The use of Direct Current in greenhouse horticulture appears to be a very promising alternative. A pilot in the greenhouse horticulture sector demonstrated a positive business case for the use of Direct Current (DC) for greater durability of components, as well as cost and material savings. DC also supports the idea of climate-neutral greenhouse horticulture, as demonstrated in the Direct Current Roadmap.
The DC Roadmap, presented last Friday, is a report compiled by Berenschot at the order of RVO.nl for the Energy Top Sector and TKI Urban Energy. This DC Roadmap focuses on ‘DC microgrids’ and seven specific areas of application. A microgrid is defined as follows: ‘a system of interconnected sources and users that can operate, either independently or linked, on a higher-level grid and can exchange energy’.
Greenhouse horticulture comprises a DC microgrid
The various DC microgrids are, with respect to the innovation phase, at the beginning of the S curve: there is a great deal of uncertainty and there are numerous, divergent opinions and ideas about the value (social or otherwise) of DC microgrids. The report, however, revealed that DC is highly promising in greenhouse horticulture; only second to the market for public lighting. The reporters visited greenhouses whose entire indoor electrical system is set to DC. In this, a single, centralised AC to DC transformer is used, to which a lighting system with DC light fixtures (SON-T or LED) and in some cases a CHP unit is connected.
Advantages of DC in comparison to AC
The use of DC in greenhouses extends the life of the light fixtures. Using thin film condensers instead of electrolytic condensers allows greenhouse growers to opt for components with a longer useful life. In addition to this, material savings can be achieved because a DC system uses cables that are smaller in diameter, which therefore require less copper. Researchers also reported that DC makes the integration and control of systems easier. It enables light fixtures to be dimmed individually because the DC cabling simultaneously allows for the control of lighting (powerline communication). Lastly, the centralised conversion of AC to DC will ensure that less energy is lost in comparison to local conversion per lamp (2 - 3%) at the start of operations.
Rounding off the pilot phase
The Roadmap predicts that the pilot phase for using DC in greenhouse horticulture will be rounded off soon. Sustained growth is possible due to the increasing demand for sensors and PV systems. The first successful pilot was completed in the Netherlands and demonstrated a positive business case. This pilot is being conducted at the Jaap Vreeken bouvardia nursery. The pilot is currently being continued at a larger scale.
Conducive to LED systems
Newly built or renovated greenhouses can now also be fitted with DC electrical systems. This applies primarily to nurseries with DC-fed SON-T or LED (in the near future) light fixtures. It is anticipated that using DC will also decrease the costs of LED systems. In the future, priority will be attached to the use of PV panels and the integration of smart innovations (such as controllable light fixtures and smart sensors) in greenhouse horticulture. The integration of these technologies can strengthen the benefits of a DC microgrid.
Hortinergy is an online software package for designing energy-efficient greenhouses by simulating energy consumption and comparing technical solutions.
Energy is a major expense in greenhouse horticulture. There are currently several solutions on the market that can help reduce your energy bill. The dilemma is how to choose the best configuration adapted to the climate outside and inside the greenhouse and the crops grown in it. This is the first online software solution to simulate the energy consumption of an existing or planned greenhouse anywhere in the world.
Suitable for a wide range of users, from growers to consultants and greenhouse equipment manufacturers, it is user-friendly and it takes less than 15 minutes to enter your parameters. To simplify the user experience, equipment manufacturers can spotlight their branded products for selected pre-set parameters. Hortinergy is a decision-making tool for sizing equipment and optimising investments: users can compare energy efficiency and technical scenarios with a simple online interface.
Stand number: 12.132
The LED lamps in the light fittings underneath the top growing layer shine brightly on the plants in the cultivation greenhouse at phalaenopsis growers De Vreede in Bleiswijk in the west of the Netherlands. The light may look white but actually it’s the right combination of colours. It’s one of the innovations that brothers Herman and John de Vreede are working on as part of their drive to supply large volumes of uniform quality orchids more sustainably. They did most of the preliminary research into the right light spectrum themselves.
The phalaenopsis nursery moved to Bleiswijk in 1995. The brothers soon bought the nursery next door and then another two sites 300 metres and 2 kilometres away, making a total of 12.5 hectares of growing space. Each of the sites is equipped for a specific purpose.
The cultivation greenhouse, where the plants spend their first 35 weeks, is heated to a temperature of 28ºC. Then they move to the spike induction site, where they stay until about week 55. Here the plants start off warm and after a few weeks the temperature is reduced to 19ºC to induce flowering. In this phase, the plants are spaced wider apart, staked and sorted by flower size, colour and number of buds. Finally, they are transferred to the finishing site for three to four weeks. Orders are packed and shipped from there.
De Vreede produces 12 million plants per year. Even Herman de Vreede finds it hard to get his head around those numbers. A massive 200,000 young tissue culture plants arrive from various locations every week and leave the nursery again as adult plants more than a year later.
De Vreede specialises in eight outstanding orchids – exclusive varieties with a long life span and offering great value for money. They come from two breeders, with most of their stock supplied by Anthura. “We test about 30 varieties a year, including from other breeders. We want to keep up with the latest innovations.”
The brothers work with large volumes. “We are equipped to fulfil orders of 500,000 units at a time. The biggest challenge for us is getting all the plants to the same stage at the right time. Much of what we do is automated now. Soon we plan to install industrial Fanuc robots which will enable us to respond even more efficiently to market demand.”
