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energy savings

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Stem topple is a serious problem in the flower bulb sector. Bent stems are unsaleable and are therefore something to avoid if at all possible. It has long been known that this problem is associated with transpiration from the plant, and for many years the advice has therefore been to ventilate well during the forcing phase so as to prevent the RH from rising above 70%. Researchers have tested whether this is really necessary throughout the whole forcing period, since less transpiration means much lower energy usage.

The main energy gobblers in tulip forcing in two- or three-tier greenhouses are light, transpiration and, in particular, heat loss to the outside. Researchers Jeroen Wildschut and Martin van Dam of Wageningen University & Research in the Netherlands investigated the potential for light reduction a while back. A tulip doesn’t need photosynthesis light or grow light because the bulb itself is a plentiful source of carbohydrates. Only steering light is needed to keep the plant standing upright and to enable it to develop good colour. In 2005 these findings paved the way for the current system of multi-tier cultivation, a method that enables forcers to expand their production area vertically. This system leads to energy savings of as much as 40-50% per crop, not least thanks to the high utilisation rate.

In Next Generation multi-tier cultivation, bulbs are forced in six or more tiers in well insulated cells. The facility is no longer a greenhouse but more like a warehouse lit with low-energy LED lights. The different climate requirements needed in each growth phase are met by compartmentalising the forcing area.

Ventilating costs energy

Following on from their research into light reduction, Wildschut and Van Dam set their sights on transpiration and ventilation. Wildschut: “Moving from a single tier to a multi-tier system cut energy consumption by 50%. In this new situation, ventilating to remove moisture uses the most energy as the system that warms up the outdoor air and then removes the moisture runs day and night. Our research shows that you can save roughly another 50% by ventilating less. By reducing transpiration, in other words by removing as little moisture as possible, you quickly start saving energy.”

In multi-tier systems with two or three tiers, the researchers have observed that energy is consumed at a rate of 300 MJ per 1,000 bulbs. The figure for rooms with six tiers and balanced ventilation is just 180 MJ per 1,000 bulbs – an encouraging outcome. However, as the researchers emphasise, savings are all well and good but they should never come at the expense of quality. So the project started off by looking into the transpiration needs of the plant in each growth phase.

Susceptible phase

Van Dam: “In the first phase of the project, we explored the susceptibility of some of the main cultivars to topple. Leaf and stem topple, along with fading and too lightweight or too short plants, are the result of poor transpiration and calcium deficiency in the cells. Calcium deficiency causes the cell membranes to become more permeable, so the cells burst and moisture leaks out, forming the point at which the stem or leaf topples over. We know that the growing parts of the flower absorb calcium as the stems elongate rapidly in the growth phase. During this phase, transpiration draws calcium from sources including water to wherever it is needed in the plant via the sap flow. Sufficient transpiration is therefore crucial in this phase.”

Also noteworthy is the fact that toppling doesn’t actually happen in this phase per se. The flowers can be standing proudly upright at the time of harvesting but the symptom can rear its head at the auction or even in the vase. “Our research shows that the degree of susceptibility to topple is clearly cultivar-specific. For example, excessive RH was found to have no effect at all in the cultivar Barcelona. Strong Gold and Seadov turned out to be much more susceptible: the number of plants affected by topple at 83% RH was very high. The middle growth phase was found to be the most susceptible to high RH.”

Low RH in middle week only

The researchers established which phase was most susceptible to topple by dividing twenty “pin trays” of tulips between two compartments. The RH in the first compartment was kept at about 100%, with below 70% in the second. The researchers switched the trays round every two days. As soon as some plants started to display signs of toppling, they were logged and removed. This trial was carried out in three forcings. The same picture emerged in each cycle. Out of the roughly 20 days the bulbs spend in the greenhouse, it is the middle week that determines whether the plant will be susceptible to topple later on.

