Agriculture Reference
In-Depth Information
calculating the capacity of the heating sys-
tem to be installed.
To analyse the energy balance of the
'greenhouse complex' it is possible to divide
it into different subsets, for instance: the
soil, the crop, the interior air volume and
the cover. Then the energy balances of each
one of them can be analysed independently,
which is easier, integrating them later.
In practice, simplifications are used
which do not consider some elements of the
energy balances that have less influence
overall; this allows for sufficient approxi-
mation of the energy balance.
The soil surface absorbs part of the solar
energy, exchanges energy by IR radiation
with the crop canopy, with the heating pipes
and with the walls, cover and other ele-
ments of the greenhouse, and by convection
with the greenhouse air. The soil surface is
cooled by water evaporation and by exchang-
ing energy with the deeper layers of the soil,
cooling or heating itself depending on the
season.
The vegetation absorbs an important
part of the solar energy that it receives and
exchanges energy by IR radiation with the
soil surface, with the heating pipes, with
the wall, cover and other elements of the
greenhouse, and by convection, with the
greenhouse air. The vegetation, in addition,
loses energy by transpiration and may, even-
tually, gain energy by condensation.
The heating pipes absorb some solar
energy and, if the boiler is on, they may
receive energy from the hot water. The pipes
exchange heat mainly by convection with
the air and by radiation with the vegetation,
the soil and the cover.
The interior air exchanges energy
mostly by convection with all the green-
house surfaces: soil, plants, heating pipes
and cover. The renewal of the interior air by
external air, normally drier and cooler, pro-
duces a decrease in its enthalpy (energy
content).
The cover absorbs a small amount of
the received solar radiation, exchanging
energy by IR radiation towards the interior
of the greenhouse and towards the exterior.
In addition, it exchanges energy by convec-
tion with the external air, through its
Table 5.1.
Characteristics of absorptivity to solar
radiation and emissivity of several surfaces (at
13°C), both expressed per unit (adapted from
Aldrich and Bartok, 1994).
Absorptivity to
solar radiation
emissivity
(at 13°C)
Surface
Concrete
0.60
0.88
Red brick
0.55
0.92
Glass
0.03
0.90
White paint
0.35
0.95
Dry soil
0.78
0.90
Wet soil
0.90
0.95
Aluminium
Commercial
0.32
0.10
Painted in white
0.20
0.91
Painted in black
0.96
0.88
Galvanized steel
Commercial
0.80
0.28
Painted in white
0.34
0.90
bees and bumblebees are sensitive, may
induce problems in their mobility, affecting
flower pollination (see Chapter 4).
The heat exchanges by radiation are
essential in greenhouses. The surfaces of a
greenhouse exchange heat by radiation
between them. The greenhouses are heated
absorbing an important part of solar radia-
tion and get cooler radiating energy towards
the sky.
The heating pipes, besides heating the
plants directly by convection, also do it
directly by radiation.
5.2
Heat Exchanges by Air Renewal
in the Greenhouse
The interior air of the greenhouse is usu-
ally warmer and more humid than the
outside air. The renewal of the interior
air with external air involves a decrease
of its energy content (enthalpy; see
Appendix 1).
5.3 Heat Exchanges in the
The calculation of the energy balance of
a greenhouse is useful, especially when
