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diffuse glass

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Diffuse glass is becoming routine in new build greenhouses – and much more quickly than anticipated. But it is still difficult for growers to estimate precisely what added value it has for their crops. So researchers Silke Hemming and Tom Dueck are calling for growers to share much more of their experiences. Here they look at some of the issues faced by growers in practice.

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Golden Fresh Farms, based in Wapakoneta, Ohio, USA, is a high-tech tomato producer selling to leading grocery retailers in North America. In Greenhouses spoke to Luis Chibante, president and CEO of the company, to learn more about the company’s activities and high-quality, innovative approach.

The history of Golden Fresh Farms, managed by Luis Chibante (president) and Paul Mastronardi (vice president), began around four years ago. It was set up as a sister company of Golden Acre Farms near Kingsville, Ontario, Canada. “When looking to expand our operations,” explains Chibante, “I decided to build a new company here in Wapakoneta, Ohio, for several reasons. Firstly, this location in the Midwest of the USA gives us access to around 200 million people within easy driving distance, which is beneficial in terms of logistics and shelf life. Secondly, the winters are cold here, which we actually like because it helps to keep pests down to a minimum without spraying, and we aim for pesticide-free as much as we can. And thirdly, there is a high energy availability at low cost in Ohio. In fact, we don’t generate our own energy because it makes better financial sense to buy it in than produce it ourselves. That’s a big difference from Canada.”

Locally grown

Sustainability is an important theme for the company, which has a strong focus on quality and freshness. “We strive to grow the best products using sustainable methods. We’re positioned at the higher end of the market, but once our retail customers know that we’re an efficient greenhouse company they’re happy to enter into long-term partnerships with us,” comments Luis.
Nowadays, both retailers and consumers are increasingly realizing that cheaper is not always the best approach. It’s more a matter of how you can improve your product quality, taste, longevity and appearance. That enables you to sell 100% of your product rather than just 80% of your product with the other 20% going to waste, and waste is such a huge issue in retail. “So retailers might have to pay us a bit more, but they’re ultimately profiting a lot more. It took us a little while to create that new mindset among our customers, but they’ve now seen that it achieves real results. And the consumer is actually the winner.”
The company is already selling to most of the big retailers in the USA: Kroger, Meijer, Metro, Sobeys. “In fact, now that we’re a US company a lot of retailers are actually coming to us saying they want to stock our product because it’s locally grown and the sustainability and the product quality are better than they’re used to.”

Tradition of innovation

Now that the first phase of the construction project has been completed, Golden Fresh Farms is a 7 hectare hydroponic greenhouse facility with 75 employees. It has a production capacity of approximately 200,000 tomato plants and an annual yield of 4.5 million kilos of three types of tomatoes: beef, cluster and cocktail. At the end of expansion phases 2 and 3, which will take another 10 to 15 years, the facility will comprise around 32 hectares.
Chibante continues: “To ensure freshness and quality, it’s a very high-tech facility with a fully integrated operation from seeding and harvesting to fully automated packaging and shipping. We’ve definitely benefited from the tradition of innovation at Golden Acre Farms in Canada. That was one of the first greenhouses in North America to work with a trough system. We’re talking 18 years ago.”
In another example, before diffuse glass was invented, Golden Acre Farms was actually one of the first greenhouses in Canada to install two curtain systems, one for energy and one for diffuse light. “So we were doing diffusion before it even became popular. And then five or six years ago, we were one of the first high-tech facilities in North America in terms of lighting and robotised packaging.”

Improving efficiency

The company Thermo Energy Systems has played a key role in those innovative developments, and was instrumental in the design and construction process of the Ohio facility too. “I’ve known Henry Froese, president of the company, for more than 20 years. Even back in those early days we were both very interested in improving efficiency, so we bounced ideas off of one another – him as an engineer and me as a grower – to make his company better and to make my company better, and our partnership has evolved ever since,” recalls Chibante.
“He’s done a lot for us in Canada over the years so it was only logical for us to contact him and his company when we were planning this new facility. In the end, they built the whole greenhouse as their first-ever turnkey project. We architected the facility together – it took around a year and a half to do all the drawings – and then we found the right location. The construction work was completed in just eight months, believe it or not. We’ve been able to use all the technical knowledge gained in Canada when designing our Ohio facility.”

