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In this study, the effects of six operating conditions on the performance of a 3 kW- ORC (organic Rankine cycle) system were investigated. The results of experiments show that, despite differences in the physical parameters of the three working fluids used, the performance of the ORC system was similar. Further, the cooling water temperature (CWT) was strictly controlled, but the experimental results were affected by the condensation temperature, however the experimental system can maintain stable operation. Since generators are affected by different factors, the variation in generator generation fluctuates over a wide range during steady state operation of the system. The theoretical shaft work of the expander and generator power generation of the system using R245fa exceeds that of the other two working fluids due to the density. It had a maximum generator power conversion efficiency of 64.651% under C1 condition, maximum exergy efficiency of 27.346% under C3 condition; the system has a maximum cycle efficiency of 9.543% and a cold energy utilization efficiency of 5.633%, under C6 conditions; although maximum power generation (1.019 kW) and maximum net work (1.321 kW), the total exergy loss of the system also reached its maximum value (13.756kw) under C5 condition.

This article aims to study entropy generation and heat transfer due to free convection. Two types of base fluids (water and kerosene oil) are taken with a suspension of titanium alloy nanoparticles. An external magnetic field is applied in a perpendicular direction and the induced magnetic field is neglected. Scientific analysis is performed on magnetohydrodynamic flow through a Darcy medium. Free convection and the sudden motion of the heated plate cause the fluid to flow. The problem is formulated in terms of differential equations with associated physical conditions. Relations for entropy generation and Bejan numbers are also provided. The Laplace transform technique has been used for finding the exact solution to the problem. Results are plotted using Mathcad software and a comparison is made between water-titanium alloy and kerosene oil-titanium alloy nanoparticles for velocity, temperature, entropy generation, and Bejan number. It is concluded that kerosene oil base fluid has a greater velocity and temperature profile in all parametric studies as compared to water-based fluid. While in the case of entropy generation and Bejan number, near to the plate and for away the plate the behaver is distinct. Entropy generation and Bejan number are boosting up via using different base fluid. For larger estimation of Brinkman number, both Bejan number and entropy rate have the opposite effect. The volume fraction of nanofluid enhance the rate of heat transfer in case of both nanofluid. While the water base nanofluid enhance the rate of heat transfer up to 19.14% and kerosene oil base fluid is enhanced up to 30.01%.

The purpose of the review is to find the best currently available correlations for calculating heat transfer and pressure drop in the main heat-transfer equipment items in organic Rankine cycle (ORC) units. The search is limited to the designs of apparatuses, which are the best ones in the opinion of the authors of this paper, for a conventional two-circuit ORC-unit, where thermal oil cools a heat source in the first circuit and transfers heat to refrigerant in the vapor generator (hereinafter referred to as the evaporator). Besides the evaporator, the second circuit of the unit includes a “refrigerant–water” or “refrigerant–air” condenser and a regenerative heat exchanger which heats up liquid refrigerant upstream of the evaporator with the exhaust vapor of the turbine (or expander). The criteria are presented for selecting working fluids for such units depending on the heat source temperature. The working fluids that have found the widest application at each temperature level (such as cyclopentane, benzene, toluene, MM, MDM, R1233zd, R245fa, R601, R601a, RC318, R134a) are listed, and their characteristics and thermodynamic properties are presented at specified condensation (25°C) and boiling (200, 120, and 70°C) points. The analysis of these data, including information on the proposed working fluids, has yielded nominal parameters of ORC-units. Thousands of fundamental and engineering works are devoted to the study of boiling and condensation processes, the interest in which has been growing over the past 10–15 years. The development of new energy conversion technologies and the appearance of new working fluids, materials, and methods of surface treatment has given a second wind. This paper reviews correlations for heat-transfer coefficients and hydraulic resistance factors in apparatuses with refrigerant boiling in round tubes, condensation in tubes and channels and in the shell side (on tube bundles), and heating and cooling of single-phase refrigerant in tubes and channels. The correlations for engineering calculation of the main heat-transfer equipment of ORC-units, which are the most convenient ones in the authors’ opinion, are presented.

