Hot Topics at the 18th SDEWES Conference in 2023: A Conference Report

Abstract

The present paper reviews the hot topics at the 18th Conference on Sustainable Development of Energy, Water, and Environment Systems (SDEWES) held from 24 to 29 September 2023 in Dubrovnik, Croatia. The selected papers, including aspects of biomass energy application, energy saving in building, low-carbon development and climate change adaptation, hybrid application of sustainable energy, energy storage system, and other measures in sustainable development, are published in this Special Issue of Energies. The 12 selected papers in Energies and corresponding literature that relates to the above topics and has been published in the most recent year are reviewed. In particular, diversifying energy applications are trending towards comprehensive integration with more efficient clean energy capacities for global energy procurement, which can also offer increased resilience and sustainability, reduce reliance on fossil fuels, mitigate environmental impacts, and enhance energy security through a mix of renewable sources and innovative technologies. Meanwhile, energy storage has become increasingly important, which can manage the intermittent of renewable energy, stabilize the grid, and improve energy security and resilience against disruptions. In addition to conventional solar energy storage, biomass energy storage, building energy storage, water storage, etc., can also reduce the overall energy costs, deferring investments in additional transmission infrastructure.

1. Introduction

The 18th Conference on Sustainable Development of Energy, Water, and Environment Systems (SDEWES) took place from 24 to 29 September 2023 in Dubrovnik, Croatia [1]. SDEWES 2023 gathers 646 scientists, researchers, and sustainable development experts from 58 countries and 6 continents. A total of 507 attended in person, and 137 participated online. The event featured 568 oral presentations and 56 poster presentations, spread across 51 onsite regular sessions and 71 online sessions, along with 13 special sessions. Additionally, there were 4 keynote plenary lectures and 3 panels delivered by renowned experts. The Project Exchange Event, a highlight of the onsite program, showcased 28 projects and generated significant interest among attendees.

Through the organization of the SDEWES conference, scholars can engage with international peers, not only showcasing their latest achievements on a global platform but also gaining access to cutting-edge information in their field and understanding the trends in the energy sector, further promoting the development of related theoretical and practical research. For example, through presentations, Q&A sessions, and interpersonal interactions at the conference, scholars can gain academic nourishment that differs from what they would receive from reading books and papers, which can inspire them. Additionally, the SDEWES international conference helps scholars gain recognition and acknowledgment from their international peers, enhancing their personal academic standing and increasing their international influence. Furthermore, the SDEWES conference has the potential to narrow the research gap between countries, promoting the specialization and standardization of development in the energy field.

The cooperation between the Energies and SDEWES conference is robust and mutually beneficial. Energies publishes selected papers from the SDEWES conference after qualification, showcasing cutting-edge research in sustainable development, energy, and environmental sciences. This collaboration also enhances the dissemination of conference findings to a broader academic audience and promotes ongoing dialogue in these critical fields.

This editorial aims to review selected papers in this Special Issue, providing more reflective, opinionated, and focused guidance to readers. The present paper also summarizes the area of accepted papers in this Special Issue between Energies and SDEWES and reviews the previously related publications reported in the SDEWES series. This paper could provide context and highlight key themes and topics covered, express the journal’s stance on current trends and developments, and engage readers with certain practices. The most recent studies (especially published in 2023 and 2024) with the topics of biomass energy application, hybrid application of wind and solar energy, energy storage systems, emission reduction and climate change, energy saving in buildings, etc., are included.

2. Biomass Energy Application

Biomass energy continues to play an important role in global energy supply [2]. Biomass energy is derived from organic matter, which contains carbon absorbed through photosynthesis. When biomass is used for energy generation, the carbon is released during combustion and returns to the atmosphere [3]. With increased biomass production, an equivalent amount of carbon is absorbed, making modern biomass energy a nearly zero-emission fuel [4]. Biomass energy is the largest source of renewable energy globally, accounting for 55% of renewable energy and over 6% of the global energy supply [5]. According to the Energy Information Administration (EIA) in the United States, biomass energy is projected to constitute 2.27% of renewable electricity generation in 2024, decreasing to 2.06% by 2025. In 2023, biomass energy generation reached 21.6 billion kilowatt-hours (kWh), which is expected to rise to 22.8 billion kWh in 2024 and slightly decrease to 22.5 billion kWh by 2025 [6]. The power sector had 2.9 gigawatts (GW) of waste biomass capacity and 2.3 GW of wood biomass capacity at the end of 2023, with these capacity levels expected to remain stable [7]. Main research on biomass energy includes efficient biomass burners and reactors [8], renewable energy [9] and usable material production [10], waste disposal [11], etc.

Bioethanol burners in Europe are expanding but are mainly seen as design elements, with their potential in household energy systems often overlooked [12]. J Ryšavý et al. [13] proposed a method to compute the heat output and flue gas composition of a bioethanol fireplace with a vortex flame. Three ethanol-based fuels were tested using a burner with various regulation rings. Heat output ranged from 3.97 to 2.22 kW without rings, decreasing by 35–40% with rings. The rings extended combustion time by up to 195%. They positively impacted CO emissions but negatively affected NOx levels. Costa and Piazzullo [14] developed a three-dimensional computational fluid dynamic model of a syngas-fueled internal combustion engine in order to compare engine performance considering four biomasses. Two woodchips of different moisture content and Lower Heating Value were considered (Case A: 11.2 w/w% and 15.7 kJ/kg; Case B: 8.9 w/w% and 16.2 kJ/kg), as well as a mix of woodchip and briquettes of olive pomace and pre-processed green wastes (Case C: 8.4 w/w% and 18.3 kJ/kg; Case D: 12.4 w/w% and 25.5 kJ/kg). They focused their study on the effect of the inert species in the syngas composition. In fact, the inert species reduce the primary energy that can be exploited through combustion, but they also have a thermal insulation effect on the cylinder walls. This could be useful to increase the average value of the combustion temperature and reduce the heat losses. The authors state that it is important to evaluate the effect that such inertia has on the thermal dynamics and the overall efficiency of the conversion process. At this aim, the presented 3D model, based on a detailed kinetic mechanism of combustion, can reproduce the combustion cycle of a co-generative engine fueled by syngas with different compositions as a function of considered biomasses. Experimental measurements were performed under real operations to validate the results of the numerical results achieved by the proposed model. The main conclusions show that the optimal global conversion efficiency of an internal combustion engine-based plant supplied by biomass is obtained in the case of high-quality syngas with low inert gases. In fact, the biomass in Case C, with the highest N₂ mass fraction equal to 73.85%, achieves the lowest global conversion efficiency of 70.5% with respect to the biomass in Case B, with the lowest N₂ mass fraction equal to 62.45%, achieving the highest global conversion efficiency of 79.4%. However, the inert gases decrease the syngas lower calorific value but simultaneously have a beneficial insulating effect along the liner walls.

