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The roofs of many new greenhouses now contain glass with an anti-reflection (AR) coating, which improves light incidence. For a long time there was no such solution for existing greenhouses. But that has changed. Van Adrichem Kwekerijen is one of the first cohort of companies to gain experience with this new coating. The result: yields are up by 5%.

When sunlight falls on the greenhouse roof, three things happen. Much of it passes through the glass; this is transmission. Some reflects off the greenhouse roof and is therefore lost to the crop. This is reflection. The third thing that happens is absorption, which causes the material to warm up slightly. Together, transmission, reflection and absorption always make up 100% of the solar radiation that falls on the greenhouse roof. There is not much that can be done about absorption; that’s simply a property of the material. So when you’re looking to increase transmission, reducing reflection is a good place to start.

The discovery that applying an anti-reflection coating impacts positively on the crop was made in research done by Wageningen University & Research (WUR) back in 2006. Since then, growers installing new greenhouses have almost always had the glass factory-coated with an AR coating.

Practical experiences

Twelve years on and there is now a solution available for existing greenhouses as well. Measurements by the WUR LightLab have shown that the effect of this coating is comparable to that of AR glass. So an anti-reflection coating is an option for everyone.

The results of the first practical experiences were awaited with great anticipation, and they are now in. In March 2017, Van Adrichem Kwekerijen had a Mardenkro AntiReflect coating applied to an unlit section of its greenhouse in Steenbergen near the southern Dutch border, where they grow the large cluster tomato Merlice. “We wanted to see for ourselves what the effect would be, so we went about the process very rigorously,” commercial manager Loek van Adrichem says. “We had one 1-hectare section coated and left the one next to it untreated. We also selected 20 rows for comparison in each section. They were in the middle to rule out any impact from the side walls.”

Nothing at all happened to begin with. No matter how closely van Adrichem monitored the crop, he really didn’t see any differences. Not in leaf colour, leaf size, vegetative or generative growth, or fruit development. “Our cultivation operations were exactly the same, and in both sections, which can be controlled independently of one another, we did exactly the same with the climate, screens, ventilation and so on, because there was no reason to do things differently. Also, we had a bit of tomato russet mite here and there but we saw no differences between the rows in the weekly counts.”

Get coating in early spring

But gradually something began to catch the eye in the row records: yields in the coated section were rising compared with the uncoated section. Van Adrichem: “By July we were 2% up. By mid-October this had risen to 5% in the 20 rows we were measuring. This was a pleasant surprise. Because we prune all the trusses down to five fruits, we didn’t actually pick more tomatoes, but the fruits were slightly bigger. The only caveat is that the tomato russet mite infestations may have distorted the picture a little. That could narrow the difference slightly.”

In fact, it would have been better to apply the coating immediately after planting in December. But that wasn’t possible from a technical point of view as it was too cold and wet. “As a rule, the extra light would have the most impact on young plants,” he says. “So we lost three months of light gain because we applied the coating late. We mainly picked more kilograms in the summer, which is a bit of a pity. But in the autumn it also added up nicely. And that’s a good thing, as this isn’t a cheap solution. It needs to earn its keep. If you pick more kilograms in both the spring and the autumn, it’s a very attractive proposition.”

Light measurements

When the coating was applied in Steenbergen, six individual glass panes were also sprayed. The WUR LightLab measured their light transmittance. Compared with untreated samples, they allowed 3.6% more hemispherical light through.

This year the manufacturer plans to measure the effect in the greenhouse setting using specially designed measuring units. This is complicated because measuring PAR light under practical conditions is very difficult; there’s a reason why WUR carries out all its measurements under standardised conditions.

Meanwhile, more than 50 hectares of glass in the Netherlands have been treated with the new anti-reflection coating. The nurseries concerned grow tomatoes, sweet peppers, roses, chrysanthemums and lilies. In all cases, Mardenkro was present when the glass was coated and it also carries out light measurements. “We do this because the application process is a lot more precise than for other coatings,” says account manager Paul van Gils. “Only contractors certified by us can work to our strict protocol and the weather conditions have to be right. We then guarantee an increase in light incidence of at least 2.5%, but in practice we are seeing averages of more than 3%.”

Production up by 5%

In 2016, the research institute calculated what the crop could do with this extra light. A range of increases in production were arrived at: 3% in tomatoes, 3% in sweet peppers, 5% in cucumbers, 2.5% in roses, 2.5% in gerberas and 3% in chrysanthemums. Van Adrichem is therefore well above the calculated figure ¬– thanks in part to his green fingers.

His intention this year is to scale up to 200 hectares in the Netherlands, Belgium, Germany and the UK, again with strict oversight during application. Meanwhile, measurements taken by the laboratory show that after one year, the degree of light transmittance is still more or less maintained.

