Greenhouse Energy Conservation Strategies
Greenhouse Thermal Curtains
On a clear night, heat energy is lost rapidly from the earth into space via long wave radiation. On the other hand, energy loss diminishes during a cloudy night because the clouds trap long wave radiation, serving as a natural blanket. Thermal curtains, or screens, as they are sometimes called, are fabrics that are pulled across the roof and are sometimes used to cover the sidewalls inside the greenhouse to reduce nighttime heat loss in cold weather. When the curtain are closed, there is less volume air to heat and the long wave radiation bounces back to the soil and plants night so that it cannot leave the greenhouse.
Installation of Greenhouse Thermal Curtains
Thermal curtains are typically installed just below the gutter and are retractable so they can be closed during the night and retracted during the day. Depending on greenhouse orientation, design and grower preferences, curtain systems can operate from gutter-to-gutter or from truss-to-truss.
Gutter-to-gutter Curtain System
In a gutter-to-gutter curtain system, there is a single panel of fabric per house instead of multiple panels as in a truss-to-truss system. Though the volume of greenhouse space that is heated is minimized in this configuration, the amount of cold air above the system is maximized. This makes it harder to mix and reheat the air above the system when it uncovers in the morning.
Truss-to-truss Curtain System
With a truss-to-truss system, the panels of curtain material move across the distance between trusses. There are three ways to configure the truss-to-truss system. First, it can be flat at gutter height, minimizing heated areas and making installation easy.
Greenhouse Curtain Support Systems
Greenhouse curtain systems support the panels of curtain material on wires or nylon monofilament lines parallel to the direction of movement of the curtain. These lines are uniformly spaced across the greenhouse at distances of eighteen inches to four feet on center, depending on the design of the system. Greenhouse curtains can be supported two ways: they can slide on top of support wires or they can be suspended from wires.
Mechanical Drive Systems for Thermal Curtains
Thermal curtains can be controlled manually or automatically. In small greenhouses, the curtain can be opened and closed manually with a hand crank device to reduce costs. In automated systems the two types of drive systems used are the cable drive and the rack-and-pinion drive. The cable drive has several cable loops that run the length of the greenhouse. A gearbox drive turns a drive shaft (usually a 2- to 3-inch pipe) that runs the length or width of the greenhouse, depending on the type of curtain system.
Operation of Thermal Curtains
The motors are often controlled by a computer system that evaluates light conditions at plant level (for shading purposes) and outside weather conditions (for energy conservation purposes). The advantage of a computer system over a timer clock is that it can be programmed to open the curtain in stages to minimize potential cold stress on the plants.
Several materials on the market are made specifically for greenhouse curtain applications. The main types of curtain materials are nonporous, porous, and semi-porous. The disadvantage of nonporous materials is that when condensation drips off the roof onto the curtain, the added weight can cause the curtain support to fail. Therefore, nonporous materials are not recommended for greenhouse curtains. Porous materials allow condensation to drip through, but they also allow significant air exchange between the underside and topside of the curtain.
Thermal Curtains Performing Multiple Functions
Curtain perform multiple functions: (1) heat retention on winter nights, (2) partial sun screening on bright summer days, and (3) total exclusion of light for lengthening the night in summer for photoperiodic crops. If all three functions are required in a greenhouse, two automatic screen-pulling systems will have to be installed, which is often done.
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