A modeling study was conducted on the effects of CO2 dosing and ventilation on CO2 concentration and crop photosynthesis in the greenhouse. After all, in the (near) future, CO2 will become scarce due to horticulture becoming fossil-free. As a result, CO2 will become increasingly expensive, and CO2 dosing will have to be more economical. How can you achieve the highest possible concentration in the greenhouse while dosing as little as possible? Dennis Medema shares the results of the modeling study on Kas Als Energiebron.
Saving on CO2 by artificially increasing the ventilation rate in the greenhouse?
Can you actually get CO2 from outside by increasing ventilation? And what exactly is the effect of changing CO2 concentration on photosynthesis and thus crop production? An answer to these questions was obtained through a modeling study of the effects of CO2 dosing and ventilation on CO2 concentration and crop photosynthesis in the greenhouse.
Air exchange with outside air (ventilation), CO2 dosage, and CO2 uptake by the crop (for photosynthesis) are the three main components to be calculated. The greenhouse must be ventilated to cool the greenhouse or remove moisture. As a result, much of the dosed CO2 escapes to the outside air. If little or no dosing takes place, the CO2 concentration soon drops below the outside value and ventilation can also be used to raise the CO2 concentration to that of the outside air (400-420ppm; 400 ppm has been calculated). Partly by opening windows further and partly by mechanically increasing ventilation.
These situations of no or low dosing were put into a model by researchers using cucumber as an example crop and further calculated to answer the above questions, initially at some fixed light intensities such as 200, 500, and 1000 μmol/m2/s and then some sample days with variable light. Calculations were made with a 6-meter-high greenhouse so the ventilation rate (number of complete air changes per hour) differs by a factor of 6 from the ventilation rate in m3/m2/hour. Read the study by Botany on techniques to increase ventilation rates to save CO2 here.
Misunderstanding
First of all, a persistent misunderstanding needs to be cleared up. Photosynthesis does not respond to the kilos of dosage put into the greenhouse but to the effect of the kilos of dosage on the CO2 concentration in the greenhouse. For most crops, the saturation level is around 800 ppm, and for cucumber, it is more likely to be 700 ppm. So dosing more than 800 ppm usually makes no sense. This produces roughly 30% more crop photosynthesis for most crops.
However, reaching 800 ppm may require little or many kilos of dosing depending on the ventilation rate and light level. If dosed and ventilated little, the CO2 concentration in the greenhouse will quickly drop towards 300 ppm, causing a 20% reduction in photosynthesis compared to a plant standing at 400 ppm. Ventilating more in that situation will bring more CO2 from the outside air into the greenhouse, which will increase CO2 concentration. However, because of CO2 uptake by photosynthesis, the outside value is never reached and they have to settle for some production loss. If the greenhouse CO2 concentration could be increased to 380ppm via ventilation, this would leave a loss of photosynthesis of 2.5% compared to 400ppm.
Unfortunately, achieving this concentration (20 ppm less than the outside air concentration) turns out to be very difficult. Indeed, depending on the light level, this requires a ventilation rate of 15 to 25 /hour (ventilation rate of 90-150 m3/m2/hour, see Figure 1). This is very much. In fact, the natural ventilation rate is more likely to be around 9 /hr (windows fully open and wind speed of 3-4m/s) and drops by using insect netting. If you accept a loss of photosynthesis of 5% compared to photosynthesis at outdoor air concentration, a ventilation rate of 14 /hour is still needed, regardless of PAR level. This means an additional mechanical ventilation of 6-8 /hour (36-48 m3/m2/hour) is needed on top of natural ventilation to contain production losses. For reference, the flow rate of dehumidification plants is around 10-15m3/m2/hour.
Since this increase in ventilation rate is quite substantial and also costs some production, researchers have continued to calculate the effect of very low doses. If a small amount of CO2 is dosed anyway (say 30 kg/ha/hour), much less ventilation is already needed to get close to 400 ppm. At a CO2 dosage of 60 kg/ha/hour, the CO2 concentration in the greenhouse is always higher than in the outside air because the dosage is higher than the crop photosynthesis of cucumber at 1000 μmol/m2/s PAR, resulting in a loss of CO2 to the outside air. If a CO2 concentration of 400 ppm is controlled, ventilation no longer has any effect on the concentration in the greenhouse because there is no longer a concentration difference with the outside air. The dosage required for this is equivalent to the CO2 uptake (photosynthesis) of the crop. This dosage is between 0 and 60 kg/ha/hour and depends on light intensity.
From a sustainability point of view, it seems optimal if CO2 dosage and uptake are equal and no CO2 is lost to the outside air. But because up to 30% more crop photosynthesis takes place at higher CO2 concentrations, from an economic point of view a higher dosage will still be more desirable as long as the price of CO2 allows it.
Free CO2
In short, cutting back on CO2 will quickly cost production, according to the study. The effectiveness of dosed kilos depends heavily on ventilation rate and light intensity. Theoretically, mechanically increasing the ventilation rate is an option to exploit free CO2 from the outside air but it requires a substantial air displacement capacity to contain production losses. Currently, controlling for the same concentration of CO2 as in the outside air is the simplest way to save (a lot of) CO2. But even this is not without production losses compared to the optimal concentration. Meanwhile, research is already being conducted into 'direct air capture', among other things. The price of CO2 and that of the product will ultimately decide which method will be used in the future.
This consultancy was carried out by Plant Lighting and funded and coordinated by Kas als Energiebron, an innovation program of LVVN and Glastuinbouw Nederland. Made possible in part by Kennis in je Kas (KijK).
Click here for the full final report.
Source: Greenhouse as a source of energy