is particularly effective at sites with low water velocity or low head
because it generates high starting torque even at small flow speeds. Its vertical-axis design
allows it to capture flow from any direction without alignment systems, and its construction
is simple, low-cost, and reliable. These features make it ideal for applications in rural or
decentralized hydro setups.
2. How is torque measured by a rope brake dynamometer?
Torque is determined using the frictional resistance created by a rope wound around the
brake drum of the turbine shaft. One end of the rope carries adjustable weights, while the
other is attached to a spring balance. The net difference in forces from the two ends
multiplied by the drum radius gives the braking torque, which is equal to the turbine torque
at equilibrium.
3. Why are both weights and a spring balance used?
Weights apply a known adjustable braking load, while the spring balance measures the
slack-side tension. Without the spring balance, only the applied load would be known but
not the exact resisting force. Together, they provide the net force acting on the rope,
ensuring accurate torque evaluation.
4. What factors govern turbine efficiency?
Efficiency depends on fluid velocity (since power input scales with velocity cubed), turbine
blade geometry, number of rotor buckets, and the tip speed ratio. Mechanical losses like
bearing friction and hydraulic losses from turbulence also reduce performance. For
Savonius turbines, the optimum efficiency is obtained at low tip speed ratios.
5. Differentiate between drag and lift turbines.
Feature Drag Turbines (e.g., Savonius) Lift Turbines (e.g., Kaplan)
Principle Fluid drag pushes blades Aerodynamic lift drives rotation
Blade Speed
Slower than fluid (TSR < 1) Faster than fluid (TSR > 1)
(TSR)
Efficiency Low, typically 15–25% High, often 40–50% or more
Starting Torque High, self-starting Low, usually requires external push
Dominant Force Drag force Lift force
Blade Shape Simple curved buckets Aerofoil profile
Complexity Easy to construct and maintain Complex design and maintenance
Low-speed, small-scale, decentralized Large-scale, high-efficiency power
Applications
uses generation
