High-temperature heat pumps (HTHPs) are high-potential technologies for the decarbonization of low- and medium-temperature industrial heating processes. The conventional HTHPs can deliver heat up to 150-160 degrees C, while the process heat production at higher temperatures is a challenge that has attracted a lot of research in the last year. The goal of this work is to conduct a detailed investigation of different configurations of supercritical CO2 reverse Brayton HTHPs, aiming to determine the most efficient and promising designs. This analysis investigates different process heat production from 150 degrees C up to 250 degrees C, while the HTHPs are driven by low-grade waste heat in the range of 50 -120 degrees C. This work is performed with developed mathematical thermodynamic models in Engineering Equation Solver, which are verified with literature data. According to the results of this analysis, the recompression is a proper solution for low heating production temperatures (mainly at 150 degrees C), while at higher heating production temperatures, the Reheating with an internal heat exchanger has to be selected. The application of the internal heat exchanger enhances the coefficient of performance up to 8.13% and the exergy efficiency up to 6.54%. For the typical case with source temperature at 100 degrees C, the average COP enhancement is found at 3.9% with internal heat exchanger, at 12.5% with Reheating with internal heat exchanger and at 15.5% with Double reheating with internal heat exchanger compared to the Simple cycle.

The electrification of the industrial sector is an important pathway to decarbonizing the industry and achieving a sustainable society. High-temperature heat pumps (HTHPs) are critical devices for providing industrial heat and consuming green electricity. The goal of the present work is the theoretical thermodynamic analysis of a reverse Brayton HTHP that operates with novel working fluids. Specifically, the idea of using mixtures of working fluids with CO2 is studied for the first time with the aim of suggesting new candidates to increase the performance of industrial HTHPs. A model of an HTHP with an internal heat exchanger is developed and verified in the MATLAB programming language. Nine different mixtures are studied: CO2/R152a, CO2/R1234ze(E), CO2/Propane, CO2/Butane, CO2/Isobutane, CO2/Pentane, CO2/Isopentane, CO2/Hexane and CO2/Heptane. The examined industrial heat production temperatures are 150 degrees C, 200 degrees C and 250 degrees C, while the waste heat stream temperatures that drive the heat pump are considered to be 80 degrees C and 100 degrees C. The results prove that the application of the mixtures can enhance the COP, especially in the case of lower temperature lifts. CO2/R152a seems to be a promising choice compared to pure CO2, presenting performance enhancements ranging from 4.12% to 64.02% among the studied scenarios.

Beam-down concentrating solar technology is a novel and promising technology consisting of two consecutive reflections, concentrating the available solar irradiation at ground level, where the receiver is located. Solar rays fall on the primary mirror field, which redirects them towards a secondary reflector, and finally, they are reflected and focused on the receiver. In this work, the integration of a beam-down linear Fresnel reflector and a compound parabolic collector is investigated. The novelty of this work lies in presenting the design of a new parabolic-segmental secondary reflector aiming to combine construction cost reduction and efficiency improvement compared to the parabolic design. Furthermore, the receiver is a complex of two compound parabolic concentrators. The optimization of the receiver's cavity was conducted using the Bezier polynomial parametrization method. The specific receiver is designed for the maximum utilization of solar irradiation and is intended for medium to high-temperature applications. Linear and flat reflectors comprise the primary mirror field. The maximum optical efficiency is equal to 34.83%. The system's geometrical concentration ratio is equal to 16.20, whereas the concentration ratio of each compound parabolic concentrator is 3.04. The thermal and exergy analyses were carried out for various operating scenarios, developing the system's performance curves, and the maximum thermal and exergy efficiencies were found to be equal to 33.70% and 16.93%, respectively. Furthermore, it is assumed to lead in potential construction cost reduction due to its simpler design and less manufacturing processes required, which is not claimed as an evidence of this study.

