A CO2 Control System for a Greenhouse with a High Ventilation Rate

A simple CO2 control system was developed for a greenhouse with a high natural or forced ventilation rate. This CO2 control system maintains the difference in CO2 concentration between inside and outside the greenhouse, ∆C, during daytime being small (ca. 5 μmol mol), by supplying a variable amount of CO2 gas with time. Results showed that this system worked satisfactorily, while the CO2 concentration outside the greenhouse varied with time in a range between 350 and 450 μmol mol with an average of 380 μmol mol. In this system, the CO2 supply rate was determined based on the time course of ∆C only, and does not consider any physical or physiological mechanisms of CO2 balance in the greenhouse. Theoretically, the CO2 supply rate is equal to the net photosynthetic rate of greenhouse crops (Pn), if both rates of CO2 emitted from the substrate/floor and CO2 exchange between inside and outside the greenhouse are negligibly small. Hence, this CO2 control system will be useful not only for monitoring and increasing the Pn in a greenhouse with a high ventilation rate, but also as a diagnosis tool for monitoring the physiological status of greenhouse crops. INTRODUCTION A significant depression of CO2 concentration (up to ca. 100 μmol mol) often occurs on a fine day in a greenhouse with crops, even when its natural or forced ventilation rate is high (Yabuki and Imazu, 1965; Ito, 1970; Hanan, 1998). This depression of CO2 concentration reduces the net photosynthetic rate of the greenhouse crops (Pn). This occurs because Pn decreases almost linearly with a decrease in the CO2 concentration in a range roughly between 100 and 500 μmol mol under wide ranges of photosynthetic photon flux and air temperature. To prevent a delay in crop growth due to the depression of CO2 concentration inside the greenhouse, CO2 control is necessary. A loss of CO2 released to the outside, resulting in a decrease in the economic profit of the CO2 control, is proportional to the difference in CO2 concentration between inside (Cin) and outside (Cout), ∆C, and the greenhouse ventilation rate. The loss of CO2 can be minimized by maintaining Cin at the same level as Cout, regardless of the greenhouse ventilation rate. The CO2 control with Cin ≈ Cout for a greenhouse with open ventilators is widely used in European countries, such as UK and The Netherlands, during summer. Extension of this CO2 control method is still limited in moderate and hot climate regions, especially in Asian countries such as Japan, Korea, and China, because of the high costs of investment in the CO2 control system and liquid CO2 gas (170 Japanese Yen/kg or 1.5 USD/kg). The objective of this study was to extend the CO2 control method of Cin ≈ Cout for a greenhouse with a high ventilation rate in moderate and hot climate regions. To realize this CO2 control method, a simple CO2 control system was developed and tested in a small experimental greenhouse. CONCEPT OF THE PROPOSED CO2 CONTROL METHOD AND ITS BENEFIT The goal of the abovementioned CO2 control method is to increase Pn with minimal and efficient use of CO2 in a greenhouse located in moderate and hot climate regions during daytime. During CO2 control, the consumption rate of CO2 can be given as the sum of Pn and the rate of CO2 loss. The rate of CO2 loss is given as the ventilation rate multiplied by Proc. IC on Greensys Eds.: G. van Straten et al. Acta Hort. 691, ISHS 2005 650 ∆C. Generally, in a greenhouse located in moderate and hot climate regions, the ventilation rate is high due to the necessity of lowering air temperature by ventilation. Hence, to decrease the consumption rate of CO2 and simultaneous increase in Pn, Cin should be maintained at almost the same level as Cout (or ∆C ≈ 0). Although an increase in Cin by supplying CO2 is relatively small (up to ca. 100 μmol mol), Pn increases significantly. For example, when CO2 gas was supplied to the greenhouse in order to increase Cin of 300 μmol mol up to Cout of 380 μmol mol, and the CO2 compensation point of the crops is 100 μmol mol, the expected increase in Pn is 40% (Fig. 1). In this case, the CO2 gas supplied to the greenhouse never escapes from the inside, and all of the CO2 gas supplied was fixed by the greenhouse crops through photosynthesis, regardless of the ventilation rate. With this CO2 control method, the CO2 supply rate is equal to Pn, if both rates of CO2 emitted from the substrate/floor and the CO2 exchange between inside and outside the greenhouse are negligibly small compared with Pn. In other words, by using this CO2 control system, Pn can be estimated simultaneously based on the CO2 supply rate using no CO2 balance model. By means of this estimation of Pn, the effect of environmental conditions (e.g., air temperature, shortwave radiation flux, etc.) on Pn can be evaluated readily (Matthews et al., 1987). Estimation of Pn can also be useful for monitoring or controlling crop growth. It should be noted that this CO2 control system can be used not only for monitoring and increasing Pn, but also as a simultaneous diagnosis tool for the physiological status of greenhouse crops. This CO2 control system can be operated independent of other environmental control devices, such as heating, ventilation, shading, and watering devices, because only ∆C or Cin and Cout are measured and used for estimating the CO2 supply rate. This operation method will contribute to simplifying the structure of the CO2 control system, and thereby, to reducing the investment required for the CO2 control system. It is reported that the CO2 enrichment at a high concentration (generally, more than 1000 μmol mol) causes physiological disorders in several plant species. For example, the stomatal function of eggplants is remarkably sensitive to high CO2 concentration, resulting in less translocation of calcium and boron (Hanan, 1998). However, this CO2 control method will not be confronted by these problems, because Cin is equal to Cout or lower than the concentration reported to cause these physiological disorders. A PROTOTYPE CO2 CONTROL SYSTEM The prototype CO2 control system developed in this study consists of an infra-red type CO2 analyzer, gas sampling lines made of Teflon, three solenoid valves, a liquid CO2 gas container with a pressure regulator, and a float-type flow controller with a needle valve (Fig. 2). The CO2 control system monitored Cin and Cout alternatively every 1 min, and determined the CO2 supply rate. The CO2 supply rate was regulated with the flow controller. Measurements and controls of Cin were made with a data controller. The CO2 control system was tested in a naturally ventilated greenhouse with roof and side ventilators (4.8 m x 5.5 m x 3.4 m) at Chiba University, Japan (lat. 35o50’, long. 139o50’). In the present experiment, only the roof ventilator was used to simulate a large-scale greenhouse, where the ventilation from the roof ventilator was dominant compared with that from the side ventilator. In the greenhouse, tomato (Lycopersicon esculentum Mill. “House Momotaro”) plants, each with about 13 true leaves, were grown in pots at a planting density of 5.8 m. Irrigation was made with a nutrient solution once or twice a day by using an automatic irrigation system. In the first trial (May 11, 2004), liquid CO2 was not supplied to the greenhouse to investigate how much Cin was decreased compared with Cout. Although the roof ventilator of the greenhouse was fully opened, Cin reached a concentration of 334 μmol mol when Cout was 369 μmol mol. In this condition, expected increase in Pn with the CO2 control system was 13% (= ((369 5) 100)/(334 100)). The value of ∆C decreased with time and became stable at ca. 13 μmol mol between 11:30 and 14:30 (Fig. 3). This decrease in ∆C was because the air temperature inside the greenhouse was beyond an optimum range of Pn