Next Issue
Volume 14, September-2
Previous Issue
Volume 14, August-2
 
 
energies-logo

Journal Browser

Journal Browser

Energies, Volume 14, Issue 17 (September-1 2021) – 373 articles

Cover Story (view full-size image): Anaerobic digestion (AD) is an attractive bioprocess for the valorization of organic wastes via a complex microbial consortium and subsequent conversion of metabolic intermediates to hydrogen and methane. The biodegradability of fed substrate significantly affects the gaseous products, the efficiency, and the system performance. Therefore, the most appropriate system, i.e., single- vs. two-stage AD, must be selected for a given feedstock to maximize the biofuel yield of the overall process. As proven, the AD of easily biodegradable waste streams, such as Cheese Whey, in a two-stage system is more stable and energy productive compared to single-stage digestion. In contrast, the AD of recalcitrant substrates, such as Liquid Cow Manure, leads to the conclusion that a simple one-stage system is preferable. View this paper
  • Issues are regarded as officially published after their release is announced to the table of contents alert mailing list.
  • You may sign up for e-mail alerts to receive table of contents of newly released issues.
  • PDF is the official format for papers published in both, html and pdf forms. To view the papers in pdf format, click on the "PDF Full-text" link, and use the free Adobe Readerexternal link to open them.
Order results
Result details
Section
Select all
Export citation of selected articles as:
Article
Direct-Connect Test of Solid Scramjet with Symmetrical Structure
Energies 2021, 14(17), 5589; https://doi.org/10.3390/en14175589 - 06 Sep 2021
Cited by 1 | Viewed by 715
Abstract
The solid scramjet has become one of the most promising engine types. In this paper, we report the first direct-connect test of a solid scramjet with symmetrical structure, carried out using boron-based fuel-rich solid propellant as fuel. During the test, which simulated a [...] Read more.
The solid scramjet has become one of the most promising engine types. In this paper, we report the first direct-connect test of a solid scramjet with symmetrical structure, carried out using boron-based fuel-rich solid propellant as fuel. During the test, which simulated a flight environment at Mach 5.6 and 25 km, the performance of the solid scramjet was obtained by measuring the pressure, thrust, and mass flow. The results show that, due to the change in the combustion area of the propellant and the deposition of the throat in the gas generator during the test, the equivalence ratio gradually increased from 0.54 to 0.63. In a solid scramjet, it is possible to obtain a symmetrical distribution of the flow field within the combustor. Moreover, in a multi-cavity combustor, the combustion state expands from the cavity to the center of the flow channel. The performance of the solid scramjet increased during the test, reaching a combustion efficiency of about 42%, a total pressure recovery coefficient of 0.35, and a thrust gain specific impulse of about 418 s. The solid scramjet with symmetrical structure is feasible. The cavity configuration adopted in this paper can reduce the ignition delay time of fuel-rich gas and improve the combustion efficiency of gas-phase combustible components. The shock trains in the isolator are conducive to the recovery of the total pressure. The performance of the solid scramjet is limited by the low combustion efficiency of the particles. Full article
Show Figures

Figure 1

Article
Sand Production of the Shale Gas Well in Different Production Periods: Structure and Component
Energies 2021, 14(17), 5588; https://doi.org/10.3390/en14175588 - 06 Sep 2021
Cited by 2 | Viewed by 581
Abstract
Complex geology and fracturing operations have led to frequent sand production problem in the shale gas well. Sand production brings huge engineering risks and seriously affects the normal production of the shale gas well. In order to study the property and source of [...] Read more.
Complex geology and fracturing operations have led to frequent sand production problem in the shale gas well. Sand production brings huge engineering risks and seriously affects the normal production of the shale gas well. In order to study the property and source of the yielded sand, sand samples in three production periods of flowback, production test and gas production are collected from Sichuan Basin of China. Combining the methods of particle size analysis, microscope observation, scanning electron microscope, CT scanning, infrared spectroscopy and energy dispersive spectrum analysis, the multi-scale structure and composition characteristics of the yielded sand from different production periods were investigated. Results show that the sand size is the largest in the production test period and the smallest in the gas production period. The large-size sand is blocky in the flowback period, while it is flaky in the period of production test and gas production. The roundness of sand becomes worse as the sand size decreasing. Sand composition has the characteristics of fracturing proppant and shale mineral. Cementing material between large-size sands has the network structure and the higher content of aluminum and iron. Organic chemicals are found to be adhered to the sand surface in all three periods. Both shale fracture and proppant failure can generate particles that provide the material source for sand production. This research provides the source of the yielded sand and a theoretical guidance for the sand production mechanism. Full article
Show Figures

Graphical abstract

Article
A Simulation Environment for Training a Reinforcement Learning Agent Trading a Battery Storage
Energies 2021, 14(17), 5587; https://doi.org/10.3390/en14175587 - 06 Sep 2021
Cited by 3 | Viewed by 1217
Abstract
Battery storages are an essential element of the emerging smart grid. Compared to other distributed intelligent energy resources, batteries have the advantage of being able to rapidly react to events such as renewable generation fluctuations or grid disturbances. There is a lack of [...] Read more.
Battery storages are an essential element of the emerging smart grid. Compared to other distributed intelligent energy resources, batteries have the advantage of being able to rapidly react to events such as renewable generation fluctuations or grid disturbances. There is a lack of research on ways to profitably exploit this ability. Any solution needs to consider rapid electrical phenomena as well as the much slower dynamics of relevant electricity markets. Reinforcement learning is a branch of artificial intelligence that has shown promise in optimizing complex problems involving uncertainty. This article applies reinforcement learning to the problem of trading batteries. The problem involves two timescales, both of which are important for profitability. Firstly, trading the battery capacity must occur on the timescale of the chosen electricity markets. Secondly, the real-time operation of the battery must ensure that no financial penalties are incurred from failing to meet the technical specification. The trading-related decisions must be done under uncertainties, such as unknown future market prices and unpredictable power grid disturbances. In this article, a simulation model of a battery system is proposed as the environment to train a reinforcement learning agent to make such decisions. The system is demonstrated with an application of the battery to Finnish primary frequency reserve markets. Full article
(This article belongs to the Special Issue Energy Management of Prosumer Communities)
Show Figures