Escalating climate change and the increasing frequency of weather extremes pose a threat to the resilience of urban environments and human health, highlighting the urgent need for implementing energy-efficient interventions and reducing building cooling loads. This study investigates the passive building envelope retrofit technologies of external shading, electrochromic windows, and thermochromic windows through a multi-criteria evaluation analysis based on energy savings, economic performance, and indoor thermal comfort improvement. Thermochromic windows are discerned by a mean colour transition temperature of 34 degrees C and operate throughout the entire year, while electrochromic windows are activated only during cooling periods. Both technologies present total solar transmittance indices of 72.6% and 8.4% in the bleached and tinted state, respectively. External shading devices are either static or movable, applied with an inclination angle, and are either standalone interventions or combined with chromogenic glazing. Eight retrofit scenarios are investigated for a single-story, fully electrified residential building in Athens, Greece. The building features south- and east-oriented windows, which is an appropriate case to assess the effectiveness of these passive envelope cooling technologies in regulating solar heat gains. Thermal comfort is assessed using Fanger's PMV (predicted mean vote) and PPD (Predicted Percentage of Dissatisfied) indices. The combination of electrochromic windows and movable external shading yields the highest annual electricity savings at 22.2% and reduces the PPD by 15.8%. Local static shading, on the other hand, ranks as the optimal retrofit solution in terms of economic performance, with a life-cycle cost of & euro;6378, a 9.3% improvement in thermal comfort, and a corresponding reduction of 626 thermal discomfort hours. While the proposed multi-criteria framework can be applied to other buildings and climates, the quantitative results reported here are linked to the specific case examined: a residential building with south- and east-facing glazing in Athens, Greece, representing Mediterranean climatic conditions.

This study investigates the combined impact of building retrofitting and distributed energy resource (DER) deployment on the efficiency and stability of urban low-voltage distribution networks. A synthetic residential district was developed using a bottom-up approach, aggregating dynamically simulated building energy profiles and mapping them to feeder nodes. Two models were used: a steady-state investment optimization model to size photovoltaics (PV) and battery systems under net billing, and a detailed dynamic simulation to assess power flows and voltage regulation. This enables a spatially resolved analysis linking building-level demand, district energy planning, and grid dynamics. Results show that retrofitting reduces total electricity demand by 32.7 %, heating loads by over 50 %, and peak load from 201 kW to 113 kW. Aggregated PV-battery systems are more cost-effective than decentralized installations. In the decentralized setup, weighted self-consumption rates reached 46.2 % and 56.1 % in the reference and renovated cases, while the aggregated configuration achieved 51.2 % and 60.2 %, respectively, with shorter payback periods (similar to 6.4 years vs. 7.4-9.9 years). Despite the shared use of resources, electricity imports per building decreased under aggregation, reflecting higher autonomy; for instance, imports in unrenovated apartments dropped by 10.7 %, while the allocated CAPEX share declined from 4085 (sic) to 3730 (sic) (-8.7 %) due to more efficient system sizing and economies of scale. Dynamic simulations showed that Q/V control reduced RMS voltage deviation by 10.7 %, variability by 16.7 %, and ensured 100 % compliance with voltage limits. The proposed framework demonstrates the benefits of coordinated retrofitting and DER aggregation in enabling cost-effective decarbonization, improved self-consumption, and enhanced power system resilience.

The main purpose of the present paper is to investigate a multiple energy production and storage system to fulfill the needs of a residential building, considering thermodynamic, economic, and environmental factors. The proposed configuration combines the concept of the "Carnot battery" with polygeneration. It includes a multistage heat pump, an organic Rankine cycle module, and water storage tanks that store heat at numerous temperature levels, i.e., a low-temperature cooling storage, a medium, and a high-temperature storage. The system is driven by photovoltaics, while the integration of a battery is also tested as a secondary energy storage technique. The configuration is examined parametrically considering different values of the photovoltaic area, storage tank volumes, and battery capacities, while the integrated components are optimally dimensioned. The results reveal that the annual round-trip efficiency reaches the value of 81.9 %. A multi-objective evaluation indicates a heating coverage of 67.5 %, a cooling coverage of 95.8 %, a DHW coverage of 95.9 %, and an electricity coverage for appliances & lighting of 87.7 %, at the final design, whereas the mean load coverage is computed at 86.7 %. Finally, the total net present cost is determined at 78125.7 , whereas the annual carbon emissions savings are calculated at 8599.0 kgCO2-eq.

