Without the process of photosynthesis, light energy would not be able to convert carbon dioxide into oxygen, which is essential for life and our crops.
The CO2 level must be continuously monitored to ensure the yield of the greenhouse crop and the quality of the harvest.
How can we ensure accurate CO2 control?
Did you know that in addition to oxygen, our plants also generate sugar?
Photosynthesis is the source of the oxygen we breathe, but also the food we eat. Without the photosynthesis process, the light energy would not convert carbon dioxide into oxygen.
This unique process is more or less efficient depending on several parameters, including the carbon dioxide concentration in the surrounding air.
As well as carbon dioxide, the plant needs sugar to grow. And the key point is that it indeed creates sugar by itself. Minerals, water and light are the other components required.
Photosynthesis reaction is then as follows:
CO₂ + H₂O + Light → Sugar + O₂
The plant more precisely uses this sugar as a fuel. It allows it to generate new cells and, in a way, to breathe.
The answer is straightforward: in order to optimise the photosynthesis process thus stimulating and controlling the growth of plants.
Greenhouse crop production is now a growing and global reality with an estimated 405 000 ha of greenhouses spread throughout Europe.
The last 20 years have seen a revolution in greenhouse cultivation and technologies.
Just a few years ago, a tomato yield of 100 tonnes/ha in a greenhouse was considered a good performance. Today, a harvest of 600 tonnes/ha is not unusual in high-tech greenhouses.Hans Dreyer, Director of Plant Production and Protection Division at Food and Agriculture Organization of the United Nations
One may think that world areas where the sunshine is abundant do not require greenhouses. But this is not true.
Depending on the plant cultivated, here again, CO2, as well as temperature and air speed, is a key parameter, and its optimal level varies.
The CO2 concentration in ambient air, is famous for increasing dramatically since the industrial revolution, and faster and faster nowadays.
However, its average level is currently around 400 ppm (parts per million) which means 0,04% of the air we breathe.
Whereas, for instance, under adequate light and temperature conditions, tomatoes grow best at 900 ppm and cucumbers at 700 ppm.
It appears then obvious that CO2 controlled the atmosphere, thus greenhouses are to be developed at any place in order to meet the challenge of human nutrition in the coming years.
The Netherlands are well known as the pioneer country for crop growth in climate-controlled houses. With the huge and still growing number of 9000 large greenhouses, which occupy 0.25% of the total land area, this market represents a significant part of the country’s GDP.
150 000 workers are employed and 80% of the products are exported.
Spain is also famous for having one of the largest greenhouses in the world. This is in Almeria, where greenhouses cover almost a 200km2 area.
Supplementary CO₂ should be used during periods of sunny weather, but not in cloudy weather or at night.
It can be extracted from burners using oils or natural gas. In such cases, care must be paid to avoid the presence in the greenhouse of toxic gases – whether for plants (SO2, ethylene etc.) or humans (carbon monoxide).
Alternatively, pure liquid CO2 purchased from commercial suppliers may be used.
The most common method of CO2 enrichment for greenhouse applications is the combustion of fossil fuel. And the most used fuel for CO2 enrichment is natural gas.
With the combustion of 1 m3 of natural gas, approximately 1.8 kg CO2 is generated.
Then supplying CO2 may lead to local variations in CO2 concentration throughout the greenhouse. Horizontal, and vertical gradients in environmental conditions are disadvantageous but inevitable. Most importantly is to prevent a decrease in the homogeneity of plant growth and crop production.
For instance, with a distribution network, a high CO2 concentration is found near the distribution tubes and a low level close to the ridge, or near the opened ventilation windows. It is then recommended to place the CO2 distribution lines on a low level near the crops. This way, the natural diffusion of the carbon dioxide to the top of the greenhouse will ensure CO2 enrichment homogeneity on the vertical axis.
The horizontal distribution is also a challenge since the whole surface of the greenhouse should also contain the same amount of CO2, so that all plants grow at the same speed and the maturity and quality are homogeneous throughout the whole culture.
To ensure a volumetric (both horizontal and vertical) homogeneity of CO₂ concentration in the greenhouse, the best strategy is to measure it at several places in the greenhouse.
This can be done with several analysers and/or making a multipoint sampling with one single analyser, depending on the greenhouse size, and the available budget.
In the case of a large greenhouse, several CO2 monitors will be used to cover the whole volume. And to ensure the best representativity of all plants' atmosphere, each monitor will measure simultaneously several (usually 4 or 6) smaller areas.
This optimised strategy allows controlling that the CO2 is equally spread to all crops.
The Fuji Electric ZFP CO2 monitor for greenhouses is a dedicated NDIR (Non-Dispersive Infra-Red) gas analyzer. It was designed years ago for this purpose and has been improved with experience.
More than 10,000 ZFP CO2 monitors are currently in use throughout Europe to optimize our food production by enhancing photosynthesis through CO2 fertilization.
Equipped with its internal filter and internal pump, this Infrared analyser is able to suck the ambient air around its own position, but also from remote areas through a network of sample pipes. A usual strategy like illustrated below consists in sucking the air from several areas to ensure the homogeneity of CO2 in the targeted area.
Installation of the CO₂ ZFP controller is straightforward, and its unique stability allows an annual calibration frequency.
Fuji Electric's non-dispersive infrared technology is famous since the 1960s for its robustness and signal stability under the hardest climatic conditions.
The sensor works by an infrared (IR) lamp directing waves of light through a tube filled with a sample of air. This air moves toward an optical filter in front of an IR light detector.
The IR light detector measures the amount of IR light that passes through the optical filter.
The band of IR radiation also produced by the lamp is very close to the 4.26-micron absorption band of CO2.
Because the IR spectrum of CO2 is unique, matching the light source wavelength serves as a signature or "fingerprint" to identify the CO2 molecule.
As the IR light passes through the length of the tube, the CO2 gas molecules absorb the specific band of IR light while letting other wavelengths of light pass through.
At the detector end, the remaining light hits an optical filter that absorbs every wavelength of light except the wavelength absorbed by CO2 molecules in the air sample tube.
Finally, an IR detector reads the remaining amount of light that was not absorbed by the CO2 molecules or the optical filter.
The difference between the amount of light radiated by the IR lamp and the amount of IR light received by the detector is measured.
Since the difference is the result of the light being absorbed by the CO2 molecules in the air inside the tube, it is directly proportional to the number of CO2 molecules in the air sample tube.
This data is then treated by the internal DSP board and then output, most usually as a 4-20 mA signal to be used for the process control: the CO2 enrichment system here in our case.
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