In semi-closed greenhouses, the treated air is supplied either in situ, when heat exchangers are used or via an air-distribution system. Thus, in relation to the air treatment and distribution inside the greenhouse, two main semi-closed greenhouse types could be distinguished: (a) semi-closed greenhouses with an air treatment corridor where the air is treated in one side of the greenhouse and after is distributed via perforated tubes usually installed below the crop gutters or below the benches and (b) semi-closed greenhouses with decentralized fan coil units, usually used in greenhouses with cultivation benches for pot plants.
Except for the above (a) and (b) systems, the equipment of a semi-closed greenhouse includes roof vents and a pipe rail and a grow pipe or other heating system. In addition, it may also include a shading and an energy saving screen and a fog system.
2.2.1. Air Treatment Corridor and Air Distribution System
The main components of a semi-closed greenhouse with air treatment corridor and air distribution system are presented in Figure 2
The air enters the greenhouse trough an evaporative cooling pad. In order to prevent insects’ intrusion in the greenhouse, an insect proof screen is placed at the outside segment of the evaporative cooling pad and in the roof vents of the greenhouse. After passing the cooling pad, the air does not directly enter in the crop section, as it would be the case for a conventional greenhouse, but in a separate section/compartment that is named air treatment or air exchange corridor.
The air treatment corridor is the part of the greenhouse where the air is treated before it is distributed inside the greenhouse. It is a separate compartment located on the outer gable end walls of the greenhouse, usually to the direction of the ridge, with length of around 1.5–2.0 m and width equal to the width of the greenhouse. The air is moved by means of fan-blowers installed in the air treatment corridor, connected to the air distribution system i.e., one per polyethylene perforated tube. The air treatment corridor can be also equipped with a roll gable, to control the air flow; heat exchangers, which allow to further regulate the temperature of the air distributed to the greenhouse; a misting system for humidification and a CO2
injection system (e.g., see [17
Since the air is being treated in the air treatment corridor and the properties of the air that flows in the greenhouse are defined in this section, its construction and operating characteristics have a decisive influence on the overall performance of the semi-closed greenhouse. The air flow characteristics may be affected by different parameters such as the air pressure profile of the porosity of the insect proof screen, the pad surface characteristics [19
], the fan speed, and the position of the roll gable. These parameters, together with physical properties of outside air, finally determine the amount of the internal greenhouse air that will enter the air treatment corridor and get mixed with the outside air (that enters the greenhouse passing through the evaporative pad). The efficiency of a semi-closed greenhouse equipped with air treatment corridor in reaching the air temperature, humidity, and CO2
concentration set-point values inside the greenhouse is influenced by three groups of factors related to the air treatment corridor: (a) the design of the air treatment corridor, (b) the type of the systems installed in the air treatment corridor, the overall control and efficiency of the systems installed in the air treatment corridor, and (c) the outside climate.
The air, after its treatment in the treatment corridor, is distributed inside the greenhouse via perforated tubes. The amount of treated air discharging from any opening in a pipe is dependent on the discharge coefficient of the opening and the static pressure difference across the opening. If both of these quantities remain constant along the perforated tube, then uniform discharge will result. In order to achieve this, the cross-sectional area of the pipe has to be so large so that it acts as a plenum chamber. In practice the static pressure difference is varying, due to [20
the friction of the air with the tube wall, which causes a decrease of pressure to the direction of flow and is characterized by the friction factor;
the reduction of the air momentum in the tube as the air is discharged at the openings, which results in an increase of pressure in the tube across each opening. This has been named the ‘diffusion’, ‘inertia’, and ‘static regain’ effect, and is characterized by the static regain coefficient, which is the change in static pressure expressed as a decimal part of the velocity pressure change.
The relative magnitudes of these processes determine whether over any section of tube there is a net increase or decrease in the static pressure. Obviously, the uniformity of the air pressure between different tubes of the same greenhouse compartment is important for the proper operation of the system. However, while this is considered to be the case, it is not always true. The static pressure measured in the center of the cross section of perforated tubes, one meter before their end in several rows in a tomato greenhouse is presented in Figure 3
], indicating that differences can occur in commercial greenhouses.
The performance of the air distribution system is crucial for the overall performance of the semi-closed greenhouse, because it should be able to uniformly distribute the air inside the greenhouse under different ventilation rates. This is the reason why different hole patterns and diameters of the perforated tubes should be designed considering: the length of the tube, the range of the ventilation rate that should be achieved, and the operational characteristics of the fans.
How it works (Figure 4
): as long as the capacity of the coolers and dehumidifiers of the greenhouse is enough to maintain the set point air temperature and relative humidity, the greenhouse is closed and no air is entering the greenhouse. The greenhouse air is entering the air treatment corridor, is dehumidified, heated enriched in CO2
, and then distributed again inside the greenhouse by means of the perforated tubes. The pipe rail and grow pipe heating system of the greenhouse may be used during this period to maintain the set point air temperature inside the greenhouse while the thermal screen may be also used for energy saving (Figure 4
a). In the case where there is a heat exchanger in the air treatment corridor, then this is also used to control the greenhouse air temperature.
In cases where outside air temperature and relative humidity levels are appropriate to lead, after some air treatment, to optimal conditions inside the greenhouse, avoiding the dehumidification of the greenhouse air, the pad gable is opened, and outside air is entering the greenhouse air treatment corridor. Then, after heating, dehumidification, and CO2
enrichment, the air is distributed inside the greenhouse (Figure 4
b). An overpressure is created inside the greenhouse and the roof vents are opened to release the air outside the greenhouse. The above takes place when the cost for the treatment of the outside air is lower than keeping the greenhouse completely closed.
During periods when no heating is needed, a mixture of internal and outside air may be created in the air treatment corridor. Then, after CO2
enrichment, the mixture is distributed inside the greenhouse through the perforated tubes and exits the greenhouse though the roof vents (Figure 4
During warm periods, the evaporative cooling pad is used to cool the outside air before its entrance to the air treatment corridor. Then, after CO2
enrichment, the air is distributed inside the greenhouse through the perforated tubes and exits the greenhouse though the roof vents (Figure 4
It has to be noted that several greenhouse companies have been working on the development of semi-closed greenhouses around the world and that is why the systems developed may have some or many differences from what is presented above. Nevertheless, to the best of the authors’ knowledge, the above operation concept gives an average presentation of the variations available. Although it is not possible to present all the available designs, Figure 5
shows two design types of air treatment corridors in semi-closed greenhouses [21
]. The ventilation unit at Figure 5
a greenhouse has three intake ducts while that presented in Figure 5
b has two intake ducts. This difference makes possible the mixing three or two air streams, by controlling the valves of the inlet air ducts to regulate the air supplied in the greenhouse. In the case of Figure 5
a, one inlet is positioned outside of the greenhouse, allowing outside air to enter the greenhouse when dehumidification or cooling is required. The other two air inlets are positioned inside, below and above the thermal screen. The lower air inlet (below the thermal screen and the anticondensation foil) is used for recirculation, while the upper one (above the thermal screen) can supply dry air for dehumidification, as the air above the screen is less humid due to condensation on the cold greenhouse cover (especially during winter), [16
]. In the case of Figure 5
b, the first inlet is installed inside of the greenhouse below the thermal screen and the second one at the outside, supplying outdoor air. The unit is equipped with an air-to-air heat exchanger, which is used to recover heat from the outgoing indoor airstream and to deliver it to the incoming cold outdoor air airstream. In both systems, the air supplied to the greenhouse will flow along a low temperature heat exchanger before entering the ventilator.