In their report, the researchers state that the tulips need to be able to transpire for between 30 and 65% of the forcing period. Therefore, during this short period (which can be between three and nine days, depending on the cultivar used) the grower can influence calcium uptake and should keep the RH sufficiently low. The tulip is not affected by high RH during the first and last weeks of forcing. Wildschut: “By keeping the RH below 70% in the middle week only, when the tulips are at their most susceptible to topple, you can therefore save a lot of energy without making the problem worse.”

Adjusting the cultivation strategy

Last January, Van Dam shared the findings of the research with attendees at the Day of the Tulip industry event. “I could see that there was a lot of interest among the audience. Obviously growers aren’t going to change tack straight away, but it is important for them to think about it. They will only start making adjustments to their own cultivation strategy if they believe that it works. That’s fair enough – and it’s also why it will be important to follow up on this research. We want to be able to offer practical guidance. That’s the only way our efforts up to now will actually lead to a reduction in energy consumption and therefore in CO2 emissions in the flower bulb sector.”

Once the researchers secure financing for the follow-up research, they plan to investigate transpiration in the phase most susceptible to topple in more depth. The latest research revealed that the RH doesn’t have to be low 24 hours a day at this time. They expect that it should be possible to switch the ventilation system off for between three and 10 hours a day in this phase. If that is the case, growers could save up to as much as 47% of the energy they use on dehumidification by allowing the plants to transpire for between 18 and 21 hours a day during the susceptible phase. Now that’s definitely worth investigating.


Ventilating less can save energy. However, if the RH is allowed to get too high, this can result in leaves bursting open or stems collapsing (toppling). Researchers looked into what effect reducing transpiration in different growth phases during forcing would have. They discovered a distinct period in which tulips are more susceptible to topple and in which adequate transpiration is essential in order to maintain quality. In the days before and after this period, growers can quite safely allow the RH to increase slightly without making the problem worse.

Text: Jojanneke Rodenburg.
Images: Studio G.J. Vlekke.

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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.

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Intensive lighting in winter is common in Phalaenopsis growing. A long-running Dutch research project is seeking to answer the question of whether less lighting could be used at that time of year. The trials in the first three years revealed that turning the lights up or down could cut electricity bills by as much as 30% without loss of quality or production. The results from the fourth trial year, with practical trials at Ter Laak Orchids, show that the limit has been reached in terms of quality – at least in the top segment.

The trials run in 2014-2016 by the specialist Dutch research companies Plant Lighting and Plant Dynamics, with support from growers, delivered some surprising insights. One of these was that timing is more important than the light sum in the vegetative and generative phases. They also discovered that a long day of 16 hours produces more CO2 uptake than a day of 11½ hours. Dimming the lights at the beginning and end of the lighting period seems to be possible without loss of production or quality. That generates electricity savings of more than 30%.

Dimming in cooling phase

Based on these results, two follow-on studies were run in the winter of 2016/2017. The first took place in climate chambers equipped with daylight simulators and SON-T lights from Plant Lighting in Bunnik, the Netherlands. In these chambers, the dimming treatment, which can cut electricity usage by up to 30%, was also applied in the cooling phase for the first time. In another room, the plants were lit in line with the biorhythm, starting at 05:00 instead of 01:00. This can save as much as 43% in electricity because it makes better use of free daylight.

Researcher Sander Hogewoning explains: “The CO2 uptake was the same in all three treatments. Yet something caught our eye: CO2 uptake ended relatively late: it didn’t stop until 2½ hours after the lights went on. So the plant rhythm is not always the same, and we have no idea why that is. This indicates that it is important to keep on taking measurements with sensors, otherwise you are taking a risk. We also noticed that the plants were a week behind in the treatments with dimmed light. That can be explained by the fact that the plant temperature was 0.5°C lower on average because the SON-T lamps were used less. In practice, you would compensate for that by turning up the heat. Although the percentage of double spiked plants was just as high, the percentage of branched ones was lower. With these treatments, you’re reaching the limits in terms of quality.”

