The ISO++ double screen system from Bom Group is unique in the horticultural industry. It is an innovative double screen system which lets more light through and saves more energy.
The two parts of this patented double screen can be operated individually despite being just 6 cm apart. The two fabrics are attached to the upper beam of the greenhouse trellis girder, keeping the lower beam free from attachments so it can be used for other mechanical installations such as crop wires or a third screen.
Minimum light loss
Because the two screens are so close to each other, when they are both closed a cushion of stationary air forms between the two screens, saving energy and producing an insulating effect. The proximity of the two screens also helps minimise light loss.
Stand number: 08.108
Next Generation Growing has played a major role in the choices Dutch orchid grower Rob Olsthoorn of OK Plant is making for the new greenhouse he is building in Naaldwijk. “We want to grow in a closed environment as far as possible. The three screens in the greenhouse are helping us achieve that.”
The bottom screen is a transparent, energy-saving fabric. The middle screen is a shading fabric with an open structure which shades 55% of the light. For the top screen, Olsthoorn opted for a light reduction fabric. This screen keeps 99% of the assimilation lighting inside the greenhouse and he can also use it as a blackout when necessary.
The glass is highly diffuse. “This way, I get good light distribution with my triple screen,” the Westland-based orchid grower says, walking through his new greenhouse. “The middle screen shades 50% of the sunlight. That’s plenty. I’m not a fan of screening too heavily. It’s better to have too much light and to have to add in the other two screens than to have too little and not be able to adjust it. It’s important for us to be in complete control so we can create the growing conditions that are best for us. Everything revolves around quality.”
As closed as possible
Olsthoorn grows Phalaenopsis in 9 cm pots on 6 hectares, the last 2.5 hectares of which are currently being built. He has another 2 hectares on which he grows seasonal products like cyclamen, campanula and Primula obconica. “We are targeting the higher segment, such as wholesalers and garden centres. We don’t supply the mass market but specialise in plants with solid added value. And that needs us to be completely focused on quality. Some customers are so strict that they raise the alarm as soon as they see a bad leaf or a mark.”
Next Generation Growing plays a major role in improving quality in Olsthoorn’s greenhouse. “The climate, RH and energy consumption must be as stable as possible so that we can grow in as closed an environment as possible.”
So the nursery opted for a triple screen system which was built into the structure of the greenhouse. You won’t find any end strips made of fabric there. In this part of the nursery, the greenhouse builders Technokas built white steel plates into the greenhouse structure at each end of the screen installations. As a result, the greenhouse is completely energy-efficient and light-proof for its entire service life.
“Sitting down with the right partners in the preliminary phase makes a big difference. This system is actually a screen system 2.0,” says Jeroen de Jonge of Peter Dekker Installaties (PDI). “It saves us a lot of work. Our installers used to only be able to start making the fixed strips once the construction stage was finished. They had to squeeze in around heaters, water pipes and lamps to secure the strips for the three screens to the greenhouse separately. With this new system, the greenhouse builder hangs up the metalwork as the building work gets under way. Then all we need to do is pull up the screen, secure it – and that’s it. This system is much less prone to breakdowns and it’s maintenance-free.”
The grower is also hugely impressed with the system. “I’m ready for the next 20 years now. It’s an investment but it will easily pay for itself in the long term. The closed system keeps the greenhouse climate much more stable and it’s quite a lot easier to maintain. I no longer have any flaps that keep coming loose when I spray down the greenhouse.”
The triple screen they went for in combination with highly diffuse glass also serves a purpose. “It means we can use a transparent energy screen that lets in maximum light. It allows the sunlight in in a much more evenly distributed way.”
More resilient crop
The plants under the screens warm up more slowly, enabling them to absorb more sunlight and therefore more UV radiation, the orchid grower believes. “That produces a stronger plant and therefore a more resilient crop.” He has seen it in Asia with his own eyes. “They grow in plastic greenhouses there. I took my light meter with me and I discovered that they let in a lot more light than we usually do here. And yet the plants were still a rich green colour.”
To prevent the climate from becoming too humid, he is installing a dehumidifier unit in the new greenhouse which blows dry air in from outside. “This means I won’t have to adjust the screens so much and the temperature inside the greenhouse will stay more stable.”
