Multifunctional microgrid integrating membrane distillation driven Si+ based hydrogen powered system for energy-water-resource nexus

The predominant reliance on grey, blue, and green (via renewables) hydrogen (H₂) production pathways perpetuate fossil fuel and chemical dependencies, creating significant water stress and carbon lock-in effects that directly contradict decarbonization objectives and exacerbate climate vulnerabilities. This underscores the need for a nexus approach to H₂ production and utilization. The H₂ economy requires integration of innovative physical and chemical processes for sustainable resource management to increase the likelihood of achieving energy-water-resource (EWR) nexus. A multi-functional microgrid (MfMG) aimed to operate on the circular economy (CE) principles (namely resource recovery, circular supplies and product life extension) for producing energy, water and resources (even on on-demand basis at the site of utilization without involving logistic bottlenecks) is conceptualized as part of our Hong Kong’s Innovation and Technology Commission’s Partnership Research Programme with EPRO Advance Technology Limited. The conceptualized MfMG integrates membrane distillation (MD) for wastewater treatment for portable/non-portable water with the Si+ chemical process for low-carbon H₂ generation coupled with a systematic chemical recovery process, the generated H₂ is then fed into the polymer electrolyte membrane (PEM) fuel cells for electricity, and the retired batteries can store the electricity. The objective is to investigate the MfMG’s design and operational feasibility to have better understanding on the scale up potential as well as its role in managing EWR nexus at different scales with an overarching goal of maximizing the utilization of high-quality MD permeate for portable/non-portable water production rather than directing it towards H₂ production. To realize this, we employed a multi-stage approach, beginning with theoretical modeling and lab-scale experiments, followed by scaling up MfMG with rated capacities ranging from 5 kW to 1 MW. Theoretical modeling considering the optimistic scenario indicates that the MfMG, integrating physical and chemical processes under CE principles in action, effectively addresses the EWR nexus suggesting that MfMG by design has the capability. Experimental results confirm significant energy, water, and resource generation capabilities indicating the scale-up potential in achieving desired operational performance for EWR nexus management but highlighting the importance of MD operational parameters (such as feed temperature and flow rate, and feedwater characteristics), role of Si+ reaction rate (here we observed up to 90%), and the resource recovery ratio for energy (50-80%)-water (90-100%)-resources (84-90%) especially when technological and process inefficiencies are considered. Our analysis on the scaled-up systems, from the energy management context, MfMG due to its dominance in exothermic nature of operations generated a significant heat ranging from 1,000 MJ to over 200,000 MJ across designated capacities. Even with at least 50% heat recovery via a combined heat and power cycle, this heat showed the potential to fully meet both the thermal and electrical energy demands of MD. For e.g., 5 kW MfMG, requiring 0.0324 m3 of MD-treated water (10 wt.% NaCl brine at 75 °C), exhibited thermal and electrical energy consumption of 3.132 MJ/m3 and 0.05832 kWh/m3, respectively. In terms of water management, the MfMG achieved a significant water surplus, producing nearly four times the initial water feed as a byproduct. This demonstrates substantial water circularization potential, eliminating water dependency for H₂ production from the second batch onward. For instance, 5 kW MfMG produced 0.12961 m3 of water while requiring only 0.0324 m3 of water from MD for H₂ production via the PEM fuel cell. In terms of resource management, the MfMG exhibited the potential for dry sodium silicate recovery i.e., 3.285 kg of per 1 kg of Si+ at 90% reaction rate from the initial recovery stage resulting ~165 kg of sodium silicates in 5 kW MfMG. Moreover, a resource nexus approach reveals at least 80% NaOH recovery potential (after accounting for losses) by applying circular economy principles to the recovered sodium silicates. This enables circularization, sustains continuous H₂ production, and reduces reliance on external NaOH sources from the second batch onward. Also, the life cycle assessment results indicated low-carbon H₂ production with virgin Si+ and carbon-neutral production with recycled Si+ and by maximizing the renewables utilization in the Si+ value chain. Overall, the MfMG designed with physical and chemical processes integration showed a potential to operate with near neutral in terms of emissions, providing sustainable electricity and water solutions by managing the EWR nexus.