The Kalina cycle and Organic Rankine Cycle (ORC) are among the most promising technologies for utilizing low to medium temperature heat sources for electricity generation. In the present study, the Kalina cycle, along with subcritical and transcritical ORC configurations using novel and environmentally friendly working fluids, are investigated thermodynamically, considering geothermal sources across a temperature range of 75-300 degrees C. The scenarios examined include: the Kalina cycle; subcritical ORC with R600a, R601a, R1233zd(E), and the R601a/ R600a mixture; and transcritical ORC with R600a, R601a, and R1233zd(E), with corresponding models created using the Engineering Equation Solver (EES) and MATLAB coupled with the CoolProp tool. Each system is optimized under its respective decision variables to maximize electricity generation, and the optimized systems are compared across the full temperature range. The findings show that the Kalina cycle is the most effective option at low source temperatures, achieving up to 50% higher power generation than subcritical ORC with pure fluids and around 5% more than the zeotropic mixture at the lower end of the range. At medium temperatures (150-175 degrees C), the subcritical ORC performs best, with R601a/R600a being the optimal choice at 150 degrees C and R600a at 175 degrees C. At higher temperatures, the transcritical ORC configurations are the best performing, with R1233zd(E) proving to be the most effective working fluid. Overall, different systems are favored at different source temperature ranges, highlighting the importance of identifying the most suitable option for maximizing electricity generation.

The authors wish to make the following correction to this paper [...]

A closed-loop Reverse-Brayton High Temperature Heat Pump with air working medium is studied and optimized considering real operating conditions for a ceramic industry. The heat pump was considered to upgrade the waste heat of the kiln's exhaust gases into useful heat for further heating up the combustion air. All the necessary thermodynamic variables are determined by calculating the compressor and turbine work, the input power, the coefficient of performance, and the exergy efficiency. First, a detailed optimization procedure is carried out for various inlet preheated air temperatures in the range of 100-200 degrees C, with the primary aim being the maximization of the heat pump performance (COP maximization) by identifying the optimal working-medium mass flow rate for each preheated air temperature level. Secondly, for the defined optimized operating conditions, a dynamic analysis was performed for a typical week of operation, considering real data with inlet preheated air temperatures within the range of 106.8-179 degrees C (typical week fluctuation) and considering operation under optimized mass flow rate values. It was found that the integration of HTHP leads to 1.36% primary energy savings and 5.44% CO2 emissions reduction on a yearly basis, ensuring a slightly lower annual operating cost than the respective of natural gas combustion.

Building energy renovation planning should be based on a multi-criteria evaluation that targets both reduced energy consumption and a high-quality indoor thermal environment. The present study investigates the building energy retrofit technologies of thermal insulation, highly insulative windows, mechanical ventilation for cooling purposes, and shading, aiming to identify the optimum energy retrofit strategy for different building typologies. Indoor thermal comfort is evaluated with the thermal comfort indexes of the predicted mean vote (PMV) and the Predicted Percentage of Dissatisfied (PPD). Each renovation scenario is evaluated in terms of thermal performance and thermal comfort, while an optimum retrofit scenario is defined as the one that simultaneously achieves the maximum decrease in the yearly energy demand and the greatest decrease in the building’s indoor thermal discomfort. The multi-objective analysis is performed using the EnergyPlus simulation engine, which is used to perform yearly dynamic simulations and provide accurate results. This study considers a typical one-story apartment building located in the city of Athens, Greece. According to the calculations, the retrofit strategy that combines all four examined interventions results in an 11.8% and 56.1% decrease in the building’s heating and cooling energy demand, respectively, while an annual enhancement of 16.6% in the building’s thermal comfort PPD index is calculated.

