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Environmental control for Phalaenopsis culture

 
Chiachung Chen, Biosystems Engineering Laboratory
National ChungHsing University
 
 

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.

I. Controlling equipment function

The controlling equipment could be classified according to their function.

A. Temperature

1. Heating: greenhouse effect, hot-water heating, hot-air heating

2. Cooling: natural ventilation, mechanical ventilation, evaporative cooling, and mechanical refrigerator.

B. Humidity

1. Humidifying: misting, spraying on the ground

2. Dehumidifying: heating, ventilation, dehumidifier

C. Light intensity

1.  Reducing light: shading nets, curtains.

2. 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.

 

Ⅱ. Environmental control 

A. Light control

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.

B. Temperature

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 follows:

1. Natural ventilation:

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℃.

2. Mechanical ventilation:

The outside air is sucked into the greenhouse by the mechanical pressure difference. The final inside temperature is equal to the outside air temperature.

3. Evaporative cooling:

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.

4. Mechanical refrigerator

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 environment.

 

Ⅲ. Measurement equipment

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 greenhouse.

A. Radiation meter:

Measures wavelength ranges from 280 to 3000 nm. The reading value represents the received solar energy on the ground of the greenhouse.

B. 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.

C. PAR-meter:

Measures wavelength ranged from 380 to 760 nm, is similar to the plant’s sensitivity. The unit of PAR meter is μmol/m2-sec. It is recommended for Phalaenopsis, especially with the artificial lights.

4. 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.

1. Simple controller

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.

2.  Staging controller

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 shutters.

The disadvantages of the staging controller are lack of data recording and it cannot cope with the interaction of internal microclimates.

 

3. Computer controller

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.