When a product is submitted to sun drying or in a direct or
mixed-mode solar dryer, the overall energy balance to the product
can be described by Ref. [9] (Phoungchandang and Woods, 2000):
d ðm Cp TÞ
dt
¼ a ApIðtÞ h AsðT TaÞ d ðl mwÞ
dt
As ε s F ðT4 T4a
Þ (1)
The left term of the previous equation corresponds to the rate of
energy gained by the dried product, and the parcels in the right side
are the absorbed radiant energy, the convective heat loss, the
evaporative heat loss and the radiation heat loss, respectively.
I(t) is the radiation flux density at a certain day time, and may
be obtained through meteorological records. When the recording
time interval is very large, it may be shortened through appropriate
meteorological models. A model developed by Charles-
Edwards and Acock (1977) [10] can be adopted, taking into