Van Adrichem is continuing with the same coating for now. He is waiting for the results from the current season and will then decide how to proceed. “If we pick slightly bigger fruits from the outset and achieve higher yields early in the season, then 5% is a nice figure to aim for.”

Summary

Van Adrichem Kwekerijen has been using a new anti-reflection coating that can be applied to existing greenhouses for almost a year now. The coated glass lets 3.6% more light through. The nursery has achieved a 5% increase in production with Merlice tomatoes and is continuing the trial with the same coating this season.

Text: Tijs Kierkels.
Images: Wilma Slegers.

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In the past, greenhouse roof light transmittance has always been measured when the roof was dry. But a greenhouse roof spends half the year wet from condensation, and the type of condensation determines how much light passes through the glass. Now there is a measurement protocol that can quantify that. An anti-condensation coating is an option for existing greenhouses and Dutch tomato grower Paul van Paassen is the first grower to try it out.

Measuring the light transmittance of greenhouse roof materials accurately is essential for estimating how much natural light the crop can use. At the beginning of this century, ‘measuring accurately’ still meant measuring light entering the greenhouse perpendicularly. But this did not give the grower an exact picture because most of the light does not enter perpendicularly but at an angle. In that case, two things change: the path of the incident light is longer and when light enters at an angle, more is reflected off the roof. On top of that, when the weather is cloudy the light comes from all directions, so measurements need to be taken from all possible angles.

Hemispherical transmittance

Measuring hemispherical transmittance is standard practice these days. But that doesn’t provide a reliable picture for the grower either, says Gert-Jan Swinkels, light measurement expert at Wageningen University & Research in the Netherlands. There is generally a difference between the transmittance of wet glass and dry glass. The type of condensation on the material also makes a big difference. Droplets reduce transmission, whereas a water film can actually increase it. “In the winter months, the roof of a greenhouse in which vegetables are grown is wet almost 100% of the time. That is precisely the time when the amount of light is the limiting factor for good production. The transmittance of the greenhouse roof when wet should therefore be an important consideration in the choice of material for anyone building a new greenhouse,” Swinkels says.

Erratic measurements

In 2010, a team led by Cecilia Stanghellini took a closer look at the effects of condensation. To do so they built a small greenhouse with a replaceable roof in a climate chamber, where they were able to study condensation in perfectly controlled conditions. The result was surprising. On average across all the materials tested, the loss of light through a roof with condensation was as much as 9% compared with a dry roof. In practice this number started to take on a life of its own as there were big differences between the various materials. For ordinary greenhouse glass, the reduction in light was 5%. Diffuse glass let in almost the same amount of light in dry and wet conditions.
This research demonstrated that it makes a big difference whether the roof is wet or dry. Furthermore, model calculations revealed that greenhouse roofs spend half the year wet to some degree.
The research was useful for raising awareness, but the method involving a trial greenhouse in a climate chamber is clearly too cumbersome for measuring transmittance in practice. There is also another problem, Swinkels says: “Two years ago we measured the amount of PAR light in the Winterlight greenhouse under a dry and wet diffuse roof, using PAR sensors. Over a longer period, the trends were fairly accurate but the measurements were erratic; for example, we saw a progression across the whole day, even in cloudy weather. That told us that these measurements are really too unreliable to use as the basis for decisions and take too long.”

Condensation meter

The conclusion was that a standardised measurement protocol was needed which could ultimately become a standard (or could be incorporated into NEN 2675). So they built a special device which does not yet have an official name but is provisionally known as a “condensation meter”. Swinkels and his colleagues used this device to optimise the measurement method.
“We concluded that we can use this device to accurately measure transmission through dry and wet materials at an adjustable angle, i.e. the slope of the greenhouse roof. Because you can adjust the slope, condensation will drain off and you get a realistic picture of what happens in a real greenhouse. The measurements are reasonably stable but because a lot of factors affect condensation, repetitions sometimes produce slightly different results. For this reason we are not classifying the effect of condensation in percentage terms at present but in classes, such as negative, very negative, positive and very positive,” says Swinkels.

Glass more hydrophobic

The protocol was developed using six samples: four diffuse materials and two clear glass types. “In the case of the diffuse materials, transmittance in wet conditions increased by 0 to 1.5%, while with clear glass we saw a decrease of 1 to 3%,” he says. A slightly different magnitude from the five percent seen in the Stanghellini project, therefore. But Swinkels has an explanation for that: “We noticed that new glass is more hydrophobic – water-repellent – so it produces larger droplets. Glass that has been on a greenhouse for many years and has been frequently cleaned becomes more hydrophilic, so you tend to get a water film on it.”
The device is already in operation. Swinkels hopes that awareness of the importance of wet transmittance will grow and that manufacturers will start paying more attention to the condensation behaviour of their materials.

