Notes · Environmental ScienceUK · A-Levels

Energy resources

This chapter surveys the resources from which energy is obtained and weighs them against one another. It covers energy demand and the meaning of energy density and renewability, the non-renewable fossil and nuclear resources, the range of renewable resources and how they work, and the evaluation of energy resources against reliability, cost, carbon emissions and impact, closing with the efficiency, storage and transition needed to secure future supply.

4 sections~14 min reading time3 competenciesLevel Foundation 1 · Standard 2 · Advanced 1

T·101010 / 16

Exam profile

AO1 · Describe the range of energy resources and their characteristicsAO2 · Calculate energy outputs, efficiencies and capacity factors from dataAO3 · Evaluate the sustainability and trade-offs of different energy resources and energy mixes

Operators:describeexplaincalculateevaluatecompareassess

basic level

AS-Level expects you to describe the main renewable and non-renewable resources, how they work and their broad advantages and disadvantages.

higher level

The full A-Level requires calculation of efficiencies and capacity factors and the reasoned evaluation of energy resources and the energy mix.

Depth

Text

Contents · 4 sections

Energy resources

Energy demand, energy density and resource types#

FoundationAQA 7447 3.3

Types of energy resource

Fig. 1Energy resources are classified as non-renewable or renewable; many renewables are also low-density.

Modern society depends on a large and growing supply of energy for electricity, heat, transport and industry, and demand has risen with population and living standards. Energy resources are classified first as non-renewable, those that exist in a fixed stock that is used up faster than it forms (fossil fuels and nuclear fuels), and renewable, those replenished by natural processes on a human timescale (solar, wind, water, geothermal and biomass). A third useful idea is the low-density resource: one where the energy is spread thinly, so a large area or volume must be harvested to gather a useful amount, as with solar and wind.

Energy density, the amount of energy stored per unit mass or volume, is a key property. Fossil and nuclear fuels have very high energy densities, so a small mass carries a great deal of energy, which is why they are so convenient for transport and for concentrated power generation. Many renewables are diffuse: the energy in sunlight or wind arriving on a given area is modest, so capturing a useful amount needs large collectors spread over a wide area. This difference in density explains much about how each resource is used and why the transition to renewables changes the scale and appearance of energy infrastructure.

It is also worth distinguishing primary energy, the energy in the resource as found (the coal, the sunlight, the wind), from secondary energy such as electricity, which is a convenient carrier made from a primary source. Electricity is not itself a resource but a way of delivering energy; how cleanly it is generated depends entirely on the primary sources in the mix. This distinction prevents the common error of calling electricity clean or dirty without asking how it was made.

The choice between resources is never made on one criterion alone. Reliability, energy density, cost, the carbon emissions of using the resource, and its wider environmental and social impacts all matter, and they often pull in different directions. Framing the whole chapter as an exercise in weighing these criteria, rather than as a list of resources, is the approach that leads to strong evaluative answers.

Worked example

Reasoning about energy density

Explain why petrol is used for cars while wind is not, referring to energy density.

Petrol
Petrol has a very high energy density, so a small, light tank carries enough energy for a long journey.
Wind
Wind is a diffuse, low-density resource that cannot be carried on board; it must be captured over a large area by fixed turbines.
Conclude
High energy density makes petrol convenient for mobile use, whereas wind suits fixed generation feeding electricity to a grid.

Result: Petrol's high energy density suits mobile use; wind's low density suits fixed generation, so they are used differently.

Typical mistakes

Calling electricity a renewable or clean resource in itself; it depends on the primary sources used to generate it.
Forgetting that many renewables are low-density, so they need large areas to gather a useful amount of energy.

Active revision

Explain why a coal power station occupies a small site for its output while a solar farm of similar output covers a large area.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Non-renewable resources: fossil fuels and nuclear#

StandardAQA 7447 3.3

Comparing fossil fuels and nuclear power

Fig. 2The non-renewable resources compared on the criteria that matter for energy choices.

Fossil fuels, coal, oil and natural gas, are the remains of organisms buried and altered over millions of years, so they are stores of ancient solar energy fixed by photosynthesis. They have high energy densities and established infrastructure, which is why they still dominate world energy. Their central drawback is that burning them releases carbon dioxide, the main driver of the enhanced greenhouse effect, transferring carbon from a slow geological store to the atmosphere; they also release other pollutants such as sulfur dioxide and nitrogen oxides, and their extraction damages land and water. Of the three, gas releases the least carbon dioxide per unit of energy and coal the most.

