Greenhouse Ventilation and Cooling
Greenhouse shading is a procedure for cooling which attempts to reduce the amount of solar radiation, which reaches the plants. The heat load is created by the infrared part of the radiant energy from the sun. Plants are photosynthetically active in the blue to red range (380 – 700 nanometers) of the total light spectrum. Wavelengths greater than 770 nanometers result in increased temperatures in the greenhouse. Elevated day temperatures within a greenhouse can impact plant production and quality. Ventilation alone is frequently insufficient to control the greenhouse temperature, especially during warm months. Some form of shading is usually used to restrict the solar radiation entry. There are two primary options for providing shade: shading compound or whitewash; or shade cloth either used externally (See Figure 3.12) or as movable refractive screens or curtains inside the greenhouse (See Figure 3.13). Each of these options has distinct advantages and disadvantages, which warrant consideration when deciding how to reduce light and temperature in the greenhouse. Where radiation levels are relatively high, shading does not have to have an adverse effect on plant growth during sunny weather. Light charts have been developed that generalize crop quality responses according to daily light integrals. Provided that sufficient daily light is delivered, one can expect to grow a good or high quality plant from a photosynthetic potential perspective.
Shading Compounds (White Wash)
Greenhouse temperatures can be reduced by applying semi-permanent shading compound, or white wash to the outer glazing of the greenhouse (See Figure 3.14). The magnitude of shading for shade compound is dependent on how much the original material is diluted. A higher dilution of shade compound results in less shade, whereas a higher concentration of compound increases shading.
Greenhouse Shade Curtains
Shade curtains are the other primary choice for reducing heat and light inside a greenhouse. Several factors must be considered when designing a shade curtain system for greenhouses, including shade placement, shade curtain materials, and system configuration.
Although the same mechanical support system can be used for energy savings, shade screens often serve dual and even triple roles they play in the greenhouse. In addition to reducing light intensity, screens are used to diffuse light entering the greenhouse, conserve heat energy during the heating season, and increase the length of the dark period for photoperiodic crops. The composition of the screen will vary according to the role or combination of roles served.
Materials for Shade and Heat Retention. Covering materials for shade and heat retention include knitted white polyester, non-woven bonded white polyester fiber, and composite fabrics manufactured specifically for use in greenhouse curtain systems. The white polyester fabrics offer excellent durability. White polyester has largely been superseded by composite fabrics of alternating strips of clear and aluminized polyester or acrylic held together by a finely woven mesh of threads.
Blackout Materials. Growers of photoperiodic crops need to be able to block out light to provide longer nights for control of flowering. Screens for this purpose have a black underside to absorb light. Typically 99.9 percent of light is blocked. Blackout screens include polyethylene film, knitted polyester, and composite fabrics where all the strips are either aluminized or opaque. Most blackout materials attempt to reduce heat buildup when the curtain system is covered for day-length control in summer. Knitted polyester is available with a powered aluminum reflective coating bonded to one surface.
Operation of Greenhouse Shade Curtains
Greenhouse curtains can be controlled manually or automatically or by a time clock or smart controller that can detect solar levels. In this way, screening is applied only during the hours when it is needed. The advantage of a clock or a controller is that no one has to be present to have the curtain open or close.
Shade curtains are either drawn from gutter-to-gutter or from truss-to-truss. Gutter-to-gutter curtains run the length of a greenhouse and, as the name implies, are drawn from one gutter to another. One disadvantage of this system is that it may be difficult to accommodate superstructure like supplemental lights, electrical conduit and ventilation tubes. There are three types of configurations used for truss-to-truss curtain systems: flat-at-gutter-height, slope-flat-slope, and slope-slope.
Mechanical Drive Systems
Three types of drive systems are used to move the screen, cable and drum, push pull, and chain and cable. The cable and drum system operates like a clothesline. It uses a gear motor that rotates a shaft near the center of the greenhouse. A stainless steel cable wrapped around a grooved drum connects shafts at each bay. The cable is attached to the leading edge of the screen. It extends the screen when the cable pulls the leading edge. It gathers and retracts the screen when moved in the other direction.
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.
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