This study investigates a solar-driven absorption heat transformer (AHT) with the LiBr/H2O pair. Four different solar thermal technologies are examined for producing useful heat which is then stored in a tank and feeds the AHT for industrial heat production. The examined solar thermal collectors are the simple flat plate collector (SFPC), the advanced flat plate collector (AFPC), the evacuated flat plate collector (EFPC), and the evacuated tube collector (ETC). Practically, the heat transformer upgrades the solar useful output through a thermochemical absorption cycle. This work uses a validated thermodynamic program created in Engineering Equation Solver, while the unsteady investigation is carried out through TRNSYS software by connecting the tools properly, exploiting their interoperability. During this analysis, the impact of some critical parameters on the system behavior is studied to calculate the industrial heat production and the system efficiency by conducting a dynamic simulation of a typical summer typical week in Athens (Greece). Specifically, the impact of the solar collectors' mass flow rate, the load mass flow rate, the solar field area, and the tank's volume are defined for all the collector types. Finally, a yearly simulation is conducted for the main scenario (100 m2 solar field with a tank of 8 m3) indicating that the selection of EFPC is the best choice energetically and economically compared to others. Specifically, in this scenario, the yearly industrial heating production is calculated at 36879 kWh, the yearly system energy efficiency at 21.17 %, the yearly system exergetic efficiency at 5.86 %, the levelized cost of heating at 0.0702 /kWh, and the yearly CO2 emissions avoidance at 8504 kg CO2. The reported data shows that the proposed systems are more sustainable compared to a benchmark scenario with a natural boiler system. Finally, it is useful to state that the use of ETC and AFPC are also sustainable choices, but they are less effective compared to EFPC.

Industrial heat demand is a major source of CO2 emissions, making the decarbonization of this sector essential for achieving sustainability. This study explores and compares different methods for supplying useful heat to the industrial sector through a multi-criteria approach that considers technical performance, economic viability, and environmental impact. Both conventional and alternative systems are examined, aiming to develop sustainable designs. These include solar-based systems using parabolic trough collectors, supported by either electric heaters or natural gas boilers. In addition, a high-temperature heat pump (HTHP) utilizing waste heat is analyzed, also combined with either electric heaters or gas boilers as backup. For reference, a conventional natural gas boiler system is included as a baseline case. In total, five scenarios are evaluated for applications in the chemical industry. Each scenario is assessed in terms of energy and exergy efficiency, cost, and CO2 emissions. A multi-criteria analysis is then applied to determine the most sustainable option under varying electricity and waste heat price conditions. The results indicate that the configuration combining a high-temperature heat pump with electric heaters (Scenario 3) achieves the highest performance, with energy and exergy efficiencies of 0.952 and 0.666, respectively. The lowest CO2 emissions are observed in the case of using solar collectors with electric heaters (Scenario 1), reaching 4154 tons per year. From an economic perspective, Scenario 3 emerges as the most favorable option at lower electricity prices (0.10 €/kWh), with a levelized cost of heating (LCOH) of 0.0555 €/kWh. At higher electricity prices, the optimal design shifts to Scenario 2, which combines solar collectors with a natural gas boiler, resulting in an LCOH of 0.0603 €/kWh.

