One of the bottlenecks within the energy transition is the CO2 supply to greenhouse horticulture companies that do not use natural gas. Where residual CO2 is not available and companies rely on pure CO2, it threatens to become a scarce, thus expensive growth factor. Moreover, much of the gas is lost once vents open to remove excess heat and/or moisture.
"In normal greenhouses, of which the Venlo greenhouse is the best-known manifestation, a grower has to dose a lot of CO2 to ensure crop growth," says Danny Monté, Greenhouse Cultivation Expert at KUBO. "CO2 is becoming more expensive, and for emissions through the vents, a grower will eventually have to pay for it as well. So you don't want to use more of it than is strictly necessary."
A distinctive feature of the Ultra-Clima® greenhouse is its very high internal ventilation capacity combined with few vents and permanent overpressure in the growing space. The vents do not serve to allow air exchange, but only to exhaust warm, humid greenhouse air when necessary.
The idea came from an American grower, who developed it with KUBO. The grower was looking for a greenhouse that could effectively keep insects out (few vents, insect netting, overpressure) combined with a high capacity for air treatment (dehumidification, cooling and heating) and air circulation. The air treatment takes place in façade units that pass the treated air (outside air and greenhouse air) to a so-called climate corridor, which is physically separated from the cultivation area. In the climate corridor, the air is mixed with air coming from the greenhouse and further brought to the desired temperature. If desired, enrichment with CO2 also takes place here.
Powerful sustainable fans in the intermediate walls then bring the treated air into the cultivation space via air distribution hoses under the cultivation gutters. The humid greenhouse air is either returned to the climate corridor for renewed treatment or blown off to the outside through the few vents and outlets.
In standard greenhouses, when vents are closed, there is usually limited internal ventilation and little air movement. To compensate for CO2 uptake by the crop and avoid deficiencies, it is common to dose to a greenhouse concentration of 600-900 ppm.
In the most recent trial at the Blue-lab test center in 's-Gravenzande - an Ultra-Clima greenhouse of 1,500 m2 for research purposes - greenhouse staff grew tomatoes virtually without the input of supplemental CO2. Only during the first months of cultivation, when the vents were permanently closed and ventilation was mainly internal, was consumption by the plants supplemented to the normal outside air level of 400 ppm via the OCAP connection. Dosing was started as soon as the lower limit of 375 ppm was hit. For the vast majority of the cultivation, the CO2 requirement was covered exclusively by outside air, which was continuously blown into the greenhouse in large quantities under the crop.
With a CO2 concentration of up to 400 ppm, you might expect production to be significantly lower compared to normal dosing practice. This was not the case.
Blue-lab operator Emiel van Velden: "For this trial we collaborated with Kwekerij Schenkeveld. At their growing location in Schiphol Zuid and at our test location, vine tomatoes of the same variety, with the same planting date, were grown synchronously. There this was done in a Venlo greenhouse with active dehumidification via AVS, here with our own air treatment system. The Venlo greenhouse allows a bit more light through than our test location. Because this was an unlit crop in which we were aiming for equal growth conditions, we only used our LED system to keep the daily light sums the same in both greenhouses. We are grateful that Schenkeveld was willing to share all the data with us."
That data showed that energy consumption for heating and ventilation was 22% lower than at the reference farm. In the test greenhouse, during the entire cultivation period, 3 kg/m2 of CO2 was dosed coming-from OCAP, versus 39 kg/m2 at the reference farm (coming from OCAP and CHP). "That amounts to a savings of 92%, or 360,000 kg/ha/year," Monté calculates. "Production came out at over 76 kg/m2, just 1.5 kilograms or 2% less than at Schenkeveld. That's a much better result than expected."
Water consumption also came out lower, about 7% on balance. That's a nice by-product, which the crop specialists attribute to a slightly better "water use efficiency" of the crop due to the stable growing climate. "Stress situations are rare in this greenhouse, partly because we can also cool with humidified pads in the suction boxes that are in the gables," Van Velden says.
According to the cultivation specialists, the explanation for the high production level at a CO2 concentration of "only" 400 ppm lies with the stomata of the leaves combined with the permanent supply of fresh air. "The high refreshment rate of the air guarantees that the CO2 level around the stomata is continuously maintained," notes Monté. "In a conventional greenhouse, even when there is active internal ventilation, there is much less air movement. You do then have to overdose to prevent the CO2 concentration around the stomata from dropping too far and the plant not being able to absorb enough of it."
"Research has also shown that plants create fewer stomata as they are exposed to high CO2 concentrations and that those stomata are less active. If the CO2 concentration does drop once, for example when the vents open and no dosing takes place, the plant cannot absorb enough of it. The situation in Ultra-Clima is substantially different. The continuous air movement around the leaf surface creates an active climate in which the stomata present - which occur in greater numbers - function optimally. "As a result, the plant utilizes the available CO2 much more efficiently," says Monté.
In August, cultivation consultant Peter Klapwijk of 2 Harvest visited the trial and was given access to the data from both crops. He is enthusiastic about the cultivation concept that has been applied. "In this greenhouse you basically grow without vents at a very high internal ventilation and input of outside air," he said.
"The absence of vents keeps the CO2 concentration at the natural level and the uptake from the boundary layer around the leaves is compensated very quickly," Klapwijk concludes. "In winter, the proportion of outside air is lower and CO2 is dosed to 400 ppm. The input required for this is minimal. That the plants are not short of anything is shown by the fact that the production and fruit quality of the unexposed vine tomato cultivation in the trial were at the level of the reference farm especially when you set that against the modest input of energy, water and CO2. It is an inspiring and innovative concept."
In the Netherlands, KUBO has so far realized two projects with the discussed greenhouse type: one for tomato grower RedStar (2012) and one for lettuce grower Fromboer (2020), both located in Dinteloord.
"This greenhouse has far fewer vents than a Venlo greenhouse," says the grower. The air exchange capacity is 75 m3/m2/hour, ten times more than the AHUs at their previous farm could provide. "With that, you can create a slight overpressure and keep pests out of the greenhouse without losing a lot of heat and CO2."
He continues, "Importantly, based on air handling and internal ventilation, we can efficiently maintain an active climate and remove moisture under closed windows. We use a heat pump for heating and cooling and store the heat released during cooling in a buffer."
Purchasing power for the LED system became too expensive due to the energy crisis. So in 2022, a CHP was still installed. "During running hours, we also utilize the released CO2," Boer adds. "In summer, we suffice with CO2 from outside air; thanks to our high ventilation capacity, it takes little effort to stay at 400 ppm."