Introduction

Heat pumps are a key technology for decarbonizing space heating and domestic hot water production. The selection of an appropriate refrigerant strongly influences system efficiency, operating range, safety, and environmental impact. Carbon dioxide (CO₂) has attracted considerable attention as a natural refrigerant due to its negligible Global Warming Potential (8GWP); however, its transcritical operation introduces challenges at high heat sink temperatures. In parallel, low-GWP alternatives such as propane (R290) and hydrofluoroolefins (HFOs), including R1234ze, have emerged as promising candidates for subcritical heat pump applications. A consistent and physically sound comparison between these refrigerants remains essential, particularly under water-to-water operating conditions relevant to building heating systems. The objective of this study is to develop a thermodynamic modelling framework for a water-to-water heat pump to compare the performance of CO₂, R290, and R1234ze under identical boundary conditions.

Methodology

A steady-state lumped-parameter model of a water-to-water heat pump was developed in MATLAB. The evaporator and condenser (or gas cooler in the case of CO₂) were modelled using prescribed water inlet and outlet temperatures, allowing heat transfer to be calculated directly from the water side. Compressor performance was modelled using a constant isentropic efficiency. For subcritical refrigerants, realistic operating conditions were imposed, including a small degree of superheating at the compressor inlet and subcooling at the condenser outlet. For CO₂, transcritical operation was considered, with the gas cooler outlet temperature determined by the heat sink conditions. Expansion was assumed to be isenthalpic for all cases. The model was first validated against experimental data obtained from a CO₂ water-to-water heat pump test rig. After validation, the same modelling assumptions, heat exchanger boundary conditions, and efficiency parameters were applied to all refrigerants to ensure a consistent basis for comparison.

Results

The model successfully reproduced the main thermodynamic trends observed in the experimental CO₂ heat pump, confirming the reliability of the modelling approach. Under the investigated operating conditions, the simulated coefficient of performance (COP) for CO₂ was approximately 4.9, which is consistent with values reported for similar experimental systems. When the same boundary conditions were applied to alternative refrigerants, significant differences in cycle performance were observed. The simulations predicted COP values of approximately 10 for both R290 and R1234ze, indicating substantially lower compressor work compared with the CO₂ cycle under the same heat sink conditions. In addition to efficiency differences, the refrigerants exhibited distinct thermodynamic behaviour. Subcritical refrigerants operated with compressor inlet states close to the saturation line and moderate pressure levels, whereas CO₂ required significantly higher operating pressures due to its transcritical cycle. The results also showed that CO₂ performance is strongly influenced by the gas cooler outlet temperature, whereas the performance of subcritical refrigerants is more stable under the same operating range. These findings highlight the importance of operating conditions and refrigerant thermophysical properties in determining heat pump performance.

Conclusions

A unified thermodynamic modelling framework for water-to-water heat pumps has been developed and validated using experimental CO₂ data. The main novelty of the study lies in the consistent comparison of transcritical and subcritical refrigerants using identical boundary conditions, modelling assumptions, and compressor performance parameters, enabling a fair evaluation of refrigerant performance. The comparative analysis indicates that, under the investigated operating conditions, R290 and R1234ze achieve significantly higher COP values than CO₂, primarily due to lower compression work and favourable thermodynamic properties in subcritical operation. However, the results also demonstrate that CO₂ performance is highly sensitive to gas cooler outlet temperature and may become more competitive at higher heat sink temperatures. The proposed modelling framework provides a robust tool for evaluating alternative refrigerants and supports the assessment of environmentally friendly working fluids for future heat pump applications.

Introduction

Seasonal smog has become a major environmental and public health concern in the transboundary Punjab region of Pakistan and India. Extensive open field burning of crop residues upon harvesting of major crops like rice and wheat are the significant causes of air pollution, greenhouse gas emissions and the loss of recoverable bioenergy. This research is intended to test the impact of the socio-spatial features of crop residue burning, its connection with smog formation and the loss of bioenergy in the transboundary regions of the Indo-Pak Punjab border.

