2.2. Greenhouse AC

Agricultural plants are living things; they grow, move, eat, and reproduce. They are basic entities for food production and associated industries. Plants are sensitive to temperature and humidity similar to human beings and, therefore, require proper AC.

However, plants' AC phenomena are quite complicated due the photosynthesis and evapotranspiration processes by which they require CO2 and/or O2 from the air. Therefore, plants' AC is quite different in day times (active photosynthesis and evapotranspiration) and night times (active evapotranspiration only) [2]. According to a study [31, 32], plants grow well when its vapor pressure deficit (VPD) is ranging from 0.45 kPa to 1.25 kPa, and ideal yield could be obtained for VPD = 0.80–0.90 kPa. In this regard, influence of greenhouse conditions on plants' VPD has been determined as shown in Figure 2(a). Moreover, the figure provides a comparison of ideal VPDs for different growth stages of tomatoes [33]. It can be noted that plants are very sensitive to air conditions and require dynamic humidity and temperature at different growth stages. Similarly, using the concept of VPD, the ideal AC zones can be formulated for various greenhouse products as shown in Figure 2(b). It can be seen that each agricultural product requires typical thermal conditions, which may or may not be achievable for many AC systems.

Figure 1. Animals' air-conditioning (AC): (a) optimum ventilation rate for livestock building [26, 30]; and (b) optimum air temperature and humidity levels.

Figure 2. Greenhouse AC: (a) influences of greenhouse conditions on plants' vapor pressure deficit (VPD); and (b) optimum air temperature and humidity levels.
