Search

CO2 refrigeration and heat pump systems have gained widespread attention due to their environmentally friendly features. However, their application is limited by high operating pressure and limited refrigeration performance. To address this, the technology of adding other refrigerants to CO2 to form a CO2-based mixture has emerged as an effective solution, as it reduces operating pressure and improves energy efficiency. Although CO2-based mixtures can be mixed in any ratio, there is an optimal composition ratio that maximizes performance under given working conditions. However, as working conditions change, the optimal composition ratio also changes, and fixed compositions cannot achieve the best energy efficiency throughout the year. Therefore, this paper studies four types of CO2-based mixtures, namely CO2/R32, CO2/R290, CO2/R1270, and CO2/R1234yf， and proposes a CO2-based mixture refrigeration and heat pump systems composition adjustment scheme that adjusts the system's composition according to different working conditions. The paper designs two composition adjustment strategies, studies the limits of composition adjustment using different methods, and evaluates the energy-saving performance of different adjustment methods. The results show that the maximum range of composition adjustment is between.25 and.97， and the energy-saving of up to 27.2% can be achieved through composition adjustment in some areas. [ABSTRACT FROM AUTHOR]

Abstract Thermal-integrated pumped thermal electricity storage (TI-PTES) could realize efficient energy storage for fluctuating and intermittent renewable energy. However, the boundary conditions of TI-PTES may frequently change with the variation of times and seasons, which causes a tremendous deterioration to the operating performance. To realize efficient and flexible energy storage in operating conditions, a novel composition-adjustable TI-PTES is proposed, and the operating performance is investigated and compared with composition-fixed TI-PTES. Simulation results show that, compared to composition-fixed TI-PTES, the energy storage efficiency of TI-PTES could be enhanced by the absolute value of 4.4–18.3% by introducing composition adjustment method under various boundary conditions. Besides, tuning sub-system composition could simultaneously adjust the capacities of power input, heat storage and power output, realizing a more flexible operating range for TI-PTES. A case study for an isolated energy community shows that composition-adjustable TI-PTES could realize 100% conversion of off-peak electric energy and reduce daily investment by 35.6% compared with composition-fixed TI-PTES.

Thermal-integrated pumped thermal electricity storage (TI-PTES) could realize efficient energy storage for fluctuating and intermittent renewable energy. However, the boundary conditions of TI-PTES may frequently change with the variation of times and seasons, which causes a tremendous deterioration to the operating performance. To realize efficient and flexible energy storage in operating conditions, a novel composition-adjustable TI-PTES is proposed, and the operating performance is investigated and compared with composition-fixed TI-PTES. Simulation results show that, compared to composition-fixed TI-PTES, the energy storage efficiency of TI-PTES could be enhanced by the absolute value of 4.4–18.3% by introducing composition adjustment method under various boundary conditions. Besides, tuning sub-system composition could simultaneously adjust the capacities of power input, heat storage and power output, realizing a more flexible operating range for TI-PTES. A case study for an isolated energy community shows that composition-adjustable TI-PTES could realize 100% conversion of off-peak electric energy and reduce daily investment by 35.6% compared with composition-fixed TI-PTES. [ABSTRACT FROM AUTHOR]

Thermoelectric materials are widely used in refrigeration chips, thermal power generation, catalysis and other fields. Mg3Bi2-based thermoelectric material is one of the most promising thermoelectric materials. Herein, the Mg3Bi2-based samples were prepared by high temperature synthesis, and the influence of Mg/Sb content on the electrical transport properties and semi-conductivity/semi-metallicity of the materials has been studied. The results indicate that the efficiency of introducing electrons from excess Mg prepared by high temperature synthesis is lower than that introduced by ball milling, due to the high vapor pressure of Mg. The doping of Sb/Te at the Bi site would make it easier for the material to change from p-type conduction to n-type conduction. With the increase in Mg content, the semi-conductivity of the material becomes weaker, the semi-metallicity becomes stronger, and the corresponding conductivity increases. With the increase in Sb content, the samples exhibit the opposite changes. The highest power factor of ~1.98 mWm−1K−2 is obtained from the Mg3.55Bi1.27Sb0.7Te0.03 sample. [ABSTRACT FROM AUTHOR]

Thermoelectric materials are widely used in refrigeration chips, thermal power generation, catalysis and other fields. Mg3Bi2-based thermoelectric material is one of the most promising thermoelectric materials. Herein, the Mg3Bi2-based samples were prepared by high temperature synthesis, and the influence of Mg/Sb content on the electrical transport properties and semi-conductivity/semi-metallicity of the materials has been studied. The results indicate that the efficiency of introducing electrons from excess Mg prepared by high temperature synthesis is lower than that introduced by ball milling, due to the high vapor pressure of Mg. The doping of Sb/Te at the Bi site would make it easier for the material to change from p-type conduction to n-type conduction. With the increase in Mg content, the semi-conductivity of the material becomes weaker, the semi-metallicity becomes stronger, and the corresponding conductivity increases. With the increase in Sb content, the samples exhibit the opposite changes. The highest power factor of ~1.98 mWm−1K−2 is obtained from the Mg3.55Bi1.27Sb0.7Te0.03 sample.

