What it shows
Hill villages in Uttarakhand, Himachal and the North East run small hydro plants on streams. A bucket wheel turned by a jet of water drives a generator. The project builds a Pelton-style wheel from spoons or cups, drives a small motor as a generator and measures how output power changes with head and flow, and finds efficiency.
The science
Water at height h has potential energy that becomes the kinetic energy of the jet, with speed v = sqrt(2gh) for an ideal nozzle. The cups reverse the jet so that nearly all of its momentum goes to the wheel. Hydraulic power is ρ g Q h and the electrical output is V times I. Efficiency is output over hydraulic power. Maximum power is obtained when the wheel rim speed is about half the jet speed.
Materials
Cost: under Rs 1,500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A wheel made from a plastic or wooden disc with 12 steel or plastic spoons or cups
- A 12 V DC motor to act as a generator, a pulley or direct coupling
- A water tank or a 20 litre drum raised on a stand, a nozzle of 4 to 6 mm from a pipe cap with a drilled hole
- Pipe, valve and a bucket with a stopwatch to measure flow
- A rheostat or resistor load and two multimeters
- A pump-free recirculating tray or a drain outdoors
Safety
For this project: Keep water away from electrical connections, and use a low-voltage motor only. An adult must handle drilling and the raised tank to stop it from toppling. Keep the floor dry to prevent slipping.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
11 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Calculate the jet speed and flow for a 5 mm nozzle at heads of 0.5, 1.0 and 1.5 m using the formula.
Build the wheel with 12 cups on a disc of 20 cm diameter and balance it on a shaft with bearings.
Fix the wheel to the motor shaft or couple it with a belt, and mount the wheel so that the jet strikes the cups tangentially.
Set up the tank at 1 m head with a valve and nozzle, and measure the flow by collecting water for 30 seconds in a bucket.
Run the jet on the wheel with no load and record open-circuit voltage and rpm.
Add loads with the rheostat and record voltage and current at five load settings.
Calculate electrical power and hydraulic power, and then efficiency.
Repeat at heads of 0.5 and 1.5 m and with nozzles of 4 and 6 mm.
Plot power against load resistance to find the maximum power point.
Plot power against head and compare with the head raised to the power 1.5 that theory gives for the ideal case with constant nozzle area.
Write the report with a village-scale design example, calculating the head and flow needed for 1 kW.
What you should see
The model produces a few watts at 1 m head, lighting a 12 V LED lamp. Output rises with head and nozzle size, and overall efficiency is typically 5 to 25 percent for a home-made wheel and generator.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is head?
The vertical height from the water source to the turbine, which gives the water pressure.
2Why does a Pelton wheel suit high head and low flow?
It uses a fast jet striking cups, which works best where pressure is high but the volume is small.
3What limits the efficiency of the model?
Splashing, friction in bearings, poor cup shape and the low efficiency of a small motor used as a generator.
4What is the rim speed condition for maximum power?
The cups move at about half the jet speed.
Take it further
Compare the cup shape of spoons with a half-ball plastic cup and a split cup and measure the gain in efficiency.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Science projects
Mini and Major Project Guide
For diploma and BTech students: choose, plan, build, document and present