The air above an energy screen is much drier than that below it. Currently around 20 companies in The Netherlands are making use of this fact to dehumidify the greenhouse. However, until recently, hardly anyone had tried vertical ventilation in an unlit crop of tomatoes. Tomato grower van den Broek, partly stimulated by German legislation, is a pioneer in this field. The initial experiences are positive.
The horticultural company run by Paul, Marcel and Jolanda van den Broek has very gradually changed from a Dutch one into a German one. It was logical, based on decisions made over the years, that the company, originally from Venlo, should eventually end up in Germany.
“The buyers were always almost all German. From a very early stage we have always wanted to provide exactly what the customer wants. At that time there was always a lot fuss amongst the Venlo growers if they were told at the last minute what sort of packaging to pack the product in; we only saw opportunities. We concentrated on quality instead of volume but it was difficult then at the auction in Venlo to distinguish ourselves from the rest. Therefore it was logical to go directly to an auction in Germany; there, customers do look at the name of the grower. If you do it well you can earn more,” explains father, Marcel van den Broek.
When producer organisations were formed in the Netherlands the van den Broek family kept their distance. Instead, in 2004 they were able to become a member of Tomatengärtner Rheinland, another new step towards Germany. “The trend ‘local for local’ – the preference for products from your own region – was at that point not very strong. Even so, it made more sense to produce in Germany if you were a member of the Rheinland organisation,” he says. Therefore, in 2007 they bought a building plot in Straelen, sold the company in Venlo and built a 4 ha greenhouse. “Thus we became real German growers and that’s how we feel today too,” he says.
As the crow flies they are just four kilometres from the border but producing in Germany is very different to that in the Netherlands. Firstly, with their four hectares they were for a long time one of the largest horticultural companies in the region. Furthermore, both the energy market in Germany and strict energy legislation are strong determining factors. Indeed these actually drove them towards Next Generation Growing.
Last year the greenhouse was expanded to 6.5 ha. The new greenhouse (with polycarbonate walls, low haze roof 26%) was planted at the end of December. They are growing cluster tomatoes (Lyterno) and mini-Roma tomatoes (Strabena). In the early days the older greenhouse was heated using a coal-fired boiler of 3.5 MW. “Natural gas in Germany is twice as expensive as in the Netherlands; then it’s quickly a choice of coal or wood. But we did look into how we could make it more environmentally friendly. Now we hire a co-generator (CHP) of 1.6 MW from Weltec Biopower that is run on biogas. We use the heat and the green energy goes into the grid. We’ve just installed another such CHP and then 90% of the heating will be from biogas. We’ll only burn coal to handle the peaks,” explains his son, Paul.
The decision to lease the CHP was stimulated by the German legislation: “If you install your own CHP you have to pay an energy tax on the electricity that you generate; then it’s not very lucrative,” he says.
They’ve been applying the principles of Next Generation Growing for some time already: “In the past we used the minimum pipe rail much more,” explains the father. “Now we shade more and accept that sometimes there’s a day when the temperature is somewhat lower.” They now use 31 to 32 m3 gas in the greenhouse that is green for 49.5 weeks per year.
The energy efficiency program in the State of North Rhine-Westphalia has a big influence on the decisions too. “Greenhouse horticulture companies have to achieve energy savings and every measure taken scores a certain number of points: the polycarbonate walls, the energy screen, the climate computer. To achieve the standard we were virtually obliged to install a second screen in the new greenhouse even though we would use it very little. The investment was out of proportion to the number of hours it would be used.”
Because the moisture in the old greenhouse often became too high, they approached Greenco, of Middenmeer and Someren, the Netherlands, about the possibility of using the VentilationJet-system by Hinova. It consists of two fans; an upper fan draws dry air from above the screen to below it. The lower fan mixes the air and spreads it evenly. In this way dehumidification and an homogenous greenhouse climate are achieved together. Furthermore, this system counts towards the necessary energy points.