CO 2 is a promising natural working fluid for its environment-friendly, property-stable and low-cost, and CO 2 -based zeotropic mixture could enhance the comprehensive performance of combined cooling and power cycle (CCP). Generally, CCP operates in a wide temperature range, and the fixed operating composition could not realize the optimal match for two sub-cycles. Zeotropic mixture composition adjustment could realize targeted thermal match of sub-cycles and further enhance the performance of CCP. However, systematic research on composition-adjustable CCP is limited in theoretical simulation, and there is still a lack of experimental evidence for the performance enhancement of composition adjustment on CCP. Besides, the feasibility of composition adjustment on CCP has not been fully verified through experimental method yet. In this study, an experimental prototype of CO 2 -based composition-adjustable CCP is established, the approach to realize composition adjustment is investigated, and the systematic test is carried out. Experimental results show that the introduction of composition adjustment is beneficial to the performance of CCP. Under the same boundary conditions and similar refrigeration conditions, composition-adjustable CCP could export 13.72 % more net power than composition-fixed CCP when the initial charged CO 2 mass fraction equals 0.246, and the improvement is more apparent when the initial charged composition gets larger. • Experimental investigation on composition-adjustable combined cooling and power cycle (CCP) is carried out. • A new heat transfer balance method is proposed to measure the operating composition. • Comparative experimental study is carried out between composition-adjustable CCP and composition-fixed CCP. • The introduction of composition adjustment could enhance the performance of CCP by 13.72 %. [ABSTRACT FROM AUTHOR]

Abstract Organic Rankine cycle (ORC) is recently receiving increasing attention and regarded one of the most promising low-grade heat-to-power technology. Compared to the conventional high-grade fossil energy-to power technologies, ORC is much more sensible to the environmental temperature variation due to the low temperature of heat sources. A conventional ORC is typically designed under a given condition and mostly operate under off-design conditions, resulting in low annual average performance. This study proposes a novel zeotropic ORC in which the mixture composition can be tuned in response to the environmental temperature variation using liquid - separation condensation. A thermodynamic optimization model is formulated to optimized the mixture composition according to the environmental temperature. A concept design of liquid-separation condensation -based composition tuning system is conducted. The results show that the proposed ORC can significantly improve the overall efficiency without extra cost increment, which can be improved by up to 9.15 % compare to the conventional ORC for 100 oC heat source. [ABSTRACT FROM AUTHOR]

The decrease in fossil energy reserves and the increase in energy costs have resulted in a strong interest in power generation using renewable heat sources or waste heat. Organic Rankine cycle (ORC) is a promising heat-to-power conversion technology. Although the ORC using zeotropic mixture is superior to ORC using pure fluid in thermodynamic performance due to low irreversibility during the evaporation and condensation of zeotropic mixture, the improvement in thermodynamic performance is usually achieved at the cost of poor economic performance. The studies on improving the thermo-economic performance are limited. In the present study, a novel zeotropic ORC coupling liquid separation condensation and multi-pressure evaporation is proposed. Multi-pressure evaporation is presented to improve heat match between the heat source and working fluid. Liquid separation condensation is applied to control the composition of the mixtures that enter the evaporation processes. Thermodynamic analysis and optimization model of the novel ORC is developed. The superiority of the proposed novel ORC over the traditional simple zeotropic ORC and traditional multi-pressure evaporation zeotropic ORC is elaborated for different mixtures. The contribution of mixture composition adjustment and condensation enhancement through liquid-vapor separation on the cycle performance improvement is investigated. Sensitivity analysis of heat-sink inlet temperature, heat source inlet temperature, vapor quality, heat source specific heat capacity, and heat-sink temperature rise on the cycle performance are conducted. The case results show that net power output of the novel ORC is 13.05–26.18% higher than that of the simple zeotropic ORC. The contribution of mixture composition adjustment on improving the net power output can be up to 3.57% compared with traditional multi-pressure evaporation ORC for mixture R245fa–R365mfc. When the heat transfer enhancement through liquid separation is incorporated into the thermodynamic optimization, the net power output of the novel zeotropic ORC can be increased by 8.22% compared with the traditional multi-pressure evaporation ORC under the same economic constraint. [ABSTRACT FROM AUTHOR]

