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Greenhouse cultivation has evolved from simple covered rows of open-fields crops to highly sophisticated controlled environment agriculture (CEA) facilities that projected the image of plant factories for urban agriculture. The advances and improvements in CEA have promoted the scientific solutions for the efficient production of plants in populated cities and multi-story buildings. Successful deployment of CEA for urban agriculture requires many components and subsystems, as well as the understanding of the external influencing factors that should be systematically considered and integrated. This review is an attempt to highlight some of the most recent advances in greenhouse technology and CEA in order to raise the awareness for technology transfer and adaptation, which is necessary for a successful transition to urban agriculture. This study reviewed several aspects of a high-tech CEA system including improvements in the frame and covering materials, environment perception and data sharing, and advanced microclimate control and energy optimization models. This research highlighted urban agriculture and its derivatives, including vertical farming, rooftop greenhouses and plant factories which are the extensions of CEA and have emerged as a response to the growing population, environmental degradation, and urbanization that are threatening food security. Finally, several opportunities and challenges have been identified in implementing the integrated CEA and vertical farming for urban agriculture.

Feedstock particle sizing can impact the economics of cellulosic ethanol commercialization through its effects on conversion yield and energy cost. Past studies demonstrated that particle size influences biomass enzyme digestibility to a limited extent. Physical size reduction was able to increase conversion rates to maximum of ≈ 50%, whereas chemical modification achieved conversions of >70% regardless of biomass particle size. This suggests that (1) mechanical pretreatment by itself is insufficient to attain economically feasible biomass conversion, and, therefore, (2) necessary particle sizing needs to be determined in the context of thermochemical pretreatment employed for lignocellulose conversion. Studies of thermochemical pretreatments that have taken into account particle size as a factor have exhibited a wide range of maximal sizes (i.e., particle sizes below which no increase in pretreatment effectiveness, measured in terms of the enzymatic conversion resulting from the pretreatment, were observed) from <0.15 to 50 mm. Maximal sizes as defined above were dependent on the pretreatment employed, with maximal size range decreasing as follows: steam explosion > liquid hot water > dilute acid and base pretreatments. Maximal sizes also appeared dependent on feedstock, with herbaceous or grassy biomass exhibiting lower maximal size range (<3 mm) than woody biomass (>3 mm). Such trends, considered alongside the intensive energy requirement of size reduction processes, warrant a more systematic study of particle size effects across different pretreatment technologies and feedstock, as a requisite for optimizing the feedstock supply system.

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