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Dynamic simulations can accurately estimate the thermal demands for space heating and cooling in buildings, as well as the energy and economic performance of specific energy refurbishment actions. This study aims to evaluate the energy and economic savings resulting from the adoption of particular energy measures applied to a cluster of residential condominium buildings, also considering some possible Italian funding policies. To this scope, dynamic simulation models of several buildings with different features in terms of geometry, shape, and thermo-physical properties are considered. The selected buildings are representative of the most common buildings in the city of Naples, Southern Italy. Two scenarios regarding the possible penetration of the refurbishment actions are considered: the “25% scenario”, where 25% of buildings in the Naples municipality adopt the selected measures, and the “100% scenario”, where all buildings adopt such energy refurbishment actions. The results of the simulations, reported over different time periods, compare the economic, energy, and environmental benefits of the specific energy measures. This study evaluates the replacement of conventional natural gas-fired boilers with natural gas-fired condensing boilers, as well as the use of thermal insulation on the external walls of the buildings. The primary energy demand for space heating decreased by 28% when the proposed energy measures were implemented in all buildings of the Naples municipality.

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.

This paper aims to develop a dynamic simulation model for the reduction of energy consumption through the use of organic waste from a residential district, supplied by a hybrid renewable energy plant. The proposed layout is based on a novel paradigm of a renewable energy community focused on the biocircular economy and a sustainable approach. The novelty with respect to the majority of papers developed in the literature on renewable energy communities lies in the use of both solar photovoltaic production and the organic fraction of municipal solid waste collected by the community. Energy production by biomass conversion and by photovoltaic fields shared among the buildings is used to satisfy in a sustainable manner the community loads for heating, cooling, and power. The district heating network is based on water loop heat pumps and air-to-air heat pumps and it includes the power-to-heat energy storage strategy. The biogas produced by the anaerobic digestion process is cleaned in order to supply a solid oxide fuel cell for the production of additional power, mainly during the hours of poor or null solar energy production. Then, the layout integrates several innovative topics, such as the power-to-heat strategy, the biocircular economy, the low-temperature district heating, the use of a solid oxide fuel cell, and a renewable energy community. The dynamic model of the proposed hybrid renewable layout is developed in the TRNSYS environment, but some innovative energy components, such as anaerobic digestion, the biogas upgrading unit, and the solid oxide fuel cell, are dynamically modeled in MATLAB and then integrated into the whole plant model. The proposed plant has been confirmed to be extremely profitable and able to obtain important energy savings, considering the achieved payback period of 4.48 years and the primary energy saving of 23%. This layout resulted in an interesting solution for pushing the development of smart and sustainable cities.

This paper addresses the problem of the reduction in the huge energy demand of hospitals and health care facilities. The sharp increase in the natural gas price, due to the Ukrainian–Russian war, has significantly reduced economic savings achieved by combined heat and power (CHP) units, especially for hospitals. In this framework, this research proposes a novel system based on the integration of a reversible CHP solid oxide fuel cell (SOFC) and a photovoltaic field (PV). The PV power is mainly used for balancing the hospital load. The excess power production is exploited to produce renewable hydrogen. The SOFC operates in electrical tracking mode. The cogenerative heat produced by the SOFC is exploited to partially meet the thermal load of the hospital. The SOFC is driven by the renewable hydrogen produced by the plant. When this hydrogen is not available, the SOFC is driven by natural gas. In fact, the SOFC is coupled with an external reformer. The simulation model of the whole plant, including the reversible SOFC, PV, and hospital, is developed in the TRNSYS18 environment and MATLAB. The model of the hospital is calibrated by means of measured data. The proposed system achieves very interesting results, with a primary energy-saving index of 33% and a payback period of 6.7 years. Therefore, this energy measure results in a promising solution for reducing the environmental impact of hospital and health care facilities.

The current applications in the energy sector are based largely on fossil fuels which release greenhouse gas emissions to the atmosphere. To face the issue of global warming, the energy sector has to transfer to and develop sustainable energy solutions that do not release carbon emissions. This is one of the primary motivators for the SDEWES conference as well as for this review, and previous ones, examining the most recent works based on sustainable and green energy production in such fields. The 17th Conference on the Sustainable Development of Energy, Water, and Environment Systems (SDEWES) was held on 6–10 November 2022 in Paphos, Cyprus. The SDEWES conference aims at solving complex and ongoing concerns that approach a long-term perspective and supporting innovative solutions and continuous monitoring and evaluation. This review paper aims at collecting the main presented papers focused on the following hot topics: low-carbon technologies based on renewable and clean-energy systems, including mainly biomass, solar, and wind energy applications; energy storage systems; hydrogen-based systems; energy-saving strategies in buildings; and the adoption of smart management strategies using renewable energy systems. These topics are investigated in order to propose solutions to address the issues of climate change, water scarcity, and energy saving. From the analyzed works, we note that some key issues for sustainable development remain to be further addressed: such as novel and advanced energy storage systems, green hydrogen production, novel low-temperature district heating and cooling networks, novel solar technologies for the simultaneous production of power and high temperature heat, solar desalination for hydrogen production systems, and agrivoltaic systems for the production of power and food.

