Gertjan van der Spek is the first tomato grower without artificial lighting in the Netherlands to have two transparent energy screens and no dehumidification. This was his next step to save energy after having reduced the use of the minimum rail and ventilating above the screen, instead of making gaps in the screen. In this way he hopes to use no more than half cubic metre of gas per kilo tomato.
The idea of using a double energy screen for vegetable production is not new. A growing number of pepper growers are doing this. “Pepper plants grow less quickly so the screens can stay closed for longer. The switch to using two screens is less great for them,” says climate specialist Paul Arkesteijn, of screen manufacturer Svensson.
To show that possibilities exist for tomato growers as well, a demonstration trial was run at the Delphy Improvement Centre in Bleiswijk, the Netherlands, last year. This trial compared an area with two moveable transparent screens with a standard transparent screen with a fixed anti-condensation foil.
The top layer was the ordinary Luxous 1347 FR transparent energy screen; the bottom layer was the 1347 FR H2no with an anti-condensation property. The condensation droplets flow out of the latter screen. According to Arkesteijn both transmit 80% light. When both are closed, they still transmit 64% of the light. The crops in both greenhouses grew well. The trial greenhouse saves an extra 4 m3 gas/m2.
Gertjan van der Spek, of greenhouse company Solyco, with two locations near Rotterdam, grows Roma tomatoes on 4.3 ha. He is part of a horticultural cluster comprising six companies. They have a joint boiler house which has three energy sources available: waste heat from the ROCA-power station; two CHPs; and a boiler. The latter also serves as a backup if there is a breakdown in the CO2 supply.
“Our cluster wanted to purchase a heat pump to further cool the flue gases from the CHPs. Now the flue gases are around 45 to 50ºC and we want the heat pump to cool them to 23ºC. The two CHPs use 900 m3 gas per hour and the recovered heat corresponds to approximately 130 m3 of gas. To be eligible for a subsidy scheme run by the Netherlands Enterprise Agency the cluster had to save in total 15 per cent on energy. That meant each grower had to make extra effort,” explains the tomato grower.
This took place at the time that Van der Spek became one of two growers to join the supervisory commission for the trial at the Improvement Centre. Hence the reason for him being able to pay extra attention to the trial with the two transparent screens.
Second energy screen
During the spring the trial appeared to be running so well that Van der Spek dared in August last year to install a second energy screen under his existing screen. The fact that his first screen of eight years old was becoming more porous and therefore was in need of replacement also played a role. He had to invest in a second wiring system for the second screen but that wasn’t a problem, even though this option was not taken into account during the original construction. “Screen installer Alweco came up with the solution of attaching the second screen half way across the trellis. For safety we ensure that the cloths are not moved at the same time because it’s during opening and closing that most of the force is placed on the walls.”
The grower chose the light-transmitting Luxous 1347 FR, without anti-condensation properties. “The anti-condensation property is not necessary for us since we mostly screen at night.”
1,500 double screening hours
The planting date was 1 December and up until the end of April he regularly used two energy screens. The grower always closed the new screen first. Up to week 20 he had accumulated 2,200 hours of screening. He used the old screen for 1,500 hours.
Compared to last year he used 0.5 m3 more gas during this period for a slightly longer production. “In terms of energy consumption, we are about equal to last year, but then it was very mild compared with this year.” When he compares his energy consumption with that of colleagues he uses about 2 m3 less than growers with a fixed AC-foil and 5 m3 of gas less than growers with a single screen.
During the first six weeks he hardly made any savings compared with growers with the fixed AC-foil. “During this period the main advantage of a moveable screen is flexibility. I already had some production advantages because with a moveable screen you have fewer problems with moisture. We never wanted to have a fixed foil. I always had the idea that it only became cold when the foil was removed.”
The trial in Bleiswijk also used the double screens intensively during the autumn months, from October. That achieved extra savings of 1 to 2 m3. “During the last few weeks of the cultivation period the temperature needs to be high enough to allow the tomatoes to ripen properly.”
The New Thinking
In 2014, when the winter was mild, the grower used 27 m3 gas per m2 and yield was 63 kg. In a ‘normal’ year that is around 32 m3. “We now want to make these savings with the double screen cloth and we hope again to have a yield of 63 kg,” says the grower.
Van der Spek has already made huge steps on his nursery over the years. When he started in 1992 he used 72m3 gas. After building a new greenhouse in 2000 consumption dropped to 50 m3 gas. In 2005 that was reduced further to 40 after the installation of his first screen. The step to 32 m3 happened mainly thanks to new insights into production. Yield increased from 50 kg in 1992 to around 63 kg of tomatoes today. “Production rose to more than 65.5 kg thanks to grafting but we’ve scarified some of that as we save energy.”
The result is impressive: from 1.5 m3 gas previously to now 0.5 m3 gas per kg tomato. Arkesteijn attributes the new cultivation insights to The New Thinking, a derivative of the Next Generation Growing. “They used to make a gap in the screen to release moisture but consequently suffered a cold dump. That resulted in horizontal temperature differences. The climate is controlled based on the coldest spots. Now growers keep their screens closed for longer and ventilate the moisture away through the cloth screen. The big advantage is the uniform climate. “That is not only beneficial for saving energy, but also for product quality.”
Use minimum rail sparingly
The tomato grower says he has applied this strategy for two years. Firstly he only ventilated on the sheltered side. Now, when using two energy screens at once, he ventilates on both sides to achieve good air movement above the screen, so that the air is drawn through the screen cloth. And instead of opening the windows just a little he now dares to open them wide. Another adjustment is that the grower uses the minimum pipe very sparingly. “We have dropped from 50ºC to 40ºC and now to 30ºC.”
Arkesteijn: “Previously we believed you had to heat the crop for it to continue to transpire. What matters now is that the crop transpires sufficiently during the day. The energy screen closes at 80 to 100 watt radiation. The crop is activated the next day by the sun that shines through the particularly transparent screen."
Tomato grower Gertjan van der Spek is the first tomato grower in the Netherlands who doesn’t use artificial light to have two transparent energy screens without any dehumidification system. After a series of energy saving measures, such as the first screen, ventilating above the screen cloth and seldom use of the minimum rail, he took the decision to install a second wiring system and a moveable screen. He hopes this will result in gas consumption of 27 m3 and yield of around 63 kg per m2.
Text and images: Marleen Arkesteijn