Considering the environmental challenges humanity faces in the 21st century, it is obvious that there is an enormous need for change of the global energy map. Under these circumstances, new energy sources and intermediates must be considered as options to limit the greenhouse gases emissions and mitigate climate crisis. Biohydrogen production is one of the most appealing options, due to hydrogen's multiple applications and zero emissions as a fuel, to empower a future hydrogen circular economy. In this review article we focus on two methods that are not widely used at industrial scale but have many future possibilities and growth margins: (a) photo-fermentation and (b) bio photolysis. Both methods are light dependent and need photobioreactors to function and produce significant amounts of biohydrogen. Based on an extensive literature search and systemic analysis of the findings, presentation of the different reactants, operating conditions and biohydrogen productions, key factors and effecting parameters were discussed. Temperature, pH, light intensity and photobioreactor operation and design are some of the most significant factors that define the biohydrogen production rates and yields. Innovative solutions and approaches are presented including biotechnological and genetic engineering modifications to microorganisms, as well as combinations of some hybrid biohydrogen producing methods, especially dark and photo fermentation. For implementing a biohydrogen circular-economy, different wastes were explored as potential feedstocks, and overcoming of major bottlenecks that biophotolysis and photo-fermentation face in the transition to a sustainable biohydrogen economy, were discussed.

A techno-economic assessment of two circular economy scenarios related to fluidized bed gasification-based systems for combined heat and power (CHP) generation, fueled with rice processing wastes, was conducted. In the first scenario, a gasification unit with 42,700 t/y rice husks capacity provided a waste management industrial symbiosis solution for five small rice-processing companies (SMEs), located at the same area. In the second scenario, a unit of 18,300 t/y rice husks capacity provided a waste management solution to only one rice processing company at the place of waste generation, as a custom-made solution. The first scenario of a cooperative industrial symbiosis approach is the most economically viable, with an annual revenue of 168 €/(t×y) of treated rice husks, a very good payout time (POT = 1.05), and return in investment (ROI = 0.72). The techno-economic assessment was based on experiments performed at a laboratory-scale gasification rig, and on technological configurations of the SMARt-CHP system, a decentralized bioenergy generation system developed at Aristotle University, Greece. The experimental proof of concept of rice husks gasification was studied at a temperature range of 700 to 900 °C, under an under-stoichiometric ratio of O₂/N₂ (10/90 v/v) as the gasification agent. Producer gas’s Lower Heating Value (LHV) maximized at 800 °C (10.9 MJ/Nm³), while the char’s Brunauer Emmet Teller (BET) surface reached a max of 146 m²/g at 900 °C. Recommendations were provided for a pretreatment of rice husks in order to minimize de-fluidization problems of the gasification system due to Si-rich ash. With the application of this model, simultaneous utilization and processing of waste flows from various rice value chain can be achieved towards improving environmental performance of the companies and producing energy and fertilizer by using waste as a fuel and resource with value.