The latest Intergovernmental Panel on Climate Change (IPCC) highlighted the ineffectiveness of current CO₂ reduction efforts in achieving the 1.5 °C limit, necessitating radical measures [15]. Baldelli et al. [16] investigated the enhanced hydrogen production from wet residual biomass by integrating anaerobic digestion with thermochemical processes. This method is part of a hybrid power supply, combining an electric grid, a photovoltaic (PV) plant, and thermal energy storage (TES). A Simulink/Simscape model shows that this integrated approach improves hydrogen yield to 5.37% kgH₂/kg biomass, significantly better than single processes. TES notably reduces the plant’s grid energy demand by nearly half, demonstrating improved efficiency and independence. The anaerobic digestion of municipal solid waste’s organic fraction and the subsequent biogas production offer environmental benefits, reducing landfill waste and greenhouse gas emissions [17]. Calise et al. [18] analyzed the biogas production in a plug flow reactor, using a MATLAB-implemented simulation model to control process temperature and predict microbial growth and decay. The thermal and biological processes were simulated simultaneously. The reactor’s thermal energy was supplied by solar thermal sources and electrical demand by photovoltaics. The model, integrated into a TRNSYS environment, performed dynamic yearly simulations, showing a 35% thermal and 50% electrical demand met by renewables, with a payback time of about 5 years. Calise et al. [19] investigated a renewable Power-to-Gas system using photovoltaic and anaerobic digestion technologies to produce synthetic renewable natural gas. Biogas from municipal waste, including 35% CO₂, was upgraded and combined with hydrogen from solid oxide electrolysis in a multistage methanation reactor. A novel control strategy could ensure its stable operation and the MATLAB models for the electrolyzer and reactor were integrated into TRNSYS for year-long dynamic simulations. It was indicated that the thermo-economic analysis considered the oxygen sales, revealing significant primary energy savings (30.33 GWh/year) and CO₂ reductions (6330 tCO₂eq/year). Economic viability was proven to be strong with a 2.63-year payback, bolstered by oxygen sales. System efficiency reaches 0.75, notably high for such configurations.

On the other hand, converting bio-waste into valuable aromatics represents a sustainable approach to waste management and resource utilization, offering economic, environmental, and societal benefits [20]. Valizadeh et al. [21] explored how to convert sewage sludge into valuable aromatics (BTEX) via catalytic pyrolysis using modified HZSM-5 catalysts under different conditions. HZSM-5 yielded more BTEX in a methane medium (20.91 wt.%) compared to HY (17.24 wt.%) and Hβ (11.70 wt.%) due to its proper acidity and higher methane activation. Under methane decomposition, HZSM-5 produced 27 wt.% BTEX, enhanced by surplus in situ hydrogen and methane involvement. Using Ni/HZSM-5, Mo/HZSM-5, and Ni-Mo/HZSM-5 increased BTEX might yield further due to improved acidity and physical properties. This method could offer an efficient way to convert sewage sludge into valuable biofuels. Meanwhile, ash management when combusting biomass is crucial for optimizing efficiency, protecting the environment, and maximizing the potential benefits of biomass energy production [22]. Zhu et al. [23] conducted a combined experimental and modeling study on ash melting-induced slagging in biomass-fired boilers. An ash viscous deposition model based on ash fusion, viscosity characteristics, and the critical velocity criterion was developed. Sampling from a 130 t/h biomass-fired boiler was applied to validate the model using ANSYS FLUENT with a user-defined function and dynamic mesh. Results indicated that smaller particles (20 μm) led to less viscous adhesion. Deposition efficiency and growth correlated with temperature, with the top furnace dominated by viscous deposition of molten ash. In the high-temperature superheater, viscous deposition accounted for only 20.1%, suggesting a shift to gaseous condensation and fly ash capture.

3. Energy Saving in Buildings

In 2023, the European Union made the latest amendments to the Energy Performance of Buildings Directive (EPBD), aiming to improve building energy efficiency and ultimately achieve the goal of zero carbon emissions in the building sector [24]. Member states will also ensure that the average primary energy consumption of residential buildings is reduced by 16% by 2030. In 2024, the Chinese government released the action plan of 2024–2025 Energy Conservation and Carbon Reduction [25], which is an important initiative aimed at reducing energy consumption and carbon dioxide emissions. The building sector, as one of the main areas of energy consumption and carbon emissions, has been assigned significant tasks and goals.

The fifth-generation district heating and cooling systems are gaining popularity for their low-temperature heat transfer fluids, enhancing renewable energy use. These systems require precise design and management, achievable with advanced simulation and optimization tools [26]. Buonomano et al. [27] introduced a simulation tool for designing and optimizing these systems, capable of assessing each building-plant system’s impact on the entire network. The tool was validated using an experimental prototype, demonstrating significant primary energy savings (10.3 MWh/year, 6.5%) through a predictive control logic optimizing water loop temperature. The residential and tertiary sectors account for 29.5% of a city’s energy use. The Basque Government’s Building Quality Control Lab promotes innovation and sustainability in buildings through an experimental facility [28]. Mancinelli et al. [29] measured the domestic hot water and heating systems using a condensing boiler and aerothermal heat pump, analyzing efficiency and costs using thermo-economics. This approach addresses maintenance challenges in building thermal systems by utilizing real sensor data for cost-effective improvements and maintenance solutions.

Air-cooled chillers in commercial sectors benefit from variable-speed drives in compressors and condenser fans for improved part-load efficiency [30]. Current systems vary fan speeds to maintain set condensing parameters. Catrini et al. [31] explored the energy-efficient control strategies and highlighted the chiller performance under different fan speeds and loads. Tests on a 50-kWc chiller reveal energy efficiency ratio increased with fan speed, peaking at an 8.8% gain. Findings showed that by varying fan speed impacts capacity and efficiency, optimal results at 980 rpm could be reached for some configurations. Meanwhile, implementing tailored fan control strategies could save up to 12.1% in energy use, emphasizing potential savings through optimized management. Zini et al. [32] introduced a systematic approach for developing a user-friendly monitoring method based on predictive models applied to HVAC systems in an Italian healthcare facility. This method could detect subtle changes in energy demand efficiently. Despite requiring extensive data collection, its reproducibility and integration potential into existing management systems make it a practical tool for enhancing building energy efficiency. Cirillo et al. [33] utilized the SUSSTAIN-EL rotary heat pump, employing NiTi SMA and air as the heat transfer medium, which was evaluated using a 2D finite element model under various conditions. Results indicate strong energy performance in both open and closed-loop configurations, supporting its potential for widespread use in cooling and heating applications at a larger scale.