Figure 1

Article
Economic and Energy Efficiency of Farms in Poland
Energies 2021, 14(17), 5586; https://doi.org/10.3390/en14175586 - 06 Sep 2021
Cited by 2 | Viewed by 717
Abstract
Climate change and negative environmental effects are results of a simplified understanding of management processes, i.e., assuming economic effects as the basis for development, without taking into account external costs. Economically efficient facilities are not always environmentally efficient. Due to the existing conflict [...] Read more.
Climate change and negative environmental effects are results of a simplified understanding of management processes, i.e., assuming economic effects as the basis for development, without taking into account external costs. Economically efficient facilities are not always environmentally efficient. Due to the existing conflict of economic and environmental goals, it seems necessary to look for measures that would include both economic and environmental elements in their structure. The above doubts were the main reasons for undertaking this research. One of the important sectors of the economy accepted for research, where energy is an essential factor of production, is agriculture. Agricultural production is very diversified both in terms of inputs and final products. Depending on the production direction, the processes of conversion of energy accumulated in inputs into energy accumulated in commodity products have different natures and relationships. Taking into account the importance of agriculture in the national economy and the current environmental needs of the world, the types of farms generating energy surplus and those in which the surplus is the least cost-consuming were indicated. The research used the economic and energy efficiency index, which makes it possible to jointly assess technical and economic efficiency. Assuming the need to produce food with low energy consumption and a positive energy balance, it is reasonable to develop a support system for those farms showing the highest economic and energy efficiency indicators. Full article
(This article belongs to the Special Issue Energy Sources from Agriculture and Rural Areas)
Show Figures

Figure 1

Article
A New Evaluation of Skin Factor in Inclined Wells with Anisotropic Permeability
Energies 2021, 14(17), 5585; https://doi.org/10.3390/en14175585 - 06 Sep 2021
Cited by 2 | Viewed by 679
Abstract
Oil and gas well productivity can be affected by a number of different skin factors, the combined influences of which contribute to a well’s total skin factor. The skin caused by deviated wells is one such well-known factor. The present study aimed to [...] Read more.
Oil and gas well productivity can be affected by a number of different skin factors, the combined influences of which contribute to a well’s total skin factor. The skin caused by deviated wells is one such well-known factor. The present study aimed to investigate skin effects caused by deviated well slants when considering vertical-to-horizontal permeability anisotropy. The research employed computational fluid dynamics (CFD) software to simulate fluid flows in inclined wells through the injection of water with Darcy flow using 3D geometric formations. The present work investigates the effects of four main characteristics—namely, the permeability anisotropy, wellbore radius, reservoir thickness, and deviation angle—of open-hole inclined wells. Additional investigations sought to verify the effect of the direction of perforations on the skin factor or pressure drop in perforated inclined wells. In the case of an inclined open hole well, the novel correlation produced in the current study simplifies the estimation of the skin factor of inclined wells at different inclination angles. Our comparison indicates good agreement between the proposed correlation and available models. Furthermore, the results demonstrated a deviation in the skin factor estimation results for perforated inclined wells in different perforation orientation scenarios; therefore, existing models must be improved in light of this variance. This work contributes to the understanding and simulation of the effects of well inclination on skin factor in the near-wellbore region. Full article
(This article belongs to the Section L: Energy Sources)
Show Figures

Figure 1

Article
The Icing Distribution Characteristics Research of Tower Cross Beam of Long-Span Bridge by Numerical Simulation
Energies 2021, 14(17), 5584; https://doi.org/10.3390/en14175584 - 06 Sep 2021
Viewed by 665
Abstract
The cross beam of a long-span bridge will freeze in low temperature. When the temperature rises, the ice on the cross beam will thaw and fall off. If the ice is too heavy, it may cause vehicle damage and casualty. In order to [...] Read more.
The cross beam of a long-span bridge will freeze in low temperature. When the temperature rises, the ice on the cross beam will thaw and fall off. If the ice is too heavy, it may cause vehicle damage and casualty. In order to reduce the risk of falling ice, a scale model of the cross beam was taken as an example, and a kind of numerical simulation method is presented to study the icing distribution characteristics on surface of the cross beam. This paper simulates the ice accretions process of the cross beam by Fluent module and FENSAP-ICE module of ANSYS and investigates the influence of wind and temperature in the process. This is a new numerical simulation method for studying ice accretions of buildings. The results indicate that water freezes mainly on the windward surface, and the thicker ice is near the top and bottom edge of windward surface. According to the results of numerical simulation, a measure of ice melting based on electric heating method is proposed in this paper, and the feasibility and effectiveness of this method are verified by numerical simulation. The results show that the icing distribution characteristics are accord with the fact and the ice-melting measure is feasible and effective. Full article
Show Figures

Figure 1

Article
Influence of LPG and DME Composition on Spark Ignition Engine Performance
Energies 2021, 14(17), 5583; https://doi.org/10.3390/en14175583 - 06 Sep 2021
Cited by 4 | Viewed by 573
Abstract
This article presents a detailed analysis of the potential of dimethyl ether (DME) fuel applications in SI engines. This paper presents the tests results completed on an 1.6-dm3 Opel Astra engine fueled by gaseous fuel as a mixture of LPG and DME. [...] Read more.
This article presents a detailed analysis of the potential of dimethyl ether (DME) fuel applications in SI engines. This paper presents the tests results completed on an 1.6-dm3 Opel Astra engine fueled by gaseous fuel as a mixture of LPG and DME. Dimethyl ether is a fuel with properties similar to liquid LPG fuel. In addition, DME is very well miscible with LPG, hence the possibility of creating a mixture with any DME divisions. The assessment of the possibility of using DME as a component of the mixture was carried out with the use of a chassis dynamometer and equipment, enabling an analysis of the changes taking place inside the cylinder. The results of the analyses are the parameters of the thermodynamic processes describing changes in the engine cylinder. Full article
(This article belongs to the Special Issue Renewable Fuels for Internal Combustion Engines)
Show Figures

