Home Posts Tagged "greenhouse covering"

greenhouse covering

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One of the main tasks of the Smart Materials project was completed last year. Greenhouse simulation models were used to explore the potential effect of smart covering materials with switchable optical filters on microclimate, use of resources and crop performance.

Until recently, the only way to modify the amount and quality of light reaching the crop in a greenhouse was to use temporary coatings or screens. Three main groups of filters were identified and simulated: filters reflecting both PAR and NIR radiation, filters selectively reflecting only the NIR part, and filters reflecting the FIR spectrum.

Different climate regions and types of coverings were analysed to factor in a range of scenarios. The results highlight a significant potential improvement in microclimate and yield associated with the use of switchable optical filters for all the analysed climates, even if the optical properties are less than ideal.

For some of the simulated filters there are other alternatives available which perform equally well (thermal/energy saving screens). Further research is needed to analyse the technical and economic feasibility of these theoretical filters.

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A strategic partnership contract was signed by Theo Sanders and Harm Maters of the Hortivation Foundation and Sjaak Bakker of the Greenhouse Horticulture division of Wageningen UR on 19 April.

Aside from individual support offered by the Greenhouse Horticulture division of the Wageningen University Research Centre to the members of the Hortivation Foundation, the partnership will include the joint set-up and execution of various innovation-oriented projects. Hortivation’s members include Alcomij, Boal, Certhon, Dalsem, Gakon Kassenbouw, Glasimport Greenhouses, Maurice Kassenbouw, Metazet, Prins Kassenbouw and Van der Hoeven.

NEN for light transmission

The first project to be undertaken jointly between Hortivation and Wageningen UR is to set a standard for the optical properties of diffuse and other greenhouse covering and screening materials. This will result in an NEN standard for the light transmission of transparent greenhouse materials. Immediately following, the priorities will be set for the strategic partnership in the 2016-2017 period.

The Hortivation Foundation (formerly Storeka) focuses on technical innovation and knowledge management for greenhouse horticulture applications. Its internationally operating members are active in the development and worldwide supply of greenhouse production systems. In collaboration with and for the benefit of its members, Hortivation aims to secure the highest position in the Netherlands in the field of integrated growing systems.

Source: Wageningen UR. Photo: Mario Bentvelsen.

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Van Uffelen Flowers held an open-doors day to show off its newly delivered four-hectare chrysanthemum greenhouse at Herenwerf in Maasland on Saturday 16 April, together with its builders and installers.

The newly designed Greenhouse was built by Technokas. The greenhouse cover was executed in diffuse glass, with a haze factor of 70. Another interesting detail is the double screening system by Svensson (Harmony 2515) and Bonar (energy-saving blackout cloth), executed by Huisman Scherming. The lighting system was provided by Hortilux Schréder (1,000 Watt SON-T narrow-angle lighting fixtures, 10,000 lux), and the other water and electrical systems were by Stolze.

Next Generation Cultivation

The greenhouse is prepared for the installation of air handling units for mixing the air in the greenhouse with air blown in from outside, in accordance with the basic principles of Next Generation Cultivation. ‘This system allows the entire air content of a greenhouse to be complete renewed approximately once every hour,’ explains Hans van Tilborgh of Technokas. ‘It will not be replacing the air vents, but will comprise a useful addition to them.’ Before the outside air is blown into the greenhouse through a large hose, it is heated to the greenhouse temperature. This is to prevent climate differences in the greenhouse.

However, the system does not provide in heat recovery, like tomato grower Ted Duijvesteijn’s ID Greenhouse. ‘That would mean installing a much more complex system. It also saves energy. In combination with a double screen this greenhouse will allow us to save 30 to 40 per cent more energy than in a conventional chrysanthemum greenhouse,’ continues Van Tilborgh.

Production halls

In addition to the greenhouse, Technokas also supplied Van Uffelen Flowers with hoistable heating frames and production halls, designed by the Poortinga & Zwinkels architecture firm. According to architect Hester Poortinga, Van Uffelen aims to have its new building reflect the brand identity and values of Zentoo: transparent, unifying and innovative. Zentoo is the trademark under which Van Uffelen chrysanthemums are marketed. The chrysanthemum varieties are supplied by Fides and Deliflor.

Other technical tours de force at Van Uffelen Flowers are the Robur fully automated spray boom, the ISO Group peat block planting machine and the Bercomex harvester. Once harvested, the flowers are transported to the shed on underground conveyor belts. The cooling facilities with pre-cooling units were supplied by Hamelink Koeling BV.

Text/photos: Mario Bentvelsen.

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Growers know that a diffuse roof – with or without an anti-reflective coating – offers added value compared with traditional greenhouse glass. However, it is not so simple to decide which one to choose. Researcher Silke Hemming lists all the relevant aspects. An underestimated factor is measuring samples taken from good representative spots for new large-scale building projects.

In the Dutch and Belgian horticultural sector, growers building new greenhouses opt almost unanimously for a roof made of diffuse glass. Also, depending on the light requirement of their crops the glass panes in the roof are often covered in one, two - and with double glass - even three or four AR-coatings. Although concepts such as haze factor, hemispheric transmission and UV-transmission are well established, researcher Silke Hemming of Wageningen UR Greenhouse Horticulture, notes that in practise there is still much uncertainty and confusion. The recent introduction of the new term F-scatter doesn’t make it any easier. What should growers pay attention to in order to make an objective decision?

