Energy efficiency by reducing heat transfer [Print] Print page
Reducing heat transfer is one way of improving energy efficiency. Sometimes we want to keep things cool. In summer we use air conditioners to keep our homes and offices cool and comfortable. Electrical energy is saved if heat entering our rooms is minimized by good insulation. Similarly, energy is also saved when refrigerator walls are well-insulated. In winter, we wear thick clothes to keep ourselves warm. Heat loss from our body to the environment is reduced by our clothing. Vacuum flasks and thermal cookers also reduce heat loss to keep their contents hot.

Fig. 1 We use a thermal cup to keep drinks hot. Fig. 2 A thermal cooker is effectively a large thermal cup.
In the processes we mentioned above, notice that energy flows from a region of higher temperature to a region of lower temperature. The flow of energy resulting from a difference in temperature between two objects (or two regions) is called heat. There is continuous heat transfer from the hotter object to the colder object (or regions) until the two objects reach the same final temperature.

Fig. 3 (a) Two bodies A and B are of different temperatures, the temperature of A is higher than that of B. (b) When they are in contact, heat is transferred from A to B. (c) Heat transfer will stop when both A and B reach the final temperature.
In order to reduce heat transfer, we must first understand the processes of heat transfer and the factors that affect its rate. There are three main processes of heat transfer, conduction, convection and radiation. We will describe these processes below and see how heat loss can be minimized in daily applications.

Conduction

When you put a steel pot above a stove, the inside of the pot and the food gets hot. Heat is transferred through the metal bottom of the pot to the inside. This is an example of heat transfer by conduction.

When a hot object is in contact with a cold object, or there is a temperature difference between different regions of an object, heat will transfer from the hot side to the cold side through conduction.

Fig. 4 As the heated particles collide with their neighbours, energy is transferred from the hot side to the cold side of the object by conduction.
Conduction is related to the movement of particles inside matter. The particles (these may be atoms, molecules or ions) in matter are kept in constant random motion by the energy they possess. Temperature is an indication of the average kinetic energy of this random motion. The higher the temperature, the higher the average kinetic energy. For example, the particles on the hotter side of an object vibrate more rapidly than those on the colder side. As a rapidly vibrating particle collides with its less rapidly vibrating neighbour, part of its kinetic energy is transferred to the neighbouring particle. Through the continuous collisions of these neighbouring particles, energy is transferred from the hotter side of an object to the colder side. This is the microscopic explanation of conduction.

Solids conduct heat better than liquids, which are in turn better conductors than gases. The particles in a solid are most tightly bond and their positions are more or less fixed relative to each other (Fig.5a). The force between adjacent particles is strong, making heat transfer by collision the most efficient. The particles in liquid can move around within it, which means that the force between particles is not as strong (Fig.5b). Thus liquids are usually poor conductors of heat. In a gas, the particles are far apart, making energy transfer by collision very inefficient (Fig.5c). Thus gases, like air, are very poor conductors of heat.

Fig. 5 (a) In a solid, particles are fixed relative to each other. (b) In a liquid, particles are free to move. (c) In a gas, particles are far apart and moving at high speeds.
The speed of heat transfer by conduction is different for different materials. Metals like copper and aluminium are good conductors of heat because they contain many free electrons. The free electrons are not bond to particular metal atoms but can move freely around them. Free electrons are effective in transferring heat by collision. Non-metals like glass, wood and polystyrene are usually poor conductors of heat (good insulators) because they transfer energy through the collision of atoms or molecules, not through the collision of free electrons.

Fig. 6 The ability of some common materials to conduct heat

Convection

When enjoying a hotpot meal, have you ever noticed that the food keeps moving up and down in the water even when the water is not boiling? This shows that there is bulk movement of water when it is being heated. The rising of smoke above a burning candle is also due to the bulk movement of heated air. This bulk movement is known as convection. It is an effective means of heat transfer.

Convection is the process of heat transfer by the bulk movement of a fluid, i.e., liquids or gases.

Fig. 7 When a liquid is heated from below, there is bulk movement of the liquid, forming a convection current.
Convection cannot occur in a solid because the position of particles in solids are fixed relative to each other.

When convection occurs, the part of the fluid near the heat source gets hot, the density of the hot fluid decreases and this part rises above the rest of the body of fluid. Colder fluid nearby comes to take the original place of the hot fluid, forming a convection current as shown in Fig. 7.

Heat convection Watch videoWatch videoDownload video: 12.7mb
Domestic heaters are usually placed near the ground. This facilitates the rising of warmed air and sets up a convection current that circulates around the whole room. On the other hand, air conditioners are usually placed near the ceiling. This facilitates the downward flow of cooled air to set up a convection current.

Fig. 8 Air conditioners are usually placed near the ceiling. Fig. 9 Domestic heaters are usually placed on the floor.
Wind can be produced by air convection. Coastal wind, for example, is caused by convection due to the temperature difference between the air above the sea and the land.

Radiation

Fig. 10 We receive great amount of radiation energy from the sun.

Fig. 11 In an electromagnetic wave, the oscillating electric field and magnetic field are perpendicular to each other.
Do you ever wonder how the sun’s energy reaches the Earth? The space between the sun and the Earth is a vacuum. So the sun’s energy cannot be transferred to the Earth by either conduction or convection because both processes require need matter as a medium. The sun’s energy is actually transferred to the Earth through a process called radiation.