“It was difficult to judge if it would work for us because Greenco uses lighting and has a second screen which we don’t have. Nevertheless an extra tool for moisture control seemed attractive. We don’t really like to be a front-runner but because we were the first unlit tomato grower to have such a system we have had to discover everything for ourselves in terms of greenhouse climate. Luckily in the meantime two other members of our growers’ organisation also have such a system,” says Marcel van den Broek.
The new greenhouse now has 52 VentilationJets and 52 separate Hinovators; the latter is a fan that pulls greenhouse air upwards and spreads it like a blanket over the crop ensuring even mixing: the air distribution fan.
Check by eye
They have only been using it for three months but they speak very positively about it. “The temperature distribution in the greenhouse is much better. You see that from the crop. I get goose bumps from such evenly coloured tomatoes,” says Paul van den Broek. “The aim is keep the relative humidity at 87% under the closed screen; that works well. “In the old greenhouse, we need to open the window vents above the closed screen much more often."
In the evening (18.00 - 23.00) everything is switched off. “Then the plant needs to relax and unwind and that’s not possible if the fans stimulate transpiration. After that the system restarts and the computer determines the force and capacity. We check it by eye: the crop needs to have fresh-looking tops. If they start to turn grey we intervene.”
An homogenous climate gives a somewhat higher production; then the gas consumption per kilo of product is lower. “In addition, we can use the screens for slightly longer. Together that could lead to about 10 per cent energy savings per kilo of product," he says.
Tomato grower Van den Broek has gradually changed from a Dutch into a German company. An interest in Next Generation Growing as well as German energy legislation have driven some noteworthy decisions, such as a leased CHP that runs on biogas and a system with two fans placed under each other. The initial experience with this system is very positive: the climate is more uniform and the crop too.
Text: Tijs Kierkels. Images: Wilma Slegers.
Reducing energy consumption in greenhouses is associated with higher humidity levels. Many growers are concerned that this could make the crop less active, impacting on transpiration from the top in particular. Last year, a Dutch research project was launched with the specific aim of measuring that. The conclusion: rather than a reason for concern, intensive screening is actually a way of improving the crop.
The “Transpiration from the Top” study took place at the Wageningen University & Research greenhouses in Bleiswijk, the Netherlands. Over two months, greenhouse climate and energy researcher Feije de Zwart tested a measurement method in a mature tomato crop and evaluated the quality of the technique. “We looked at the extent to which we could specifically measure transpiration at the top of a crop using a thermal imaging camera. And it worked. But this method requires an extremely accurate camera and a lot of attention. So for the time being, although this camera is excellent in the lab, it is not really suitable for use in the commercial setting.”
Low-energy cultivation not only requires better greenhouse insulation, but it also means the crop has to be grown in higher humidity levels. Growers tend to use their screens more often and don’t often open gaps in them. This increases the humidity in the greenhouse. “One of the reasons why growers are reluctant to accept this situation is that they are concerned the crop may not transpire enough,” de Zwart says. “After all, the more humid the air in the greenhouse, the lower the difference in vapour pressure between the crop and the greenhouse air, and the less the crop will transpire. They are most concerned about the top of the plant.”
These concerns stem from the fact that inadequate transpiration can affect nutrient transport to growing tips. He adds: “To measure is to know, so we developed a measurement system that can accurately determine transpiration at the top. A sensor of this kind could put growers’ minds at ease and could result in wider acceptance of higher humidity levels, particularly in vegetable cultivation.”
Camera plus artificial leaf
To test the system, the researchers installed a thermal camera above the crop. Besides the leaves on the crop itself, there were always two artificial leaves in the camera’s line of sight. One of these artificial leaves was fitted with a PT-100 temperature sensor to check the temperature registered by the camera. With this setup they were able to compare the temperature of a leaf at the top of the plant with that of an artificial leaf that was not transpiring, in the same conditions. The lower the temperature of the real leaf compared with the non-transpiring one, the greater the transpiration.
De Zwart again: “In fact, we noticed that the temperature of the real leaf fell quite a bit below that of the artificial leaf at night and that the temperature difference increased as the humidity level dropped. The best thing was that the behaviour of the real leaves was very much in line with our expectations based on our calculations. When we took another close look at the calculations, we noticed that the reduction in radiated heat loss brought about by screening really does increase the temperature at the top of the crop quite significantly. And this in turn leads to higher levels of transpiration at the top.”
