Excess Renewable Energy Research Articles

Core‐Shell Catalyst Pellets for CO2 Methanation in a Pilot‐Scale Fixed‐Bed Reactor

AbstractPower‐to‐methane (PtM) offers an efficient opportunity for surplus renewable energy storage, but heat management is a major challenge for conducting the highly exothermic methanation reaction. To address this challenge, this study presents the first successful demonstration of core‐shell catalyst pellets in a pilot‐scale reactor for CO2 methanation. Experiments in a wall‐cooled fixed‐bed reactor are conducted with diluted and undiluted reactant feed under systematic variation of cooling temperature and inlet gas flow rate. The results show that core‐shell catalyst pellets significantly reduce the hot‐spot temperature while maintaining comparable reactant conversions to uncoated catalyst pellets at the same conditions. Additionally, core‐shell catalyst pellets allow for undiluted reactant feed at comparably low hot‐spot temperatures (approx. 600 °C).

In-Situ CO2 Release from Captured Solution and Subsequent Electrolysis: Advancements in Integrated CO2 Capture and Conversion Systems

The electrochemical reduction of carbon dioxide (CO2ER) offers a compelling strategy for sustainable energy and carbon management by transforming CO2 into valuable chemicals and fuels. This approach is particularly attractive when powered by surplus renewable energy, aiding in the closure of the carbon cycle. However, the significant energy requirements for CO2 isolation, pressurization, and purification from capture media pose challenges for industrial-scale implementation. To overcome these obstacles, recently innovative electrolyzers have developed that directly convert reactive carbon solutions, such as bicarbonate-rich effluents from carbon capture units, into higher-value products. This electrolyzers produces CO2 in situ by reacting (bi)carbonate with acid generated within the electrolyzer, facilitating efficient CO2ER at the cathode surface and eliminating the need for costly CO2 recovery and compression steps. This study details recent efforts in advancing this type of electrolyzer, focusing on CO2 sources for capturing step, technical aspect consideration of integrated systems, electrolyzer design consideration and membrane designs for integrated systems. Herein, we emphasize the need for a permeable cathode that allows efficient (bi)carbonate ion transport while maintaining a high catalytic surface area. Additionally, we discuss the critical role of electrolytes, including the impact of (bi)carbonate concentration, their effect on CO2 utilizations and CO2ER selectivity. We also demonstrate the state-of-the-art performance metrics for electrolyzers that utilize CO2-captured solutions, such as Faradaic efficiency, experimental validation of CO2 sources, current density, and CO2 utilization efficiency, a guideline for future studies. Collectively, we believe that this analysis will contribute to the development of industrial-scale electrochemical reactors for CO2 conversion, advancing towards a sustainable and closed-loop carbon cycle.

Comparative Study of Mesophilic Biomethane Production in Ex Situ Trickling Bed and Bubble Reactors

Biomethane production via biogas upgrading is regarded as a future renewable gas, further boosting the biogas economy. Moreover, when upgrading is realized by the biogas CO2 conversion to CH4 using surplus renewable energy, the process of upgrading becomes a renewable energy storage method. This conversion can be carried out via microorganisms, and has attracted scientific attention, especially under thermophilic conditions. In this study, mesophilic conditions were imposed using a previously developed enriched culture. The enriched culture consisted of the hydrogenotrophic Methanobrevibacter (97% of the Archaea species and 60% of the overall population). Biogas upgrading took place in three lab-scale bioreactors: (a) a 1.2 L bubble reactor (BR), (b) a 2 L trickling bed reactor (TBR) filled with plastic supporting material (TBR-P), and (c) a 1.2 L TBR filled with sintered glass balls (TBR-S). The gas fed into the reactors was a mixture of synthetic biogas and hydrogen, with the H2 to biogas CO2 ratio being 3.7:1, lower than the stoichiometric ratio (4:1). Therefore, the feeding gas mixture did not make it possible for the CH4 content in the biomethane to be more than 97%. The results showed that the BR produced biomethane with a CH4 content of 91.15 ± 1.01% under a gas retention time (GRT) of 12.7 h, while the TBR-P operation resulted in a CH4 content of 90.92 ± 2.15% under a GRT of 6 h. The TBR-S operated at a lower GRT (4 h), yielding an effluent gas richer in CH4 (93.08 ± 0.39%). Lowering the GRT further deteriorated the efficiency but did not influence the metabolic pathway, since no trace of volatile fatty acids was detected. These findings are essential indicators of the process stability under mesophilic conditions.