Nuclear power releases energy by the fission of heavy atoms, usually uranium, in a reactor, and uses the heat to raise steam and drive a generator. Its great advantage is that it generates large amounts of reliable electricity with very low carbon-dioxide emissions during operation, and its fuel has an extraordinarily high energy density. Its disadvantages are the production of radioactive waste that remains hazardous for a very long time and must be stored securely, the high cost and long time to build and decommission plants, and the small but serious risk of an accident releasing radioactivity. Nuclear fuel is itself finite, so nuclear power is non-renewable.

The reliability of these resources is a major reason they persist. Fossil and nuclear plants can supply a steady, controllable output regardless of weather or time of day, providing the baseload and the ability to meet peaks in demand. This dependability is exactly what many renewables lack, so comparisons between resources must weigh the low emissions of renewables against the reliability of fossil and nuclear power, a tension at the heart of energy policy.

Because fossil and nuclear resources are finite and, in the case of fossil fuels, the chief cause of climate change, the long-term direction is away from them. Yet the transition is constrained by their entrenched infrastructure, their reliability and their energy density, so they are being phased down rather than switched off overnight. Being able to set out both the advantages that keep them in use and the disadvantages that drive their replacement is the balanced understanding the examination rewards.

Worked example

Weighing nuclear power

A country wants low-carbon electricity that is available at all times. Assess whether nuclear power meets this need and state its main drawback.

Low carbon
Nuclear fission emits very little carbon dioxide during operation, so it helps cut emissions.
Reliability
A nuclear plant provides a steady baseload output independent of weather, unlike wind or solar, so it meets the always-available requirement.
Drawback
It produces long-lived radioactive waste and is costly and slow to build, so it must be weighed against these disadvantages and against renewables plus storage.

Result: Nuclear power provides reliable low-carbon electricity, but its radioactive waste and cost are the key drawbacks to weigh.

Typical mistakes

Calling nuclear power renewable because it is low-carbon; its uranium fuel is a finite, non-renewable stock.
Treating all fossil fuels as equivalent; coal emits the most carbon dioxide per unit energy and gas the least.

Active revision

Evaluate the case for building a new nuclear power station as part of a low-carbon electricity supply.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Renewable resources and how they work#

StandardAQA 7447 3.3

A wind turbine

Fig. 3A wind turbine converts the kinetic energy of moving air into electricity via the generator in the nacelle.

The renewable resources draw on continuous natural flows. Solar energy is captured either as electricity by photovoltaic cells or as heat by solar thermal collectors. Wind turbines convert the kinetic energy of moving air into electricity. Hydroelectricity uses the potential energy of water held behind a dam, and tidal and wave power capture the energy of the sea. Geothermal energy uses heat from within the Earth, and biomass, wood and other plant material and their derived fuels, releases energy stored by recent photosynthesis. Most of these ultimately derive from the Sun, which drives the wind, the water cycle and plant growth.

Their shared strength is that they are replenished naturally and, in use, emit little or no carbon dioxide, so they are central to reducing climate change. Their characteristic weaknesses follow from being diffuse and, for several, intermittent. Solar produces nothing at night and less under cloud; wind depends on the weather; so their output varies and cannot simply be turned up to meet demand. Others are more dependable: hydroelectricity can be controlled and stored behind a dam, tidal power is intermittent but highly predictable, and geothermal and biomass can supply a steady output.

Each renewable also has its own environmental impacts, so they are low-impact rather than impact-free. Large dams flood land and disrupt rivers and their ecology; wind and solar farms take land or sea space and affect landscape and wildlife; tidal barrages alter estuaries; and biomass is only low-carbon if the crops are regrown to reabsorb the carbon dioxide released, and competes with food production for land. Recognising these specific impacts is essential to an honest evaluation and guards against presenting any renewable as perfect.

A wind turbine illustrates how a renewable device works and where its limits lie. Moving air turns the blades, which drive a generator in the nacelle to produce electricity; the output depends on wind speed and ceases when the wind is too light or too strong, so the turbine only generates for part of the time. The same pattern, a clean but variable output that depends on a natural flow, recurs across the intermittent renewables and is the reason storage and a mix of sources are needed, as considered next.

Worked example

Explaining a renewable's variability

Explain why a wind farm cannot on its own guarantee a constant electricity supply, and what is needed to overcome this.

Source of variability
Wind speed varies with the weather and time, so the turbines generate a variable output and nothing when the wind is too light or too strong.
Consequence
Supply does not automatically match demand, so at times the wind farm produces too little (or too much) for the grid's needs.
Solution
Combining wind with energy storage and with other, dependable sources in a mixed grid smooths the supply so demand is always met.