Sustainable lighting solution
Orders arrive in peaks. “We supply more than half of our annual production in the first five months of the year,” de Vreede says. “There are a lot of special occasions like Women’s Day and Mother’s Day at that time of year. To accommodate peak production we decided to install a second growing layer above part of the cultivation greenhouse. We now have four hectares of growing space there instead of three. That helps make the crop more sustainable to grow because we’re maximising our space.”
It wasn’t practical to install a second growing layer directly above the original one, either in terms of climate or air circulation. So the brothers decided to put in a second layer along the sides of the three cultivation areas. It is relatively low, just 1.5 metres above the bottom layer. Lighting is needed to make up for the lack of daylight. The standard lighting with SON-T lamps used elsewhere in the nursery can’t be used here.
“There are SON-T lights above this part, but with 600W output, slightly less than the 1000W from the other lamps we use,” Herman de Vreede says. “We went with LED grow lights for the bottom layer. Not only because they generate less heat, but also because they are a sustainable solution. They use less energy and you can choose a particular combination of light colours.”
Three years of tests
At the time there was no such thing as a standard solution. So before they started building in October 2016, they ran tests over a three-year period to see which light spectrum produced the best results. “We tested the effect of different light spectra on properties such as development rate, root development and the hardiness of the plant, both inside and outside the nursery. A lot of knowledge is needed for that, as you have to see what the best result is for each situation. The light spectrum that is most suitable for the vegetative phase of phalaenopsis is not necessarily the right one for the spike induction phase, for example.”
The tests in the nursery were overseen by Simone de Vreede, who had gained a lot of experience in this area and carried out research at her parents’ nursery while still at university. Once they had decided on the light spectrum they wanted, the next step was to find out where to source the lights from. Ultimately they chose Philips GreenPower LED top lighting, which fitted the bill nicely. The lights give out light that looks white. The advantage of this is that it makes it easier to visually inspect the plants being grown in the greenhouse.
More stable climate
“Installing a second growing layer blocked out the daylight from the bottom layer,” says Stefan Hendriks of Philips. “They couldn’t use SON-T because of the short distance between the crop and the lamps: they would generate too much heat. With LED you can create a controllable climate in which phalaenopsis can be grown very efficiently with relatively little light.”
Since the second growing layer was installed in October 2016, the plant specialist has been visiting the nursery every two weeks to carry out analyses and take crop measurements, including length, leaf splitting and dry matter concentrations. In addition, the climate is intensively monitored by means of PAR, temperature and humidity sensors. These observations are linked to the climate data from the computer. “Based on this data, we want to fine-tune the use of the lamps and optimise our cultivation even further. Experience and knowledge are essential when using LEDs. That’s why we carry out a lot of in-depth analyses here,” says Hendriks.
The phalaenopsis grower is also considering buying in LED lights for the other sections when the time comes to replace the SON-T lamps there. Hendriks adds: “Besides being more energy-efficient, LEDs last longer. The life span of the models we use is given as L90. That means that after 25,000 hours of operation, the light output is still 90% of the original level. But the module will still go on working fine after that and will have many burning hours left in it.”
At De Vreede the lamps will probably wear out sooner than that, due to the number of hours they operate. With 14 hours of lighting a day, they are in use for 5,110 hours a year. But that also means that the LED lighting in the new no-daylight situation will pay for itself more quickly.
Dutch phalaenopsis growers De Vreede have 12.5 hectares divided into cultivation, spike induction and finishing sites. In order to have enough growing space available at peak times, they invested in a second growing layer above part of their cultivation area. To light the bottom layer, now in shade, they installed LED lighting with the right light spectrum for the vegetative phase, having first done their own in-situ research into which spectrum to use.
Text and images: Marleen Arkesteijn.
A giant dehumidifier in cherry tomato grower Robert van Koppen’s greenhouse in Kwintsheul (Westland, the Netherlands) keeps the RH below the critical value of 95%. The principle is simple: when moist air in the greenhouse is sucked along the cold ribs of the dehumidifier, the water vapour condenses, just like the condensation that forms on a cold bottle of beer when you take it out of the fridge. The dry, slightly warmer treated air goes back into the greenhouse.
Robert van Koppen grows cluster cherry tomatoes on four hectares. “I’m only a small grower in terms of surface area, so I want to make my company stand out. We grow ‘Delight’ cherry tomatoes which are very sweet. They have a fruit weight of 8 grams and are 30 mm in diameter. We’re constantly looking for ways to improve the climate and save energy, but quality and flavour need to be guaranteed. Saving energy means tolerating moisture because we keep the screens closed for longer. The DryGair dehumidifier prevents the humidity from rising too high.”
Van Koppen heard about the device from his Dutch supplier Royal Brinkman and, starting in January this year, decided to rent one and set up a trial in a 1,500 m2 section which can be ventilated and heated separately.
The grower is very happy with it. “We extract 1,000 litres of water from a surface area of 1,500 m2 every day. This lowers the RH by 6% and the difference in RH and temperature from one end to the other is less than 1%.” But it’s still to early to put a definite price tag on the benefits.
According to Eef Zwinkels, technical account manager at Royal Brinkman, the device is the best way to get rid of the excess moisture generated by plants transpiring in the greenhouse. Seen in that light, this technology is a good fit for Next Generation Growing, which advocates using less energy and screening more.
He shows it on the Mollier diagram, which is also used on the Next Generation Growing course. The bottom horizontal axis shows the amount of water in the air in g/kg. The vertical axis shows the temperature. The graph contains a series of RH curves from 10 to 100% and indicates how much absolute humidity (AH) the air contains at a certain temperature. If the screens are closed in the afternoon, both the RH and the AH rise. “If you actively eliminate absolute humidity from the greenhouse, the RH doesn’t rise when the temperature drops and you don’t move into the danger zone,” Zwinkels says.
