Page 55 - Energize April 2022

TECHNICAL

Absorptivity and emissivity
Convective cooling depends on wind speed and ambient
temperature and cannot be affected by conductor design.
Radiative cooling and solar heating are affected by the solar
absorptivity (α) and thermal emissivity (ε) of the conductor. These
properties can be affected by design or change in condition of the
cable.

Radiated heat loss is estimated by using Equation 2:
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P ad = s [( + 273) − ( + 273) ] W/m (2)
Where: P rad = radiation (heat) loss (W/m), = conductor
diameter, s = emissivity coefficient of conductor surface, =
Stefan Boltzmann constant, and = conductor and ambient
temperature respectively.

Solar heat gain is estimated using Equation 3 Figure 2: Increase of Ampacity with increase in emissivity 2

P olar = ∗ P ∗ ′ / (3) a conductor would be absorbed versus how much would be
reflected, with values ranging from 0,0 for perfect reflection to 1,0
Where P solar is solar heat gain, is absorption coefficient, P is total for complete absorption. Higher values of absorptivity increase the
solar and sky radiated heat flux rate, and is conductor area. heating due to solar radiation.
As a conductor ages and its surface darkens from pollution
Emissivity (ε) and oxidation, both the emissivity and the absorptivity will
The emissivity of a heated surface is the ratio of the radiant increase. 2
energy emitted by that surface to the radiant energy emitted by Increased emissivity can increase line ratings, while increased
an ideal emitter or black body. Emissivity is similar to emission absorptivity will lower line ratings. The relative effects of the
efficiency, with 0,0 representing a surface which emits no two properties depend on the operating temperature of the
radiation and 1,0 a perfect emitter (the so-called ‘blackbody”). conductor, which is dependent on the current carried. While heat
Figure 2 shows how the rating of a Falcon conductor increases emitted increases with conductor temperature, heat absorbed is
with increasing emissivity. independent of temperature and is only dependant on the level of
Aluminium is the metal of choice for overhead transmission solar radiation. Thus the effect of increasing emissivity is greater
conductors due to its low electrical resistance, weight and cost. than the effect of increased absorptivity for higher temperatures.
However, aluminium’s inherently low emissivity causes overhead Table 1 shows the effects of operating temperature on emissivity
conductors to retain heat generated during transmission of power. and absorptivity variations.
The overhead conductor’s available thermal rating is limited by For conductors operating at 90°C, the absorptivity values
aluminium’s surface emissivity. Copper conductors also exhibit a counteract the increase in emissivity, and ratings stay relatively
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low emissivity and suffer from the same problems as aluminium. stable. At higher temperatures where the emissivity dominates the
While emissivity may be the most difficult line rating variable to influence, the ratings diverge.
understand conceptually, for high temperature conductors it has a
very real impact. Weathering of conductors
Field experience has shown that “weathered” conductors of
Absorptivity (α) copper or aluminium are capable of carrying higher currents
The opposite of emissivity is the variable called absorptivity, that than new “shiny” conductors. Tests carried out on weathered
is the ratio of how much of the incident solar radiation striking and new conductors in 1956 confirmed this fact. Weathering
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1590 Falcon ACSS at 90°C 1590 Falcon ACSS at 200°C
α varies from 1 to zero ε varies from 0,24 to 0,9 α varies from 1 to zero ε varies from 0,24 to 0,9
Ampere rating 1300 1311 2397 2205
1366 1427 2423 2449
1428 1510 2449 2621
1504 1588 2513 2781
Variation 204 277 116 576
Table 1: Effect of operating temperature on ampacity change 2

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