A predictive model can forecast a building’s energy consumption based on various factors, such as historical data, building characteristics, usage patterns, and climate conditions [34]. Calise et al. [35] compared dynamic simulation and semi-stationary models for assessing residential building energy demands. Semi-stationary models, mandated for energy labeling, often overestimate savings compared to dynamic simulations using TRNSYS software version 18. They focused on a Naples residential case that demonstrated significant discrepancies: the semi-stationary method showed a 64.7% higher primary energy saving in comparison to the more accurate 43.2% from dynamic simulation. However, the limitations of regulatory methods in accurately estimating energy efficiency were also proposed, and the importance of adopting dynamic simulations was further investigated for more reliable energy assessments and economic evaluations in building energy performance studies. Olympios et al. [36] introduced the Design and Operation of Integrated Technologies (DO-IT) framework, aiming at guiding technology investments in integrated energy systems within buildings. Noteworthy for its integration of open-source modeling tools and a novel optimization approach, case studies demonstrated that over 25% cost savings compared to conventional grid-importing could be approached, and benefits such as intra-day electricity demand stabilization and seasonal energy storage via green hydrogen could be reached. The proposed framework could support the decision-making for stakeholders ranging from end-users to policymakers interested in low-carbon building technologies and relevant policies.

Dang and Voskuilen [37] presented a study regarding a parametric modeling approach for the energy retrofitting of heritage buildings, considering the Amsterdam city center as a case study. The aim of the study is to identify the minimum requirements for preparing the residential stock to use heat pumps in combination with local lower-temperature heat (LTH) from open water, air, soil, or anthropogenic residual heat sources. The developed approach consists of 4 different steps: (i) Geographical Information Systems (GIS) based on the analysis of the residential building stock providing data regarding the size and topology of the buildings, construction date, residential function; (ii) Identification and characterization of the building archetype stock based on the identification of the U values of glazing and opaque surfaces, infiltration rates and HVAC systems; (iii) Energy simulation using parametric modeling tools based on the definition of the 3D model of the building geometry model, the definition of the climatic data by the software Open Studio 3.6 and EnergyPLus 23.1. The main outputs of the energy modeling were the heating and cooling demand of the buildings, the estimation of the energy use intensity and the peak heating value; and (iv) Identification of retrofit packages and potential performance based on the monumental restrictions for each building archetype. For each building typology, the model computed 3024 retrofitting packages, totaling 9072 retrofitting combinations. For each package, the model estimates if the archetype is ready for lower-temperature heat using two criteria: criterion (A) the annual space heating demand is lower than 50 kWh/m²; criterion (B) the peak heating demand of the living room space is lower than the total heating capacity for a return temperature of 45 °C and a supply temperature of 55 °C. This condition ensures that the living room is comfortable when heated at LT. By the versatility of the model, the most impactful combination of retrofitting options can be quickly identified so that the energy-saving measures are progressively implemented by owners depending on the budget availability. This approach allows users to detect investment opportunities to reduce carbon emissions and optimize the collective use of low-temperature heat sources. It is important to carefully consider a series of limitations associated with the presented approach. For example, large deviations between the theoretical and actual demands, especially for buildings with low energy labels, are common in Amsterdam Centrum. To further improve the calibration of the model, research on the behavior of the users and detailed measured data concerning the indoor thermal conditions can be useful. However, the authors state that such models can be useful to contribute to the decision of policymakers about retrofitting strategies on a large scale for sustainable development. However, specific subsidies and incentives could support widescale retrofit initiatives, connecting fast and cost-effective solutions for protected buildings with the private market.

4. Low-Carbon Development and Climate Change Adaptation

With increasing global demand for clean, sustainable energy, the quest for renewable sources is crucial [38]. Reliable and long-established energy styles could reduce reliance on fossil fuels [39], chemical products, district heating, etc., which currently account for the dominant potential of the world’s carbon consumption, thus aiding in climate change mitigation.

The European Union aims to decarbonize chemical production despite current fossil fuel dependency. Divkovic et al. [40] investigated the transition of municipal heating from fossil fuels to low-emission sources. An optimization model using multi-objective linear programming to balance economic and ecological goals was introduced. Please note that for uncertainties like energy prices and efficiency, the model optimized district heating unit sizing and operation under varied growth scenarios and emission factors. Results could offer Pareto optimal solutions, suggesting flexible heat generation strategies and timing for infrastructure commissioning aligned with heat demand growth. Lopez et al. [41] explored the power-to-chemical solution, modeling supply chains and conducting life-cycle analyses for electricity-based polyethylene in Germany, Belgium, the Netherlands, Spain, and Finland. By 2050, local production costs range from 712 to 1406 EUR/t, with imports of electricity-based methanol and polyethylene costing 56–118% and 42–108%, respectively, compared to local production. Noted that the local production could offer 15–61% lower greenhouse gas emissions than fossil-based polyethylene, potentially achieving negative carbon footprints with careful carbon content management. Investment in Southern Europe and North Africa for low-cost electricity and limited land in Central Europe were critical factors.

Meanwhile, petrochemical products such as polyolefin are gradually developing from traditional single-time use to recycling applications in order to reduce carbon emissions to the environment [42]. Jerzak et al. [43] explored converting car tire waste from Krakow into valuable chemical products through pyrolysis at 500 °C in a hybrid reactor. Pyrolysis–gas chromatography–mass spectrometry analysis showed that layering the catalyst and tire waste favored aromatic hydrocarbon formation. The process yielded 40.8% raw carbon black at a high heating rate (500 °C/s). At a slower rate (0.17 °C/s) and higher temperature (800 °C), a similar yield was observed. The resulting carbon black, with high ash content (~50%), mostly SiO₂, needs demineralization for commercial use, but the high SiO₂ content suggests its potential as a catalyst material. Petrovič et al. [44] investigated converting vegetable oil industry residues into valuable products via hydrothermal carbonization (HTC) of hemp cake (HC) and pumpkin cake (PC) to produce hydrochar. The co-carbonization (co-HTC) using whey instead of water was also explored. The high-quality hydrochars indicated significant carbon content and calorific value, and HC yielded more hydrochar, while PC hydrochars were richer in nutrients. Co-HTC with whey improved hydrochar yield, calorific value, and nutrient retention. Calise et al. [45] investigated how to reduce electricity and space heating demands in residential districts using renewable energy communities. The paper proposed an integrating district heating network and a power-to-heat control strategy without major apartment refurbishments. Using TRNSYS software, the dynamic models were proposed in comparison to conventional layouts, showing significant profitability with a 2.2-year payback period and 38% primary energy savings.