Coupled hydro-mechanical model for underground hydrogen storage undergoing cyclic injection and production

ABSTRACT Excess renewable energy can be used to generate and store hydrogen underground, offering a secure and econimical solution to the intermittency challenges of renewable energy sources. The success of underground storage relies on understanding and controlling the geo-mechanical integrity of storage sites during cyclic loadings. While geological deformation during natural gas and carbon dioxide storage is well studied, few have investigated pore pressure and in-situ stress variations during cyclic loading, which is unique to underground hydrogen storage sites. This paper presents a predictive analytical model for the pore pressure and stress changes during cyclic loading. The model was derived based on continuous point source injection, a basic hydro-mechanical model while applying the superposition to consider the cyclic loading. The developed model was evaluated for hydrogen and natural gas subjected to cyclic injection and production. Comparing hydrogen and natural gas storage cases can offer valuable insights into the behavior of hydrogen since the deformation of natural gas storage is more studied. Finally, analytical solutions for both storage cases were validated by numerical models with discrepancies between the models amounting to less than 1%. Comparative analysis of the results from 3 to 99 cycles of injection and production for both hydrogen and methane demonstrates that rise in cycle numbers leads to a corresponding rise in stress accumulation in the system. Neglecting the stress accumulation during these cycles can lead to significant geomechanical issues, potentially compromising the storage system’s safety and integrity.

Regional Analysis and Evaluation Method for Assessing Potential for Installation of Renewable Energy and Electric Vehicles

Many countries are adopting renewable energy (RE) and electric vehicles (EVs) to achieve net-zero emissions by 2050. The indicators of RE and EV potentials are different. Decision-makers want to introduce RE and EVs; however, they need a method to find suitable areas. In addition, this is required in the time-series analysis to provide a detailed resolution. In this study, we conducted a time-series analysis in Japan to evaluate suitable areas for the combined use of RE and EVs. The results showed the surplus RE areas and shortage RE urban areas. The time-series analysis has quantitatively shown that it is not enough to charge EV batteries using surplus RE. Moreover, a ranking methodology was developed for the evaluation based on electric demand and vehicle numbers. This enables the government’s prioritization of prefectures and the prefectures’ prioritization of municipalities according to their policies.

Mining the atmosphere: A concrete solution to global warming

To neutralize anthropogenic climate impacts, excess carbon dioxide (CO2) – about 400 Gt of carbon – needs to be removed from the atmosphere. After the energy transition is accomplished, we propose that excess renewable energy can be used to extract CO2 from the atmosphere and convert it into methane or methanol, which are further processed into polymers, hydrogen, and solid carbon. End-of-life polymers are pyrolysed and part of the carbon is used to produce silicon carbide. Solid carbon and silicon carbide become then aggregates and fillers for concrete and asphalt. At the end of their lifecycle, landfilled construction materials become the final carbon sink. Up to 12 Gt of carbon could be stored per year, mostly as concrete aggregates. The synthesis of carbon-based materials in cycles of increased chemical reduction has multiple advantages, including long-term stability, high storage density of the carbon, decentralized implementation, and replacement of current CO2-emitting materials.

