Good temperature control is vital for a productive high tunnel greenhouse, and this in turn means that the ventilation system needs to be effective and efficient. Regardless of whether a grower uses fans or “natural ventilation,” it is important to make sure that the system is designed properly and working well. Here are a few pointers that will help you improve your system.
The concept of cooling a greenhouse with thermal buoyancy and wind goes back to the beginning of controlled environment. All greenhouses built prior to the 1950's had some form of vents or louvers that were opened to allow the excess heat to escape and cooler outside air to enter.
When polyethylene was developed, with large sheets covering the whole roof, placing vents on the roof proved difficult. Engineers then came up with the concept of using fans that draw outside air through louvers in one endwall and exhaust it out the opposite end. With thermostatic control, this was, and still is, the accepted method for cooling many structures where positive air movement is needed.
Growers with high tunnels have found that roll-up sides work well for warm season natural ventilation. Both manual and motorized systems are available. A location with good summer breezes and plenty of space between houses is needed. It helps to have greenhouses designed with a vertical sidewall up to the height of the attachment rail to reduce the amount of rain that can drip in.
Greenhouses with roof and sidewall vents operate on the principle that heat is removed by a pressure difference created by wind and temperature gradients. Wind plays the major role. In a well designed greenhouse, a wind speed of 2-3 miles/hour provides 80% or more of the maximum possible ventilation. Wind passing over the roof creates a vacuum and sucks the heated air out the vent. If sidewall vents are open, cool replacement air enters and drops to the floor level. If the sidewall vents are closed, cool air enters the bottom of the roof vent and the heated are escapes from the top of the vent.
Buoyancy, the effect of warm, moist air rising, also aids ventilation. Heavy cool air near the floor becomes lighter as it is heated and rises towards the roof. On cool days the large temperature difference creates excellent air exchange. On hot days the temperature difference can be only 5 or 10 degrees and the buoyancy effect is almost nonexistent. The trend toward taller greenhouses has helped in because it allows the hot air to be above the plants. Horizontal air flow fans should be shut off to avoid destratifying the warm air.
Roof and side vents on conventional greenhouses need to be large enough to get good air movement. The American Society of Agricultural Engineers recommends that the combined sidewall vent area should equal the combined ridge vent area and each should be 15 to 20% of the floor area. The best orientation for the greenhouse is to have the normal summer wind direction blow over the ridge so that it creates a vacuum on the leeward ridge vent. For summer ventilation, the windward side vent opening should equal the leeward ridge vent opening.
Until the development of the open-roof greenhouse concept, cooling large gutter-connected structures was difficult especially in southern climates. Area for sidewall vents is usually limited, and passing cool outside air and warm inside air through the roof vents usually results in uneven cooling.
Open-roof greenhouses are structures that allow the entire roof to be retracted – leaving the plants open to the sky. These are available from most major manufacturers and have been growing in popularity. Most designs use standard vent hardware and controls to operate the roof system. Some have roof panels that are hinged at the gutter and open upward. Others have panels that are hinged at the ridge and one gutter and slide sideways on Teflon bearings. The size of the opening can be controlled from 0% to about 75%. Most designs use rubber gasketing to seal the joints.
Advantages of Open-Roof Greenhouses
- During warm weather, the temperature inside the greenhouse can be maintained within a degree or two of outside temperature with little or no energy needed. Many growers have found that this shortens production time and produces a better quality plant.
- In the spring, plants can be hardened off by opening the roof on nice days. This saves considerable labor of moving plants outside.
- Energy costs are reduced. Fan ventilation can use from 0.5 to 1 kilowatt hour/sq ft/year.
- Depending on design and orientation, the crops may receive more light during the middle of the day than in a conventional greenhouse or less light in early morning or late afternoon due to more layers of glazing that it has to pass through. Further research is needed in this area.
- Irrigation is reduced due to more uniform temperature and the potential for natural rainfall.
Adding side vents allows cooling and air movement when high winds or rain prevent the roof from being opened. The guillotine vent, available from a couple of manufacturers, eliminates the conventional vent with arms that interfere with inside or outside work area.
To get adequate cooling on hot, sunny days, a shade system may be needed. It should be porous so that the heat generated below can escape up through the shade material. Evaporative cooling, either a fog system or portable evaporative coolers, can give added cooling. A large number of hanging baskets tends to reduce natural cooling.
Continued developments in the design of natural ventilation systems are giving growers better control of temperature and humidity at lower cost. Proper sizing, orientation, and operation can provide better control than with fan systems.
