Topic 6 of 6
Energy resources for electricity
Compare an energy resource by explaining how it supplies electricity and how well that supply meets a particular demand.
Efficiency compares useful output with total input. Power describes how fast energy is transferred. Neither measure alone establishes cost, reliability or environmental impact.
Non-renewable resources, including fossil and nuclear fuels, have finite supplies that are not replenished on the timescale of their use. Renewable resources are replenished by ongoing natural processes. Renewable does not mean that a site can be used at any extraction rate, that output is constant, or that there are no environmental effects.
Follow the transfer route to electricity
A turbine receives a mechanical transfer from moving gas or water and drives a generator. The generator makes energy available electrically. Several resources use this route, but the original store and the way the turbine is driven differ. Photovoltaic solar cells provide a direct route from incoming electromagnetic waves to electrical output without a turbine.
Non-renewable
Fossil fuels
Route: Coal, oil or natural gas supplies a chemical store. Burning the fuel heats gas or produces steam that drives a turbine and generator.
- Efficiency
- Only part of the fuel input becomes electrical output. Energy is also transferred to the surroundings by heating; plant design and operating conditions affect the fraction converted.
- Cost
- Include construction, fuel purchases and transport, operation, maintenance and pollution controls.
- Reliability
- Output can be controlled when fuel and a working plant are available. Fuel supply interruptions and maintenance still matter.
- Environmental impact
- Combustion releases carbon dioxide and other pollutants. Extraction and transport also have effects; these depend on the fuel and controls used.
Non-renewable
Nuclear fuel
Route: Fission changes nuclear stores and supplies energy to heat a working fluid. Steam drives a turbine and generator.
- Efficiency
- A thermal transfer route is involved, so not all the energy supplied from the fuel becomes electrical output. Include energy transferred to the surroundings.
- Cost
- New plants require substantial initial investment and long construction programmes. Include fuel, operation, waste management and eventual decommissioning.
- Reliability
- A suitable operating plant can provide steady electricity. Fuel availability and planned or unplanned shutdowns still affect supply.
- Environmental impact
- Electricity generation has low greenhouse-gas emissions, but radioactive waste needs management. Mining, construction and decommissioning also matter.
Renewable when the biological supply is replenished
Biofuel
Route: Plant or other biological material can be burned, or processed into a fuel. Its chemical store supplies a thermal process that drives a turbine and generator.
- Efficiency
- Compare electrical output with the fuel energy supplied. Thermal and other transfers to the surroundings reduce the fraction reaching the electrical output.
- Cost
- Include growing or collecting the material, processing, transport, storage and the generating plant.
- Reliability
- Stored fuel can be used when needed, but a continuing supply depends on feedstock, land and collection or production arrangements.
- Environmental impact
- Burning releases emissions. Land use can affect food production and biodiversity. Replanting alone does not establish that the whole process is carbon neutral.
Renewable
Wind
Route: Moving air supplies kinetic energy to a rotor. The rotor drives a generator.
- Efficiency
- Compare electrical output with the defined kinetic-energy input from the wind. Not all the wind energy is extracted, and the machinery has further losses.
- Cost
- There is no purchased fuel, but turbines, installation, maintenance and grid connection cost money. Variable output can add storage or backup needs.
- Reliability
- Output depends on wind conditions. A windy annual average does not guarantee the required output at every time.
- Environmental impact
- No fuel is burned during generation. Manufacturing, landscape and noise effects, and possible bird and bat impacts still require consideration.
Renewable
Tides
Route: Tidal streams can drive turbines. A tidal barrage can also use a water-level difference to drive flow through turbines; the turbines drive generators.
- Efficiency
- Identify whether the input is moving-water energy or energy from a water-level difference. Only part reaches the electrical output after the turbine and generator.
- Cost
- Include coastal structures, turbines, installation, maintenance in a marine setting and grid connection.
- Reliability
- Tidal timing is predictable, but output is cyclic. The times of greatest supply need not match the times of greatest demand.
- Environmental impact
- Barrages can alter estuarine conditions, habitats and navigation. Effects depend on the design and site; a tidal-stream turbine is not identical to a barrage.