Practical trials

The limits were explored and found in the second trial as well. This study took place in the Ter Laak Orchids trial greenhouses in Wateringen in the west of the Netherlands, in both the vegetative and cooling stages. The generative stage took place in the production greenhouse. The researchers and growers chose four varieties for the trials: Sacramento, Donau, Jewel and Las Palmas. Martin van Dijk of Ter Laak: “We grow more than 100 varieties here, so we wanted to know what effect a different lighting regime would have on different varieties. The quality and the number of double spikes must remain the same. That’s essential for us.”

In the one 80 m² trial greenhouse, the plants were lit for the usual 16 hours. The plants in the other trial greenhouse were also lit for 16 hours but with the dimming treatment used in the previous trial. The only difference was that the start was delayed until 03:00 in order to make better use of the daylight. To check the quality, the root weight and above-ground weight were measured three times. At the end of the generative stage, the researchers counted the number of double spikes and the number of flowers.

The same or marginally lower

The results? The quality of the plants was the same or marginally lower. With the dimming treatment, the roots in three of the four varieties were lighter than in the control treatment, although the weight of the leaves and flowers was comparable. The number of flower buds was also the same.
Another indicator of quality is the percentage of multiple spikes. With the dimming treatment, only Jewel showed significantly lower results by the end of the generative stage. The percentage was slightly lower in the other two varieties but not to a statistically significant extent. The differences are not massive, but they do show that the limits of dimming were reached in this trial as well.

Hogewoning: “My conclusion after these trials is that dimming saves a lot of electricity and produces the same or slightly lower quality. We advise growers in the top segment not to push the boundaries when looking for savings but to stop a little way from the limit. However, the quality differences are small. Growers with fewer lamps will find that switching the lights on later saves them money. By making better use of free daylight, they will reach their light sum more easily at the time of day that is most important for the plants. And finally, bear in mind that any differences in quality are very much magnified because we are simulating winter for 30 weeks of cultivation. In reality it’s not always December.”

Different choices

With the benefit of hindsight, the growers involved are making various choices depending on the capacity of lamps, but also depending on whether they have a CHP plant or have to buy in electricity, which is expensive. At Ter Laak Orchids they are biding their time. Van Dijk: “With the results of the study and the experience we gained last year, we are waiting to see what the results of next year’s sensor study will be. This winter we plan to turn the lights up and down incrementally, but starting at 01:00 as normal.”

Honselersdijk-based Levoplant has been switching the lights on later since last year. Cultivation manager Erwin van Vliet, a long-standing member of the supervisory committee: “We used to start at 01:00 and we would stop suddenly at 16:00, sometimes even earlier. Now we start at 04:00 in October and finish at 19:00. In November and December we start at 03:00 in order to achieve our light sum. We also turn the lights up and dim them incrementally. That works very well for us because it makes the climate in the greenhouse more uniform. An additional advantage last year was that we had less of an issue with premature spiking. The quality is every bit as good as before.”

Developing knowledge

Are these insights resulting in energy savings? Van Vliet: “We are not saving as much as in the study. We are lighting for longer, although we are saving energy by dimming. Before the study, the trend was heading towards 100% lighting for 16 hours. But we now know that really isn’t necessary. So we need to continue to develop our knowledge – by working together.”

The Pot Orchid Growers Cooperative this year invested in the development of robust, affordable sensors to provide ongoing information on the plants’ light usage. Van Dijk and Ter Laak are certainly convinced of the benefits. “We are installing a wireless network in the new Daylight Greenhouse we are building to allow for the use of wireless sensors in the future.”


The fourth year of the research into lighting Phalaenopsis in winter has confirmed the previous years’ results. The orchid needs a long day, but that can be achieved by gradually turning up and dimming the lights in the vegetative, cooling and generative stages. Switching on the lights later saves more electricity. The quality of the plants in the dimming treatments is the same or marginally lower. The limits of the savings thus seem to have been reached.

Text: Karin van Hoogstraten. Images: LD Photography.

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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

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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.

Large volumes

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.

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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.

