Greenhouse Structures and Design
Open-Roof Greenhouse Systems
Open-roof greenhouses are becoming more popular with commercial growers. The open-roof greenhouse offers maximum natural light and ventilation to the greenhouse crop. Similar in construction to that of the Venlo, the open-roof is constructed of two or more roof peaks per greenhouse bay. Each roof section is hinged at the gutter and is connected to a rack-and-pinion or push-pull drive system. Computer-controlled motors push the roof sections open, in effect making the entire roof into a vent. Open-roof greenhouse designs can often control the greenhouse temperature sufficiently without exhaust fans. Natural ventilation works primarily on the principle that wind blowing across a roof vent will create a low-pressure zone that will suck the air out of the greenhouse while letting air in the side vents.
Advantages and Disadvantages of Open-Roof Greenhouses
Growers cite many benefits to open-roof greenhouses, but the biggest is the improvement in crop quality and production efficiency due to the improved range of environments they provide. A traditional greenhouse offers good climate control during inclement weather, but during warm, sunny weather, it can be a struggle to maintain the best temperature and humidity much of the year without expensive, sophisticated environmental controls. Because it allows the grower to essentially grow crops outdoors when the roof is open, this design provides a superior environment for hardening and conditioning crops, resulting in less shipping damage, improved longevity on the retail bench, and better performance in the garden.
Open-Panel Greenhouse Systems
Open-panel greenhouses use the entire greenhouse roof like a ridge vent (Figure 1.10). These open-panel structures are typically hinged at the gutters, allowing the glass, rigid-panel, or double-polyethylene glazing to be raised to a vertical position. Covering materials used in open panel greenhouses include glass, polycarbonate, and double polyethylene, but not fabric. When closed, the structure functions like a covered greenhouse with typical heating and cooling requirements and typical snow and wind loads. Open-panel systems using glass are generally the most expensive of the open roof designs. A less expensive option uses double-poly film glazing instead of rigid panels or glass, but regular replacement of the poly film would be required.
Retractable-Film Greenhouse Systems
Retractable-roof greenhouses act by folding the glazing as the glazing is retracted (Figure 1.11). The covering is suspended from hooks that slide along heavy-gauge, stainless steel wires and is opened and closed by a motor drive system. When opened, the curtain folds accordion-style against the frame of the structure, allowing maximum light and air to the crop. To operate bays independently, individual motors are needed for each bay (increased installation cost). When open, retractable-roof systems provide over 95 percent ventilation.
Flat-Roof Greenhouse Systems
Flat-roof greenhouses are very similar to retractable-film systems, but they do not have trusses, which significantly reduces structural, installation, and glazing materials costs (Figure 1.12). Without trusses, flat-roof systems may not be compatible with all types of greenhouse automation equipment, or installation adjustments may be needed to use automation equipment. For example, boom irrigation systems cannot be hung from the trusses, so they must be mounted to sidewall posts. In addition, the glazing is typically designed to allow water to pass through.
Low-Profile Greenhouse Systems
Low-profile greenhouses are either very wide-span systems with low-pitch roofs, or they may have sawtooth-style roofs. The low-pitch roofs often use specialized film glazing that contains hundreds of small channels that direct rainfall or melted precipitation to the gutters. They typically require the use of expensive energy trusses to design them for reasonable wind and snow loads. Their ventilation can be as good as retractable-film systems if proper sidewall ventilation is installed. The main advantage of low-pitch roofs is the limited number of posts required to support the roof, which may improve labor efficiency inside the structure.
Rolling-Roof Greenhouse Systems
The rolling-roof greenhouses typically have bows mounted on lattice trusses spanning each bay. Tube-type motors drive aluminum pipes that roll the covering up or down on each side of the bow. The covering is typically a nylon mesh film enclosed within two layers of plastic and provides from 15 to 25 percent shade when closed.
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