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.