The manufacturer recommends using one unit per 1,500 to 5,000 m2, depending on the amount of transpiration from the crop and how much space there is above it. It goes without saying that a fully grown tomato crop transpires more than a pot mum on an ebb and flow system.
Besides extracting water actively from the air, the units also circulate about 22,000 m3 of air in the greenhouse per hour. This makes for a homogeneous climate, which in turn translates into a uniform crop. In addition, air movement is responsible for plant activity, which aids calcium uptake.
Zwinkels installed the dehumidifier last year in a large number of nurseries with a wide range of crops such as patio plants, pot plants, tomatoes, mother plants, geraniums, lettuce in gutters and anthuriums, he reports. The units work best between 10 and 30°C.
Van Koppen has seen a number of benefits in the crop in the few months since the trials began. “Dehumidifying the air has given us a more generative crop status, which is producing more fruit. We try to grow the sweetest cluster cherry tomatoes. We don’t go for quantity but quality. The more generative crop status also helps produce a better flavour.”
Due to the lower RH, the pressure from diseases is lower, especially from fungal attacks. "Over the past five years we have constantly been cutting our energy consumption, so the risk of fungal attacks has been increasing. That’s why I focus on reducing moisture. You can see that there is less moisture in the air now.”
An additional benefit is that the grower is keeping more of the CO2 inside the greenhouse because he has to vent less. That promotes the growing process. And because the crop is slimmer, there is a better balance between roots and leaves and Van Koppen has no problems with high root pressure. He is going to continue to use the dehumidifier, although he has not yet decided how many units to buy and when.
Cherry tomato grower Robert van Koppen is trialling a dehumidifier in a 1,500 m2 greenhouse section. Warm, moist greenhouse air condenses against the cold ribs of the dehumidifier. Not only does he get 1,000 litres of moisture out of the air every day, which reduces the RH by 6%, the RH and temperature are also more evenly distributed. Other benefits of the lower RH include a more generative crop status, less disease pressure and lower energy consumption.
Pot mum grower Ruud Nederpel:
“Efficient dehumidifier eliminates need for minimum vent position and drying the air with heat”
Brothers Theo and Ruud Nederpel from Wateringen in Westland, the Netherlands, grow pot mums on 4 hectares. They use an ebb and flow system on concrete floors. Production takes 9 to 10 weeks and the crop is under a blackout screen for 13 hours a day for 7½ weeks. Because the RH rises quickly under the blackout and energy screens in winter, in early December 2016 they decided to rent a dehumidifier for an 8,000 m2 section of the greenhouse to see whether it would make a difference.
"In our case, the RH was rising to more than 93% in the winter. This causes problems with diseases such as Botrytis and rust because the plants are no longer active. In the other sections we are still using a minimum pipe at 45ºC, leaving gaps in the blackout and energy screens and venting. This costs energy. In the trial section with the dehumidifier, the RH is down to between 83 and 86% and we no longer have to remove any moisture,” says Ruud Nederpel. He removes about 54 litres of water per hour. The benefit is likely to be greatest in the autumn and winter when there is a lot of dark, dull weather.
The device is also very user-friendly, the grower says. “It’s easy to install. You simply plug it into the wall socket and the condenser removes the water. You can easily route the water to the return water in the ebb and flow system and re-use it on the crop.”
Having had such positive results with the dehumidifier, the brothers have now bought the first one they rented. With winter behind them, they are thinking about whether to invest in dehumidifiers for the other sections in the autumn. "It's simply about the humidity in the autumn and winter as it’s drier and hotter in the summer. There is a price tag issue: how much energy will it cost me to remove the moisture? How many pests and diseases will it prevent? How much will it improve crop quality? It’s different for every grower. It’s an attractive solution for us because we want to keep the greenhouse completely closed in winter. Removing the moisture keeps the plants more active. And an active plant is more resilient than a plant that is stressed.”
The Nederpel brothers are reckoning on a payback time of three to four years. "This is an assumption. We don’t yet know how much energy we will save compared with previous years.”
Text and images: Marleen Arkesteijn.
The focus of energy savings usually lies in technology. But the crop itself offers numerous opportunities to economise on energy consumption. Many research results are still waiting to be translated into practice.
Significant differences in energy consumption per square metre or per kilo of product regularly exist between nurseries with similar greenhouses and the same crop. These can be attributed to different views on production among the growers. One grower likes to play it safe, the other looks more into the possibilities of the plant. Within the scope of The Next Generation Growing other ways to deal with the characteristics of the crop is gaining more attention. Previous research results form the basis for this.
Transpiration is the driving force behind essential processes such as mineral uptake, transport in the plant, maintaining cell tension and development of fruits. But the plant exaggerates: In the greenhouse it often transpires much too much. This 'luxury' transpiration brings an excess of moisture into the air, which then needs to be removed. And draining off moisture always costs energy. It would be very advantageous to be able to slow down transpiration. Various studies have shown that it can be reduced by 30 to 35% in tomatoes without any cost to production.
Less minimum heat
Yet in practise, growers are still reluctant to slow down the transpiration. They want an ‘active crop’ and are afraid that root growth will lag behind. But ‘active’ means that the crop fully assimilates; this can also be done with less transpiration. And you can stimulate root growth better with a lower greenhouse temperature.
Over the last few years commercial growers have been assessing much more critically the use of the minimum heating pipe to reduce air humidity. Actually, raising the temperature encourages the crop to transpire even more. And then the windows have to be opened to lower the air humidity.
