The Main Performance and Characteristics of Glass Greenhouses
The glass greenhouse is the earliest developed modern greenhouse type, which is highly transparent and aesthetically pleasing, but has a higher construction cost. It is suitable for areas with poor light conditions or better economic conditions. Glass is a rigid transparent material, so general glass greenhouses are in the form of a pointed roof. Depending on the structure, glass greenhouses are generally divided into three types: portal steel frame structure greenhouses, truss roof structure greenhouses, and Venlo structure greenhouses.
1. Portal steel frame structure glass greenhouse:
(1) Structural characteristics: The portal steel frame structure greenhouse generally has a span of 6.0 meters, 6.4 meters, and 8.0 meters, an opening of 3.0 meters, an eave height of 2.5-3.0 meters, and a roof slope of 1:2. The 8.0-meter-span greenhouse has an additional tie rod on the roof beam to enhance the overall structural strength of the greenhouse. The columns and roof beams of the greenhouse are connected by steel, forming a whole, with a large indoor space. However, this structure uses a lot of steel, and the cost is correspondingly high, so it is less used in current production.
(2) Equipment configuration: The main feature of this greenhouse is the long windows on both sides of the ridge. The early window opening mechanism was a four-link window opening, and after the gear rack window opening mechanism was used, most of them were changed to gear rack window opening. Each row of skylights is driven by 1 motor and controlled independently. Therefore, this greenhouse uses more motors for roof windows. Basic insulation curtains were not installed in the early greenhouses, so the insulation performance of the greenhouse was poor. In order to improve the insulation performance of the greenhouse, double-layer glass is often used for the envelope structure of the greenhouse in cold regions.
2. Truss roof structure glass greenhouse:
(1) Structural characteristics: The roof beams of this greenhouse use a truss structure or the entire roof uses a truss structure. Due to the roof structure's reinforcement, the greenhouse's span can generally be more than 10 meters, up to 24 meters, which is especially suitable for large-span greenhouses. Some livestock buildings and aquaculture greenhouses also adopt this form, and some ecological greenhouses and ornamental greenhouses with larger span requirements also adopt this structure.
(2) Equipment configuration: This greenhouse is generally a full-time production greenhouse, with equipment including heating, cooling, ventilation, etc. Because the roof ventilation window area is small, the greenhouse must be equipped with side wall ventilation.
(3) Performance characteristics: The biggest feature of this greenhouse is the large indoor operating space, which is convenient for mechanized operations. But because the roof is large, only windows are opened on the ridge, the ventilation area is obviously insufficient, and the ventilation and cooling of the greenhouse need other auxiliary cooling measures such as wet curtains and fans to achieve. In addition, due to the large roof structure, the greenhouse is relatively high, the corresponding structural materials are large, and the wind load to be borne is also large, especially the uneven light in the greenhouse.
3. Venlo structure glass greenhouse:
The glass greenhouse is a type of greenhouse that rose in large numbers in Western countries after the 18th century with the development of the world's glass industry. Among these greenhouse forms, the Venlo type small roof glass greenhouse is the most typical structure type. The word "Venlo" comes from the name of a small town in the Netherlands, and the Venlo greenhouse was born here in the 1950s. the Venlo greenhouse is celebrated for its small component sizes, ease of installation, long lifespan, and low maintenance needs.
3.1 Structural Characteristics of the Venlo Type Greenhouse The Venlo greenhouse typically spans 6.40 meters, 9.60 meters, or 12.80 meters, with a roof unit span of 3.20 meters, a standard roof height of 0.80 meters, and commonly used eave heights ranging from 3.00 meters to 6.00 meters. The structural materials are defined by hot-dip galvanized light steel structure for the steel columns and side wall purlins, a horizontal truss structure for the roof beam, and a special aluminum alloy profile for the roof beam joint in a "herringbone" shape. The greenhouse gutter is formed from relatively small hot-dip galvanized steel or aluminum alloy. The roofing and sidewall materials consist of 4 mm thick float glass. Reinforced concrete is usually used for the foundation, connected by brick walls or supported concrete protective panels around the perimeter.
3.2 Load-Bearing Characteristics of the Venlo Type Greenhouse The Venlo greenhouse employs a typical purlin-free roof system. The load borne by the glass is directly applied to the longitudinal gutters of the greenhouse, from which it is transferred directly to the columns or the roof beam joints. Therefore, the gutter bears the uniformly distributed load and concentrated maintenance load from the roof system. The external forces borne by the roof beam mainly come from the concentrated forces at certain nodes and other equipment loads suspended from the truss, both of which are transmitted from the gutter.
3.3 Functional Features of the Venlo Type Greenhouse
① High Light Transmittance and Uniform Illumination: Thanks to its load-bearing structure, the Venlo greenhouse uses high-transmittance glass as its lighting material. Combined with the use of specialized aluminum profiles as roof beams, the section size of the roof beam is greatly reduced, eliminating purlins and connectors, thus reducing the shading of the entire roof system and significantly increasing the greenhouse's light transmittance. This feature is particularly beneficial in the weak winter light conditions of Europe, making it a crucial factor in its widespread use.
② Excellent Sealing: The Venlo greenhouse uses a special aluminum alloy with matching rubber strips and injection parts as the glass inlay components, significantly improving the greenhouse's sealing. Good sealing helps to reduce heat loss due to convection, and it facilitates efficient roof drainage.
③ Large Ventilation Area: The Venlo greenhouse, with a high ratio of roof surface to ground projection, can have 2-4 pairs of roofs or 2-4 ridges and gutters per span, thus achieving a ventilation rate equivalent to other greenhouses with the same span when using an interval window opening method. When using continuous windows, the ventilation rate doubles.
④ High Roof Drainage Efficiency: Due to the high number of gutters in each span of the Venlo greenhouse (2-4), compared with other greenhouses of the same span, each gutter's catchment area is reduced by 50% to 83%.
⑤ High Usage Flexibility: The application of truss-type roof support beams gives the Venlo greenhouse greater flexibility in use. The 350-600 mm height of the truss support beam provides sufficient installation space and support positions for window opening mechanisms, curtain pulling systems, crop suspension systems, and other equipment.
⑥ High Component Universality: The consistency of the Venlo greenhouse roof units and the similarity of the node structures offer the maximum potential for component standardization. This minimizes variations and quantities of components caused by different greenhouse spans, reducing greenhouse production and management costs while also facilitating after-sales service and maintenance.
In conclusion, the evolution of the greenhouse structure - from primitive shelters to the advanced Venlo type greenhouse – is a testament to human ingenuity and our persistent pursuit of a better life. Whether it's the primitive pit dwelling greenhouses that originated in the Shang and Zhou dynasties or the more modern solar greenhouse and Venlo glass greenhouse, each structural advancement has played a crucial role in improving agricultural productivity and meeting the food demand of an ever-growing population.
Moreover, with the advent of more modern, sustainable, and energy-efficient models, it is clear that the future of greenhouse farming is bright. In this era where climate change and sustainability have taken center stage, the development and evolution of greenhouses will continue to be an important aspect of human resilience and food security. As we look forward, it is undeniable that greenhouses will continue to be a significant component of our agricultural future, combining traditional wisdom with modern technology to ensure food availability for generations to come.