Volumetric Heating Capacity Research Articles

Thermodynamic and economic analysis of a solar-assisted ejector-enhanced flash tank vapor injection heat pump cycle with dual evaporators

This paper proposes a solar-assisted ejector-enhanced flash tank vapor injection heat pump cycle with dual evaporators (SEFVIC), designed for heat pump drying application. Compared to the standard flash tank vapor injection heat pump cycle (FVIC), the SEFVIC uses an additional high-temperature evaporator to further minimize irreversible losses during the heat transfer process. Moreover, it integrates solar energy to improve the drying quality and heating capacity of the cycle. Meanwhile, the introduction of an ejector improves the system's performance by reducing throttling loss. The advantages of SEFVIC over FVIC are evaluated using theoretical models based on the first and second laws of thermodynamics. Results show that SEFVIC reveals a 34.1 % improvement in the heating coefficient of performance and a 192.8 % enhancement in volumetric heating capacity compared to FVIC under typical operating condition. Notably, SEFVIC maintains superior performance even at lower evaporating temperatures. Moreover, exergy research indicates that 77.1 % of the total exergy destruction in SEFVIC is attributed to the solar collector. Economic analysis indicates that the integration of solar energy into the SEFVIC is economically beneficial. These findings highlight the prospective application of SEFVIC and guide combining the multi-temperature heat pump drying system and auxiliary solar source.

Performance analysis of a modified solar assisted ejection-compression heat pump cycle for drying application

In this paper, a modified solar assisted ejection-compression heat pump cycle (SEHPC) is proposed for drying application. Based on the basic ejection-compression heat pump cycle (BEHPC), an auxiliary flash tank and an internal heat exchanger are introduced to improve its heating performances. The introduction of flash tank can improve the heat absorption in evaporator and increase the volumetric heating capacity in condenser, which eventually reduce the throttling losses and enhance the performance of the SEHPC. The performances of the modified cycle with refrigerant R1234yf are analyzed and compared to those of basic cycle by developing the thermodynamic models based on the energy, exergy and economic analysis. Moreover, the main parameters, including evaporation temperature, condensation temperature, generation temperature, ejector back pressure, and the solar radiation intensity, are selected as variables for research. The theoretical results indicate that compared to BEHPC, the mechanical coefficient of heating performance COPw, the thermal coefficient of heating performance COPt, the volumetric heating capacity qhv and the exergy efficiency ηex of SEHPC are increased by 5.2%, 8.4%, 17.1%, and 8.0%, respectively, under a specified working condition. Furthermore, the SEHPC outperforms BEHPC under all given working conditions, especially at high condensation temperature and high generation temperature. The economic analysis results show that the cost per unit of exergy production by SEHPC is 6% lower than that of BEHPC. Additionally, the maximum exergy destruction occurred in the solar collector, which means great optimization potential in solar collector. This study demonstrates that SEHPC has practical performance improvement potential for heat pump drying application.

Optimal refrigerant selection and parameter analysis of a wastewater heat pump using CO2 zeotropic mixtures

Heat pump technology can effectively recover waste heat from low-grade wastewater, which is crucial for energy conservation and carbon emission reduction in buildings. In this study, zeotropic mixtures were used to recover heat from low-grade wastewater and produce high-temperature water in a heat pump. The deep recovery of waste heat was realized by using a CO2 zeotropic mixture with a large glide temperature. Furthermore, this method ensures an excellent temperature matching between the heat transfer fluids and effectively reduces irreversible losses. To obtain an optimal zeotropic mixture, a method based on heuristic screening of the optimal CO2 zeotropic mixture was proposed. Furthermore, a model of a CO2 zeotropic mixture heat pump was established and experimentally validated. Based on the validated model, a sensitivity analysis of the various operating parameters of the heat pump was conducted. The results show that the zeotropic mixture CO2/R1234yf with a mass fraction of 0.17/0.83 is selected as the optimal refrigerant for the heat pump with the coefficient of performance, exergy efficiency and volumetric heating capacity of 7.46, 0.37, and 7978 kJ⋅m−3, respectively. The sensitivity analysis of operating parameters shows that the inlet temperature of hot water has more crucial influence on the coefficient of performance and exergy efficiency than various operating parameters, with sensitivity coefficients of −0.3868 and 0.2157, respectively. The volumetric heating capacity and system total exergy destruction per heating capacity unit are most sensitive to the flow rate of wastewater and hot water, with sensitivity coefficients of 0.1121 and −0.4985, respectively. These results provide a foundation for the design and optimization of a CO2 zeotropic mixture heat pump system.