Radiation is the process of heat transfer by electromagnetic waves.

Electromagnetic waves are oscillatory electric and magnetic fields which can travel in a vacuum. The sun emits many different kinds of electromagnetic radiation. Visible light is only part of it. Radiation carries energy from the sun to the Earth.

Hot objects give out energy through radiation in the form of electromagnetic waves. Warm objects like the human body mainly radiate infrared radiation. This is why infrared sensitive cameras can “see” people clearly even in the dark.

The appearance of the surface of an object determines the rate of emission and absorption of electromagnetic waves for that object. Black or dark-coloured objects emit and absorb electromagnetic waves at higher rates than shiny, white or light-coloured objects

Fig. 12 The space shuttle and the space suit of the astronaut are white. Photo courtesy: NASA/JPL Fig. 13 The surface of this airship is shiny.
The Earth also loses energy to space by emitting infrared radiation. The atmosphere contains small percentages of gases called greenhouse gases like water vapour, carbon dioxide, nitrous oxide, methane etc. that absorb part of this radiation and reduce the rate of heat loss, helping to warm the Earth. This is called the greenhouse effect. Although most greenhouse gases occur naturally in the atmosphere, scientists believe that human activities are increasing the levels of these gases. For example, carbon dioxide in the atmosphere has increased mainly due to the burning of fossil fuels and deforestation. Nitrous oxide and methane level are also increasing due to agricultural, industrial and other activities.

An increase of about 0.5 oC has been observed in the average global temperature since the late 19th century. The melting of glaciers and the decrease of snow cover in the northern hemisphere seem to confirm this global warming phenomenon.

Scientists generally agree that the rising levels of greenhouse gases in the atmosphere are contributing to the observed global warming phenomenon. Watch the animation below to learn more:

Flash animation: Radiation energy transfer from sun to Earth and global warming
The issues of global warming and greenhouse effect are controversial,. To what extend the increased level of greenhouse gases is responsible for the observed global warming phenomenon is difficult to determine. References [1] and [2] may help you learn more on these issues.

Summary

Before proceeding to discuss applications which achieve energy efficiency by reducing heat transfer, let’s click on the following animation for a summary of the microscopic processes that occur in conduction, convection and radiation.

Flash animation: Heat transfer processes

Energy transfer and energy efficiency

In everyday life, there are many things that have to be kept hot or cold. This is achieved through minimizing the three ways of energy transfer. If the rate of energy transfer is low, less energy is required to keep something hot or cold.

Vacuum flask

A vacuum flask is designed to prevent energy transfer between the content inside and its surroundings outside. With re-heating or cooling unnecessary, a vacuum flask is a convenient energy saving container.

Fig. 14 The internal structure of a vacuum flask.

Fig. 15 A thermal cooker.
Do you know how a vacuum flask is made? Take a look at the photo on the right (Fig.14). It has a double glass shell with a vacuum in between. The shell is coated with a layer of silvery reflective material. The outer casing is made of metal or plastic.

The vacuum prevents any energy transfer through conduction and convection. The silver coating reflects much of the radiation and thus radiation energy transfer is also minimized. So the contents inside a vacuum flask can be kept at a more or less constant temperature for a long time. But the vacuum flask cannot completely stop heat transfer; it can only reduce the rate of the transfer. Moreover, there is still some heat loss through the stopper at the top. Conduction also occurs along the glass shell where the inner and outer walls are connected, bypassing the vacuum.

Thermal cooker

Chinese people like slow cooking processes (e.g. stewing). People may think that when food is stewed it absorbs heat at a slow rate. In fact this is not the case. When stewing, food is kept at a constant high temperature for a long time. Since much of the heat supplied by the stove is actually lost to its surroundings, a continuous supply of heat is required.

Thermal cookers provide a convenient means to stew food in an energy efficient way. A thermal cooker consists of two main parts: a steel inner pot and an outer vacuum flask. The food is first heated as usual in the inner pot. The inner pot is then put inside the outer insulating container which is designed to reduce heat transfer as much as possible. Well-insulated from the surroundings, food inside the inner pot can be kept at a high temperature for a long time without the use of energy. A typical thermal cooker can keep the food inside above 70 oC for over 8 hours.

The following activity will help you understand more about the working principle of a thermal cooker.

Activity: Energy efficiency of a thermal cooker

Refrigerator

A refrigerator has many features which lower conduction, radiation and convection energy transfer to reduce the consumption of electrical energy.

Fig. 16 Flexible seals at the edge of a refrigerator door
Refrigerators usually have a light coloured outer surface to reflect radiation and so reduce heat entering the refrigeration compartment. The edges of a refrigerator’s doors have flexible seals to prevent cold air inside from mixing with hot air outside, thus reducing convection. The flexible seals are made of good insulating material to further reduce energy transfer through conduction. The thick walls and doors of a refrigerator are also well-insulated to reduce heat gain by conduction.

Clothes and blankets

Fig. 17 Furry clothes help keep us warm in winter.
Without suitable clothing and blankets in cold weather, we would need to consume a great deal more energy for indoor heating to keep us warm and comfortable.

Clothes and blankets are usually made of materials such as cotton, wool and down that can trap air. Since air is a poor conductor of heat, trapped air reduces heat loss through conduction from our body. The materials themselves are also poor heat conductors.