Last year the focus in the Dutch 2SaveEnergy greenhouse was on high-wire cucumbers. Over two production cycles, Wageningen University & Research investigated whether it was possible to grow a crop that could intercept and use winter light to the full. This trial was a preliminary study in advance of upcoming trials in the Winterlight greenhouse, a design that lets in 10% more light in the winter.
The crop in the innovative greenhouse was very successful, say crop researchers Jan Janse and Frank Kempkes. It was cleared at the end of November and the total yield is estimated at 110 kg, with 260 cucumbers. “And all with minimal energy input,” Janse says. “We used around 17.5 m3 of gas over the entire year. That once again puts us well below the average for the sector.” Yet again, this new greenhouse proves what it is capable of. The concept clearly demonstrates that Next Generation Growing can be made even more energy-efficient without affecting production or quality.
Steering the crop
This high-insulating greenhouse at the Energy Innovation and Demonstration Centre in Bleiswijk features clear glass and a permanent, high light transmitting, diffuse layer of plastic film parallel to the glass. The greenhouse is fitted with a dehumidifier unit with outdoor air entering via ducts under the gutter.
The researchers planted the cucumber variety Hi-Jack in the greenhouse on 29 December 2015. The crop was steered by varying the row width and plant density and by thinning out the fruits, the optimum having been modelled in advance. Over two production cycles the team intensively monitored light interception, cultivation (crop, production and quality) and energy consumption.
Different row widths
Janse: “We decided to start with a plant density of 1.67 plants/m and three different row widths: 1.4, 1.6 and 1.8 m. Among other things, we wanted the trial to tell us which row width would produce the best crop, would be best for light utilisation and would be easiest to work in. After all, you have to be able to move through the cucumbers on the high-wire trolley without damaging the plants too much. Each setup consisted of three ‘carousels’ (growing gutters). Regular crop observations were carried out on one carousel in each setup.”
To record the observations, Kempkes took photographs from a fixed position above the crop at the same time once a week. Using an image processing program, the researchers were then able to track the development of the crop and the projected leaf surface area to get an idea of the amount of light being intercepted. Kempkes: “If you can see a lot of the floor or the gutter on the photos, light interception is not as good as it could be.”
45% less gas
At the end of January last year, the plants were doubled by pinching out. “At one point we had a crop with highly generative growth and small leaves, but we still harvested a lot of cucumbers from it. Over six weeks (weeks 15-20) the crop produced as many as ten cucumbers per week, or 4.5 kg/m2. So clearly the plants were using their assimilates very efficiently.”
According to Janse, even the two growers on the supervisory committee were looking enviously at the crop. “The plants were strong and production was high – better than in a commercial greenhouse, in fact. We still have some work to do to find out the exact reasons for this. It may be partly down to good crop care. What was also striking was the temperature achieved: it averaged 1ºC higher than in a commercial greenhouse. This meant that the cucumbers got going very quickly. Over the entire period, the development time was around 14 days at an average greenhouse temperature of 21.4ºC. The row widths of 1.4 and 1.8 m yielded the highest production. In addition, gas and pure CO2consumption worked out at 12.7 m3and 4.5 kg/m2 respectively. This represents savings on gas consumption of around 45% compared with commercial greenhouses.”
Later second planting
The team set up a new crop in mid-July. This time they used Hi-Power with a plant density of 2.25 plants/m2, again with the same three row widths. In the period with the most light, there was therefore a gap of three weeks between the end of the first crop and the start of the second one. “We deliberately planted it slightly later because we wanted to test the crop in the dark period as far as possible. After all, this was a preliminary trial for the Winterlight greenhouse. By the end of October we had already harvested almost 100 cucumbers from this second crop, with an average fruit weight of around 420 grams. The crop finished off well and we achieved excellent overall production of good quality cucumbers.”
This time too, the row widths of 1.4 and 1.8 m yielded the best results. So the objective was fulfilled. An excellent crop can indeed be achieved with a relatively small leaf surface area, in other words small leaves, a characteristic of the variety. Because the rows were oriented east-west and care had been taken to distribute the wires evenly, the crop clearly intercepted enough light and a good proportion of the assimilates went to the fruits. There was barely any fruit abortion. Smaller leaves also mean less transpiration, which saves energy in cold periods.
The research into an efficient Winterlight crop was brought to a successful conclusion. On to phase two: in late December 2016 a new high-wire crop was planted in the Winterlight greenhouse, this time with a row width of 1.8 m. Kempkes: “Both 1.4 and 1.8 produced good results but there was more plant damage with 1.4 m spacing.” The scientist, who also manages the Winterlight greenhouse project, is expecting a lot from the follow-on trial. “The Winterlight greenhouse was handed over recently and is really very nice.”
The entire structure is painted with a white powder coating with an increased reflection factor of 90%. The glass used is SmartGlass, a new type of diffuse glass in panes measuring 300 x 167 cm. Light transmission remains constant even if the glass is wet or covered in condensation. The integrated ISO++ screen system is fitted in a W shape for optimum light transmission when the screen is closed. In addition, the greenhouse is fitted with a new, highly transparent screen cloth with even better light transmission.
The greenhouse is equipped with an Air in Control climate system. The expectation was that the greenhouse would let in at least 10% more light. “Initial measurements have indicated that this light gain has in fact been achieved. The next crop will prove whether we can achieve a 10% rise in production too.”
Cutting energy consumption
In the meantime, the 2SaveEnergy greenhouse has been adapted for a new research project entitled “A strong crop with little gas”. The assumption is that it should be possible to go yet another step further in reducing energy consumption. Janse: “We will be trying this out with a dehydration system that recovers heat using a heat pump. This will enable us to not only recover tangible but also latent heat. There will also be three movable screens in the greenhouse and some adjustments will be made to the control strategy for the tomato crop.”
Both the 2SaveEnergy greenhouse and the Winterlight greenhouse are financed by the Greenhouse as a Source of Energy programme, the innovation and action programme of LTO Glaskracht Nederland and the Dutch Ministry of Economic Affairs.
A high-wire cucumber crop was grown in the high-insulating 2SaveEnergy greenhouse last year. The aim of the project was to optimise a vegetable crop by making use of the available scarce winter light. Over two cycles the researchers intensively monitored light interception, cultivation (crop, production and quality) and energy consumption. The crop performed well and the trial clearly demonstrates that both energy savings and higher production are achievable.
Text: Jojanneke Rodenburg. Images: Studio G.J. Vlekke.