District heating utilizing waste heat of a data center: High-temperature heat pumps

Data centers are energy-intensive facilities with substantial low-grade waste heat. High-temperature heat pumps can be critical in boosting the data center’s waste heat for district heating, improving the system-level energy efficiency of data centers, and reducing CO2 emissions in district heating. This study built thermodynamic models to assess high-temperature heat pumps with six configurations using low global warming potential refrigerants to supply heat up to 120 °C. The heat pump configurations include single-stage or two-stage cycles with advanced components, such as internal heat exchanger, economizer, flash tank, or parallel compressor. The refrigerants include R1234ze(Z), R1233ed(E), R1224yd(Z), R600, and R600a, and R245fa is used as a reference. A case study was carried out to recover the waste heat from the Frontier high-performance computing data center and provide hot water for district heating at the US Department of Energy’s Oak Ridge National Laboratory campus. The optimized performance of high-temperature heat pumps is characterized with various effectiveness of internal heat exchangers, and the operating parameters of economizer or flash tank, as well as their combination. The results show that the configurations of two-stage cycles with internal heat exchanger + flash tank and internal heat exchanger + economizer/parallel-compressor provide the highest coefficient of performance under scenarios of the maximum allowable value and a fixed value (0.3) of the internal heat exchangers’ effectiveness, respectively. R1234ze(Z) and R600a are the most promising refrigerants, considering trade-offs between the coefficient of performance and the volumetric heating capacity. The single-stage cycle with internal heat exchanger + economizer/parallel-compressor using R1234ze(Z) is recommended for utilizing Fronter’s waste heat in district heating. A one mega-watt high-temperature heat pump will reduce 33,100–33,200 metric tons of CO2 emission annually, corresponding to 85.4 %–85.6 % of equivalent CO2 emissions from natural gas boilers. This study provides good guidelines for designing and deploying high-temperature heat pumps to support sustainable data centers and decarbonize district heating in the US.

Natural refrigerant mixtures in low-charge heat pumps: An analysis of the potential for performance enhancements

The study investigated the characteristics of different natural refrigerants and their mixtures in a residential heat pump with low refrigerant charge. Three main evaluation criteria were utilized for comparing different mixtures: Coefficient of Performance (COP), Volumetric Heating Capacity (VHC), and a newly proposed indicator, the heating capacity at charge limit. Propane was used as a reference refrigerant. It was found that some mixtures significantly improved both COP and the heating capacity at charge limit while maintaining similar volumetric heating capacity and operating conditions. Alternative multi-criteria decision-making techniques were adopted to rank the best refrigerant mixtures. Mixtures rich in Dimethyl Ether (DME), such as DME-CO2 [0.96-0.04] and DME-Propylene [0.75-0.25] were found consistently among the best options. Those were followed by Propylene-rich mixtures such as Propylene-CO2, Propylene-Isobutane and Propylene-Butane. The levelized cost of heat (LCOH) could be improved by up to 12 %. To accelerate the transition to natural refrigerants, without compromises on efficiency and costs, further research and certification activities on non-fluorinated refrigerants based on Dimethyl Ether (R-E170), CO2 (R-744) and Propylene (R-1270) are recommended.

4E assessment of ejector-enhanced R290 heat pump cycle with a sub-cooler for cold region applications

Currently, the worldwide implementation of the Montreal Protocol has accelerated the development of low GWP refrigerants. R290 as an alternative refrigerant with low GWP has great advantages over R134a for heat pump applications. However, the use of expansion valve throttling in the heat pump cycle has a large irreversible loss. Therefore, an ejector-enhanced sub-cooler vapor injection heat pump cycle (ESVIC) with R290 for water heating is proposed in this paper to improve the energy efficiency by applying ejector. The thermodynamic models are established based on the 4E (energy, exergy, economic and environmental) analysis to compare its performance with the basic ejector-enhanced heat pump cycle (BEEC) and the sub-cooler vapor injection heat pump cycle (SVIC). The energy analysis results show that compared with the BEEC and SVIC, the volumetric heating capacity of the ESVIC is improved by 42.0 % and 21.2 %, and the heating coefficient of performance (COPh) of the ESVIC is improved by 27.6 % and 5.3 %, respectively. The exergy analysis results indicate that in BEEC, SVIC and ESVIC the exergy destruction of the expansion valves is 0.3 %, 13.5 % and 0.8 %, respectively. It is indicated that the adoption of ejector can significantly decrease the exergy destruction of expansion valves. Moreover, the environmental analysis results reveal that the carbon emissions of ESVIC is 21.5 % and 5.0 % lower than those of BEEC and SVIC, respectively. Besides, for the ESVIC system, the carbon emissions of the life cycle climate performance when using R290 are 9.9 % lower than that of using R134a, indicating that the R290 is a potential substitute for R134a in heat pump applications. The economic analysis results demonstrate that the unit exergy production costs of ESVIC is 21.6 % and 5.1 % lower than BEEC and SVIC, respectively.

