Cooling by radiant absorption is where the surfaces of the cooling unit have the ability to emit and absorb radiation. The ability to omit and/or absorb radiation is measured as an epsilon value, whereby 1.0 is the highest possible and 0.001 the lowest. Typically, unpainted aluminum has an epsilon value of 0.2 and matt white powder-coated metal 0.95 and matt black powder-coated metal 0.97. The surface area and surface temperature are also critically important.

Both traditional Chilled Ceilings and X-Wing® Radiant Passive cooling units have large surface areas of powder-coated metal (0.95 / 0.97 epsilon value) that are cooler than the surfaces of the room and occupants being cooled. Whereas fin coil batteries used for Convective Only Passive Chilled Beams have unpainted aluminum fins (0.2 epsilon value).

Radiant Equation

Surface Area x Surface Temperature x 5.67 x epsilon value

Note: 5.67 is a constant value, known as the Stephan Boltzmann constant.

Radiant Heating is by omitting heat energy, Radiant Cooling is by absorbing heat energy as the surface with the highest temperature interacts with opposing surfaces of a lower temperature, giving its higher energy to the surface with the lower energy to try to become in equilibrium. This is another reason as to why fin coil batteries have little by way of radiant cooling (or heating) as all-aluminum fins largest surface areas are facing one another (not facing the room space) and all fins are already the same surface temperature, hence no radiant exchange possibilities even if the aluminum had a higher epsilon value if painted.

Frequently Asked Questions