Benefit heat from HPS

So it obviously has diffuse glass, energy curtains and Golden Fresh Farms was set up for semi-cooling if the company decides to grow in summertime in the future. “So basically we have everything we need at our fingertips. We work with HPS lighting rather than LEDs, for two main reasons. Firstly, the energy costs are so low in Ohio that LED cost-savings aren’t a consideration, and secondly the winters here are so cold – with night-time temperatures of down to minus 5°C and daytime temperatures of below zero – that we actually benefit from the heat produced by the HPS lighting.”

Pioneering technology

Another innovative solution that Thermo Energy Systems installed at Golden Fresh Farms is a pioneering fan system by Dutch manufacturer Van der Ende Group. The unique setup of the manufacturer’s Enfan horizontal fans in conjunction with the Verti-Fan vertical fan system combines vertical and horizontal airflows to compensate for any temperature and humidity variations.
This maintains an optimal climate throughout the entire greenhouse, activating crop growth while also helping to save energy. “The horizontal fans are common over here but it’s unique to combine them with the vertical system. Everything works on sensors and is fully computer controlled, there’s no manual intervention. The need for constant air circulation depends on so many factors – the climate and light level outside, whether the curtains are closed, whether the HPS lighting is on – and for each factor the computer knows precisely when to implement the fans. This optimally balances the climate from one end of the greenhouse to the other – which is 430 metres in length and has 49 bays of 8.5 metres each – and there’s a constant air flow without any wind effect.”

Further expansion

The first crops were planted in the Golden Fresh Farms greenhouse in February 2017, so the coming season (September 2017-August 2018) will be the first real test, but Golden Fresh Farms’ president is pleased with the initial results. “The production numbers look really good and we’re already seeing results in the crops in terms of quality. In fact, we like the innovative air circulation system so much that we’re planning to install it in our Canadian facility too in the longer term.”
In the meantime, he has got his work cut out supervising the further expansion of the company in its mission to ensure that its US customers continue to receive the freshest, highest-quality, most environmentally sustainable and locally grown produce throughout the year.


Golden Fresh Farms, based in Wapakoneta, Ohio, USA, has recently completed phase one of what will become an 32 hectare high-tech greenhouse facility for tomatoes. This innovative hydroponic facility includes a pioneering climate control system combining horizontal and vertical fans to achieve optimal airflow the entire length of the greenhouse.

Text: Lynn Radford. Images: Golden Fresh Farms.

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The winter light greenhouse aims to enhance light transmission by more than 10% during the European winter. In September a trial greenhouse of 500 m2 was prepared at a site at Wageningen University & Research in Bleiswijk, the Netherlands. Among other things it includes a special light transmitting cloth and diffuse, hydrophilic glass. “We’ve pinched a few per cent from everywhere. Our goal is to maximise the amount of light in the greenhouse between October and March.”

Even on a grey July day you had to squint your eyes when walking into the trial greenhouse during the construction work. In that respect it seems mission is accomplished. “Even the people working here say it is very light,” says project leader Frank Kempkes, researcher at Wageningen University & Research. He's been working on this project since 2014.


Kempkes organised the building together with a consortium of companies: Glascom Tuinbouw and DA Glass provided the glass; Ludvig Svensson the screen material; Bayer CropScience the most suitable cucumber variety; and Bom Group was responsible for the greenhouse and screen construction. The latter company was a logical part of the project, says its CEO Mike Vermeij. “Our company is always focused on innovation. We work closely with Wageningen University & Research on several projects. In this project too, we wanted to test our practical capability with their theoretical knowledge. Together we achieve more.”
Kempkes explains the reason for this project: “Nearly everywhere in the world natural light is the limiting factor for growth and production for part of the year. In the winter the prices are at their best. Therefore, we are searching for the ideal winter light greenhouse and a greenhouse that is also more energy efficient, because it uses more, free, solar energy."

