For calculation of cyclic rating factors it is not necessary to accurately know the value of thermal diffusivity and it is generally satisfactory to use the tabulated functions given in the standard IEC 60853 which are based on the value of 5*10-7 m$^2$/s, and correspond roughly to a soil having a thermal resistivity of 1 K.m/W, and a moisture content of around 7% of dry weight. If no data are available on the soil, use a value of diffusivity of 5*10-7 m$^2$/s.
We allow to select the soil diffusivity for each system separately, even when the system is within a backfill area. This feature must be handled with care because for long transient periods it may cause reversive effects due to the given defined overall thermal resistivity of the soil. For very long transient periods, the rating may even be lowered when the load factor is decreased, especially when the chosen soil diffusivity with its corresponding thermal resistivity differs greatly from the defined overall thermal resistivity of the soil.
The values for soil diffusivity corresponding with the thermal resistivities 0.7, 0.9, 1.2 and 2.5 K.m/W as given in the IEC have been adjusted to be on the interpolated trend line for all values.
|Value||Soil diffusivity||Thermal resistivity|
|8e-07||0.80E-6 m²/s||0.5 K.m/W|
|7e-07||0.70E-6 m²/s||0.6 K.m/W|
|6.5e-07||0.65E-6 m²/s||0.7 K.m/W|
|6e-07||0.60E-6 m²/s||0.8 K.m/W|
|5.5e-07||0.55E-6 m²/s||0.9 K.m/W|
|5e-07||0.50E-6 m²/s||1.0 K.m/W|
|4.5e-07||0.45E-6 m²/s||1.2 K.m/W|
|4e-07||0.40E-6 m²/s||1.5 K.m/W|
|3e-07||0.30E-6 m²/s||2.0 K.m/W|
|2.5e-07||0.25E-6 m²/s||2.5 K.m/W|
|2e-07||0.20E-6 m²/s||3.0 K.m/W|