Carbon dioxide may cause heat to increase within the atmosphere, leading to glacier melting, sea level rise, extreme weather, and other consequences and significantly impacting the Earth’s climate [46]. Bielka et al. [47] focused on reducing anthropogenic carbon dioxide emissions through Carbon Dioxide Capture and Storage (CCS). Technologies were explored for CO₂ capture, separation, dehydration, and transport, emphasizing selection criteria for geological storage sites. A dehydration and compression system was designed for captured CO₂, featuring multistage compression and cooling with triethylene glycol (TEG) for dehydration. Energy efficiency was optimized by managing gas temperatures and integrating heat recovery. The study also highlighted the importance of reducing operational costs and enhancing plant efficiency in CCS installations. Gerres et al. [48] discussed the energy-intensive production and emission challenges of key industrial materials like steel, cement, and chemicals, stressing the need for decarbonization to limit global warming. The lack of commercially viable low-emission processes and the necessity for policy support were concluded, which will promote the adoption over the coming decades. The TRANSid model was introduced to analyze conditions for scaling climate-friendly material production. A case study on cement illustrates how the model optimized the carbon capture technology investment by 2050 and suggests its potential application across other industrial sectors for policy analysis.

Aiming to outline a cost-effective pathway for Italy’s energy system decarbonization by 2050, crucial actions were achieved to zero-emission goals. Pastore et al. [49] explored various cost scenarios for renewables like photovoltaics, wind power, and lithium-ion batteries using the H2RES model. Results indicated that Italy could achieve a 100% renewable energy system, with Power-to-X technologies playing a pivotal role in balancing intermittent generation. The study also analyzed the sensitivity of long-term energy models to cost forecasts, especially for batteries and hydrogen technologies, influencing optimal strategies for national decarbonization efforts. Mainar-Toledo et al. [50] investigated the environmental impacts on geothermal power plants (GPPs) with high non-condensable gases (NCGs), focusing on Kizildere 3 U1 in Turkey where CO₂ constitutes 99% of NCGs. An innovative CO₂ reinjection strategy was proposed to mitigate atmospheric emissions. Life-cycle assessment (LCA) calculations were conducted to assess current (baseline) and potential (reinjection) environmental impacts. The construction showed significant environmental impact due to materials, while operational impacts stem from geothermal fluid composition. Meanwhile, the CO₂ reinjection could prevent 1700 tons·year⁻¹ emissions at the pilot site, constituting 10% of total emissions over the plant’s lifespan.

Weger et al. [51] identified the barriers and possibilities for the development of short-rotation coppice as an agroforestry system for adaptation to climate change in central European conditions. They presented an economic methodology for assessing the competitiveness of biomass production in agroforestry systems under the current identified market conditions. The paper compared different selected environmental, economic, and production features of agroforestry systems in a standard (alley cropping) and a newly proposed design with fast-growing trees grown in short-rotation coppice. Agroforestry systems improve the parameters of biodiversity by the abiotic conditions diversification. Agroforestry systems, along with other woody plants and perennial crops, such as windbreaks, perennial energy crop plantations, short-rotation coppice, and hedges, generate a so-called transition ecosystem between dominant ecosystems. Agroforestry systems provide suitable conditions for several organisms. Among these techniques, short-rotation coppice is the farming of fast-growing trees for species such as poplars and willows, which can also be implemented in agroforestry systems. Selected tree species with stump coppicing capability can be used to generate woody biomass for material and energy purposes. To identify the barriers and potentials of these modern management methods, two case studies of the Czech Republic were investigated. Adopting the minimal price principle from the farmer’s point of view, the economic efficiency of agroforestry systems was evaluated. In particular, (i) alley cropping agroforestry systems with cherry and walnut trees in single rows (tree strips) with 28 m-wide arable fields between them (crop strips), and (ii) coppiced tree belt agroforestry systems with poplars and willows and 25 m-wide arable fields between them (crop strips) were analyzed. The production characteristics of trees were evaluated considering the experimental plantations of the Czech Republic of previous research projects. The economic data were achieved from long-term experimental plantations and combined with the present operation and energy costs. Results show that agroforestry systems with short-rotation coppice achieve comparable production and economic results as annual crops when suitable sites with appropriate quality of agronomy were selected. The products from short-rotation coppice, mainly in terms of energy woodchips, can be evaluated as strategic, increasing the producer’s independence from purchased energy fuels. Without a significant subsidy (EUR 4353/ha and EUR 754/ha/year (5 years) alley cropping agroforestry systems with fruit trees would not have a high economic feasibility for farmers due to the maintenance costs in the first years. Without subsidies, fruit prices with respect to the values of 2020 would need to be increased by up to about 55%. The results achieved in this analysis are applicable to other countries with similar growing conditions (Slovakia, Poland, and Western European countries).

Wagner et al. [52] presented the activities and results achieved by the “Schools4Future” project in order to promote sustainability and climate protection in German schools. This project aims to promote educational work for students to understand the effects of climate change and measures to reduce greenhouse gas emissions. In this framework, schools belong to a sector that features high energy consumption: school buildings are frequently in bad condition, an important amount of energy is wasted, and renewable technologies are hardly exploited. The “Schools4Future” project is focused not only on the reduction of greenhouse gas emissions in schools but also on the comprehensive involvement of students and teachers in the transformation toward a sustainable future. They were able to draw up their own CO₂ balances, detect wasted electricity, and identify weaknesses in the building, determining the potential for using renewable energies. The emissions data regarding the school cafeteria, school trips, and paper consumption were identified by the students themselves to increase their awareness about the climate balance of the different sectors of the school. The paper shows the results of the carbon footprint assessments of the 12 pilot schools. A very crucial factor is the time of the last refurbishment of the building and the age of the building which leads to higher or lower greenhouse gas emissions. However, the emissions deriving from building energy use surpassed the emissions related to the other sectors. To compare all the schools, the CO₂ emissions per student were estimated. The most climate-friendly school emits only about 300 kg of CO₂ per student per year, while the least climate-friendly school emits almost 1000 kg of CO₂. Several energy actions were identified to reduce greenhouse gas emissions. For example, the high emissions from meat dishes decreased by adjusting the menu with meat served only two days a week. The food waste was avoided by flexibly adjusting portion sizes. To reduce energy consumption, the fit of the windows was improved by replacing the sealing lips. In terms of mobility for class trips, several schools avoided air travel and switched to more climate-friendly modes of transport.

