Energy resources

CORE VS EXTENDED GUIDE

• Core students study only the unlabelled sections.
• Extended students must study everything, including Extended Extended points.
• Extended = Core + Supplement.

Electrical Power from Fossil Fuels

Fossil fuels are non-renewable energy resources formed over millions of years from the remains of dead organisms. The three fossil fuels are coal, oil, and natural gas. Each stores chemical energy that is released by burning (combustion).

The energy transfer pathway in a fossil-fuel power station follows a fixed chain. The fuel burns in a boiler, transferring energy by heating to water, which produces high-pressure steam. The steam drives a turbine [a device with blades that spin when hit by steam or moving fluid], which spins a generator [a machine that converts kinetic energy into electrical energy by electromagnetic induction].

Energy is therefore transferred from the chemical store of the fuel to the thermal store of steam, then to the kinetic store of the turbine, and finally transferred electrically by the generator.

MisconceptionStudents often write "the generator makes energy." A generator does not create energy — it transfers energy from the kinetic store to electrical energy. Exam cue: always state the transfer pathway, never say energy is "created" or "made."

Electrical Power from Biofuels

Biofuels are fuels derived from recently living biological material, such as wood, ethanol from crops, or methane from animal waste. Biofuels store chemical energy, which is released by combustion.

A biofuel power station operates in the same way as a fossil-fuel station: the biofuel burns in a boiler, steam drives a turbine, and the turbine spins a generator. The key difference is the source of the fuel. Because new crops or biological material can be grown to replace what is burned, biofuels are classified as renewable — provided the rate of replanting matches the rate of consumption.

Burning biofuels still releases carbon dioxide into the atmosphere. However, the plants absorbed carbon dioxide while growing, so biofuels are sometimes described as carbon-neutral overall, although this depends on production and transport methods.

Examiner InsightCIE papers often ask students to compare biofuels with fossil fuels. The critical distinction is renewability: biofuels can be regrown, whereas fossil fuels cannot be replaced on a human timescale. Exam cue: state "renewable because new biological material can be grown to replace what is used."

Energy from Water: Waves, Tides and Hydroelectric

Water can be used as an energy resource in three distinct ways: wave power, tidal power, and hydroelectric power. Each harnesses a different aspect of water movement or storage.

Wave power uses the kinetic energy of surface waves on the sea. Floating devices on the ocean surface bob up and down as waves pass. This movement drives a generator, transferring energy from the kinetic store of the waves to electrical energy. No boiler or turbine is needed in most wave-power devices, although some designs use the wave motion to compress air through a turbine.

Tidal power exploits the regular rise and fall of sea levels caused by the gravitational pull of the Moon (and, to a lesser extent, the Sun). A tidal barrage [a dam-like barrier built across a river estuary] traps water at high tide. As the water flows out through channels at low tide, it drives turbines connected to generators. Energy is transferred from the gravitational potential store of the raised water to the kinetic store of the flowing water, then to electrical energy via the turbine and generator.

Hydroelectric power uses water stored behind a dam at height. The water has energy in its gravitational potential store. When released, water flows downhill through pipes, driving a turbine and generator. The energy transfer follows: gravitational potential → kinetic → electrical. A boiler is not used, but a turbine and generator are essential components.

Energy from Geothermal Resources

Geothermal resources provide energy from the thermal energy stored in hot rocks deep underground. The Earth's interior remains hot due to radioactive decay of elements within the Earth's core and mantle.

In geothermal power generation, cold water is pumped down through pipes into hot underground rock. The water absorbs thermal energy from the rock, producing steam or very hot water. This steam rises to the surface and drives a turbine connected to a generator. In some regions, naturally occurring hot water or steam vents (geysers) provide a direct source.

The energy transfer pathway is: thermal store of hot rocks → thermal store of water/steam → kinetic store of turbine → electrical energy via generator. A boiler is not needed because the Earth itself heats the water, but a turbine and generator are used.

Geothermal energy is only available in regions with suitable geological conditions, such as volcanic areas or places where hot rocks are close to the surface. This limits its geographic availability.

Energy from Nuclear Fuel

Nuclear fuel, such as uranium or plutonium, stores energy in the nuclear store of its atoms. This energy is released by nuclear fission [the splitting of a heavy nucleus into two smaller nuclei, releasing a large amount of energy].

In a nuclear power station, the nuclear fission reaction occurs inside a reactor. The energy released by fission heats water in a boiler to produce steam. The steam drives a turbine, which turns a generator. The energy transfer chain is: nuclear store of fuel → thermal store of water/steam → kinetic store of turbine → electrical energy via generator.

Nuclear power stations produce no carbon dioxide during operation, so they do not contribute to the greenhouse effect during generation. However, they produce radioactive waste that remains hazardous for thousands of years and requires safe, long-term storage. Nuclear fuel is non-renewable because uranium and plutonium are finite resources extracted from the Earth.

