Although their revolutionary greenhouse has had a superb first year, the brothers Duijvestijn see room for improvement. Technically there are still gains to be made. And, although the goal to save 50% on energy by using low-grade heat was achieved in the first year, the businessmen take little pleasure from that. The system’s partners watch and learn. Where can they still make improvements?
The ID greenhouse that Duijvestijn Tomatoes of Pijnacker, the Netherlands, started running in December 2013 is performing well. In the first quarter, the crop was mostly grown in the conventional way in order to eliminate any technical teething troubles without taking any major risks. Then in the very light and closed greenhouse, step by step, they switched to the Next Generation Growing. This greenhouse concept, jointly developed by Technokas, Boal and Scheuten Glass, lends itself excellently to this purpose.
Noteworthy features of the new greenhouse are: A double glazed roof with large format glass (2x3m); insulation glass with four anti-reflective coatings and a structured layer to scatter the incoming light; air conditioning using air drawn in from outside; heating with low grade (primary) and high grade heat (secondary); continuous ridge ventilation with the opening on the top side; and a construction without any light-intercepting trellises. Instead it uses relatively heavy columns and angled sidewalls to support the greenhouse. The screens are tucked under the gutters.
Ted Duijvestijn calls 2014 a peculiar year, because the reference crop of plum tomato Prunus, performed exceptionally well. “As a result we weren’t able to achieve any extra yield in the new greenhouse,” he said. “If all the components had immediately performed as we wanted them to, we certainly could have got more out of it.”
Apart from the later planting date and the conventional start to the crop, a few other factors contributed to the setback. One of these was the CO2-supply in the new greenhouse. “We had a central control for the CO2 in which a two way valve distributed the CO2 between the conventional greenhouse and ID greenhouse,” explains Duijvestijn. “Measurements inside the greenhouse showed that sometimes too little CO2 entered the new greenhouse so that the concentration measured next to the closed windows was sometimes under the level outside. Then you know you are missing your chance regarding kilos. This year the new greenhouse had its own CO2-control system.”
Insect mesh removed
Another disappointment was that on warm days there was a shortfall in the ventilation capacity. That was directly related to the insect mesh in the ridge ventilation, which the growers had chosen to install at the last minute. “The mesh had a big impact on the ventilation capacity,” says the grower. “On very warm days, which were quite often last year, the plants were sometimes exposed to stress.”
Therefore, the mesh was removed during the crop changeover. Duijvestijn says that it is now more difficult to manage the secondary goal, which is emission-free production, but the ridge ventilation can now very quickly and in line with the original calculations achieve the desired level of cooling.
“The vents open from the top side to obtain the maximum chimney effect,” he explains. When the windows are fully open it’s possible to refresh about 150 m3 air per m2. Due to the diffuse glass that doesn’t have to happen very often but it is good to have such a fast and powerful system available. The capacity to draw in outside air is 10 m3/m2 per hour and is used mostly for the fine-tuning.
The advanced and expensive double glazed roof largely meets expectations although Duijvestijn would like to see less condensation.
“That certainly has our attention,” says product manager horticulture Laurens Vlaar, of Scheuten Glass. “We are all convinced that double glazing adds value, but we also know that glass is extremely differentiated and complex. Together with the users and research institutions we want to initiate improvements where possible and offer tailor-made solutions. These could include different structural treatments that you can apply in order to achieve the desired scattering of light. Every type of structure has its advantages and disadvantages. With hindsight we can see that the pyramid structure in this greenhouse roof is a little too coarse. A finer structure would probably lead to fewer problems with condensation.”
Peter Zwinkels, of Technokas, adds: “This has also opened my eyes. In the past condensation was often seen as an inevitable side-effect but in recent years it has started to receive the attention it deserves. Nowadays we always test glass samples that are wet and the differences can be amazingly large. Wageningen has a great setup for taking measurements which we thankfully can use.”
One thing that Zwinkels would like to do differently in this context is take measurements when the glass is at an angle, just like when it is on the greenhouse roof. Currently glass samples are only measured when they are horizontal and that, certainly with respect to condensation, can give a distorted picture.
Glass producer Scheuten, as a project partner, is closely involved in several new greenhouse concepts in the Netherlands. Despite the still relatively high cost, Vlaar sees a lot of potential for well-insulated, double glazed greenhouse roofs using diffuse tempered glass with AR-coatings.
“The projects which we have been able to achieve so far are fairly small scale and therefore relatively expensive,” he says. “Before there’s a further roll-out, the project volumes need to increase to enable a serious decrease in price. For projects of several hectares that is feasible, certainly now that we are better able to deal with the particular nature of horticultural projects.”
Wageningen UR Greenhouse Horticulture was not only consulted for determining the glass specifications, it also provides the monitoring program. “That’s very valuable,” says Duijvestijn. “Before we invested in this greenhouse I was intensively involved in all kinds of trials for three years. However, I’ve actually learned the most from using it, not least, thanks to the monitoring by the researcher.”
Invest in research
Together with Zwinkels and Vlaar he stresses that the sector and the government should keep up their joint investment in research. The relative reticence that has occurred over the last few years gives him reason for concern. “There have been many cut backs and in my opinion potential financial partners sometimes say no too easily to subsidy applications for research,” according to Zwinkels. “That is partly due to a lack of strategic focus and expertise."
The greenhouse builder cites Next Generation Growing as an example. “I have already seen several requests fail because they were not thought innovative enough. However, many variations are being designed. Growers must be able to make informed choices and that requires comprehensive research and testing. Growers also want to pay but it makes a difference if other parties continue to make a contribution and thereby reduce the business risk to an acceptable level. That is what happened here. Without external help this greenhouse may never have been built."
The energy target (50% energy savings) was achieved in the first year of production, but by a combination of circumstances no extra yield was achieved in the ID Greenhouse. Now the greenhouse has its own CO2 control system and the insect mesh over the ridge ventilation has been removed, so that may now be possible. The well insulated, double glazed roof made of diffuse glass has excellent transmission properties. The condensation behaviour, in relation to the treatments used to make the glass diffuse, deserves extra attention in future projects.
Text/photos: Jan van Staalduinen