Chapter 7

Carbon Dioxide in Greenhouses

Carbon Dioxide Supplementation in Greenhouses

Without additional input of pure carbon dioxide, the carbon dioxide content of the atmosphere in the greenhouse can be reduced to less than 50 percent in some cases of its normal content in air. This shortage of carbon dioxide reduces the efficiency of photosynthesis and can have several negative effects on the health and development of greenhouse crops. Supplementation of carbon dioxide is seen as the only method to overcome this deficiency. In addition, many growers will increase the level of carbon dioxide above 340 ppm to further enhance the growth of greenhouse crops. Depending on the crop variety cultivated, an additional supply of carbon dioxide can have several beneficial effects including earlier flowering, higher fruit yields, improved stem strength, and flower size.

When to Supplement with Carbon Dioxide

Carbon dioxide enrichment is not required as long as the crops are growing and developing to the complete satisfaction of the grower, or if high ventilation rates make carbon dioxide enrichment uneconomical. Carbon dioxide enrichment should be considered, however, if crop production and quality are below required levels. In general, crop production times from late fall through early spring increases the potential need for carbon dioxide enrichment as it coincides with reduced ventilation rates due to colder outdoor air temperatures.

Methods of Carbon Dioxide Supplementation

There are several methods of carbon dioxide supplementation in a greenhouse environment. Once the decision has been made that carbon dioxide supplementation will enhance the productivity of the greenhouse, the grower must understand the advantages and disadvantages of each system.

Ventilation

Ventilation is the most economic method to limit carbon dioxide depletion in the greenhouse air, but it only allows the maximum to reach levels close to those of the external air (350 ppm). Besides, in many cases ventilating is not desirable, for other reasons. Therefore, artificial enrichment is a usual practice.

Carbon Dioxide Supplementation via Combustion

As carbon dioxide is one of the products of combustion, this process can be used to introduce carbon dioxide into the greenhouse (See Figure 7.1). The major concern with using combustion is carbon dioxide is only one of the products of combustion.

Natural Gas Carbon Dioxide Generators

Carbon dioxide generators (See Figure 7.2) using hydrocarbon fuels (e.g., natural gas, propane, and kerosene) are common carbon dioxide sources in the greenhouse. Some manufacturers make burners in which either natural gas or propane can be used, as well as units with adjustable outputs. Carbon dioxide generators are hung above head height along the center of the greenhouse. Carbon dioxide produced in some units rises out of the burner into the greenhouse atmosphere, where convection currents move the gas about the greenhouse.

Boiler Stack Carbon Dioxide Recovery Systems

Boiler stack recovery system requires a clean burning high output boiler and a system to recover the carbon dioxide from the exhaust stack for distribution to the crop (See Figure 7.3). The boiler should be equipped with a flue gas condenser to reduce the flue gas temperature and take the moisture out of the flue gases, avoiding their entrance in the greenhouse.

Liquid Carbon Dioxide Supplementation

Liquid carbon dioxide is another alternative for supplementation (See Figure 7.4). Pure carbon dioxide is delivered in bulk by truck to the greenhouse. The distribution system for liquid carbon dioxide in the greenhouse is simpler to design and install. For a small greenhouse operation the carbon dioxide may be supplied in cylinders.

Control of Carbon Dioxide Levels in the Greenhouse

The simpler forms of carbon dioxide injection control use either a time clock or a light sensor to turn the carbon dioxide generator on in the morning and off in the evening. During the day, the carbon dioxide generator is automatically turned off when the ventilating fans are on. In the event of roof ventilation, mechanical switches are installed on the ventilators to allow the carbon dioxide generator to operate only when the vents are open less than 2 inches (5 cm). More sophisticated carbon dioxide injection systems usually consist of a carbon dioxide generator, a control system, and a “feedback” monitoring system for monitoring carbon dioxide levels in the greenhouse. The monitoring device is usually an infra-red gas analyzer (IRGA).

Recommended Greenhouse Carbon Dioxide Concentrations

The level to which the carbon dioxide concentration should be raised depends on the crop, light intensity, temperature, ventilation, stage of the crop growth, and the economics of the crop. Target concentrations are typically between 400 and 1,200ppm. In some operations, target levels were up to 1,700 ppm, however, sustained carbon dioxide concentrations above 1,500ppm have been shown to adversely impact on plants.

Distribution of Carbon Dioxide in the Greenhouse

It is important to have an adequate distribution system for carbon dioxide within the greenhouse. The distribution of carbon dioxide depends mainly on air movement within the greenhouse(s), as carbon dioxide does not travel very far through diffusion. There are two main methods for the distribution of pure carbon dioxide. In the first instance, the carbon dioxide is forced through a central header connected to flexible, polyethylene tubes (with evenly spaced holes) placed low in the crop canopy or in the case of bench crops, under the bench.

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