Compared with the pure fluids, the zeotropic mixtures can balance the requirements of environmental protection, heat source matching and system safety, and exhibit excellent thermodynamic performance. However, compared to the widespread applications of pure fluids, zeotropic mixtures are rarely exploited in thermodynamic cycles, and there is a lack of targeted summary on refrigeration systems, organic Rankine cycle systems and combined power and refrigeration systems. In the recent years, zeotropic mixtures are developing at an unprecedented pace, while the working fluids components are inevitably explored in the process. In this paper, the research progress of zeotropic mixtures in the field of refrigeration systems, organic Rankine cycle systems and combined power and refrigeration systems are reviewed. Based on the review of zeotropic working mixtures, the reasonable predictions can be proposed. In the future, environmental problems will still be one of the most important concerned issues. Therefore, the zeotropic mixtures consisting of natural hydrocarbons and carbon dioxide, which are environmentally friendly, have great potential for development. Furthermore, zeotropic mixtures of natural working fluids can improve comprehensive energy efficiency of combined systems and will play an important role in future carbon emission reduction technologies.

This experimental investigation evaluates the thermal performance enhancement of evacuated tube solar water collectors through the implementation of binary nanofluid formulations using acetone as the base fluid. Three metal oxide nanoparticles, including magnesium oxide (MgO), copper oxide (CuO), and titanium dioxide (TiO₂), were systematically tested both individually at 0.25 wt% concentration and in binary combinations to assess synergistic enhancement effects. Mono‐nanofluid analysis revealed that MgO exhibited superior performance among single‐nanoparticle formulations, achieving 36.2% maximum thermal efficiency and 52.1°C maximum water temperature, representing 33.6% efficiency improvement over the pure acetone baseline. Binary combinations demonstrated synergistic enhancement exceeding individual constituent performance. The MgO‐CuO combination achieved 41.1% maximum efficiency with 51.6% improvement, while MgO‐TiO₂ exhibited favorable temperature stability during afternoon hours. Parametric investigation of MgO‐TiO₂ mixing ratios identified that 0.30 wt% MgO combined with 0.20 wt% TiO₂ achieved superior overall performance with 49.2% maximum thermal efficiency and 60.9°C maximum water temperature, representing 81.5% efficiency improvement and 38.1% temperature improvement relative to baseline. This enhancement is attributed to the synergistic combination of high thermal conductivity from MgO, enabling rapid heat absorption, and elevated specific heat capacity from TiO₂, providing improved thermal retention. The findings demonstrate that strategic binary nanoparticle combinations with appropriate mixing ratios can overcome inherent limitations of single‐component systems, offering a practical pathway for solar thermal efficiency enhancement. Illustration of the work.

With the continuous increase in global greenhouse gas emissions, the impacts of climate change are becoming increasingly severe. In this context, geothermal energy has gained significant attention due to its numerous advantages. Alongside advancements in CO2 geological sequestration technology, the use of CO2 as a working fluid in geothermal systems has emerged as a key research focus. Compared to traditional water-based working fluids, CO2 possesses lower viscosity and higher thermal expansivity, enhancing its mobility in geothermal reservoirs and enabling more efficient heat transfer. Using CO2 as a working fluid not only improves geothermal energy extraction efficiency but also facilitates the long-term sequestration of CO2 within reservoirs. This paper reviews recent research progress on the use of CO2 as a working fluid in Enhanced Geothermal Systems (EGS), with a focus on its potential advantages in improving heat exchange efficiency and power generation capacity. Additionally, the study evaluates the mineralization and sequestration effects of CO2 in reservoirs, as well as its impact on reservoir properties. Finally, the paper discusses the technological developments and economic analyses of integrating CO2 as a working fluid with other technologies. By systematically reviewing the research on CO2 in EGS, this study provides a theoretical foundation for the future development of geothermal energy using CO2 as a working fluid.