Porous Monolithic Perovskite Structures for High-Temperature Thermochemical Heat Storage in Concentrated Solar Power (CSP) Plants and Renewable Electrification of Industrial Processes

A novel approach towards thermal energy storage of surplus renewable energy (RE) is introduced via a hybrid thermochemical/sensible heat storage concept implemented with the aid of porous structures made of redox metal oxides, capable of reversible reduction/oxidation upon heating/cooling in direct contact with air, accompanied, respectively, by endothermic/ exothermic heat effects and demonstrating fully reversible dimensional changes under cyclic operation. The proposed modular storage units can be heated during the day to a level exceeding the metal oxide’s reduction onset temperature either by hot air streams from air-operated Concentrated Solar Power (CSP) tower plants or via surplus/cheap RE-electricity from photovoltaics, wind, or other renewable sources (“charging”/energy storage step). When this RE sources become non-available or upon demand, the fully charged system can transfer its energy to a controlled airflow that passes through the porous oxide block and initiates the exothermic oxidation of the reduced metal oxide. Thus, a hot air stream is produced which can be used to provide electricity or exploitable heat for industrial processes. The present work elaborates on the operating principles and the potential application of this concept and reports progress in the preparation and shaping of reticulated porous ceramics (RPCs also known as “ceramic foams”) from CaMnO3-based perovskite compositions and their preliminary testing with respect to cyclic reduction-oxidation.

Developing hydrogen energy hubs: The role of H2 prices, wind power and infrastructure investments in Northern Norway

Hydrogen is seen as a key energy carrier to reduce CO2 emissions. Two main production options for hydrogen with low CO2 intensity are water electrolysis and natural gas reforming with Carbon Capture and Storage, known as green and blue hydrogen. Northern Norway has a surplus of renewable energy and natural gas availability from the Barents Sea, which can be used to produce hydrogen. However, exports are challenging due to the large distances to markets and lack of energy infrastructure. This study explores the profitability of hydrogen exports from this Arctic region. It considers necessary investments in hydrogen technology and capacity expansions of wind farms and the power grid. Various scenarios are investigated with different assumptions for investment decisions. The critical question is how exogenous factors shape future regional hydrogen production and export. The results show that production for global export may be profitable above 90 €/MWh, excluding costs for storage and transport, with blue hydrogen being cheaper than green. Depending on the assumptions, a combination of liquid hydrogen and ammonia export might be optimal for seaborne transport. Exports to Sweden can be profitable at prices above 60 €/MWh, transported by pipelines. Expanding power generation capacity can be crucial, and electricity and hydrogen exports are unlikely to co-exist.

Insights into Ir-Based Trimetallic Nano-Catalyst for Acidic Water Electrolysis

In the pursuit of a sustainable and environmentally friendly energy future, water electrolyzers emerge as indispensable players in the landscape of renewable energy storage. At the core of their significance lies the ability to convert surplus renewable energy, such as solar or wind power, into hydrogen - a clean and versatile energy carrier. Through the process of electrolysis, water molecules are split into hydrogen (via Hydrogen evolution reaction, HER) and oxygen (via Oxygen evolution reaction, OER) using electricity, typically derived from renewable sources. However, current hydrogen production is still heavily relying on fossil fuels, with natural gas reforming as the primary source. Despite its high cost, iridium oxide (IrO2) is the most commonly used catalyst for the sluggish OER. Combining Ir with other non-noble metals is one strategy to reduce Ir loading, while maintaining and even increasing catalyst activity. Bimetallic alloying of Iridium-based nanomaterials has the potential to enhance its intrinsic catalytic properties towards acidic OER than that of the single element doped catalyst. The mechanisms include providing more reaction active sites, tuning the electronic structure of the matrix, and synergistic effects that promote the catalytic reaction by facilitating the formation and stabilization of active oxygen intermediates. Although many transition metals, such as Ni, Co, Fe, Mn, W, Sn, Nb, and Ti, have been doped into Ir to from bimetallic and trimetallic catalyst structures, no studies have explored the fundamental relationship between dopant and catalyst activity. This work systematically investigates the effect of doping Iridium-Cobalt matrix with a third, fourth-period transition metals including Mn, Cr, Fe, Co, Ni and Cu, towards acidic OER activity. A facile one pot chemical reduction synthesis is used to synthesize the trimetallic nanomaterials. Half-cell measurements are conducted to evaluate the electrochemical performance and stability of the catalysts. Lastly, physio-chemical methods are used to characterize the composition and structure of the catalysts. At the ECS meeting, we will share our results to provide new insight into structure-function relationships of our bimetallic-doped Ir catalysts.