One step further than economical use of the minimum pipe is dehumidification with external air in combination with more screening. Then you control the air humidity independently of the window position. But this of course requires extra technology.
A very effective means to drastically reduce crop transpiration is to pick leaves on a major scale. With a leaf area index (LAI = m2 leaf surface area per m2 ground) of 3 to 4 you already have sufficient light interception. Any number above that means you have a superfluous amount of leaf in the greenhouse.
It’s normal to pick the leaves of tomato plants but it would also be a good idea for sweet peppers too. The lower leaves only transpire and don’t contribute any more to photosynthesis. Picking could also be an option for different ornamental crops. Of course you need to consider whether the extra labour outweighs the energy savings.
In practise there are many fixed views about the necessary temperature gradient during the day. Tomato production is definitely a crop that is very dependent on the temperature strategy. But some of these opinions lead to very high energy consumption. If you heat before sunrise, when the outside temperature is at its lowest point, it costs a lot of gas. If you want to achieve a sharp drop in temperature at the end of the day, and therefore open the windows, all the heat that you’ve just put in is simply lost.
Retain the heat
The question therefore is whether the temperature gradient during the day needs to be so precise. To find out, a study compared three regimes: Heat up quickly in the morning and cool down quickly in the evening; heat up and cool down slowly; and a middle road in which the house was heated slowly and cooled down quickly. The researchers followed the crop for an entire season, critically observed by a growers group. What happened? Looking at the crop you couldn’t tell which treatment had taken place and yield hardly differed. However, the steady strategy did save energy.
For the growers group it was a question of ‘seeing is believing’. They applied the regime to their own nurseries. Seen from a plant perspective the results were not surprising: The plant responds sooner to the mean 24-hour temperature than to a specific gradient during the day. So as a grower of fruit vegetables you can easily heat the house adapted to the amount of light and keep the heat in at the end of the day. You achieve the same 24-hour temperature with less energy.
Another point is that at the end of the day leaves and fruit cool off at different speeds. The leaf temperature follows the greenhouse temperature; the fruit temperature lingers behind. The effect of this could be that the fruit attracts more assimilates. The differences are so small, that it’s hardly noticeable. Research has shown no differences in fruit weight between the different cooling strategies.
In pot plants, where the shape of the total crop is important, phenomena like DIF (the difference between day and night temperatures) and DROP (a sudden drop in temperature) can indeed affect the elongation or the compactness. Then it’s worth having a temperature regime during the day.
Light and lighting
If you look at light from an energy point of view, you arrive at two questions: How do you best utilise the natural light and when does it pay to use assimilation lighting? The answer to the first question was always: Ensure that the greenhouse has the highest light transmittance possible. Based on the research over recent years we can now add: Diffuse light almost always pays off. This light penetrates much deeper into the crop, the horizontal distribution of light is more uniform and both result in more assimilation.
The answer to the second question requires some more explanation. With respect to temperature, the plant responds to the average over the day, or over a few days. The latter forms the basis for temperature integration. With light however, there is an immediate response. At the same time, there are reasons why the plant, despite a lot of light, assimilates very little, for example, because the stomata are closed for one reason or another. It is therefore very useful to know the reason why. Then you know when the assimilation lights have an effect.
A grower can already determine the photosynthetic activity himself with instruments such as the Plantivity, but these measure just a very small piece of leaf. New methods are being developed that measure the photosynthesis (actually the fluorescence) of a square metre of leaf surface area.
A better understanding of photosynthesis can save energy because then the grower can adjust the lighting and CO2-dosing according to the activity of the crop.
A different growing strategy is a potential key to saving energy. An important part of this is to slow down transpiration. Furthermore, the precise temperature gradient over the course of the day is often not that important. The plant responds more to the average for the day (or several days). This response also offers a basis for saving energy. Finally, better utilisation of natural and assimilation light is possible.
Text and images: Ep Heuvelink (Wageningen University), Anja Dieleman (Wageningen UR Greenhouse Horticulture) and Tijs Kierkels.
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.
Assimilation lights give off a lot of heat which stays at the top of the greenhouse when you would rather have it down near the crop. And closed screens also bump up the temperature too much. Fresh Valley in the south-east of the Netherlands has solved both these problems with a combination of two vertical fans. After a successful trial, tomato grower Bert van den Brand has also installed the system at his second site where it is enabling him to limit light emissions with no side-effects.
When Bert van den Brand increased the assimilation lighting in his nursery in Uden from 8,000 to 13,000 lux in late 2014, he encountered a problem. “We are right next door to a residential area there so we have to use screens to prevent light emissions. But with such high light levels the night-time temperature rises so high that it affects your 24-hour temperature. So you end up with thin, puny plants, poor fruit setting and the risk of scorching on the shoot apex”, he says. He wanted to find a relatively simple solution to the problem.
As it happened, an energy efficiency trial involving a combination of two fans had been running at his Maasbree site since the beginning of that year, run by Wageningen University & Research in collaboration with Dutch suppliers Vostermans Ventilation and Hint Installatietechniek. The results were so good that the grower decided to try out the system at his site in Uden as well.
Fresh Valley has two sites: 2.7 hectares in Uden and 6.3 hectares in Siberië, Maasbree. The nursery mainly grows Juanita, a small, sweet truss tomato sold under their own brand name, L’Amuse. It also supplies Kumato, a golden-brown tomato grown by other growers. Half the area in Maasbree is lit with 10,000 lux and the other half with 13,000 lux. Half the power needed for this system comes from the nursery’s own CHP unit and the rest is bought in. They opted for this arrangement to avoid generating a lot of excess heat.
