Physics 3.13 - 3.14 - Energy resources and trends
Compare how fossil fuels, nuclear fuel, bio-fuels, wind, water, tides and sunlight supply useful energy. Then use numerical evidence and physical reasons to explain why the mix of resources changes over time.
Resources, carriers and timescales
Start with the system: people need energy transferred for heating, transport and electrical devices. An energy resource is something available in nature that can supply those transfers. Coal, moving air and sunlight are resources; the useful output might be heating, movement or electricity.
Energy resource
An energy resource is a source from which energy can be transferred for a useful purpose.
Electricity needs special care. It is an energy carrier, not a primary resource: electricity is generated by transferring energy from a resource such as wind, nuclear fuel or natural gas. It therefore makes sense to say "electricity generated using wind", but not to classify electricity itself as renewable or non-renewable.
The classification depends on the supply of the resource and the timescale on which people use it.
Renewable and non-renewable
A renewable resource is replenished naturally on a timescale comparable with, or shorter than, the timescale on which it is used.
A non-renewable resource is a finite stock that is not replenished on a useful human timescale.
Renewable does not mean infinite or continuously available. A forest used for bio-fuel can be depleted if trees are cut faster than they regrow, and sunlight is not available at night. Conversely, a large stock of uranium can last for years in a power station but remains non-renewable because mined uranium is not naturally replaced on a human timescale.
It is also the resource, not energy itself, that can run out. Energy is conserved as it is transferred between stores; after use, much of it is dissipated to the thermal stores of the surroundings and is less useful for doing the intended job.
Non-renewable resources
The named non-renewable resources are fossil fuels and nuclear fuel. Both can be stored until needed, so the rate at which many power stations use them can be controlled to help match demand. Their energy stores and consequences are different.
Fossil fuels
Coal, crude oil and natural gas formed over millions of years from ancient biological material. Their chemical stores are depleted much faster than new fossil fuels form, so they are non-renewable.
| Fossil fuel use | How it is used | Useful outcome |
|---|---|---|
| Electricity generation | Fuel is burned; heating produces steam or hot gases that turn a turbine and generator | Electrical energy is transferred to the grid |
| Heating | Natural gas, oil or coal is burned in a boiler, furnace or cooker | Energy is transferred to thermal stores |
| Transport | Petrol, diesel or aviation fuel made from crude oil is burned in engines | Energy is transferred to the kinetic store of a vehicle and to the surroundings |
Fossil fuels are concentrated, transportable and supported by extensive infrastructure. A gas-fired power station can increase its output when demand rises. However, reserves are finite, extraction changes environments, and combustion releases carbon dioxide. Other combustion pollutants can also damage health and environments. These consequences provide reasons for reducing fossil-fuel use even before a reserve is physically exhausted.
Nuclear fuel
Nuclear power stations use a mined fuel such as uranium. Energy transferred from the nuclear store heats water; steam turns a turbine connected to a generator. The fuel is not burned, but it is finite and so is non-renewable.
Nuclear stations can produce large, steady amounts of electricity without depending on wind or sunlight. They do not release carbon dioxide from fuel combustion during operation because there is no combustion. Building and decommissioning stations are expensive, output cannot be changed as quickly as at some fossil-fuel stations, and radioactive waste must be isolated and stored safely for long periods. Mining, construction and waste management also have environmental impacts, so "no combustion emissions during operation" must not be shortened to "no environmental impact".
The uses are not identical. Fossil fuels are used directly for transport and heating as well as electricity generation; nuclear fuel is used mainly to generate electricity. This is why replacing fossil-fuel electricity does not by itself remove fossil fuels from every part of society.
Renewable resources
The five named renewable groups are bio-fuel, wind, hydroelectricity, tides and the Sun. Each supplies energy in a different way, so each has different uses and availability.
| Resource | Where the energy is available | Main uses | Availability and consequences |
|---|---|---|---|
| Bio-fuel | Chemical store of recently living material, such as wood or crops | Burned for heating or electricity; processed into transport fuels | Fuel can be stored, but it is renewable only if biomass is regrown at least as fast as it is used. Burning releases carbon dioxide, and growing crops uses land. |
| Wind | Kinetic store of moving air | Turbines turn generators to produce electricity | No fuel is burned during operation, but output varies with wind speed. Turbines need suitable sites and can affect views, noise levels and wildlife. |
| Hydroelectricity | Gravitational store of water at height, replenished by the water cycle | Falling water turns a turbine and generator | Water flow can be controlled and output can respond quickly where a reservoir is available. Suitable valleys are limited, and dams flood land and alter river habitats. |
| Tides | Kinetic and gravitational stores of seawater moved by tides | Tidal streams or barrages turn turbines and generators | Tidal timing is predictable, but generation is intermittent and suitable coastal sites are limited. Barrages and turbines can alter estuary habitats and navigation. |
| The Sun | Radiation from the Sun | Photovoltaic cells generate electricity directly; solar thermal collectors heat water | No fuel is needed during operation. Output varies with daylight, cloud and season; panels and large solar farms require materials and space. |
Wind, sunlight and tidal motion are flows: the immediate resource cannot be stockpiled like a pile of coal before conversion. A hydroelectric reservoir can store water at height, while bio-fuel material can be stored and burned when required. Electricity generated during a surplus can also be transferred into a separate storage system for later use.
No entry in the table is simply "good" or "bad". Renewable sources avoid depletion of a finite fuel when managed sustainably, and most avoid combustion during operation, but every installation uses materials, occupies a site and can affect ecosystems. A useful comparison must name the criterion being compared.
Comparing an energy mix
An energy mix is the combination of resources used by a country, region or organisation. The mix changes because decision-makers weigh several criteria, and the importance of each criterion depends on the purpose.
| Criterion | Physical question | Why it changes a decision |
|---|---|---|
| Renewability | Is the resource replenished as fast as it is used? | A finite reserve may become harder or more expensive to obtain as accessible stocks decline. |
| Availability and control | Can output be produced when demand occurs? | Wind and solar output vary; stored fuels and controlled water flow can supply electricity at chosen times. |
| Response time | How quickly can output change? | Rapid response helps match short peaks in electricity demand. |
| Environmental effects | What is emitted, disturbed or left as waste? | Carbon dioxide, air pollution, radioactive waste, flooded habitats, land use and wildlife effects are different costs. |
| Cost and infrastructure | What must be built, fuelled, connected and maintained? | A source with no fuel cost can still need expensive construction, grid connections or storage. |
| Location and scale | Is a suitable resource close enough, and how much output is possible? | A tidal scheme needs an appropriate coast; hydroelectricity needs suitable relief and water; solar output depends on light. |
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