Maximizing thermal performance in triangular honeycomb thermochemical reactors: Structural and operating parameter studies

Using thermochemical energy storage methods to store energy is increasingly vital for boosting the share of renewable energy in consumption within buildings and industries. This entails harnessing off-peak excess renewable energy, such as electricity or waste heat energy, for storage purposes. While previous research emphasizes the importance of improving reactor performance, comprehensive analysis of the new honeycomb reactor designs is lack. Innovative advancements in maximizing thermal performance within triangular honeycomb reactors are elucidated in this study. This study employs a validated 3-D model to explore the effects of these parameters on heat and mass transfer within a triangular honeycomb reactor. By investigating structural and operational parameters, a novel equilibrium is achieved, amplifying heat exchange efficiency and channeling thermal energy with unprecedented precision. Through a rigorous examination of reactor air velocity distribution, air temperature lift, and outlet air absolute humidity, this research unveils pivotal insights into enhancing reactor efficacy. Notably, the study identifies that increasing the channel thickness by 1.2 times, resulting in energy storage density of 463 kJ/kg. This innovation positions triangular honeycomb reactors at the forefront of sustainable energy management, offering a potential solution for decarbonization such as storing industrial waste heat and harnessing surplus off-peak electricity in buildings.

Techno-economic assessment of waste heat recovery for green hydrogen production: a simulation study

AbstractThe demand for hydrogen as a green energy carrier is increasing as energy sources shift towards sustainable solutions. Alkaline electrolysers offer a clean method to produce hydrogen, though their limited efficiency results in significant energy loss. This study explores the potential to enhance electrolyser efficiency through waste heat recovery. It examines the technical and economic aspects of using excess heat from an alkaline electrolyser, powered by surplus renewable energy, as a feed-in source for district heating. Utilising a simulation framework for renewable power plants, the study integrates a validated electrolyser model. The analysis focuses on the impact of heat utilisation and heat sales on system efficiency, economic viability, and hydrogen pricing. Findings show improved efficiency with heat supply, especially for smaller electrolyser configurations. Heat sales lead to a slight reduction in hydrogen costs and the study demonstrates their viability for smaller electrolysers. Additionally, it highlights the need for an advanced cooling strategy for larger systems. Overall, the results underscore the potential of integrating electrolysis with district heating, offering valuable insights for future renewable-powered energy systems.

Planning the customer service process of a logistics company based on implementing green technologies

The peculiarities of planning environmentally oriented customer service for a logistics company based on implementing "green" technologies are discussed in the article. It has been researched that planning is an integral part of the successful management of any company, especially in the logistics sector, where customer satisfaction and operational efficiency are key factors. Under modern conditions, to respond to customer requests, logistics companies must take into account the impact of their activities on the environment and implement appropriate methods to minimize the negative impact on the environment, and accordingly, the process of planning customer service in a logistics company should include an environmental component. Given this, the importance of integrating environmental factors and business processes at each stage of planning to ensure sustainable development of a logistics company based on forming a cyclic approach to planning environmentally oriented customer service is substantiated. The method of iterative planning that allows a logistics company not only to improve the quality of customer service but also to integrate environmental practices into its activities, which meets the modern requirements of sustainable development and social responsibility of business has been established. The developed model of environmentally-oriented planning of customer service combines business processes of logistics service and "green" technologies, transforms the operational efficiency of a logistics company in the context of sustainable development, substantiates the integration of environmentally sustainable solutions at all stages of the logistics chain - from initial planning to the final delivery of goods to consumers. To support the proposed model, the implementation of Power-to-X technology is recommended, which allows the conversion of excess renewable energy into energy specifically for logistics and transportation purposes. The implementation of this measure enables the logistics company to reduce emissions and increase energy efficiency, to form a comprehensive approach to the development and integration of environmentally friendly technologies in the company's logistics operations to increase efficiency and competitiveness. It is emphasized that the proposed model demonstrates a comprehensive approach to building a highly efficient and at the same time environmentally sustainable logistics system of the company by integrating innovative "green" technologies.

