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Abstract Economically harvesting energy from a microbial fuel cell (MFC), increasing its electrical power production, and developing its role as a practical energy supply, needs a low-cost and high-performance design of the MFC compartments. According to this strategy, a novel monolithic membrane electrode assembly (MEA) was fabricated and evaluated as an air–cathode in a single-chamber MFC (SCMFC). The MEA was made of bacterial cellulose (BC), conductive multi-walled carbon nanotubes (CNT), and nano-zycosil (NZ). BC, as a nano-celluloses with oxygen barrier property, can maintain anaerobic conditions for the anode compartment. Binder-less CNT coating on BC avoids costly binders such as poly-tetra fluoro ethylene (PTFE) and Nafion and decreases the MEA charge transfer resistance. NZ, as a very cheap modifier, not only prevents the anolyte leakage but also provides more MEA’s active sites for the oxygen reduction reaction (ORR). The electrochemical performance of the MEA was compared to a PTFE- based gas diffusion electrode (GDE) in the SCMFC. The MEA cell provided a pulse power density of 1790 mW/m2, roughly twice as high as the pulse power density of GDE (920 mW/m2). SCMFC’s internal resistance decreased from 1.84 KΩ (with GDE) to 0.8 KΩ (with MEA). Also, the cell’s columbic efficiency increased from 4.2% (with GDE) to11.7% (with MEA). Additionally, the capacitance of the MEA (65 mF) was much higher than the value for GDE (0.73 mF). Thus, the MEA compared to the GDE showed higher performance in the SCMFC for electricity generation and wastewater treatment at a lower cost.

Abstract Small Mediterranean islands are remote, off-grid communities characterized by carbon intensive electricity systems coupled with high energy consuming desalination technologies to produce potable water. The aim of this study is to propose a novel dynamic, multi-objective optimization approach for improving the sustainability of small islands through the introduction of renewable energy sources. The main contributions of our approach include: (i) dynamic modelling of desalination plant operations, (ii) joint optimization of system design and operations, (iii) multi-objective optimization to explore trade-offs between potentially conflicting objectives. We test our approach on the real case study of the Italian Ustica island by means of a comparative analysis with a traditional non-dynamic, least cost optimization approach. Numerical results show the effectiveness of our approach in identifying optimal system configurations, which outperform the traditional design with respect to different sustainability indicators, limiting the structural interventions, the investment costs and the environmental impacts. In particular, the optimal dynamic solutions able to satisfy the whole water demand allow high levels of penetration of renewable energy sources (up to more than 40%) to be reached, reducing the net present cost by about 2–3 M€ and the CO2 emissions by more than 200 tons/y.

Abstract The fast economic and population growth in the African continent will lead to an important increase in demand for energy and water resources. Unfortunately, very few studies have addressed water use for energy production in Africa. This study focuses on water consumption and withdrawals throughout the different stages of energy production (fuel production, power plant construction and operation) in African countries. An in-depth analysis of water loss through evaporation in hydropower reservoirs is also performed due to the important role it plays in many countries and its severe impacts on electricity generation during the increasingly frequent droughts in Africa. The results indicate that in the year 2016, a total of 42 billion cubic meters of water was lost through evaporation in hydropower reservoirs compared to 1.2 billion cubic meters from all the other fuel types combined. Oil extraction and refining dominate water use for fuel production and non-hydro renewable energies have an almost negligible impact on the overall water use (10 million cubic meters). Fuelwood is shown to be a high consumer of water accounting for 4.5 billion cubic meters. The use of non-hydro renewable energies instead of fossil fuels can contribute significantly to reduce water use while covering the growing energy needs in Africa. Modern technologies that substitute fuelwood use in households would also reduce the impacts on water resources. The hydropower potential remains largely untapped in several regions of the continent. Nevertheless, new hydropower developments need to be carefully considered especially in regions characterized by severe water scarcity.