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Last year the focus in the Dutch 2SaveEnergy greenhouse was on high-wire cucumbers. Over two production cycles, Wageningen University & Research investigated whether it was possible to grow a crop that could intercept and use winter light to the full. This trial was a preliminary study in advance of upcoming trials in the Winterlight greenhouse, a design that lets in 10% more light in the winter.

The crop in the innovative greenhouse was very successful, say crop researchers Jan Janse and Frank Kempkes. It was cleared at the end of November and the total yield is estimated at 110 kg, with 260 cucumbers. “And all with minimal energy input,” Janse says. “We used around 17.5 m3 of gas over the entire year. That once again puts us well below the average for the sector.” Yet again, this new greenhouse proves what it is capable of. The concept clearly demonstrates that Next Generation Growing can be made even more energy-efficient without affecting production or quality.

Steering the crop

This high-insulating greenhouse at the Energy Innovation and Demonstration Centre in Bleiswijk features clear glass and a permanent, high light transmitting, diffuse layer of plastic film parallel to the glass. The greenhouse is fitted with a dehumidifier unit with outdoor air entering via ducts under the gutter.
The researchers planted the cucumber variety Hi-Jack in the greenhouse on 29 December 2015. The crop was steered by varying the row width and plant density and by thinning out the fruits, the optimum having been modelled in advance. Over two production cycles the team intensively monitored light interception, cultivation (crop, production and quality) and energy consumption.

Different row widths

Janse: “We decided to start with a plant density of 1.67 plants/m and three different row widths: 1.4, 1.6 and 1.8 m. Among other things, we wanted the trial to tell us which row width would produce the best crop, would be best for light utilisation and would be easiest to work in. After all, you have to be able to move through the cucumbers on the high-wire trolley without damaging the plants too much. Each setup consisted of three ‘carousels’ (growing gutters). Regular crop observations were carried out on one carousel in each setup.”
To record the observations, Kempkes took photographs from a fixed position above the crop at the same time once a week. Using an image processing program, the researchers were then able to track the development of the crop and the projected leaf surface area to get an idea of the amount of light being intercepted. Kempkes: “If you can see a lot of the floor or the gutter on the photos, light interception is not as good as it could be.”

45% less gas

At the end of January last year, the plants were doubled by pinching out. “At one point we had a crop with highly generative growth and small leaves, but we still harvested a lot of cucumbers from it. Over six weeks (weeks 15-20) the crop produced as many as ten cucumbers per week, or 4.5 kg/m2. So clearly the plants were using their assimilates very efficiently.”
According to Janse, even the two growers on the supervisory committee were looking enviously at the crop. “The plants were strong and production was high – better than in a commercial greenhouse, in fact. We still have some work to do to find out the exact reasons for this. It may be partly down to good crop care. What was also striking was the temperature achieved: it averaged 1ºC higher than in a commercial greenhouse. This meant that the cucumbers got going very quickly. Over the entire period, the development time was around 14 days at an average greenhouse temperature of 21.4ºC. The row widths of 1.4 and 1.8 m yielded the highest production. In addition, gas and pure CO2consumption worked out at 12.7 m3and 4.5 kg/m2 respectively. This represents savings on gas consumption of around 45% compared with commercial greenhouses.”

Later second planting

The team set up a new crop in mid-July. This time they used Hi-Power with a plant density of 2.25 plants/m2, again with the same three row widths. In the period with the most light, there was therefore a gap of three weeks between the end of the first crop and the start of the second one. “We deliberately planted it slightly later because we wanted to test the crop in the dark period as far as possible. After all, this was a preliminary trial for the Winterlight greenhouse. By the end of October we had already harvested almost 100 cucumbers from this second crop, with an average fruit weight of around 420 grams. The crop finished off well and we achieved excellent overall production of good quality cucumbers.”
This time too, the row widths of 1.4 and 1.8 m yielded the best results. So the objective was fulfilled. An excellent crop can indeed be achieved with a relatively small leaf surface area, in other words small leaves, a characteristic of the variety. Because the rows were oriented east-west and care had been taken to distribute the wires evenly, the crop clearly intercepted enough light and a good proportion of the assimilates went to the fruits. There was barely any fruit abortion. Smaller leaves also mean less transpiration, which saves energy in cold periods.

