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3.2 Energy from Wind and Water

GCSE Physics · AQA · Chapter 3: Energy Resources

A free GCSE Physics lesson for AQA (8463) from OBA School: wind turbines, wave generators, hydroelectric and pumped storage schemes, and tidal barrages. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Explain what makes wind, waves, hydroelectricity and the tides renewable energy resources
  • Describe how a wind turbine and a wave generator generate electricity
  • Describe how hydroelectric power stations and pumped storage schemes work, and calculate the energy using Ep = m g h
  • Describe how a tidal barrage generates electricity
  • Compare the reliability and environmental effects of these resources

Key words

  • Renewable: Replaced by natural processes as fast as it is used, so it will never run out.
  • Wind turbine: A generator on top of a tall tower, turned by blades that the wind pushes round.
  • Hydroelectric: Electricity generated by water flowing downhill from a reservoir through turbines.
  • Pumped storage: A hydroelectric scheme that pumps water uphill when demand is low and releases it when demand is high.
  • Tidal barrage: A dam across an estuary that traps water at high tide and lets it flow out through turbines.
  • Reliable: Able to supply electricity whenever it is needed, whatever the weather.

Renewable energy from wind and water

Wind, waves, flowing river water and the tides are renewable: natural processes keep replacing them, so they will never run out. They need no fuel, so they release no carbon dioxide while they generate electricity.

  • The Sun drives wind (uneven heating of the air), waves (made by the wind) and rain (water evaporates, then falls on high ground).
  • The tides are caused mainly by the gravitational pull of the Moon (and the Sun) on the oceans.

Key fact: All four resources in this section use a turbine to turn a generator, just like a power station. The difference is what turns the turbine: moving air or moving water instead of steam.

Wind power

The wind pushes the blades of a wind turbine round. They turn a shaft connected to a generator at the top of the tower. The faster the wind, the greater the power output. Groups of turbines are called wind farms; offshore wind farms are built at sea, where winds are stronger and steadier.

  • Unreliable: no electricity when there is little or no wind, and turbines are switched off in very strong winds to avoid damage.
  • Environment: no fuel and no CO₂ while running, but some people find turbines noisy or unsightly, and blades can kill birds and bats.

Going further: if the wind speed doubles, twice the mass of air passes the blades each second and each kilogram has 4 times the kinetic energy (Ek = ½ m v²). So the power of the wind goes up by 2 × 4 = 8 times: it is proportional to v³.

Wave power

Wave generators float on the sea. As waves pass, parts of the generator move up and down (or rock), and this motion turns a generator. Cables carry the electricity to the shore.

  • Output depends on the size of the waves, so supply is not constant – calm seas give little electricity.
  • Generators must survive storms, and long cables and buildings along the coast are needed, which can spoil the coastline.
  • They may change the flow of water and affect the habitats of sea life and birds.

Hydroelectric power and pumped storage

Rain fills a reservoir behind a dam in hilly or mountain areas. Water flows down pipes to turbines at the bottom, turning generators. Energy is transferred from the gravitational potential energy store of the water, to its kinetic store, to the turbine, and then electrically.

Worked example: power from a hydroelectric station

50 000 kg of water flows down through the turbines each second, falling 120 m. The station is 85% efficient. (g = 9.8 N/kg)
Ep lost each second = m g h = 50 000 × 9.8 × 120 = 5.88 × 10⁷ J, so input power = 5.88 × 10⁷ W
useful output = 0.85 × 5.88 × 10⁷ W = 5.0 × 10⁷ W

Answer: about 50 MW

A pumped storage scheme has an upper and a lower reservoir. When demand for electricity is low (e.g. at night), spare electricity from other power stations pumps water up. When demand is high, the water flows back down to generate electricity within seconds. It is a way of storing energy, not a source of energy: it gives back less energy than was used to pump the water up.

Key fact: Hydroelectricity is reliable and starts very quickly, unless a drought lowers the reservoir. But building a dam floods a valley, destroying habitats (and sometimes homes and farmland).

Tidal power

A tidal barrage is a long dam built across a river estuary. At high tide, water is trapped behind the barrage. When the tide outside has gone out, the trapped water is let out through turbines in the barrage, turning generators. (Some barrages also generate as the tide comes in.) Estuaries that become narrow funnel the tide and make it rise higher, which suits a barrage.

  • Reliable and predictable: there are two high tides every day, at times we can predict years ahead. But the height of the tides changes over a monthly cycle, so the output varies.
  • Environment: it floods mudflats where birds feed, affects fish and can block shipping.
  • Tidal stream turbines on the sea bed, turned by tidal currents, are an alternative.

Comparing wind and water resources

Exam tip: in an “evaluate” question, compare reliability, cost, environmental effects and location, then give a conclusion.

  • Wind; Reliability unreliable: depends on wind speed; Main environmental effects noise, visual impact, risk to birds
  • Waves; Reliability unreliable: depends on wave height; Main environmental effects coastline spoilt by cables; habitats affected
  • Hydroelectric; Reliability reliable unless drought; starts in seconds; Main environmental effects valleys flooded; habitats lost
  • Tidal; Reliability predictable, but output varies with tide height; Main environmental effects estuary habitats changed; shipping blocked

Common mistakes

  • Saying pumped storage “produces” energy. It stores energy and returns less than was put in.
  • Saying tidal power depends on the weather. Tides are caused by the Moon’s gravity and are predictable; wind and waves depend on the weather.
  • Forgetting that 1 m³ of water has a mass of 1000 kg when using Ep = m g h.
  • Describing renewable resources as having “no environmental impact”. They release no CO₂ when running but affect habitats, land and views.

Revision questions

Try these, then check your answers below each one.

Question 1. Which energy resource does not depend on energy from the Sun? (Choose from: wind, waves, hydroelectricity, tides.)

Answer: tides

Working: Tides are caused mainly by the Moon’s gravity. Wind and waves come from the Sun heating the air, and hydroelectricity depends on rain from water evaporated by the Sun.

Question 2. In a hydroelectric power station, 3.0 × 10⁴ kg of water falls through a height of 150 m every second. (g = 9.8 N/kg)<br/>Calculate the gravitational potential energy lost by the water each second, in MW.

Answer: 44.1 MW

Working: Ep = 3.0 × 10⁴ × 9.8 × 150 = 4.41 × 10⁷ J each second = 4.41 × 10⁷ W = 44.1 MW.

Question 3. True or false? A pumped storage scheme gives back more energy than is used to pump the water uphill.

Answer: False

Working: False. Pumped storage only stores energy; some is wasted by the pumps, pipes and turbines, so it gives back less than was put in.

Related lessons

Continue in the full course

This lesson is section 3.2 (AGCPH0302) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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