De Zwart’s conclusion is therefore that transpiration at the top of the plant simply continues when screens are used, even if humidity is higher. “Intensive screening can limit transpiration from the crop as a whole but, conversely, stimulates it from the top of the crop.”
Vertical differences in the crop
These results could perhaps explain why good yields were achieved in all those practical trials with Next Generation Growing, despite the expectation that the high humidity would cause problems. De Zwart again: “If you look at water uptake, for example by comparing the amount irrigated and the drain, or by using a weighing gutter, you can barely see the effect of screening at all. But if you look at the increase in temperature in the crop, then you can see that closing the screen increases the temperature at the top, while often lowering it slightly further down in the canopy. This is because the use of the screen means less heating is needed further down.”
In any case, the temperature gradient across the crop drops, making transpiration more even throughout the crop. “That last factor, the evenness, had never really occurred to me,” he adds. “So rather than being a reason for concern, intensive screening is actually a way of improving the crop. This realisation is essentially the most important outcome of our research. It is such interesting information that it has been added to the Radiation Monitor.”
Growers and other interested parties who attended the Next Generation Growing course in the Netherlands will already be familiar with the Radiation Monitor. This online simulation model calculates the effects of screening and greenhouse covering materials on energy consumption and vertical temperature distribution.
De Zwart used the same model to establish the expected difference in temperature between a transpiring and a non-transpiring leaf. However, the data obtained from the project mentioned above demonstrated that the original calculation method was too inaccurate. Following improvements, the program now calculates transpiration at each layer of the crop.
The basis for this is that the difference in vapour pressure between the greenhouse air and the leaves plays a bigger role in driving transpiration than the local leaf temperature. The program can be used via the greenhouse horticulture models website.
De Zwart is slightly less enthusiastic about the results of the original project setup. “The measuring equipment was quite tricky to set up. We didn’t have a problem collecting images with a thermal camera, but focusing the lens was difficult. The plant was growing, so we had to constantly refocus the lens. Also, you have to use artificial leaves. We now know that a tomato leaf transpiring at the normal rate at the top of the plant is around 0.4°C cooler at night than a non-transpiring leaf in the same place.”
If this measurement method is used to distinguish normally transpiring leaves from leaves transpiring at a lower rate, the temperature differences measured should be in the magnitude of 0.2°C. These are such small differences that you would need to know exactly how warm a non-transpiring leaf would be in that position. That is why you need artificial leaves and a very accurate camera. He adds: “Actually, you do wonder whether the information you get is really worthwhile. After all, we now know that transpiration at the top of the plant simply continues when screens are used intensively, even if the air humidity is higher.”
Low-energy cultivation means a lot of screening hours and higher humidity in the greenhouse. Research shows that although higher humidity causes transpiration to decrease, the use of screens does not affect transpiration from the top of the plant. The project used a thermal camera and artificial leaves. The setup worked and, besides providing figures for the top of the crop, it also highlighted the vertical temperature distribution in the crop. The data was integrated into the online Radiation Monitor.
Text: Jojanneke Rodenburg.
Images: Wageningen University & Research and Jan van Staalduinen.
Screen suppliers each have their own methods of determining the energy-saving performance of their screens, making it very hard for the grower to compare them. But this is about to change. Wageningen University & Research in the Netherlands has been working with screen manufacturers to develop an objective measurement method.
There are many different types of screen fabrics: from fully transparent to completely dark, from woven to knitted, with open or closed structures and with or without aluminium components. Screens are also used for different purposes: saving energy, reducing solar radiation, diffusing light or as a blackout. But whatever your reason for screening, as soon as you close a screen you start saving energy.
You would think it would be difficult to compare the energy-saving credentials of such a wide variety of screens. But that’s not the case. How much energy can be saved depends on a number of physical phenomena which can be measured in all screens. However, if all manufacturers apply their own criteria, the results will always be difficult to compare.
