Direct Current highly promising alternative in horticulture

Direct Current highly promising alternative in horticulture

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 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.


Online software for energy-efficient greenhouses

Online software for energy-efficient greenhouses

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


Right light spectrum on bottom layer with half the energy

Right light spectrum on bottom layer with half the energy

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.


Dehumidifier boosts yields, cuts disease pressure and saves energy

Dehumidifier boosts yields, cuts disease pressure and saves energy

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.

RH curves

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.

Uniform climate

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.

Price tag

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.


Slowing down crop transpiration cuts energy consumption

Slowing down crop transpiration cuts energy consumption

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.

Picking leaves

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.