- In new modern state of the art greenhouses, a computerised plant control system which controls the heat and ventilation of the greenhouse is used. It is likely that there are different requirements for the system throughout the year. Alteration on the computer programme will allow the greenhouse environment to be adjusted which will help the growth of certain plants. Building a state of the art greenhouse with a computerised environmental control system aids in creating the best possible conditions for the plants. There is an array of climate condition which the ventilation control creates. Moreover a computerised environmental control system helps towards greater savings of energy and additionally helps the progress of growth and plant management through a computerised control system.
IMPORTANCE OF GOOD CONTROL
Problem in greenhouse climate control is that temperature changes occur rapidly and are depending on solar radiation, outside temperature, relative humidity and production systems. These changes affect the overall energy efficiency of greenhouse production systems. The aim of every producer is to reduce the energy input per unit of production, and maintain and increase the quality of the final product.
Accurate controlling systems and their coordination can reduce direct energy inputs enclosed in fuel and electricity for heating. Automated controls increase the productivity of workers enabling them to attend to more important tasks. This can improve the overall energy efficiency of greenhouse system. The most important function of good controlling system is the information available to the grower that can be used in terms of better management decisions. Growers report reduction of overall water use as much as 70% if operating with good controlling equipment. In terms of water management significant savings were obtained in fertilizer application and its effectiveness.
More precise control of temperature and relative humidity helps in minimizing plant stress and diseases reducing the need for fungicides and other chemicals.
Advantages of good climatic control can be seen in healthier plants, less susceptible to diseases and insects. In terms of technical systems and equipment, good control prevents over-cycling of equipment and increasing of its operating hours.
REGULATION AND CONTROL SYSTEMS
The general principle of regulation is as follows:
The value of the parameter to be regulated is quantified, by direct measurement or by calculation. For instance, to regulate the opening of the vents as a function of the greenhouse air temperature, the air temperature is measured, which happens to be 25°C.
This measured value is compared with the parameter’s set point, which can be predefined by the user or be the result of a calculation or the application of a pre- established rule. The air temperature set point value, in this example, can be fixed in 23°C, which is lower than the measured value (25°C).
Finally, by means of one or several actuators, one or more pieces of equipment begin to operate, to decrease the difference between the measured and the set point values of the parameter. In this example, the motors opening the vents would start oper- ating, opening the vents to ventilate and approximate the air temperature of the greenhouse to the set point (23°C).
In classical control, the processes to be regulated are considered as input–output systems. It may occur that there is more than one input or output . The inputs can be of two types: (i) control inputs; and (ii) exogenous inputs (or disturbances).
schematizes an input– output system (Fig. 21.3.1a), an input–output system with a disturbance , and the inputs and outputs for the climate control of a greenhouse . In this last case, CO2 enrichment, heat supply and vent opening for ventilation are considered as inputs of the system. The disturbances are the outside temperature, the outside wind direction and velocity, and the external global radiation, humidity and CO2. The outputs are the inside temperature, humidity and CO2. Considering radiation as a distur- bance, even though it is essential for photosynthesis, is due to the fact that it is not a value that can be controlled by the user.
- Scheme of control systems:
- input–output system;
- input–output system with a disturbance
- greenhouse climate control system that details the inputs, outputs and the disturbances (external climate parameters)
There are two methods of regulation: manual and automatic. Manual regulation is not in use for the central boiler heating system and for fertigation, but it is still used for ventilation, shading and humidification. The disadvantages of manual regulation are: (i) that it is not possible without an operator; (ii) it is imperfect if there are no measurement instruments; and (iii) manual switches are imprecise (e.g. to set the duration of humidification, or the opening rate of the vents). However, manual regulation is essential when the climate conditions are exceptional (e.g. intense frost); its possible use must be foreseen.
Automatic regulation can be electromechanical or electronic. In electromechanical regulation, the parameter in question is regulated as a function of the set point value of the parameter. In electronic regulation, the parameter can be regulated as a function of the values of one or several parameters (i.e. night air temperature as a function of the previous day’s radiation).
There are two types of regulation: ‘closed loop’ or ‘open loop’. ‘Closed loop’ regulation takes into account only the average values of the parameter to be regulated. ‘Open loop’ regulation also considers the values of other parameters (e.g. to regulate the air temperature, it also considers the wind velocity or radiation).