The production of biogas by means of the anaerobic digestion process is becoming increasingly attractive in the green economy context. When municipal organic waste is used to produce biogas, a further positive effect on urban waste disposal is obtained. Starting from the anaerobic digestion model n.1, an accurate analysis of the temperature effects on the anaerobic digestion process in a plug flow reactor is performed. This paper aims at presenting a comprehensive and integrated one-dimensional biological and thermal model for a plug flow reactor. Partial differential equations with respect to time and space are considered to model the heat transfer between the reactor and the internal heat exchanger and between the reactor and the environment. In this scope, a suitable simulation code was developed in MATLAB and validated using the data available in literature. The results of the calculations show that temperature plays a crucial role in the anaerobic digestion process, since it strongly affects the kinetic rates of the microbial species and the methane production. The results obtained in terms of temperature fields and biogas production are compared with the ones available in literature, dealing with a continuously stirred tank reactor. The comparison is conducted considering that both reactors process a volumetric waste flow rate of 20 m3/d and have the same structural characteristics. The plug flow reactor resulted better performance with a produced biogas flow rate equal to 2300 Nm3/year.

In the current energy and environmental framework, the environmental impact of the road transport sector and the urban waste management and disposal are extremely important for highly crowded cities. This work assesses the energy, economic and environmental performance of an innovative paradigm for the full decarbonisation of the road transport sector. This problem is integrated with the management of the organic fraction of municipal solid waste. In particular, the proposed technology is based on an anaerobic digestion plant coupled with a biogas upgrading unit, for the production of biomethane. In addition, photovoltaic panels and solar thermal collectors are also considered for matching electrical and thermal demands, in order to achieve a fully-renewable system. To this scope, the system also includes suitable thermal and electric storages. The economic analysis also considers specific public funding policies, currently available for this technology. This system aims to be a novel paradigm in the energy scenario of waste disposal and road transport sector refurbishment. TRNSYS software was adopted to perform an accurate dynamic simulation for a one-year operation of the system. The anaerobic digestion model is developed by the authors in MatLab and integrated in TRNSYS, for dynamic simulation purpose. Results show that the plant is almost self-sufficient due to the integration of storage systems for both the thermal and electric energy. The photovoltaic system is able to reduce by 45% the energy dependence from the grid. Energy and environmental analyses show a Primary Energy Saving of 126% and a reduction of CO2 equivalent emissions by 112%. The economic feasibility analysis shows a promising Simple Payback period of 6 years.

This paper regards the design and dynamic modelling of a greenhouse coupled with renewable energy technologies to obtain a hybrid renewable energy plant as an optimal solution in the green farm framework. The considered technologies are PV panels, solar thermal collectors, and a biomass auxiliary heater. The system is also coupled with a pyrogasifier, supplied by wood and agricultural waste in the framework of a biocircular economic approach. To supply the investigated user, with a “green farm” located in Castelvolturno (Naples, South of Italy) reducing the energy consumption and operating costs, all of the main components of the plant were suitably designed. The operation of the designed components was simulated by a dynamic simulation model developed by TRNSYS software and validated by means of the literature results. A comprehensive energy, economic, and environmental analysis of the greenhouse is presented. The main results suggest that the investigated renewable plant reduces the total equivalent CO2 emissions by 148.66 t/y. Considering the current high increases in energy prices as a result of the energy crisis due to the war, the system shows very significant profitability with a simple payback of only 1.7 years.

Nowadays, almost 30% of total energy consumption (130 EJ) is consumed for the operation of buildings, mainly by space heating/cooling and ventilation systems, hot water preparation systems, lighting, and other domestic appliances. To improve the energy efficiency of buildings, several countries are promoting the use of renewable energy. The most promising systems include active and passive solar installations. In passive systems, the solar energy is collected, stored, reflected, or distributed by the roof ponds, natural convective loops, and the most popular direct gain walls and thermal storage walls (known as Trombe walls). This paper reviews the experimental and numerical studies devoted to the different solutions of Trombe walls, including solar chimneys integrated on the vertical walls, classic Trombe walls, Trombe walls with incorporated phase change materials, and photovoltaic Trombe walls. The actual state of the art is presented in the context of reducing energy consumption and enhancing thermal comfort. Most of the analyzed studies showed that the application of thermal storage walls allowed achieving these goals, led to lower emissions of greenhouse gases, and improved living standards. Nevertheless, there is a need for more detailed feasibility studies, including cost and environmental indicators.

