Agriculture Reference
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humidity increased. When the wet pad operated crop transpiration rate was nearly
independent of external humidity and ventilation rate.
The fog cooling system consists of spraying very small water droplets from noz-
zles positioned above the crop area. The drops should be small enough to evaporate
fast, and before reaching the foliage (Fig. 10.7b ). Several techniques were proposed
for droplet generation (Arbel et al. 1999 ; Li and Willits 2008 ) that ranged from
twin-fluid nozzles combining compressed air and water; low pressure systems; and
high pressure systems (von Zabeltitz 2011 ).
The advantage of fog cooling systems, as compared to fan and pad, is the pos-
sibility to operate in both forced and natural ventilation greenhouses, and the more
uniform temperature and humidity distributions in the greenhouse. For example,
Arbel et al. ( 2003 ) studied a greenhouse equipped with a forced ventilation sys-
tem combined with fogging. The results revealed that inside the greenhouse an
air temperature and relative humidity of 28 °C and 80 %, respectively, were main-
tained at noon during the summer. Furthermore, the high uniformity of the climat-
ic conditions (the same magnitude of temperature measurements error ± 0 · 5 °C),
within the greenhouse, in the lengthwise (north-south) and vertical directions were
reported. Uniform microclimatic conditions are preferable since they induce uni-
form crop growth, yield and quality.
Temperature, humidity ratio and CO 2 concentration gradients can also devel-
op in fan-ventilated greenhouses without evaporative cooling. Teitel et al. ( 2010 )
measured and modeled horizontal gradients in a greenhouse in which pepper was
grown. The model results showed that the largest gradients are to be expected at
around midday (11:00-12:00), when the intensity of solar radiation is greatest.
Vertical gradients in greenhouses were also investigated by Zhao et al.
(2001) who measured vertical gradients of temperature and humidity in a pepper
greenhouse grown under different ventilation conditions. Their experiments were
conducted in a full-scale, commercial greenhouse, under closed and naturally ven-
tilated conditions. A comparison was made between ventilation by continuous roof
openings only and ventilation by opening both roof and side windows. Two cases
were considered for each of the ventilation modes: (i) the plants in the greenhouse
were mature and big, and (ii) the plants were young and small. With mature plants,
the gradients of temperature and humidity ratio before opening the ventilation win-
dows were considerable and they remained so after the windows were opened (ei-
ther roof only or both roof and side windows). Smaller gradients were observed
with only roof ventilation, than with ventilation via roof and side openings. With
young, small plants the gradients were much smaller than with mature plants and
they could be assumed negligible for either ventilation mode. Both Teitel et al.
( 2010 ) and Zhao et al. (2001) results illustrate the interactive effects between plants
and greenhouse microclimate.
Greenhouse heating is a common approach in northern countries or in regions
susceptible to frost. It is also used in subtropical regions to keep nighttime and even
daytime temperature at the biological optimum. Under certain climatic conditions
greenhouse heating is necessary, however, it is an economic problem due to the
high energy costs. In addition to the direct biological effects of rapid growth and
earliness, heating is important in reducing the relative humidity, thus lowering the
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