Double top wires
The triple screen has one downside: the screens are very close together. On a 60 cm high truss there are three axles which can rotate independently. If the fabric blows up there is a risk that it can become trapped in the turning axles. “Of course, three screens are always riskier than a single system,” says De Jonge. “So we have installed systems that mitigate that risk. For example, we fitted an axle guard on the axles, like a kind of emergency brake. If there is a malfunction, the motors stop, preventing any consequential damage. Fitting double sets of top wires to prevent the fabric from being blown up is also a must with a triple screen.”
No matter how well the greenhouse is equipped with screens, diffuse glass and a dehumidification system, vents remain a problem. To solve this, Olsthoorn has opted for one-sided ridge ventilation. “Fortunately we were able to build the greenhouse in such a way that the vents could be fitted on the north-eastern side of the roof. If the sun shines at noon and the vents are open, no direct sunlight enters the greenhouse, so the plants don’t get burned.” He is not bothered about the wind. “Ninety percent of the time it comes from the west.”
Not only are they building a new greenhouse, they are also adapting the existing one to improve the quality of the crop. The grower also has a triple screen there: an 88% screen fabric, a 66% fabric and a diffuse, transparent screen. “This plus the combination of float glass and 50% chalk screening isn’t enough to further optimise crop quality,” Olsthoorn explains. So fitters are also installing an outdoor screen above the greenhouse roof. The space between the greenhouse roof and the screen can keep the temperature inside the greenhouse around 6-7ºC cooler than outside.
Olsthoorn opted for a 50% screen fabric with an open structure. “That means I don’t need to use forced cooling so much and I need to use less artificial light in dark weather in the summer. That saves a lot of power,” he says.
This may be difficult to justify financially compared with his chalk screening, he admits. “But it will pay off in the long term. The outdoor screen gives us complete control over the weather conditions. If it only helps us deliver fractionally better quality, we will have achieved what we set out to achieve with the outside screen.”
Grower Rob Olsthoorn of OK Plant in the Netherlands deliberately opted for a triple screen for his new greenhouse so as to make it as closed an environment as possible. To improve climate equality even further, he integrated the screen into the greenhouse structure, producing a greenhouse that will be completely energy-efficient and light-proof for its entire service life. The existing greenhouse is being fitted with an outdoor screen which will make the weather conditions completely controllable. The aim of all these measures is to improve quality.
Text and images: Marjolein van Woerkom.
A new online app quantifies the effect of radiated heat loss on crop temperature and energy loss from the greenhouse in a simple, user-friendly way. The radiation monitor is a handy tool for growers who want to get a better understanding of aspects such as the use of screens and greenhouse cover materials. More knowledge of physical and phytophysiological processes in the greenhouse and the crop can help the grower produce even better results.
Anyone with a PC can use the radiation monitor. The app was launched recently, and Aat Dijkshoorn, Next Generation Growing (NGG) project manager in the Netherlands, is very happy with the result. “This program makes it easy to calculate the effects of screening on energy consumption and vertical temperature distribution. It helps growers take concrete decisions – such as whether or not to close the screens tonight – and supports the trend towards energy-efficient growing.”
Know your temperatures
Knowing the plant temperature helps the grower grow more efficiently and accurately. As crop adviser Peter Klapwijk recently put it on HortiNext: “When I talk to growers about their climate strategy, I often realise that they still see the greenhouse air temperature as the most important reference variable. Many of them understand the importance of plant temperature but dismiss it because it’s ‘so difficult to measure reliably’. So people don’t tend to pay much attention to it. But this is a misconception because it is essentially the temperature of the plant that determines the crop’s growth rate and how it is steered.”
Measuring plant temperature is by no means easy, Dijkshoorn admits. Temperature is a result of all the energy flows that occur inside and outside the greenhouse. A simple sensor unit or thermal camera will only capture part of all that data. Then there’s the problem that the equipment needs to be incredibly accurate to register the differences, which are often only a matter of decimal places. I’m convinced that the radiation monitor makes that a thing of the past as well. The simulation model gives an excellent picture of cause and effect, which makes the plant temperature much easier to steer accurately.”