The combination of fans was left in place after the trial (3,000 m2). “The bottom fan, a Multifan V-FloFan, draws the cold air upwards from below and distributes it horizontally so that it passes along the lights and warms up. When the lights are on, this fan is always running,” van den Brand says. It’s the same at the Uden site.
Better temperature distribution
The top (axial) fan draws cold, dry air downwards through the closed screen. This air bounces off a plate and is then distributed by the bottom fan. The top fan is only used if it gets too hot or too humid under the screen,” says Guus Vostermans, sales engineer at the company of the same name.
The result achieved with the combination, named Ventilation Jet, is better mixing of the greenhouse air and therefore fewer horizontal and vertical temperature fluctuations. Van den Brand: “Before the trial I had not expected it to work as well as it did. But not only did Wageningen University & Research’s measurements indicate better temperature distribution, the crop is also more even.”
There are 50 fan combinations per hectare in Maasbree and 11 per hectare in Uden. Because the top fans there are double capacity, that works out at the equivalent of 22 per hectare. So that is still quite a lot fewer. “In Uden we have an Obscura blackout screen that is 95% closed. So you actually always have a small gap of five percent. That’s why we could get by with fewer there,” he says. They also have ducts with fans hanging under the gutters, which also help improve air circulation. “If we didn’t have those, we would need more fan combinations.”
Preventing light emissions
Van den Brand has not yet decided to kit out the whole greenhouse in Maasbree with the system. That will probably only happen when the screens need replacing. At the time of the trial, which focused on energy saving, two highly insulating screens were installed in the section with the fans. Because of the low post height (4.5 m), they run across the same wire bed so they can’t both be closed at the same time. There is a blackout screen (XLS 10 Revolux) and a transparent screen (XLS SL 99 Revolux W/W).
But saving energy is not van den Brand’s main objective. “Our primary concern at Uden is to prevent light emissions. The Ventilation Jet is the only way we can keep the air cool enough in our situation. The second argument in its favour is a more even climate.”
The system does in fact deliver energy savings. Van den Brand puts the savings in the section in Maasbree (3,000 m2) at around 10-15% in winter, thanks to a lower minimum pipe temperature, fewer gaps and less venting. “The minimum pipe temperature is about 4°C lower, so 36°C instead of 40°C, for example. But the plant always comes first. If it needs 40°C, it gets 40°C.”
The transparent screen stays closed for the first eight weeks of cultivation. “This also benefits the plant. With the darker screen we don’t need to leave gaps at night up to an outside temperature of 12°C, and you hardly ever get that in winter”, he says.
Without the fans he would have to vent more when the screen is closed. “That’s not ideal, of course: you’re getting rid of heat and wasting energy”, he says.
Mixed feelings about NGG
So there is evidence that this system does save energy. But as already mentioned, that is not van den Brand’s primary concern. He mainly tends to sidestep Next Generation Growing (NGG): “With an unlit greenhouse, NGG can save you a lot of energy, but we have lighting everywhere. There are definitely good things in the NGG approach, but I can also see accidents happening. You take big risks to save a few cubic metres of gas, but that can be at the expense of quality and flavour, as flavour can deteriorate if you inhibit transpiration. We can’t allow that to happen with our own brand. So we have very mixed feelings about NGG. It depends a lot on whether you are production- and cost-oriented or market-oriented, as we are. We would rather not risk it.”
Fresh Valley was one of the first nurseries to use vertical fans. The concept has been refined since then. Vostermans says that following customer feedback they reduced the noise level: “People found it particularly annoying at harvest time and while working on the crop. The current generation is quieter and also a lot more energy-efficient.”
Fresh Valley uses a combination of two vertical fans. The bottom fan is always running when the assimilation lights are on. It draws cold air upwards from below and mixes it with the warm air at the top of the greenhouse. The second fan draws cold, dry air downwards through the closed screen. Together they create a better greenhouse climate with fewer temperature fluctuations and save energy as well. At one site, the main reason for installing the fan system was to prevent light emissions without causing problems.
Text: Tijs Kierkels. Images: Wilma Slegers.
Energy efficient production according to the principles of Next Generation Growing, without any additional investment, is the aim of pepper trial being carried out at the Delphy Improvement Centre (IC), Bleiswijk, the Netherlands. Armed with two energy screens and fans the trial participants want to save 30% on energy and still achieve good fruit quality.
The main barriers raised by pepper growers to grow as energy efficiently as possible in practice are doubts about the impact on crop health and fruit quality. This therefore was the reason for running two climate trials this year with peppers, one at the Improvement Centre and the other at neighbouring Wageningen UR Greenhouse Horticulture.
The one at the IC is being carried out in an area covered with a standard greenhouse roof and two energy screens. The other trial is taking place simultaneously in a VenlowEnergy greenhouse with double glazing. The red variety Maranello was planted in both greenhouses on 7 December 2015. A Supervisory Committee, which includes four pepper growers, is following the trials closely.
The trial at the IC uses two transparent energy screens, namely Luxous 1547 D FR and Luxous 1347 H2no FR. The H2no property ensures that when the screen is used during the day it also allows a lot of light to penetrate even when it is wet due to condensation. In addition the area is equipped with horizontal and vertical fans and is sparsely heated. Although the energy consumption on commercial nurseries is usually about 30 m3 per m2 the trial participants are aiming for 20 m3 per m2. That is a saving of more than 30%.