High temperature heat pumps (HTHPs) are critical technologies for industrial electrification and decarbonization. This work investigates an innovative configuration that produces useful industrial heat in the range of 200 degrees C to 300 degrees C, driven by electricity and low-grade waste heat in the range from 90 degrees C to 130 degrees C. Specifically, the present work studies a cascade HTHP which includes an absorption heat transformer with LiBr/water working pair in the low-stage and a water/steam mechanical compression heat pump in the upper stage. The system is optimized aiming to maximize its exergy efficiency. The analysis is conducted with a developed and verified model in Engineering Equation Solver. The results showed that the suggested system outperforms other HTHPs according to the literature evidence. The reported system coefficient of performance is found to range from 1.81 to 9.41, while the exergy efficiency ranges from 58.78% to 66.24%. Also, a comparative analysis with a conventional natural gas boiler, the environmental and economic superiority of the suggested examined system was reported. Specifically, the levelized cost of heating for the boiler case was found to be 0.1017 & euro;/kWh, while that of the present system was in the range of 0.0372 to 0.0876 & euro;/kWh.

The present study concerns the design and simulation of a hybrid Organic Rankine cycle (ORC) powered by solar and geothermal energy. The system is designed for maximized geothermal source utilization through partial evaporation. A dynamic simulation is performed considering the climate conditions of Athens, Greece, and hexane as the working medium in the system. The hybrid ORC is evaluated for four medium-grade geothermal water temperatures, 100 degrees C, 120 degrees C, 140 degrees C, and 160 degrees C, and six solar collecting areas from 100 to 600 m2 and its performance is compared with the corresponding standalone solar and geothermal systems. The results show that the hybrid system achieves higher annual electricity production across the majority of examined cases, with a maximum increase of 8.1% over the separate systems, and a lower Levelized Cost of Energy (LCOE), ranging from 0.037 /kWh to 0.119 /kWh. A multi-criteria analysis is also conducted to identify the optimal solar collecting area for each geothermal source by minimizing the distance to the ideal solution on a Pareto front defined by two objectives: maximum electricity production and minimum LCOE. The findings highlight that source hybridization under a careful design is a solution of high potential for the development of effective and sustainable units for electricity generation.

The Carnot battery, as an advanced energy storage technique, can play a key role in the global transition toward decarbonization and sustainability. On the other hand, multigeneration systems enable the simultaneous production of multiple useful energy outputs with high overall efficiency. The present study aims to review, categorize, and compare energy systems that integrate the concepts of Carnot battery and multigeneration, offering a comprehensive overview of their technological potential. Since the existing literature has not sufficiently examined this combination, the review introduces a novel research perspective that addresses an important scientific gap. The configurations are classified according to the type of Carnot battery employed, while their energy sources, energy products, and integrated components, such as thermal energy storage devices and thermodynamic cycles, are thoroughly discussed. The main outputs include electricity, heat, cooling, domestic hot water, hydrogen, and freshwater. All systems are evaluated and compared based on thermodynamic and economic performance indicators, including power-to-power efficiency, energy and exergy efficiencies, and key economic figures. The results indicate that both energetic and exergetic efficiency, as well as economic feasibility, are enhanced when multiple energy products are generated. Moreover, Brayton-based units exhibit high energy efficiency, whereas configurations with liquid air energy storage show greater economic viability. Overall, this study highlights the significant potential of integrated Carnot battery and multigeneration systems as promising pathways toward sustainable, efficient, and flexible energy solutions for future low-carbon applications.

It is urgent to turn to the broader utilization of renewable energies instead of fossil fuels to effectively tackle this widely recognized challenge of transition to sustainable energy. The present study aims to provide insight into existing understanding and develop approaches toward advances in the working fluids, namely nanofluids, along with turbulators for enhancing heat transfer processes related to energy applications. It gives a general introduction with an overview of the existing information from the literature, then addresses outstanding issues for implementing new ideas in energy systems and solar collectors to enhance the heat transfer rate, efficiency measures, and design for the future. This visionary paper outlines the key hurdles to be conquered if such technologies significantly impact future sustainable energy systems. These are inclusively outlined as novel material development, performance enhancement, long-term stability, life cycle methodology, and cost reduction in implementing innovative technologies into large-scale industrial applications. The present work concludes with the design of a road map that integrates these advanced technologies into sustainable energy systems and identifies huge potential in these technologies to make considerable contributions towards the global transition towards renewable energy sources.