The combined cooling and power cycle has wide applications ascribed to the feasibility of simultaneously providing cooling and electricity. CO 2 -mixture based combined cooling and power cycle is a promising technology for remarkable performance. However, optimal compositions of working fluid in sub-cycles are usually different when operating with CO 2 -mixture. In off-design conditions, the mismatch between compositions and sub-cycles may further aggravate. This study employs composition-adjustable combined cooling and power cycle to solve the above problem and off-design performance in various boundary conditions is investigated. A refrigeration truck operated in highway condition is selected as the research objective. The results show that the optimal compositions of working fluid in sub-cycles are disparate in different engine operating conditions and the composition-adjustable system performs better than composition-fixed system in most engine operating points. Beyond satisfying the demand of 7.73 kW (−20 °C) cooling capacity, the total net power output of composition-adjustable system could reach 2.41 kW in the whole operating conditions, which is 7.11% higher than composition-fixed system (2.25 kW). In addition, composition-adjustable system has a much broader operating feasibility to the shift of heat sink inlet temperature and is more suitable for the application with large cooling requirements. [Display omitted] • Off-design performance of composition-adjustable combined cooling and power cycle is investigated. • Comparison between composition-adjustable system and composition-fixed system is carried out. • Optimal compositions of mixture in sub-cycles are obtained in off-design conditions. • Composition regulation process is developed when boundary condition changes. [ABSTRACT FROM AUTHOR]

Barium titanate-based lead-free piezoceramics are considered as a key material for the fabrication of dielectric capacitors due to their large relative dielectric constant (ε r) and are also identified as a remarkable electrocaloric material on account of their low Curie temperature (T C) and high spontaneous polarization (P s). At this work, to optimize the electrical performances of BaTiO 3 -based Pb-free piezoceramics, chemical doping was used to tailor their composition. The aim is to boost Curie temperature while sustaining decent electrical properties. Besides, to enlighten the underlying physical mechanisms, first-principles calculations based on atomic coupling, orbital hybridization as well as electron localization were carried out. The experimental results show that the dielectric, ferroelectric, and piezoelectric properties are mainly determined by the orthorhombic-tetragonal (O-T) two-phase coexistence and grain size. Notably, a boosted dielectricity, ferroelectricity, and piezoelectricity accompanied by a raised T C is achieved at x = 0.10 mol%. Moreover, the highest T C ∼125.40 ℃ is obtained at x = 0.40 mol% along with a degradation in dielectricity, ferroelectricity, and piezoelectricity. • O-T phase boundary is present in all samples at the vicinity of room temperature. • O-T phase boundary and grain growth contribute to piezoelectricity together. • First-principles calculations are used to reveal the inherent mechanisms. • Superior ferroelectricity P r ∼13.49 μC/cm2 is gained at x = 0.16 mol%. • An improved T C = 125.40 ℃ is obtained at x = 0.40 mol%. [ABSTRACT FROM AUTHOR]

Thermodynamic cycle can realize maximum efficiency when temperature profiles of working fluid and heat source/sink match exactly, such as Carnot cycle and Lorenz cycle. Ascribed to the discrepancy of real working fluid and ideal working fluid, generally, the best temperature match of different heat exchange processes cannot be obtained meantime. To surmount this issue and give full play to zeotropic mixtures' advantage, transformation of combined cooling and power cycle after introducing composition adjustment is discussed in this study. The phenomenon of temperature mismatch happened in a composition-fixed combined cycle is investigated first, it shows that optimal mixture compositions among various heat exchange processes are disparate. Then, an ideal composition tunable combined cycle is evaluated without considering the specific adjustment process; results show that, under the same cooling requirement, net power output of the combined cycle could be improved by 3.93% owing to the adjustment of composition operated in sub-cycles and enhanced temperature match. To realize the required composition obtained during ideal analysis, a two-step composition adjustment method is proposed and the desired composition could be acquired by suitable separation quality and rebuilding proportion. However, due to the restraint of pinch point, the practical composition tuning structure could realize 2.30% improvement. • Optimal compositions for various heat exchange processes are disparate. • Potential of composition adjustment is proved by an ideal composition tunable structure. • A two-step composition adjustment method is proposed to obtain required compositions. • Performance of practical composition tuning structure is restricted by pinch point. [ABSTRACT FROM AUTHOR]