Calculations in preliminary phase

In order to design the right greenhouse, in 2014 and 2015 researchers in Bleiswijk first looked at all the parameters. In consultation with the greenhouse designer and suppliers nine designs finally emerged. Further calculations were made using an optical simulation model created by researcher Gert-Jan Swinkels. This resulted in a greenhouse design that on paper provided 12% extra light. How? It was achieved by a combination of factors: The greenhouse construction and equipment; the glass; the screen; the screen installation; and the variety.

Larger glass panes

Starting with the greenhouse construction, it is situated east-west. This is better than a north-south position during the winter. “That is slightly less favourable in the summer but then the light is not a limiting factor. In the end you gain that back in the winter,” says the project leader.
The roof is still the ‘normal’ symmetrical Venlo roof, 5.60 m wide with extra large glass panes of 3.00 x 1.67 m. The researcher explains how this evolved. “Based purely on theory an asymmetrical saw tooth roof seemed better for the winter light greenhouse. But in practise there were disadvantages. Therefore we still went for a symmetrical roof.”
The large glass panes also caused headaches. “The disadvantage of larger glass is that it bends more. Then you need wider rods, but they intercept the light. Therefore we chose thicker glass of 5 mm instead of 4 mm. That absorbs slightly more light, but the effect is less great than wider rods."
Regarding the greenhouse construction, the upright Twinlight columns are not solid, but have a smarter, leaner, yet sturdy construction. In addition, all gutters, rods, trusses and columns are covered in a white powder coating with a reflection factor of 90%, a big improvement on out-dated aluminium that has a factor of less than 60%. “That results in just a few tenths of a percentage, but we do everything we can to achieve more light," says the researcher.

Diffuse and hydrophilic glass

The biggest gain comes from the light transmission through the diffuse glass. Compared with clear float glass – the current standard-horticultural glass – the SmartGlass used yields more than 7% extra light. A whole process was involved in its selection. First of all, researchers established that after taking measurements diffuse glass was a better choice for the crop than clear glass, even in winter. The hemispheric transmission – a weighted average of incident light taken from all angles – of this glass is more than 90% (perpendicular 96%), while that of standard glass is 84% (perpendicular 91%).
Usually diffuse glass is structured glass. In this case it is etched glass, the speciality of glass manufacturer DA Glass. Measurements showed that both types of glass performed well, as long as the glass is hydrophilic (water attractant). “Condensation always occurs on the glass in winter. If droplets form it is at the expense of light transmission. When a water film forms the transmission remains virtually the same or even increases. The type of glass produced by this supplier causes a water film to form and there is hardly any loss in light transmission,” explains the project leader.
Actually, Kempkes expects to see many more innovations in glass. “Solar panels are covered with glass. This market is growing fast so there’s a demand for improvements and therefore it continues to develop. The horticulture sector could piggyback on this.”

Better basic material for cloth

In addition to the clever construction and SmartGlass, the winter light greenhouse prototype is fitted with screens by Ludvig Svensson. These lead to at least a 4% gain in light compared with the manufacturer’s 'standard' energy screen. The greatest gain is achieved by having a better starting material. “The screen is more transparent. After testing many samples we eventually ended up with this material,” says Kempkes. Because the H2NO-technique has been applied to this energy screen the screen remains transparent even when wet from adherent condensation.
In addition to this alternative material the fitting is also special; it’s not horizontal, but forms a W-shape. “This was the screen supplier’s idea. Just like the W-shape is better for optimum light transmission through the roof, it’s the same for the screen.”
Another novelty is the Iso++ screen installation, in which two screens are about 6 cm apart. The cavity space is therefore small, so the air virtually stands still and as a result forms a good insulating layer. This is not so important for the light transmission, but it is important for the energy efficiency of the greenhouse. That also applies to the innovative dehumidifier with heat recovery, the Air in Control system by Bom Group. “We have also further optimised the forced ventilation in the greenhouse by using this system,” says Mike Vermeij.