Methods

The study focuses on four divisions in India (Amritsar, Jalandhar, Ludhiana, and Patiala) and four divisions in Pakistan (Lahore, Gujranwala, Faisalabad, and Sahiwal). The method used in the estimation of crop residue generation was crop-to-residue ratio using crop production. The NASA FIRMS provides satellite-based fire data, which was examined with Geographic Information System (GIS) tools in order to determine the presence of residue burning hotspots and temporal changes in the past decade. Air quality indicators such as Air Quality Index (AQI) and particulate matter (PM) were also studied to determine the correlation with the occurrence of the residue burning events.

Results

It is revealed that about 15–22 million tonnes of crop residues are burnt in the study area every year. According to satellite reports, this coincides with the high levels of AQI and PM during periods of peak smog. The amount of energy lost in open field burning is estimated to be between 210 and 330 petajoules (PJ) every year, which is a huge loss of potential bioenergy resources.

Conclusions

The results indicate the close relationship between the burning of crop residues, poor air quality and wasted bioenergy resources within the transboundary region of the Punjab. The research also suggests that biomass briquetting may be an alternative solution to the utilization of crop residues that would help decrease emissions and promote energy recovery in the region and the transition to sustainable energy.

Introduction

Desalination of saline waters to augment freshwater supplies has become an important component of the water supply portfolio of many communities around the world. Because desalination technologies typically require electricity and/or thermal energy for the conversion of saline water into freshwater, nuclear power plants can serve as ideal sources of heat and electricity for these plants. However, the cost of nuclear energy has increased in recent years due to the shifts in supply chains and global economy, and the inclusion of renewable energy sources such as solar and wind energy conversion technologies is often considered to provide cost-effective solutions. The research objective of this study is to assess various water/power schemes.

Methods

Several water/power scenarios including nuclear, solar and conventional fuel sources were studied. The technoeconomic feasibility of a plant was modeled using the DEEP software created by the International Atomic Energy Agency (IAEA) (DEEP, 2018). The DEEP program allows for the modeling of desalination output and water costs powered by various energy sources. The program analyzes the process based on process variables and configurations, and it is effective in evaluating the feasibility scenarios. Several trials were completed to compare the variance in factors affecting the proposed facility along with the present configuration options. The trials were compared to determine the most economically feasible configuration for the needs and characteristics of the proposed location.

Results

A technoeconomic feasibility study performed on a coastal community in the USA has shown that the desalination option is expensive. When compared with the conventional water supply options, freshwater from nuclear desalination plants integrated with RO, MED, and MSF processes costs $0.8, $1.08 and $1.56 per m³, respectively. These values are up to 3.5-, 4.7- and 6.8-fold higher than the conventional water supply option. In addition, seasonal water temperatures have a significant effect on the freshwater and power costs. For example, for the MED process, the 9.5 ​°C increase in temperature decreased the cost of water by twelve cents per cubic meter. For each configuration, the cost to produce nuclear power increased from 66.8 $/MWh at the coldest temperature to 68.7 $/MWh at the warmest temperature.

Conclusions

This analysis indicates that further evaluation with nuclear and renewable energy combination should be conducted to reduce the water and power costs while enhancing energy resilience and environmental sustainability. This presentation will include (i) nuclear and renewable energy integration; (ii) integration of nuclear and wastewater systems for reuse; and (iii) integration of nuclear and bioenergy for agricultural systems. The scenarios will present the synergies, technoeconomics and environmental benefits of these innovative power/water schemes. Recommendations for developing nuclear energy-based sustainable power-water systems will be discussed.

Introduction

In Castilla-La Mancha, the meat processing industry generated approximately half of the 104,431 tonnes of agro-industrial waste in 2019 (Castilla-La Mancha, 2024). These are highly biodegradable, making them ideal for methane production through anaerobic digestion (Rodríguez-Abalde et al., 2019). However, the hydrolysis of these lipids and proteins can inhibit this process (Rodríguez-Méndez et al., 2017) due to the release of ammonia and long-chain fatty acids (Rodríguez-Abalde et al., 2017). To avoid this, various strategies can be employed, such as co-digestion or a two-stage anaerobic digestion process. In the latter case, the hydrolysis and acidogenesis stages would occur first, obtaining H₂, volatile fatty acids (VFAs), and ammonium (NH₄⁺). NH₄⁺ is produced through ammonification under anaerobic conditions where protein hydrolysis occurs (Deng et al., 2024). Therefore, developing strategies to intensify the anaerobic digestion process, such as thermal pretreatment at different temperatures, will promote the formation of renewable gases (H₂ and CH₄), as well as NH₃, which is considered an energy vector and a carrier of H₂.