Application to climate management
Climate management allows for simultaneously maintaining the set of climate factors (temperature, humidity, CO2) close to pre-established set point values, respecting certain rules (absolute or conditional prohibitions, priorities, time delays) imposed by the user. The climate computer manages the climate. We can distinguish several levels of climate management:
Level 1: (base level) The time scale is very short (about 1 min). It excludes processing of the information. Most of the actual climate control computers employ this level of management nowadays.
Level 2: The time scale is of the order of 1 h or a whole day. The objective is the management of the physiological functions involved in the growth and development of the plants in the short term (photosynthesis, transpiration). It involves the use of models.
Level 3: The time scale is longer than 1 day. This level is the bio-economic optimization and strategic decision support. It allows solutions to be obtained that are close to the economic optimum, in each case. The realization of level 3 is ideal and has not yet been achieved in practice.
Types of controllers
The controllers can be classified depending on the type of regulation, which is the way in which the correction calculation can be made.
There are two main types of regulators: (i) non-progressive controllers that only regulate fixed positions of the controlled device; and (ii) progressive controllers that control any position.
- Non-progressive controllers
In the ‘on/off’ type, the actuator can only take two positions: on or off. Mechanical ventilation, for instance, starts if the interior temperature exceeds 23°C and stops if it decreases below 20°C, pre-fixed set values. A set point value can be used for a ‘dead zone’, detailed later.
The ‘on/off’ mode is usual in dynamic ventilation, CO2 injection, humidification, shading and hot air heating.
A disadvantage of the ‘on/off’ mode is the frequent starts and stops around the set point value. There are three ways to avoid it:
Using a time delay: After the equipment starts, it cannot stop until a certain minimum time has lapsed. It is used, for instance, in air heating. Once the equipment starts (for example, because the air temperature is at 14°C, lower than the fixed set point of 15°C) it has to operate, for instance, for 5 min before stopping, although the set point is reached again before this period ends. Therefore, the air temperature will exceed the set point value, delaying the next start.
Using a dead zone: Around the set point value a dead zone is fixed (x), so the regulated equipment for a certain set point (c) starts when the value (c − x) is reached and stops when (c + x) is exceeded. In this way frequent starts and stops can be avoided. In the previous example, we could fix a set point value (c) at 15°C, and the dead zone (x) at 1°C, so the system would start at 14°C and stop at 16°C.
Using average values: These are used for those parameters that can change a great deal, such as wind velocity or the light. As average values are used as set points, the variability is highly cushioned. For instance, as the wind velocity oscillates a lot because the wind is frequently gusty, the average value of the measurements of a set period is used instead of the last instantaneous air velocity measurement.
In the progressive type of controllers, the operation of the equipment is modulated to maintain the parameter values to be controlled inside the interval of pre-established set point values.
The most common progressive controllers are: (i) the proportional control (P); (ii) the proportional integrated control (PI); and (iii) the proportional integrated derivative control (PID).
In the ventilation process of high temperature or high humidity, when the temperature or the humidity reaches the set point the vent opening is operated. If this opening is operated proportionally to the measured temperature or humidity excesses, in relation to the threshold value, a proportional regulation is applied. Thus, if the difference is small, the vents will open a little and if the difference is very large they will open 100%.
The band of proportionality opening must be defined. If the band is 6°C and the set point temperature is 20°C, the vents will open 50% when the greenhouse temperature is 23°C, and will open 100% when it is 26°C. These band values may be increased or decreased, depending on the external temperature and wind velocity. Proportional regulation is also used in thermosiphon heating systems although in a more complex way.
Proportional Integrated Control (PI)
In a proportional controller (P) the amplitude of the action (the percentage opening of the vents in the previous ventilation example) is proportional to the difference between the average value and the set point value (temperature in the ventilation example).
In a PI the amplitude of the action is proportional to the integral of the differences between the average value and the set point. It eliminates the deviations step by step
In a similar way, the PI control is used in thermo syphon heating systems. If the interior temperature control is not linked to the external climate conditions (temperature and wind velocity), the control operates in feedback mode, so it only acts when the interior temperature changes and induces, after checking the set point, activa- tion of the equipment if applicable. If, on the contrary, the alteration of the external climate parameters influences a priori the control (before it affects the internal temperature) the control operates in ‘feed forward’ mode. For example, a heating system by feed forward control can start or increase the energy supply induced by an increase in the wind velocity (which contributes to the cooling of the greenhouse), although the interior temperature has not yet decreased (which will happen after a certain time). In these cases, the rule or model that relates the wind velocity with the later predictable greenhouse air temperature changes must be prefixed.