Assessing the balance half way through the trial it would seem that the goal is achievable. Energy consumption is even slightly lower. Maximum use of the energy screens and omission of the minimum pipe rail therefore have a huge impact.
Screening based on radiation
The principles of Next Generation Growing (NGG) were applied during the trial. The use of the energy screens is the dominating factor. The upper screen opens when the radiation is 100 watt per m2. The second screen opens at the moment that the temperature above the screen differs by four degrees from the desired heating temperature. This small difference should prevent a cold dump.
There were moments this spring that the lower screen was still closed when the radiation was 300 to 500 W/m2. The intensive use of the screens has, from the start of the cultivation to early April, led to 14% light loss. That was difficult for the growers to get used to as they prefer to allow in as much light as possible.
“But the crop was growing to our liking,” says Rick van der Burg, crop manager at the IC. “We noticed that the room temperature was quickly a degree higher than what is usual in practice," adds Arie de Gelder, researcher at Wageningen University & Research.
Drain off moisture
At the same time the screens play a major role in the removal of moisture. At the moment that the RH becomes too high, cool dry air is supplied via the vents above the screens. The moisture is then removed to the outside via transport through the screens. Therefore the usual method of making a gap in the screen is not used,
The fans ensure a uniform temperature and moisture distribution in the greenhouse. At the start of the cultivation this was achieved by just using the horizontal fans. As the crop becomes taller the vertical fans are used too.
The trial participants are not completely satisfied with the air currents and thus the temperature distribution that occurs in the section. Bubble wrap is attached to the walls to rule out influences from outside and from the adjacent much warmer section. Because so little heating is used the temperature differences between the walls has relatively large impact. “We’ve noticed that strong air currents occur,” says De Gelder. That will be different in a practical situation.
When Van der Burg made the first assessment in mid April, it revealed that 2,500 hours of screening were with a double screen. That saved a lot of energy especially in March and April.
A net radiation sensor was hung in the top of the greenhouse. This shows how much radiation enters the greenhouse and how much radiation is emitted from the crop. The double screen in the night leads to an important reduction in the radiation emitted.
Screen out the light
As the radiation increases, the screens will be used as a tool to screen out excessive light. The greenhouse does not have a solar reflective coating. When the radiation is more than 700 W/m2 the upper light diffusing screen closes 80% and the lower screen 40%. They are positioned so that they overlap each other. De Gelder: “In this way we want to keep the humidity as well as the CO2 as much as possible at the right level."
Initially ventilation only happened when the greenhouse temperature was more than 27°C. Since the greenhouse temperature rose rapidly at high radiation it was decided to slightly reduce the temperature. Van der Burg: “We noticed that the fruits then become wet and we have to prevent that.”
24-hour temperature based on radiation
The desired greenhouse temperature is very dependent on the radiation. During dark periods the 24 hour temperature is 18.5ºC. When the light sum is 1,000 joules the 24-hour temperature should be 20.5ºC and at 2,000 joules it should be 22.5ºC. The light sum of the previous day determines the night temperature that follows.
It’s noteworthy that no minimum pipe rail is used. Heating is only used when there is a need for energy. Incidentally, plant temperature is well monitored.
What is now interesting is how the crop responds to these climate settings. In particular the growers in the Supervisory Commission, who are willing to push to the limits, have been amazed at the crop condition. They didn’t expect the crop to look so good after so much screening and the subsequent loss of light. During the first setting some fruits aborted so the trial participants didn’t have to consider thinning out. The first setting started to develop a little later than in commercial nurseries but the differences weren’t shocking. Harvesting started in week 12 and by week 18 the yield was 6.10 kg/m2.
The second part of the cultivation will be interesting when the radiation rises even higher and the crop develops further. Then the emphasis will be more on the vertical temperature distribution in the greenhouse. Of course the growers and researchers are closely following the quality of the fruit. Everyone is wondering what the final fruit quality will be like and what affect the climate regime has on the total yield.
A pepper trial with NGG in an existing greenhouse in the Netherlands shows that during the first half year a lot of screening has no adverse effects on the crop or yield. Up to now it has been easily possible to save 30% on energy. The two energy screens limit the radiation during the night so the crop temperature remains higher.
Text and images: Pieternel van Velden
Phalaenopsis growers sometimes use artificial lighting for 16 hours per day, totalling 8 to 10 mol/m2/day, especially in the winter. For a crop with a cultivation period of 40 to 50 weeks, lighting is therefore a major expense. Measurements show that the correct timing of artificial lighting is more important than the total light sum. Additional research should show whether growers can indeed save around 30% on the lighting hours. Before making such a big change in mindset and operation, growers want absolute certainty.
Since 2012 light specialists Sander Hogewoning and Govert Trouwborst of research centre Plant Lighting, of Bunnik the Netherlands, have been researching the ideal lighting for phalaenopsis. They’ve been doing that with colleagues from Plant Dynamics. It’s rewarding work because very little scientific research has been carried out on this subject. As a result there’s plenty to discover. The Dutch program Kas als Energiebron (Greenhouse as Energy Source) – whose goal is to save energy – was the most important investor in the trials. The crop alliance for pot orchids, of the growers association, LTO Glaskracht Nederland, also contributes and is actively involved in the research.
How much lighting makes sense?
Hogewoning outlines the situation from the beginning. Phalaenopsis is a CAM-plant, just like pineapple, agave and cactus. This means that photosynthesis is different to ordinary C3-plants. CAM-plants are slow growers who only open their stomata in the afternoon. They take up CO2 during the afternoon and night and store it in cells in the form of malate. Storage space is thereby limited. Photosynthesis takes place during the day because light is needed for this. When the malate is all used up photosynthesis for the most part stops.
