The greenhouse for the Phalaenopsis culture is
to protect plants against extreme weather conditions such as heavy rain
and hurricanes and provide a favorite environment for profitable
production of Phalaenopsis.
The greenhouse must have three environmental sections
for Phalaenopsis, for growing, spiking and flowering. The
temperature of the growing section needs to be high to enhance growth
and inhibit premature spiking. The spiking section needs a lower
temperature to stimulate spiking. The flowering section also needs a
lower temperature to control flowering date and quality.
Besides control the environment, the integral control
system could help in watering, fertilization and ventilation, by using
the integral light intensity to reset the temperature and isolating the
insects with screens.
Controlling equipment function
The controlling equipment could be classified
according to their function.
Heating: greenhouse effect, hot-water heating, hot-air heating
Cooling: natural ventilation, mechanical ventilation, evaporative
cooling, and mechanical refrigerator.
Humidifying: misting, spraying on the ground
Dehumidifying: heating, ventilation, dehumidifier
light: shading nets, curtains.
Supplementing light: artificial lights.
The most expensive techniques for environmental
control for Phalaenopsis are for cooling at night and artificial
light supplementation. At night, the only way to cool the inside air is
by refrigerator. The light supplement requires a lot of energy.
The reducing ratio of the solar intensity depends on
the transmittance of cover materials and the shading percentage of
internal and external shading nets. The increasing light intensity is
affected by the amount and capacity of artificial light. The positions
and height of artificial light source also influence light intensity.
The air temperature in the greenhouse could be
increased by hot-air heater and hot-water heaters. The energy efficiency
of hot-water heater is higher than that of hot-air heater. However, the
hot-air heater provides a rapid response in heating. The selection of
the type of heater is affected by the greenhouse region and size.
The cooling capacity of controlled equipments is as
The outside air is induced into greenhouse by the
temperature difference between inside and outside greenhouse air or the
wind-driven force. The inside air temperature is greater than that of
the outside air temperature by at least 5℃.
The outside air is sucked into the greenhouse by the
mechanical pressure difference. The final inside temperature is equal to
the outside air temperature.
The cooling ability is influenced by the efficiency
of cooling equipment and outside-air relative humidity. The lower the
outside-air humidity is the better the cooling ability. The inside-air
temperature could be maintained at 20℃ in Southern California. However,
the inside air temperature should > 25℃ in Florida, because of the high
relative humidity, the evaporative cooling ability is limited.
The cooling ability is affected by the capacity of
equipment and the closeness (isolation) of greenhouse walls.
From the above discussion, the installation of
control equipment needs to consider the climate conditions of the
greenhouse location and the targets temperatures of the control
The environmental control system needs to measure the
microclimate date for comparing and controlling the greenhouse
environment. The collection information includes temperature, relative
humidity, light radiation and wind speed.
1. Temperature measurement
Thermometers used to detect temperature include
glass, bio-metal, metal-resistance and thermistor thermometers. The
selection of these devices needs to consider the accuracy, measuring
range, measuring span and response time.
The glass thermometer is the simplest. However, the
read value cannot connect to the control device. The bio-metal
thermometer is stable and has long-term durability, but inaccurate and
has slow response. The thermistor thermometer has a narrow measurement
range. Considering the greenhouse environment, the metal-resistance type
sensor is adequate for temperature detection and control.
2. Relative humidity measurement
Three types of instruments are often used to measure
the air humidity in greenhouse: dry and wet bulb thermometer (psychrometer),
capacitive type meter and resistance type meter.
The fixed-type of psychrometer was traditionally used
in the greenhouse. The bulb of the wet thermometer is enclosed by a wet
wick so that the free water can be evaporated. The temperature of the
wet thermometer is then reduced because of the absorption of heat from
the air. When the reading values with the dry and wet thermometer reach
equilibration, the relative humidity of air can be found in a table or
by the relative humidity equation.
The accuracy of the psychrometer is affected by
thermometer performance, the wind speed passing the wet bulb and the
available of water. The instrument needs to be used carefully.
The capacitive and resistance relative humidity meter
operates to measure the change in capacitance and resistance of the
sensing element with a change in air humidity. As the moisture in the
air is condensed at the surface of the sensing element, the capacitive
relative humidity meter becomes damaged. So the resistance-type relative
humidity meter is more suitable for subtropical regions.
3. Radiation meter and light sensors
There are three kinds of light meters used in the
A. Radiation meter:
Measures wavelength ranges from 280 to 3000 nm. The
reading value represents the received solar energy on the ground of the
Visible light meter:
This meter is usually called Lux meter. It measures
the wavelength of 400-700nm. The required wavelength for plants is
different by the human eye. This meter is less suitable for the
environmental control of the greenhouse.
Measures wavelength ranged from 380 to 760 nm, is
similar to the plant’s sensitivity. The unit of PAR meter is
It is recommended for Phalaenopsis, especially with the
Wind speed meter
The wind speed measuring device is usually called an
anemometer. There are two types: the hot-wire meter is suitable for
measuring the low wind speed from 0 to 3.0m/s, and the dynamo type is
good for measuring the outside wind speed. This dynamo type meter is
usually installed on the roof of the greenhouse.
Ⅳ. Controlling device
Many commercial devices have been adopted as
environmental control devices for Phalaenopsis greenhouses. From
simple to complex, the device could be as simple as a timer, thermostats
and hygrostates or as complex as a computer system.
The typical simple controller is a timer or
thermostat. For example, a thermostat is installed to control the water
temperature of the heating pipe or to the action of exhaust fans. A
simple controller is cheap and easy to use. However, it cannot treat the
interaction of environmental factors.
This type of controller is still adopted for many
greenhouses. The user makes several temperature settings to control the
sequence of several equipments. The typical application is the control
of fans and pad.
For example, with four temperature settings, T1
(23℃), T2 (26℃), T3 (28℃), and T4
(30℃), if the inside air temperature (Ti) is less than T1,
all equipments is not operating. With Ti > T1, one –third of
the fans begin to operate. With T2 > Ti > T1, the
others third of the fans operate. With T3 > Ti > T2,
the pumps of the pad begin to supply the water. With T1 > Ti
> T1, the residual fans are working.
Besides the motion of the fan and pad, the staging
controller could be used to control the shading nets or curtains, the
hot-water temperature of boilers, or the active motion of fan jets and
The disadvantages of the staging controller are lack
of data recording and it cannot cope with the interaction of internal
A computer control system is complex. The measurement
data and control signals can be recorded and further analyzed. The
interaction of environmental factors such as temperature and relative
humidity can be analyzed by the software in the computer. The optimal
control strategy can be evaluated. This system requires high investment
and a specialist to maintain.