The task of energy-efficient heat supply and removal in thermal control, heating and cooling systems is very relevant for many branches of technology. The paper presents the results of the development and study of a 21 m long loop heat pipe (LHP) that is a passive heat-transfer device operating on a closed evaporation-condensation cycle and using capillary pressure to pump a working fluid. These devices can be used in systems where the heat source and the heat sink are removed from each other by a distance measured in meters and even tens of meters, without the use of additional energy sources. The device has a 24 mm diameter evaporator with a 188 mm long heating zone, a vapor line and a liquid line (external/internal diameters of 8/6 mm and 6/4 mm). A 310 mm long pipe-in-pipe heat exchanger equipped with a cooling jacket was used as a condenser. The tests were conducted with the LHP in a horizontal position. Heat was removed from the condenser by forced convection of a water-ethylene glycol mixture with temperatures of 20 and –20°C and a flow rate of 6 dm 3 /min. The heat load supplied to the evaporator from the electric heater increased from 200 to 1700 W in the first case and to 1300 W in the second. The vapor temperature at the outlet of the evaporator varied from 25 to 62°C and from 24 to 30°C, respectively. Its maximum temperature difference along the length of the vapor line did not exceed 4°C. Such devices can be used in energy-efficient systems for utilizing low-potential heat, heating or cooling remote objects, and for uniformly distributing heat over a large surface area of heat sinks.

A multi-objective optimization based on the non-dominated sorting genetic algorithm (NSGA-II) is carried out in the present work for the basic organic Rankine cycle (BORC) and regenerative ORC (RORC) systems. The selection of working fluids is integrated into multi-objective optimization by parameterizing the pure working fluids into a two-dimensional array. Two sets of decision indicators, exergy efficiency vs. thermal efficiency and exergy efficiency vs. levelized energy cost (LEC), are adopted and examined. Five decision variables including the turbine inlet temperature, vapor superheat degree, the evaporator and condenser pinch temperature differences, and the mass fraction of the mixture are optimized. It is found that the turbine inlet temperature is the most effective factor for both the BORC and RORC systems. Compared to the reverse variation of exergy efficiency and thermal efficiency, only a weak conflict exists between the exergy efficiency and LEC which tends to make the binary objective optimization be a single objective optimization. The RORC provides higher thermal efficiency than BORC at the same exergy efficiency while the LEC of RORC also becomes higher because the bare module cost of buying one more heat exchange is higher than the cost reduction due to the reduced heat transfer area. Under the heat source temperature of 423.15 K, the final obtained exergy and thermal efficiencies are 45.6% and 16.6% for BORC, and 38.6% and 20.7% for RORC, respectively.

The purpose of this paper is to provide the review details on the research attempt made in the field of ejector systems. This review paper provides details on design methodology, geometrical parameters, operating parameters effect, CFD studies, turbulence model selection, working fluid, and irreversibility of the ejector system. The journey of two-stage ejectors with their geometrical details and auxiliary entrainment positions is also presented. It gives a higher entrainment ratio as compared with a single-stage ejector. The new techniques, constant rate of momentum change and constant rate of kinetic energy, also came into the knowledge to design physics-based single and two-stage ejectors. This method helped in the design to create variable area geometry of the nozzle, mixing, and diffuser. This helps to remove the thermodynamic loss or irreversibility of conventional ejectors due to sudden area change at the exit/inlet of the diffuser section. In addition, the performance of the ejector, including entrainment ratio, nozzle exit position, and back pressure effect, is also presented. Finally, the effect of different working fluids on the performance of the ejector and application with various fields is also reviewed.

Supercritical CO2 (S-CO2) thermodynamic power cycles have been considerably investigated in the applications of fossil fuel and nuclear power generation systems, considering their superior characteristics such as compactness, sustainability, cost-effectiveness, environmentally friendly working fluid and high thermal efficiency. They can be potentially integrated and applied with various renewable energy systems for low-carbon power generation, so extensive studies in these areas have also been conducted substantially. However, there is a shortage of reviews that specifically concentrate on the integrations of S-CO2 with renewable energy, encompassing biomass, solar, geothermal and waste heat. It is thus necessary to provide an update and overview of the development of S-CO2 renewable energy systems and identify technology and integration opportunities for different types of renewable resources. Correspondingly, this paper not only summarizes the advantages of CO2 working fluid, design layouts of S-CO2 cycles and classifications of renewable energies to be integrated but also reviews the recent research activities and studies carried out worldwide on advanced S-CO2 power cycles with renewable energy. Moreover, the performance and development of various systems are well grouped and discussed.