CO₂ Enrichment

Abstract

The CO₂ concentration inside a greenhouse can drop significantly below outside level when a dense crop is growing, even if the greenhouse is well-ventilated. The concentration can drop to less than 200vpm during winter in mild climate regions.

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The CO₂ concentration inside a greenhouse can drop significantly below outside level when a dense crop is growing, even if the greenhouse is well-ventilated. The concentration can drop to less than 200vpm during winter in mild climate regions. As the CO₂ concentration limits the photosynthesis of most vegetable species, the productivity decreases. The optimal CO₂ concentration for growth and yield seems to be 700–900vpm (De Pascale and Maggio 2005, 2008). The CO₂ concentration should be kept to at least the outside level, but CO₂ enrichment is not a current practice in mild climates up to now.

The production loss due to CO₂ depletion may be higher than the production loss due to a reduced temperature through ventilation (Stanghellini et al. 2008). The enrichment of greenhouse air with CO₂ leads to better plant growth, shorter cropping times, and higher quality. Therefore a combined control of ventilation and CO₂ enrichment with low-cost CO₂ sources may result in improved and economically viable methods for crop growth in greenhouses.

Production losses in greenhouses are influenced by two main factors:

• Sufficient ventilation to avoid CO₂ depletion.

• Maintaining a higher temperature by heating on sunny, chilly days in spite of CO₂ depletion.

It is necessary to assess running and installation costs for CO₂ enrichment or heating to find out the optimal strategy for climate control. The compensation of CO₂ depletion by increased ventilation or even by CO₂ enrichment seems to be cheaper than compensation of production loss by heating. A good management strategy can be to ventilate as much or as little as necessary for temperature and humidity control, and to control CO₂ concentration inside the greenhouse up to outside level when ventilation is being used, and to higher levels when no or little ventilation is required for temperature control (Stanghellini et al. 2008, 2009).

The CO₂ concentration can be raised as follows (von Zabeltitz 1999):

• Technical carbon dioxide from bottles or tanks.

• Exhaust gases from gas burner for CO₂ enrichment with simultaneous heat production.

• Exhaust gases from directly fired air heater with gas burner.

• Straw between the plant rows, enriched with fertiliser and wetted. CO₂ is released during decomposition, but the amount of CO₂ cannot be controlled.

Exhaust gases from oil and coal heaters must not be used because of the content of sulphur dioxide.

Exhaust gases from gas burners can be led directly into the greenhouse (Fig.16.1). Gas will be burned, and the CO₂ is blown with the circulating air into the greenhouse. Special control systems are necessary, and care must be taken that no carbon monoxide is formed. It is practical to mix the exhaust gas with fresh air. The water vapour and heat production, as well as the maximum allowed concentration, have to be taken into consideration.

Fig.16.1

Gas burner for CO₂ enrichment

CO₂ from an air heater (Fig.16.2) can only be produced while it is in operation and when heating is needed, which is normally necessary only at night in mild climate regions. Some of the combustion gases are tapped off and used for CO₂ enrichment.

Fig.16.2

Directly fired gas burner for heating with CO₂ discharge for enrichment