Abstract This work presents a performance analysis of a waste-heat-to-power Reverse Electrodialysis Heat Engine (RED-HE) with a Multi-Effect Distillation (MED) unit as the regeneration stage. The performance of the system is comparatively evaluated using two different salts, sodium chloride and potassium acetate, and investigating the impact of different working solutions concentration and temperature in the RED unit. For both salt solutions, the impact of membrane properties on the system efficiency is analysed by considering reference ionic exchange membranes and high-performing membranes. Detailed mathematical models for the RED and MED units have been used to predict the thermal efficiency of the closed-loop heat engine. Results show that, under the conditions analysed, potassium acetate provides higher efficiency than sodium chloride, requiring a smaller MED unit (lower number of effects). The maximum thermal efficiency obtained is 9.4% (43% exergy efficiency) with a RED operating temperature of 80 °C, KAc salt solution, adopting high-performing ion exchange membranes, and with 12 MED effects. This salt has been identified as more advantageous than sodium chloride from a thermodynamic point of view for the RED-HE technology and is also recommended for a cost-effective technology implementation.

Abstract Multigeneration systems, owing to their efficient fuel utilization, are recognized as one of the best technical and economical methods of energy saving and climate control. In this paper, a multigeneration system is proposed for the production of power, heating/cooling, and desalinated water. The proposed system was first studied by means of an energy, exergy, exergoeconomic, and environmental analyses and the obtained results were compared with that of multigeneration systems described in the literature (the selected multigeneration systems are based on a gas turbine cycle as prime mover). In addition, a parametric study was used to investigate the effects of primary thermodynamic quantities such as air pre-heater outlet temperature, pinch-point temperature difference in evaporator, evaporator temperature of cooling cycle, and evaporator temperature of desalination system on cycle performance. Results indicated that the proposed cycle’s power, heating, cooling, and desalinated water production is 30.5 MW, 40.8 MW, 1 MW, and 0.364 kg/s, respectively. In addition, the cycle’s total cost and total CO2 emissions are 1943.5 $/h and 0.163 kg/kWh. The parametric survey showed that the air pre-heater outlet temperature and the gas turbine inlet temperature are the most influential parameters in changing the system’s CO2 emissions. In this way, an increase of the pre-heater outlet temperature causes a 26% reduction in the cycle’s CO2 emissions, whereas an increase of the gas turbine inlet temperature leads to a 53% increase in CO2 emissions.

Abstract Small run-of-river hydropower may significantly contribute towards meeting global renewable energy targets. However, exploitation of river flows for energy production triggers environmental impacts and conflicts among stakeholders, thereby requiring optimal water resources allocation strategies. The variety of interests at stake demands instruments to quantitatively assess manifold effects of water management choices. In this work, analytic tools to guide design of run-of-river power plants when incommensurable objectives must be jointly maximized are presented. The approach is grounded on the concept of Paretian efficiency and applied to a hypothetical case study in Scotland, where energy production could compete with regionally relevant ecosystem services. We found that a multi-objective design complying with predefined environmental regulation entails significant economic losses without safeguarding ecological functions. Conversely, if the environmental flow is regarded as a decision variable subject to minimum lawful values, economically appealing and ecologically effective plant configurations emerge. Our findings suggest the existence of broadly valid alternative strategies for designing small run-of-river hydropower while preserving ecological functions, associated to small or large plant capacities. Local hydrologic conditions and target ecosystem services determine the most effective strategy for specific case studies. The analysis indicates that larger power plants are sometimes the most effective way to preserve ecosystems services through economically viable projects. Therefore, renewable energy policy should avoid incentive schemes that penalize a priori larger installations. The approach offers an objective basis to identify effective hydropower design, management and policies when additional ecosystems services are considered, thus supporting a sustainable intensification of run-of-river energy production.