SafetyNuclear fuel is radioactive. Reactor shielding (thick concrete and lead) is essential to protect workers and the environment from ionising radiation.

MisconceptionStudents sometimes write that nuclear power stations produce no pollution. While they produce no greenhouse gases during generation, they do produce radioactive waste, which is a significant environmental concern. Exam cue: always mention radioactive waste as a disadvantage.

Energy from Solar Cells

Solar cells (photovoltaic cells) convert light energy from the Sun directly into electrical energy. No boiler, turbine, or generator is required — the conversion is direct.

When light falls on a solar cell, energy is transferred from the radiation of sunlight to electrical energy in the circuit. Solar cells are silent in operation, produce no greenhouse gases, and require very little maintenance. They are renewable because sunlight is continuously available.

However, solar cells only generate electricity during daylight hours and their output decreases on cloudy days, making them weather-dependent and less reliable in some climates. Large arrays of solar cells are needed to generate electricity on a useful scale, which requires significant land area. The initial cost of manufacturing and installing solar panels can be high, although running costs are very low.

Examiner InsightCIE papers distinguish between solar cells and solar panels. Solar cells generate electricity from light. Solar panels heat water using infrared radiation. Do not confuse the two. Exam cue: if the question says "solar cell," the answer must involve generating electrical energy from light.

Energy from Solar Panels and Wind

Solar panels (solar thermal panels) absorb infrared radiation and other electromagnetic waves from the Sun to heat water directly. Water flows through pipes in the panel, absorbs thermal energy, and is used for domestic heating or hot water supply. Solar panels do not generate electricity — they transfer energy by heating. No boiler, turbine, or generator is involved.

The Sun is also the source of wind energy. The Sun heats the Earth's surface unevenly, because land and sea absorb radiation at different rates. This uneven heating causes convection currents [large-scale circular movements of air caused by temperature differences] in the atmosphere, which produce wind. Wind turbines have large blades that are turned by the moving air. The rotating blades drive a generator to produce electrical energy. A turbine and generator are used, but no boiler is needed.

Wind energy is renewable, produces no greenhouse gases during operation, and can be installed on land (onshore) or at sea (offshore). However, wind is intermittent — turbines only generate when wind blows, making output unreliable. Wind farms require large land areas and some people consider them visually intrusive or noisy.

Advantages and Disadvantages of Energy Resources

Each energy resource has distinct advantages and disadvantages that can be assessed using five criteria: renewability, availability, reliability, scale, and environmental impact.

MisconceptionStudents sometimes state that renewable resources have "no environmental impact." Every energy resource has some impact — even solar cells require materials and land. Exam cue: always identify at least one specific environmental effect, even for renewable resources.

Examiner InsightCIE questions often ask candidates to "discuss" or "compare" resources. Structure answers using the five criteria above. A strong answer gives one advantage and one disadvantage with explicit reasoning, not vague statements like "it is good for the environment." Exam cue: name the specific criterion (e.g. reliability, renewability) in your answer.

Efficiency of Energy Transfer

Every energy transfer involves some energy being transferred to stores that are not useful — most commonly the thermal store of the surroundings. Efficiency describes how much of the total energy input is transferred to useful output.

A device with high efficiency wastes little energy. A device with low efficiency transfers a large proportion of its input energy to non-useful stores (usually thermal energy dissipated to the surroundings by heating). No real device is 100% efficient because some energy is always dissipated.

Qualitatively, efficiency increases when the proportion of useful energy output increases relative to the total energy input. Reducing friction, improving insulation, or using streamlined designs all increase efficiency because they reduce the energy transferred to non-useful thermal stores.

MisconceptionStudents sometimes write that energy is "lost" during a transfer. Energy is never lost — it is transferred to non-useful stores (usually the thermal store of the surroundings), where it dissipates and becomes less useful. Exam cue: write "dissipated" or "transferred to the thermal store of the surroundings," not "lost."

Extended The Sun as the Main Energy Source

Radiation from the Sun is the main source of energy for all our energy resources except geothermal, nuclear, and tidal.

Solar radiation directly powers solar cells and solar panels. The Sun's radiation also drives the water cycle (evaporation → condensation → rain → rivers), which provides energy for hydroelectric and wave power. Uneven solar heating of the atmosphere causes convection currents that produce wind, so wind energy originates from the Sun. Fossil fuels and biofuels store chemical energy that originally came from photosynthesis, which requires sunlight.

Geothermal energy comes from radioactive decay inside the Earth, not from the Sun. Nuclear energy comes from the nuclear store of uranium or plutonium, which formed in ancient stellar processes, not from current solar radiation. Tidal energy comes primarily from the gravitational pull of the Moon on the Earth's oceans.