Variety with small leaves

According to theory all these components add up to 12% extra light. The researchers are now going to accurately measure if this extra light transmission is realised in practise. The first autumn crop of cucumbers was planted at the beginning of September and will be tested. Similarly, the first winter crop of cucumbers - what the trial is really about - will be planted at the end of December.
“Together with Bayer we decided to plant Hi-Jack, a variety for a high wire winter crop. It produces smaller leaves, so that the light can better penetrate the crop. We've also tested which planting distance is best for light penetration. In this greenhouse we choose 1.86 metres in one trellis and 1.77 m in the other," says Kempkes. How the crop develops in the winter light greenhouse will be closely followed.


A winter light greenhouse has been built at the Innovation- and Demo Centre for Energy in Bleiswijk, the Netherlands. The greenhouse and screen construction, the equipment, glass and screen have been designed to optimise light transmission in the winter. The researchers, together with the participating companies, are striving for 10% extra light between October and March. A winter crop of cucumbers will be planted at the end of December. The autumn crop has already been planted.

Text: Karin van Hoogstraten
Images: Leo Duijvestijn

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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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'We should not depreciate the closed greenhouse', 'Greenhouse growers should rely less on feelings and more on knowledge' and 'In the greenhouse sector of 2050 gas is no longer relevant'. These were some of the remarkable statements that were made at the well-attended EnergiekEvent 2016 in Bleiswijk, where the 10th anniversary of Kas als Energiebron was also celebrated.

Looking back, Kas als Energiebron (Greenhouse as a Source of Energy), the innovation and action programme for energy efficiency and sustainability in the greenhouse horticulture sector, was found to be successful. The energy efficiency of the Dutch greenhouse growers has increased significantly, while the energy sources are becoming more sustainable. Especially thanks to geothermal and residual heat, wind energy, (semi-)closed greenhouses, diffuse glass, LED lighting and - the manual is in need of a reprint - Next Generation Cultivation.


The best news of the EnergiekEvent was that the increased energy efficiency does not affect the quality or production. But it does require more data, knowledge and insight of the growers. "Before, there used to be one measuring unit on each hectare. In the future we want to change this to smaller units. Maybe even to micro-controllers for every plant, not only for the production, but also for the quality. For example, think of LED lighting to enhance certain plant components," says Sjaak Bakker, manager of Wageningen UR Greenhouse Horticulture.

Bakker outlined a future in which electricity will play a leading role. "We are rapidly moving towards an all-electric situation. Just look at electric cars, batteries and what's happening in LED lighting. They are already thinking about lighting through foils or glass cells."

Our CO2 supply should also be different in 2030 or 2050, Bakker said. "If we no longer burn gas in the future, where would we get our CO2 from? An example might be extracting CO2 from the air."

Closed greenhouses

Next Generation Cultivation is being succesfully applied in various crops, including tomato and gerbera. An estimated 15 to 20% of the Dutch growers is using this method. A lot of knowledge has been gained in closed greenhouses and during previous research, Bakker said. "In Next Generation Cultivation many things that were already developed have been put together: think of temperature integration, research on screens, air movement, etc."

The closed greenhouse is disappearing, it seems. But Wilco Wisse, chairman of Kas als Energiebron and staffmember of Lans Tomaten, thinks that closed greenhouses should not be depreciated. "We ourselves also have an Optima greenhouse of 1.5 hectares and we know better than anyone how difficult it is to make profits with it. It is actually a huge solar collector though, it stores excess heat of the summer and supplies it in the winter. Therefore, we should not depreciate the closed greenhouse.”