This analysis focuses on exploiting solar energy to meet industrial energy needs. It is based on upgrading solar heat production using surplus electricity from renewables to generate high-temperature industrial process heat. The approach involves the use of efficient evacuated flat plate solar thermal collectors coupled with a thermal storage tank to power a reverse air Brayton heat pump for high-temperature heat production. A Matlab algorithm has been developed to dynamically analyse energy balances across the system. The analysis is conducted for the location of Komotini, Greece and the results regard the operation of a system for a typical summer week. For the baseline design point with a 1000 m2 collecting area and 50 m3 thermal tank volume, the average daily process heat production for the industry is 8.63 MWh, while the average daily coefficient of performance (COP) for the heat pump is 1.478. Moreover, the present work includes results regarding the parametric performance of the system for different combinations of collecting area and thermal tank volume. It was demonstrated that with the optimization of the solar thermal collector area and thermal tank volume, the daily average COP can be increased up to 23.1%. Finally, the influence of the units of transfer area of the heat exchangers on the coefficient of performance has been investigated.

Heating systems using R744 refrigerant are attractive and sustainable choices for heat pump configurations for space heating. The unique feature of this study is that it compares multiple fundamental transcritical CO2 heat pumps to identify the most energy-efficient and environmentally favorable option for heating purposes. The reference system includes an evaporator, compressor, throttle device, and gas cooler. It is evaluated against a system with an internal heat exchanger, an ejector, double-stage compression, and a cascade compression system. The importance of this study lies in the comprehensive optimization and parametric analysis across the ambient temperatures from -20 degrees C to 20 degrees C. The systems are coupled with either fan coils or radiators, producing heat at return/supply water temperatures 40/45 degrees C or 60/75 degrees C, respectively. A comparison with conventional heat pump units is also performed. The systems are applied to a residential building in Athens, Greece, serving its heating energy loads, and the analysis is performed with MATLAB-developed models, validated according to other studies in existing literature. Results indicate that all investigated systems outperform the basic system across all operation conditions. The cascade CO2/R600 compression system achieves the highest electricity savings of 29.3% but significantly increases the total equivalent CO2 emissions up to 423.8%. The ejector-assisted system combines energy efficiency, up to 13.2% COP improvement, and a decrease in the total equivalent warming impact, up to 6.5%. The systems with the internal heat exchanger and the double-stage compression achieve maximum COP improvement of 8.0% and 8.8% and total equivalent warming impact reduction of 4.5% and 4.0%, respectively.

The Organic Rankine Cycle (ORC) is an effective method for transforming low- and medium-grade heat into electricity that has recently gained significant attention. Several review studies in the literature are focused on working fluids, system architecture, and the individual utilization of renewable and alternative heat sources in ORCs, like solar irradiation, geothermal, biomass, and waste heat energy. However, no studies have yet investigated ORC systems driven by two of the aforementioned sources combined. This work aims to review and explore multiple aspects of hybrid ORC systems. Such systems are categorized based on source combinations and configurations, and the results regarding their thermodynamic, thermo-economic, and environmental performance are discussed. The source arrangements follow the following three main configurations: series, parallel, and heat upgrade. Most of the examined systems include solar energy as one of the sources and only four cases involve combinations of the other three sources. The reported results show that hybrid ORCs generally perform better thermodynamically compared to their respective single-source systems, exhibiting an enhancement in power production that reaches 44%. An average levelized cost of energy (LCOE) of 0.165 USD/kWh was reported for solar–geothermal plants, 0.153 USD/kWh for solar–biomass plants, and 0.100 USD/kWh for solar–waste plants. Solar–biomass plants also reported the lowest reported LCOE value of 0.098 USD/kWh. The payback periods ranged from 2.88 to 10.5 years. Further research is proposed on multiple source combinations, the in-depth analysis of the three main configurations, the integration of polygeneration systems, the incorporation of zeotropic mixture working media and experimental research on ORCs with combined sources.