It is well known that there are three heat transfer modes:
- Conduction
- Convection
- Radiation
Emissivity is the key material surface property related to radiative heat transfer. In radiative heat transfer, a surface exchanges heat with the surroundings via radiation (electromagnetic wave):
- The surface emits radiation to the surroundings (characterized by its emissivity)
- The surface absorbs radiation emitted by the surroundings (characterized by its absorptivity)
- The surface reflects radiation emitted by the surroundings (characterized by its reflectivity)
There are two relationships:
- Emissivity = Absorptivity (Kirchhoff’s law of thermal radiation)
- Absorptivity + Reflectivity = 1 (conservation of energy)
It is easy to calculate the absorptivity and reflectivity, when the emissivity is known.
Most natural surfaces are with high emissivity, around 0.9. Reflective metal surfaces are with low emissivity, around 0.05 or lower. Listed in the table below are the performances of high emissivity and low emissivity surfaces:
| High emissivity surface | Low emissivity surface | |
|---|---|---|
| Radiation emission to surroundings | Emits more radiation | Emits less radiation |
| Absorption/Reflection of radiation from surroundings | Absorbs more radiation Reflects less radiation | Absorbs less radiation Reflects more radiation |
| Overall radiative heat transfer with surroundings | Stonger radiative heat transfer | Weaker radiative heat transfer |
In summary, emissivity is a material surface property characterizing its radiative heat transfer ability. A surface with high emissivity has stronger radiative heat transfer with the surroundings; a surface with low emissivity has weaker radiative heat transfer with the surroundings.
For insulation applications, surfaces with low emissivity are preferred, due to the weaker radiative heat transfer (and therefore better insulation).