Examiner InsightCIE frequently tests the three exceptions — geothermal, nuclear, and tidal. A common error is forgetting tidal energy. Exam cue: memorise the three non-solar resources as a group.

Extended Nuclear Fusion in the Sun

Energy is released by nuclear fusion in the Sun. Nuclear fusion is the process in which two light nuclei [such as hydrogen nuclei] join together to form a heavier nucleus, releasing a large amount of energy.

Inside the Sun, extremely high temperatures (approximately 15 million °C) and pressures cause hydrogen nuclei to fuse together to form helium nuclei. Each fusion reaction releases energy, which is radiated outward as electromagnetic radiation — including light and infrared. This is the source of all solar energy reaching Earth.

Research is being carried out to investigate how energy released by nuclear fusion can be used to produce electrical energy on a large scale. Fusion has the potential to provide enormous quantities of energy from abundant fuel (hydrogen from water), with minimal radioactive waste compared to fission. However, sustaining the extremely high temperatures and pressures needed for controlled fusion on Earth remains a major engineering challenge, and no fusion power station currently operates commercially.

MisconceptionStudents often confuse nuclear fusion with nuclear fission. Fusion joins light nuclei together; fission splits heavy nuclei apart. Both release energy, but the processes and fuels are fundamentally different. Exam cue: if the question mentions the Sun, the answer is fusion; if it mentions uranium or a power station reactor, the answer is fission.

Extended Calculating Efficiency

Key Equations

Efficiency (energy form):

$$\text{efficiency}=\frac{\text{useful energy output}}{\text{total energy input}}\times 100\%$$

Variables:

• useful energy output = the energy transferred to the desired store, in J
• total energy input = the total energy supplied to the device, in J
• efficiency = a percentage (%) or a decimal (no unit) if not multiplied by 100

SI unit: % (when multiplied by 100) or no unit (as a ratio)

Rearrangement for useful energy output:

$$\text{useful energy output}=\frac{\text{efficiency}}{100}\times \text{total energy input}$$

Rearrangement for total energy input:

$$\text{total energy input}=\frac{\text{useful energy output}}{\text{efficiency}÷100}$$

ProportionalityFor a fixed total energy input, efficiency is directly proportional to useful energy output. Doubling the useful output doubles the efficiency.

Efficiency (power form):

$$\text{efficiency}=\frac{\text{useful power output}}{\text{total power input}}\times 100\%$$

Variables:

• useful power output = the rate of useful energy transfer, in W
• total power input = the total rate of energy supplied, in W

SI unit: % (when multiplied by 100)

Rearrangement for useful power output:

$$\text{useful power output}=\frac{\text{efficiency}}{100}\times \text{total power input}$$

Rearrangement for total power input:

$$\text{total power input}=\frac{\text{useful power output}}{\text{efficiency}÷100}$$

ProportionalityFor a fixed total power input, efficiency is directly proportional to useful power output.

Efficiency is defined as: (%) efficiency = (useful energy output) ÷ (total energy input) × 100%, or equivalently (%) efficiency = (useful power output) ÷ (total power input) × 100%.

Either the energy form or the power form may be used, depending on the data given in the question. Efficiency can never exceed 100% because useful output cannot exceed total input.

Worked Example 1 — Using the energy form

A light bulb receives 80 J of electrical energy and transfers 12 J to the light (visible radiation) store. Calculate the efficiency of the bulb.

Finding the efficiency

Equation used

$$\text{efficiency}=\frac{\text{useful energy output}}{\text{total energy input}}\times 100\%$$

Given

$$\text{useful energy output}=12\text{ J}$$

$$\text{total energy input}=80\text{ J}$$

Substitution:

$$\text{efficiency}=\frac{12}{80}\times 100\%$$

$$\text{efficiency}=0.15\times 100\%$$

$$\text{efficiency}=15\%$$

Worked Example 2 — Using the power form with rearrangement

A motor has an efficiency of 60% and a total power input of 500 W. Calculate the useful power output.

Finding the useful power output

Equation used

$$\text{efficiency}=\frac{\text{useful power output}}{\text{total power input}}\times 100\%$$

Rearranging for useful power output:

$$\text{useful power output}=\frac{\text{efficiency}}{100}\times \text{total power input}$$

Given

$$\text{efficiency}=60\%$$

$$\text{total power input}=500\text{ W}$$

Substitution:

$$\text{useful power output}=\frac{60}{100}\times 500$$

$$\text{useful power output}=0.60\times 500$$

$$\text{useful power output}=300\text{ W}$$

Examiner InsightCIE questions may give efficiency as a decimal (e.g. 0.60) or as a percentage (e.g. 60%). Check whether the question asks for a percentage answer — if so, multiply the decimal by 100. Exam cue: always include the % symbol or state "as a decimal" in your final answer.