The anaerobic digestion of the organic fraction of municipal solid waste and the biogas production obtained from its stabilization are becoming an increasingly attractive solution, due to their beneficial effects on the environment. In this way, the waste is considered a resource allowing a reduction in the quantity of it going to landfills and the derived greenhouse gas emissions. Simultaneously, the upgrading process of biogas into biomethane can address the issues dealing with decarbonization of the transport. In this work, the production of biogas obtained from the organic fraction of municipal solid wastes in a plug flow reactor is analyzed. In order to steer the chemical reactions, the temperature of the process must be kept under control. A new simulation model, implemented in the MatLab® environment, is developed to predict the temperature field within the reactor, in order to assess how the temperature affects the growth and the decay of the main microbial species. A thermal model, based on two equilibrium equations, is implemented to describe the heat transfer between the digester and the environment and between the digester and the internal heat exchanger. A biological model, based on suitable differential equations, is also included for the calculation of the biological processes occurring in the reactor. The proposed anaerobic digestion model is derived by the combination of these two models, and it is able to simultaneously simulate both thermal and biological processes occurring within the reactor. In addition to the thermal energy demand, the plant requires huge amounts of electricity due to the presence of a biogas upgrading process, converting biogas into biomethane. Therefore, the in-house developed model is integrated into a TRNSYS environment, to perform a yearly dynamic simulation of the reactor in combination with other renewable technologies. In the developed system layout, the thermal energy required to control the temperature of the reactor is matched by a solar thermal source. The electrical demand is met by the means of a photovoltaic field. In this work, a detailed thermoeconomic analysis is also proposed to compare the environmental impact and economic feasibility of a biomethane production plant based on a plug flow reactor and fed by renewables. Several economic incentives are considered and compared to determine the optimal solution, both in terms of energy and economic savings. The plant is designed for the treatment of a waste flow rate equal to 626.4 kg/h, and the biomethane produced, approximately 850 tons/years, is injected into the national gas grid or supplied to gas stations. In the proposed plant, a solar field of an evacuated tube collector having a surface of approximately 200 m2 is able to satisfy 35% of the thermal energy demand while over 50% of the electric demand is met with a photovoltaic field of 400 m2. A promising payback time of approximately 5 years was estimated.

The energy sector, along with the conditions of the natural and built environment, are significantly affected by the tremendous increase in total energy consumption [...]

Sustainability has become a broad societal goal, aiming to ensure that human beings coexist safely and harmoniously with nature over a longer time. The influence of the COVID-19 pandemic on the global economy is coming to an end. The development and merits of sustainable energy supply, advanced technology, and economic features have received significant attention over the last few decades. However, significant gaps still exist with respect to how to design, construct, and implement hybrid and optimal energy systems with the lowest investment and cost. Since 2002, the Sustainable Development of Energy, Water, and Environment Systems (SDEWES) conferences have become a significant meeting venue for researchers to introduce, discuss, share, and disseminate novel concepts and ideas. This paper presents an overview of published articles in the Special Issues (SIs) dedicated by the series SDEWES conferences, especially those published in Energies recommended by the 16th SDEWES Conference, which was held on 10–15 October 2021 in Dubrovnik, Croatia. This SI in Energies focused on four main topics, including the application of renewable bioenergy, component enhancement in renewable systems, sustainable development for buildings and economic analysis and evaluation for sustainability. The collected papers provide insight into the topics related to recent advances in improving sustainable efficiency, including studies on waste-to-wealth techniques, utilization of hybrid bioenergy systems, heat exchangers and other components for performance enhancement, energy supply and demand analysis, low-temperature DHC systems, techno-economic assessment, and environmental evaluation.

During the 20th century, the global energy system was mainly based on the use of fossil fuels, such as oil, natural gas, and coal [...]

Organic Rankine Cycles (ORC) are often based on the utilization of special dry or isentropic fluids, namely substances with a vertical or positive slope of the saturated steam curve.Several heat transfer correlations for boiling process are available in literature.However, only a few of them are specifically developed and validated for the case of the organic fluids. Such correlations are tested and extrapolated by means of suitable experimental devices and with a specific organic fluid, presenting non-negligible percentage error variations when compared with experimental results concerning different organic fluids.This paper investigates the impact of the overall heat transfer coefficient evaluated by means of different correlations available in literature on the main output parameters of an Organic Rankine Cycles, using a zero-dimensional numerical model.Two case studies are investigated, both of them powered by medium-enthalpy geothermal source which provides a constant thermal load. The first case study concerns a 1.20 MWel ORC, powered by geofluid at 160 °C and 7.00 bar and using n-pentane. The second ORC case study is supposed to provide 8.00 kWel and to be powered by geofluid available at 95.0 °C and 3.00 bar and using R245fa, in agreement with the results available in literature. Both of the models are developed and simulated in Engineering Equation Solver (EES) environment. Keywords: Heat transfer correlations, Organic Rankine Cycle, Geothermal, Numerical model, Organic fluids, Shell&Tube heat exchanger

The renewable energy technologies attracted 70% global energy investment in 2021, but the global CO2 emission is increased by 1 [...]

The COVID-19 pandemic has had a significant impact on the supply chains of traditional fossil fuels. According to a report by the International Energy Agency (IEA) from 2020, oil-refining activity fell by more than the IEA had anticipated. It was also assumed that the demand in 2021 would likely be 2.6 million bpd below the 2019 levels. However, renewable markets have shown strong resilience during the crisis. It was determined that renewables are on track to meet 80% of the growth in electricity demand over the next 10 years and that sustainable energy will act as the primary source of electricity production instead of coal. On the other hand, the report also emphasized that measures for reducing environmental pollution and CO2 emissions are still insufficient and that significant current investments should be further expanded. The Sustainable Development of Energy, Water and Environment Systems (SDEWES) conference series is dedicated to the advancement and dissemination of knowledge on methods, policies and technologies for improving the sustainability of development by decoupling growth from the use of natural resources. The 15th SDEWES conference was held online from 1–5 September 2020; more than 300 reports with 7 special sections were organized on the virtual conference platform. This paper presents the major achievements of the recommended papers in the Special Issue of Energies. Additionally, related studies connected to the above papers published in the SDEWES series are also introduced, including the four main research fields of energy saving and emission reduction, renewable energy applications, the development of district heating systems, and the economic assessment of sustainable energy.