Comprehensive performance evaluation method of working fluids for high temperature heat pump based on multi-objective optimization

The working fluid has garnered significant attention as a critical factor influencing the efficiency of high-temperature heat pumps. A novel method based on multi-objective optimization has been introduced for assessing the performance of heat pump working fluids. The method enhances the clarity of the objective and flexibility in addressing various operating conditions, which helps to improve the efficiency of working fluid screening, especially for the complex mixture system. This study utilizes eight pure refrigerants as the initial fluid set and evaluates 560 binary mixtures with a 5% composition variation. The NSGA-II algorithm is combined with the theoretical heat pump cycle model to obtain Pareto optimal solutions regarding the mixtures, with the coefficient of performance and volumetric heating capacity as the objective functions. Then the evaluation of the optimal solutions is conducted by considering their flammability, toxicity, environmental performance, and temperature glide properties. Subsequently, a comprehensive performance index is derived. The evaluation reveals the dominant solution set for each pure refrigerant. The findings suggest that the blend R142b/R1233zd(E) with a mass fraction of R142b between 0.15 and 0.25 demonstrates a comprehensive performance index of 0.322–0.350, indicating superior performance during the basic cycle. Furthermore, various cycle configurations have been investigated. In the vapor injection configuration, R600a/R601(0.6/0.4) shows good performance in all aspects except flammability. The mixture R245fa/R601(0.2/0.8) exhibits superior overall performance in the internal heat exchanger configuration. It displays excellent temperature glide matching characteristics and good environmental performance. Nevertheless, it shows suboptimal performance in terms of toxicity and flammability. This approach replaces the previous qualitative comparison-based decision making methods, offering high efficiency and practicality.

Thermal design and optimization of high-temperature heat pump integrated with district heating benchmarked in Denmark for process heat supply

This work aims to assess and optimize the performance of cascade high-temperature heat pump (HTHP) integrated with district heating (DH) to produce 1 MW steam at 160 °C for the industrial processes. The heat available in the primary loop of the DH network at 80 °C is considered as the heat source; which is cooled down 70 °C through the HTHP evaporator, before supplying the DH secondary loop. The use of alternative hydrocarbons in the low-temperature loop are examined; and considering the gas compressor limitation, the HTHP performance of using each refrigerant is optimized and compared to each other.The optimization results reveal that pentane- hydrocarbon with the highest critical temperature- is the most promising refrigerant to be paired with steam in the high-temperature loop, reaching the highest COP of 2.66. However, concerning safety and compressor sizing issues, butane is an excellent candidate; with volumetric heating capacity (VHC) of about two times more than that of pentane, in the expense of just about 4 % reduction in the HTHP COP. In addition, water injection theoretically controls the steam compressor discharge temperature successfully, with just 0.9 K superheating at the compressor outlet; and reduces its power consumption and the HTHP COP up to 4.3 % and 1.7 %, respectively. Moreover, techno-economic analysis demonstrates that the HTHP technology shows a better business case compared to the conventional natural gas and electric boilers.