“Growers now use a lot of light even if the plant isn’t using the light. How much lighting is actually useful for a CAM-plant? And when?” asks the researcher.
Long night unfavourable
In a study in 2014 the researchers showed that a light sum of 6.5 mol/m2/day is enough to fulfil the maximum storage for malate, under the condition that the leaves remain fairly horizontal. More lighting does not yield any more photosynthesis. In addition, the trials showed that allowing in more light during the later phase of cultivation compared with the early phase did not lead to any extra photosynthesis. Another interesting conclusion: Lighting fully in the early morning and afternoon doesn't make sense: During those hours electron transport in the leaves is low.
Trouwborst: “Nevertheless, it is also unwise to provide no light during these inefficient hours. Continuously lighting for 16 hours yielded better results than maintaining a long night and providing 12 hours of light. Growers in practise experience the same. Therefore we wanted to look for the best lighting recipe.”
Step-by-step or low intensity?
This was the focus of the research in 2015. Hogewoning explains his goal. “The aim is to save energy by using less lighting during the hours when the plant hardly uses the light. The question is how far can you dim in the morning and/or the afternoon without loss of production?"
The researchers trialled the variety Sacramento over two sessions. One trial ran from mid March to end May, the second ran from the start of June to the end of July 2015. They started with plants that had already been through the cultivation and cooling phases. They divided them over eight climate units, each of 25 plants. The units simulated daylight similar to that in a winter situation.
Each unit received its own light recipe using SON-T-lighting for about eight weeks. One of these was the control that received lighting similar to that normally used in practise. This was 7 mol/m2/day dosed over 16 hours. Four treatments were dynamic and based on previous research. “For these treatments we maintained a day length of 16 hours. During the inefficient hours in the morning and evening we dimmed the SON-T light to a greater or lesser extent. We switched them on and off in different ways. This saved between 8 and 33 per cent electricity. In addition, two treatments had a day length of 11.5 hours.”
Dimming the light works
The results were very positive. They confirmed the hypothesis that dimming at certain hours is possible and agreed with the results of previous research. The total CO2 uptake for each of the four dynamic treatments was the same as the control treatment of 7 mol/m2/day over 16 hours - even the same as the treatment that saved more than 30% on energy.
Hogewoning: “The savings are substantial; in practise that can amount to hundreds of thousands of euros, not only on electricity costs, but also because growers can replace the bulbs less often.” Trouwborst adds: “Therefore it’s not about achieving the correct light sum but the right timing. You have to use the right light intensity at the right moment.”
Hogewoning points out another important result. “A long night came out the worst, which we also discovered in 2014. When we continuously lit for 11.5 hours, 7 mol/m2/day, the CO2-uptake by the plant is less than in the dimming treatment with a 16 hour day length and a light sum of 5.2 mol/m2/day.”
Questions from commercial growers
These are spectacular results, but will phalaenopsis growers dare to dim the SON-T-lights early in the morning and afternoon? Trouwborst: “We recently presented these results to the supervisory commission and the crop alliance for pot orchids. Their first question was: ‘Does anyone do this in practise already?’ With millions of euros of plant material at stake growers won’t so easily alter their lighting methods. That’s why it is important to slowly scale up this research. The crop alliance is following this very closely.”
Growers, of course, want to know if a good uptake of CO2 translates into plant quality and number of buds. The researchers have also measured that but Hogewoning points out: “The number of plants was too small to draw conclusions. However we do see that the number of flowers is in line with the CO2-uptake. The treatment with the least uptake – a long night, 5.2 mol/m2/day – clearly led to fewer flowers.”
Because they want to see these positive results confirmed on a larger scale, the researchers are running a repeat trial with more plants. For this they invested in new climate units of over 2 m2, in addition to their existing smaller units. “In these new units we can even let the sun slowly rise and set. All year around we can mimic the light situation in the winter, without being dependent on the weather conditions,” explains Hogewoning.
The trial was due to run until the end of May 2016. In consultation with the growers the researchers ran the two most successful treatments as well as the control treatment, which is similar to that currently used in practise. Hogewoning: “The crop alliance is very curious about the results. They are waiting with baited breath. In the meantime growers who have questions can discuss it within the group.”
Commercial phalaenopsis growers use a lot of artificial lighting. Research indicates that the timing of the lighting is more important than the total light sum. It seems that growers can dim the lighting early in the morning and in the afternoon without any negative effects. If these initial results are confirmed it appears that at least 30% can be saved on electricity.
Text: Karin van Hoogstraten. Images: Gert Janssen (Vidiphoto)
Gertjan van der Spek is the first tomato grower without artificial lighting in the Netherlands to have two transparent energy screens and no dehumidification. This was his next step to save energy after having reduced the use of the minimum rail and ventilating above the screen, instead of making gaps in the screen. In this way he hopes to use no more than half cubic metre of gas per kilo tomato.
The idea of using a double energy screen for vegetable production is not new. A growing number of pepper growers are doing this. “Pepper plants grow less quickly so the screens can stay closed for longer. The switch to using two screens is less great for them,” says climate specialist Paul Arkesteijn, of screen manufacturer Svensson.
To show that possibilities exist for tomato growers as well, a demonstration trial was run at the Delphy Improvement Centre in Bleiswijk, the Netherlands, last year. This trial compared an area with two moveable transparent screens with a standard transparent screen with a fixed anti-condensation foil.
