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ABSTRACT
Taiwan is located in the subtropical
region. The existence of high solar energy leads to heat accumulation in
greenhouses in summer caused mal-formation of flowers and increased risk
of disease, reducing the quality and quantity of greenhouse crops.
Phalaenopsis is a member of the orchidaceae. Because of the richness of
cultivars and favorable climate, this orchid has become the most
economically important plant in the flower industry
in Taiwan. To meet the increased demand, the production of
Phalaenopsis needs industrial management. The development of the
greenhouse structures and environment control systems was the key
technique for this orchids industry. The important items for this
advanced technology included: 1) the design of the closed-type
greenhouse for subtropic weather, 2) the development of the longitudinal
variations model in temperature and relative humidity for evaporative
cooling greenhouse, 3) the control strategy of microclimate control, 4)
the installment and layout of the internal ventilation fans, 5) the
design of the cooling house for orchids flower induction, and 6) the
development of the orchids physiological sensing systems. These
techniques had made the significant contribution for the orchids
industry.
I. Introduction
Taiwan is located in the
subtropical region. The existence of high solar energy leads to heat
accumulation in greenhouses in summer caused mal-formation of flowers
and increased risk of disease, reducing the quality and quantity of
greenhouse crops. The average maximum day temperature goes up to nearly
36℃.
The production of many horticultural products in summer becomes
impossible in open fields because of higher temperature and strong solar
radiation.
The greenhouse industry
has been developed for many years in Europe countries. However, the
greenhouse designed for temperate weather can not meet the requirement
of subtropic countries. The most important function of greenhouse is to
cool the inside temperature in summer.
There are four ways to
deal with the heat accumulation problem in greenhouse: 1) shading, 2)
ventilation, 3) evaporative cooling, and 4) mechanical refrigeration (Garzoli,
1989; Giacomell, et al., 1985; Montero and Anton, 1994). The high cost
and heavy energy requirement make the refrigeration technique only be
used for special operation. Shading could decrease the solar energy,
however, the minimum requirement of light intensity for plants limits
the extent of shading rates. The air exchange between the inside and
outside of a greenhouse by natural ventilation is based on the
difference in temperature between the greenhouse and the ambient air or
the difference in pressure between the greenhouse and the outside air.
The inside temperature of the greenhouse is usually 5 - 15℃
higher than the temperature of outside air because of insufficient air
exchange rates. The power rating and number of fans could control
mechanical ventilation rate. The lowest temperature within a greenhouse
controlled by only fans could at most be the same as that of the outside
air. Many orchids would still not grow well in the mechanically
ventilated greenhouse in summer. Considering the efficiency and cost of
the environmental control techniques, evaporative cooling is an adequate
way to reduce the greenhouse temperature lower than that of the ambient
air. However, the design standards for fan and pad systems of temperate
weather did not fit to subtropic weather.
The popular way of
evaporative cooling system adopted by growers is fan and pad system in
Taiwan. As the air passed through the pad by the suction of fans, the
evaporation of the water decreased the air temperature and increased the
relative humidity. The temperature gradient across the greenhouse leads
uneven growth rates and maturation rates for plants.
Microclimate model for
greenhouse is very useful to describe the relationship between
microclimatic conditions and the affecting factors. Kano and Saddle
(1988) reviewed greenhouse models. Most greenhouse models were developed
to consider the heating requirement. All models from literatures assumed
the uniformity of the temperature and humidity inside greenhouse (Bonland
and Baille, 1993). Spatial thermal variations in the air mass were not
considered. The experimental data for validating the model was usually
measured at the central position of the greenhouse. However, non-uniform
distribution of the temperature and humidity could be found in
greenhouse with the operation of ventilation devices. As the solar
radiation increased or the ventilation capacity was lower, the
microclimatic gradient was more significant. Chen (2003) proposed a
greenhouse model that incorporated the effects of ventilation and
evaporating cooling. This model that considered the gradient of air
temperature and humidity had been developed and validated and now served
as a tool to evaluate the performance of controlling equipments for
Taiwan.
II. The
characteristic of the weather in Taiwan
The special
characteristics of the weather in Taiwan were listed as follows：
(1)
Long-term rain season in spring
(2)
High solar radiation, heavy rains,
and stronger wind power in summer.
(3)
Higher night temperatures from the
spring to autumn.
(4)
Cold current in some winter periods
(5)
Higher insect and microorganism
population.
To maintain the demands
for orchids production, the needs of greenhouse structures and main
functions of environmental controlling systems are：
(1)
Resist the heavy wind loads from
typhoon.
(2)
Endured the extreme conditions of
high solar radiation and heavy rainfall for the long-term season.
(3)
Cool the inside temperature at the
optimum conditions during the periods of the higher temperature of
outside air.
(4)
Maintain the uniformity of the
temperature and humidity distribution.