Figure 1

Article
Effect of NACA0012 Airfoil Pitching Oscillation on Flow Past a Cylinder
Energies 2021, 14(17), 5582; https://doi.org/10.3390/en14175582 - 06 Sep 2021
Cited by 1 | Viewed by 612
Abstract
The flow past a cylinder is a classical problem in flow physics. In a certain range of Reynolds number, there will be Karman vortex street phenomenon in the wake of a cylinder, which will greatly increase the pressure drag of the cylinder. By [...] Read more.
The flow past a cylinder is a classical problem in flow physics. In a certain range of Reynolds number, there will be Karman vortex street phenomenon in the wake of a cylinder, which will greatly increase the pressure drag of the cylinder. By controlling the vortex shedding phenomenon, drag reduction of the cylinder could be effectively realized. In this paper, a NACA0012 airfoil with pitching oscillation is placed downstream of the cylinder. Based on the tight coupling method, kinematics equations and Navier–Stokes equations in the arbitrary Lagrangian–Eulerian form are solved. Firstly, the effect of airfoil oscillation period and the distance between airfoil leading edge and cylinder center (x/D) are studied respectively, especially considering the aspects of vortex shedding and drag reduction effect. Besides, the vortex interaction in the flow field around the airfoil and cylinder is analyzed in detail. It is found that the NACA0012 airfoil with pitching oscillation can change the period of vortex shedding. Moreover, it can also increase the drag reduction rate to as high as 50.5%, which presents a certain application prospect in the engineering drag reduction field, e.g., for launch vehicles, ship masts, submarine pipelines, etc. Full article
Show Figures

Figure 1

Article
Development of a Unified Taxonomy for HVAC System Faults
Energies 2021, 14(17), 5581; https://doi.org/10.3390/en14175581 - 06 Sep 2021
Cited by 4 | Viewed by 890
Abstract
Detecting and diagnosing HVAC faults is critical for maintaining building operation performance, reducing energy waste, and ensuring indoor comfort. An increasing deployment of commercial fault detection and diagnostics (FDD) software tools in commercial buildings in the past decade has significantly increased buildings’ operational [...] Read more.
Detecting and diagnosing HVAC faults is critical for maintaining building operation performance, reducing energy waste, and ensuring indoor comfort. An increasing deployment of commercial fault detection and diagnostics (FDD) software tools in commercial buildings in the past decade has significantly increased buildings’ operational reliability and reduced energy consumption. A massive amount of data has been generated by the FDD software tools. However, efficiently utilizing FDD data for ‘big data’ analytics, algorithm improvement, and other data-driven applications is challenging because the format and naming conventions of those data are very customized, unstructured, and hard to interpret. This paper presents the development of a unified taxonomy for HVAC faults. A taxonomy is an orderly classification of HVAC faults according to their characteristics and causal relations. The taxonomy includes fault categorization, physical hierarchy, fault library, relation model, and naming/tagging scheme. The taxonomy employs both a physical hierarchy of HVAC equipment and a cause-effect relationship model to reveal the root causes of faults in HVAC systems. A structured and standardized vocabulary library is developed to increase data representability and interpretability. The developed fault taxonomy can be used for HVAC system ‘big data’ analytics such as HVAC system fault prevalence analysis or the development of an HVAC FDD software standard. A common type of HVAC equipment-packaged rooftop unit (RTU) is used as an example to demonstrate the application of the developed fault taxonomy. Two RTU FDD software tools are used to show that after mapping FDD data according to the taxonomy, the meta-analysis of the multiple FDD reports is possible and efficient. Full article
Show Figures

Figure 1

Article
All-SiC ANPC Submodule for an Advanced 1.5 kV EV Charging System under Various Modulation Methods
Energies 2021, 14(17), 5580; https://doi.org/10.3390/en14175580 - 06 Sep 2021
Cited by 1 | Viewed by 933
Abstract
This work is focused on the design and experimental validation of the all-SiC active neutral-point clamped (ANPC) submodule for an advanced electric vehicle (EV) charging station. The topology of the station is based on a three-wire bipolar DC bus (±750 V) connecting an [...] Read more.
This work is focused on the design and experimental validation of the all-SiC active neutral-point clamped (ANPC) submodule for an advanced electric vehicle (EV) charging station. The topology of the station is based on a three-wire bipolar DC bus (±750 V) connecting an ac grid converter, isolated DC-DC converters, and a non-isolated DC-DC converter with a battery energy storage. Thus, in all types of power converters, the same three-level submodule may be applied. In this paper, a submodule rated at 1/3 of the nominal power of the grid converter (20 kVA) is discussed. In particular, four different modulation strategies for the 1.5 kV ANPC submodule, exclusively employing fast silicon carbide (SiC) MOSFETs, are considered, and their impact on the submodule performance is analyzed. Moreover, the simulation study is included. Finally, the laboratory prototype is described and experimentally verified at a switching frequency of 64 kHz. It is shown that the system can operate with all of the modulations, while techniques PWM2 and PWM3 emerge as the most efficient, and alternating between them, depending on the load, should be considered to maximize the efficiency. Furthermore, the results showcase that the impact of the different PWM techniques on switching oscillations, including overvoltages, can be nearly fully omitted for a parasitic inductance optimized circuit, and the choice of modulation should be based on power loss and/or other factors. Full article
(This article belongs to the Special Issue Power Electronics and Energy Management for Battery Storage Systems)
Show Figures

Graphical abstract

Article
State of Power Estimation of Echelon-Use Battery Based on Adaptive Dual Extended Kalman Filter
Energies 2021, 14(17), 5579; https://doi.org/10.3390/en14175579 - 06 Sep 2021
Cited by 3 | Viewed by 598
Abstract
Owing to the degradation of the performance of a retired battery and the unclear initial value of the state of charge (SOC), the estimation of the state of power (SOP) of an echelon-use battery is not accurate. An SOP estimation method based on [...] Read more.
Owing to the degradation of the performance of a retired battery and the unclear initial value of the state of charge (SOC), the estimation of the state of power (SOP) of an echelon-use battery is not accurate. An SOP estimation method based on an adaptive dual extended Kalman filter (ADEKF) is proposed. First, the second-order Thevenin equivalent model of the echelon-use battery is established. Second, the battery parameters are estimated by the ADEKF: (a) the SOC is estimated based on an adaptive extended Kalman filtering algorithm, that uses the process noise covariance Qk and observes the noise covariance Rk , and (b) the ohmic internal resistance and actual capacity are estimated based on the aforementioned algorithm, that uses the process noise covariance Q?,k and observes the noise covariance R?,k. Third, the working voltage and internal resistance are predicted using optimal estimation, and the SOP of the echelon-use battery is estimated. MATLAB simulation results show that, regardless of whether or not the initial value of the SOC is clear, the proposed algorithm can be adjusted to the adaptive algorithm, and if the estimation accuracy error of the echelon-use battery SOP is less than 4.8%, it has high accuracy. This paper provides a valuable reference for the prediction of the SOP of an echelon-use battery, and will be helpful for understanding the behavior of retired batteries for further discharge and use. Full article
Show Figures