Sustainable future

Leo Oprel, currently working as a policy advisor at the Ministry of Economic Affairs - who is considered as the founder of Next Generation Cultivation at Wageningen UR - outlined a sustainable vision of Dutch horticulture, in which:

  • new greenhouses will look very similar to existing greenhouses
  • gas has been replaced by sustainable electricity and heat
  • virtually no chemical pesticides are used
  • the use of CO2 for growth is limited
  • knowledge is crucial
  • climate control will include energy and ventilation management
  • heat exchangers will become commonplace to transfer energy from the moist greenhouse air to the incoming drier air
  • cultivation is done with more humidity, with a very homogenuous greenhouse climate
  • light is the key for temperature control
  • the diffuse greenhouse roof transmits more light and there are more screens hanging above each other
  • super translucent greenhouses will determine the production - and the existence - in the winter
  • light will be flexibly captured above a certain level in the summer and will be available for energy storage
  • excess radiation is immediately absorbed without the greenhouse air heating up too much
  • assimilation lighting is used on the basis of the requirements of the plant
  • the artificial light is also dimmable because it works with direct current
  • the heat requirement is reduced to 10 cubic meters of gas per m2
  • geothermal energy and heat pumps with heat/cold storage supply the residual heat
  • the backup will consist of batteries
  • offices and industrial buildings supply additional electricity with roof-integrated solar panels
  • windmills are a regular appearance in horticultural areas
  • proud entrepreneurs have created a sustainable future!

Kas als Energiebron, the innovation and action plan for energy efficiency and sustainability in greenhouse horticulture of LTO Glaskracht Nederland and the Ministry of Economic Affairs, will continue until the end of next year. Both the greenhouse horticulture industry and the government want to continue with the program. The ministry of Economic Affairs wants to maintain the 50/50 cost sharing – but is still negotiating with LTO.

At the end, the participants visited the workshops and greenhouses at Wageningen UR/Delphy Improvement Centre, where they were informed of the latest research.

Text/photos: Mario Bentvelsen.

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Linflowers is the first chrysanthemum nursery in the Netherlands to have diffuse glass. It was a difficult choice because apart from a lack of commercial experience no research results were available either. The brothers Van Tuijl particularly wanted high light transmission and received diffuse light as a bonus. So far their experiences have been positive.

As you walk around the new greenhouse of Linflowers, Bommelerwaard, in the middle of the Netherlands when the sun is shining it’s obvious that the glass provides super light transmission. The light level is so high that sunglasses are almost essential when harvesting the white flowers.

Afraid of scorching

“I wasn’t very positive about diffuse glass. I thought it was more for vegetable crops. Although you know you might achieve extra yield if the light penetrates further into the crop, I expected the extra production to come mainly in the summer and for a chrysanthemum grower that’s of no value,” says David van Tuijl. However, the greenhouse builder remained insistent. He now has low-iron glass with an AR-coating that allows in 5% more light. “Now that is interesting. And in terms of price, having opted for tempered glass it made no difference if we chose to have it diffuse or not. But we thought it might be beneficial because we were a little afraid of scorching under so much light.”
David, together with his brother Rochus, owns Linflowers which comprises three locations (15 ha in total). With help from a restructuring scheme in the Bommelerwaard region in 2014 they dismantled one location to make more room for colleagues. In exchange they could demolish an old nursery in Zuilichem and construct a new greenhouse of 7.6 ha. They grow 15 different kinds of spray chrysanthemums for the East European market which prefers a heavy quality.

Is extra investment worth it?