This problem has been around in the sector for some time. A previous project by two Dutch research institutes, Wageningen University & Research and TNO in Delft, and two screen manufacturers resulted in a method of objectively determining screen emissivity, but this method is not suitable for quantifying the screen’s energy-saving performance.
“The amount of energy saved by a screen fabric depends on three components: radiation exchange (transmissivity and emissivity), air permeability and water vapour transport through the fabric. If you can measure these three properties, you can compare screen fabrics,” says Silke Hemming, head of the WUR Greenhouse Technology research team. Hemming headed up the project which, besides Wageningen University & Research (WUR), also involved Cultilene, Ludvig Svensson, Low & Bonar and Novavert.
“The manufacturers sent us 29 different fabrics,” Hemming explains. “Because the aim of the project was to develop a measurement protocol, we selected seven that were very different, ranging from very dense to highly permeable, for example. We used these to develop effective measurement methods.”
This resulted in a quest for which the WUR LightLab even had to build its own equipment in order to determine one of the properties. Hemming: “We started off with radiation exchange. There was already a device available for this: the TNO emissivity meter. The total emissivity (radiated heat loss) depends on the transmissivity of thermal radiation through the fabric, the reflectivity of radiation on the underside and the absorption and emissivity of radiation on the upper surface.”
Some screen fabrics allow high levels of thermal radiation through, whereas a highly aluminised screen does not. The combination of low thermal radiation transmissivity and low emissivity on the upper surface traps the radiated heat on the inside, maximising energy savings and minimising cooling of the crop.
The second part of the measurement protocol is air permeability. If it is warm inside the greenhouse and cold outside, air permeability always means a loss of energy (sensible and latent heat). This property was particularly tricky to measure. Hemming: “First we investigated what air speeds were commonly found in greenhouses. They turned out to be very low, which meant that we were unable to carry out measurements in a wind tunnel. So the LightLab developed a piece of equipment especially for us: an air suction device, or permeability meter.”
This consists of a round steel tube in which small discs of the screen fabric can be clamped. Air is sucked through the fabric at different speeds, causing a slight pressure difference. By measuring the air pressure on both sides of the fabric you can quantify its permeability.
Water vapour transmission
There is no standard for the two properties mentioned above (emissivity and air permeability). However, there is one for the third property, water vapour transmission: ASTM E96. This standard originates in the textile industry, where it is used for applications such as developing breathable clothing. Water vapour transmission is determined using the cup method. “But this standard turned out to be irrelevant to greenhouses. In a greenhouse, not only is there a difference in moisture concentrations on either side of the screen, there is always a temperature difference as well. The cup method therefore underestimates water vapour transmission in greenhouse situations. And then there is the issue of condensation forming on the fabric. So we brought in the Swedish institute Swerea, which can measure all relevant properties of water vapour transport through textiles,” the project leader explains.
Screen fabrics differ quite considerably. Some let a lot of moisture through, which benefits growers who want to use this method to remove moisture. On the other hand, a screen that is completely impermeable to moisture saves a lot of energy. “But that means you need another way to remove moisture, such as a mechanical dehumidification system,” Hemming says.
The three fabric properties are not entirely independent of one another. A completely wet screen is impermeable to thermal radiation, for example, and this was taken into account in the project. The main aim of the project was to develop replicable measurement methods. If manufacturers were to start using these methods as a standard and pass on the measurement results to their customers, every grower would be better placed to decide which screen to use. Simply making a more informed choice can deliver energy savings of 5%.
Once they had developed the measurement methods, the researchers entered the results into the KASPRO climate and energy model to work out the overall energy saving performance for each screen. The model crop was tomato, and the screens were only used at night to enable all the fabrics to be compared under standard conditions. “The highest scoring screen saved around 26% year round compared with the reference (no screen), while the worst performing screen only saved around 9% as it was highly permeable to light and moisture. Looking at the savings achieved during the night-time screening hours alone, the highest scoring screen scored almost 50% and the lowest 13%,” she says.