Analysis of an ejector-assisted flash tank vapor injection heat pump cycle with dual evaporators for dryer application

In this paper, an ejector-assisted flash tank vapor injection heat pump cycle with dual evaporators (EFVIC) for heat pump dryer applications is proposed based on a basic flash tank vapor injection heat pump cycle (FVIC). An ejector is introduced to recover the expansion work of the system, and a high temperature evaporator is added to utilize the ejector's entrainment ability to reduce the irreversible loss caused by lower temperature difference. The performance improvements of EFVIC are evaluated through energy analysis, exergy analysis and comparisons with FVIC. The results indicate that the volumetric heating capacity and heating coefficient of performance of EFVIC with R1234yf are improved by 66.99 % and 18.88 % compared with those of FVIC using R1234yf, respectively. In addition, the presented cycle achieves better exergy efficiency because the exergy destruction proportion of expansion valves in EFVIC is 75.98 % lower than that in FVIC. Furthermore, the compressor has the highest exergy destruction among all components for EFVIC, accounting for 33.22 % of the total exergy destruction, hence it has the greatest optimization potential. The advantages of the presented cycle performance reveal its energy saving characteristics and application potential in heat pump dryer.

Performance comparison of pure, binary and ternary refrigerants considering different systems

Mixed refrigerants are widely used in heat pumps; thus, it is necessary to analyze the performance advantages of pure, binary and ternary refrigerants. In this paper, 14 kinds of pure refrigerants were selected, and 24,024 kinds of pure, binary and ternary refrigerants were obtained and analyzed. The refrigerant performance in terms of the COP and volumetric heating capacity were compared in normal, vapor injection with internal heat exchanger and with flash tank systems. In this paper, 40 °C was used as the temperature lift, and four cases with heating production temperatures of 70, 80, 90 and 100°C were analyzed. The Pareto fronts for the different refrigerant types and compositions were obtained by using exhaustive method. The COP and volumetric heating capacity exhibited contradictory trends in all cases. The results showed that mixed refrigerants had significant advantages over pure refrigerants in achieving the Pareto fronts. However, in all cases, when the COP was higher than 5, the binary refrigerants had similar advantages to ternary refrigerants; thus, ternary refrigerants were not necessary. When the COP was lower than 5, it was difficult to further increase the volumetric heating capacity by sacrificing the COP.

Energy, exergy and economic evaluation of a solar enhanced ejector expansion heat pump cycle

Recently, heat pump technology has become the preferred method in drying field. However, most existing heat pump cycles use the single-stage vapor compression with expansion valve, which has the problem of large throttling loss. The use of ejector in the heat pump cycle can recover part of the expansion work and enhance cycle performance. Therefore, this paper proposes a solar-enhanced ejector expansion heat pump cycle (SEEHP) for drying application. Based on the conventional ejector expansion heat pump cycle (EEHP), the introduction of solar energy could not only improve the heating capacity of the cycle, but also improve the performance of the ejector. The energy, exergy and economic evaluation methods are used to theoretically analyze the EEHP and SEEHP. Compared to the EEHP, the energy analysis shows that the SEEHP with R134a refrigerant has a 23.64%–39.82% increase in heating coefficient of performance (COPh) and 32.79%–33.96% increase in volumetric heating capacity (Qcv) for the given condensing temperature range. Exergy analysis results reveal that the exergy destruction of SEEHP is mainly concentrated in the solar collector, indicating that the solar collector has the greatest potential for optimization in practical application. Economic analysis shows that investing in solar energy in the SEEHP is beneficial.

The thermodynamic potential of high-temperature transcritical heat pump cycles for industrial processes with large temperature glides

Industrial heat pumps up to 200 °C are an emerging technology with the potential to reshape the industrial heating supply. For large heat sink temperature glides, transcritical cycles are able to increase the power-to-heat efficiency. Its potential is however yet to be unlocked. To examine this potential, a thermodynamic optimization model is proposed. The model includes robust cycle optimization, is able to screen a large set of working fluids, and includes proper post-processing. This model is applied to three highly relevant industrial cases, namely thermal oil heating, superheated steam drying and spray drying. The heat sink temperature glides for the respective case studies are 60 K, 81 K and 105 K. The results show that a temperature glide larger than 60 K is desired to achieve a better coefficient of performance (COP) with transcritical cycles compared to the classical subcritical cycles. Moreover, potential working fluids were identified for these high operational temperatures. For the case study with a heat sink temperature glide of 81 K, transcritical cycles allowed for a COP increase of 4.6%, whereas this increased to 7.3% for a heat sink temperature glide of 105 K. Furthermore, transcritical cycles introduce a much larger volumetric heating capacity, a lower compressor discharge temperature and a substantially lower pressure ratio. In addition, the best performing working fluids for subcritical cycles are highly flammable, which is only the case for some transcritical working fluids. Therefore, these cycles can be beneficial for temperature glides below 60 K. The compressor for transcritical cycles should however be able to cope with pressures up to 60 bar. If these compressors are available, transcritical cycles are shown to be superior compared to classical subcritical cycles.