The production of “green hydrogen” is currently one of the hottest topics in the field of renewable energy systems research. Hydrogen storage is also becoming more and more attractive as a flexible solution to mitigate the power fluctuations of solar energy systems. The most promising technology for electricity-to-hydrogen conversion, and vice versa, is the reversible solid-oxide cell (SOC). This device is still very expensive, but it exhibits excellent performance under dynamic operating conditions compared to the competing devices. This work presents the dynamic simulation of a prototypal renewable plant combining a 50 kW photovoltaic (PV) field with a 50 kW solid-oxide electrolyzer cell (SOEC) and a compressed hydrogen tank. The electricity is used to meet the energy demand of a dwelling located in the area of Campi Flegrei (Naples). The SOC efficiency is simulated by developing a mathematical model in MATLAB®. The model also calculates the cell operating temperature as a function of the input current. Once the optimal values of the operating parameters of the SOC are calculated, the model is integrated in the transient system simulation tool (TRNSYS) for dynamic analysis. Furthermore, this work presents a parametric analysis of the hydrogen storage system (HSS). The results of the energy and environmental analyses show that the proposed system can reach a primary energy saving by 70% and an amount of saved CO2 of 28 tons/year. Some possible future market scenarios are considered for the economic analysis. In the most realistic case, the optimal configuration shows a simple pay back lower than 10 years and a profit index of 46%.

Anaerobic Digestion (AD) is a well-established process that is becoming increasingly popular, especially as a technology for organic waste treatment; the process produces biogas, which can be upgraded to biomethane, which can be used in the transport sector or injected into the natural gas grid. Considering the sensitivity of Anaerobic Digestion to several process parameters, mathematical modeling and numerical simulations can be useful to improve both design and control of the process. Therefore, several different modeling approaches were presented in the literature, aiming at providing suitable tools for the design and simulation of these systems. The purpose of this study is to analyze the recent advancements in the biomethane production from different points of view. Special attention is paid to the integration of this technology with additional renewable energy sources, such as solar, geothermal and wind, aimed at achieving a fully renewable biomethane production. In this case, auxiliary heat may be provided by solar thermal or geothermal energy, while wind or photovoltaic plants can provide auxiliary electricity. Recent advancements in plants design, biomethane production and mathematical modeling are shown in the paper, and the main challenges that these fields must face with are discussed. Considering the increasing interest of industries, public policy makers and researchers in this field, the efficiency and profitability such hybrid renewable solutions for biomethane production are expected to significantly improve in the next future, provided that suitable subsidies and funding policies are implemented to support their development.

In this research, a technoeconomic comparison of energy efficiency options for energy districts located in different climatic areas (Naples, Italy and Fayoum, Egypt) is presented. A dynamic simulation model based on TRNSYS is developed to evaluate the different energy efficiency options, which includes different buildings of conceived districts. The TRNSYS model is integrated with the plug-in Google SketchUp TRNSYS3d to estimate the thermal load of the buildings and the temporal variation. The model considers the unsteady state energy balance and includes all the features of the building’s envelope. For the considered climatic zones and for the different energy efficiency measures, primary energy savings, pay back periods and reduced CO2 emissions are evaluated. The proposed energy efficiency options include a district heating system for hot water supply, air-to-air conventional heat pumps for both cooling and space heating of the buildings and the integration of photovoltaic and solar thermal systems. The energy actions are compared to baseline scenarios, where the hot water and space heating demand is satisfied by conventional natural gas boilers, the cooling demand is met by conventional air-to-air vapor compression heat pumps and the electric energy demand is satisfied by the power grid. The simulation results provide valuable guidance for selecting the optimal designs and system configurations, as well as suggest guidelines to policymakers to define decarbonization targets in different scenarios. The scenario of Fayoum offers a savings of 67% in primary energy, but the associated payback period extends to 23 years due to the lower cost of energy in comparison to Naples.

The global COVID-19 pandemic has had strong impacts on national and international freight, construction and tourism industry, supply chains, and has resulted in a rapid decline in the demand for traditional energy sources. In fact, research has outlined that urban areas depend on global supply chains for their day-to-day basic functions, including energy supplies, food and safe access to potable water. The disruption of global supply chains can leave many urban areas in a very vulnerable position, in which their citizens may struggle to obtain their basic supplies, as the COVID-19 crisis has recently shown. Therefore, solutions aiming to enhance local food, water and energy production systems, even in urban environments, have to be pursued. The COVID-19 crisis has also highlighted in the scientific community the problem of people’s exposure to outdoor and indoor pollution, confirmed as a key element for the increase both in the transmission and severity of the contagion, on top of involving health risks on their own. In this context, most nations are going to adopt new preferential policies to stimulate the development of relevant sustainable energy industries, based on the electrification of the systems supplied by renewable energy sources as confirmed by the International Energy Agency (IEA). Thus, while there is ongoing research focusing on a COVID 19 vaccine, there is also a need for researchers to work cooperatively on novel strategies for world economic recovery incorporating renewable energy policy, technology and management. In this framework, the Sustainable Development of Energy, Water and Environment Systems (SDEWES) conference provides a good platform for researchers and other experts to exchange their academic thoughts, promoting the development and improvements on the renewable energy technologies as well as their role in systems and in the transition towards sustainable energy systems. The 14th SDEWES Conference was held in Dubrovnik, Croatia. It brought together around 570 researchers from 55 countries in the field of sustainable development. The present Special Issue of Energies, specifically dedicated to the 14th SDEWES Conference, focuses on four main fields: energy policy for sustainable development, biomass energy application, building energy saving, and power plant and electric systems.