Result: Wind is intermittent, so it must be combined with storage and other sources to guarantee a constant supply.

Typical mistakes

Presenting renewables as having no environmental impact; each has specific impacts (land use, habitat, landscape).
Treating all renewables as intermittent; hydro is controllable, tidal is predictable, and geothermal and biomass are steady.

Active revision

Compare the reliability of wind power and hydroelectric power, and explain why the difference matters for supplying electricity.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Evaluating energy resources and securing future supply#

AdvancedAQA 7447 3.3

Capacity factors of electricity sources

Fig. 4Approximate capacity factors (illustrative values): dependable sources deliver a higher fraction of their maximum output.

Energy resources are evaluated against several criteria at once: reliability, energy density, cost, the carbon emissions of using them, and their wider environmental and social impacts. No single resource scores best on all of them, which is why countries use an energy mix rather than relying on one source. A useful measure of how much a power source actually delivers is the capacity factor, the fraction of its maximum possible output that it produces over a year; intermittent renewables have lower capacity factors than dependable fossil or nuclear plants, which affects how much capacity must be built to meet demand.

Efficiency is central to using any resource well. The efficiency of a conversion is the useful energy output as a fraction of the total energy input, and because no conversion is perfect, energy is always lost, usually as heat. Improving efficiency, in power stations, buildings, transport and appliances, reduces the amount of primary energy needed for the same service, cutting both cost and emissions, and is often the cheapest way to close a supply gap. Energy conservation, simply using less, works alongside efficiency to reduce demand.

The intermittency of many renewables is met by storage and by diversity. Energy storage, such as pumped-storage hydroelectricity and batteries, stores surplus energy when generation exceeds demand and releases it when demand exceeds generation, smoothing the supply. A diverse mix of sources spread over a wide area also helps, because the wind is usually blowing or the sun shining somewhere, and dependable sources cover the gaps. Together, storage, diversity and better grids make a largely renewable supply workable despite the variability of individual sources.

Securing future supply therefore means reducing demand through efficiency and conservation, expanding low-carbon generation, and building the storage and grids to integrate it, while managing the decline of fossil fuels. Evaluating any proposal means weighing its contribution to reliability, cost, emissions and impact, and recognising the trade-offs: a resource that is cheap may be high-carbon, one that is low-carbon may be intermittent or costly. The strongest answers reach a justified overall judgement rather than listing points, and connect the energy transition to the climate and sustainability chapters.

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

Efficiency

The useful output as a percentage of the input; the rest is lost, usually as heat. No conversion is 100% efficient.

\text{capacity factor} = \frac{\text{actual output over a year}}{\text{maximum possible output}} \times 100

Capacity factor

How much of its theoretical maximum a source actually delivers; intermittent renewables have lower capacity factors than dependable sources.

Worked example

Calculating a capacity factor

A wind farm with a maximum (rated) output of 100 MW generates 300 000 MWh of electricity in a year. Taking a year as 8760 hours, calculate its capacity factor.

Find the maximum possible output
Maximum = rated power x hours = $100 \times 8760 = 876\,000$ MWh.
Apply the capacity factor equation
$\dfrac{300\,000}{876\,000} \times 100$ .
$\frac{300\,000}{876\,000} \times 100 = 34.2\%$
Interpret
A capacity factor of about 34% is typical for wind and reflects its intermittency: it delivers about a third of its theoretical maximum, so extra capacity and storage are needed to meet demand.

Result: The capacity factor is about 34%, typical of wind and reflecting its intermittency.

Typical mistakes

Confusing efficiency (useful output over input) with capacity factor (actual over maximum possible output).
Concluding that one resource is simply best; the criteria pull in different directions, so a mix and a balanced judgement are needed.

Active revision

A power station takes in 500 MW of energy from fuel and delivers 190 MW of electricity. Calculate its efficiency and suggest where the rest of the energy goes.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

References & sources

Sources

AQA

AQA AS and A-level Environmental Science (7447) specification

Department for Education

GCE AS and A level subject content

Energy resources

4 sections~14 min reading time3 competenciesLevel Foundation 1 · Standard 2 · Advanced 1

Energy resources

Key points

Reasoning about energy density

Key points

Weighing nuclear power

Key points

Explaining a renewable's variability

Key points

Calculating a capacity factor

Sources

Energy resources

Key points

Reasoning about energy density

Key points

Weighing nuclear power

Key points

Explaining a renewable's variability

Key points

Calculating a capacity factor

Sources