Díaz and Bartolomé [53] presented a comprehensive review of climate change adaptation indicators for urban design. They state that there is a necessity to enhance risk management, taking into account climate resilience in urban policy design. Although improved urban climate monitoring, relatively few scientific papers are focused on climate change adaptation in urban areas. In fact, it is crucial the identification adaptation indicators that represent a basis for decision-making, as well as methods for the evaluation of the effectiveness of implemented and planned measures in municipalities. The identified indicators are as follows: vulnerability, resilience, universal thermal climate, exposition, habitability, cooling effect index, comfort index, bare soil index, dangers indicators, urban sustainability, vegetation indicators, precipitation index, thermal comfort index, blue city indicators, city blueprint index, social indicators, thermobioclimatic index, index of discomfort. All these indicators can help urban planners and policymakers identify areas of vulnerability and develop effective adaptation and mitigation strategies to protect urban populations from the impacts of climate change. The results of the review also suggest that research and publication on the topic of climate adaptation is more prevalent in Asia and Europe compared to other regions of the world. In the climate adaptation literature, climate change, urban heat, and floods are the most studied risks. On the other hand, drought and cyclones are less investigated in published research, even though they are sometimes an effect of climate change. Climate change was considered to be a general risk. Nonetheless, the interconnectedness between risks is not assessed (e.g., an increase in temperature results in an increase in droughts).

5. Hybrid Application of Sustainable Energy

The integrated application of wind energy [54], solar energy [55], hydraulic energy [56], hydrogen [57], geothermal [46], etc., is a key approach to achieving a sustainable energy future and addressing climate change. Through technological innovation, policy support, and market demand, the development and application of these renewable energies can be further promoted.

Energy and water scarcity are global challenges requiring collaborative solutions. Floating photovoltaic systems (FPV) are increasingly accepted due to benefits like land preservation, water saving, and enhanced efficiency [58]. Elminshawy et al. [59] compared FPV and land-based photovoltaic systems (LPV) in a Mediterranean climate, analyzing electrical and thermal performance, evaporation mitigation, and economic aspects at different tilt angles. It was shown that a 10° tilt FPV reduces module temperature by 7.24 °C and evaporation by 83.33%, outperforming a 20° LPV by 8.92% in power generation. FPV could generate electricity at USD 0.059/kWh, saving 2.19 m³/m² of water vapor and mitigating 5.20 kg CO₂/m² annually. Marrasso et al. [60] focused on a mixed-use district in Southern Italy’s industrial zone, optimizing energy systems with evacuated tube collectors, photovoltaic panels, wind turbines, and a hydrogen electrolyzer. Two new assessment indicators, including the Positive Balance Check and Carbon Neutrality Check, were proposed to evaluate energy and environmental impacts. The district achieves surplus renewable energy and carbon neutrality, emphasizing the importance of multi-vector systems in decarbonizing electricity and heating.

Europe anticipates a rapid shift to renewable energy, including a 25-fold increase in offshore wind power capacity by 2050. Although the Mediterranean Sea’s potential is less significant than the North and Baltic Seas, it will host numerous wind projects soon. The development of offshore wind farms begins with identifying suitable sites, a complex task requiring efficient methods due to diverse criteria. Polykarpou et al. [61] introduced a new optimization approach is introduced for this purpose, integrating technical factors like wind speed, water depth, and proximity to shore. Applied in the central Aegean Sea, known for its high wind energy potential, the method identifies extensive areas suitable for floating wind structures. It matches the accuracy of exhaustive search methods but with lower computational costs, highlighting its potential for broader spatial analyses and as a decision support tool in offshore wind farm development. Neshat et al. [62] explored the integration of hybrid offshore renewable energy platforms, combining wind turbines and wave energy converters (WECs) on single foundations to optimize energy production and reduce costs. The DMOGWA was introduced, which is a novel multi-objective swarm optimization method, enhancing solution quality by balancing WEC power output and wind turbine nacelle acceleration. Compared with existing methods, DMOGWA demonstrated superior convergence speed and solution quality, significantly improving power output (up to 138.5% increase) and reducing nacelle acceleration (41%) across operational conditions. Ahmed et al. [63] assessed the benefits of wind energy, such as increased energy output, extended operational life, and contributions to net-zero goals. Technological advancements such as variable hub heights and upgraded turbine classes, optimize repowering potential were concluded. On the other hand, the time-limited permits and financial disincentives, necessitating stable financial support, and adaptive energy laws were also proposed as challenges. Environmental sustainability was emphasized through recyclability and community collaboration. Martinez et al. [64] investigated the economic feasibility of floating offshore wind projects in the European Atlantic and Mediterranean regions using site-specific levelized cost of energy analysis. The energy costs, detailing expenditures, and calculating production based on local wind conditions were studied. It should be noted that the key cost drivers include wind resource availability, turbine quantity, rated power, and infrastructure expenses. Regions with the lowest energy costs (~95 EUR/MWh) included Great Britain, Ireland, the North Sea, the NW Iberian Peninsula, the Gulf of Lyon, and the Aegean Sea, owing to their high wind potential. Rusu [65] investigated the past and future wind power dynamics in the Mediterranean until the century’s end, using data from two Regional Climate Models under scenario RCP4.5. The historical wind data from 1976 to 2005 was analyzed, compared to ERA5 data, and examines average and extreme wind conditions for 2041–2070 and 2071–2100.

Green hydrogen shows promise in decarbonizing energy systems and mitigating deforestation by replacing biomass fuels. Integrating waste heat recovery from hydrogen production into district heating networks can enhance decarbonization efforts despite challenges like the cost of hydrogen transfer to distant industries. Moradpoor et al. [66] assessed the aforementioned complexities, highlighting the potential benefits of coupling hydrogen production with district heating networks. The author quantified the national benefits of efficient building renovations and explored how integrating hydrogen could amplify these gains. Results indicated modest economic benefits from waste heat integration but significant potential to reduce biomass use. Transport costs for hydrogen in Finland were estimated at 0.1–0.2 EUR/kg, underscoring both challenges and opportunities in sustainable energy transitions. Weiss et al. [67] explored the integration of Hydrogen Direct Reduction of Iron (HDRI) with renewable electricity for low-carbon steel production. A new techno-economic model for optimizing Power-to-X plants applied to an HDRI plant in Finland under various market scenarios was introduced. It should be noted that the production costs are projected at 373 EUR/t for current conditions and 351 EUR/t for 2025–2030. EU renewable fuel regulations might increase costs by 30–46 EUR/t and impact hydrogen storage requirements. The flexibility of the direct reduction shaft significantly influenced storage needs and overall costs, offering insights crucial for designing future low-carbon steel plants.

