Supplying pure CO₂ is the ideal method of CO₂ enrichment in greenhouses. In contrast to enrichment by burning fuel, enrichment with pure CO₂ is not coupled to heat production. Pure CO₂ is clean, the supply is direct and easily controllable and does not require enormous investments. The drawback usually is the (very) high costs of pure CO₂. This article discusses purity, techniques and quantities needed for supply.

PURITY

‘Pure’ CO₂ may contain small amounts of impurities, depending on how the CO₂ is produced. CO₂ can be obtained as a by-product of chemical industries or biochemical processes, or generated by burning fossil fuel or gained from natural CO₂ sources. Some contaminations are totally harmless, like nitrogen which is a normal compound of air. Others are dangerous, e.g. sulphur dioxide and nitrogen oxides cause damage or yield reduction, while ethylene is even fatal for plants. Ethylene or a similar compound in bottled CO₂ would make it unsuitable for enrichment. There are no guidelines available, but from research in Europe it can be derived that pure CO₂ for enrichment to plants should contain not more than 0.005 % ethylene, 0.1 % NO, 0.05 % NO₂, 0.005% ozone and 0.10% NH₃ (the author and HortResearch accept no responsibility for these figures).

TECHNIQUES

Pure CO₂ is supplied into the greenhouse from refillable aluminium or steel cylinders (bottles) or from tanks, containing CO₂ in liquid and gas phase under a pressure of about 6 MPa. The modern so-called cryogenic tanks have a constant pressure of about 2 MPa, either maintained by vacuum insulation or refrigeration (below -18 ^oC). They contain between 3 and 30 tons of CO₂. There are also cryogenic ‘minitanks’ or ‘flasks’ holding 300 kg of CO₂. In the Netherlands, where greenhouse properties are concentrated, some growers together can have one large CO₂ tank with a distribution system to all users, in order to benefit from a lower price.

There are two methods for the distribution of pure CO₂. Liquid CO₂ can be inserted at unreduced pressure into the air flow of a fan, connected to a distribution net with lay-flat ducts (a system normally used for distribution of flue gas CO₂, see earlier article). Alternatively, the liquid CO₂ passes through a vaporiser, that either uses heat from the ambient air or electric energy. After passing a pressure reducer the CO₂ gas then flows by its own pressure through a main duct (about 20 mm wide) towards the greenhouse. For good distribution, there should be a small pvc or nylon tube (6-8 mm) in every greenhouse bay or plant bed, e.g. 1.5 or 2m apart.

A special form of using pure CO₂ is adding it to the irrigation water. A carbonator (e.g. trade mark 'carborain') is used to dissolve CO₂ in the irrigation water. There are some reports that this would improve root growth and nutrient uptake, but it is not clear what would cause these effects. The general understanding is that CO₂ supply to irrigation water has no effect on photosynthesis and only a minimal overall effect on the plants.

AMOUNTS OF CO₂ LOSS BY VENTILATION

The amount of CO₂ to be supplied equals the amount lost by ventilation plus the amount taken up by the plants (the photosynthesis). The loss by ventilation is very high at high CO₂ levels and low at low CO₂ levels. When the CO₂ level in the greenhouse is below the ambient level, there is even CO₂ flowing into the greenhouse (see figure 1). Another situation is when the CO₂ concentration inside the greenhouse equals the outside level (340 ppm): then there is no net loss of CO₂, even when the windows are wide open.

At higher CO₂ levels, the loss of CO₂ depends on the rate of ventilation. For example at 700 ppm CO₂ when the windows are closed, the losses of CO₂ will be 2 to 4 g CO₂/m²/h or more in a very leaky greenhouse. With the windows open at 20% and with a wind speed of 4 m/s the losses will be in the order of 15 g CO₂/m²/h.

AMOUNTS OF CO₂ TO BE SUPPLIED

At 340 ppm CO₂ (ambient CO₂) the rate of crop photosynthesis varies from less than 1 g CO₂/m²/h during very dark weather, to up to 5 g CO₂/m²/h under favourable light conditions. At the ambient CO₂ level there is no loss of CO₂ to the outside. Therefore maintaining this CO₂ level only requires the photosynthesis to be compensated, for which up to 5 g CO₂/m²/h is needed.

At 700 ppm CO₂ the photosynthesis varies from 3 g CO₂/m²/h during dark weather, to 6 or 7 g CO₂/m²/h at high light. The loss by ventilation varies from 2 to 4 at closed windows, to 15 g/m²/h and far more at ventilation (see above). The total amount of CO₂ to be supplied should be 5 g CO₂/m²/h on an overcast day with no ventilation, while on sunny days with ventilation, a rate of 20 g CO₂/m²/h or more is needed for maintaining 700 ppm.

CONCLUSIONS

A generally recommended minimum supply rate is 5 g CO₂/m²/h. This is considered sufficient to maintain a high CO₂ level in a closed greenhouse (winter situation) and also sufficient to cover the CO₂ uptake by the crop in summer, thus preventing CO₂ levels below ambient (CO₂ depletion). For maintaining higher CO₂ levels during ventilation, the supply rate should be at least twice or three times as much. As such high supply rates are expensive, the level targeted (or achieved anyway) will normally be not much beyond 340 ppm during ventilation. Higher supply rates are only feasible at a very low cost price for CO₂ and/or high market prices for the product.

It is generally worthwhile, also during ventilation, to target 340 ppm and so to avoid depletion. As shown in figure 1, plant growth is significantly reduced by such low CO₂ levels. Moreover, when the CO₂ concentration is below the outside level, there is no loss of CO₂ to the outside, and all the CO₂ supplied is used by the plants.

The amount of CO₂ used on an annual basis varies according to the CO₂ supply strategy. An absolute minimal strategy requires about 3 kg/m²/year, while a bit more generous strategy requires already 10 kg/m²/year. The estimated increase in annual production from these two CO₂ strategies will be in the order of 2 to 8 %. However, in periods of the year with little ventilation and clearly raised CO₂ levels, the effect of CO₂ enrichment may be in the order of 10 - 20% increased growth or production.

CO2 flows between greenhouse atmosphere and ambient atmosphere.

This graph shows the crop production (in %) at various levels of CO₂ (in ppm). The production at the normal ambient CO₂-level (340 ppm) is taken as the standard production level (100%), and production obtained at other CO₂-levels are compared to that standard production. The band accounts for the variation in response for various crops and various conditions. This graph is based on data from experiments done at various places with many different vegetable crops.

A to Z guide for using carbon dioxide in the greenhouse:

1. CO₂ enrichment in greenhouses
2. CO₂ for growth
3. How crops respond to CO₂ enrichment
4. CO₂ enrichment in tomatoes
5. CO₂ enrichment from burning fuel
6. CO₂ from the central burner
7. Enrichment with pure CO₂