Agriculture Reference
In-Depth Information
•
The reduction of greenhouse heat losses
Table 7.4.
Yield results (early and total, in g m
−2
)
of a 126 day-cycle climbing bean crop, sown at the
beginning of November, for different temperature
set points (using hot air heating with propane,
direct combustion). (Source: López
et al
., 2003b.)
is achieved, mainly, by decreasing the
heat exchange surfaces (roof cover and
sidewalls) with the exterior and the
losses per unit area, using proper cover-
ing materials and insulation devices
(thermal screens, double covers) and
windbreaks.
Greenhouse heating can be directed to
Yield (g m
−2
)
b
estimated
energy use
(MJ m
−2
)
Treatment
a
early
Total
•
T-14/T-12
1074a
2869a
180
the aerial part of the crop, to the roots
(soil or substrate) or to both.
Hot air heating is usually performed by
T-14
954a
2863a
250
T 14-12
795a
2767a
180
•
T-12
231b
1952b
120
means of fan coils or using hot air gen-
erators with direct or indirect combus-
tion. The heat distribution takes place
by convection. They have low thermal
inertia and mainly heat the aerial parts
of the crop.
Hot-water heating systems distribute
Control (no
heating
at all)
58b
1123c
-
a
Treatments: T-12, minimum set point 12°C air tempera-
ture; T-14, minimum set point 14°C; T-14/T-12, set point
14°C/12°C, during the vegetative productive stages of the
cycle; T 14-12 (split), set point 14°C during first half of the
night, 12°C during the second half.
b
Numbers in the same column followed by a different
letter (a, b, c) indicate significant differences (
P
= 0.95).
•
the heat through pipes by radiation and
convection. They have more thermal
inertia than hot air systems; they heat
the air and also the soil or substrate.
The systems that use water at high tem-
perature (50-80°C) are more expensive
than the low temperature systems
(40-50°C).
Soil or substrate heating transmits the
in heated greenhouses than in unheated
ones (Meca
et al
., 2003). In low-cost parral-
type greenhouses, their implementation is
difficult, due to the large number of internal
supports, and so their efficiency is lower. In
this case, again consideration of profitabil-
ity will determine whether their use is
worthwhile.
It is necessary to generate information
on the set point values that optimize climate
control (heating, CO
2
, humidity), from the
economic point of view in the different crop
growing conditions.
•
heat by conduction and it normally
uses hot water (at low temperature) or
electrical resistances.
The ideal fuel is natural gas, followed by
•
LPG (butane and propane) and diesel,
but their prices are high. Other fuels (fuel
oil, coal, wood, vegetable waste) may be
of interest, for economic reasons.
Temperature management, when heating
•
is applied, must be done using economic
criteria, because usually the optimum
temperatures for the plant's growth do
not coincide with those at which the
highest profitability for the grower is
achieved. Therefore, temperature man-
agement must be adapted to the specific
growing conditions of each area.
7.5
Summary
•
To limit energy losses in the green-
house, heat losses must be reduced and
natural inputs favoured, and if the lat-
ter are insufficient use heating.