(5)
Heating the inside temperature and
maintain the optimum relative humidity in the winter months.
(6)
Provide the optimum quantity,
quality and lighting period of the irradiance.
(7)
Keep out the insect invasion by
screen nets or other methods.
III.
The stage of greenhouse
introduced from other countries
Since 1960, some foreign
greenhouses were begun to be introduced into Taiwan. The trussed-roof,
steel-constructed, Japanese type greenhouse was the first type to be
built. The characteristics of this greenhouse were the complicated
ridged vents and sidewall vents. This greenhouse usually prepared multi
sidewall vents for the controlling of the natural ventilation.
The first disadvantage
of this greenhouse was the limitation of the air exchange rate. The
outside air was sucked into the greenhouse by the force due to the
temperature difference between outside air and inside air. The maximum
air exchange rate was only 0.35 min .
The air temperature in the greenhouse still higher 5 to 20℃
than that of the outside air. The heat accumulation was very serious in
sunny days.
The other problem of
this greenhouse was the damaged of structure. Owing to the higher
temperature in summer, the tightness of ridge vent and sidewall vent can
not be maintained properly. As the typhoon passed this area, the leakage
of vents became the pass way for heavy wind to enter the greenhouse, and
then damaged the greenhouse.
The venlo-type
greenhouse began to be introduced in 1987. The“two
half glass pane”ventilation
window was the characteristic of this greenhouse. The span and bay was
3.2m ×
4m. Some drawbacks were found and must be modified to cope with the
weather conditions of Taiwan.
(1)
The function of ventilation-windows
The ventilation of venlo-type
greenhouse was operated by the ventilation-windows. The wind speed and
direction influences the air exchange rate of natural ventilation.
However, the instability of these weather conditions in Taiwan limited
the performance of this device. The air exchange rate was too low. The
tightness of this ventilation window was easy to be damaged by the
strong winds.
(2)
The air exchange rates of mechanical
ventilation
The design criteria of
air exchange rates for venlo-type greenhouse was ranged from 0.7 to 0.8
min(ASAE,
1996). This standard is enough to maintain the ventilation rate temple
Europe countries. However, a serious temperature and humidity gradient
was found owing to the inadequateness of the air exchange rate. The
difference of the air temperatures between pad and fan position was
usually 6 to 8℃
in Summer.
(3)
The duration of pad materials.
The growth of the algae
was very quick. This biological materials damaged the structure of pads,
retarded the exhaust air passed the pad materials, and reduced the pad
efficiency.
(4)
The control strategy of the
controlling device
Only the temperature
controlling strategy was introduced. These control systems did not
consider the requirement of the relative humidity and irradiance
control.
IV.
The development of subtropic
greenhouse
To develop the
greenhouse production system, a microclimate model was developed and
validated (Chen, 2003).
The model was validated by the data
measured in a 25.6 m*56 m greenhouse. A close agreement was found
between the measured and predicted values of air temperature and
relative humidity. The predictive performance for temperatures is 2℃
and for relative humidity is 6 %.
From the simulated
results of model, higher ventilation rate and lower solar radiation had
the significant effect to reduce the inside temperature. As the length
of the greenhouse was too long, the temperature and relative humidity at
the back section of the greenhouse could not meet the growth requirement
of crops. This model then was applied to describe the air temperature
and relative humidity variations along the length of a greenhouse with a
fan-pad evaporative cooling system. The model can be applied to evaluate
the effects of ventilation rates and solar radiation on the thermal
microclimate of a greenhouse.
As the microclimate
model of the subtropical greenhouse was developed, several design
criteria were established by sensitive analysis.
1.
The air exchange rate: the air
exchange rate was simulated by the model, then the performance and
numbers of the exhaust fan could be determined.
2.
The area of pads: the required area
of pads can be calculated by the ventilation rate of fans.
3.
The shading ratio of shading nets:
the specification of shading nets was determined by the required shading
ratio that evaluated by the requirement light intensity of orchids and
microclimate model.
4.
Roof without vents: the ridge vent
or ventilation-window can not resist to the subtropical weather. The
ventilation rate of natural ventilation was too low to be applied, the
novel roof of subtropic greenhouse was designed as the close-type.
According to the
microclimate model, the effects of outside atmosphere conditions and the
performance of controlling devices could be evaluated. The novel control
strategies included the setting parameters of solar irradiance,
temperature, and relative humidity was developed. The control strategies
was executed by a single chip micro processed installed in a controller.
V. The
advanced technologies of the environment control systems
Some novel
technologies of environment control system has been developed and
commercialized in recent years. There are:
(1)
The algae prevention technology for
pad materials.
(2)
The adoption of the slide-type
sidewall window.
(3)
The installment and control of the
internal circulating fan.
(4)
The installment of internal energy
saving curtains.