Figure 1

Article
Performance Optimizations of the Transcritical CO2 Two-Stage Compression Refrigeration System and Influences of the Auxiliary Gas Cooler
Energies 2021, 14(17), 5578; https://doi.org/10.3390/en14175578 - 06 Sep 2021
Viewed by 623
Abstract
To optimize the performance of the transcritical CO2 two-stage compression refrigeration system, the energy analysis and the exergy analysis are conducted. It is found that higher COP, lower compression power, and less exergy destruction can be achieved when the auxiliary gas cooler [...] Read more.
To optimize the performance of the transcritical CO2 two-stage compression refrigeration system, the energy analysis and the exergy analysis are conducted. It is found that higher COP, lower compression power, and less exergy destruction can be achieved when the auxiliary gas cooler is applied. Moreover, the discharge temperature of the compound compressor (HPS) can be reduced by decreasing the temperature at the outlet of the auxiliary gas cooler (Tagc,out). When the Tagc,out is reduced from 30 to 12 °C, the discharge temperature of the compound compressor (HPS) can be decreased by 13.83 °C. Furthermore, the COP and the exergy efficiency can be raised by enhancing the intermediate pressure. Based on these results, the optimizations of system design and system operation are put forward. The application of the auxiliary gas cooler can improve the performance of the transcritical CO2 two-stage compression refrigeration system. Operators can decrease the discharge temperature of the compound compressor (HPS) by reducing the Tagc,out, and increase the COP and the exergy efficiency by enhancing the intermediate pressure. Full article
Show Figures

Figure 1

Article
A Vision for Energy Decarbonization: Planning Sustainable Tertiary Sites as Net-Zero Energy Systems
Energies 2021, 14(17), 5577; https://doi.org/10.3390/en14175577 - 06 Sep 2021
Cited by 3 | Viewed by 721
Abstract
The power system is changing towards a decarbonized one. The Kyoto protocol and the Paris climate agreement have prompted many nations to approve energy policies based on volatile renewable energy sources (RESs). However, the integration into the grid of the power generated by [...] Read more.
The power system is changing towards a decarbonized one. The Kyoto protocol and the Paris climate agreement have prompted many nations to approve energy policies based on volatile renewable energy sources (RESs). However, the integration into the grid of the power generated by RESs as well as the electrification of the heating, gas and transportation sectors is becoming a huge challenge. Planning industrial and tertiary sites as net-zero energy systems (NZESs) might contribute to advance the solutions of fully integrating volatile RESs into the power system. This study aims to point out the importance of planning large energy consumer sites such as NZESs, and to depict a holistic modeling approach for this. The methodology is based on a multi-layer approach, which focuses on on-site power generation by RESs, on the improvement of energy efficiency, and on the increase of system flexibility. A qualitative case study has been conducted. It considers the planning of a Net-Zero Energy Data Center located in Germany. Results point out that new interdisciplinary and in particular social analysis methods are necessary. They might be used for accelerating the decision making process during the planning of RES-based on-site power generation systems. Besides, for computation and cooling systems, new technologies that are continuously emerging in the market should be taken into account. If well designed, they contribute to significantly decrease the whole energy demand of data center. Finally, optimal sizing of energy storage systems (electric and thermal) as well as an expedient choice of performance indicators to evaluate technology options are identified as the key factor for decreasing the external energy demand of tertiary sites, such as data center. Full article
(This article belongs to the Special Issue Alternative Energy Policy)
Show Figures

Figure 1

Article
Shrinking-Core Model Integrating to the Fluid-Dynamic Analysis of Fixed-Bed Adsorption Towers for H2S Removal from Natural Gas
Energies 2021, 14(17), 5576; https://doi.org/10.3390/en14175576 - 06 Sep 2021
Cited by 3 | Viewed by 976
Abstract
Natural gas sweetening is an essential process within hydrocarbon processing operations, enabling compliance with product quality specifications, avoiding corrosion problems, and enabling environmental care. This process aims to remove hydrogen sulfide (H2S), carbon dioxide, or both contaminants. It can be carried [...] Read more.
Natural gas sweetening is an essential process within hydrocarbon processing operations, enabling compliance with product quality specifications, avoiding corrosion problems, and enabling environmental care. This process aims to remove hydrogen sulfide (H2S), carbon dioxide, or both contaminants. It can be carried out in fixed-bed adsorption towers, where iron oxide-based solid sorbent reacts with the H2S to produce iron sulfides. This study is set out to develop a fluid-dynamic model that allows calculating the pressure drop in the H2S adsorption towers with the novelty to integrate reactivity aspects, through an iron sulfide layer formation on the solid particles’ external skin. As a result of the layer formation, changes in the particle diameter and the bed void fraction of the solid sorbent tend to increase the pressure drop. The shrinking-core model and the H2S adsorption front variation in time support the model development. Experimental data on pressure drop at the laboratory scale and industrial scale allowed validating the proposed model. Moreover, the model estimates the bed replacement frequency, i.e., the time required to saturate the fixed bed, requiring its replacement or regeneration. The model can be used to design and formulate new solid sorbents, analyze adsorption towers already installed, and help maintenance-planning operations. Full article
Show Figures

Graphical abstract

Article
Optimization Tool for the Strategic Outline and Sizing of District Heating Networks Using a Geographic Information System
Energies 2021, 14(17), 5575; https://doi.org/10.3390/en14175575 - 06 Sep 2021
Cited by 1 | Viewed by 597
Abstract
The implementation of district heating networks into cities is a main topic in policy planning that looks for sustainable solutions to reduce CO2 emissions. However, their development into cities is generally limited by a high initial investment cost. The development of optimization [...] Read more.
The implementation of district heating networks into cities is a main topic in policy planning that looks for sustainable solutions to reduce CO2 emissions. However, their development into cities is generally limited by a high initial investment cost. The development of optimization methods intended to draft efficient systems using heating consumption profiles into a prescribed geographic area are useful in this purpose. Such tools are already referred to in the scientific literature, yet they are often restricted to limit the computational load. To bridge this gap, the present contribution proposes a multi-period mixed integer linear programming model for the optimal outline and sizing of a district heating network maximizing the net cash flow based on a geographic information system. This methodology targets a large range of problem sizes from small-scale to large-scale heating networks while guaranteeing numerical robustness. For sake of simplicity, the developed model is first applied to a scaled down case study with 3 available heating sources and a neighborhood of 16 streets. The full-scale model is presented afterwards to demonstrate the applicability of the tool for city-scale heating networks with around 2000 streets to potentially connect within a reasonable computational time of around only one hour. Full article
Show Figures