Even after they were offered the diffuse glass at no extra cost they still didn’t make the decision immediately. “We turned the question around: What were the disadvantages? Together with consultant Theo Roelofs and the glass supplier we spoke to Tom Dueck, the diffuse expert at Wageningen UR Greenhouse Horticulture.”
But there weren’t any research results available for chrysanthemums. Indeed together they came up with more questions: You make the light softer but you lose a little through the diffusivity. What happens then? The low-iron glass allows in as much extra red as blue light. Would the chrysanthemums become shorter if you make the light diffuse, or not?
By using a growth model, chrysanthemum specialist Theo Roelofs of DLV Plant, tried to determine if the extra investment in the glass with high light transmission (compared with ordinary float glass) could be economically justified. He was able to translate 5% more light into 1.2 kg/m2 of chrysanthemums or 15 stems/m2 or 4 g/stem. “We didn’t take into account the diffusivity. That will certainly have a large effect in the winter but due to a lack of data we assumed that to be neutral,” says Roelofs.

Took the gamble

The calculation brought some comfort but still Van Tuijl had three scenarios in his head: “Firstly, you achieve five per cent more growth. That is okay. Secondly, there is no extra growth and you lose your investment. The third possibility is that you have peculiar crop growth. Then you have a real problem because removing the glass is not an option.”
In addition, there was still the question about how light transmission will develop over time. “Vegetable growers who installed the first types of diffuse glass are facing this now. The glass becomes dirtier than expected because it’s less easy to clean and therefore less light than expected enters the greenhouse,” says Roelofs. In the meantime the way in which the glass is made diffuse has changed making dirty glass much less of a problem.
In short: all the evaluations did not provide a clear picture for chrysanthemums. “In the end we simply took the gamble,” says Van Tuijl. The glass is supplied by Horti Glass. They opted for a relatively low haze, namely 50%, to minimise the light loss that occurs when the glass is made diffuse.

More yield than expected

They have already produced a few crops and therefore have gained some experience but it is still too early to give a balance for the year in terms of kilos. “However, I am quite positive. If I had to make the decision again today I would do the same,” says the grower.
The brothers can compare two of their own locations although they have to keep in mind that the soil structure of the new nursery is different and there were some running-in issues. So with some reservations, he says: “So far we’ve managed to achieve well above five per cent extra yield and the weight is better than expected. We target the East European market so the stem weight has to be well above 80 gram; during the winter we were regularly hitting 90 grams.”
This provides opportunities to adjust the planting density or speed of production. They decided to play safe with the first crops. They maintained the same growing speed and the consequence of this appears to be a heavier crop. Visually the crop is just the same as at the other location and the way of inhibiting growth is the same.
In principle the energy consumption should be more favourable because more light enters but the grower has not noticed any distinct differences. “I am particularly pleased with the extra growth,” he says.


A diffuse roof does allow you to be more dynamic with the cultivation strategy, says the crop advisor. Even though no one else has experiences with chrysanthemums under diffuse glass, diffuse coatings are being applied. “When Redufuse was launched, we noticed that the types that are sensitive to wilting quickly, such as Bacardi, had less of a problem when the sun suddenly broke through. It offers new possibilities. A number of nurseries have already fitted very clear glass. These growers were noticing that the crop wilted suddenly and they had the feeling that growth was below optimal. Then a diffuse coating offers a solution.”
Furthermore a diffuse roof makes it possible to use the black out screen as a shading screen. “If you close it partially under a normal roof you get a sharp separation between the light and shadows. With a diffuse roof the difference is much less,” says the advisor. In the mean time two other chrysanthemum nurseries have also decided to fit diffuse glass with high light transmission for the same reasons as Linflowers. These are currently being built.


Despite lack of research and commercial experience chrysanthemum nursery, Linflowers, opted for diffuse glass, although this was just a bonus on top of high light transmission. On balance it’s calculated to bring 5% extra yield. It has lived up to expectations during the first six months. Also, the diffuse roof gives the opportunity to grow the crop in a different way.

Text: Tijs Kierkels. Photos: Wilma Slegers

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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

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In part 2 of this interview, tomato grower Ted Duijvestijn speaks about how he aims to operate his business without any need for fossil fuels, reduce environmental pollution caused by packaging waste and grow even healthier tomatoes using LED lighting.

Would combining a CHP unit with geothermal heat be viable?