(5)
The uniformity distribution of hot
air heater or hot-water system.
(6)
The installment of silver shading
nets.
(7)
The installment of insect screens.
(8)
The development of the cooling house
for flower inducing.
(9)
The physiological sensing systems
for orchids.
The detail information
of these novel technologies were described as follows：
(1)
The algae prevention technology
The exterior of pad
surfaces was covered by shading nets to reduce the light irradiance.
Before the pumps stopped, all the water existed in the pad was sucked
out to reduce the opportunity for algae development.
(2)
The slide-type sidewall vents
As the electric energy
was interrupted, all exhaust fans were stopped. The heat accumulation in
greenhouse will be a serious problem. The special design of slide-type
sidewall vents had the emergence function to be opened manually, and
then the open area of the sidewall can release the heat accumulation. At
the normal condition, the slide-type windows can maintain in tightness
condition to keep out the outside air.
(3)
The internal circulation fan
The industrial
ventilation fan was adopted to circulate the internal air without
inspire the outside air. With the special criteria for the numbers and
installing angle of these fans, the temperature and relative humidity of
inside air could be uniformly and reduced the opportunity of the disease
development.
(4)
The internal energy saving curtains
The moveable systems
included plastic curtains, insulating curtains, driven mechanism and
control devices. It could help to decrease the required internal space
for heating or cooling. The required energy cost could be reduced.
(5)
The uniformity distribution of
hot-air heater or hot water heater
The hot air exited from
heater was distributed by the perforated-tube air circulation system. To
ensure the uniformity distribution of hot air, the size and distance of
holes in the perforated-tube was studied by the fluid theory. A software
program was developed to calculate the required open area and to decide
the position of holes. The air distribution with the hot water heater
was assisted by the internal fans to ensure the uniformly temperature
distribution.
(6)
The silver shading nets
The novel silver shading
nets was applied to replace the black shading nets. The quality of solar
irradiance could be modified by this shading net.
(7)
The insects screen
The installment of the
insect screen can keep out the insects entering the greenhouse. However,
the air resistive characteristics could reduce the ventilation capacity.
A special design for the screen installment was finished to avoid the
air cladding problem.
(8)
The cooling house for flower
induction.
The cooling ability of
pad and fan system was limited by the wet bulb temperature of outside
air. Because of the higher humidity environment, the internal
temperature was 7-8℃
lower than the temperature of outside air. The cooling ability for air
temperature of greenhouse was usually 27-28℃
in Summer. The required flower inducing temperature was lower than that
of this range. To keep the inside temperature to meet the requirement of
spike inducing induction temperature, a special cooling house was
designed and installed in the greenhouse. A refrigeration tons of
mechanical refrigeration systems can maintain 12-13 m
cooling area.
(9)
The physiological sensing systems.
With the development of
a closed chamber system and several gas sensors, the physiological
functions, such as photosynthesis, respiration, and transpiration of
orchids at various environments could be determined rapidly and
accurately. The optimum growth conditions for orchids could be
determinate by an orchids growth model.
VI.
Conclusion
The subtropical
greenhouse industry was developed since 1990. Owing to the growth of the
orchids industry, the technology of greenhouse industry had significant
progress. With the development of the structure and covering materials,
the orchids could be protected to keep out the treated of extreme
weather. The internal microclimate could be maintained at the optimum
conditions for three phases of orchids. As the standardization of
structure units, the building cost was reduced. These techniques had
made the significant contribution for the orchids industry.
References
1.
Albright, L. D. (1991).
Environmental control for animals and plants. 1st ed. ASAE,
MI. USA.
2.
ASAE. (1996). ASAE Standard. 43rd
ed. EP406.2 OCT95. Heating, ventilating and cooling greenhouse. St.
Joseph, MI: ASAE.
3.
Boulard, T. and A. Baille (1993). A
simple greenhouse climate control model incorporating effects of
ventilation and evaporative cooling. Agricultural and Forestry
Meteorology, 65(2):145-157.
4.
Chen, C. 2003. Prediction of
longitudinal variations in temperature and relative humidity for
evaporative cooling greenhouses. Agricultural Engineering Journal
12:143-164.
5.
Garzoli, K. V. (1989). Cooling of
greenhouse in tropical and subtropical climates. Acta Horticulturae,
257:93-100.
6.
Giacomelli, G. A., M. S. Giniger, A.
E. Krass and D. R. Mears (1985). Improved methods of greenhouse
evaporative cooling. Acta Horticulturae, 174:49-55.
7.
Kano, A. and E. Sadler (1985).
Survey of greenhouse models. J. Agricultural Meteorology, 41(1):75-81.
8.
Montero, J. I. and A. Anton (1994).
Greenhouse cooling during warm periods. Acta Horticulturae, 357:49-61. |