Figure 1

Article
GHG and NH3 Emissions vs. Energy Efficiency of Maize Production Technology: Evidence from Polish Farms; a Further Study
Energies 2021, 14(17), 5574; https://doi.org/10.3390/en14175574 - 06 Sep 2021
Cited by 3 | Viewed by 620
Abstract
The paper determines the effect of selected cultivation technologies, including production chain energy inputs (growing, harvest, heap forming) on greenhouse gas emissions (GHGs) to the atmosphere. The data for the study was collected from 13 actually operating family farms ranging in size from [...] Read more.
The paper determines the effect of selected cultivation technologies, including production chain energy inputs (growing, harvest, heap forming) on greenhouse gas emissions (GHGs) to the atmosphere. The data for the study was collected from 13 actually operating family farms ranging in size from 2 to 13 ha, located in the Podlaskie voivodship (Poland). GHG and ammonia (NH3) emissions from natural and mineral fertilisation as well as GHGs from energy carriers in a form of fuels (ON) were estimated. The average GHG emissions from the sources analysed were 1848.030 kg·CO2eq·ha−1 and 29.492 kg·CO2eq·t−1 of the green forage yield. The average NH3 emissions per hectare were 15,261.808 kg NH3 and 248.871 kg NH3·t−1 of yield. The strongest impact on the environment, due to the GHG emissions to the atmosphere, thus contributing to the greenhouse effect, is due nitrogen fertilisation, both mineral and natural. On average, in the technologies under study, 61% of the total GHG emissions came from fertilisation. The GHG emissions were correlated with the energy efficiency, calculated at the previous research stage, of the production technologies applied. There is a negative correlation (r = −0.80) between the features studied, which means that the higher the energy efficiency of the silage maize plantations, the lower the air pollution emissions in a form of the GHGs from the sources under study. It is so important to prevent environmental degradation to continue, conduct in-depth, interdisciplinary research on reducing the energy consumption of crop production technologies and striving to increase energy efficiency. Full article
(This article belongs to the Special Issue Advances in Sustainable Energy and Environmental Economics)
Show Figures

Figure 1

Article
Solar-Powered Thermoelectric-Based Cooling and Heating System for Building Applications: A Parametric Study
Energies 2021, 14(17), 5573; https://doi.org/10.3390/en14175573 - 06 Sep 2021
Cited by 3 | Viewed by 788
Abstract
Thermoelectric (TE) based cooling and heating systems offer significant advantages over conventional vapor compression systems including no need for refrigeration or major moving parts, high controllability, and scalability. The purpose of the present study is to provide an energy and economic assessment of [...] Read more.
Thermoelectric (TE) based cooling and heating systems offer significant advantages over conventional vapor compression systems including no need for refrigeration or major moving parts, high controllability, and scalability. The purpose of the present study is to provide an energy and economic assessment of the performance of a TE-based radiant cooling and heating system for building applications. It is considered that TE modules are integrated in the ceiling to lower/increase the ceiling temperature through the Peltier effect during the hot/cold season to provide thermal comfort for the occupants via radiation and convection. The study explores the possibility of using rooftop PV panels to produce electricity required for the operation of TE modules. An actual office building located in Melbourne, FL, USA is considered for a test study, and the hourly cooling and heating loads of the building are calculated through building energy simulation in eQuest. Various operating conditions, including different input voltages and temperature gradient across TE modules, are considered, and the system is sized to properly address the year-around cooling/heating demand. It is shown that a nominal cooling capacity of 112.8 W and a nominal PV capacity of 31.35 W per unit area of the building is required to achieve the target goal when the system operates at the optimal condition. An economic analysis is also performed, and estimated cost, as well as potential savings, are calculated for each operating condition. The optimal operating condition with minimum cost is selected accordingly. The results demonstrated that the initial cost of the proposed system is considerably higher than conventional heating/cooling systems. However, the system offers other benefits that can potentially make it an attractive option for building cooling/heating applications. Full article
(This article belongs to the Topic Sustainable Energy Technology)
Show Figures

Figure 1

Article
Data-Driven Virtual Inertia Control Method of Doubly Fed Wind Turbine
Energies 2021, 14(17), 5572; https://doi.org/10.3390/en14175572 - 06 Sep 2021
Cited by 3 | Viewed by 565
Abstract
This paper presents a data-driven virtual inertia control method for doubly fed induction generator (DFIG)-based wind turbine to provide inertia support in the presence of frequency events. The Markov parameters of the system are first obtained by monitoring the grid frequency and system [...] Read more.
This paper presents a data-driven virtual inertia control method for doubly fed induction generator (DFIG)-based wind turbine to provide inertia support in the presence of frequency events. The Markov parameters of the system are first obtained by monitoring the grid frequency and system operation state. Then, a data-driven state observer is developed to evaluate the state vector of the optimal controller. Furthermore, the optimal controller of the inertia emulation system is developed through the closed solution of the differential Riccati equation. Moreover, a differential Riccati equation with self-correction capability is developed to enhance the anti-noise ability to reject noise interference in frequency measurement process. Finally, the simulation verification was performed in Matlab/Simulink to validate the effectiveness of the proposed control strategy. Simulation results showed that the proposed virtual inertia controller can adaptively tune control parameters online to provide transient inertia supports for the power grid by releasing the kinetic energy, so as to improve the robustness and anti-interference ability of the control system of the wind power system. Full article
(This article belongs to the Special Issue Energy Efficiency, Low Carbon Resources and Renewable Technology)
Show Figures

Figure 1

Article
Krasovskii Passivity and μ-Synthesis Controller Design for Quasi-Linear Affine Systems
Energies 2021, 14(17), 5571; https://doi.org/10.3390/en14175571 - 06 Sep 2021
Cited by 6 | Viewed by 650
Abstract
This paper presents an end-to-end method to design passivity-based controllers (PBC) for a class of input-affine nonlinear systems, named quasi-linear affine. The approach is developed using Krasovskii’s method to design a Lyapunov function for studying the asymptotic stability, and a sufficient condition to [...] Read more.
This paper presents an end-to-end method to design passivity-based controllers (PBC) for a class of input-affine nonlinear systems, named quasi-linear affine. The approach is developed using Krasovskii’s method to design a Lyapunov function for studying the asymptotic stability, and a sufficient condition to construct a storage function is given, along with a supply-rate function. The linear fractional transformation interconnection between the nonlinear system and the Krasovskii PBC (K-PBC) results in a system which manages to follow the provided input trajectory. However, given that the input and output of the closed-loop system do not have the same physical significance, a path planning is mandatory. For the path planning component, we propose a robust controller designed using the μ-synthesis mixed-sensitivity loop-shaping for the linearized system around a desired equilibrium point. As a case study, we present the proposed methodology for DC-DC converters in a unified manner, giving sufficient conditions for such systems to be Krasovskii passive in terms of Linear Matrix Inequalities (LMIs), along with the possibility to compute both the K-PBC and robust controller alike. Full article
(This article belongs to the Special Issue Advanced Control Strategies for Electric Power Management)
Show Figures