‘We only use them as a back-up for geothermal heat. Our tomatoes aren’t grown with assimilation lighting, so we don’t need the CHP units for that. We do have one CHP unit that’s constantly in operation. It runs on gas from the heat well. The water from the geothermal well apparently contained methane gas. An old 0.9 kWh CHP unit was used to assess the possibility of using this by-product. As soon as it became apparent that we could, we bought a new CHP unit with a little more capacity that could be run at full capacity on the by-product. With this CHP unit in addition to the geothermal heat we have exactly as much energy as we need. We can independently provide in 100% of our energy need.’

Do you really need that back-up?

‘The geothermal heat well initially met all our expectations, but we experienced a problem in March 2015 when the pump got disconnected. It was anticipated that this pump was located 400 meters below the surface and we had to “fishing” for it. We were able to retrieve the motor, but the pump had sunk to a depth of 2,400 metres. We tried to get it our using a cable with a grapnel. After fourteen attempts we decided to give it one last try and were successful.

‘That’s the problem with new technology; you simply don’t have everything fully under control. Initially everything was running smoothly, with a flow rate of 90 up to 185m3/hr. After some time, the return water was no longer absorbed as easily into the ground; the rate dropped to 60m3/hr. We then discovered that it contained gas. The deeper you get, the warmer the gas becomes and it expands, which makes pumping more and more difficult. We then halted operations. Some relief was brought by a separator, which is now being used to retrieve the gas from the supply pipe that fuels the CHP unit. All of this caused us quite a headache. We had to adjust the process three times before it finally worked properly.’

Were you taking too big a risk when you switched to geothermal energy?

‘Innovation is always paired with risks. You could just as easily say: “I prefer not to take that risk”, but that only leads to stagnation. Fortunately, the government can lend a helping hand by supporting, stimulating and acting as a safety net. Ultimately, all the risks such a business case entails are covered sufficiently. It is very important that innovative projects are backed by the government, because they serve a higher purpose, after all. The entire sector can benefit from the know-how thus gained. Besides that, you can use geothermal energy for a multitude of purposes. Look at our oven-dried tomatoes, for example.’

What else can you do with geothermal energy?

‘Numerous conventional uses come to mind: supplying energy to third parties, such as nearby residences. What will the distribution of geothermal energy be like five, or even ten years from now? The real estate market is continually evolving. It is becoming energy neutral, and as a result discussions are taking a completely new turn. If you enter the market for residential energy supply, you may notice that the situation has changed entirely within the space of five or ten years. You have to calculate that into your plan, too.’

Your geothermal energy project got a lot of attention.

‘That was unprecedented. The first year alone drew over 4,000 visitors from all wakes of life: from ministers to students and from colleagues to interested parties from numerous branches of industry, all with a common interest: sustainable cultivation. From this, you notice that sustainability is becoming increasingly important. These visitors are very important to our firm. The discussions you have with them often lead to unexpected and new insights. They ask smart questions, which forces you to come up with smart answers!’

You are participating in a project for wind turbines. Why? You’re already growing tomatoes on an energy-neutral basis.

‘We aim to become even more sustainable. We want to be entirely fossil fuel-neutral; and that extends to our electricity consumption. We brainstormed on this topic, too: how can this be achieved? We came to the conclusion that you should investigate wind energy, specifically. We learned about a wind turbine initiative in the direct vicinity and decided to participate. This project met with some resistance, so the outcome is still rather uncertain.’

Duijvestijn Tomaten also collaborated on a project that makes cardboard packaging from waste foliage, joining forces with other growers as well as the Smurfit Kappa cardboard factory and the Van Vliet waste treatment company.

Why make packaging from your own waste foliage?