The PID control improves the performance of the PI control. In the control process of a certain parameter, the control system starts the actuators or equipment when it must correct the values of the parameter. The values of the parameter are still periodically measured and this information allows, if necessary, to correct the actions. In this way, the system can correct the control actions by means of ‘feedback’ control.
When a disturbance occurs, and its effect on the parameter being controlled is known, immediate action can take place, preventatively. This action is the ‘feed forward’ control, as already detailed.
Frequently, in the management of the process both feedback and feed forward intervene simultaneously. For instance, when the temperature of a greenhouse exceeds the set point, due to a radiation increase, the computer opens the vent (feedback). If later the wind velocity or the external temperature increases, the control system can adjust the vent opening in advance (feed forward).
Another type of control configuration is the cascade configuration that is used in complex processes.
- Graphical representation of the performance of a good ventilation control system depending on the temperature.
- Proportional control (P);
- proportional integrated control (PI).
Selection of the type of automatic control
In the simplest systems, non-progressive controllers (on/off) can be used. To select the type of progressive controller, it is recommended to choose the P type (simple proportional control) whenever possible. If the set points are not to be exceeded, the PI controllers must be used. When the speed of the process requires it, PID must be used.
Once the type of controller has been chosen, the management value of the parameters to control must be selected.
A model is a simplified representation of a system or one of its parts. The greenhouse, the crop and its management constitute a system. Normally, a model is represented by a number of mathematical equations.
There are a large number of model types. A static model is a set of equations that relate several aspects such as, for instance, heat losses, or ventilation, that occur at a time when, essentially, the sys- tem is balanced; thus, a static model can be considered a steady-state model. In these models the equations are based on physical laws, so they are called mechanistic models.
A dynamic model incorporates the time variable. These models are necessary when a process whose response is slow is represented, such as the heating of the soil. They are stochastic models.
The term heuristic or stochastic refers to the mediums used in the resolution of the models, thus, heuristic models are solved by exploration or by means of trial and error, whereas stochastic models are solved using statistical methods.
The feed forward control systems use models, which determine the predictable effects of a disturbance in the regulated process and, preventatively, adjust the set points to this new situation.
In greenhouses, two groups of models can be distinguished: (i) physical models, which focus on the greenhouse microclimate as a function of the external climate; and (ii) physiological models that focus on the plants and their relations with the greenhouse microclimate.
Simulation models can be of any kind, from the simplest to the most complex, and are of great use, if they are well conceived and validated, to simulate several real situations at a low cost. A very simple example, in Mediterranean greenhouses, is the simulation model of transmissivity to solar radiation (Soriano et al., 2004b). This has been of great use in designing new low-cost greenhouse structures that are more efficient in capturing solar radiation at a low cost. At a more complex level, there is a diversity of simulation models, both for energy balance and for crop growth and production.
Use of models
Models have constituted a very useful tool for research of the greenhouse physical environment and the crop’s growth and production (Challa, 2001).
At the beginning, obviously, simple stationary models were used. The use of models in the design and management of control systems has been widespread and very positive, but its application on a commercial scale has been limited and restricted to well-equipped greenhouses (Gary, 1999).
The simplest models, such as the rule of thumb, and scale models, have been used in Mediterranean greenhouses (Soriano et al., 2004b). In these greenhouses, at different levels of complexity and from the practical point of view, the models which have attracted the most interest are those of irrigation control and analysis of yield potential of the crops. Nowadays, in well-equipped greenhouses, the control of the air temperature (that is regulated depending on the available light) is widely used and this is based on a simple model.
However, there are reservations in using models, from the user’s side, and these stem from the need for simple, robust and universal models (Bailey, 1999). In addition, previous work gathering relevant information (for instance, on assimilates distribution) needs to be done prior to the application of a model, and in many cases this information is not available (Gómez et al., 2003).
A primary aspect to be considered in greenhouse climate control models is that the grower’s goal is to maximize the profit. Therefore, and given the normally existing variability in a crop, the grower/user must be the one who finally makes the decisions.