Figure 1

Article
Application of the Harmonic Balance Method for Spatial Harmonic Interactions Analysis in Axial Flux PM Generators
Energies 2021, 14(17), 5570; https://doi.org/10.3390/en14175570 - 06 Sep 2021
Cited by 2 | Viewed by 760
Abstract
In this paper, an application of the Harmonic Balance Method (HBM) for analysis of Axial Flux Permanent Magnet Generator (AFPMG) is carried out. Particular attention was paid to development of mathematical model equations allowing to estimate the machine properties, without having to use [...] Read more.
In this paper, an application of the Harmonic Balance Method (HBM) for analysis of Axial Flux Permanent Magnet Generator (AFPMG) is carried out. Particular attention was paid to development of mathematical model equations allowing to estimate the machine properties, without having to use quantitative solutions. The methodology used here allowed for precise determination of Fourier spectra with respect to winding currents and electromagnetic torque (both quantitatively and qualitatively) in steady state operation. Analyses of space harmonic interaction in steady states were presented for the three-phase AFPMG. Satisfactory convergence was between the results of calculations and measurements which confirmed the initial assumption that the developed circuit models of AFPMG are sufficiently accurate and can be useful in the diagnostic analyses, tests and the final stages of the design process. Full article
(This article belongs to the Special Issue Design and Application of Electrical Machines)
Show Figures

Figure 1

Article
Gasification of Coal by CO2: The Impact of the Heat Transfer Limitation on the Progress, Reaction Rate and Kinetics of the Process
Energies 2021, 14(17), 5569; https://doi.org/10.3390/en14175569 - 06 Sep 2021
Cited by 2 | Viewed by 545
Abstract
This paper presents the impact of thermal lag on the progress of different coal types’ gasification by CO2. The analysis was performed using thermogravimetry and numerical modeling. Experiments were carried out at a heating rate of 1–50 Kmin−1 and a [...] Read more.
This paper presents the impact of thermal lag on the progress of different coal types’ gasification by CO2. The analysis was performed using thermogravimetry and numerical modeling. Experiments were carried out at a heating rate of 1–50 Kmin−1 and a temperature ranging from 383 to 1173 K. The developed numerical model enabled the determination of a true sample temperature considering the gasification process to consist of two single-step consecutive reactions. Analysis revealed that the average thermal lag in CO2 is about 11% greater than that in N2, which is related to the properties of CO2 itself and the occurrence of the char–CO2 reaction. The onset temperature of the reverse Boudouard reaction depends on the type of fuel; however, no simple relationship with the coal rank was found. Thermal lag has an impact on the kinetic parameter Aα0.5 describing devolatilization, up to 19.8%, while in the case of the char–CO2 reaction, this influence is expected to be even greater. The performed analysis proved that disregarding thermal lag may significantly hinder the interpretation of the analyzed processes; thus, TG experiments should be carried out with a low heating rate, or at the post-processing stage, a thermal lag model needs to be employed. Full article
(This article belongs to the Special Issue Modelling and Calculation of Raw Material Industry)
Show Figures

Figure 1

Article
Thermodynamic Analysis of CNG Fast Filling Process of Composite Cylinder Type IV
Energies 2021, 14(17), 5568; https://doi.org/10.3390/en14175568 - 06 Sep 2021
Viewed by 689
Abstract
Due to ecological and economic advantages, natural gas is used as an alternative fuel in the transportation sector in the form of compressed natural gas (CNG) and liquefied natural gas (LNG). Development of infrastructure is necessary to popularize vehicles that use alternative fuels. [...] Read more.
Due to ecological and economic advantages, natural gas is used as an alternative fuel in the transportation sector in the form of compressed natural gas (CNG) and liquefied natural gas (LNG). Development of infrastructure is necessary to popularize vehicles that use alternative fuels. Selected positive factors from EU countries supporting the development of the CNG market were discussed. The process of natural gas vehicle (NGV) fast filling is related to thermodynamic phenomena occurring in a tank. In this study, the first law of thermodynamics and continuity equations were applied to develop a theoretical model to investigate the effects of natural gas composition on the filling process and the final in-cylinder conditions of NGV on-board composite cylinder (type IV). Peng–Robinson equation of state (P-R EOS) was applied, and a lightweight composite tank (type IV) was considered as an adiabatic system. The authors have devised a model to determine the influence of natural gas composition on the selected thermodynamic parameters during fast filling: Joule–Thomson (J-T) coefficient, in-cylinder gas temperature, mass flow rate profiles, in-cylinder mass increase, natural gas density change, ambient temperature on the final natural gas temperature, influence of an ambient temperature on the amount of refueled natural gas mass. Results emphasize the importance of natural gas composition as an important parameter for the filling process of the NGV on-board composite tank (type IV). Full article
Show Figures

Figure 1

Article
Is Green Recovery Enough? Analysing the Impacts of Post-COVID-19 Economic Packages
Energies 2021, 14(17), 5567; https://doi.org/10.3390/en14175567 - 06 Sep 2021
Cited by 11 | Viewed by 1435
Abstract
Emissions pathways after COVID-19 will be shaped by how governments’ economic responses translate into infrastructure expansion, energy use, investment planning and societal changes. As a response to the COVID-19 crisis, most governments worldwide launched recovery packages aiming to boost their economies, support employment [...] Read more.
Emissions pathways after COVID-19 will be shaped by how governments’ economic responses translate into infrastructure expansion, energy use, investment planning and societal changes. As a response to the COVID-19 crisis, most governments worldwide launched recovery packages aiming to boost their economies, support employment and enhance their competitiveness. Climate action is pledged to be embedded in most of these packages, but with sharp differences across countries. This paper provides novel evidence on the energy system and greenhouse gas (GHG) emissions implications of post-COVID-19 recovery packages by assessing the gap between pledged recovery packages and the actual investment needs of the energy transition to reach the Paris Agreement goals. Using two well-established Integrated Assessment Models (IAMs) and analysing various scenarios combining recovery packages and climate policies, we conclude that currently planned recovery from COVID-19 is not enough to enhance societal responses to climate urgency and that it should be significantly upscaled and prolonged to ensure compatibility with the Paris Agreement goals. Full article
(This article belongs to the Special Issue Energy Systems Analysis and Modelling towards Decarbonisation)
Show Figures