Winterlight greenhouse

The research into an efficient Winterlight crop was brought to a successful conclusion. On to phase two: in late December 2016 a new high-wire crop was planted in the Winterlight greenhouse, this time with a row width of 1.8 m. Kempkes: “Both 1.4 and 1.8 produced good results but there was more plant damage with 1.4 m spacing.” The scientist, who also manages the Winterlight greenhouse project, is expecting a lot from the follow-on trial. “The Winterlight greenhouse was handed over recently and is really very nice.”
The entire structure is painted with a white powder coating with an increased reflection factor of 90%. The glass used is SmartGlass, a new type of diffuse glass in panes measuring 300 x 167 cm. Light transmission remains constant even if the glass is wet or covered in condensation. The integrated ISO++ screen system is fitted in a W shape for optimum light transmission when the screen is closed. In addition, the greenhouse is fitted with a new, highly transparent screen cloth with even better light transmission.
The greenhouse is equipped with an Air in Control climate system. The expectation was that the greenhouse would let in at least 10% more light. “Initial measurements have indicated that this light gain has in fact been achieved. The next crop will prove whether we can achieve a 10% rise in production too.”

Cutting energy consumption

In the meantime, the 2SaveEnergy greenhouse has been adapted for a new research project entitled “A strong crop with little gas”. The assumption is that it should be possible to go yet another step further in reducing energy consumption. Janse: “We will be trying this out with a dehydration system that recovers heat using a heat pump. This will enable us to not only recover tangible but also latent heat. There will also be three movable screens in the greenhouse and some adjustments will be made to the control strategy for the tomato crop.”
Both the 2SaveEnergy greenhouse and the Winterlight greenhouse are financed by the Greenhouse as a Source of Energy programme, the innovation and action programme of LTO Glaskracht Nederland and the Dutch Ministry of Economic Affairs.


A high-wire cucumber crop was grown in the high-insulating 2SaveEnergy greenhouse last year. The aim of the project was to optimise a vegetable crop by making use of the available scarce winter light. Over two cycles the researchers intensively monitored light interception, cultivation (crop, production and quality) and energy consumption. The crop performed well and the trial clearly demonstrates that both energy savings and higher production are achievable.

Text: Jojanneke Rodenburg. Images: Studio G.J. Vlekke.

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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.”

Hotting up

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.

Dehumidification technology

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

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Roughly 60 to 70 greenhouses in the Netherlands have invested in air handling systems and they are all different. Growers who are interested in the Next Generation Growing can no longer see the wood for the trees. Specialist advisers believe it can be much simpler and cheaper as well as more standardised.

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The cultivation of crops in the city, or Urban Farming, is beginning to arouse increasing interest worldwide. This often involves Vertical Farming, in which multiple cultivation layers are grown in closed-off spaces under LED lighting. In addition, vegetables are grown on top of buildings, sometimes in greenhouses or simply in the open air.

Many people are convinced that the cultivation of vegetables on stacked layers, or Vertical Farming, is the future. Tall buildings will increasingly be used for growing crops rather than for industrial manufacturing. Those crops do not grow in the sun, but under LED lights; and not in the soil, but in a thin layer of water or mist. Shielded from the influence of the changing seasons, the year-round cultivation of these crops grown under constant conditions is possible.

North America and Japan

New developments always take place outside the traditional areas. The same applies to Vertical Farming. You will not come across it in the Netherlands with its high-tech greenhouse horticulture, but mostly in Japan and America, where mainly young entrepreneurs are attracted to Vertical Farming. There is one exception in the Netherlands: lettuce farmer Deliscious in Beesel is starts its crops in a seven-layer system under LEDs. Later on, they are transplanted and transferred to a greenhouse for further growth in mobile gutters.