In this study, batch tests were performed to investigate the potential of producing renewable gases (H₂, NH₃, and CH₄) from meat processing waste. This will support the development of future strategies to intensify the anaerobic digestion process.

Material and Methods

Three different concentrations (12.5%, 25% and 50%) of the selected waste (manure, blood, wastewater, and sludge from the company wastewater treatment plant (WWTP)) were used to assess the potential of producing renewable gases. Tests were conducted according to Hollinger et al. (2016) and Carrillo-Reyes et al. (2020) for evaluating CH₄ and H₂ generation, respectively.

Results and Discussion

As shown in the analysis, blood and manure have a high COD. Blood also has a high NH₄⁺ content. As the concentration of the wastewater and sludge increases, CH₄ production increases. In the case of wastewater, CH₄ production is too low due to the presence of recalcitrant and/or toxic compounds that are detrimental to methanogenic archaea. However, increasing the organic load of the sludge multiplies CH₄ generation by almost fivefold, from 50 to 217 mL CH₄. However, when blood is added, CH₄ production decreases as blood concentration increases, due to the accumulation of volatile fatty acids (VFAs), as evidenced by the detection of H₂. Furthermore, the high N-NH₄⁺ content in blood may also contribute to the inhibition of archaea. In the case of manure, the maximum amount of CH₄ was produced when the waste concentration was 25%, resulting in an improvement in CH₄ production from 95 to 146 mL CH₄. Higher concentrations would lead to inhibitory processes, possibly related to an overload of organic matter. NH₄⁺ concentration was analysed throughout the trials, increasing along the different tests, highlighting the transformation of organic nitrogen into ammoniacal nitrogen. Nevertheless, an inhibitory concentration of NH₄⁺ (>1.5 g/L; Deng et al., 2024) was only achieved when blood was used. On the other hand, no H₂ production was observed during the biochemical H₂ test due to the unacclimatised inoculum used and the need for an initial waste pre-treatment. Future work includes waste pretreatment, co-digestion, co-fermentation, and using acclimated sludge to obtain H₂.

Conclusions

Anaerobic digestion is a suitable method of treating waste to produce renewable gases (CH₄, H₂, and NH₃). The results obtained showed that WWTP sludge and manure were the most suitable materials for CH₄ production, while blood had high potential for NH₃ generation. However, intensification strategies such as co-digestion and two-stage anaerobic digestion are needed to optimise the production of these gases.

References

Carrillo-Reyes, J., Buitrón, G., Moreno-Andrade, I., Tapia-Rodríguez, A. C., Palomo-Briones, R., Razo-Flores, E., ... & Zaiat, M. (2020). Standardized protocol for determination of biohydrogen potential. MethodsX, 7, 100754.

Castilla-La Mancha. (2024). Plan de prevención y gestión de residuos de Castilla-La Mancha 2030.

Deng, Z., Sierra, J. M., Ferreira, A. L. M., Cerqueda-Garcia, D., Spanjers, H., & van Lier, J. B. (2024). Effect of operational parameters on the performance of an anaerobic sequencing batch reactor (AnSBR) treating protein-rich wastewater. Environmental Science and Ecotechnology, 17, 100296.

Holliger, C., Alves, M., Andrade, D., Angelidaki, I., Astals, S., Baier, U., Bougrier, C., Buffière, P., Carballa, M., & De Wilde, V. (2016). Towards a standardization of biomethane potential tests. Water Science and Technology, 74(11), 2515–2522.

Rodríguez-Abalde, Á., Flotats, X., & Fernández, B. (2017). Optimization of the anaerobic co-digestion of pasteurized slaughterhouse waste, pig slurry and glycerine. Waste Management, 61, 521–528.

Rodríguez-Abalde, Á., Guivernau, M., Prenafeta-Boldú, F. X., Flotats, X., & Fernández, B. (2019). Characterization of microbial community dynamics during the anaerobic co-digestion of thermally pre-treated slaughterhouse wastes with glycerin addition. Bioprocess and Biosystems Engineering, 42, 1175–1184.