Motorized Vent Controllers
These should be cleaned and lubricated several times a year. To prevent damage to hinges and vent arms, a wind sensor should be used. This will close the vents when wind speed gets too high.
Before the heating season begins vents should be adjusted so they close evenly and tightly. Poor closing vents result in significant heat loss during the winter.
Fan systems can provide well-controlled air movement through the greenhouse under all weather conditions. As the fans exhaust the heated air, a slight vacuum is created that draws in cooler outside air through louvers, open doors and cracks. The main advantage of fan systems over natural ventilation is that they provide precise airflow that operates regardless of the outdoor windspeed.
Improper sizing of fans is a major cause of poor ventilation in many greenhouses. The fan system should be sized to provide a maximum one volume air exchange per minute to a height of 8 feet (2.5 meters) for summer ventilation. This will result in an 8 to 10 degrees F. (5 to 6 degrees C.) rise from the intake louver to the fan. For example, for a 25-foot by 96-foot greenhouse the fans should have a capacity of 25 feet by 96 feet by 8 feet = 19,200 cubic feet per minute. In southern climates, a height of 10 feet (3 meters) is sometimes used to get a greater ventilation rate.
If the greenhouse is not used during the summer, for instance, greenhouses used for bedding plant production, the capacity can be reduced to 3/4 air changes per minute. For winter ventilation a capacity of 1/4 volume air change per minute is adequate. Because the ventilation needs vary from season to season, it is best to provide for several rates of ventilation. This can be done by using two-speed fans in small houses and multi-fan installations in larger houses. Energy savings and better control will result from using two-stage thermostats, temperature controllers or a computer system, rather than a single-stage thermostat.
Fan systems work best if the draw (distance from the inlet to the fan) is less than 150 feet. For most greenhouses this means installing the fans on one end wall with louvers placed in the opposite end. In longer houses the fans should be located along the sidewalls so that they can draw air in through louvers in both ends.
Where possible, fans should be located to allow them to work with the prevailing summer wind. A reduction in output of 10% or more occurs where a fan is exhausting into the wind.
To provide adequate air for the fans, the intake louver area should be at least 1.25 times the fan area, especially in poly-covered houses. Ideally, the inlet should be controlled by a pressure sensor that opens and closes the inlet to maintain a steady pressure difference from outside the house to inside. This keeps the airflow patterns within the greenhouse steady, and helps prevent “dead air” spaces.
Although more expensive, a continuous louver or several smaller louvers will give more uniform temperatures within the greenhouse. If, when the fans are operating, the plastic is being pulled tightly against the greenhouse frame or if the door is difficult to open, the intake area is not adequate. The louvers should be operated by motorized dampers or solenoids and be connected to the thermostat that operates the fan. Occasionally in small, tight houses when the fan turns on it creates a negative pressure quickly and the motorized damper does not open. This can be corrected by using a time delay relay to keep the fan from starting until the louver is open.
Locate the fan so that the air flows over and through the plant canopy rather than under the benches or in the ridge of the greenhouse. The bottom of the fan or louver should usually be located about 3 feet above the floor.
Fan and vent motors are usually controlled by thermostats. Often these have a wide differential between the off and on position, sometimes as much as 6 to 8 degrees F. Using this type of thermostat can result in a high electric bill. For example, if the thermostat is set at the desired setpoint temperature of 75°F, a +/- 2º thermostat will shut the fan off at 73 degrees F., whereas a +/- 5-degree thermostat will allow the fan to cool the greenhouse to 70 degrees F. You can check a thermostat by turning the control dial and listening to it when it clicks on and off, noting the temperature difference between the on and off position. At the same time check the accuracy of the thermostat dial setting by placing an accurate thermometer next to the sensing bulb.
To get the most accurate temperature control, thermostats should be located near the center of the greenhouse at plant height. When purchasing new fans, select those that have been tested in accordance with Air Movement and Control Association (AMCA) standards. Fan output varies considerably between manufacturers.
Also compare the Ventilating Efficiency Ratio (VER). This is the ratio of the volumetric rate of air movement to the rate of energy consumption. This varies from about 10 - 20 cubic feet per minute/watt. Fans having a VER of 15 or higher are desirable. A database of fan performance data is available online at http://bess.illinois.edu/.
