How is the efficiency of an energy conversion calculated?

Efficiency equals useful energy output divided by total energy input, multiplied by 100 for a percentage. The same ratio can be calculated with power instead of energy.

An electric motor takes in 500 J and delivers 400 J of useful kinetic energy. Calculate its efficiency and the energy wasted.

The motor is 80% efficient and wastes 100 J. The working is 400 ÷ 500 × 100 = 80%, and 500 − 400 = 100 J, lost as heat and sound.

Why can no machine have an efficiency above 100%?

An efficiency above 100% would mean the machine delivers more energy than it receives. The machine would then be creating energy, which the Law of Conservation of Energy forbids.

How many joules are there in one kilowatt-hour?

One kilowatt-hour equals 3.6 MJ. A power of 1,000 W for 3,600 s gives 3,600,000 J.

CCEA A-Level Environmental Technology 3930

Fossil Fuels

Topic overview Coal, oil and gas in modern society Reserves, lifespan and accessibility Fossil fuel combustion and the IPCC evidence Carbon trading schemes Fossil fuel demand, emerging economies and fuel security Environmental impacts of fossil fuel use

Global Action on Energy Conservation and Climate Change

Topic overview Sustainable development and climate agreements Emissions and energy efficiency targets for 2020 Renewable energy and demand reduction in policy Nuclear power as an alternative

Macrogeneration

Topic overview Conservation of energy and the simple generator Thermal power stations and steam turbines Direct and indirect renewable electricity Electricity transmission through the National Grid Smart grids and interconnectors Combined heat and power

Microgeneration

Topic overview Microgeneration technologies Benefits of home and community microgeneration The heat pump cycle and coefficient of performance Air source and ground source heat pumps

Energy From Wind

Topic overview Horizontal and vertical axis wind turbines Kinetic energy, wind power and the Betz limit Power curves, rated output and energy losses Swept area, blade length and turbine mass Air density, hub height and turbine siting Power control, yawing and survival speed Choosing a location for a commercial wind farm

Energy From the Sun

Topic overview Solar radiation, tracking and concentrating solar power Flat plate and evacuated tube collectors Sizing a domestic solar thermal system Photovoltaic cells and semiconductor wafers Types of photovoltaic module compared Planning regulations and solar incentives Passive solar design in new and existing buildings

Energy From Biomass

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Energy Storage

Topic overview Intermittency and reliability of renewable supply Why renewable energy needs storing Pumped hydro and compressed air energy storage Where energy storage is most beneficial

Plastics for the Future

Topic overview Crude oil as a fuel and a feedstock Refinery gases and the making of polymers Persistent plastic waste and the Great Pacific Garbage Patch Toxic gases from incinerating plastics Biodegradable, photodegradable and compostable plastics Testing photodegradable plastic against polythene Bio-derived polyethylene and a sustainable plastics future

Renewable Energy Investigations

Topic overview Desktop research on renewable energy technologies Choosing an investigation and its variables Apparatus and risk assessment for energy practicals Accuracy and reliability of energy measurements Presenting and analysing energy data Evaluating a method and site-based recommendations

Sustainable Development

Topic overview Population growth and demand for resources The I = PAT equation and the role of technology Ecological footprints, carbon footprints and One Planet Living

Waste Management

Topic overview Landfill dependence and the landfill tax Waste types, priority waste streams and EU directives The waste management hierarchy Recycling at a Materials Recovery Facility Landfill decomposition, methane and leachate Engineered landfill: dry tomb and bioreactor designs

Waste to Energy Technologies

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Land Contamination and Remediation

Topic overview Contaminated land, risk and traditional remediation Bioremediation principles and bioreactor systems Micro-organisms that degrade named pollutants Bioremediation benefits and genetically engineered microbes Phytoremediation of metals and other elements Phytoextraction with Indian mustard and alpine pennycress Biohydrometallurgy and phytomining of ores

Hydrogen Fuel Cells

Topic overview Hydrogen as a fuel: properties and storage Hydrogen production: steam reforming, electrolysis and photocatalysis How a fuel cell works and the PEM cell Alkaline, phosphoric acid, molten carbonate and solid oxide fuel cells Fuel cell applications and the challenges of hydrogen

Transport System Challenges

Topic overview Four key challenges for future transport New vehicle technologies and their infrastructure Bioethanol production and the heat energy of ethanol Biodiesel and the biofuel debate Strategies to reduce transport demand

Energy Performance of Buildings

Topic overview Energy conservation and residents' behaviour Insulation, air tightness and glazing U-values and the heat flow equation The zero carbon homes hierarchy BREEAM, the Code and upgrading existing homes

Emerging Technologies

Topic overview Tidal stream and tidal barrage power Wave energy converters Environmental impacts of wave and tidal energy Smart materials and smart systems Carbon capture and storage Geo-engineering, direct air capture and bio-photovoltaics

Sustainable Urban and Rural Communities

Topic overview Energy and buildings in sustainable urban developments Transport, waste and brownfield sites Water, drainage and urban green space Energy, water and waste for isolated rural dwellings Local food and communication technology in rural areas

The Code for Sustainable Homes

Topic overview Purpose, levels and assessment of the Code Energy, carbon dioxide and water categories Materials, surface water run-off and waste categories Pollution, health and wellbeing, management and ecology categories Measuring the performance of an existing building Performance profiles, improvements and evaluation

Macrogeneration

Conservation of energy and the simple generator

3 things to know
AS 1

Overview

The conservation of energy and wasted energy, efficiency and kilowatt-hour calculations, Sankey diagrams, electromagnetic induction, the parts of an alternating current generator and how a simple wind generator works.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

State three ways of increasing the voltage produced by a generator.

Key term

State the Law of Conservation of Energy.

Key term

If energy is always conserved, why is some energy described as wasted?

Questions from this subtopic

Each one with the model answer underneath.

1
State the Law of Conservation of Energy.
Model answer
Energy cannot be created or destroyed, only transferred or converted from one form to another. The total energy of a closed system stays constant.
2
If energy is always conserved, why is some energy described as wasted?
Model answer
Wasted energy is not destroyed but spread into the surroundings, mostly as low-temperature heat. Energy in that dissipated form can no longer do useful work.
3
How is the efficiency of an energy conversion calculated?
Model answer
Efficiency equals useful energy output divided by total energy input, multiplied by 100 for a percentage. The same ratio can be calculated with power instead of energy.
4
An electric motor takes in 500 J and delivers 400 J of useful kinetic energy. Calculate its efficiency and the energy wasted.
Model answer
The motor is 80% efficient and wastes 100 J. The working is 400 ÷ 500 × 100 = 80%, and 500 − 400 = 100 J, lost as heat and sound.
5
Why can no machine have an efficiency above 100%?
Model answer
An efficiency above 100% would mean the machine delivers more energy than it receives. The machine would then be creating energy, which the Law of Conservation of Energy forbids.
6
How many joules are there in one kilowatt-hour?
Model answer
One kilowatt-hour equals 3.6 MJ. A power of 1,000 W for 3,600 s gives 3,600,000 J.

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Fossil Fuels

Global Action on Energy Conservation and Climate Change

Macrogeneration

Microgeneration

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