Analysis of low GWP ternary zeotropic mixtures applied in high-temperature heat pump for waste heat recovery

In this study, the novel low GWP ternary large glide temperature zeotropic mixtures were proposed to apply in the high-temperature heat pump for waste heat recovery. The ternary zeotropic mixtures were composed of two retardants (CO2 and R1233zd(E)) and one flammable, including (R600a, R290, R1270, and RE170). First, a novel method, which combines the operated pressure, glide temperature, flammability, safety, energy, and exergy efficiency, were proposed to select the proper ternary zeotropic mixture applied in the high temperature heat pump. Besides, the effect of operated temperature parameters, including the subcooling degree (Tsc), superheating degree (Tsh), dew point temperature in evaporator (Tdew,evap), and dew point temperature in condenser(Tdew,cond), on the performance of the high temperature heat pump using the selected optimal ternary zeotropic mixture was conducted further. The results show that the ternary zeotropic mixture CO2/R600a/R1233zd(E) was selected to applied in the high temperature heat pump with the mass fraction of 0.2/0.7/0.1, and the maximum heating coefficient of performance (COPh), volumetric heating capacity (qv) and exergy efficiency (ηex) are 3.22, 3017 kJ/m3 and 0.402, respectively under the selected condition. Besides, compared with the Tsc and Tsh, the Tdew,cond and Tdew,evap affect the performance notable for the selected ternary zeotropic mixture, especially for the Tdew,cond, which affect the condensation pressure significantly and should be gain more attention for the high temperature application. Among the Tdew,cond and Tdew,evap, The COPh and qv are sensitive to the Tdew,cond and the Tdew,evap, respectively. The results hope to promote the application and research of low GWP zeotropic mixtures in the high-temperature heat pump.

Technical-economic-environmental analysis of high temperature cascade heat pump with R718 (high stage) and six different low global warming potential refrigerants (low stage)

High temperature cascade heat pump with two separate refrigerants is a crucial system for decarbonizing industrial heating through electrification. The selection of optimal working refrigerants and temperature lifts relies on not only the thermodynamic analysis outputs such as coefficient of performance but also economic and environmental analyses. In this study, a technical–economic-environmental investigation was conducted for the proposed high-temperature cascade heat pump employing water as the high-stage refrigerant and six low global warming potential refrigerants in the low stage. To facilitate a detailed economic-environmental analysis, precise equipment sizing was undertaken. A comprehensive heat transfer model was developed to size the falling film shell-and-tube cascade heat exchanger, and empirical correlations were employed to size the internal heat exchanger at various cascade temperature lifts. The cascade heat exchanger model is validated against experimental data with less than 5 % error. The distribution of refrigerant charged mass was illustrated using instantaneous density and temperature data. To consolidate all thermal, economic, and environmental sub-indicators into a single parameter, the coefficient of performance, volumetric heating capacity, total annual cost, and total equivalent warming impact were combined as (COP × VHC)/(TAC × TEWI). This proposed comprehensive indicator can be applied to high-temperature cascade heat pumps to identify the optimum working pair and conditions. The results highlighted that R718 + R1234ze(Z) and R718 + R600 were the best working refrigerant pairs, considering all energy, economic, and environmental criteria.

Vapor-liquid equilibrium measurement and heating performance modeling on eco-friendly zeotropic blends of CO2/R1234ze(Z) and CO2/R1336mzz(E)

In the context of sustainable development, CO2 and hydrofluoroolefins (HFOs) are regarded as promising refrigerant candidates. Despite recognition of application potential, the knowledge of their blends is insufficient. In this paper, vapor–liquid equilibrium (VLE) properties of two eco-friendly zeotropic CO2/HFO blends, namely CO2/R1234ze(Z) and CO2/R1336mzz(E), were measured at temperature from 253.15 K to 333.15 K at CO2 concentration from 0 to 1. Peng-Robinson (PR) model and Soave-Redlich-Kwong (SRK) model are identified as suitable models for describing VLE of CO2/HFO with deviation less than 4.5%, whereas Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) model is shown to have larger deviation. Furthermore, a heat pump water heater is modelled to demonstrate the working potential of CO2/HFO in terms of thermodynamics and safety, while the newly fitted binary interaction parameter kij being used. Both CO2/R1234ze(Z) and CO2/R1336mzz(E) show more than 150% advantage in volumetric heating capacity and comparable heating efficiency compared to R134a. The optimal concentration of CO2 is higher than 0.6, eliminating the flammability of mixture. Additionally, the operating pressures of CO2/HFO are reduced to lower than 3.2 MPa at typical working condition due to HFO addition. The results contribute to the accurate VLE description of CO2/HFO and further investigation on the applications of eco-friendly refrigerants.

