Farmers that operate greenhouses in the summer

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Natural Ventilation In Greenhouse During Summer

Farmers that operate greenhouses in the summer - when ventilation is a continual need - can save energy and money by replacing mechanical ventilation with natural air exchange. Per year, wall fans and horizontal airflow can consume 0.5-1 kWh/SF of greenhouse floor area.
Natural Air Circulation
In rigid and film-glazed greenhouses, natural ventilation uses wall and roof vents for air circulation and exchange rather than powered fans. This configuration is based on pressure variations caused by wind and temperature gradients. To maximize air movement, sidewall ventilation may be adequate, or a mix of side wall and roof ventilation may be used. It is not suggested to rely solely on roof ventilation.
Energy Savings and Return
The amount of time it takes for the project to pay for itself through reduced electrical use and savings is determined by various factors, including the growing season and mechanical ventilation run-time, greenhouse size, cost of installation (retrofit vs. new construction), and electrical demand charges. For most greenhouse growers, sidewall vents or rollup sides are a low-cost retrofit option.
Natural ventilation could pay for itself in four years. Energy and cost savings are most significant when: 1. little or no mechanical ventilation is used to supplement natural ventilation, and 2. mechanical ventilation is designed into new greenhouses rather than retrofitted.
Tip for Saving Energy
Greenhouse Passive Ventilation
Using passive ventilation and natural air movement instead of mechanically powered fans can help farmers save energy and money.
Specifications and guidelines for side wall vents
Depending on the location, use of additional vents, and prevailing wind, roll-up sides can act as both an input and an outflow for ventilation. This ventilation technique is commonly performed by running sheets of plastic the length of the tunnel and rolling them up around a pipe to the desired degree of opening. The rolling mechanism might be manual (hand crank) or  motorized roll-up motors and  controllers for greenhouse automation  .
Vents on the Ridge and Peak
For poly-film greenhouses, side wall ventilation can be fitted as a roll-up side wall and as a hinged vent for glass or polycarbonate buildings. Side wall vents are typically installed at the ground level and are two to three feet tall. Because natural ventilation depends on wind or pressure differences, side wall vents should be put along the length of a greenhouse's sides and on walls free of outside obstructions such as vegetation or other structures. Constructing new greenhouses with side wall vents facing regular summer breeze is best, allowing wind to flow into the greenhouse naturally.
Vents on the Ridge
Airflow through a greenhouse can be increased by combining side wall vents with the roof or ridge ventilation. Ridge ventilation can be a retractable, hinged rigid-glazed vent or a framed-poly film-hinged vent. Ridge ventilation can also be used to create a roll-up roof. Warm air escapes via an open ridge, making a pressure difference within the greenhouse that draws air in through side wall vents. Ridge vents should face the wind and be 15-20% of the size of the greenhouse floor area. When side wall and roof vents are combined, the side wall vent openings should be greater than the area of the roof vent opening. 2 Ridge vents are the most cost effective when used in new construction. They can be installed through greenhouse retrofits, although their payback times (estimated at 15-20 years) vary depending on several factors. The following factors influence costs: manual vs. automated installations, solid vs. film-type glazing materials, ridge vent height, and installation issues connected to irrigation systems, heat currents, and so on.
Controls and Procedures
Natural ventilation roll-up or hinge operation can be done manually or with motorized control.
Computer controls can be implemented to predict high wind speeds and other weather phenomena, assisting in preserving the intended greenhouse atmosphere. It is not suggested to run Horizontal Air Flow (HAF) fans when vents are open to maximize natural ventilation airflow (and energy savings). Vents should also be thoroughly sealed to reduce heat loss during the heating season.

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