Agriculture Reference
In-Depth Information
Air Flow and Ventilation
Greenhouses
One significant effect of covering crops is the modification in air movement near
the canopy. The reduced air velocity may have positive or negative effects on crop
production as it reduces physical damage to the foliage and fruit, increases the
thickness of leaf boundary layers and suppresses the turbulence level of the flow,
thus reducing the exchange rates of gases between leaves and their environment
and allows for potential water savings. On the other hand, reduced air velocity may
reduce the ventilation rate which may avoid sufficient supply of CO
2
for plant pho-
tosynthesis and adequate removal of excess heat and water vapor. Hence the design
of any cover should take into account these effects on the crop.
Greenhouses are ventilated either by natural or by forced ventilation systems.
Natural ventilation is generally a reliable, low-cost and maintenance and energy-
efficient method to keep temperature and humidity inside agricultural buildings
within safe and comfortable limits. Natural ventilation can be generated by two
different effects. The first is the buoyancy force (stack effect) which results from
density differences between the internal and external environment due to tempera-
ture and humidity differences. The second is wind-driven flow which may enhance
or hinder the buoyancy-driven flow, depending on the locations and sizes of the
openings and the wind speed and direction (Allard
1998
). Natural ventilation sys-
tems are mostly used in greenhouses located at mild winter climates where climate
control needs are moderate.
Forced ventilation systems in greenhouses are mostly based on mechanical fans
with some optional cooling devices like a wet pad or a fogging system (Linker et al.
2011
). Usually, in such systems, fans suck air out on one side and openings on the
opposite side allow for air flow in. In order not to hinder interaction with wind-
induced natural ventilation, it is preferable to install the fans on the leeward side of
the greenhouse. In forced ventilation systems the air flow rate and hence the venti-
lation rate can be controlled; Teitel et al. (
2004
) showed that controlling fan motor
speed saved electrical energy. Kittas et al. (
2001
) compared between forced and
natural greenhouse ventilation systems and found that forced ventilation increased
significantly the aerodynamic conductance, but did not influence significantly wa-
ter consumption when compared with natural ventilation, because of the negative
feedback between canopy-to-air VPD and stomatal conductance.
Natural ventilation processes in greenhouses were studied using experiments,
modeling and Computational Fluid Dynamics (CFD) simulations. In the low invest-
ment greenhouses and plastic tunnels, natural ventilation is a cheap and dominant
way to manage and control greenhouse climate, such as natural CO
2
enrichment to
secure normal crop growth (Luo et al.
2005
), and water vapor removal to reduce the
risk of pest epidemics (Kofoet and Fink
2007
).
Natural ventilation in greenhouses is applied through either roof or side openings,
or both. Roof openings are usually applied in large greenhouses where ventilation