Baseline Econometric Model and Solar Energy Requirement for Potential Hydrogen Demand in Tarlac, Philippines

This study aims to build a baseline econometric model for the possible development of green hydrogen production in Tarlac. Scenario analysis will consider hydrogen produced from renewable solar energy as an alternative energy carrier in the fuel mix for electricity generation in Tarlac to achieve sustainable economic growth and improve energy security while minimizing carbon emissions and improving resource efficiency. Tarlac is the chosen area to be considered in this study because this is where New Clark City is being established. New Clark City is a 9450-hectare disaster-resilient, environment-friendly, integrated urban development metropolis. It is important to build the baseline econometric model before proceeding with scenario generation. The parameters used are determined from available historical data through statistical procedures. The basic assumption is that the relationships between the independent and dependent variables that existed in the past will continue to be true in the future. The energy demand data and the continuous predictors were used to build a baseline econometric model for potential hydrogen demand in Tarlac, Philippines. Model equations were obtained using stepwise regression analysis. These model equations were used to forecast the energy demand up to 2032. These forecasted energy demands will be used in scenario generation to integrate green hydrogen production. Preliminary assessments of green hydrogen scenarios that are based on the assumption of renewable energy surplus generated from solar power plants in Tarlac are discussed. The forecasted demand will be used to assess the scenario generation in integrating green hydrogen in New Clark City. The proposed energy management strategy of storing surplus renewable energy, such as green hydrogen, for the power sector has the potential to impact various stakeholders, including industries and the government, significantly. This strategy aligns with transitioning to cleaner and more sustainable energy sources, contributing to national and international environmental targets. Adopting green hydrogen storage can contribute to energy security by diversifying the sources of power generation.

Superimposed Positive Sequence Impedance for Detecting Unintentional Islanding in Microgrid

Incorporation of environmentally friendly energy sources (RESs) into the electricity grid has many benefits, including economic, technological, and environmental. However, excessive renewable energy sources (RES) in the power grid provide technical problems, including equipment protection, DG operation, and islanding detection. One of the most serious challenges is the islanding phenomenon. Islanding can cause several problems, such as frequency instability and voltage fluctuations resulting in damage to electrical equipment or threatening utility workers who may be working/accessing the equipment. This research proposes an efficient islanding detection algorithm to lessen the impact of such threats. This novel passive islanding detection scheme is based on superimposed positive sequence impedance (SPSI). For calculating the superimposed positive sequence impedance (SPSI), the voltage and current signals are obtained from targeted DG points. The scheme’s performance is tested on multiple bus systems across islanding and non-islanding conditions using a MATLAB/Simulink environment. It is shown that even in the presence of noise, the algorithm can determine an islanding decision with high accuracy and a short detection time of 84 ms. In comparison to other algorithms, it operates at zero power mismatch (ZPM) and does not affect power quality.