These figures relate to the fabric properties. The actual energy savings to be achieved in practice depend very much on how the grower uses the screen (the number of screening hours and the time at which the screen is closed) and on the type of screening system they use. “If you only look at the energy savings, the best fabric is one that allows no thermal radiation, moisture or air through at all. But without adjustments this would simply result in condensation on the crop. The grower will always have to find a balance between saving energy and the amount of moisture they want to remove through the screen,” Hemming says.
The researchers are calling on screen manufacturers to routinely include five fabric properties in their product information: emissivity, thermal radiation permeability, air permeability, water vapour transport and a performance indicator for total energy savings. At present, product brochures tend to only give the highest figure: the energy that can be saved on the coldest night of the year. The new method makes it possible to present a realistic and comparable indicator for energy-saving performance.
Hemming expects the new measurement protocol to have many benefits: it will enable growers to make better informed choices and manufacturers will gain new insights into aspects such as air permeability, which was previously poorly quantified. They will then be able to incorporate these findings in their product development work.
Researchers have been working with screen manufacturers to develop an objective method of determining the energy saving performance of screen fabrics. The method looks at three parameters: radiation exchange, air permeability and water vapour transport through the fabric. If manufacturers elevate this method to a standard, it will enable growers to make more informed choices when buying screens.
Text: Tijs Kierkels. Images: Wilma Slegers.
In a Cappricia crop in the 2SaveEnergy Greenhouse, we are seeing how far we can go with limited ventilation. Among other things, an average 24-hour temperature of 21.4ºC was achieved in April.
The crop tolerated this for quite a while, but in early May we had to start providing more ventilation. But venting more during the day means that less CO2 and moisture are retained in the greenhouse. In the 2SaveEnergy greenhouse, we have also been trying to save as much energy as possible without affecting the strength of the crop. That has worked well so far. Up to April, we were using 5 m3/m2 (planting date: 5 January) plus 10 kWh electricity for the heat pump.
This greenhouse has a double glass roof with an F-clean film and is also equipped with a double aluminium screen and a transparent (Luxous) screen. To limit outgoing radiation, the screen is closed quite early at the end of the day, at around sunset. In winter and spring we don’t generally vent off heat towards the evening. During dehumidification, we recover both the sensible and latent heat from the air and we also use the heat from the heat pump for heating the greenhouse air.
Hans Houben’s initial reason for trying out Next Generation Growing was to save energy. “But that shouldn’t really be your main objective: you need to focus on the plant,” the cucumber grower says. This has led to a higher 24-hour temperature, growing with the light, more screening and adjusting for outgoing radiation. Oh, and lower gas bills too.
Every year is different when you use the Next Generation Growing (NGG) method. You keep taking one step further and all of a sudden you’ve created a completely new way of growing. “I am always very keen to keep up with the latest developments, but to begin with I was quite sceptical. It all sounds very logical but it takes courage to do it. Right now I’m starting venting on the wind side, for example. It is working out well: you get a much more even climate and it’s easier to keep the humidity at the right level. But I really wouldn’t have done it this way two years ago,” Hans Houben says.
His screening hours have also increased: he is now screening twenty per cent more than in his second NGG year. “During the first couple of years you tend to be a bit wary of doing things this way. Before I started I used to mainly keep an eye on relative humidity, but now it’s all about absolute humidity, humidity deficit, vapour pressure and outgoing radiation.”
Back to the plant
Hans and Carla Houben’s cucumber business Mellantas in Sevenum (4.7 ha) in the south-east of the Netherlands is on its third season of high-wire cucumbers. At their old site they had two crops of cucumbers per year, followed by autumn tomatoes. After moving to their new location they introduced high-wire cultivation with two crops per year, first Topspin and then Kurios. The plants grow in rockwool that lies on the ground.
Gas consumption is currently at 28.5 m3/m2 for production of 230 cucumbers per square metre. A traditional crop would use 34-35 m3/m2 for 180-195 fruits. “The power of Next Generation Growing lies in the fact that you are going back to the plant,” Houben says. “We have started growing more quickly, with a higher 24-hour temperature, but we keep the plant load at no more than 6-7 cucumbers per plant. From 11 am onwards we allow the temperature to get higher than before, light permitting.”
