TECHNICAL ASPECTS, MANAGEMENT AND CONTROL OF CO₂ ENRICHMENT IN GREENHOUSES – REFEREED PAPER –

This paper outlines the present state of the art of CO₂ enrichment as it is practiced in commercial greenhouses, particularly in moderate climates.
Some scientific work is discussed and conclusions and practical guidelines are presented.

With respect to CO₂ sources liquid (pure) CO2 is considered superior to other sources, mainly because of its purity and because enrichment is not necessarily connected with heat production.
Some aspects of liquid CO₂ are briefly discussed.
Insertion via irrigation water is not recommended.

Because of the relatively high price of liquid CO₂, enrichment by combustion of fossil fuel is a more feasible alternative, which is widely applied.
Fuels appropriate for CO₂ enrichment are low-sulphur oils, natural gas, kerosene (paraffin) and propane.
Two main types of burners are distinguished, namely hot-air heater and CO₂ generators located inside the greenhouse and, on the other hand, the central burner outside.

The most striking disadvantage of burners inside the greenhouse is the simultaneous insertion of heat and CO₂. Because the temperature mostly has priority above the CO₂ concentration, the CO₂ level is either supra-optimal (in winter) or sub-optimal (in summer conditions).

A central heating system, provided with a fan and a transport and distribution system for CO₂, can be used for CO₂ enrichment and heating independently.
Often the burner can be turned-down to about 1/10 of its full capacity, in order to prolong the period of CO₂ enrichment (on summer days) or to have a more constant combustion (on winter days). A heat storage tank enables CO₂ enrichment by combustion at day time, while the heat is used at night.

In CO₂ management three levels are distinguished: decisions on investments, stategies (here meant as set point generation) and control (set point realization). With respect to investments an adjustable supply capacity ranging from about 4 to 15 g.m^-2.h^-1 CO₂ is recommended.

CO₂ strategies should be based on physiological aspects (CO₂ level of 1000 ppm favorable), economic criteria (costs for enrichment increase at increasing ventilation) and practical limitations (risk of accumulation of toxic gases, undesired stomatal closure at higher CO₂ levels).

In many commercial climate control computers the grower can determine a CO₂ strategy by choosing the settings.
One economic principle is commonly practiced: decreasing supply at increasing ventilation.
A dynamic, economic optimization strategy, which generates a CO₂ set point on the basis of calculated costs and estimated yields offers perspectives.
Intermittent enrichment and reduced enrichment time are considered not preferable.

IV International Symposium on CO2 in Protected Cultivation

E.M. Nederhoff

https://doi.org/10.17660/ActaHortic.1990.268.11