‘It creates residual value. We recently designed packaging to our innovative “Silky Pink” cocktail tomatoes, in collaboration with Rijk Zwaan and The Greenery. I always consider waste a thorn in my side. I like to prevent waste, and so we hit upon the idea of a box made from waste foliage. Nevertheless, the challenges you meet with are becoming more and more complex. You need a different perspective and a different approach. And you need to look at things from a different angle: not from an economical point of view, but by thinking about sustainability. You are doing something society believes in: the bio-based economy, sustainability. These are demands you can respond to wholeheartedly.’

To you, sustainability is not simply a catchword.

‘Absolutely not. There is nothing I hate more than wasting food. You have put everything you have into your product: time, energy, labour, nutrients, attention, love - and that’s being thrown away as if it were nothing! I therefore wholeheartedly support the “Kromkommer” Project (ed. Kromkommer is a contraction of ‘crooked’ and ‘cucumber’), in which odd-looking vegetables and fruit are put to use instead of thrown away. Why throw them away? There’s nothing wrong with them. Supermarkets only want perfect products, and that’s only becoming worse. Many products are therefore not suited for sale to supermarkets, while consumers actually couldn’t care less how straight or crooked their cucumbers actually are.

Do you derive inspiration from this for your own firm?

‘Kromkommer is endeavouring to create a more honest chain to alleviate strain on the environment. From the same perspective we also develop products, test them and assess their market potential. Consider our oven-dried tomatoes. You could fill a container of these only halfway, fill the rest with oil, and offer it at a cheaper price. That puts you in a more competitive position, but we’re convinced that this approach is not viable in the long run. Our preferred target audience is composed of conscious consumers. This group may not be large now, but it is growing. Even senior citizens are becoming more quality-conscious and can afford luxury products. And health freaks may be willing to pay a good price for a good product, but the information you present alongside your products has to appeal to them.’

Is this why you produce extra healthy tomatoes?

‘We are investigating this. Wageningen University Research Centre discovered that LED lighting on sprouting tomato bunches produces fruit with a higher vitamin content. We are now trying this out in the ID Greenhouse®. We are testing the effect of LED lighting on the fruit and expect this to bring us even farther in the future. We have also launched a photosynthesis study among LEDs with a view to further optimising our production. However, if all you are taking into consideration is the cost price, this won’t bring you a step further.’

You regularly mention Wageningen University Research Centre.

‘We have a joint venture with the research centre and with various schools. Many of our visitors are students. I like that fact that they don’t pass instantaneous judgement; they never say: “that’s impossible”. An attitude like that allows you to transcend borders. If you promote that, you will go far. You can build on that. And it’s good training for them. Training and education are becoming increasingly important.’

Can you, as a business, keep abreast of all the new developments?

‘Developments come and go at a rapid pace. This puts a lot of pressure on you. However, you can never keep up with all the developments. You simply have to let go. You shouldn’t let this take the upper hand. You may think you can take control of all the developments, but if you’re not careful they will be taking control of you. The dividing line is very thin: innovation is fun, but it brings a lot of risks with it. On the other hand: if you don’t jump aboard the bandwagon, you won’t be able to travel far.’

Duijvestijn Tomaten in Pijnacker was elected ‘the world’s best tomato grower’ in the Crop & Process Technology category in 2015 by a jury who also presented the accompanying Tomato Inspiration Award. The jury was composed of experts Gene Giacomelli (University of Arizona, USA), Ep Heuvelink (Wageningen University, the Netherlands), Stefanie de Pascale (University of Naples, Italy) and Tadahisa Higashide (NARO Institute, Japan). Duijvestijn Tomaten grows 14.5 hectares of primarily round and plum tomatoes. Besides these tomatoes, they also grow Silky Pink, an exclusive new variety of pink tomato with an exceptionally fruity flavour.

Visit the website of Duijvestijn Tomaten-Nursery.

Download the complete interview with Ted Duijvestijn on innovative projects such as geothermal energy, the ID Greenhouse®, CHP units, LEDs, geothermal energy-dried tomatoes and packaging made from waste foliage (login required).

 Copy/photo: Tuinbouwteksten.nl/Theo Brakeboer.