Chapter 4

Greenhouse Environmental Control Systems

Types of Environmental Control Equipment

Greenhouse controls typically feature specialized programs for the maintenance and management of horticultural crops. They are optimized for the control of highly dynamic horticultural environments and specialized equipment control strategies employed to enhance plant growth processes. This may include: assimilation and photoperiodic lighting; complex irrigation and nutrient control; mist, fog, or pad & fan evaporative cooling; precision humidity management; shade and thermal curtain systems; carbon dioxide supplementation; diurnal or multi day climate settings; heat storage; tank management; and a range of other crop specific control requirements.

Thermostats and Timers

Thermostats allow manual adjustment of on/off settings. Timers are used as on/off switches. These simple devices are low cost and provide limited control. A typical greenhouse zone may require 3 or more individual thermostats to control heating and cooling functions, plus timers for irrigation and lighting control. Additional relays are often necessary to interconnect fans and louvers and other devices that must work together. A simple zone will generally require 2 or 3 thermostats (1 for heat and 1 or 2 for cooling stages).

Stage Controllers

The name “stage” controller has an origin from the ability to stage the control in multiple stages. Stage controllers bring two benefits to basic temperature control: automatic sequence of operation, and remote sensing. A single stage controller takes the place of several thermostats. Stage controls use a single sensor element to control both heating and cooling functions in a greenhouse zone. That sensor can be located among the plants while the controller can be located more conveniently and safely outside the plant environment. These controllers divide the actions of the greenhouse heating and cooling equipment into stages, called a sequence of operation. A typical example would be the following six-stage temperature control system. Half of the unit heaters are turned on at a temperature setting of 60 degrees F. (16°C) and in the event that these cannot supply the required heat, the remainder heaters are turned on at 58 degrees F. (14°C).

Computer Zone Controllers

Computer zone controllers bring the benefits of computerization to the stage controller concept, providing greater control flexibility and programmability, and the ability to provide improved accuracy and better equipment coordination. With computers it is possible to integrate a range of devices for measuring few parameters at the same time. Their accuracy is quite good. They are generally designed to provide control and coordination of temperature in a single zone, although some companies offer units that are expandable to 2 or 3 zones.

Integrated Computer Control (ICC)

Integrated computer controls combine the capability of several stage or computer zone controllers and various other individual control devices such as timers, into a single, integrated computer system. Integrated computer controls can provide coordinated control of virtually all greenhouse environment functions, including heating, cooling, irrigation, fertilization, boiler control, lighting, carbon dioxide management, alarms, and much more. The ventilation, for example, is not only controlled by the temperature, but is at the same time influenced by the humidity, outside temperature, solar radiation, wind speed) wind direction, and rain. The integrated computer controls the ventilation, heating, cooling, air circulation) carbon dioxide dosing, boilers, pump units, and irrigation valves based on multiple settings entered by the grower. In this system, the on/off control has been advanced to a modulating control that can apply varying proportions of any input. The end result is more stable and accurate climate (within 1°F [0.5°C] of the desired setting), energy conservation, and improved crop quality.

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