Figure 1

Article
Consumer versus Organic Products in the COVID-19 Pandemic: Opportunities and Barriers to Market Development
Energies 2021, 14(17), 5566; https://doi.org/10.3390/en14175566 - 06 Sep 2021
Cited by 13 | Viewed by 1912
Abstract
The main objective of this study was to determine the behavior of the Polish consumer of organic products during the COVID-19 pandemic and to identify preferred channels of distribution of organic products in the situation of restricted freedom of movement as well as [...] Read more.
The main objective of this study was to determine the behavior of the Polish consumer of organic products during the COVID-19 pandemic and to identify preferred channels of distribution of organic products in the situation of restricted freedom of movement as well as to assess what information displayed on the labels of organic food was most important to the customer. The research was conducted on a sample of 1108 respondents with the use of CAWI technique collected in an online survey carried out in February–August 2020. To analyze the obtained results, cluster analysis, linear regression model and duplication method were used to verify the substitute channels for purchasing organic goods. The pandemic has intensified the health value of consumers when making decisions about choosing food products. Consumers are sensitive shoppers who read the content of the labels and pay attention to the ingredients of the products they buy. The price is also of significant importance for consumers; however, it is less important than, for example, the expiration date of the purchased product. With the use of PCA analysis, it was possible to identify 18 factors that could be divided into three segments: marketing, practical and sensory. The proposed factors, according to the respondents, had an effect on the purchase of organic products by Polish consumers. Regarding the preferred purchasing channels, the Internet is becoming more and more important. Almost one-quarter of the respondents confirmed that they bought organic products via the above-mentioned distribution channel. Nearly 17% of the surveyed consumers considered the Internet to be an alternative way of doing their shopping. The results obtained in the research can be used in the sector of organic food producers to design marketing strategies and to adapt their offer to the proposed four groups of purchasers of organic products: eco-activists, eco-dietitians, eco-traditionalists, eco-innovators. Full article
Show Figures

Figure 1

Review
Electrification of Compact Off-Highway Vehicles—Overview of the Current State of the Art and Trends
Energies 2021, 14(17), 5565; https://doi.org/10.3390/en14175565 - 06 Sep 2021
Cited by 4 | Viewed by 1209
Abstract
Electrified vehicles have undergone great evolution during the last decade because of the increasing attention paid on environmental sustainability, greenhouse gas emissions and air pollution. Emission regulations are becoming increasingly tight, and governments have been allocating multiple funds to facilitate the spreading of [...] Read more.
Electrified vehicles have undergone great evolution during the last decade because of the increasing attention paid on environmental sustainability, greenhouse gas emissions and air pollution. Emission regulations are becoming increasingly tight, and governments have been allocating multiple funds to facilitate the spreading of the so-called green mobility. In this context, steering towards electrified solutions not only for passenger vehicles, but also for compact off-highway vehicles extensively employed, for instance, on construction sites located in urban areas, warehouses, and greenhouses, is essential even if seldom considered. Moreover, the electrification of compact off-highway machinery may allow manufacturers to increase their expertise in and lower the costs of these alternative solutions, while gathering useful data to be applied in bigger and more remunerative off-highway vehicles. In fact, while electric automobiles are as of now real alternatives for buyers, off-highway vehicles, regardless of the application, are mostly in the research and experimental phase, with few of them already on the market. This delay, in comparison with the passenger automotive industry, is caused by different factors, mostly related to the different tasks of off-highway vehicles in terms of duty cycles, productivity performance parameters and user acceptability. The aim of this paper is to give an overview of the many aspects of the electrification of compact off-highway vehicles, to highlight the key differences between on-highway and off-highway vehicles and to summarize in a single source of information the multiple solutions investigated by researchers and manufacturers. Full article
Show Figures

Figure 1

Article
A Study to Investigate the Effect of Valve Mechanisms on Exhaust Residual Gas and Effective Release Energy of a Motorcycle Engine
Energies 2021, 14(17), 5564; https://doi.org/10.3390/en14175564 - 06 Sep 2021
Viewed by 587
Abstract
The purpose of this study was to investigate the effect of valve mechanisms on the exhaust residual gas (ERG) and effective release energy (ERE) of a motorcycle engine. Here, a simulation model and the estimation a new valve mechanism design is presented. An [...] Read more.
The purpose of this study was to investigate the effect of valve mechanisms on the exhaust residual gas (ERG) and effective release energy (ERE) of a motorcycle engine. Here, a simulation model and the estimation a new valve mechanism design is presented. An AVL-Boost simulation model and an experiment system were established. The classical spline approximation method was used to design a new cam profile for various valve lifts. The simulation model was used to estimate the effect of the new valve mechanism designs on engine performance. A new camshaft was produced based on the research data. The results show that the engine obtained a maximum engine brake torque of 21.53 Nm at 7000 rpm, which is an increase of 3.2% compared to the engine using the original valve mechanism. In addition, the residual gas was improved, the maximum engine effective release energy was 0.83 kJ, the maximum engine power was 18.1 kW, representing an improvement of 7.2%, and the air mass flow was improved by 4.97%. Full article
Show Figures