The aim of the present paper is to develop a reliable and accurate model of the wastewater biochemical treatment process and to explore the behaviour through a general dynamic simulation environment, namely the INtegrated Simulation Environment Language (INSEL), for the analysis of the energy demand of the whole wastewater treatment plant. In particular, the presented model pays special attention to the chemical kinetics involved in the activated sludge process for the reduction of nitrogen and carbon compounds. According to the best practices, the plant configuration considered in this work includes the denitrification-nitrification process, performed by completely mixed reactors. In particular, the process analysed in this paper is based on the Ludzak-Ettinger process. The biological process is simulated according to the well-known method widely used in the literature, namely the Activated Sludge Model No 1 (ASM1). The model includes a set of equations for the calculation of aerobic growth of heterotrophs, anoxic growth of heterotrophs, aerobic growth of autotrophs, decay of autotrophs, ammonification of soluble nitrogen, hydrolysis of entrapped organics, and hydrolysis of entrapped organic nitrogen. All these equations, along with energy and mass balances, are solved by the explicit Euler method. The developed model is validated using literature data, showing a great accuracy (deviation below 1%). As for the temperature, results show that, between 15 and 25 °C, in the initial part of the process, transport effects dominate the consumption ones. When the temperature is higher than 30 °C, nitrate consumption is so fast that biomass growth is limited by this effect. Conversely, in case of low temperatures (5–10 °C), biomass growth is not limited by nitrate availability. Finally, results also showed that temperature significantly affects the denitrification process, whereas the effect on the oxygen is lower.

Hospitals are very attractive for Combined Heat and Power (CHP) applications, due to their high and continuous demand for electric and thermal energy. However, both design and control strategies of CHP systems are usually based on an empiric and very simplified approach, and this may lead to non-optimal solutions. The paper presents a novel approach based on the dynamic simulation of a trigeneration system to be installed in a hospital located in Puglia (South Italy), with around 600 beds, aiming to investigate the energy and economic performance of the system, for a given control strategy (electric-load tracking). The system includes a natural gas fired reciprocating engine (with a rated power of 2.0 MW), a single-stage LiBr-H2O absorption chiller (with a cooling capacity of around 770 kW), auxiliary gas-fired boilers and steam generators, electric chillers, cooling towers, heat exchangers, storage tanks and several additional components (pipes, valves, etc.). Suitable control strategies, including proportional–integral–derivative (PID) and ON/OFF controllers, were implemented to optimize the trigeneration performance. The model includes a detailed simulation of the main components of the system and a specific routine for evaluating the heating and cooling demand of the building, based on a 3-D model of the building envelope. All component models were validated against experimental data provided by the manufacturers. Energy and economic models were also included in the simulation tool, to calculate the thermoeconomic performance of the system. The results show an excellent economic performance of the trigeneration system, with a payback period equal to 1.5 years and a profitability index (ratio of the Net Present Value to the capital cost) equal to 3.88, also due to the significant contribution of the subsidies provided by the current Italian regulation for CHP systems (energy savings certificates).

This work presents a thermoeconomic comparison between two different solar energy technologies, namely the evacuated flat-plate solar collectors and the photovoltaic panels, integrated as auxiliary systems into two renewable polygeneration plants. Both plants produce electricity, heat and cool, and are based on a 6 kWe organic Rankine cycle (ORC), a 17-kW single-stage H2O/LiBr absorption chiller, a geothermal well at 96 °C, a 200 kWt biomass auxiliary heater, a 45.55 kWh lithium-ion battery and a 25 m2 solar field. In both configurations, electric and thermal storage systems are included to mitigate the fluctuations due to the variability of solar radiation. ORC is mainly supplied by the thermal energy produced by the geothermal well. Additional heat is also provided by solar thermal collectors and by a biomass boiler. In an alternative layout, solar thermal collectors are replaced by photovoltaic panels, producing additional electricity with respect to the one produced by the ORC. To reduce ORC condensation temperature and increase the electric efficiency, a ground-cooled condenser is also adopted. All the components included in both plants were accurately simulated in a TRNSYS environment using dynamic models validated versus literature and experimental data. The ORC is modeled by zero-dimensional energy and mass balances written in Engineering Equation Solver and implemented in TRNSYS. The models of both renewable polygeneration plants are applied to a suitable case study, a commercial area near Campi Flegrei (Naples, South Italy), a location well-known for its geothermal sources and good solar availability. The economic results suggest that for this kind of plant, photovoltaic panels show lower pay back periods than evacuated flat-plate solar collectors, 13 years vs 15 years. The adoption of the electric energy storage system leads to an increase of energy-self-sufficiency equal to 42% and 47% for evacuated flat-plate solar collectors and the photovoltaic panels, respectively.