Adaptive step size quantized simulation method for gas–electricity integrated energy systems

Gas–electricity integrated energy systems (GE-IES) offers a promising solution for enhancing energy efficiency and accommodating renewable energy sources. Accurate dynamic simulation is essential for optimizing and controlling GE-IES. However, the presence of various local controllers introduces prominent discrete characteristics, posing challenges for the dynamic simulation of the GE-IES. This paper investigates the dynamic simulation method in GE-IES with discrete characteristics. Firstly, we propose an adaptive step size simulation method based on quantized state system theory. This method maintains the event-driven characteristics of the quantized state integration algorithms, while enhancing computational speed through adaptive step size adjustments. Secondly, we establish an event-driven simulation framework that facilitates interactions of different subsystems during the dynamic simulation, improving the compatibility with various models and solving algorithms. Finally, we validate the accuracy, efficiency, and scalability of the proposed method and the framework using two typical GE-IES cases with different scales. Simulation results demonstrate the effectiveness on the dynamic simulation of GE-IES and highlight the feasibility of natural gas networks in consuming and storing surplus renewable energy.

Digital twin-driven architecture for AIoT-based energy service provision and optimal energy trading between smart nanogrids

This article explores integrating digital twin technology and blockchain within smart grids to optimize energy trading among prosumers and consumers in smart nanogrids. Our platform employs a multi-objective optimization strategy, including Particle Swarm Optimization (PSO), to delineate energy trading routes between nanogrids, optimizing parameters such as route distance, surplus renewable energy, and energy power loss. Our platform ensures efficient and effective energy trading services by meticulously considering factors such as surplus energy amount, energy price, route distance, and time. The proposed digital twin-based architecture comprises seven layers, each tailored to address specific functionalities and services for energy management within smart nanogrids. At the apex lies the application layer (digital twin services), leveraging the digital twin's capabilities to optimize energy trading, manage surplus energy, and efficiently meet energy demand. This layer facilitates informed decision-making and resource optimization. Integrating a digital twin-driven architecture with a blockchain-based platform tackles challenges inherent in decentralized energy trading. The digital twin offers real-time energy resource monitoring and optimisation, ensuring efficient utilisation and autonomous decision-making. Concurrently, leveraging blockchain technology ensures secure and transparent transactions, fostering trust among participants and facilitating peer-to-peer energy exchange. Task generation, device virtualization, task mapping, scheduling on edge devices, and task assignment layers further streamline task execution and resource utilization, enhancing the efficiency of energy management processes. The predictive optimal energy control layer also orchestrates the entire architecture, enabling predictive and optimized energy control within smart nanogrids. Furthermore, the Security as a Service (SECaaS) layer enhances security and trustworthiness using blockchain technology, incorporating components such as consensus management, real-time distributed ledgers, and identity management. This layer enhances the security and transparency of energy-related transactions and data within the digital twin framework. The results showcase a remarkable 53% reduction in peak load, emphasizing the optimized energy consumption and demand achieved. Furthermore, our platform has significantly increased the utilization of renewable energy resources by 24%, highlighting its contribution to sustainable energy resource management. Rigorous assessment of the prediction and optimization modules reveals their high accuracy and precision, with mean absolute percentage error (MAPE) values of 15.125 and 14.369, respectively. These findings underscore the efficacy and reliability of our digital twin-based approach, surpassing existing solutions and benchmarks.

Consuming the redundant renewable energy by injecting hydrogen into the gas network: A case study of electricity-gas-hydrogen coupled system of Ireland

Converting surplus electricity into hydrogen is considered a promising way for accommodating large-scale wind or solar power. However, it is expensive to establish a new-built hydrogen network or storage facilities at the current stage. Hydrogen-enriched compressed natural gas (HCNG), a mixture of natural gas and hydrogen, can be an alternative approach to consuming surplus renewable energy in the short-medium term. This paper investigates the feasibility of establishing an electricity-gas-hydrogen coupled energy system by modeling the HCNG network, power grid, and hydrogen electrolyzer in Ireland with consideration of pipeline characteristics. The economic analysis based on the planning results of the coupled system is presented in detail. When the hydrogen injection ratio reaches 20% at the HCNG stations, the system operating cost and carbon emission can be reduced by 24.2% and 6.4%, and 100% wind can be consumed.

