Agriculture Reference
In-Depth Information
external surface, and with the interior air,
through its internal surface. It may collect
energy when water vapour condenses on
the cover and will cool when the condensed
water evaporates.
A relevant fact to consider is the ther-
mal inertia of the greenhouse, which will
depend on what its components are made
of. Therefore, the relationship between the
thermal capacities of the air/cover/plants/
and soil (up to a depth of 20 cm) of the
greenhouse is of the order of 1/3/10/100
(Day and Bailey, 1999), which means that
the air thermal inertia of the greenhouse is
minimal and therefore its temperature
responds quickly to energy balance changes
(as the air is heated), whereas the response
of the soil temperature is slow, since its
thermal inertia is much larger.
Table 5.2.
'Global heat transfer coefficient' (
K
in
W m
−2
°C
−1
) for some greenhouse covering
materials, measured under normalized conditions
(temperatures: exterior:
−
10°C, interior: +20°C,
wind: 4 m s
−
1
). (Source: Nisen and Deltour, 1986.)
Clear
sky
Overcast
sky
Cover
Single
Pe
8.8-9.0
7.1-7.2
eVA
7.8
6.6
PVC
7.6
6.4
Polyester
7.2
6.2
Glass (4 mm)
6.1
5.5
Double
Pe + Pe
6.4
4.2
PC (6 mm)
3.5
3.2
Glass + glass
3.1
2.8
(for
calculations
on
air
renewal
see
Appendix 1 section A.4.5)
For the approximate calculation of the
heating in the night when the requirements
are higher, the following simplified energy
balance equation can be applied (solar radi-
ation being nil) (Montero
et al
., 1998):
Heating = Overall losses + Air renewals
(see Appendix 1)
5.4
Simplified Greenhouse
Energy Balances
If we consider all the greenhouse heat
exchanges overall, by radiation, conduc-
tion and convection, through the cladding,
we may quantify their amounts (per time
unit) as:
Qc
=
K
(
Ti
−
Te
)
Sc
+
m Cp
(
Ti
−
Te
) (5.2)
Where:
Qc
= Heating requirements (W)
m
= Air mass renewed per unit time (kg s
−1
)
Cp
= Air specific heat (J kg
−1
°C
−1
)
Some authors estimate the heating
requirements under conditions of closed
ventilators (i.e. when air renewals are only
by infiltration, and this renewal represents
only 10% of the heating requirement)
(Boodley, 1996), as follows:
Q
=
K
(
Ti
−
Te
)
Sc
(5.1)
Ti
= Interior temperature (°C)
Te
= External temperature (°C)
Q
= Amount of heat exchanged between the
interior and the exterior (W)
Sc
= Cladding surface (m
2
)
K
= Global heat transfer coefficient of a
greenhouse covering material, characteristic
of each covering material (W m
−2
°C
−1
) (see
Table 5.2 and Appendix 1 section A.4.9).
To simplify, it could be assumed that
the solar energy penetrating the greenhouse
is responsible for heating the greenhouse
and for evapotranspiration, neglecting the
energy used for photosynthesis, among other
simplifications, the instantaneous energy
balance would be approximately:
Qc
= 1.1
K
(
Ti
−
Te
)
Sc
(5.3)
5.5
Summary
•
There are three fundamental modes of
energy exchange in the form of heat:
(i) conduction; (ii) radiation; and
(iii) convection (with or without change
of state).
Energy is transported through a medium
Solar radiation − Evapotranspiration
•
+ Heating = Overall losses
at rest by means of conduction. In
greenhouses the heat exchanges by
+ Air renewal
