Agriculture Reference
In-Depth Information
greenhouse) increases with the height of the window opening and the wind speed,
and decreases with the solar radiation.
An important issue in natural ventilation systems is the estimation of the ventila-
tion rate, which is responsible for the adequate removal of excess heat and water
vapor and supply of CO
2
(Boulard
2006
). Boulard and Baille (
1995
) derived an
expression for the ventilation rate, based on the Bernoulli equation, which depends
on both the buoyancy and wind effects. Kittas et al. (
1997
) investigated the rela-
tive contribution of the two factors to greenhouse natural ventilation and found
that under the conditions of their experiments, the wind effect predominated on the
buoyancy effect when
u
∆> where
u
is external wind speed and
∆
T is the
temperature difference between inside and outside of the greenhouse.
In naturally ventilated greenhouses the control of CO
2
enrichment is largely re-
lated to the climatic conditions. For example, greenhouse CO
2
enrichment in warm
climates is restricted by the need to ventilate, leading some growers to intermit-
tent enrichment, where enrichment and ventilation alternate several times an hour.
This strategy relies on the heat and CO
2
capacity of the system, characterized by a
heating time constant of the order of 10 min, during which period ventilation may
be suspended (Ioslovich et al.
1995
). The latter authors have demonstrated that for
slowly changing weather, the optimal CO
2
enrichment is basically not intermittent,
but rather approximately stable. Seginer (
1990
) considered the combined effect of
CO
2
enrichment and shading and concluded that under desert conditions, where
ventilation is mandatory during most daytime hours, CO
2
enrichment was effective
only during the morning, before ventilation rate had to be increased in order to cool
the crop.
/
1,
Screenhouses and Screen Covers
Another type of structures which become popular among growers in mild winter
climates is the screenhouse or screen cover (Tanny
2013
). These structures are
much cheaper than greenhouses and under certain climatic conditions may provide
the adequate protection for the crop. Since the screenhouse is a semi-open structure,
air flow and ventilation are strongly influenced by the external climatic conditions.
A major effect of porous screens is to increase the resistance to airflow which de-
creases the internal mean air velocity.
In an attempt to characterize the effect of horizontal screen covers or screenhous-
es on air velocity, several field measurements established relationships between in-
side air velocity and outside wind speed (e.g., Waggoner et al.
1959
; Tanny et al.
2006
; Siqueira et al. 2012; Tanny
2013
). In few cases, the vertical gradient of air
velocity was also considered (Allen
1975
; Tanny et al.
2010
). Obviously, denser
screens with lower porosity would induce higher resistance to air flow and hence
will diminish inside air velocity more than screens with higher porosity. However,
Tanny (
2013
) suggested that several additional factors (e.g. screen deployment
