Comparison of Three Ventilation Rate Measurement Methods under Different Window Apertures in Winter and Spring

Abstract

Real-time photosynthetic rate monitoring is crucial for managing crop cultivation in greenhouses. Nederhoff and Vegter (1994) accordingly presented a canopy photosynthesis measurement method that enabled the accurate estimation of the greenhouse CO2 balance. The photosynthesis of cultivated plants in a greenhouse is directly related to the ventilation rate, which also affects the air temperature and humidity. Takakura et al. (2017) proposed a method for directly estimating the canopy photosynthetic rate by introducing the ventilation rate, determined from the greenhouse environmental parameters, into the CO2 balance equation. The ventilation is very complex as it is the result of the heat transfer processes of conduction, convection, and radiation occurring in a naturally ventilated greenhouse. Additionally, the ventilation rate has been found to be influenced by the presence of crops as well as the structure and design of the greenhouse, and has been observed to constantly fluctuate throughout the day (Mashonjowa et al., 2010). Therefore, it is necessary to continuously measure the ventilation rate in greenhouses used for cultivation. Various ventilation rate measurement techniques have been studied extensively, such as the tracer gas (TG), heat balance (HB), and water vapor balance (WVB) methods. The TG and HB methods are the most widely adopted for greenhouse ventilation rate measurement (Fernandez and Bailey, 1992). In previous research, the TG method has exhibited highly accurate air exchange rate measurement under leakage conditions (i.e., with the window apertures closed) and with the smallest window apertures (Fernandez and Bailey, 1992; Nederhoff et al., 1985; Baptista et al., 1999; Muñoz et al., 1999). Other studies have shown that the HB method achieves high accuracy with larger window apertures (Fernandez and Bailey, 1992; Baptista et al., 2001). However, the WVB method was found to estimate the ventilation rate more accurately than the TG method with small window apertures (Boulard and Draoui, 1995) and has been applied in a greenhouse used to cultivate mature plants (Harmanto et al., 2006). It is important to note that the TG method is not suitable for long-term, continuous ventilation rate measurement (Sherman, 1990) because it requires that a considerable amount of the TG be present in a greenhouse under cultivation, and SF6, which is often used as a TG, is quite expensive. Meanwhile, the HB technique requires numerous variables to measure the ventilation rate even when it is possible to do so continuously (Baptista et al., 1999). There are also several challenges associated with the WVB method related to the i) direct measurement of the transpiration rate parameter using a lysimetric device (Kittas et al., 2002); ii) overestimation of the ventilation rate at night (Mashonjowa et al., 2010); and iii) evaluation of the error