Energy Efficiency by Reducing Heat Transfer  Printable view

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 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.

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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.

The speed of heat transfer by conduction is different in different materials. Solids usually conduct heat better than liquids, which are in turn better than gases. Metals like copper and aluminum are good conductors of heat because they contain a lot of 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. Non-metals like glass, wood and polystyrene are usually poor conductors of heat (good insulators).

Fig. 3   The ability of some common materials to conduct heat