Heat Transfer

Heat transfer is the transfer of thermal energy from a hotter mass to a cooler mass. When an object is at a different temperature to its surroundings or another object, then heat transfers from the hot object to the cooler one and continues until they are at the same temperature. Heat transfer is slowed down with the use of insulating materials, however it can never be stopped completely.

There are three modes of heat transfer, conduction, radiation and convection. In solar systems normally all three modes are present in one form or another.

Conduction

Conduction is defined as the transfer of thermal energy between neighboring molecules in a substance due to a temperature gradient. It always takes place from a region of higher temperature to a region of lower temperature, and attempts to equalise temperature differences. While conduction can take place in solids, liquids and gases, solids are most easily understood.

Heat causes the molecules in a substance to vibrate faster, these faster molecules start to interact with their neighbours. The neighbouring molecules start to vibrate faster and so on, and gradually heat energy is transferred.

Radiation (Thermal)

Thermal radiation is electromagnetic radiation emitted from a material due to the heat of the material.

The radiation is madeup of a spread of wavelenghts, the hotter the object, the shorter and more intense the wavelenght. A good example of thermal radiation is a person near a raging bonfire will feel the radiated heat of the fire, even if the surrounding air is very cold. Or in very cold, but bright sunny conditions, it can often feel quite warm.

Convection

Heat convection is where a heat fluid travels, and while because the (heated) fluid itself is moving, heat transfer takes place. Generally there are two types of convection.

Forced Convection
A fan blows air (or fluid) past the hot object, the air is heated as it comes in contact with the object and is blown / transferred into the room.

Natural Convection
This is where the heat itself causes the air (fluid) motion (e.g. hot air rises through bouyancy forces), a good example is the slight draft than can be felt with a hot "radiator". in fact most heat transfer from house radiators is via convection air currents.