Topic H: Energy and Use of Energy (Elective)

This elective applies the physics of heat and electricity to the practical problem of supplying and using energy. You will revisit domestic electricity and power, study how heat moves through and out of buildings, learn how to measure and improve efficiency, and compare the renewable and non-renewable sources that power modern life along with their environmental costs.

Electricity at home and electrical power

Homes in Hong Kong are supplied with alternating current at about 220 volts. Each appliance is rated with a power in watts, and the electrical power it uses is P = I V, the rate at which it converts electrical energy. The energy consumed over time is E = P t, and electricity bills measure this in kilowatt-hours, where one kilowatt-hour is the energy used by a one kilowatt appliance running for one hour. To find the cost, multiply the energy in kilowatt-hours by the unit price. Knowing an appliance's power rating allows you to estimate its running cost and to choose the correct fuse, which should be rated just above the appliance's normal operating current.

Heat transfer in buildings

A building loses or gains heat through its roof, walls, windows and floor by conduction, convection and radiation. Conduction carries heat through solid walls and glass, convection moves heat as air circulates and through gaps and draughts, and radiation passes through windows as infra-red and sunlight. In a hot climate like Hong Kong the main concern is keeping heat out to reduce air-conditioning loads, while in cold climates the aim is to keep heat in. Understanding which mechanism dominates in each part of a building tells engineers where the largest energy savings can be made, and why windows, roofs and poorly sealed openings are often the weakest points.

Insulation and reducing heat loss

Insulation reduces unwanted heat transfer. Materials that trap pockets of still air, such as fibreglass, foam and double glazing, are good insulators because air is a poor conductor and the trapped air cannot circulate to carry heat by convection. Cavity walls and loft insulation cut conduction and convection through the building fabric, while reflective foils and light-coloured or shiny surfaces reduce radiative heat gain by reflecting infra-red. Double-glazed windows place a layer of air or low-pressure gas between two panes to slow conduction. By comparing the cost of installing insulation with the money saved on heating or cooling, a payback time can be calculated to judge whether the measure is worthwhile.

Energy efficiency and the Sankey diagram

Efficiency is the fraction of input energy that is converted into useful output, efficiency = useful energy output over total energy input, often expressed as a percentage. No device is perfectly efficient because some energy is always wasted, usually as heat to the surroundings. A Sankey diagram shows this clearly: the width of each arrow is proportional to the energy it carries, with the useful output flowing on and the wasted energy branching away. For example, an old filament lamp converts most input energy to heat and only a little to light, so it is inefficient, while an LED lamp converts a much larger fraction to light. Improving efficiency saves money and reduces environmental impact.

Energy conservation in daily life

Conserving energy means using less of it to achieve the same result, which lowers cost and reduces the burning of fuels. Practical measures include switching to efficient LED lighting and high-rating appliances, turning off devices instead of leaving them on standby, setting air conditioners to sensible temperatures, using natural light and ventilation, and improving building insulation. Energy-efficiency labels help consumers compare appliances, and smart meters help households track and reduce their use. On a larger scale, reducing peak demand eases the load on power stations. These everyday choices, multiplied across a city, make a significant difference to total energy use and to greenhouse gas emissions.

Non-renewable energy sources

Non-renewable sources cannot be replaced within a human lifetime once used. Fossil fuels (coal, oil and natural gas) are burned to release heat that raises steam to drive turbines and generators, and they currently supply most of the world's electricity because they are energy-dense and relatively cheap. However, burning them releases carbon dioxide, the main greenhouse gas driving climate change, along with pollutants that harm health. Nuclear fission of uranium releases large amounts of energy with no carbon dioxide, but produces radioactive waste with long half-lives and carries the risk of serious accidents. The limited supply and environmental costs of non-renewable sources are major reasons for developing alternatives.

Renewable energy sources

Renewable sources are naturally replenished and do not run out on a human timescale. Solar cells convert sunlight directly into electricity and solar panels heat water; wind turbines use moving air to turn generators; hydroelectric schemes use falling water; and tidal and wave power harness the sea. Most renewables produce little or no carbon dioxide during operation, which makes them attractive for tackling climate change. Their drawbacks include intermittency (the Sun does not always shine and the wind does not always blow), the large land or sea area needed, high initial costs, and dependence on geography. A practical energy strategy usually combines several sources and improved efficiency to balance reliability, cost and environmental impact.