Chapter 8

Carbon Dioxide in Greenhouses

Carbon Dioxide Supplementation in Greenhouses

In general, carbon dioxide supplementation is the process of adding more carbon dioxide in the greenhouse, which increases photosynthesis in a plant. Carbon dioxide supplementation is also called “carbon dioxide enrichment” or “carbon dioxide fertilization.” Although benefits of high carbon dioxide concentration have long been recognized, advances in new technologies and automation in the greenhouse industry has dramatically increased the need for supplemental carbon dioxide. With the development of improved lighting systems, environmental controls and balanced nutrients, the amount of carbon dioxide is the only limiting factor for maximum growth of plants. 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. In addition, over the past 10 years, we have also seen greenhouse growers seal up their greenhouses in an effort to control their heating bills during the winter. An apparent result of tightly sealing the greenhouse is a reduction of carbon dioxide levels within the greenhouse below ambient levels found outdoors. Thus, keeping the other growing conditions ideal, supplemental carbon dioxide can provide improved plant growth. However, carbon dioxide supplementation does not always translate into increased profits in the greenhouse due to other limiting factors such as adequate levels of nutrients, temperature, water, and light. The grower must understand that if there is one limiting factor for production then increasing one factor alone will not always increase overall production. Only if the grower is supplying all the other factors and the only limiting factor in the production regime is carbon dioxide, will carbon dioxide supplementation increase production.

Advantages and Disadvantages

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. Supplementing carbon dioxide at an early age reduces the number of days to maturity and plants can be harvested earlier. In flower production, supplemental carbon dioxide increases the number and size of flowers, which increase the sales value because of higher product quality.

When to Supplement with Carbon Dioxide

In general, carbon dioxide enrichment systems should be turned on during sunrise, and turned off several hours before sunset; however, additional carbon dioxide enrichment may be needed if supplemental lighting is used is used at night to insure adequate 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.

Recommended 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. For most crops the saturation point will be reached at about 1,000 to 1,300 ppm under ideal circumstances. Carbon dioxide levels exceeding this range have little additional positive effects, and levels exceeding 1,500 ppm may hinder plant growth

Sources of Carbon Dioxide

Practical application of supplemental carbon dioxide to the greenhouse usually requires a relatively pure source of carbon dioxide, a distribution system, and a monitoring and control system capable of maintaining set levels. The more practical sources of carbon dioxide include the clean combustion of fuels, usually propane or natural gas or pure tank carbon dioxide. The economics of equipment and fuel costs often dictate choices for a greenhouse size and location. Carbon dioxide is heavier than air. Therefore, distribution systems should maintain enough turbulence to keep added carbon dioxide evenly mixed with the greenhouse air. Monitoring and control systems vary in sophistication but should be able to measure carbon dioxide levels at several locations in the greenhouse, compare current concentrations to a set point, and adjust the concentration by adding carbon dioxide when required.

Carbon Dioxide Generators

Carbon dioxide generators (See Figure 8.1) 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. The amount of carbon dioxide produced depends on the type and purity of fuel. The carbon dioxide burner capacity ranges from 20,000 to 60,000 Btu per hour and can produce 8.2 pounds of carbon dioxide per hour by burning natural gas.

Boiler Stack Carbon Dioxide Recovery Systems

Alternatively, a portion of the flue gas from natural gas boilers connected to hot water heating systems can be directed into the greenhouse as a means of supplementing carbon dioxide to the crop (See Figure 8.2). 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. They have monitoring systems that safe guard against flue gas introduction when the carbon monoxide level is higher than a set level.

Liquid Carbon Dioxide Supplementation

Liquid carbon dioxide is another alternative for supplementation (See Figure 8.3). Liquid carbon dioxide is stored in pressurized tanks, usually just outside the greenhouse. Pure carbon dioxide is delivered in bulk by truck to the greenhouse and stored in refrigerated tanks. The compressed carbon dioxide is in a liquid state and must be vaporized through vaporizer units before entering the greenhouse. The distribution system for liquid carbon dioxide in the greenhouse is simpler to design and install.

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 closed. 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).

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