Existing greenhouses

Understanding the condensation properties of the roof comes too late in the day for existing greenhouses. It had already been suggested in the 2010 report that an anti-droplet coating on existing greenhouses would be a good way of preventing light loss caused by condensation droplets. Mardenkro has developed just such a coating for glass under the brand name AntiCondens, a further development of a formula that has been used on polytunnels for many years.
Paul van Paassen in Bleiswijk is the first Dutch grower to use it. He grows cluster tomatoes (Merlice) on coco substrate on 2.2 hectares. “It was pretty damp last year and the windows stayed wet for a long time. My crop advisor Willem Valstar of Stargrow Consultancy suggested trying out the new coating. We trialled it on six windows in May. The results were impressive so I put in an order after the crop had finished,” he says.

World of difference

Then it was a matter of waiting for a nice dry day once the crop had ended, as the coating has to be applied on a clean, dry roof and then left to dry for a while. Because Van Paassen had to turn up the heating quite high to get rid of the vapours from the new paint job, the coating also dried straight away. The coating was applied with a spray cart with the nozzles close to the glass. It is left there for a season. If the windows are cleaned with fluoride, the coating will be removed as well.
Van Paassen is delighted with the result. “It makes a world of difference. It looks completely different too. The condensation is still there but you can’t see it at all. It is visibly lighter inside the greenhouse now, although it is difficult to say what impact that will have on production and quality.”
A patch of roof was accidentally missed in the middle row. You can still see condensation droplets there. Mardenkro measured the light incidence at this point with a PAR sensor and compared it with the light incidence under the coating. The difference is striking: on three consecutive days in May, the coating improved the daily light sum by 6 to 7% (see figure).

Drying time

Van Paassen uses a form of Next Generation Growing which involves using more screens than in the past but activating the plants properly in the morning. He never had much problem with mildew anyway so he can’t say whether the coating reduces disease pressure. “In theory, it should be easier to dry the air with heat. What’s more, the water film also flows into the condensation gutter more easily,” account manager Paul van Gils adds.
The greenhouse should also be heating up a little faster in the morning as more light is entering. “But you can’t measure that,” the grower says. “But I do think it works and I get the feeling it has saved me money. I will be ordering some more at the end of the year, but I do have to make sure I apply it at the right time as I can’t gauge whether it is still worthwhile if I have to heat up the greenhouse specially just to dry the glass first.”
Van Gils recognises that the biggest stumbling block at the moment is the need for drying time once the product is applied. This won’t be a problem for growers who switch crops during the warm months. The product is not phytotoxic in the form in which it is applied, although there are problems in roses if the buds get wet. Special equipment is needed to apply the product over a crop, however.

Summary

Existing measurement protocols for greenhouse roof materials show the light transmittance of a dry roof. But the roof is covered with condensation half the time and the type of condensation has a significant impact on how much light passes through. Now a measurement protocol has been developed for wet greenhouse roofs at an angle. A tomato grower is the first to gain experience with an anti-condensation coating for glass. There is clearly more light in his greenhouse, although the need for the roof to be dry during application is still an obstacle.

Text: Tijs Kierkels. Images: Tijs Kierkels and Wageningen University & Research.

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Due to the great success of the first book, Mardenkro publishes a second book called 'ReduSystems: The bright side of growing'. The book combines Mardenkro’s best read articles that were published during 2016/2017 and provides more insight into the technology behind the products.

Mardenkro, known for his ReduSystems products, likes to share his knowledge with customers. Since the publication of the first book, 2 years ago, a lot has changed within the company. As a specialist in light and heat-resistant coatings, Mardenkro is expanding into a product line that enables growers to optimize their greenhouse climate throughout the year. The new book highlights the new product AntiReflect and explains how this product ensures a light gain of approximately 3% on existing horticultural glass.

Smart chemistry

The new book combines Mardenkro’s best read articles that were published during 2016/2017 and provides more insight into the technology behind light transmission optimization and other topics. Jort Gerritsen, CEO Mardenkro: “We started as a chalk company and now, about 25 years later, we are developing smart chemistry so that every grower can create a perfect greenhouse climate. During 2018, we will launch a product package that, combined, ensures that up to 10% more light will enter the greenhouse.”

The new book will be officially presented during the first day of the IMP-fair in Essen, Germany. You can pick up a free copy at stand number 3C39 of Mardenkro or download the book via www.thebrightsideofgrowing.com/ (available from Tuesday 23 of January).

Source/photo: Mardenkro.

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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.

Summary

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