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Domestic use of electricity• electrical appliance: convert electrical energy to other

forms of energy.

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• The end-use energy efficiency is defined as the amount of useful energy output per energy input.

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Lighting• kinds of lighting :

1. incandescent lamps,

2. gas discharge lamps, and

3. light emitting diodes (LED).

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• When an atom is transited from an excited state to a lower state, a photon is emitted.

Emission of light

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• An incandescent lamp produces light from a tungsten filament.

• Another type of incandescent lamp is tungsten halogen lamp, which is the improved type.

Incandescent lamp

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Gas discharge lamp

• Fluorescent lamps (FTL) are usually made in the form of a long glass tube.

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Light emitting diode

• Early light emitting diodes (LED) could only produce light in a narrow band of wavelengths.

• LED lighting was made possible from the mid 1990s with the development of white light LEDs.

An application of LEDs Red, blue and green LEDs

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• LED is made of two semiconductor materials joining together to form a junction.

• A forward voltage across the materials causes electron to ‘jump’ across the junction and visible light is emitted.

• The energy efficiency of LEDs is very high.

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• Our sense of colour is due to the response of our eyes to light of different wavelengths.

Response of eye to light of different wavelengths

• Our sense of the colour of a light source depends on– the spectrum of the light

emitted by the source, and– the varying sensitivity of

our eyes to light of different wavelengths.

The sensitivity of three kinds of cone cells to light of different wavelengths

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Illumination measurement

• Luminous flux is the amount of light energy emitted per unit time by a light source, with the sensitivity of human eye taken into account.

• The unit of luminous flux is the lumen (lm).

Light output and efficiency of a light source

A light source of higher luminous flux gives out more light energy per unit time.

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• Luminous efficacy measures how efficient an electrical light source produces light. Mathematically,

• The unit of luminous efficacy is the lumen per watt (lm W –1).

The CFL has a higher efficacy than the incandescent lamp of similar light output as the CFL consumes less electrical power.

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• Illuminance E is defined as the luminous flux Φ falling on a unit area of the surface.

where A is the area of the surface.

• The unit of illuminance is lm m–2 or lux.

Illumination on a surface

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A portable illuminance meter

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• The illuminance of a light source on a surface is

provided that the light source is small in size and the light source emits light evenly in all directions.

• The illuminance E produced by a light source decreases with the square of the distance r.

• This is known as the inverse-square law.

Inverse-square law and Lambert’s cosine law

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• Note that the equation applies only when the light source is small in size (ideally a point source), and the light source emits light evenly in all directions.

In these cases the inverse-square law does not hold.

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• If a light beam is incident at an angle θ on a surface, the illuminance is

which is known as Lambert’s cosine law.

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• Also, illuminance on a small surface at X:

where d is the perpendicular distance of the surface from the source.

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Cooking without a flame

• An electric hotplate makes use of long coiled heating elements to produce heat. It is not very energy efficient.

Electric hotplate Long coil in an electric hotplate

Electric hotplate

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• An induction cooker works by the principle of electromagnetic induction. It has a high energy efficiency.

Induction cooker

Induction cooker

21Working principle of an induction cooker

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• A microwave oven makes use of microwaves to heat up food. It has a high energy efficiency.

Microwave oven

Microwave ovens

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Microwaves cause the water molecules to ‘flip up and down’.

electric field of the microwaves

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Moving heat around

• An air conditioner extracts an amount of heat QC from a cold reservoir and delivers an amount of heat QH to a hot reservoir.

• Work W is provided by electricity. By conservation of energy, we have

Air conditioner

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• The cooling capacity of the air conditioner is the heat removed per unit time:

• The efficiency of an air conditioner may be measured by the coefficient of performance (COP):

• COP can also be expressed in terms of the cooling capacity and input electrical power:

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• An air conditioner consists of a compressor, an evaporator and a condenser.

Refrigerant: a fluid used to transport heat through the system

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• The basic principle of a central air-conditioning system is similar to

a domestic air conditioner.

Central air-conditioning system

The compressor and condenser are located outdoors as a chiller.

The evaporator is located indoors in an AHU.

Heat is exchanged between the chiller and AHU by circulating water.