Graphical abstract

Article
Bioenergy Production through Mono and Co-Digestion of Tomato Residues
Energies 2021, 14(17), 5563; https://doi.org/10.3390/en14175563 - 06 Sep 2021
Cited by 1 | Viewed by 574
Abstract
The agro-industry of tomato generates three types of residues: ripe rotten tomato (unfit for consumption) (RT), green (unripe) tomato (GT), and tomato branches including leaves and stems (TB). These materials are commonly wasted or used as feed for livestock. Energy production through anaerobic [...] Read more.
The agro-industry of tomato generates three types of residues: ripe rotten tomato (unfit for consumption) (RT), green (unripe) tomato (GT), and tomato branches including leaves and stems (TB). These materials are commonly wasted or used as feed for livestock. Energy production through anaerobic digestion is an alternative way to manage and simultaneously valorise these materials. Initially, the operating conditions of mono anaerobic digestion were investigated using RT. Thus, a design of experiments based on a two-level fractional factorial design with resolution V was performed to determine the factors that affect biochemical methane potential (BMP). The substrate to inoculum ratio (SIR), total volatile solids concentration (VSt), working volume (WV), presence of nutrients (Nu), and the pre-incubation of the inoculum (Inc) were investigated. The results showed that SIR is the most important factor. The maximum BMP for RT was 297 NmLCH4/gVS with SIR = 0.5; tVS = 20 g/L; WV = 20%; no pre-incubation and the presence of nutrients. Using these optimum operating conditions, co-digestion was investigated through a mixture design approach. The substrates RT and GT presented similar BMP values, whereas TB led to a significantly lower BMP. Indeed, when high concentrations of TB were used, a significant decrease in methane production was observed. Nonetheless, the highest BMP was achieved with a mixture of 63% RT + 20% GT + 17% TB, with a production of 324 NmLCH4/gVS, corresponding to a synergetic co-digestion performance index of about 1.20. In general, although the substrate RT generates the highest BMP, the mixture with GT did not impair the methane yield. Overall, the co-digestion of tomato residues must be conducted with SIR close to 0.5 and the content of tomato branches in the reaction mixture should be kept low (up to 20%). Full article
Show Figures

Figure 1

Article
Model for 400 kV Transmission Line Power Loss Assessment Using the PMU Measurements
Energies 2021, 14(17), 5562; https://doi.org/10.3390/en14175562 - 06 Sep 2021
Viewed by 718
Abstract
This paper presents an advanced model for monitoring losses on a 400 kV over-head transmission line (OHL) that can be used for measured data verification and loss assessment. Technical losses are unavoidable physical effects of energy transmission and can be reduced to acceptable [...] Read more.
This paper presents an advanced model for monitoring losses on a 400 kV over-head transmission line (OHL) that can be used for measured data verification and loss assessment. Technical losses are unavoidable physical effects of energy transmission and can be reduced to acceptable levels, with a major share of technical losses on transmission lines being Joule losses. However, at 400 kV voltage levels, the influence of the electrical corona discharge effect and current leakage can have significant impact on power loss. This is especially visible in poor weather conditions, such as the appearance of fog, rain and snow. Therefore, loss monitoring is incorporated into exiting business process to provide transmission system operators (TSO) with the measure of losses and the accurate characterization of measured data. This paper presents an advanced model for loss characterization and assessment that uses phasor measurement unit (PMU) measurements and combines them with end-customer measurements. PMU measurements from the algorithm of differential protection are used to detect differential currents and angles, and this paper proposes further usage of these data for determining the corona losses. The collected data are further processed and used to calculate the amount of corona losses and provide accurate loss assessment and estimation. In each step of the model, cross verification of the measured and calculated data is performed in order to finally provide more accurate loss assessment which is incorporated into the current data acquisition and monitoring systems. Full article
(This article belongs to the Special Issue Advances in Synchronized Measurements Technologies in Smart Grids)
Show Figures

Figure 1

Article
Modeling Wind Speed Based on Fractional Ornstein-Uhlenbeck Process
Energies 2021, 14(17), 5561; https://doi.org/10.3390/en14175561 - 06 Sep 2021
Cited by 2 | Viewed by 1042
Abstract
The primary task of the design and feasibility study for the use of wind power plants is to predict changes in wind speeds at the site of power system installation. The stochastic nature of the wind and spatio-temporal variability explains the high complexity [...] Read more.
The primary task of the design and feasibility study for the use of wind power plants is to predict changes in wind speeds at the site of power system installation. The stochastic nature of the wind and spatio-temporal variability explains the high complexity of this problem, associated with finding the best mathematical modeling which satisfies the best solution for this problem. In the known discrete models based on Markov chains, the autoregressive-moving average does not allow variance in the time step, which does not allow their use for simulation of operating modes of wind turbines and wind energy systems. The article proposes and tests a SDE-based model for generating synthetic wind speed data using the stochastic differential equation of the fractional Ornstein-Uhlenbeck process with periodic function of long-run mean. The model allows generating wind speed trajectories with a given autocorrelation, required statistical distribution and provides the incorporation of daily and seasonal variations. Compared to the standard Ornstein-Uhlenbeck process driven by ordinary Brownian motion, the fractional model used in this study allows one to generate synthetic wind speed trajectories which autocorrelation function decays according to a power law that more closely matches the hourly autocorrelation of actual data. In order to demonstrate the capabilities of this model, a number of simulations were carried out using model parameters estimated from actual observation data of wind speed collected at 518 weather stations located throughout Russia. Full article
Show Figures

Figure 1

Article
The Consumption of Renewable Energy Sources (RES) by the European Union Households between 2004 and 2019
Energies 2021, 14(17), 5560; https://doi.org/10.3390/en14175560 - 06 Sep 2021
Cited by 16 | Viewed by 1047
Abstract
The paper provides the analysis of fuel and energy transition in households sector and its sustainable development in the period 2004–2019. The main purpose of the paper is to determine the development trends in the use of renewable energy sources (RES) in the [...] Read more.
The paper provides the analysis of fuel and energy transition in households sector and its sustainable development in the period 2004–2019. The main purpose of the paper is to determine the development trends in the use of renewable energy sources (RES) in the EU countries household sector in 2004–2019, to recognize the state of development and functioning of the studied area as well as to indicate their successes and shortcomings in observed reality. The article employs the results of Energy balance sheets from Eurostat. The research entity were households from 28 European Union countries, with particular emphasis on households from Poland and selected neighboring countries. The research subjects there were different sources of renewable energy used by households, i.e., solar thermal system, geothermal technologies, primary solid biofuels, charcoal, biogases, blended biogasoline, blended biodiesels, ambient heat (heat pumps). To achieve the research objective a number of statistical measures ands methods, including cluster analysis and linear trend indicator applied. In the analyzed 16 years, an absolute and relative increase in the use of RES in the household sector was noticed. Taking into account the specificity of using RES in households, 6 clusters of countries were distinguished. In Poland, it was noted that there was a significant increase in the use of RES in households, with stagnation in the use of non-renewable energy sources, such as, for example, hard coal. Full article
(This article belongs to the Special Issue Renewable and Sustainable Energy: Current State and Prospects)
Show Figures

Figure 1

Previous Issue
Back to TopTop