The valorization of typical household food waste (HFW) produced at municipality level was studied for the production of electricity in a microbial fuel cell (MFC) from its extract, and methane, through anaerobic digestion of the solid extraction residue. HFW, after heat drying and shredding, was subjected to extraction using warm water, which resulted in a liquid fraction (extract) and a solid residue. The rich in soluble chemical oxygen demand extract was used for electricity production in a four air- cathodes single chamber MFC, operating under different organic loading rates, while the solid residue from the extraction process was used as substrate for methane production in biochemical methane potential experiments. On the basis of the energy outputs estimated for the optimum operational conditions of both aforementioned processes, it can be concluded that the exploitation of dried HFW is quite appealing as it leads to promising energy recovery.

Abstract Green Roofs (GR) represent a sustainable technological solution for reducing the environmental footprint of urban areas. Despite their benefits, traditional GRs have been criticized regarding their economic feasibility, suggesting to develop advanced hybrid engineering solutions able to simultaneously maximize their hydrological and energetic benefits. In this view, there is a need of numerical models able to describe their complete hygrothermal behavior. Thus, the main aim of this study was to assess the suitability of the one-dimensional mechanistic model HYDRUS-1D in providing an accurate and comprehensive description of the coupled water-heat-vapor transport in a field-scale Non-Vegetated Green Roof (NVGR) in the south of Italy. A complete calibration framework, which encompassed the Particle Swarm Optimization (PSO) algorithm and the combined Global Sensitivity Analysis-Generalized Likelihood Uncertainty Estimation (GSA-GLUE) method, was used to estimate the substrate thermal properties and assess the model predictive uncertainty. The calibrated model was exploited to examine the cooling efficiency of a combined Stormwater Reuse-NVGR system in the warm season. The analysis revealed that deeper substrates are positively correlated with thermal lag and attenuation, and that the irrigation can be properly designed to trigger the evaporative and convective cooling of the NVGR. The Response Surface methodology was finally used to optimize the watering regime on an 8 cm-deep NVGR. The exploitation of the evaporative cooling effect of the NVGR by means of a model-based irrigation optimization led to a reduction of the average soil bottom temperature of 4 °C. The coupled system was able to maximize the energetic benefits of GR.

Abstract In many emerging countries over the past few years some phenomena, such as a better welfare state, industrial growth and a development in agriculture, led to a significant increasing of the demand concerning fresh water. In order to face this ever-growing demand, one of the possible solutions to counterbalance the lack of water resources, is the desalination of sea water. For this specific goal solar energy, as a resource, is the process which has more reliance since it allows a low-cost production of desalted water (without using any valuable energy resources such as fossil fuels) and in a complete respect of the environment. This first study has the purpose to analyze from an energetic perspective whether it is possible or not to reach process temperatures over 100 °C, through the use of solar ponds and heat transformers, in order to produce desalinated water. The final aim of this work is to quantify the surface of solar ponds needed to a production (expressed in cubic meters) of desalinated water. An absorption heat transformer is a thermal machine that while extracting heat from a source (at an available temperature) is able to ennoble a portion of the heat collected/obtained, making it available at higher temperatures. This process occurs at the expenses of the remaining portion of heat whose temperature degrades by lowering its values. The portion of heat will be then transferred to a thermal well. Hence an absorption heat transformer can use the solar energy stored in solar ponds as an energy source at an average temperature. Process temperatures which are higher than 100 °C for a whole year can take place only under certain chained conditions such as: source temperature with steady values during the entire season obtainable through solar ponds; condensation process occurring at sufficiently low temperatures through the use of sea water; exertion of heat transformers. The heat which is usually available at these temperatures could be used for common thermal processes during the desalination of seawater. In this work we want to demonstrate that it is possible, energetically speaking, to produce desalinated water by exploiting the solar energy stored in solar ponds and the technology of absorption heat transformers. We can notice how for every m3 of desalinated water produced in one day we need ponds with an area ranging between 1000 and 4000 m2, this depends on the amount of heat flux drawn. The analysis we carried out represents a first attempt to face this kind of problem. In future studies we will examine both technical and economic feasibility.