The aim of this work is the development of a simulation model for the anaerobic digestion process of source-sorted organic fractions of municipal solid wastes. In particular, a detailed model simulating both biological and thermal behaviors of the process was developed. The biological model is based on the Anaerobic Digestion Model 1 (ADM1), which allows one to evaluate the dynamic trends of the concentrations of the main components and the biogas production as a function of the digester operating temperature. The work also includes a detailed thermal model which is developed considering the geometrical and structural features of the digester. The thermal behavior of the digester was also modeled, considering a purposely designed heat exchanger immersed inside the digester. Therefore, the thermal behavior of the process was evaluated by the well-known heat exchange equations and thermal energy balances. The combination of these two models is used to analyze the different possible operating conditions of the system. The model is also able to consider that the reactor operating temperature and the biogas production dynamically depend on a plurality of parameters: inlet hot water temperature and flowrate of the heating system, outdoor temperature, flowrate of organic fraction. The numerical resolution of the obtained differential equations and thermal balances of the model was carried out in the MATLAB® environment. The result shows that the calculated biogas production is 0.132 Nm3 per kg of OFMSW. In addition, the model also shows that the inlet hot water temperature must be increased by about 1.5 °C, to increase by 1.0 °C the digester temperature.

This paper presents a dynamic simulation model and a parametric analysis of a solar-geothermal hybrid cogeneration plant based on an Organic Rankine Cycle (ORC) powered by a medium-enthalpy geothermal resource and a Parabolic Trough Collector solar field. The fluid temperature supplying heat to the ORC varies continuously as a function of the solar irradiation, affecting both the electrical and thermal energies produced by the system. Thus, a dynamic simulation was performed. The ORC model, developed in Engineering Equation Solver, is based on zero-dimensional energy and mass balances and includes specific algorithms to evaluate the off-design system performance. The overall simulation model of the solar-geothermal cogenerative plant was implemented in the TRNSYS environment. Here, the ORC model is imported, whereas the models of the other components of the system are developed on the basis of literature data. Results are analyzed on different time bases presenting energetic, economic and exergetic performance data. Finally, a rigorous optimization has been performed to determine the set of system design/control parameters minimizing simple payback period and exergy destruction rate. The system is profitable when a significant amount of the heat produced is consumed. The highest irreversibilities are due to the solar field and to the heat exchangers.

During the last years, special attention has been paid to renewable polygeneration technologies, able of simultaneously producing thermal, cooling, electrical energy and desalinated water from seawater. This paper focuses on an innovative polygeneration system driven by renewable energy sources, including the following technologies: hybrid photovoltaic/thermal collectors, concentrating parabolic trough (CPVT), a biomass heater, a single-stage absorption chiller and a multiple-effect distillation desalination system. The system is designed to cover the base load of an isolated small community. In previous papers, the dynamic simulation model about plant operation is discussed. In this paper, a detailed exergy, economic and environmental analysis of the plant is presented. In addition, the plant was optimized using different objective functions, applying the Design of Experiment (DoE) methodology which evaluates the sensitivity of the different objective functions with respect to the selected design parameters. The results show that an increase of the storage volume is generally negative, whereas increasing the solar field area involves an increase of the exergy destruction rate, but also an improvement of the CPVT exergy output provided; the final result is an increase of both the exergy efficiency and the economic profitability of the polygeneration system.

Two solar polygeneration systems were investigated for electricity, cooling and fresh water production. In the first scenario ( LF PS ), the linear Fresnel (LF) solar field was used as thermal source of the Organic Rankine Cycle (ORC), absorption chiller (ACH) and multi-effect desalination (MED) unit. In the second scenario ( PV PS ), photovoltaic (PV) panels were considered as the electricity source to supply the electricity load that is required for lighting, electrical devices, compression chiller (CCH) and reverse osmosis (RO) units. A techno-economic comparison was made between two scenarios based on the land use factor (F), capacity utilization factor (CUF), payback period, levelized cost of electricity (LCE), levelized cost of cooling energy (LCC) and levelized cost of water (LCW). The calculations were conducted for four different locations in order to determine the effect of solar radiation level on the LCE, LCC and LCW of systems in both scenarios. The results showed that the LCE and LCW of PV PS is lower than that of LF PS and the LCC of LF PS is lower than that of PV PS . Also, the payback period of LF PS and PV PS systems are obtained as 13.97 years and 13.54 years, respectively, if no incentive is considered for the electricity sale.

