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.