Agriculture Reference
In-Depth Information
configuration, crop height, velocity measurement height and the travel distance of
air, or fetch) affect the inside air velocity in screenhouses.
Tanny et al. (
2006
) have shown that several important turbulence characteristics
were essentially unchanged from their external values, in a large screenhouse in
which banana was grown (Fig.
10.9
). Tanny et al. (
2010
) and Siqueira et al. (
2012
)
demonstrated that the friction velocity, which is a measure of the turbulent transport
of momentum, was nearly constant with height in the air space between the crop and
the screen. Tanny and Cohen (
2003
) and Tanny et al. (
2009a
) investigated the bound-
ary layer properties of the air flow above the screen, which controls the exchange
of gases and heat between the canopy and atmosphere and showed that screens may
inhibit these exchange processes, including ET, and hence lead to water saving.
Screenhouses of relatively light shading screens reduce the absolute velocity
of the approaching external wind but preserve the wind direction, and the turbu-
lence properties of the boundary layer (Tanny et al.
2006
,
2010
). This contrasts
with insect-proof screenhouses, which induce a more complicated internal air flow
pattern (Möller et al.
2003
) where in part of the screenhouse the air flow direction
was opposite to the external wind. This latter finding was similar to roof ventilated
greenhouses under leeward ventilation (Fig.
10.13
).
Ventilation rate of screenhouses was investigated using the water vapor as a
tracer in two insect-proof screenhouses in which pepper and banana was grown
separately. Tanny et al. (
2003
) have shown that ventilation rate depended on ex-
ternal wind speed, was significantly reduced as compared to open field conditions,
and was non-uniform within the screenhouse, demonstrating a higher ventilation
rate closer to the side walls than the center of the house. Ventilation rate estimates
were in the same order of magnitude for both crops (Tanny et al.
2006
). Teitel and
Wenger (
2010
) have shown the effect of screenhouse roof shape on the ventila-
tion rate, using CFD simulations. Their analysis showed that using pitched roofs
increased the ventilation rate as compared to flat roofs, due to higher penetration of
air into the house.
Modifications of CO
2
in Protected Environments, CO
2
Enrichment and Distribution and the Influence on Plant
Growth and Product Quality
Carbon dioxide (CO
2
) is a crucial component of photosynthesis used for biomass
production, and is indispensable for plant growth. Plants take in CO
2
through the
stomata by diffusion so the concentration of CO
2
in the greenhouse atmosphere
strongly influences CO
2
uptake by the plant. On the other hand, according to Frantz
(
2011
) the CO
2
concentration in the greenhouse can be reduced during the day
to levels as low as 175 ppm (the normal atmospheric CO
2
concentration is about
390 ppm) and this in turn leads to photosynthesis reduction. When greenhouse win-
dows are closed and no ventilation takes place a CO
2
supply from the atmosphere