Solar power usually requires significant land, leading to intensified land competition [68]. Ferreras-Alonso [69] used the WILIAM Integrated Assessment Model to represent land-use changes due to solar energy expansion. A Green Growth transition in the EU was simulated, targeting highly renewable energy by 2050 and testing various land-use policies. Results indicated that rapid solar deployment without land policies heightens land conflicts and emissions. Solar power could use 1–1.4% of total land, impacting urban areas. Implementing land protection and siting policies could reduce land use by 23% and emissions by 23–47%, highlighting the need for integrated land and energy planning. Parrado-Hernando et al. [70] proposed an hourly energy modeling named Integrated Assessment Models, which is crucial for balancing fluctuating sources like solar and wind. By integrating regression analysis for detailed hourly data extraction, experimental design, and computational efficiency could be enhanced. The model incorporated flexibility options to manage variability, reducing potential curtailment from 60% to 30%. By limiting renewable expansion, the model achieved 80% renewable penetration in electricity and a 53% cut in greenhouse gases by 2050, aligning with Europe-27’s goals. Overall, the approach proved effective in simulating energy system dynamics and policy impacts.

Venturini et al. [71] presented a simulation study based on the integration of floating photovoltaic panels (FPVs) with an Italian hydroelectric plant. As authors state, the advantages of this coupling are useful for several issues: (i) the installation of PV modules on a water basin leads to an easier cooling of the modules due to humidity, which, in turn, results in an increase of the module efficiency; (ii) the use of floating PV panels avoids the competition with agricultural and green areas; (iii) the installation of FPV plants does not alter the radiation balance (considering that the albedo effect on land typically ranges from 20 to 30%, whereas the reflection from PV modules does not exceed 5%); (iv) the use of FPVs results in partial coverage of the water leading to a reduction in water evaporation and an increasing of the water reserve for hydropower production. In the paper, the model description for the evaluation of the avoided water evaporation and electricity production is reported. The hydroelectric power plant located in Cosenza (South of Italy) is selected as a case study. The models can evaluate the additional electric energy production of the FPVs and the influence of the non-evaporated water on the electric energy production of the hydroelectric plant as a function of the basin surface coverage. Results of the simulation show that the extra production of the hydroelectricity plant is very small if compared to the non-FPV system, reaching about 3.56% for 25% basin surface coverage. At low coverage values, the yearly PV electric energy production is noticeable. The expected gain in electricity production in the case of 25% basin surface coverage with the FPV plant rises to 391% of that of the actual hydropower plant. If a vertical axis tracking system is installed, this gain increases to about 436%. The economic analysis shows that the production costs of FPV systems (USD/kWh) are comparable to those of land-based PV (LBPV) plants, becoming smaller in the case that a tracking system is installed. For 15% coverage of the lake, the optimal solution is detected. In this case, the levelized cost of electricity for the LBPVs is 0.030 USD/kWh, and for the FVPs, without and with a tracking system, it is equal to 0.029 and 0.032 USD/kWh, respectively.

Fatigati and Cipollone [72] presented a work based on the experimental validation of an ORC-based micro-cogeneration system exploiting solar thermal collectors to simultaneously generate electricity and domestic hot water (DHW). Their experimental setup includes two electrical resistances to heat a 150 L thermal storage tank able to produce the same thermal energy obtained by 15 m² flat plate solar collectors. The storage tank produces thermal energy to supply the evaporator of an ORC unit of about 150 W based on R245fa and domestic hot water. The system is designed to match the electric and thermal demands of a residential user. They present a technological solution allowing the improvement of the performance of solar ORC-based systems to avoid the operation under off-design conditions due to the intermittence of solar thermal energy. This leads to a hot source with a very variable temperature during operation. Therefore, a dual intake port system was coupled to the scroll expander. Such a system allows the widening of the angular extension of the intake phase to obtain the optimal built-in volume ratio between the intake and exhaust volume for several operating conditions. This allows the achievement of a higher mass flow rate for a given pressure difference at the expander side as well as for a given mass flow rate, a reduction of the expander intake pressure with a positive benefit on scroll efficiency. By the experimental setup and the validation of the related modeling developed in this work, the authors aim to evaluate the effect of the dual intake port system when coupled with a solar-assisted ORC-based micro-cogeneration system. Results show that the dual intake port system can elaborate a higher mass flow rate. For a given pressure difference between the intake and expander sides, the mass flow rate of the dual intake port system is 32% higher than the mass flow rate of the single intake port scroll system. This leads to an increase of 10% of the average power and an improvement of up to 5% of the expander’s mechanical efficiency.

6. Energy Storage Systems

As aforementioned, though renewable energy sources such as solar, wind, hydrogen, and water are complementary in terms of time and space, energy storage technology is crucial to overcome their intermittency and instability [73]. The energy transition requires integrating non-dispatchable renewable electricity with storage systems and demands flexibility to manage variability and uncertainty [74].

Thermal energy storage is regarded as the most potential energy storage method in sustainable energy systems. Scholliers et al. [75] investigated the high-temperature aquifer thermal energy storage (HT-ATES) systems’ integration into district heating networks for decarbonization. Through a structured approach involving five steps, the result identified the key factors, such as construction and operational impacts, including drilling and energy requirements for heat pumps. Noted that the decision-making transparency using life-cycle assessment and costing could be approached, guiding further research and practical enhancements for sustainable implementation of HT-ATES systems in district heating. Ortiz et al. [76] addressed the challenge of electricity storage for renewable energy by proposing a modular thermochemical energy storage (TCES) system based on the calcium-looping (CaL) process integrated with various renewable sources and applications, using electrical heaters for charging. It stored energy through the reversible reaction of calcium carbonate and CO₂, achieving high energy density and discharge temperatures. Simulations showed promising performance, with energy densities exceeding current molten-salt systems and competitive storage costs. This approach indicated the potential to enhance renewable energy integration and support global energy transition goals. Liu et al. [77] investigated the low-temperature thermochemical energy storage (TCES) using salt in porous matrix composites (CSPMs), exploring their suitability for fluidized-bed systems. CSPMs with salts like CaCl₂ in a mesoporous silica matrix were tested for fluidization behavior and energy storage metrics. Results show efficient fluidization with a minimum velocity of 0.01 m/s and significant heat discharge capability, particularly in CaCl₂/CMS composites, offering an energy storage density of 1508 kJ/kg. Stable performance across multiple cycles suggests potential for practical application in TCES. Mancinelli et al. [29] discussed the shift towards electrifying heating for sustainability, highlighting heat pumps as superior to traditional heaters and boilers. The environmental drawbacks of common refrigerants were emphasized, which could promote CO₂ as a natural alternative with improved efficiency. The CO₂ transcritical heat pump model with thermal energy storage (TES) to enhance performance by modifying the thermodynamic cycle was proposed, specifically targeting dairy processes for efficiency gains over existing methods like ejector-expansion systems. Meanwhile, Phase-Change Materials (PCMs) can absorb heat using latent heat, which is almost one order of magnitude higher than the conventional sensible heat of materials. Miccoli et al. [78] presented work that deals with a novel application of PCMs in a personal cooling system. The investigated cooling system was experimentally analyzed by changing the configuration of the PCM-based condenser. In the experimental layout, the commercial PCM, namely the RT35HC, produced by Rubitherm Technologies GmbH, was used. A simulation model was developed for the numerical simulation to optimize the operation of the proposed PCM condenser using a commercial solver. In particular, the numerical model is based on the enthalpy–porosity method and has been solved by means of the finite element commercial software COMSOL Multiphysics 5.5 using second-order schemes. In this model, the PCM melting process in the condenser was investigated in detail. The results identified as main characteristics of the investigated personal cooling system, the cooling capacity, and operating autonomy. Among the various arrangements studied, the best configuration in terms of refrigeration power and autonomy is the one featured by the refrigerant compressor at 50% power and the highest heat transfer surface of the heat exchanger.