Several countries have recently realized that the present development paradigm is not sustainable from an environmental and energy point of view. The growing awareness of the population regarding environmental issues is pushing governments worldwide more and more to promote policies aiming at limiting harmful effects of human development. In particular, the rapid increase of the global temperature, especially in the polar regions, and the management of human wastes, mainly plastic in seas, are some of the main points to be addressed by these novel policies. Several actions must be implemented in order to limit such issues. Unfortunately, the recent COP 24 Conference was not successful, but hopefully an agreement will be established in 2020 at the COP 26 Conference. The effort performed by policymakers must be mandatorily supported by the scientific community. In this framework, this paper aims at showing that countries worldwide are trying to negotiate an agreement to increase energy efficiency and reduce greenhouse gas (GHG) emissions. In addition, in this paper all the researchers reported can provide quantitative measures of the actions to be implemented in order to address a sustainable and efficient use of energy. Here, innovations in terms of novel efficient and environmentally friendly technologies mainly based on renewable energy sources have been also investigated. The study also highlights different sectors that have been involved for this aim, such as energy conversion systems, urban areas, mobility, sustainability, water management, social aspects, etc. In this framework, specific conferences are periodically organized in order to provide a forum for discussion regarding these topics. In this area the Sustainable Development of Energy, Water and Environment Systems (SDEWES) conference is the most ordinary conference. The 13th Sustainable Development of Energy, Water and Environment Systems Conference was held in Palermo, Italy in 2018. The current Special Issue of Energies, precisely dedicated to the 13th SDEWES Conference, is based on three main topics: energy policy and energy efficiency in urban areas, energy efficiency in industry and biomass and other miscellaneous energy systems.

This paper presents a design procedure and a simulation model of a novel concentrating PVT collector. The layout of the PVT system under investigation was derived from a prototype recently presented in literature and commercially available. The prototype consisted in a parabolic trough concentrator and a linear triangular receiver. In that prototype, the bottom surfaces of the receiver are equipped with mono-crystalline silicon cells whereas the top surface is covered by an absorbing surface. The aperture area of the parabola was covered by a glass in order to improve the thermal efficiency of the system. In the modified version of the collector considered in this paper, two changes are implemented: the cover glass was eliminated and the mono-crystalline silicon cells were replaced by triple-junction cells. In order to analyze PVT performance, a detailed mathematical model was implemented. This model is based on zero-dimensional energy balances. The simulation model calculates the temperatures of the main components of the system and the main energy flows Results showed that the performance of the system is excellent even when the fluid temperature is very high (>100 °C). Conversely, both electrical and thermal efficiencies dramatically decrease when the incident beam radiation decreases.

EU energy policy is more and more promoting a resilient, efficient and sustainable energy system. Several agreements have been signed in the last few months that set ambitious goals in terms of energy efficiency and emission reductions and to reduce the energy consumption in buildings. These actions are expected to fulfill the goals negotiated at the Paris Agreement in 2015. The successful development of this ambitious energy policy needs to be supported by scientific knowledge: a huge effort must be made in order to develop more efficient energy conversion technologies based both on renewables and fossil fuels. Similarly, researchers are also expected to work on the integration of conventional and novel systems, also taking into account the needs for the management of the novel energy systems in terms of energy storage and devices management. Therefore, a multi-disciplinary approach is required in order to achieve these goals. To ensure that the scientists belonging to the different disciplines are aware of the scientific progress in the other research areas, specific Conferences are periodically organized. One of the most popular conferences in this area is the Sustainable Development of Energy, Water and Environment Systems (SDEWES) Series Conference. The 12th Sustainable Development of Energy, Water and Environment Systems Conference was recently held in Dubrovnik, Croatia. The present Special Issue of Energies, specifically dedicated to the 12th SDEWES Conference, is focused on five main fields: energy policy and energy efficiency in smart energy systems, polygeneration and district heating, advanced combustion techniques and fuels, biomass and building efficiency.

Sport centres thermal demand is extremely high due to the large amount of sanitary hot water production. Therefore, energy savings actions must be performed in order to optimize system efficiency. In this framework, the present paper investigates the possibility to perform an energy rehabilitation of an indoor swimming pool centre by means of solar thermal collectors and heat pump technologies integrated with the existing plant. The case study consists of a university indoor swimming pool centre located in Naples, South of Italy. A dynamic simulation model is developed by TRNSYS software (Thermal Energy System Specialists, LLC, Madison, WI, USA). An experimental investigation is also performed in order to calibrate the swimming pool thermal model and the space conditioning equipment operation. Real data concerning the thermal demand of the centre are implemented and the dynamic behaviour of the swimming pool occupants is also considered. The proposed technical solutions are analysed from an energy and economic point of view. A parametric analysis aiming at determining the effect of the size of the solar field on the system performance is performed. The comparison outlines that the best energy and economic performance is achieved by evacuated solar thermal collectors. In particular, the Simple Pay Back (SPB) period results about 14 years without incentives and it decreases to 5 years considering the Italian incentive policy. For an evacuated collector field of 150 m2, the SPB without incentive results below 9 years.

Energy policies accompanying the transition towards a sustainable development process must be supported by technical analyses in which future energy scenarios are modeled and evaluated. This paper analyzes possible decarbonization scenarios in Italy for the year 2050. They envisage high electrification of transports and residential buildings, high use of renewable energies, and a modal shift towards public transport. The energy scenarios are evaluated using a software program, EnergyPLAN, starting from a reference model developed for the year 2014. Special attention has been given to the modeling of data that are unavailable in the literature, such as the time profile of heating and cooling demands, obtained with the degree-days method and validated by elaborating the results of the modeling of the residential building stock, this latter was dynamically simulated in TRNSYS. The results show that to obtain a significant decrease of greenhouse gas emissions and fossil fuel consumption, it is necessary not only to promote a deeper penetration of renewable sources, but also their integration with other technologies (cogeneration, trigeneration, power-to-heat systems, thermal storage, vehicle-to-grid operations). In fact, renewables technologies alone can raise some critical issues, such as excess and/or shortage of electricity production and non-sustainable exploitation of biomass.