For example, with 1,000 watts of incoming radiation the 24-hour temperature is 21.5ºC, and with 500 watts it is 19.3ºC. In his first two years of NGG, Houben allowed an extra 1.5ºC per 1000 joules of incoming radiation over and above the basic temperature of 18ºC. Now it is 2.5 to 3ºC extra – so quite an increase. He achieves this with a combination of heating, screening and ventilation.
“An extra 1.5ºC saves more energy, of course, but it makes the crop more sluggish. When it’s sunny we want a higher temperature, preferably 28ºC after 11 am rather than 25ºC, light permitting.” Before 11 am he aims to achieve a moisture deficit of 1.5-2 g/m3 to activate the crop; after that he works up to a higher temperature in a gradual line. “I used to turn the temperature down sometimes if there was a lot of light. But I don’t do that any more. You can tell by the top of the plant whether you are doing the right thing. If it is getting too thin, the 24-hour temperature needs to come down.”
If the temperature is higher during the day, the night temperature can be reduced slightly, although it is the overall 24-hour temperature that counts. Less use of minimum pipe prevents excessive evaporation and limits night-time energy consumption. Incidentally, the main source of heat is the grow pipe, which is always level with the fruits, and not the pipe rail.
More outgoing radiation
Over the past few years the grower has started screening twenty per cent more to limit outgoing radiation. He uses a very light Luxous energy screen from Svensson which only screens out twenty per cent of the light. A radiation meter (pyrgeometer) on the roof helps control the screens. There is also a thermal camera pointing at the crop. This isn’t connected to the climate computer but is used as an additional adjustment tool. Houben demonstrates how it works on the computer screen. “This morning there was a rain shower just after we opened the energy screen. You can see on the thermal image that the temperature at the top of the plants dropped to 15.5ºC at that point. You want activity but the tops of the plants are cold. So I closed the screen again and within ten minutes the temperature at the top of the plants had risen by 4-5ºC. That’s because you are eliminating outgoing radiation.”
The principle is simple. When outgoing radiation is higher than what is coming in, the screen is closed, even on a warm summer’s day. “In that case you close it ninety per cent. Then you can control the temperature easily and control outgoing radiation at the same time,” he explains.
The numbers always add up. For example, if there is 200 watts of radiation coming in, the screen blocks out 40 watts of that. But with a clear sky, outgoing radiation from the crop soon reaches 80 watts, and because that is more than 40, the screen has to be closed.
Houben has invested in a pyrgeometer, a thermal camera, a leaf temperature sensor and an extra sensor unit above the screen, but not in air handling units, extra fans or a second screen. “I could save an extra 2-3 m3 of gas with a second screen, but then I’d need a dehumidification system as well. The maths wouldn’t necessarily work then. So we decided not to do that just yet. We are waiting for dehumidification technology to move in a clearer direction,” he says.
With the experience he and other growers have gained, Houben sees potential to improve the system even further. “There is definitely scope to optimise the light/temperature ratio. You might be able to grow even faster with more light. If you can pluck up the courage, you could turn the temperature down more in the evening because you would still be achieving the 24-hour temperature, and that’s what counts. So an extra pipe during the day and not at night. But you could even raise the 24-hour temperature, which would enable you to maintain a higher temperature at night and make better use of the screen. And on sunny days you could also extend the day by switching to the night temperature later.”
Houben is also trying to gain a better understanding of the minimum level of evaporation needed at night. He is very happy with the knowledge shared on the LetsGrow platform. “I am learning a huge amount by looking over other growers’ shoulders. You don’t have to find it all out for yourself. You can see exactly what time other people open their screens and what that achieves. Next Generation Growing is really still in its infancy. You can get much more out of it if you focus primarily on the plant.”
High-wire cucumber grower Hans Houben is heading ever further down the path of Next Generation Growing. He has started screening more and keeps an eye on absolute humidity, humidity deficit, vapour pressure and outward radiation levels. His 24-hour temperature is up and it could even go a little higher. He is doing all this with one screen and no air handling units or extra fans.
Text: Tijs Kierkels. Image: Wilma Slegers