Agriculture Reference
In-Depth Information
and below the optimal temperature. Above
this, the growth rate decreases, so the ther-
mal time must be corrected (Villalobos
et al
., 2002).
For instance, López (2003) quantified
the base temperature for Mediterranean
greenhouse bean crops at 6°C, establishing
a thermal integral from sowing until the
beginning of harvest of 757 degree days.
The value of the base temperature may
vary, depending on the cultivar used
(Mauromicale
et al
., 1988).
DIF*
= Amount of direct solar radiation
converted into diffuse radiation inside the
greenhouse
A.4
Chapter 5
A.4.1
Conduction
The heat transmitted by conduction (Fig. 5.1)
between two parallel plane isothermal sur-
faces (1 and 2), in a perpendicular direction
to both surfaces (Urban, 1997a) is:
q
c
=
l
×
S
(
T
2
−
T
1
)/
d
(5.3)
A.3
Chapter 4
where:
q
c
= Heat transmitted by conduction through
the surface
S
per time unit (W)
T
1
= Lower temperature (°C)
T
2
= Higher temperature (°C)
d
= Distance between the two plane surfaces
1 and 2 (m)
l
= Thermal conductivity coefficient
(W m
−1
°C
−1
)
S
= Area (m
2
)
The heat flux increases with the ther-
mal conductivity and with the temperature
differences and decreases with the width of
the material.
inside a greenhouse
Plastic films that have a diffusing (haze)
effect on direct solar radiation change the
proportions of direct and diffuse radiation
inside the greenhouse, in relation to that
existing in the open field. The higher pro-
portion of diffuse light is of interest for two
reasons: (i) to increase the uniformity of the
spatial distribution of radiation inside the
greenhouse; and (ii) to improve the light-
use efficiency, because diffuse light pene-
trates the inside of the canopy better due to
its non-directional nature.
The diffuse radiation inside a green-
house can be estimated knowing the exter-
nal radiation conditions and two parameters:
(i) the diffuse radiation enrichment coeffi-
cient (
DIF
i
/
DIF
o
); and (ii) the conversion fac-
tor (
b
) from direct to diffuse radiation (Baille
et al
., 2003), which depends mainly on the
covering
Table 5.3.
Thermal conductivity coefficient (
l
) of
some common materials (Wacquant, 2000).
Material
l
(W m
−1
°C
−1
)
Steel
45-70
Aluminium
200
Polyethylene
0.3
Glass
0.9
Wood
0.2
material
and
the
greenhouse
Humid clay soil
1.2
characteristics.
(
)
DIF
−×
t
DIF
*
DIF
IDIR
A.4.2
Convection without phase change
i
dif
o
(4.1)
b
=
=
IDIR
o
o
The heat transmitted from a surface to a
moving fluid can be estimated as (Montero
et al
., 1998):
q
conv
=
h
c
×
S
(
T
S
−
T
F
)
where:
DIF
i
= Diffuse solar radiation inside the
greenhouse
DIF
o
= External diffuse solar radiation
t
dif
= Transmissivity to diffuse radiation
(equals the global transmissivity in com-
pletely cloudy conditions)
IDIR
o
= External direct solar radiation
(5.4)
where:
h
c
= Heat transfer convection coefficient
(W m
−2
°C
−1
)
S
= Surface crossed by the heat flux (m
2
)