Organic Rankine cycle (ORC) is an effective approach for low-grade energy utilization. It is important to improve the efficiency of an ORC when the heat source temperature varies. In this study, a method of selecting zeotropic mixture for a bottoming ORC with changeable heat source temperature is presented. The effects of the operation conditions of the marine engine and the ambient temperature are investigated. First, the optimal performances of the ORC using 40 pure working fluids are determined and compared. Then, two zeotropic mixtures benzene/m-xylene and cyclopentane/toluene are selected and the mechanism of temperature match with the heat source and sink is explored. Finally, the performance improvement with benzene/m-xylene using the composition adjustment method is evaluated. The results indicate that the suitable pure working fluids are isopentane and R245ca for a low exhaust temperature while toluene and m-xylene are the best when the exhaust temperature is high. Using the zeotropic mixtures benzene/m-xylene and cyclopentane/toluene can obtain a high performance over the operation range of the marine engine. When the exhaust temperature is 225 °C, the net power and exergy efficiency of benzene/m-xylene are improved by 6.9%–21.9% and 6.5%–22.0%, respectively, compared with the pure fluids benzene and m-xylene. When the exhaust temperature increases to 380 °C, these improvements decrease to 1.9%–6.8%. If a zeotropic mixture is used, the critical temperature of the component with a high-boiling point should be close to the maximum operation temperature of the heat source, and the temperature glide should first match with the temperature increase of the heat sink. If the heat sink temperature is fixed, it is impossible to enhance the ORC performance using the composition adjustment method. However, the composition adjustment method is effective when the temperature of the heat sink varies with the ambient temperature. The net power and exergy efficiency of the ORC are improved by up to 21.9% and 22% in winter using benzene/m-xylene. However, these improvements are less than 6.8% in summer. • Method of zeotropic mixture selection is proposed for exhaust heat recovery. • Effects of operation conditions of a marine engine is studied. • Feasibility of composition adjustment is evaluated. • Benzene/m-xylene and cyclopentane/toluene are suitable working fluids. [ABSTRACT FROM AUTHOR]

• A CO 2 -mixture-based ship waste heat recovery system is proposed. • The system can provide cooling output and adjustable power and heating outputs. • Subsystem configurations possess distinct energy transformation tendencies. • Composition adjustment of the mixture can modify the energy flow direction. • The system exhibits extraordinary potential for reducing fuel consumption. To ameliorate the adaptability of the ship waste heat recovery system (SWHRS) to the variable energy demand, this paper proposes a CO 2 -mixture-based SWHRS providing refrigerating output as well as adjustable heating and electrical output. The adjustability deriving from mixture composition regulation profits from liquid separation condensers. A scientific vessel YUKUN is selected as the recovery target, the energy demand characteristics of which in different seasons (spring/autumn, summer, and winter) and conditions (sailing and berthing) are exploited to determine six operating modes. Well-designed configurations of SWHRS's subsystems with different energy exportation emphases improve the matching of SWHRS to changes in energy demand. Moreover, composition adjustment corresponding to various energy requirements further modifies the energy conversion tendency of waste heat to heating or electrical energy desired by the target ship. Comparative research demonstrates that SWHRS possesses certain superiority over the non-adjustable system due to the regulation of mixture composition corresponding to variation in energy requirement. Thus, the SWHRS can reduce the fuel consumption rate by 4.9% to 17.2% compared to the original waste heat recovery system in YUKUN at different modes. Based on actual operating data, the SWHRS can realize a decline of 10.3% in the annual fuel consumption rate. Hence, a fuel charge of 245 000 USD could be saved for YUKUN equipped with SWHRS for one year. [ABSTRACT FROM AUTHOR]

• A novel composition tuning combined cooling and power cycle is proposed. • The performance of different cycle structures is compared. • Better temperature match is achieved by composition adjustment. • Proposed system obtains 5.18% increase in net power output. Combined cooling and power cycle is a promising integrated system for its multi-function to simultaneously provide cooling and electricity. In order to alleviate temperature mismatch between working fluid and heat source/sink, combined cooling and power cycle coupled with zeotropic mixtures is put forward. Generally, a composition-fixed mixture cannot optimally match refrigeration sub-cycle and power sub-cycle meanwhile, as these two sub-cycles commonly have different performance regulation with various mixture composition. This research creatively proposes a novel composition tunable combined cooling and power cycle based on liquid separation condenser to realize different operation composition in two sub-cycles. CO 2 mixture is condensed and separated to two streams with various concentrations through liquid separation condenser, one with high CO 2 concentration, the other has low CO 2 concentration. According to disparate flowing directions of two streams, two diverse structures are proposed in this study. Results demonstrate that the introduction of composition adjustment can substantially promote the comprehensive performance of the combined cycle. Taking a refrigeration truck as example, the new devised composition tunable combined cycle with initial composition of CO 2 /R32 (0.500/0.500) can recover engine waste heat of the truck to generate power in power sub-cycle with low CO 2 concentration (0.494), and provide required cooling capacity (5.43 kW) in refrigeration sub-cycle with high CO 2 concentration (0.634). In addition to neutralizing compression work consumed in refrigeration sub-cycle, the new proposed system can extra generate 12.2 kW power, which is 5.18% higher than conventional composition-nonadjustable combined cycle. Besides, the total heat transfer areas of three systems are similar. [ABSTRACT FROM AUTHOR]