Using CO2

It is necessary to distribute CO₂ around the glasshouse to achieve uniformity of supply. The distribution of CO₂ depends mainly on air movement within the greenhouse(s), as CO₂does not travel very far through diffusion.

In the case of CO₂ burners, distribution is achieved by means of integral fans which blow the products of combustion around the glasshouse. However, this inevitably results in the creation of both vertical and horizontal CO₂ gradients within the glasshouse. In the case of pure CO₂ and flue gases from natural gas boilers and CHP units, a distribution system has to be installed. Usually, this comprises a central header, from which run small bore, perforated tubes, taking CO₂ to all areas of the glasshouse.

CO2 supply lines are best sited directly in the crop canopy where active photosynthesis takes place or, in the case of benched crops, on or under the benches. It must be considered if using CO2 from boilers or heaters this will be warm and buoyant and have a tendency to move quickly into the greenhouse roof space, on the other hand pure CO2 is cool and will not rise so quickly. The potential for low CO2 levels inside a dense crop canopy (chrysanthemums) makes it beneficial to supplement within the canopy. Air movement around the plants will also improve the CO2 uptake because the boundary layer around the individual leaf is lessened bringing the CO2 molecules closer to the leaf.

PC 47 concluded that persistent air movement requires air flow rates of at least 0.2 ms-1 but less than 0.5 ms-1 otherwise the movement induced in leaves begins to adversely affect plant growth and development (Bailey, Harral & Fernandez, 1994).  Similar conclusions about the effects of air movement were reached by Bakker et al. (1995).  They reported that work in the USA suggested an air speed of 0.5 to 0.7 ms-1 was optimal for plant growth with growth inhibition above 1 ms-1 and damage to leaves above 4.5 ms-1.  Bailey et al., (1994) found the uniformity of the greenhouse environment in the absence of the crop was greatly improved by air movement but even in the presence of the crop, the variation in the environment was reduced by induced air movement. 

Cockshull & Horridge (unpublished) measured considerable vertical gradients in CO₂ concentration within Chrysanthemum crops growing at a West Sussex nursery, with the lowest concentrations being recorded in the vicinity of the uppermost leaves while the concentrations above the crop and at the base of the canopy were higher. Langton and Hamer (2003a, b) found that the air temperature within a closed Petunia crop canopy was markedly lower than that in the bulk of the air above the crop on high-irradiance days, but not at night, whilst the humidity of the air within the canopy was routinely higher than that of the air above the canopy during both the day and the night. 

The reports from PC 162, in which a computational fluid dynamics (CFD) model was used, demonstrated that there would be areas of reversed air flow and a ‘dead’ zone in conventional glasshouses (Davies, 1999, 2001, 2002).  This work was done using natural air flow. The model showed that these characteristics would be affected by external wind-speed and direction and that these effects would be associated with variation in the CO₂concentration within the greenhouse.  Fernandez & Bailey (1994) showed that in relatively still air on sunny days, spatial variations of up to 150 vpm CO₂ and 7°C were detected in a four-span Venlo, tomato greenhouse and that these were reduced to 20 vpm CO₂ and 1.6°C when fans were used. 

The Report of PC 47 concluded that air circulation could give substantial improvements in uniformity of conditions in the greenhouse. This uniformity was likely to be of significant benefit to growers seeking to eliminate problems with condensation and related problems which were made worse by spatial variations in environment. Air movement would also be of significant benefit in ensuring that inputs such as additional CO₂ are available to the whole crop and at the same concentration. The evaluation of air flow engineering options and of different approaches to analysing flow using models provided the essential basis for designing effective air movement systems for commercial greenhouses (Bailey, Harral & Fernandez, 1994). 

The Report also commented that the introduction of fans would create some non uniform air movement as the air leaving the fans would be moving faster than that approaching them. Greater uniformity was obtained by using perforated air ducts at floor level as the air is then discharged vertically and does not increase horizontal variations. The openings should be small so that the air emerges as a high-speed jet. This approach has relevance to systems using micro-turbines as co-generators of heat and power for in some designs of the latter, the exhaust gases are passed continually into the greenhouse while the micro-turbines are working.

Vertical air flow also fits conveniently with ‘hanging gutter’ systems of production for tomato where there is a considerable space between the glasshouse floor and the hanging gutter carrying the Rockwood slab and the rooted crop. High wire systems for cucumber and pepper and even bed systems for the production of cut flowers such as chrysanthemum might even be adapted to use this system of air distribution.