Selecting carbon dioxide enrichment technologies for urban farming, from the perspectives of energy consumption and cost

Global warming and global food security make it critical to develop energy-efficient, cost effective and sustainable food production systems with the least environmental footprint. Today, urban farming is supplying 15–20 % of the world's food and will play a more pivotal role in ensuring food security for burgeoning populations in the future. As one of the most critical growth-promoting agents for plants, carbon dioxide (CO2) is commonly added in urban farming to enhance the yield and productivity, which can be about 9–45 %. In order to improve energy efficiency and reduce cost, judicious selection and integration of CO2 enrichment technologies are crucial. This reviews the features and limitations of various technologies that have been used or can be used for urban farms. Key performance indicators, including CO2 purity, enrichment energy consumption and enrichment cost, have been employed for technology comparison. Combustion and decomposing technologies are characterized as low cost and simple equipment; however, the pollutants and odor produced by these technologies limit their applications in urban farms. Direct air CO2 capture technologies are emerging technologies; however, their high investment cost and energy consumption need to be reduced before they can be widely adopted. Suggestions on future research and development to advance the capabilities of urban farms are also provided.