Energy Sources and How Power Stations Work

1Follow the energy back

It is evening in Rubavu. A phone is charging from a socket, beans are cooking on a charcoal stove, and a moto rumbles past burning petrol. Each of these gets its energy from somewhere. The charcoal was once a tree that grew using sunlight. The petrol came from crude oil pumped out of the ground in another country. The electricity may have come from methane gas drawn up from the bottom of Lake Kivu a few kilometres away, from water falling through a turbine, or from a solar panel.

In S1 you learned the forms of energy and that energy is never created or destroyed. This unit asks a practical question: where does the energy we use come from, how long will it last, and what does using it do to the environment?

2Experiment 1: a model hydroelectric generator

You need: a large plastic bottle with a small hole near the bottom, a clamp stand, a cork, 8 plastic spoons, a thin rod (a knitting needle) as an axle, a small toy motor, two wires, an LED (or a voltmeter), a tray and a metre rule.

1. Push the spoons into the cork to make a paddle wheel. Push the axle through the cork and fix one end to the motor's shaft.
2. Connect the LED to the motor's terminals.
3. Fill the bottle and clamp it so that the water falls a height $h=20\text{ cm}h\; =\; 20\backslash \; \backslash text\{cm\}$ onto the paddles.
4. Watch the LED (or read the voltmeter). Repeat with $h=40\text{ cm}h\; =\; 40\backslash \; \backslash text\{cm\}$ and $h=60\text{ cm}h\; =\; 60\backslash \; \backslash text\{cm\}$, refilling the bottle each time.

Figure 1.1: The falling water turns the paddle wheel, the wheel turns the motor, and the motor, working backwards as a generator, lights the LED.

Observation: the greater the height of fall, the faster the wheel turns and the brighter the LED (the higher the voltmeter reading). If the LED does not light at all, reverse its two wires: an LED lets current through in one direction only.

Explanation: the water's gravitational potential energy becomes kinetic energy as it falls. The water pushes the paddles, turning the axle, and the generator changes this kinetic energy into electrical energy. More height means more energy per kilogram of water ($mghmgh$), so more electrical power.

3How a power station works

Almost every power station uses the same idea as the model: something makes a turbine spin, and the turbine turns a generator. The generator is a coil turning inside magnets (you will study it in A-level physics). What differs is what spins the turbine.

• In a thermal power station, a fuel (methane, peat, diesel, coal) is burned. Its heat boils water into high-pressure steam that drives the turbine, or the fuel burns inside an engine that turns the generator directly.
• In a hydroelectric power station, water stored behind a dam flows down a large pipe (the penstock) and drives the turbine.
• A solar photovoltaic (PV) panel is the exception: its cells change light directly into electricity, with no moving parts.

Figure 1.2: Energy chains in three kinds of power station. A transformer raises the voltage before the electricity is sent along the grid; solar panels also need an inverter, because they produce direct current.

No energy change is perfect. The efficiency of a power station tells what fraction of the energy put in comes out as electricity:

The rest is wasted, mostly as heat. Electrical energy is often measured in kilowatt-hours: $1\text{ kWh}=1000\text{ W}\times 3600\text{ s}=3.6\times {10}^6\text{ J}1\backslash \; \backslash text\{kWh\}\; =\; 1000\backslash \; \backslash text\{W\}\; \backslash times\; 3600\backslash \; \backslash text\{s\}\; =\; 3.6\; \backslash times\; 10^\{6\}\backslash \; \backslash text\{J\}$. This is the "unit" on an electricity bill.