Rodríguez-Méndez, R., Le Bihan, Y., Béline, F., & Lessard, P. (2017). Long chain fatty acids (LCFA) evolution for inhibition forecasting during anaerobic treatment of lipid-rich wastes: Case of milk-fed veal slaughterhouse waste. Waste Management, 67, 51–58.

Efficient small-scale hydrogen production via proton exchange membrane (PEM) electrolysis is increasingly recognized as a key pathway for producing green hydrogen and supporting the transition toward low-carbon energy systems. Achieving this goal requires a detailed understanding of the coupled electrochemical, thermal, and energetic phenomena governing stack performance under dynamic operating conditions. This study experimentally investigates a five-cell zero-gap PEM electrolyzer stack equipped with Nafion™ 117 membranes, titanium mesh electrodes, and stainless steel bipolar plates. The system is designed for laboratory-scale hydrogen production and evaluated under controlled operating conditions to quantify energy losses, thermal behavior, and hydrogen generation efficiency.

The experimental protocol includes membrane hydration, stack thermal stabilization, low-current electrochemical conditioning, and systematic polarization testing over an applied current density up to 0.3 A/cm². Stack temperatures during operation range from approximately 312.1 at to 322.8 K, for water circulating at 55±3 °C at 200 mL/min. Hydrogen production is experimentally measured using a calibrated collection bag and compared against theoretical hydrogen generation calculated using Faraday’s law. The resulting Faradaic efficiency increased at relatively moderate current densities, indicating improved electrochemical utilization, to reach 98.03%.

Infrared (IR) thermography reveals non-uniform temperature distributions across the electrolyzer stack, with inter-cell and in-plane temperature gradients exceeding 9 °C, even under relatively low-current operation. As current density increases, localized hotspots become more pronounced, highlighting the role of ohmic heating and uneven current distribution.

Energy efficiency, calculated by combining electrical input power with measured hydrogen production rates, remains above 50% at relatively moderate current densities. However, efficiency declines significantly at higher currents due to the combined effects of increased ohmic losses, charge transfer limitations, and thermal non-uniformities. The results demonstrate a clear coupling between electrochemical resistance growth, thermal gradients, and reduced hydrogen production efficiency.

Overall, this work provides a comprehensive characterization of the dynamic energetic response of a small-scale zero-gap PEM electrolyzer. By directly linking experimental hydrogen production and spatially resolved thermal behavior, the study offers valuable insights into loss mechanisms that limit efficiency and operational stability. These findings underscore the importance of optimized thermal management, material selection, and operating strategies for improving the performance and durability of small-scale PEM electrolyzers in green hydrogen applications. These findings are particularly relevant for improving the energy efficiency and durability of PEM electrolyzers used in renewable-energy-driven hydrogen production, thereby contributing to the development of more environmentally sustainable hydrogen technologies.

Introduction

Higher education institutions (HEIs) are increasingly expected to reduce their carbon footprint in response to climate change, rising energy costs, and the United Nations Sustainable Development Goals. Universities function as small urban ecosystems with substantial energy demand for electricity, heating, laboratories, and digital infrastructure. While many studies have examined campus decarbonization in developed and high-income countries, quantitative evidence from transition economies and Central Asia remains limited, particularly for campuses dependent on fossil-fuel-based district heating and centralized electricity systems. Addressing this literature gap, this study develops and evaluates a campus-scale decarbonization pathway for the Tashkent Institute of Chemical Technology (TICT), located in Uzbekistan.

Methodology

A greenhouse gas (GHG) inventory was developed using an activity-based approach aligned with the GHG Protocol. The baseline inventory represents campus operations prior to the implementation of sustainability measures and integrates energy audits, utility records, and national emission factors to quantify Scope 1 (direct fuel use), Scope 2 (purchased electricity), and selected Scope 3 emissions. The analysis includes electricity consumption in academic and administrative buildings of the TICT, natural gas use for heating, and indirect emissions associated with paper consumption and digital services.

Results

On a technical level, the institute deployed over 4,900 square meters of solar photovoltaic panels to mitigate reliance on carbon-intensive grid electricity. Simultaneously, inefficient district heating connections were replaced with autonomous, high-efficiency boiler systems, which reduced natural gas consumption by approximately 30%. These infrastructural upgrades were paired with institutional digitalization and behavioral awareness campaigns to minimize resource waste, alongside nature-based solutions such as large-scale landscaping to enhance localized carbon sequestration. Following the integration of these combined technological, institutional, and ecological measures, total campus-wide CO₂-equivalent emissions decreased by nearly 61% compared to the baseline.