- Computer Climate Management
Controls performed by greenhouse management systems
In heating, the primary goal of the control systems is to adapt the heat supply to the crop requirements. The secondary goal is to dehumidify the air. The main goal of ventilation regulation systems is to avoid the interior air temperature exceeding the fixed threshold. Secondary objectives are to dehumidify and favour the input of CO2. Temperature, humidity and CO2 sensors are needed for their management. They may be limited by the rain or the wind.
The only function of the shading control system is to decrease radiation, normally to reduce the temperature at times of high radiation load.
The supply of CO2 is only practised during the daytime, with intervention of the CO2, radiation and vent-opening sensors. Humidification tends to maintain the hygrometry, using humidity and air temperature sensors.
For the regulation of all these systems (thermal screens, dehumidification systems) several sensors can be used. The simplest control systems use clocks.
Digital control systems
Systems developed during the Second World War enabled analogue technology, which used electrical circuits to obtain inputs (measurement of environmental parameters) and calculate, automatically, outputs, to control mechanisms and equipment. The arrival of digital control systems, which could manage more complex systems at lower cost, has helped digital control systems to supersede analogue control systems.
A digital control system is basically composed of: (i) the controller, that is, the climate computer; (ii) the correction equipment (heating, ventilation, etc.); and (iii) sensors, to measure the different parameters to be regulated.
The climate control computer
The climate control computer controls different processes to regulate, mainly, temperature, humidity, light, CO2 and air circulation. Its functions are to measure different parameters, perform calculations with resident programs, and give activation orders to existing equipment, to maintain the regulated parameters within the desired values (set points).
When sensors are monitored using analogue technology, the signals must be converted into digital information before they can be interpreted by the computer. For this, an analogue–digital converter (ADC) is used. When a sensor generates a measurement signal that is not interpretable by the ADC, an interface that adapts the signal (to make it readable by the ADC) is used. For instance, a solar radiation sensor generates a potential difference, which is proportional to the incoming radiation, in the form of an analogue signal that is converted into a digital signal by the ADC converter in order to be interpreted by the computer.
The activation orders of the computer or output signals, at low tension (24 volts), activate relays that operate the different correction equipment. Until now, different computers performed the fertigation and climate management. Nowadays, the trend is to integrate them, which allows for a better joint management.
Functions of climate control computers
It is impossible to provide a full list of all the possible functions of climate control computers, because each user has specific requirements. The ones detailed below are the minimum required for a well-equipped greenhouse.
The set points are generally different during the day and the night, and can vary even during the same period (day or night). A clock can perform the day/night changes, or changes can be triggered by measurements of the radiation or by calculations of sunrise and sunset (depending on the latitude and date; i.e. the astronomical clock).
In the simplest systems, the user normally fixes a temperature below which the heating system is activated (heating set point), being common to use different set points during the day and the night. In addition, the user indicates a maximum temperature, above which the vents open (ventilation set point). Equally, the system can control water evaporation equipment (fog, pad and fan) or a shading screen. Nowadays, most systems can adjust the set points for several inde- pendent periods or recalculate them periodically.
The temperature set point can be modulated as a function of other parameters, such as radiation, increasing the set points as radiation increases. It is usual to fix the night set point temperature as a function of the radiation of the previous day.
In thermosyphon heating systems, the temperature of the heating pipes is usually controlled independently from that of the air, it being usual to maintain a minimum pipe temperature, to achieve a leaf temperature higher than that of the air with the aim of avoiding Botrytis (induced by water condensation).
The presence of a thermal screen affects the temperature set points. If soil or substrate heating is available, besides air heating, they must be controlled in coordination with each other. The management of the screens can be done with a clock, by radiation or by temperature. The opening of screens must be gradual.
In exceptional cases (heavy frost) the set points can be unreachable due to insufficient capacity of the existing heating system. In these cases survival temperature set points, lower than the usual are used.
Hot-water heating systems have considerable thermal inertia. Therefore, their operation should be scheduled in advance, using a proportional controller. In air heating systems the on/off control is used.
A typical example of the changes in heating and ventilation set point temperatures is represented in in thermosyphon heating systems. The set point temperatures change before sunrise. At sunrise transpiration increases quickly raising the humidity whereas temperature increases more slowly, causing condensation on the plants that favours fungal attacks. To avoid this situation the heating set point must be progressively increased before sunrise. At sunset a similar procedure is followed, for energy-saving purposes, progressively decreasing the heating set point from point C.
In case of contradiction between the ventilation control orders as a function of temperature and humidity, priority control by humidity is usually established.