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Energy Efficiency Labelling Scheme

• The Energy Efficiency Labelling Scheme (EELS) has two types of labels:1. Recognition Type energy labels2. Grading Type energy labels

Energy labels

Recognition Type Grading Type

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Recognition Type energy labels provide no specific information about the appliances.

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• Grading Type energy labels provide information including the energy consumption and efficiency of the appliances.

Voluntary EELS Mandatory EELS

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• Temperature control devices make use of a thermistor or thermocouple to detect temperature change.

Energy-saving devices

A bimetallic strip turns an electric kettle off automatically when water boils.

A thermistor is used in temperature sensing devices such as digital thermometers.

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• Motion control devices make use of infrared radiation to detect the presence or the motion of a human body.

Infrared sensors are installed in a corridor which is not frequently used.

Infrared sensors are used to detect the movements of a person.

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Energy transfer in buildings

• The law of conduction states that the rate of heat transfer (Qc/t) by conduction is given by

where A and d are the area and thickness of a building wall, κ is the thermal conductivity, Thot and Tcold are the temperatures on the hotter and colder sides of the wall respectively.

Heat transfer in buildings

Conduction of heat through a small area of a wall

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• The unit of thermal conductivity is W m–1 K–1.

Thermal conductivities of some common materials at 25°C

Conduction of heat through a small area of a wall

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• The thermal transmittance, also called the U-value, indicates the efficiency of heat transfer by conduction:

• The unit of U-value is W m–2 K–1.

A larger U-value indicates that the interface transfers heat by conduction more efficiently.

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• Overall Thermal Transfer Value (OTTV) is defined as the averaged rate of heat gain per unit area of the building envelope:

Overall Thermal Transfer Value (OTTV)

• The unit of OTTV is W m–2.

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• The OTTV of a building is an indicator used for assessing the energy performance of the building.

• According to the Building (Energy Efficiency) Regulation of Hong Kong, the OTTV of a building tower should not exceed 30 W m–2, while that of a podium should not exceed 70 W m–2.

The heat gain through a building depends on many complicated factors.

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• Heat transfer can be reduced by– using heat insulating building materials

Reducing heat transfer in buildings

Behind the curtain walls of Central Plaza is a layer of mineral wool for heat insulation.

Sprayed insulation(e.g. fibreglass;porous materials) is used in a house.

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• Heat transfer can be reduced by– using heat insulating building materials

Double glazing window and its structure

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• Heat transfer can be reduced by– using shading fins(遮陽裝置 )

Sunshade in the exterior of a building in the Science Park

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• Heat transfer can be reduced by– using solar control windows

Before adhering solar control films

Low-e coating(low-emissivity coating低放射性塗層 ) reflects most infrared radiation while allowing plenty of visible light to pass through.

After adhering solar control films

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• Heat transfer can be reduced by– making use of the orientation and window design

of a building

Large windows facing the south let in more sunlight in winter.

Large windows facing the west are not desirable in the late afternoon.

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Electric vehicles• Most vehicles we see on the streets are powered by internal

combustion engines which use either petrol or diesel as fuel.• These internal combustion engine vehicles (ICEV 內燃機汽車 ) cause pollution in city areas.

Emission from vehicles poses a serious problem to the environment.

Passenger cars use petrol for fuel.

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• Electric vehicles are vehicles propelled by electric motors.

Battery electric vehicles (電動車 )

A BEV looks like an ordinary passenger car.

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• A battery electric vehicle (BEV) uses a rechargeable battery to power the motor.

The schematic diagram of a battery electric vehicle

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• When a petrol car brakes, all its kinetic energy is lost as heat due to friction.

• Electric vehicles have an energy-saving device called a regenerative braking system (再生制動系統 ).

Regenerative braking system

During braking, the electrical energy generated is saved in the battery.

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• In a hybrid electric vehicle (HEV), a relatively small internal combustion engine is used together with an electric motor to propel the vehicle.

Hybrid electric vehicles (混合動力車 )

The schematic diagram of a hybrid electric vehicle

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When high power is needed, the energy stored in the battery is used to provide additional power.

When the power required is low, excessive energy developed by the ICE is stored in the battery.

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• The use of mass transportation helps relieve the traffic congestion, reduce fuel consumption and improve air quality.

Mass transportation

Many passenger cars are needed to transport the same amount of passengers that travel on one double-decker.