Conclusion

This study demonstrates that integrating renewable energy, efficient heating systems, digitalization, and nature-based solutions can significantly reduce campus greenhouse gas emissions. In this line, this work provides one of the first comprehensive GHG-Protocol-based campus decarbonization assessments in Central Asia and offers a replicable roadmap for universities in transition economies pursuing sustainable campus transformation.

1. Cities in Climate and Energy Action

Cities play a significant role in the shift from fossil fuels to renewable energy sources to address climate change because of mainly two reasons. First, cities are both major energy consumers and contributors to greenhouse gas emissions globally. More than half (≈55%) of the world’s population live in urban areas (4.6 billion people), and this proportion is projected to increase to 68% by 2050 (+2.5 billion people)[1]. At the same time, cities depend heavily on energy to deliver a wide range of essential energy services. As a result, almost two thirds (≈67%) of global primary energy use is attributed to urban areas, and this percentage is expected to rise to 73% by 2030. In turn, urban energy demand accounts for 75% of global carbon dioxide emissions, making cities critical to reduce global GHG emissions [2]. Second, cities are particularly vulnerable to extreme climate change-driven weather events due to their high concentration of people and infrastructure. Natural hazards such as floods and heatwaves can rapidly lead to ripple effect problems in energy systems through direct damage to energy infrastructure, and disruption in energy supply. Other interconnected impacts include increased energy demand and prices, and negative impacts on renewables due to changing weather patterns.

However, not all citizens face severe weather events in the same manner. Low-income groups and marginalized populations often live in inadequate housing, and concentrate in urban areas with aging infrastructure and poor services such as transportation, making them particularly vulnerable to climate change and energy system disruptions.

2. Unequal Distribution of both Benefits and Burdens of the Energy Transition

Just as climate change disasters do not affect all populations in the same proportion, the benefits and burdens of the energy transition are not equally distributed among all citizens. For example, low-income households spend a much higher proportion of their income on electricity bills due to living in energy-inefficient homes, and barriers from adopting cleaner and more affordable technologies. Lack of means and opportunities (e.g., access to adequate housing, education and job centers, transport, etc.) stems from underlying and interconnected systems (e.g., economic, institutional systems, etc.) that perpetuate such injustices and social discrimination, as highlighted by social justice theory.

However, to ensure the effectiveness and long-term viability of the energy transition in cities and all government levels, the representation and involvement of all populations is crucial. This is in line with UN SDG 7 Universal access to energy and SDG 11 Inclusive and sustainable cities. By studying how urban energy policy addresses, or creates and exacerbates different forms of social inequality and injustice among vulnerable groups globally, this research aims to answer the following question: how to make the energy transition more just and equitable in cities?

3. Energy justice, Just Transitions, and Urban Energy Transition Theories

Both energy justice and just transitions theory emerged to address the systemic inequitable outcomes and decision-making processes related to energy systems. Energy justice can be defined as a global energy system that fairly distributes both the gains and burdens of energy services, and considers the interests of all populations in an equitable and impartial manner [3]. In energy justice research, two prominent frameworks can be identified. First, energy justice is defined as having three central tenets: distribution, procedural and recognition justice [4]. Second, ten energy justice principles that can be applied to real-world energy problems are identified: availability, affordability, due process, transparency and accountability, sustainability, intragenerational equity, intergenerational equity, responsibility, resistance, and intersectionality [5]. However, energy justice studies are widely focused on the national government level, and hardly examine the urban level. Urban energy transitions theory, on the other hand, although situated at the city-level and widely focused on energy systems, is rooted in highly techno-economic theories such as Technological Innovation Systems (TIS), and the Multi-Level Perspective (MLP), hence overlooking social, political and cultural problems.

4. Knowledge Gap, Study Objectives, Methods, and Conclusion

This study addresses the knowledge gap between the energy justice framework and urban energy transitions theory. By combining energy justice theory with recognized urban policy guidelines by the American Planning Association and UN-Habitat, this study investigates how concepts of equity and fairness within energy systems can inform urban energy policy. To conclude, an urban energy justice framework is proposed, highlighting pathways towards defining contemporary applications of energy justice in an urban setting.