The top layer was the ordinary Luxous 1347 FR transparent energy screen; the bottom layer was the 1347 FR H2no with an anti-condensation property. The condensation droplets flow out of the latter screen. According to Arkesteijn both transmit 80% light. When both are closed, they still transmit 64% of the light. The crops in both greenhouses grew well. The trial greenhouse saves an extra 4 m3 gas/m2.
Gertjan van der Spek, of greenhouse company Solyco, with two locations near Rotterdam, grows Roma tomatoes on 4.3 ha. He is part of a horticultural cluster comprising six companies. They have a joint boiler house which has three energy sources available: waste heat from the ROCA-power station; two CHPs; and a boiler. The latter also serves as a backup if there is a breakdown in the CO2 supply.
“Our cluster wanted to purchase a heat pump to further cool the flue gases from the CHPs. Now the flue gases are around 45 to 50ºC and we want the heat pump to cool them to 23ºC. The two CHPs use 900 m3 gas per hour and the recovered heat corresponds to approximately 130 m3 of gas. To be eligible for a subsidy scheme run by the Netherlands Enterprise Agency the cluster had to save in total 15 per cent on energy. That meant each grower had to make extra effort,” explains the tomato grower.
This took place at the time that Van der Spek became one of two growers to join the supervisory commission for the trial at the Improvement Centre. Hence the reason for him being able to pay extra attention to the trial with the two transparent screens.
Second energy screen
During the spring the trial appeared to be running so well that Van der Spek dared in August last year to install a second energy screen under his existing screen. The fact that his first screen of eight years old was becoming more porous and therefore was in need of replacement also played a role. He had to invest in a second wiring system for the second screen but that wasn’t a problem, even though this option was not taken into account during the original construction. “Screen installer Alweco came up with the solution of attaching the second screen half way across the trellis. For safety we ensure that the cloths are not moved at the same time because it’s during opening and closing that most of the force is placed on the walls.”
The grower chose the light-transmitting Luxous 1347 FR, without anti-condensation properties. “The anti-condensation property is not necessary for us since we mostly screen at night.”
1,500 double screening hours
The planting date was 1 December and up until the end of April he regularly used two energy screens. The grower always closed the new screen first. Up to week 20 he had accumulated 2,200 hours of screening. He used the old screen for 1,500 hours.
Compared to last year he used 0.5 m3 more gas during this period for a slightly longer production. “In terms of energy consumption, we are about equal to last year, but then it was very mild compared with this year.” When he compares his energy consumption with that of colleagues he uses about 2 m3 less than growers with a fixed AC-foil and 5 m3 of gas less than growers with a single screen.
During the first six weeks he hardly made any savings compared with growers with the fixed AC-foil. “During this period the main advantage of a moveable screen is flexibility. I already had some production advantages because with a moveable screen you have fewer problems with moisture. We never wanted to have a fixed foil. I always had the idea that it only became cold when the foil was removed.”
The trial in Bleiswijk also used the double screens intensively during the autumn months, from October. That achieved extra savings of 1 to 2 m3. “During the last few weeks of the cultivation period the temperature needs to be high enough to allow the tomatoes to ripen properly.”
The New Thinking
In 2014, when the winter was mild, the grower used 27 m3 gas per m2 and yield was 63 kg. In a ‘normal’ year that is around 32 m3. “We now want to make these savings with the double screen cloth and we hope again to have a yield of 63 kg,” says the grower.
Van der Spek has already made huge steps on his nursery over the years. When he started in 1992 he used 72m3 gas. After building a new greenhouse in 2000 consumption dropped to 50 m3 gas. In 2005 that was reduced further to 40 after the installation of his first screen. The step to 32 m3 happened mainly thanks to new insights into production. Yield increased from 50 kg in 1992 to around 63 kg of tomatoes today. “Production rose to more than 65.5 kg thanks to grafting but we’ve scarified some of that as we save energy.”
The result is impressive: from 1.5 m3 gas previously to now 0.5 m3 gas per kg tomato. Arkesteijn attributes the new cultivation insights to The New Thinking, a derivative of the Next Generation Growing. “They used to make a gap in the screen to release moisture but consequently suffered a cold dump. That resulted in horizontal temperature differences. The climate is controlled based on the coldest spots. Now growers keep their screens closed for longer and ventilate the moisture away through the cloth screen. The big advantage is the uniform climate. “That is not only beneficial for saving energy, but also for product quality.”
Use minimum rail sparingly
The tomato grower says he has applied this strategy for two years. Firstly he only ventilated on the sheltered side. Now, when using two energy screens at once, he ventilates on both sides to achieve good air movement above the screen, so that the air is drawn through the screen cloth. And instead of opening the windows just a little he now dares to open them wide. Another adjustment is that the grower uses the minimum pipe very sparingly. “We have dropped from 50ºC to 40ºC and now to 30ºC.”
Arkesteijn: “Previously we believed you had to heat the crop for it to continue to transpire. What matters now is that the crop transpires sufficiently during the day. The energy screen closes at 80 to 100 watt radiation. The crop is activated the next day by the sun that shines through the particularly transparent screen."
Tomato grower Gertjan van der Spek is the first tomato grower in the Netherlands who doesn’t use artificial light to have two transparent energy screens without any dehumidification system. After a series of energy saving measures, such as the first screen, ventilating above the screen cloth and seldom use of the minimum rail, he took the decision to install a second wiring system and a moveable screen. He hopes this will result in gas consumption of 27 m3 and yield of around 63 kg per m2.
Text and images: Marleen Arkesteijn