For control of high temperature by ventilation a proportional controller is normally used. As the ventilation rate is difficult to measure, a temperature set point is used instead, corrected by the wind velocity and the interior–exterior temperature difference. The vent opening percentage is measured or estimated. Knowledge of the wind direction allows for choosing which vents to be opened: that is, the windward vents (facing the wind) or the leeward vents (opposite to the wind). Normally, the leeward vents open first. Maximum and minimum vent openings (in degrees) must be pre-established, in case of storm, frost or rain.
Scheme of the set point temperatures for heating and ventilation management in a climatized greenhouse over 24 h.
(A, Starting point of the set point increase; B, sunrise (final point of the set point increase); C, starting point of the set point decrease (near sunset); D, final point of the set point decrease; dotted lines, possible high temperatures during daylight hours. The set points for heating and ventilation are calculated with regard to sunrise and sunset, when solar radiation begins and finishes, respectively (adapted from Bakker et al., 1995).
- Scheme of the proportional control of the vent opening to ventilate, depending on the internal temperature. The slope of the line depends on the wind velocity and of the internal–external temperature difference (adapted from Bakker et al., 1995).
If the greenhouse has a dehumidification system, which is quite infrequent, it can be activated with a humidity set point, to decrease the humidity.
If the greenhouse does not have a heating system the only way to limit the humidity is to ventilate. When heating is available, the humidity excesses can be avoided by heating and ventilating, although at a high energy cost. The humidity set points are different during the day and the night.
In some crops, such as tomato, this simultaneous heating and ventilation is performed every morning to decrease the RH and avoid condensation on the plants.
The fog or pad and fan systems can be activated when the hygrometry is low, normally during the daytime.
Photoperiodic illumination and darkening screens are managed with clocks. Shading screens are controlled by means of maximum radiation or temperature thresholds. It is usual to maintain openings or slits in the screen, to avoid affecting the ventilation and to maintain the ‘chimney effect’ of ventilation. As the screens are sensitive to the wind, screens cannot be deployed when the wind is above a certain speed.
Artificial lighting, for photosynthesis, is activated by a clock or by temperature set points.
The CO2 set point can be modulated as a function of the temperature and the radiation. In practice, it can be modified depending on the wind velocity and direction and the degree of vent opening.
Thermal screens are deployed during the night and gathered in during the day. The deployment is done when the temperature difference (between the greenhouse and the exterior) exceeds a pre-fixed value.
The opening of the screen is done, at the pre-fixed time or depending of the light level, gradually to avoid a sudden fall of the cold air mass over the crop. When humidity is excessive the screen can slightly open to evacuate the excess humidity.
Shading screens are managed with two light levels, one to deploy and one to retract. When the interior temperature is excessive, an opening must be left to avoid excessive blockage of ventilation.
Darkening screens are controlled by clock and must perfectly block the light in order to effectively shorten the day length.
Alarms are essential to prevent crop and property damage. The control systems usually incorporate a series of security functions, whose thresholds are fixed by the user.
The most important climate alarms are the ones announcing a violation of the minimum and maximum temperature set points. Other alarms relate to the humidity, CO2 and screens set points.
The existence of alarms does not eliminate the need for preventative maintenance (verifying probes, circuits, etc.). The system must be protected against electromagnetic and electric disturbances, especially in zones where storms are common (e.g. To avoid damage by lightning).
Communication with the user
The user can change the program set points and also identify the factors that must be considered to respect the set points, intervention priorities, and intervention delays. Communication with the user allows for remote control of equipment when the appropriate communication interface is available. The most commonly used sys- tems are wireless communication (by radio- frequency), and phone and wire (with conventional cable) communication.
An essential aspect to take into account in communication with the user is the alarm notification in case of serious failure.
All the data registered during the day can be stored. In addition, the system may have the usual performance of a personal computer, providing it with the required elements (data visualization screens, data downloading, printer, etc.).
- Integrated control
- Integrated management of fertigation and climate control is already used in some modern commercial greenhouses.
- In the future, in order to optimize control in well-equipped greenhouses it will be necessary to also integrate plant growth management and economic aspects of production with the climate control and fertigation, so that the new generation of growers will have to be experts in interpreting and using technical information in decision making (Papadopoulos and Hao, 1997a, b). Before this happens, it will be necessary to generate the required information about plant growth and other non-documented aspects of the local conditions.