Energy can also be saved by using larger fans with smaller motors. For example, a 36-inch diameter fan with a 1/3-horsepower motor will give the same output as a 30-inch fan with a ½-horsepower motor with a saving in electricity of 180 watts/hr.
Maintenance of Fans
Maintenance should be done on a regular basis. This includes cleaning blades, removing grass or weeds in front of shutters and adjusting fan belts. Dirty, automatic shutters which do not fully open greatly restrict, airflow and drastically lower a fans ventilating efficiency. If these shutters do not close tightly when the fan is off, they will significantly increase the winter heating bill by allowing the infiltration of the cold outside air.
Optimize Fan Usage
Keep the greenhouse full of plants to increase evaporative cooling from plant transpiration.
Reduce summer fan operation time by applying shading to the outside of the greenhouse if the plants do not need the high light levels of summer.
High temperature/power failure alarms should be checked regularly. During a hot summer day it takes only a few minutes without ventilation to create temperatures in excess of 100 degrees F.
During periods of extreme heat the temperature inside the greenhouse could exceed that outside by 10 to 20 degrees F. even with a well-designed fan ventilation system. This puts a stress on plants, reducing the quality and growth.
Evaporative cooling which uses the heat in the air to evaporate the water from the leaf and other wetted surfaces can be used to cool the greenhouse as much as 10 to 20 degrees F. below outside temperature. Evaporative cooling works best when the humidity in the outside air is low. These conditions are most common in the dry Southwest, but even in the more humid northern sections of the United States there are many days in the summer when significant cooling can be obtained.
Fan and Pad Cooling
In the most common cooling system (fan and pad), the fans draw air through wet pads that extend the length of one endwall or sidewall. Aspen and coated cellulose are common pad materials that usually have a life span of one to three years. Approximately one square foot of pad is needed for 20 square feet of floor area.
Water for the pads should be clean and low in mineral content to prevent clogging and coating of the pads. A pump, pipes, and gutters are used to recirculate the water. A flow rate of 113 gallons per minute per linear foot of pad system should be provided to ensure adequate wetting.
It is desirable, especially in hard water areas, to add a wetting agent to the water to obtain more uniform wetting of the pads. A commercial material or liquid household detergent at the rate of 2 tablespoons per 100 gallons can be used.
Algae growth in the pads can reduce the effectiveness of the system and result in deterioration of the pads. The addition of an algaecide to the water supply will help in control.
Fog or Mist Cooling
An alternate system uses a fog or fine mist injected into the intake air stream. Although several commercial systems are available, growers can assemble and install their own system using a high pressure piston pump and fog nozzles. A two-stage system controlled by a two- stage thermostat allows more water to be applied on excessively warm, bright days. Temperature settings should be 5-10°F apart.
During bright, sunny days in the winter, ventilation may be needed to keep temperatures at an acceptable level for good plant growth. The fan and tube system introduced several years ago has become popular throughout the industry for this purpose. It mixes the cold outside air with the warm greenhouse air before it reaches plant level. Two types of systems are available.
The least expensive to install and operate uses a fan, usually operated at low speeds to exhaust the heated air from the greenhouse. The intake air enters through a perforated plastic tube suspended in the ridge of the greenhouse and connected to a motorized louver or large stove pipe elbow. Two tubes should be used in greenhouses wider than 25 feet to get more uniform cooling.
Tubes with 2-foot hole spacing should be used in plastic greenhouses. Tubes with 4-foot spacing can be used in glass greenhouses. The tubes should be punched so that the air is discharged horizontally into the house.
The second system, commonly referred to as fan-jet, uses a fan located in the ridge of one endwall to inflate the attached perforated tube. Air is brought in through an adjacent motorized louver. The fan is set to operate continuously, providing ventilation air when the louver is open and circulating air within the greenhouse when it is closed. The unit should be sized to provide about 1/2 cubic foot per minute per square foot of the floor area.
In both systems the end of the tube not attached to the intake louver or fan should be tied off. Although most greenhouse suppliers provide a standard prepunched tube, the correct size and number of holes is crucial for proper operation and uniform ventilation. Additional holes should be cut in the tube if the tube pops open when fan comes on or if the doors of the greenhouse are hard to open. On the other hand, if the tube does not fully inflate, some holes should be taped closed using a poly tape. Also check to see that the doors to the greenhouse are closed, so no short circuiting is occurring.
Contributors to this Article
- John Bartok, Jr., Agricultural Engineer, University of Connecticut
- Vern Grubinger, Professor, University of Vermont Extension