Vertical Farming has long outgrown the stage of amateurism. In the USA, one Vertical Farming company is opening after another, and always in the vicinity of a big city. The farms are able to achieve a significant output on a small surface area. The industry already has its own magazine, Urban Ag News, and there is an Association for Vertical Farming, AVF.

The AVF expects that within there will be a Vertical Farm in every city within the next decade. ‘It is not without reason that multinationals such as Philips, Metro, Osram, Toshiba, Microsoft, Panasonic, Fujitsu and GE are showing interest.’ Lemnis Oreon does not exclude the possibility of developing LEDs especially for Vertical Farming, ‘but we believe in top lighting in greenhouses’. A world map on the AVF website shows where Vertical Farming is being carried out. There are only a few dots in Europe, while Japan and North America, in particular, are covered with them.


Vertical Farming is attracting worldwide attention. Leafy vegetables and herbs can easily be grown in cities or on their outskirts all year round, without the influence of seasons. The weather can not throw a spanner in the works as with outdoor farming. To quote poet and writer Brian Brett: ‘Farming is a profession of hope’. However, this no longer applies thanks to Vertical Farming.

It is not surprising that the AVF will be manning a stand on GreenTech in Amsterdam on 14, 15 and 16 June. They will also be organising a meeting with several speakers, such as the big man behind the Vertical Farming developments Dickson Despommier (Columbia University/AVF) and Jasper den Besten (HAS Den Bosch), Marc Oshima (AeroFarms), Fabio Ziemssen (Metro Group), Paul Hardej (Illumitex), Steven Beckers (Lateral Thinking Factory & Building Integrated Greenhouses), Oscar Rodriguez (Architecture and food), Vincent Fesquet (New’rban view) and from the AVF: Christine Zimmerman, Max Loessl, Henry Gordon Smith, Howard Brin and Zjef Van Acker on the day before the GreenTech exhibition.

Sophisticated cultivation strategy

In North America Illumitex has doubled its LED lamp production every year in the last few years. ‘The future is growing indoors’, says Illumitex. The company claims to have a specific light recipe for each crop with just the right spectrum, intensity and frequency a plant needs for photosynthesis and the most energy-efficient way to do this. However, there are several other companies that provide the same know-how with the delivery of their cultivation system.

The company PlantLab says it distinguishes itself by entering into a partnership. ‘We guarantee output. That is quite different as ”we have a system”.’ according to PlantLab, not the technology but the plant is the guiding factor. The organisation determines for each situation how the plant can deliver the optimal results; for example in yield, substances or quality. ‘This is not just dependent on light, but on the total production process, including materials handling and labour productivity.’


Several companies are bringing their own Vertical Farming concept to market, such as AeroFarms (USA), Urban Produce (USA), Urban Crops (Belgium), Mirai (Japan), PlantLab (Netherlands), InFarm (USA), VydroFarm (UK) and Truleaf (Canada). The bottom line is that they are all trying to reinvent the wheel.

AeroFarms is the largest of these companies. It has the ambition to grow rapidly and is hoping to expand into locations on four continents. But there are more organisations wishing to do the same, like Metropolis Farms, who claim that you can realise a revenue of $250,000 to $5,000,000 per year on 140 to 930 m2 , depending on what you are growing and what can be sold locally. ‘A Vertical Farm can be built within a week and after 60 days you are ready to harvest.’

The company Edenworks, on the other hand, aims to reduce labour costs by more than 50% in a yet to be opened second Vertical Farm by automating the sowing, harvesting, washing, drying, packaging and labelling processes, because otherwise viable exploitation is not possible. AeroFarms suggests that there is a great need for safe nutritious food ‘and we are quickly scaling up to change horticulture worldwide’. According to the Wall Street Journal AeroFarms is not yet making any profit, but it states that all its companies will have a positive cash flow this year.

