CO2_2 enrichment

As described in chapter 1.2 the CO2_2 concentration of the ambient air plays an important role for photosynthesis. In the end of the 80s an air concentration of around 330 vpm was typical. Nowadays concentration reaches ca. 0.04 % (400 vpm) (Wikipedia, 2022, Carbon dioxide in Earth's atmosphere). In a greenhouse the CO2_2 concentration decreases during daytime due to the CO2_2 uptake of the plants by photosynthesis. So, under non-ventilated conditions values can drop below 200 vpm and this results in a reduction of mass production. The moment the vents open the incoming fresh air offsets this effect. At night there is an increase of CO2_2 concentration caused by the dissimilation of the plants. But we are only talking about an increase of around 100-150 vpm.

Fig. xx shows the increase in the production of greenhouse crops with increasing CO2_2 concentration (µmol mol-1 equivalent to vpm). It has to be noted that the slope decreases with increasing CO2_2 concentration. So, it always has to be calculated if an additional increase of CO2_2 is economical.

Fig. $$: Relative production (%) of greenhouse fruit vegetable crops as function of the average CO
Fig. $$: Relative production (%) of greenhouse fruit vegetable crops as function of the average CO2_2 concentration (µmol mol^-1^1) during the cultivation, with the production at 340 µmol mol^-1^1 as reference (100 %). -----  = 95 % confidence interval of the mean of the observations —— = 95 % confidence interval of the observations. Modified after Nederhoff 1994

In practice farmers increase the values to around 800 vpm CO2_2. Some farmers even go up to 1000 vpm even under the situation of open vents. For cucumber for example this should result in the effect that all fruits on the main stem develop to a harvestable fruit size. Normally some of the fruits on the main stem dieback due to the fact that there are not enough assimilates to feed all the developing fruits. There are several ways to enrich the greenhouse air with CO2_2:

  1. Using Low-NOx heaters
  2. To combine greenhouse heating and CO2_2supplement, low-NOx heaters can be used. These heaters combust natural gas or propane. The exhaust fumes are directly led into the greenhouse. A disadvantage is the dependence of producing heat and CO2_2. If low or no heating is necessary there is no CO2_2 production or the resulting concentration is too low to have a sufficient effect on photosynthesis. If a lot of heat is needed valves can lead the exhaust fumes to the outside to avoid excessive and dangerous values of CO and CO2_2 (see below: important remarks). Another disadvantage is that CO2_2 is only used by the plants at daytime, a period with relatively low need of heat and therefore relatively low production of CO2_2by the heater.

  3. Using exhaust fumes from a standard gas heating
  4. To avoid these disadvantages another system has been developed. Hot water for heating is produced during daytime using a standard gas heating. The exhaust fumes are led directly into the greenhouse. The produced heated water is stored in big insulated warm water tanks. From this storage the water is used for heating during night. This way CO2_2 production and production of warm water are decoupled.

  5. Using technical CO2_2
  6. The easiest way to increase the CO2_2 concentration is the use of technical CO2_2. This CO2_2 is stored in liquid form in tanks outside the greenhouse and can be distributed in the greenhouse by pipes which are laid out on the floor. Technical CO2_2 is free of impurities and therefore no problems of toxic gases due to combustion occur. This supply can be controlled by the climate computer using CO2_2 sensors and is independent from heating.

Figure xx: Low NOx heater (Priva Agro 2003)
Figure xx: Low NOx heater (Priva Agro 2003)
Figure xx: Insulated warm water storage tank (Fricke)
Figure xx: Insulated warm water storage tank (Fricke)
Figure xx: Tank for liquid CO

Important remarks: For all methods using combustion of gas it has to be remarked that the combustion produces NOx, CO, phytotoxic ethylene (C2_2H4_4) and hydrocarbons. So, the injection of exhaust fumes should be stopped if CO concentration in the fumes is too high. For all methods (also the use of technical CO2_2) the enrichment should be stopped by using a setpoint of ca. 1200 vpm. Warning sensors have to be placed in the greenhouse to protect the workers from too high concentrations of CO2_2 (the maximum workplace concentration of CO2_2 is 5000 vpm).

One of the control parameters in plant production is the water supply by irrigation or fertigation (Figure $$). These measures have an influence on the water content in the substrate and subsequently on the water content in the plant. As control mechanisms of action, decisions about the frequency and the amount of water/nutrient solution given per application have to be made.To avoid high losses of CO2_2 the application should be stopped if high wind speeds occur. Here the losses depend mainly on the tightness of the greenhouse construction. Additionally, a stop makes sense if the temperature in the greenhouse is near to the ventilation temperature (e.g. stopping enrichment if temperature reaches 4°C below ventilation setpoint) and if the radiation is too low to reach an effective use of the CO2_2 in the photosynthesis. So, the beginning and the end of enrichment can be controlled by e.g. using sunset and sunrise time (e.g. starting 1 h before sunrise and stopping 1 h before sunset) or absolute minimum radiation values.