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

Abstract

Continuous monitoring of canopy photosynthetic rates in a naturally ventilated greenhouse requires a method of measuring ventilation rates that can record accurate short-term responses throughout the day. It is necessary to clarify the difference between the accuracy and operability of ventilation rate methods. This study evaluated the diurnal-change of ventilation rate measured by the heat balance (HB) and water vapor balance (WVB) methods in the summer and early autumn seasons and compared two methods with the tracer gas (TG) method as a reference. The ventilation rate was determined in a single-span type experimental greenhouse with mature-stage tomato crops under different ventilator configurations to assess the accuracy of the above two methods. The ventilation rates measured via the HB and WVB methods were slightly lower than that measured by the TG method in the greenhouse without crops. However, the ventilation rates obtained using both methods exhibited similar variation trends with time. It is difficult to maintain high concentrations of TG in a greenhouse with a large ventilation opening area. However, it was easy to continuously measure the ventilation rate even in such a greenhouse using the HB and WVB methods. Practically, the WVB system is simpler than the HB method, which utilizes numerous sensors.