Energy efficient production according to the principles of Next Generation Growing, without any additional investment, is the aim of pepper trial being carried out at the Delphy Improvement Centre (IC), Bleiswijk, the Netherlands. Armed with two energy screens and fans the trial participants want to save 30% on energy and still achieve good fruit quality.
The main barriers raised by pepper growers to grow as energy efficiently as possible in practice are doubts about the impact on crop health and fruit quality. This therefore was the reason for running two climate trials this year with peppers, one at the Improvement Centre and the other at neighbouring Wageningen UR Greenhouse Horticulture.
The one at the IC is being carried out in an area covered with a standard greenhouse roof and two energy screens. The other trial is taking place simultaneously in a VenlowEnergy greenhouse with double glazing. The red variety Maranello was planted in both greenhouses on 7 December 2015. A Supervisory Committee, which includes four pepper growers, is following the trials closely.
The trial at the IC uses two transparent energy screens, namely Luxous 1547 D FR and Luxous 1347 H2no FR. The H2no property ensures that when the screen is used during the day it also allows a lot of light to penetrate even when it is wet due to condensation. In addition the area is equipped with horizontal and vertical fans and is sparsely heated. Although the energy consumption on commercial nurseries is usually about 30 m3 per m2 the trial participants are aiming for 20 m3 per m2. That is a saving of more than 30%.
Assessing the balance half way through the trial it would seem that the goal is achievable. Energy consumption is even slightly lower. Maximum use of the energy screens and omission of the minimum pipe rail therefore have a huge impact.
Screening based on radiation
The principles of Next Generation Growing (NGG) were applied during the trial. The use of the energy screens is the dominating factor. The upper screen opens when the radiation is 100 watt per m2. The second screen opens at the moment that the temperature above the screen differs by four degrees from the desired heating temperature. This small difference should prevent a cold dump.
There were moments this spring that the lower screen was still closed when the radiation was 300 to 500 W/m2. The intensive use of the screens has, from the start of the cultivation to early April, led to 14% light loss. That was difficult for the growers to get used to as they prefer to allow in as much light as possible.
“But the crop was growing to our liking,” says Rick van der Burg, crop manager at the IC. “We noticed that the room temperature was quickly a degree higher than what is usual in practice," adds Arie de Gelder, researcher at Wageningen University & Research.
Drain off moisture
At the same time the screens play a major role in the removal of moisture. At the moment that the RH becomes too high, cool dry air is supplied via the vents above the screens. The moisture is then removed to the outside via transport through the screens. Therefore the usual method of making a gap in the screen is not used,
The fans ensure a uniform temperature and moisture distribution in the greenhouse. At the start of the cultivation this was achieved by just using the horizontal fans. As the crop becomes taller the vertical fans are used too.
The trial participants are not completely satisfied with the air currents and thus the temperature distribution that occurs in the section. Bubble wrap is attached to the walls to rule out influences from outside and from the adjacent much warmer section. Because so little heating is used the temperature differences between the walls has relatively large impact. “We’ve noticed that strong air currents occur,” says De Gelder. That will be different in a practical situation.
When Van der Burg made the first assessment in mid April, it revealed that 2,500 hours of screening were with a double screen. That saved a lot of energy especially in March and April.
A net radiation sensor was hung in the top of the greenhouse. This shows how much radiation enters the greenhouse and how much radiation is emitted from the crop. The double screen in the night leads to an important reduction in the radiation emitted.
Screen out the light
As the radiation increases, the screens will be used as a tool to screen out excessive light. The greenhouse does not have a solar reflective coating. When the radiation is more than 700 W/m2 the upper light diffusing screen closes 80% and the lower screen 40%. They are positioned so that they overlap each other. De Gelder: “In this way we want to keep the humidity as well as the CO2 as much as possible at the right level."
Initially ventilation only happened when the greenhouse temperature was more than 27°C. Since the greenhouse temperature rose rapidly at high radiation it was decided to slightly reduce the temperature. Van der Burg: “We noticed that the fruits then become wet and we have to prevent that.”
24-hour temperature based on radiation
The desired greenhouse temperature is very dependent on the radiation. During dark periods the 24 hour temperature is 18.5ºC. When the light sum is 1,000 joules the 24-hour temperature should be 20.5ºC and at 2,000 joules it should be 22.5ºC. The light sum of the previous day determines the night temperature that follows.
It’s noteworthy that no minimum pipe rail is used. Heating is only used when there is a need for energy. Incidentally, plant temperature is well monitored.
What is now interesting is how the crop responds to these climate settings. In particular the growers in the Supervisory Commission, who are willing to push to the limits, have been amazed at the crop condition. They didn’t expect the crop to look so good after so much screening and the subsequent loss of light. During the first setting some fruits aborted so the trial participants didn’t have to consider thinning out. The first setting started to develop a little later than in commercial nurseries but the differences weren’t shocking. Harvesting started in week 12 and by week 18 the yield was 6.10 kg/m2.
The second part of the cultivation will be interesting when the radiation rises even higher and the crop develops further. Then the emphasis will be more on the vertical temperature distribution in the greenhouse. Of course the growers and researchers are closely following the quality of the fruit. Everyone is wondering what the final fruit quality will be like and what affect the climate regime has on the total yield.
A pepper trial with NGG in an existing greenhouse in the Netherlands shows that during the first half year a lot of screening has no adverse effects on the crop or yield. Up to now it has been easily possible to save 30% on energy. The two energy screens limit the radiation during the night so the crop temperature remains higher.
Text and images: Pieternel van Velden