Continuous Measurement of Greenhouse Ventilation Rate in Summer and Autumn via Heat and Water Vapor Balance Methods

One of the methods of increasing tomato production and quality in a greenhouse is to manage the CO2 concentration at or above the ambient level for supporting photosynthesis. CO2 fertilization enables plants to assimilate CO2 gas with high efficiency (Kuroyanagi et al., 2014) and increases fruit yield (Kimball and Mitchell, 1979; Yelle et al., 1990). Attention has also been focused on a method of applying CO2 to a similar level as the outside air in the greenhouse during the daytime in summer or autumn when the windows are sufficiently opened for temperature control (Ohyama et al., 2005). Furthermore, the continuous measurement of the greenhouse ventilation rate throughout the year allows for the long-term direct monitoring of the canopy photosynthetic rate and CO2 use efficiency using the CO2 balance method (Nederhoff and Vegter, 1994). The ventilation rate is a crucial parameter for heat and gas exchanges in a greenhouse. Ventilation regulates the air temperature and humidity in the greenhouse. Additionally, it influences CO2 concentration, which affects the canopy photosynthetic rate. Various techniques have been used to measure and predict the ventilation rate, such as the tracer gas (TG) method (Boulard and Draoui, 1995; Papadakis et al., 1996; Baptista et al., 1999), heat balance (HB) method (Fernandez and Bailey, 1992; Demrati et al., 2001; Harmanto et al., 2006a), and water vapor balance (WVB) method (Boulard and Draoui, 1995; Harmanto et al., 2006a, 2006b). The TG method has been widely used in greenhouse experiments. The ventilation rate measurement by the TG method is highly reliable in leakage and low ventilation conditions for different types of greenhouse and ventilation configurations (Nederhoff et al., 1985; Baptista et al., 1999; Muñoz et al., 1999; Katsoulas et al., 2006). The TG method exhibits good agreement with an infrared gas analyzer (IRGA), as mentioned by Nederhoff et al. (1985), and with a theoretical model based on pressure difference (Baptista et al., 1999) and wind pressure model approaches (Muñoz et al., 1999). However, in the summer season with the maximum ventilation opening area, the TG method experiences numerous disadvantages in large-scale greenhouses (Demrati et al., 2001). A large amount of CO2 gas must be supplied to maintain the CO2 concentration in a greenhouse higher than outside air for a large window aperture. Moreover, this method is expensive, and the long-term continuous measurement of the ventilation rate is extremely difficult under plant cultivation, where CO2 gas is absorbed. Hence, it may not be possible to use this technique for the continuous monitoring of the ventilation rate in summer and autumn season when windows are fully