In renewable energy systems, other storage methods include battery storage, pumped hydraulic energy storage, compressed air energy storage, mechanical energy storage, etc., which also play important roles in improving energy and economic efficiency [79]. Li et al. [80] explored to use of Carnot batteries as crucial for renewable energy integration, emphasizing high efficiency for sustainability. A novel Rankine Carnot battery using salt hydrate thermochemical storage was proposed, with models established for both basic and recuperator-enhanced versions (10 MW/5 h capacity). Results showed enhanced performance in the latter due to recuperator heat recovery, yielding efficiencies of 64.1% power-to-power and 48.94% exergy, with storage costs of USD 0.1922/kWh. Optimizations included higher heat source temperatures and alternative salt choices to lower costs. The study underscored the potential of thermochemical Carnot batteries for large-scale energy storage applications. Jahanbin et al. [81] explored integrating hybrid hydrogen-based systems into microgrids for sustainable energy solutions. Various solar-driven scenarios using battery and hydrogen storage (gaseous and metal hydride) were examined, employing dynamic simulations and optimization algorithms. Results showed enhanced renewable energy factors with hybrid systems, though the inclusion of batteries reduces hydrogen production via electrolysis. Techno-economic analysis reveals optimized electricity costs (USD 0.376–USD 0.789/kWh) and CO₂ emissions (6.57–9.75 tons), improving by up to 46.2% and 11.3%, respectively, through multi-criteria optimization. The research emphasizes the cost-effectiveness and environmental benefits of transitioning to green hydrogen technologies. Yin et al. [82] addressed the limited research on the self-heating of biomass in large-scale storage by developing a comprehensive modeling framework and conducting experiments. The paper highlighted the model’s effectiveness in predicting self-heating in coal piles and identifying critical factors such as pile height, particle size, and ambient wind velocity. It was revealed that initial biomass moisture content significantly influences microbial reactivity and oxygen consumption. Wheat straw is more prone to self-heating than rice straw. Meanwhile, microbial activity was crucial in the early heat accumulation stages.

Camas-Náfate et al. [83] compared particle swarm optimization (PSO) and gray wolf optimization (GWO) algorithms in modeling a commercial lithium-ion battery. Such algorithms have emerged as promising tools for optimizing model parameters in lithium-ion battery modeling, demonstrating their effectiveness in various engineering applications. The results of the modeling of a proposed lithium-ion battery (NCR18650B battery) were also compared to an experimental setup. The NCR18650B battery was selected as a battery widely used in laboratory studies due to its availability, low cost, and ease of gaining measurable parameters, such as state of charge, current, temperature, and voltage. The developed model relates mathematical equations and electrochemical experimental data, complemented by representation through an equivalent circuit. An RC circuit with the ability to simulate electrochemical features such as temperature inside the cell and the battery’s state of health was selected. The behavior of the battery under different conditions is simulated, considering temperature changes and charge/discharge cycles. The model is implemented using MATLAB/Simulink. The study aims at optimizing the parameter values as well as reducing the root mean square error between the experimental current outputs and simulated ones. The thermal and electric characteristics reported in related works from lithium batteries were considered and integrated into an electrical circuit model. The main results can be summarized as follows. A main factor influencing the model analysis was the battery current. Compared to the non-optimized model, both the gray wolf optimization and particle swarm optimization revealed notable enhancements when terminal voltage was assessed. Superior performance was detected by the gray wolf optimization model. In this case, the reduced root mean square error was 0.1700 (standard temperature condition of 25 °C; a charge ratio of 3.6 C, simulation time of 455 s) and 0.1705 (standard temperature condition of 25 °C; a charge ratio of 3.6 C, simulation time of 10, 654 s). With respect to the particle swarm optimization model, a 42% average root mean square error reduction was achieved. The gray wolf optimization model exhibits enhanced predictive capabilities and slightly lower root mean square error values for the evaluation of the battery current than the error values detected by the particle swarm optimization model. The analysis of algorithm execution time reveals insights into computational complexity, with both particle swarm optimization and gray wolf optimization models showing potential polynomial complexities. Particle swarm optimization exhibited greater-than-linear dynamics, suggesting a polynomial complexity of O(nk). Based on execution times from populations of 10 to 1000, the polynomial complexity of the gray wolf optimization model falls within O(nk) or O(2n)