5. References

[1] UN, “World Urbanization Prospects: The 2018 Revision,” 2019.

[2] IPCC, “Climate Change 2023: Synthesis Report.” 2023.

[3] Sovacool et al, “Energy justice: Conceptual insights and practical applications,” Applied Energy, vol. 142, pp. 435–444, 2015.

[4] McCauley et al, “Advancing energy justice: the triumvirate of tenets,” International Energy Law, vol. 32, no. 3, pp. 107–110, 2013.

[5] Sovacool et al, “New frontiers and conceptual frameworks for energy justice,” Energy Policy, vol. 105, pp. 677–691, 2017.

Introduction

Although Li-batteries remain widely used, there is a shift toward alternative energy storage solutions, such as supercapacitors (SCs), which offer high power output, long lifespan, rapid charge–discharge performance, low cost, and eco-friendliness. Supercapacitors often use transition metal oxides, such as nickel oxide (NiO), due to their high theoretical specific capacitance of 2,584 F g^-1, abundance, eco-friendliness, and chemical stability. However, due to NiO’s poor conductivity and structural limitations, it is commonly combined with carbon nanotubes (CNTs), whose high conductivity and large surface area enhance the overall electrochemical performance.

Methods

Herein, spray pyrolysis, a straightforwardand cost-effective method, was employed to deposit pure NiO and a composite of NiO and CNT (CNT@NiO) at 350 °C for use as supercapacitor electrodes. The structures and morphologies of the materials were probed using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FT-IR), and nitrogen adsorption/desorption. The electrochemical performance of the pristine and composite electrodes was tested by cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy in 2 M KOH over a 0-0.7 V potential window.

Results

SEM analysis revealed that pristine NiO nanoparticles had an irregular shape, whereas the composite exhibited a cylindrical morphology. According to Brunauer–Emmett–Teller (BET) analysis, the specific surface area of the composite was measured at 46.51 m² g^-1, which is greater than the 29.55 m² g^-1 observed for pristine NiO, and the average pore diameter was 27.37 nm, surpassing NiO’s 6.10 nm. Electrochemical testing revealed that incorporating CNTs significantly improved the composite’s electrochemical performance. The CNT@NiO electrode supplied an exceptional specific capacitance of 472.4 F g^-1 at 2 A g^-1, far greater than pristine NiO’s 18.92 F g^-1. Additionally, the composite exhibits a 68.8% rate capability (13.6% for NiO) and 93.3% cyclic stability (81.5% for NiO) at 2 A g^-1 after 1,000 cycles, indicating its suitability for high-performance supercapacitors.

Conclusion

These favorable results point to two deductions: (i) CNT@NiO is a promising candidate for energy storage, and (ii) spray pyrolysis is an affordable and effective electrode material fabrication method. Two novelties regarding the use of spray pyrolysis as a fabrication technique are presented in this work: (i) an in-depth discussion on reasons for prolonged cycle life in connection with structural modifications, and (ii) investigation and comparison of the Faradaic and capacitive charge storage contributions of the synthesized materials. Future work will focus on detailed interfacial analysis (e.g., using TEM and XPS) and long-term cycling assessments (>5,000 cycles) in full-cell configurations to further validate the practical application potential. Overall, this study presents a viable path toward locally fabricated, efficient energy storage systems that can drive the adoption of clean energy in Africa and help bridge the global energy gap.

Introduction

The production of non-centrifuged sugar (NCS) is a key agro-industrial activity in rural Latin America, contributing to local economies and employment. However, traditional processing systems rely on inefficient energy practices and fossil fuels, leading to high operating costs and environmental impacts. The integration of renewable energy technologies offers a promising pathway to improve sustainability and energy efficiency in the sector.

Methods

A technical and quantitative assessment was conducted for an NCS production plant with a processing capacity of 100 kg/h of NCS. The proposed hybrid configuration integrates photovoltaic (PV) electricity generation with the thermochemical utilization of sugarcane bagasse to meet the facility’s thermal energy requirements. The electrical demand of the plant was estimated at 17 kW, corresponding to a daily consumption of approximately 264 kWh. This demand profile reflects the operational characteristics of panela processing units, where the milling process operates for 16 hours at a power of 15 kW, while lighting and auxiliary equipment require 2 kW for 12 hours. Based on these conditions, a photovoltaic system with an installed capacity of 48–52 kWp was designed to supply most of the electrical demand. Thermal energy requirements for juice evaporation and concentration processes were estimated at 380 kWh and are supplied through the controlled combustion of bagasse generated during milling, thereby valorizing an internal process byproduct.