FarmedHere in Chicago aims to prove that Vertical Farming can also be done organically. In an abandoned of 1,500 m2 factory they combined crop cultivation with aquaculture, which provides the nutrients for growing. However, after six months they had to close down. With $13 million raised money the company is trying again. This time without fish, but with vegetable-based nutrients, which reduces costs by 30%.

Data science

AeroFarms cultivates crops from sowing to harvest on cloth made from recycled plastic. Under the cloth, an atomiser provides the plants with water and fertiliser. In Newark they are growing crops in a former paintball hall on 6.500 m2 and in a former steel plant with a surface area of 510 m2. The company grows crops in 12 layers, 20 crops per year, reaching a production of 900 tons. Through the years the company has been collecting crop data, enabling it to now cultivate the desired taste, or ‘data science meets horticulture’. The company can, for example, grow spicier watercress or sweeter lettuce.

Urban Crops in Waregem (Belgium) opened an automated factory plant in early 2016, the largest of its kind in Europe. The system works with cultivation in crates in a layer of water. The crates enter the cultivation space on a conveyer belt. Thanks to RFID technology in the crates a robot recognizes where they should be placed. The technology for fertilisation and the purification of process water (UV) is provided by Hortimax. The cultivation area has eight production layers in 4 rows measuring 10 metres each. With 448 crates and 10 plants per crate 442 crops can be harvested every day. The system can be built up to 25 layers. With 30 rows, the daily production will be 126,000 crops.

Urban Crops performs feasibility studies on request for cultivation in large buildings. They provide insight into the expenses and calculate cost price based on the possibilities. Urban Crops also provides two cultivation systems in containers; Farm Flex and Farm Pro. Farm Pro is fully automated and costs approximately €55,000. Depending on the weight of the harvest, the annual production is 29,000 crops. When cultivating herbs, production will easily be double that amount.

Vertical crops

Certhon developed the PlantyFood growth cell, in which both Vertical Farming and cultivation of vertical crops is possible. The company will be demonstrating this at GreenTech 2016 with the cultivation of cucumber, from sowing to harvest. By doing this it intends to launch a debate on what is possible. But vertical crops such as cucumber or tomato are not really suitable for Vertical Farming, says Toyoki Kozai of Japan’s Chiba University: ‘Tomatoes require about 1,200 kWh of electricity per kilogram of dry matter. That is as much as the annual consumption of the average refrigerator in America.’

Priva also developed a large container for VF cultivation, which combines its know-how on climate control in buildings with its knowledge on growing plants. Marketing is done by ‘Here There And Everywhere’ by GertJan Meeuws, former director of PlantLab. Priva: ‘This box allows the grower to take a considerable step in professionalization.’


While PlantLab is concentrating on patents, another Dutch company - BrightBox - focuses on open innovations. In a low-threshold manner, this company wants to research the best way of growing crops under LEDs. Its customers come from all over the world, from North America to Japan. They also get requests from the retail industry, to research shelf life and to determine which crops and varieties are suitable for Vertical Farming, for example. Grodan has shown rock wool to be an excellent growing medium. BrightBox is continuously working with the latest LEDs from its partner Philips Lighting. The approachability of BrightBox is partly reflected in its weekly open sessions, which take place every Thursday at 3 pm.

Philips also performs some research independently in its own climate cells. In the GrowWise Centre in Eindhoven, the company is developing light recipes for urban farming. The facility consists of eight cells with four growing layers each and a private climate regulation by Priva. LEDs in various colours (white, blue, red and far-red) are suspended above the containers.

Proeftuin Zwaagdijk (Zwaagdijk experimental garden) conducts research for seed companies in a three-layer cultivation system on how to let various crops flower and seed as quickly as possible. This is done under nine colours of LEDs with adjustable brightness. The system also has moveable ceilings with four-colour LEDs under which vertical crops can be grown. The LED lighting rises with the growth of the crop.

Text: Tuinbouwteksten.nl/Theo Brakeboer. Photo: BrightBox.

Download the complete Vertical Farming files, the pros and cons of vertical farming inluded (16 pages, pdf).

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