3.3 Power from the Sun and the Earth

A free GCSE Physics lesson for AQA (8463) from OBA School: solar cells, solar heating panels, solar power towers and geothermal energy. 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

• Describe what solar cells do, where they are useful, and their advantages and disadvantages
• Explain the difference between a solar cell panel and a solar heating panel
• Use ΔE = m c Δθ and efficiency to calculate the output of solar panels
• Describe where geothermal energy comes from and how it is used to generate electricity and heat buildings

Key words

• Solar cell: A flat device that transfers energy from light directly into electricity (a photovoltaic cell).
• Solar heating panel: A panel that absorbs radiation from the Sun to heat water flowing through it. It generates no electricity.
• Solar power tower: A power station where many mirrors reflect sunlight onto a tank at the top of a tower to make steam.
• Geothermal energy: Energy from hot rocks under the ground, heated by the decay of radioactive substances deep inside the Earth.
• Ground source heat: Using the steady warmth of the ground to heat water in buried pipes for heating buildings.

Solar cells

Solar cells transfer energy from sunlight directly into electricity, with no moving parts and no fuel. Many cells joined together make a solar cell panel. A typical panel transfers about 20% of the light energy it absorbs into electricity.

• no fuel, so no CO₂ and very cheap to run; Disadvantages expensive to buy and install
• useful where the grid does not reach: road signs, parking meters, satellites, remote farms; Disadvantages no output at night; much less on cloudy days and in winter – unreliable
• small cells power calculators and garden lights; Disadvantages a large area is needed to generate a lot of power

Worked example: electrical output of a solar cell panel

A panel of area 1.7 m² receives 1000 W of light on each square metre at midday. It is 20% efficient.
input power = 1.7 × 1000 = 1700 W
useful output = 0.20 × 1700 = 340 W

Answer: 340 W

Solar heating panels

A solar heating panel does not make electricity. Water (or another liquid) flows through pipes behind a black surface, which absorbs infrared and light from the Sun and heats the water. Even in Britain, a roof panel can supply much of a home’s hot water on sunny days.

You can use the specific heat capacity equation to find the energy transferred to the water: ΔE = m c Δθ (ΔE in J, m in kg, c in J/kg °C, Δθ in °C). If m is the mass flowing each second, ΔE is the energy per second – the power in watts.

Worked example: power delivered by a solar heating panel

Water flows through a panel at 0.020 kg each second. It enters at 12 °C and leaves at 40 °C. (c = 4200 J/kg °C)
Δθ = 40 − 12 = 28 °C
ΔE each second = m c Δθ = 0.020 × 4200 × 28 = 2352 J

Answer: about 2.4 kW is transferred to the water

Solar power towers

In a solar power tower, thousands of flat mirrors on the ground reflect sunlight onto a water tank at the top of a tall tower. The water boils, and the steam drives turbines that turn generators. A computer turns the mirrors to track the Sun across the sky, so the light always lands on the tank. These are built in hot, dry, sunny places such as deserts.

Geothermal energy

Deep inside the Earth, radioactive substances decay and release energy. This heats the surrounding rock, and energy is transferred towards the surface by heating. In volcanic areas, or where hot rocks are not too deep, a geothermal power station can be built:

• Cold water is pumped down a deep borehole to the hot rocks.
• It is heated and returns to the surface as steam (or very hot water) up a second borehole.
• The steam drives turbines that turn generators.

Geothermal energy can also heat buildings directly: hot water from underground is piped around homes, or ground source heat warms water in long pipes buried in the ground (used, for example, for under-floor heating).

• reliable – works day and night in any weather; Geothermal: disadvantages only possible in a few places with hot rocks near the surface
• renewable, no fuel; very little CO₂; Geothermal: disadvantages deep drilling is very expensive; small amounts of gases may escape

Exam tip: Geothermal energy is more reliable than solar energy because the rocks stay hot all the time, whereas the Sun does not shine at night and is blocked by cloud.

Common mistakes

• Mixing up solar cells (electricity) and solar heating panels (hot water).
• Saying geothermal energy comes from “the Earth’s core” or “volcanoes” without explaining it is energy released by radioactive substances in the Earth.
• Saying solar cells “do not work in winter”. They still work, but give less power in weak or cloudy daylight and none at night.
• In ΔE = m c Δθ, using the final temperature instead of the temperature change.

Revision questions

Try these, then check your answers below each one.

Question 1. What does a solar heating panel do? (Choose from: Heats water using energy from the Sun, Generates electricity directly from light, Stores electricity for use at night, Uses mirrors to boil water in a tower.)

Answer: Heats water using energy from the Sun

Working: A solar heating panel absorbs energy from the Sun to heat water flowing through it. Solar cells generate electricity; a solar power tower uses mirrors.

Question 2. A solar cell panel has an area of 2.0 m². Sunlight delivers 800 W to each square metre. The panel is 18% efficient.<br/>Calculate the electrical power output of the panel.

Answer: 288 W

Working: Input = 2.0 × 800 = 1600 W. Output = 0.18 × 1600 = 288 W.

Question 3. True or false? Geothermal energy comes from the decay of radioactive substances deep inside the Earth.

Answer: True

Working: True. Radioactive substances inside the Earth release energy as they decay, which heats the surrounding rocks.

Related lessons

• Previous lesson: 3.2 Energy from Wind and Water
• Next lesson: 3.4 Energy and the Environment
• In this chapter: 3.1 Energy Demands
• In this chapter: 3.5 Big Energy Issues

Continue in the full course

This lesson is section 3.3 (AGCPH0303) 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.