Results

Mass and energy balance calculations indicate that processing 1,000 kg/h of sugarcane generates approximately 380 kg/h of bagasse. With a lower heating value of approximately 7.7 MJ/kg, this biomass resource provides sufficient energy to achieve thermal self-sufficiency levels exceeding 90% for juice evaporation and concentration processes. Regarding electricity supply, the proposed 48–52 kWp photovoltaic system, designed using an average solar irradiance of 4.4 kWh/m²/day typical of the Barbosa region, ensures that the plant’s electrical demand is met during operational hours. Overall, the hybrid energy architecture enables an external energy substitution rate between 70% and 80%, resulting in an estimated reduction of 9–12 tons of CO₂ emissions annually. These results demonstrate the potential for significantly lowering the carbon footprint of panela production while increasing energy independence and operational resilience.

Conclusions

The comparative economic analysis highlights clear advantages for the renewable hybrid system. Purchasing electricity from the grid would represent an estimated annual cost of approximately USD 10,500. In contrast, the installation of a 52 kWp photovoltaic system capable of supplying around 80% of the plant’s electricity demand would generate annual energy savings close to USD 8,500, resulting in a payback period of approximately six years. Although the analysis uses the municipality of Barbosa as a geographical reference, the methodological framework and system design are transferable and scalable to other rural agro-industrial contexts across Latin America. Overall, the results confirm that hybrid renewable energy systems combining solar photovoltaic generation with biomass utilization constitute a technically feasible, economically viable, and environmentally sustainable pathway toward the decarbonization of small-scale sugarcane processing industries.

Introduction:

The design of efficient small-scale hydrogen energy storage systems requires balancing hydrogen production rate, electrical efficiency, and system simplicity. This study investigates the energetic trade-off between kinetic performance and energy conversion efficiency imposed by electrolyzer architecture and separator materials.

Methods:

A comparative experimental analysis was conducted using finite-gap (H-cell) and zero-gap electrolyzer configurations for both alkaline and proton exchange membrane (PEM) systems. Zirfon® Pearl 500 diaphragms were employed in alkaline electrolyzers (25 wt% KOH), while Nafion™ 117 membranes were used in PEM systems, operating in an acidic medium (2.55 M H₂SO₄) for finite-gap and deionized water for zero-gap configurations. The polarization behavior, hydrogen production rate, Faradaic efficiency, and apparent cell resistance were evaluated under comparable operating conditions to assess the trade-offs associated with separator properties and electrolyte conductivity.

Results:

Zero-gap PEM electrolyzers exhibited significantly reduced internal resistance and superior kinetic performance, achieving currents from 1.5 to 3.8 A at voltages below 2.5 V, and up to 10 A at 2.8 V. In contrast, finite-gap PEM systems required higher voltages, delivering only 0.73 A at 4 V and 2.86 A at 18 V. Alkaline H-cell electrolyzers using Zirfon® diaphragms showed lower current densities (0.24–0.27 A at 4 V and <2.35 A at 18 V), with improved performance observed using nickel electrodes. At 4 V, acidic H-cell systems with Nafion™ 117 membranes exhibited internal resistance (~5.5 Ω) approximately three times lower than alkaline counterparts (~14.81 Ω), resulting in hydrogen production rates up to 2.3 times higher. However, alkaline Ni/Zirfon configurations achieved higher Faradaic efficiency (~98%) and energy efficiency (~36%), attributed to reduced parasitic reactions and improved gas separation.

Conclusions:

The results demonstrate a clear trade-off between kinetic performance and energy efficiency depending on electrolyzer architecture and separator choice. While acidic PEM systems favor higher reaction rates due to lower internal resistance, alkaline systems provide superior energy utilization. This study establishes a comparative framework for selecting electrolyzer configurations tailored to decentralized hydrogen storage, highlighting the critical role of separator properties, ionic path length, and electrolyte conductivity in determining system performance.