Technology assessment for the transition to a renewable electric grid

To reduce carbon emissions, generation of electricity from combustion systems is being replaced by renewable resources. However, the most abundant renewable sources – solar and wind – are not dispatchable, vary diurnally and are subject to intermittency, and produce electricity at times in excess of demand (excess production). To manage this variability and capture the excess renewable energy, energy storage technologies are being developed and deployed such as battery energy storage (BES), hydrogen production with electrolyzers (ELY) paired with hydrogen energy storage (HES) and fuel cells (FCs), and renewable natural gas (RNG) production. While BES may be better suited for short duration storage, hydrogen is suited for long duration storage and RNG can decarbonize the natural gas system. California Senate Bill 100 (SB100) sets a goal that all retail electricity sold in the State must be sourced from renewable and zero-carbon resources by 2045, raising the questions of which set of technologies and in what proportion are required to meet the 2045 target in the required timeframe as well as the role of the natural gas infrastructure, if any. To address these questions, this study combines electric grid dispatch modeling and optimization to identify the energy storage and dispatchable technologies in 5-year increments from 2030 to 2045 required to transition from a 60% renewable electric grid in 2035 to a 100% renewable electric grid in 2045. The results show that, by utilizing the established natural gas system to store and transmit hydrogen and RNG, the deployment of battery energy storage is dramatically reduced. The required capacity for BES in 2045, for example, is 40 times lower by leveraging the natural gas infrastructure with a concomitant reduction in cost and associated challenges to transform the electric grid.

Recommendations for a positive energy district framework – Application and evaluation of different energetic assessment methodologies

Positive Energy Districts are seen as a stepstone towards climate-neutrality for European cities. The concept aims to make districts an active contributor to urban energy systems. However, the definition of PEDs is relatively loose and there is currently a lack of a common European assessment methodology, which makes it difficult to evaluate PED in practice. This research evaluates the energetic assessment methodologies developed in three PED-relevant projects in Europe – namely MAKING CITY, Zukunftsquartier Wien, and Zero Emission Neighbourhood – in order to derive recommendations for a common PED assessment framework. For this purpose, the three methodologies have been applied to case study districts in Germany. Subsequently, the application of the methodologies has been analysed based on their general practicality as well as their fulfilment of the PED objectives. The findings suggest that a positive energy balance might not be considered as a prerequisite of PEDs as this strict requirement sets a high entry barrier for districts that lack the intrinsic factors for surplus renewable energy production. For PED as an inclusive framework, the focus should be on delivering positive impacts for the districts and the wider energy systems; whilst the positive energy balance can be seen as a complementary rather than a mandatory condition.

Bioelectrochemical protein production valorising NH3-rich pig manure-derived wastewater and CO2 from anaerobic digestion

Microbial electrosynthesis (MES) cell use is an innovative approach for single-cell proteins (SCP) production. Coupling MES with the valorization of CO2 from anaerobic digestion and nitrogen from livestock effluents has beneficial environmental effects, reducing greenhouse gas emissions and nitrogen overloading. In addition, the reducing power needed can come from surplus renewable energy. In this study, MES with a biochar-functionalized cathode was tested at varying polarizations, i.e. non polarized, −0.6 V and −1.0 V vs Ag/AgCl, and biogas-derived CO2 and recovered ammonia from pig slurry was supplied.Negative polarization switched the microbial community from heterotrophic, typical of unpolarized MES, to a mix of both heterotrophic and autotrophic/electrotrophic communities at −0.6 V and to mainly autotrophic/electrotrophic at −1.0 V. The more negative polarization allowed the highest CO2 and N capture, i.e. 39 ± 2 % of the supplied CO2, and 6.7 ± 0.8 % supplied N. Microbial biomass characterization indicated a protein content on dry matter basis of 33.1 ± 1.3 % (unpolarized), 43.2 ± 0.6 % (−0.6 V) and 69.1 ± 1.0 % (−1.0 V). The amino acids profiles investigated showed a high nutritional value of the produced biomass, not far from those of conventional protein sources used for producing feed/food.