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R. Saxena et al.
5
10
15
20
25
30
35
40
45
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Temperature (°C)
Mean Max.
Temp. (°C)
Mean Min.
Temp. (°C)
Chennai
Months
Fig. 13.5 Average monthly maximum and minimum temperature for Chennai
13.9 Thermal Calculations
In order to calculate the temperature difference, which would be driving the heat
transfer to take place, the sol-air temperature needs to be defined. It is an effective
outer surface temperature which considers both radiation and convection (Duffie and
Beckman 2013). The sol air temperature (T sol ) is as follows:
T sol = T a +
α I − ε R
h o
(13.4)
where, ‘T a ’ is ambient temperature, ‘α’ and ‘ε’ are absorptivity and emissivity of
surface.
= Long wavelength radiative exchange from surface to sky.
ε. ≈ 60 W/m
2 for horizontal and 0 for vertical surfaces.
‘I’ is the global solar insolation and h o is coefficient of heat transfer.
h o is evaluated from Eq. 13.5 (McAdams (1954) relation):
h o = 5.7 + 3.8 s for 0 < s < 5 m/s
(13.5)
where, ‘s’ is wind speed.
Heat transfer (Q) is calculated using:
Q = U e f A(T sol − T wi )
(13.6)
where, ‘U ef ’ is overall heat transfer coefficient and ‘A’ is exposed area.