Green Building Primer

Heat transfer concerns temperature, the amount of thermal energy available, and the flow of heat, the movement of thermal energy from one place to another.

Heat transfer mechanisms can be grouped into 3 broad categories:

• Conduction: Regions of greater molecular kinetic energy will pass their thermal energy to regions with less molecular energy through direct molecular collisions. In the atmosphere heating by conduction is primarily important at the ground, where air warms by directly contacting the surface.


• Convection:
  • Free convection. Heat-induced fluid motion in initially static fluids (liquid or gas). If the static fluid is heated, it loses density and rises. If cooled, it will become dense and sinks. It is a gravity gradient that induces motion through buoyancy.
  
  
  • Forced convection. When the fluid is already in motion, heat conducted into the fluid will be transported away mainly by fluid convection. Pressure gradient forces drive the fluid motion.


• Radiation: Transfer of heat by electromagnetic radiation. Thermal energy is radiated at wavelengths determined by the temperature of the surface-short wavelengths for the sun and long wavelengths for sun-warmed materials such as brick. Bright, shiny materials reflect radiation while dull, black materials absorb it.