The air above an energy screen is much drier than that below it. Currently around 20 companies in The Netherlands are making use of this fact to dehumidify the greenhouse. However, until recently, hardly anyone had tried vertical ventilation in an unlit crop of tomatoes. Tomato grower van den Broek, partly stimulated by German legislation, is a pioneer in this field. The initial experiences are positive.

The horticultural company run by Paul, Marcel and Jolanda van den Broek has very gradually changed from a Dutch one into a German one. It was logical, based on decisions made over the years, that the company, originally from Venlo, should eventually end up in Germany.
“The buyers were always almost all German. From a very early stage we have always wanted to provide exactly what the customer wants. At that time there was always a lot fuss amongst the Venlo growers if they were told at the last minute what sort of packaging to pack the product in; we only saw opportunities. We concentrated on quality instead of volume but it was difficult then at the auction in Venlo to distinguish ourselves from the rest. Therefore it was logical to go directly to an auction in Germany; there, customers do look at the name of the grower. If you do it well you can earn more,” explains father, Marcel van den Broek.


When producer organisations were formed in the Netherlands the van den Broek family kept their distance. Instead, in 2004 they were able to become a member of Tomatengärtner Rheinland, another new step towards Germany. “The trend ‘local for local’ – the preference for products from your own region – was at that point not very strong. Even so, it made more sense to produce in Germany if you were a member of the Rheinland organisation,” he says. Therefore, in 2007 they bought a building plot in Straelen, sold the company in Venlo and built a 4 ha greenhouse. “Thus we became real German growers and that’s how we feel today too,” he says.

As the crow flies they are just four kilometres from the border but producing in Germany is very different to that in the Netherlands. Firstly, with their four hectares they were for a long time one of the largest horticultural companies in the region. Furthermore, both the energy market in Germany and strict energy legislation are strong determining factors. Indeed these actually drove them towards Next Generation Growing.

Green energy

Last year the greenhouse was expanded to 6.5 ha. The new greenhouse (with polycarbonate walls, low haze roof 26%) was planted at the end of December. They are growing cluster tomatoes (Lyterno) and mini-Roma tomatoes (Strabena). In the early days the older greenhouse was heated using a coal-fired boiler of 3.5 MW. “Natural gas in Germany is twice as expensive as in the Netherlands; then it’s quickly a choice of coal or wood. But we did look into how we could make it more environmentally friendly. Now we hire a co-generator (CHP) of 1.6 MW from Weltec Biopower that is run on biogas. We use the heat and the green energy goes into the grid. We’ve just installed another such CHP and then 90% of the heating will be from biogas. We’ll only burn coal to handle the peaks,” explains his son, Paul.

The decision to lease the CHP was stimulated by the German legislation: “If you install your own CHP you have to pay an energy tax on the electricity that you generate; then it’s not very lucrative,” he says.


They’ve been applying the principles of Next Generation Growing for some time already: “In the past we used the minimum pipe rail much more,” explains the father. “Now we shade more and accept that sometimes there’s a day when the temperature is somewhat lower.” They now use 31 to 32 m3 gas in the greenhouse that is green for 49.5 weeks per year.

The energy efficiency program in the State of North Rhine-Westphalia has a big influence on the decisions too. “Greenhouse horticulture companies have to achieve energy savings and every measure taken scores a certain number of points: the polycarbonate walls, the energy screen, the climate computer. To achieve the standard we were virtually obliged to install a second screen in the new greenhouse even though we would use it very little. The investment was out of proportion to the number of hours it would be used.”

Because the moisture in the old greenhouse often became too high, they approached Greenco, of Middenmeer and Someren, the Netherlands, about the possibility of using the VentilationJet-system by Hinova. It consists of two fans; an upper fan draws dry air from above the screen to below it. The lower fan mixes the air and spreads it evenly. In this way dehumidification and an homogenous greenhouse climate are achieved together. Furthermore, this system counts towards the necessary energy points.

Vertical fan

“It was difficult to judge if it would work for us because Greenco uses lighting and has a second screen which we don’t have. Nevertheless an extra tool for moisture control seemed attractive. We don’t really like to be a front-runner but because we were the first unlit tomato grower to have such a system we have had to discover everything for ourselves in terms of greenhouse climate. Luckily in the meantime two other members of our growers’ organisation also have such a system,” says Marcel van den Broek.

The new greenhouse now has 52 VentilationJets and 52 separate Hinovators; the latter is a fan that pulls greenhouse air upwards and spreads it like a blanket over the crop ensuring even mixing: the air distribution fan.

Check by eye

They have only been using it for three months but they speak very positively about it. “The temperature distribution in the greenhouse is much better. You see that from the crop. I get goose bumps from such evenly coloured tomatoes,” says Paul van den Broek. “The aim is keep the relative humidity at 87% under the closed screen; that works well. “In the old greenhouse, we need to open the window vents above the closed screen much more often.”

In the evening (18.00 – 23.00) everything is switched off. “Then the plant needs to relax and unwind and that’s not possible if the fans stimulate transpiration. After that the system restarts and the computer determines the force and capacity. We check it by eye: the crop needs to have fresh-looking tops. If they start to turn grey we intervene.”

An homogenous climate gives a somewhat higher production; then the gas consumption per kilo of product is lower. “In addition, we can use the screens for slightly longer. Together that could lead to about 10 per cent energy savings per kilo of product,” he says.


Tomato grower Van den Broek has gradually changed from a Dutch into a German company. An interest in Next Generation Growing as well as German energy legislation have driven some noteworthy decisions, such as a leased CHP that runs on biogas and a system with two fans placed under each other. The initial experience with this system is very positive: the climate is more uniform and the crop too.

Text: Tijs Kierkels.   Images: Wilma Slegers.