This paper presents a one-dimensional finite-volume model of an unglazed photovoltaic/thermal (PVT) solar collector. The unit consists of a conventional solar photovoltaic (PV) collector coupled with a suitable heat exchanger. In particular, the collector includes a roll bond heat exchanger and it is not equipped with back and frame insulation. The system is discretized along the flow direction (longitudinal) of the cogenerative collector. For each finite-volume element of the discretized computational domain, mass and energy balances are implemented. The collector geometry and materials parameters are taken from a commercially available device. An on-field experimental investigation is performed in order to validate the proposed model. The model is used to evaluate both electrical and thermodynamic parameters for each element of the domain and for fixed operating conditions. Finally, a sensitivity analysis is also performed in order to investigate the energetic performance of the cogenerative collector as a function of the main design/environmental parameters.

This Special Issue aims at collecting the recent studies dealing with polygeneration systems, with a special focus on the possible integration of different technologies into a single system, able to convert one or multiple energy sources into energy services (electricity, heat and cooling) and other useful products (e.g., desalinized water, hydrogen, glycerin, ammonia, etc.). Renewable sources (solar, wind, hydro, biomass and geothermal), as well as fossil fuels, feeding advanced energy systems such as fuel cells and cogeneration systems, are considered. Special attention is paid to control strategies and to the management of the systems in general. Studies including thermoeconomic analyses and system optimizations are presented.

This paper presents a detailed exergetic analysis of a novel high-temperature Solar Assisted Combined Cycle (SACC) power plant. The system includes a solar field consisting of innovative high-temperature flat plate evacuated solar thermal collectors, a double stage LiBr-H2O absorption chiller, pumps, heat exchangers, storage tanks, mixers, diverters, controllers and a simple single-pressure Combined Cycle (CC) power plant. Here, a high temperature solar cooling system is coupled with a conventional combined cycle, in order to pre-cool gas turbine inlet air in order to enhance system efficiency and electrical capacity. In this paper, the system is analyzed from an exergetic point of view, on the basis of an energy-economic model presented in a recent work, where the obtained main results show that SACC exhibits a higher electrical production and efficiency with respect to the conventional CC. The system performance is evaluated by a dynamic simulation, where detailed simulation models are implemented for all the components included in the system. In addition, for all the components and for the system as whole, energy and exergy balances are implemented in order to calculate the magnitude of the irreversibilities within the system. In fact, exergy analysis is used in order to assess: exergy destructions and exergetic efficiencies. Such parameters are used in order to evaluate the magnitude of the irreversibilities in the system and to identify the sources of such irreversibilities. Exergetic efficiencies and exergy destructions are dynamically calculated for the 1-year operation of the system. Similarly, exergetic results are also integrated on weekly and yearly bases in order to evaluate the corresponding irreversibilities. The results showed that the components of the Joule cycle (combustor, turbine and compressor) are the major sources of irreversibilities. System overall exergetic efficiency was around 48%. Average weekly solar collector exergetic efficiency ranged from 6.5% to 14.5%, significantly increasing during the summer season. Conversely, absorption chiller exergy efficiency varies from 7.7% to 20.2%, being higher during the winter season. Combustor exergy efficiency is stably close to 68%, whereas the exergy efficiencies of the remaining components are higher than 80%.

This paper presents numerical and experimental analyses aimed at evaluating the technical and economic feasibility of photovoltaic/thermal (PVT) collectors. An experimental setup was purposely designed and constructed in order to compare the electrical performance of a PVT solar field with the one achieved by an identical solar field consisting of conventional photovoltaic (PV) panels. The experimental analysis also aims at evaluating the potential advantages of PVT vs. PV in terms of enhancement of electrical efficiency and thermal energy production. The installed experimental set-up includes four flat polycrystalline silicon PV panels and four flat unglazed polycrystalline silicon PVT collectors. The total electrical power and area of the solar field are 2 kWe and 13 m2, respectively. The experimental set-up is currently installed at the company AV Project Ltd., located in Avellino (Italy). This study also analyzes the system from a numerical point of view, including a thermo-economic dynamic simulation model for the design and the assessment of energy performance and economic profitability of the solar systems consisting of glazed PVT and PV collectors. The experimental setup was modelled and partly simulated in TRNSYS environment. The simulation model was useful to analyze efficiencies and temperatures reached by such solar technologies, by taking into account the reference technology of PVTs (consisting of glazed collectors) as well as to compare the numerical data obtained by dynamic simulations with the gathered experimental results for the PV technology. The numerical analysis shows that the PVT global efficiency is about 26%. Conversely, from the experimental point of view, the average thermal efficiency of PVT collectors is around 13% and the electrical efficiencies of both technologies are almost coincident and equal to 15%.