Relative humidity and transpiration
It’s a well-known fact that there is a link between screening and temperature, and the use of screens has risen substantially in recent years as NGG gains in popularity. So it’s no surprise that all kinds of initiatives are being launched to attempt to shed more light on this relationship. The radiation monitor does that very well.
The program was devised by Wageningen University & Research in the Netherlands. Researcher/developer Feije de Zwart understands exactly what lay behind this assignment. “The fact is that many growers are still reluctant to use screens intensively and will only close their screens if the difference between the indoor and outdoor temperature is more than 10°, for example. They understand straight away that a screen saves energy but what they often don’t realise is that it can also bring about more homogeneous vertical temperature distribution in the greenhouse. Many growers mainly see screening as a way of increasing relative humidity. And that can be risky. After all, the more humid the air in the greenhouse is, the lower the difference in vapour pressure between the crop and the greenhouse air will be and the less the crop will transpire.”
Screening is good
Time and again, practical experience shows that intensive use of screening is not necessarily detrimental to crop quality and production. The same conclusion was reached in the “Transpiration at the head” study. This study revealed that reducing radiated heat loss by screening substantially increases the temperature at the head of the crop, leading in turn to higher levels of transpiration at the head. Intensive screening can limit transpiration from the crop as a whole but, conversely, stimulates it from the head of the crop. Increasing understanding of this phenomenon by explaining the theory, demonstrating measurements in practice and producing a software tool that quantifies the various effects could help raise awareness of the importance of screening even further.
“That’s exactly what the radiation monitor does,” Dijkshoorn points out. “The application produces the numbers to back up what we have been seeing in practice for some time, namely that the use of screens not only helps save energy but also benefits the crop. It tells you exactly when you can expect to save energy. As I said before, with this model at their fingertips, growers can optimise their use of screens even further.”
The radiation monitor calculates the energy balance (the sum of the incoming and outgoing energy flows) based on a large number of relevant parameters. De Zwart: “As everyone knows, the most important parameters are the outdoor and greenhouse air conditions, the greenhouse envelope, the number and type of screens, the crop and any lighting used. The physical properties of the greenhouse envelope, screens and lighting then determine exactly what the energy balance will be. When we wrote the program we decided to show these parameters in the extended help document but without making them editable.”
Other properties can be configured by selecting different greenhouse roofs, screens, crops or lighting systems, but not by changing the parameters at user level. “This way we can guarantee that only realistic parameters are used. The model then calculates the temperatures at various crop heights and, where applicable, at projecting parts of the plant, such as the flowers on gerbera, for example. The app also displays the energy consumption and light intensity at crop height. That is relevant when lighting or transparent screens are used during the day.”
According to Dijkshoorn, the application really comes into its own when different scenarios are compared. “Then you can select a scenario with the screens open and compare it with a scenario with one or two screens closed, for example. But it is important to choose realistic values for the greenhouse climate, especially as the RH in the greenhouse can change,” he says. Kas als Energiebron, one of the initiators of the project, envisages even more uses for the model and would like to see it extended to include more selection options. “For example, at the moment you can only choose between a closed screen or an open one,” Dijkshoorn explains. “But in practice some growers work with screens closed 80% of the way. How does leaving these gaps impact on the temperature? These are aspects that the model can fine tune even further.”
The user-friendly radiation monitor app is already available to growers. It uses a minimal number of input fields: just enough to produce useful calculations of the effect of screening while offering the user plenty of scope for selecting starting points they can recognise. Detailed instructions for use are available on the website and an instruction video is currently being produced. The people behind the app also hope to encourage growers to use the app in workshops, information sessions and through the Next Generation Growing course.
The radiation monitor displays the realistic effects of greenhouse roofs, screens, lighting and other user settings on temperature distribution in the crop and light intensity at the head of the crop. The monitor also shows temperatures on the surface of the screen and the greenhouse roof. All this gives the grower a better understanding of the effect of screening. The program is an internet application that can be operated on the PC.
Text: Jojanneke Rodenburg. Images: Leo Duijvestijn and Jan van Staalduinen.