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Advanced Technology of Subtropic Greenhouse for Phalaenopsis

 
 

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. Thetwo half glass paneventilation 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.

 

About the author:

Professor Chiachung Chen, Ph. D.

Biosystems Engineering Laboratory
National ChungHsing University

250 KuoKuang Rd.,

Taichung, Taiwan 40227

Tel: 886-4-22857562, Fax: 886-4-22857135

Web site: http://bse.nchu.edu.tw

Email: ccchen@dragon.nchu.edu.tw