Renewable
Hydropower
Route: Water released from a high reservoir decreases its gravitational store and moves through turbines connected to generators. Flowing rivers can also supply moving water.
- Efficiency
- Compare electrical output with the available water-energy input. Friction and losses in the turbine and generator transfer some energy to internal stores.
- Cost
- Dams, reservoirs, turbines and grid connections require construction and maintenance. No fuel is burned to supply the water flow.
- Reliability
- A reservoir can control when stored water is released, within water and environmental limits. Rainfall, seasons and drought affect availability.
- Environmental impact
- Dams can change river flow and fish migration; reservoirs may inundate land. Habitat effects and reservoir emissions depend on the site.
Renewable with suitable resource management
Geothermal reservoirs
Route: Hot fluid from underground supplies energy to a turbine process, either directly or through a secondary fluid. A generator produces electricity.
- Efficiency
- The route uses energy from a hot fluid. The fraction converted to electricity depends on the resource temperature and plant design; some energy goes to the surroundings.
- Cost
- Include exploration, drilling, pipes, generating equipment and operation. Suitable underground conditions are essential.
- Reliability
- A suitable, managed reservoir can provide steady electricity, but an adequate resource is not available at every location.
- Environmental impact
- Consider water use and effects of drilling. Emissions depend on the plant: a closed secondary-fluid system differs from one releasing geothermal gases.
Renewable
Solar
Route: Photovoltaic cells convert incoming sunlight directly into an electrical transfer. Solar thermal generation instead heats a fluid and uses a turbine process.
- Efficiency
- For a photovoltaic panel, compare electrical output with the sunlight energy incident on it. Only part becomes electricity; use a defined input when comparing with another technology.
- Cost
- Include panels or thermal equipment, installation, maintenance, grid connection and any required storage. Sunlight itself does not require a fuel purchase.
- Reliability
- Photovoltaic output changes with sunlight, clouds and shading, and is zero at night. Storage or another supply can help meet demand at other times.
- Environmental impact
- No fuel is burned in photovoltaic operation. Manufacturing, materials and disposal still matter. Using existing roofs can reduce additional land requirements.
Use comparable evidence
An efficiency percentage needs a defined input and useful output. Do not compare a thermal plant's fuel-energy input with a solar panel's sunlight input as if the resources and unused portions had identical costs or effects.
Supplied-data comparison
Efficiency is one criterion
Two model fuel-based generators each receive 1000 J from fuel during the stated comparison. A supplies 350 J electrically; B supplies 450 J. These values illustrate the calculation, not a claim about a particular plant.
- A: efficiency = 350 / 1000 = 35%.
- B: efficiency = 450 / 1000 = 45%.
B supplies more electrical energy for this equal fuel-energy input. That result alone does not show which has a lower total cost, fewer environmental effects or a more dependable supply.
For cost, compare construction, fuel, operation, maintenance and any grid, storage or backup needs over an appropriate period. For reliability, ask whether the required output is available when it is needed, including maintenance, weather, season and fuel or water availability.
For environmental impact, include construction and materials, resource extraction, operation, waste and effects on land and ecosystems. "No fuel burned during operation" is a narrower claim than "no environmental impact".
A choice changes when the demand changes
Worked explanation
Electricity for a building with suitable roofs
A building has unshaded roof space, substantial daytime demand and a smaller continuous night demand. It is connected to an electricity grid.
- Match resource to site: photovoltaic panels can use incoming sunlight on the available roofs. Estimate electrical output from the available sunlight and panel performance, rather than assuming all incoming solar energy becomes electricity.
- Match supply to time: daytime generation can contribute to daytime demand. It varies with sunlight and cannot directly supply the night demand, so include stored energy or another supply for that period.
- Compare total cost: include installation, maintenance, grid arrangements and any storage required. Free sunlight does not make the complete system free.
- Compare environmental effects: using existing roofs can avoid additional land occupation, but manufacturing, materials and disposal still matter.
If instead the building must operate independently with a large continuous night demand, the storage and backup requirements change. Evidence about the demand and site can therefore change the choice even though the resource is the same.
Give a reason tied to the situation. Tides are predictable but cyclic; a reservoir can shift some water use in time; wind and sunlight vary. These are different reliability characteristics, not one common label meaning "unreliable".