Wish list

Anyone considering the installation of a new greenhouse roof should firstly decide how much and what type of light he would like in the greenhouse: What is the optimum amount of light distribution? Is a lower light transmission acceptable or, by using anti-reflective coatings, do you want to increase this as much as possible? What sort of UV-transmission should the roof have? You can also take into account the insulation aspect because that determines to a large extent whether you need to think about single or double glass.
“Take time and care when making a wish list because this will determine the shortlist of options and the eventual choice of glass,” says Hemming. “What I also tell growers and advisors to do is properly determine the optical characteristics of the glazing and compare them with each other in the right way. That means on the basis of a representative sample. I’m amazed that people often only measure one or two panes of glass for projects that will cover several hectares.”

Variation and sampling

During the production of the glass, variation can occur that affects the transmission properties; a brand of glass can change over the years; one production batch can differ from the next batch; and it’s even possible to have variation within one glass pane. These differences are usually small, but can – depending on the quality control and any treatments after production such as structural treatments to the surface and coatings – become larger.
Hemming: “Sometimes we can measure differences of up to ten per cent in haze between batches and even between different places in one pane. This is not necessarily a problem; even when there is variation within a delivery, the product can still be good.”
She finds it risky to determine the optical properties of glass, that is going to cover several hectares, based on measurements taken on a single pane at a visually representative point. This value can by coincidence be good but it can also happen to be bad. “Therefore ensure that a good sample is taken based on the guidelines in the ISSO-publication 88 Quality Requirements for horticultural greenhouses,” stresses the researcher.

Haze and F-scatter

Growers as well as advisors have many questions about the haze factor. The haze factor expresses how much light is scattered. Nevertheless types of glass with the same haze factor can sometimes differ in the way in which light is scattered. This can be either wide or narrow. In principle, light penetrates the crop better when there is a wider scattering of light compared with a very narrow scattering but that can be associated with slightly more light loss. A new term, the F-scatter, was devised to explain the way in which light is scattered: The higher the F-scatter of the glass the wider the light is scattered.

Structural treatments

To give the glass the required diffuse properties, a number of structural treatments are possible. Also the surface of the glass can be modified on one or two sides – for example by rolling or etching – to give a more or less rough structure (regular or irregular) in order to scatter the incoming light. The researcher cannot say if a certain structure, such as a matt-matt or pyramid, would be the preferred option. “Compare the optical properties measured, the hemispheric light transmission, haze factor and F-scatter of the materials with each other and see if these match your requirements, she advises.
There are also many types of coatings. It is wise to know their impact on the hemispheric transmission. An anti-reflective coating increases light transmission in dry conditions by 2 to 4% per side. “In addition coatings can change the incoming light spectrum,” says the researcher from Wageningen. “This applies both to the PAR-spectrum as well as the transmission of Near Infra Red heat radiation. Both of them can affect growth and the temperature in the greenhouse.”

Condensation behaviour

The textured layer on the glass, which is often placed on the inside of the roof to limit the amount of dirt build-up, can affect the condensation behaviour of the roof. This applies too to any anti-reflective coating, which when two sides have been structurally treated is applied over the top.
“From autumn to spring, there is almost continuous condensation on the inside of the roof,” says Hemming. “It's a cheap way to dehumidify the greenhouse, but it can also lead to loss of light. Good condensation properties are important because in the winter there is a need for extra transmission and less need for light scattering."

Measurement method

Structure and coating together determine the nature of the condensation layer. If the combination is good, an even thin condensation layer occurs that can lead to 5% gain in light compared with a dry pane of glass. If the combination is poor water droplets occur which cause light loss: Undesired droplets occur on wet standard float glass that can cause 5% loss of light.
The researcher notes that the effects of wet glass and condensation, measured using the current methods, are only indicative because the glass lies horizontally on the measuring unit. That differs from an actual greenhouse in which the glass on the roof is always sloping. The results of these measurements can therefore differ somewhat from practice, but they do provide a reasonable indication for comparing one material with another.
“We still have a project running in which we comprehensively focus on the condensation behaviour in the lab and in practise. In time we can say more about this,” says Hemming.

General aspects

In addition to the technical properties of greenhouse roofs growers should also look at the long-term aspects, such as constant quality with respect to extra orders placed due to glass breakage. “In this context ask what kind of standardisation suppliers can offer,” recommends Hemming.
During and after the construction of the greenhouse, glass must also be properly maintained to prevent glass corrosion and to maintain the optical properties. Last but not least the grower should ask the glass manufacturer and/or supplier about the best way to clean the roof, both inside and outside. After all, a wrong treatment can have nasty consequences, among other things on the properties or lifespan of coatings.


To come to the right decision about choice of glass growers should make a list of their requirements and objectively have samples of glass measured on all the relevant aspects. The importance of a good and sufficiently large sample is often completely underestimated. Also, condensation behaviour deserves attention. New research results on this will be published soon.

Text: Jan van Staalduinen. Photos: Wageningen UR Greenhouse Horticulture