Scientists from King Abdullah University of Science and Technology in Saudi Arabia, University of East Anglia in the United Kingdom and University of Arizona in the USA suggested giving up the delivery of the carbon dioxide to the greenhouses by tanker trucks and instead extracting it directly from the atmospheric air. Their research indicated that the CO₂ capture systems may not only be eco-friendlier, but also cheaper than the traditional method of supplying carbon dioxide to high-tech greenhouses where plants quickly consume all the CO₂ required for photosynthesis because of air-tightness of premises.
The problem of high-tech greenhouses is common: in hot climate it is necessary to seal them hermetically almost fully and to cool constantly so as to protect plants from overheating. But inside a closed greenhouse the CO₂ concentration quickly drops (sometimes more than twofold as compared to the street air) due to which photosynthesis slows down and the crop yield decreases. It is unprofitable to open and air the greenhouse: this impairs climate control and sharply increases cooling costs. For this reason, greenhouse owners have to purchase liquid carbon dioxide which is produced as a by-product at industrial enterprises, liquefied, carried to hundreds of kilometers in special tank containers, stored under pressure and then gradually supplied inside the greenhouse. In 2025 such CO2 cost farmers approximately 267-783 dollars a ton, while transportation proper markedly increased the carbon footprint in agriculture.
The international team of scientists proposed an alternative: to receive CO₂ right on the spot with the use of technology of capturing the carbon dioxide from the air. It is based on adsorbtion: air is passed through special sorbent which ‘captures’ the CO₂ molecules and then releases them when the temperature, pressure or humidity change. Such approach relieves of complicated logistics and does not require super-pure carbon dioxide – for plants concentration 1,000 ppm is sufficient, i.e. approximately 0.1% in volume, while in industrial capture systems purity normally should exceed 95%.
Researchers tested two types of installations. The first one is a temperature-vacuum system. In it, air is passed through a layer of sorbent based on commercial resin Lewatit VP OC 1065 which binds CO₂. Thereafter the column is hermetically sealed, heated and vacuum is created inside due to which sorbent releases the accumulated carbon dioxide which is immediately supplied to the greenhouse.
The second system uses ion-exchange resin IRA-900 and employs the humidity change principle. Such sorbent absorbs CO₂ from dry air and then releases it when moistened with water. After drying the cycle is repeated. This option proved simpler and cheaper in terms of equipment, but required more water and electric energy.
To assess effectiveness, scientists modelled the work of the greenhouse with an area of 1 hectare under Jidda conditions. Using photosynthesis models for lettuce and cherry tomatoes, as well as real data on sun radiation and climate researchers calculated that the greenhouse with tomatoes needs about 419 t of CO2 per year, with lettuce – about 144 t. Then they determined the maximum price of carbon dioxide at which enrichment of air is still economically sound on account of the growing crop yield. Depending on the crop, market prices and added yield this “loss-free” cost amounted from several hundred to more than a thousand dollars for a ton of CO2.
Calculations showed that both adsorbtion systems match this range. Prime cost of carbon dioxide for them amounted to 240-252 dollars per ton, i.e. proved comparable or even less than the cost of transported liquid CO2, especially at long distances of transportation.
Detailed analysis showed that the cost of electric energy remains the key factor. It is for this reason that desert regions with cheap solar generation gain a serious advantage. The second most essential parameter was productive capacity of the sorbent, i.e. the quantity of CO₂ it can capture during one cycle of operation. Reduction of this productive capacity by 30% almost doubles the final cost of the carbon dioxide. Therefore, for the greenhouses it is more important to get the maximum volume of CO₂ than to achieve it super-high purity required for industrial storage or transportation.
Assessment of the full life cycle – from production of equipment to its disposal – indicated that in using solar energy the CO₂ direct capture systems become much eco-friendlier than traditional supplies of liquid carbon dioxide. For example, the carbon footprint in producing one kilogram of lettuce decreased from 0.50 kg of CO₂ – equivalent of power supply from the mains -to 0.076 kg when solar energy only was used. In the case of transported CO₂ it is the transportation exactly that gradually became the main source of releases.
Now scientists plan to hold a series of trials of real greenhouse complexes with integrated adsorbtion systems under the desert conditions since for the time being the majority of data was received either in a laboratory or on small experimental installations.
Besides, the researchers want to study how heat, humidity and dust influence the effectiveness of sorbents and the speed of capturing CO₂ in the long-term prospect. An individual research is related to the humidity system: in the future they intend to combine it with the collection of moisture from the air and condensate of the greenhouse itself in order to receive both carbon dioxide and water for irrigation at the same time.