Agati et al. [84] evaluated the effect of the degree of hybridization and energy management strategy on the performance of a fuel cell/battery vehicle in real-world driving cycles by dynamic simulations. Open-access ARTEMIS and VED databases were used to obtain power demand curves for a hybrid automotive system. Three energy management strategies, twelve configurations, and four values for the degree of hybridization were investigated. Please note that the degree of hybridization represents the share of the total power of the vehicle powertrain supplied by the battery. The investigated energy management strategies were set and defined as follows: (i) Battery Main—BTM: using mainly batteries to satisfy the power demand and fuel cells as backup; (ii) Fuel Cell Main—FCM: using batteries as backup and fuel cells operate continuously; (iii) Fuel Cell Fixed—FCF: using batteries as backup and fuel cells operate within a fixed range. The TRNSYS model can calculate the fuel cell predicted lifespan, battery cycles, H₂ consumption and overall system efficiency. Results show that the battery is mainly stressed in the FCF and BTM energy management strategies, while the FCM energy management strategy uses the battery only intermittently to cover load peaks. This is indicated by observing the state of charge of the battery, which shows different battery stress levels between the BTM and FCF modes. Higher stress levels are detected for the FCF energy management strategy. This is crucial in terms of the reduction of the lifetime of the battery. For the BTM and FCM energy management strategies, it is detected that the fuel cell works with variable power, while in the FCF mode, the fuel cell operates in a range between 90 and 105% of its nominal power to ensure its lifetime. In the BTM and FCM energy management strategies, H2 consumption decreases at almost the same rate as the degree of hybridization increases due to a smaller amount of hydrogen being used to recharge it and a decrease in battery capacity. Conversely, when the degree of hybridization decreases, the FCF energy management strategy results in a larger fuel consumption. In terms of FC durability, with a predicted lifetime ranging from 1815 h for DOH = 0.5 to 2428 h for DOH = 0.1, the FCF energy management strategy performs better. With a predicted lifetime of 800 to 808 h, the FCM energy management strategy has the worst performance, being almost insensitive to the degree of hybridization variation. The results of the simulations considering the performance degradation fuel cell highlighted an increase in hydrogen consumption of about 38% after 12 years.

7. Other Measures in Sustainable Development

The sustainable management of water and energy is crucial for achieving global sustainable development goals, reducing poverty, improving quality of life, and protecting the environment. With population growth and economic development, the demand for water and energy continues to rise, necessitating more efficient and sustainable ways to manage and use these resources. Despite the development of renewable energy and alternative technologies reducing dependence on oil resources, oil will remain an important energy resource for the foreseeable future [85]. Meanwhile, with the growth of the global population and the impacts of climate change, the management and protection of water resources are becoming increasingly important. Sustainable water resource management and reducing reliance on fossil fuels are significant global challenges. As a result, emissions accompanying economic development are still crucial and need to be further investigated [86].

Redutskiy and Balycheva [87] proposed a mixed integer nonlinear programming model for planning capacities and coordinating activities within the petroleum supply chain. There are several methods to improve the energy efficiency in the petroleum sector: (i) technological solutions, such as waste heat recovery for the combined production of heat and power (organic Rankine cycle); (ii) the electrification of remotely offshore platforms; (iii) the improvement of the efficiency of infrastructures, facilities, and processes, by means of a trade-off between capital investments into capacities of infrastructures and their logistical aspects optimizing their operation. This last aspect was investigated in this work by an optimization analysis of the petroleum supply in terms of efficient energy use. They proposed a method based on the strategic planning of specific infrastructural decisions and reduction of operation energy consumption. The proposed optimization model is a mixed integer nonlinear program (MINLP) that covers technological details like hydraulics (the relationship between pipeline diameters, pressures, flow rates, and pipe lengths) and pump systems’ operational efficiency. The model can identify the optimal solutions based on the technological decision-making criterion of minimizing energy consumption. The achieved results show how traditional approaches based on only the minimization of the cost of the infrastructure lead to inefficient energy use during the production and transportation sector of the hydrocarbons. Therefore, the proposed approach can represent a guide for the designers of specific projects in the energy sectors based on petroleum use.

Rosińska and Rakocz [88] presented a paper based on the microbiological safety of water during drinking-water treatments. Since it is pivotal to maintain the microbiological stability of drinking water, a suitable, effective disinfection method must be selected. This allows preventing the secondary growth of microorganisms in the water supply network. It is worth preparing water in the treatment method that is free of biodegradable organic substances when pumped into the water supply system. Assimilable organic carbon and biodegradable dissolved organic carbon can represent a source of bacterial growth and development in the water treatment process. For this reason, the authors of this work proposed a study focused on the qualitative and quantitative assessment of the composition of organic substances represented by biodegradable dissolved organic carbon and assimilable organic carbon. They analyzed the impact of the selected oxidant used in water treatment, i.e., O₃ and the combined ozonation/UV method, on the content of biodegradable organic carbon. For different types of raw waters (groundwater and surface water) on the laboratory scale, the effect of several ozonation conditions based on a specific O₃ dose and the time of contact with water was assessed. The combined method (ozonation/UV) on the levels of the biodegradable organic fraction was also investigated. Three doses of disinfectant were used: 1.6 mg/L, 5.0 mg/L, and 10.0 mg/L, and the time of contact with water was first for 4 min and second for 15 min. The UV radiation time was 10 and 30 min. The greatest alteration in assimilable organic carbon and biodegradable dissolved organic carbon for groundwater was observed at a contact time of 15 min and an O3 dose of 10.0 mg/L, 400 and 197%, respectively. Instead, for surface water, after the ozonation/UV process, the assimilable organic carbon and biodegradable dissolved organic carbon content decreased after both 10 and 30 min of radiation in comparison to the water after ozonation. The biodegradable dissolved organic carbon content decreased by 27% and 31%, respectively. The assimilable organic carbon content decreased by 33% and 22%, respectively.

8. Conclusions

In summary, biomass energy continues to play an important role in the current global energy supply while the research focuses on its highly efficient application and pollutant reduction. Meanwhile, the building sector displays as the main energy consumption and carbon emissions, and energy saving in buildings becomes urgent to alleviate the global warming effect. The application of energy-saving techniques, as well as the hybrid application of sustainable energy resources, may effectively contribute to low-carbon development and climate change adaptation. On the other hand, energy storage has become increasingly important, which can manage the intermittent of renewable energy.

Since the SDEWES conference is an international event focusing on the sustainable development of energy, water, and environmental systems, many scientists, researchers, and professional engineers from various fields joined this event. Please note that energy applications are increasingly focusing on integrating diverse and efficient clean energy solutions for global energy needs. This trend aims to enhance resilience and sustainability, decrease dependence on fossil fuels, mitigate environmental impacts, and bolster energy security through a combination of renewable sources and innovative technologies. Researchers in companies can focus on novel energy technologies to solve technical challenges encountered in practical operations, which can be applied to practical products and services, promoting the commercialization of new energy technologies [89]. This process enables the large-scale production of new energy technologies, therefore reducing costs and enhancing their market competitiveness and penetration rate. On the other hand, scientists from universities are committed to the fundamental research of new energy technologies, providing scientific theoretical support for technological advancements, nurturing talent in the field of new energy, supplying human resources to the industry, and simultaneously offering scientific basis and policy recommendations to the government [90]. Consequently, the SDEWES conference is pivotal in improving sustainability and has the potential to harness its resources and influence towards advancing social and ecological goals.