Automatic carbon dioxide enrichment strategies in the greenhouse: A review

Greenhouses constitute a proven solution for coping with environmental degradation and for increasing both the quantity and quality of agricultural products. Appropriate carbon dioxide (CO2) control not only improves crop yield and quality but also reduces the carbon footprint of greenhouses. However, CO2 enrichment control in greenhouses is a dynamic, interactive and time-delayed process. In practice, optimal CO2 concentrations in the greenhouse are difficult to maintain because CO2 is confounded with temperature, humidity, light intensity, etc.; therefore, ambient CO2 concentrations in greenhouses are often suboptimal or excessive. This paper is a review of the current theoretical and applied studies of CO2 enrichment in greenhouses and discusses the advantages and limitations of various methods. The major points addressed are as follows: 1) the five sources of CO2 enrichment in greenhouses; 2) the monitoring and data processing of CO2 concentrations; and 3) the various methods for controlling automatic CO2 enrichment. This paper discusses new challenges and perspectives and suggests future studies and methods for a greenhouse CO2 enrichment system. A new symbiotic greenhouse system requiring sensible CO2 balance is also presented.