On Integrated Fluid Screening and Turbomachinery Design for Optimized Industrial Heat Pumps

Abstract This work presents an approach to the contextual integration of fluid selection and compressor design for the cycle design of efficient industrial heat pumps. The vapor-compression cycle of an air–water heat pump operated at 42 °C source and 82 °C target temperature is investigated as a theoretical case study. An optimization study is performed, which includes the assessment of suitable refrigerants. Besides well-known single-component refrigerants, various binary mixtures are considered. The cycle optimization aims at simultaneously providing high cycle coefficient of performance and volumetric heating capacity. Cycle operation with the mixtures R-41/trans-2-butene (10, 90 mol. %) and CO2/R-161 (40, 60 mol. %) yields the highest values of these parameters, respectively. For further evaluation, centrifugal compressors operated with each of the two promising mixtures are designed with an in-house meanline program. In addition, the compressor design for the hydrofluoro-olefin refrigerant R-1234ze(Z) is considered as a reference. All designs are reviewed with respect to cycle as well as compressor design criteria and the applied methodology will assist designers in identifying key decision variables. The comprehensive design assessment suggests that CO2/R-161 (40, 60 mol. %) provides the best overall solution for an efficient cycle with a compact compressor design.

A Multi‐Criteria Study of Optimal Working Fluids for High Temperature Heat Pumps

AbstractA bi‐objective optimization with respect to coefficient of performance (COP) and volumetric heating capacity (VHC) is performed for a high temperature heat pump. For fixed temperature levels of the heat source and sink, the values for temperature in the evaporator and condenser as well as the degree of vapor super‐heating and condensate sub‐cooling are optimized. Constraints are placed on the minimal pressure in the evaporator, the maximal pressure in the condenser and the compressor discharge temperature. Results are presented for a one‐stage and a two‐stage heat pump cycle. Cyclobutane and several butene isomers are identified as working fluids with a good compromise between COP and VHC from a CoMT‐CAMD approach based on PC‐SAFT and a realistic model for the ideal gas heat capacity.

Performance evaluation of low GWP large glide temperature zeotropic mixtures applied in air source heat pump for DHW production

This study proposes the low GWP zeotropic mixtures applied in the air source heat pump for the domestic hot water production using the city water of 15 °C. The large glide temperature in zeotropic mixture side was applied to match temperature lift in hot water side in the condenser. Fifteen zeotropic mixtures, composed of the low boiling compositions (R744, R41, and R170) and high boiling point compositions (RE170, R600a, R1234yf, R1234ze(E), and R290), were compared. A new indicator named temperature match degree (TMD) was applied to evaluate the heat exchanging process. Besides, the system performances, including energy efficiency ratio (EER), exergy efficiency (ηex), volumetric heating capacity (qv, kJ/m3), are evaluated under different conditions. The results showed that the heat pumps using the zeotropic mixtures show better performance than that of the corresponding pure composition, except for qv. The maximum EER, ηex and qv reaches 4.17, 0.472 and 3702 kJ/m3 with the mole fraction of R744/RE170 of 0.3/0.7.TMD was also improved by the zeotropic mixtures, and the maximum TMD reaches 76 % with the mole fraction of RE170/R600a of 0.6/0.4, which is triple for R600a. The results prove the potential of the heat pumps using the large glide temperature zeotropic mixtures applied in domestic hot water production.

Thermodynamic analysis of a modified vapor-injection heat pump cycle using an ejector

This paper presents a modified sub-cooler vapor-injection (MSV) cycle with an ejector for air source heat pump applications. The added ejector is used to realize two condensation temperatures for the cycle by using its pressure lift ability and effectively improve cycle performance. The cycle performances are evaluated through thermodynamics methods and compared with those of a basic sub-cooler vapor-injection (BSV) cycle. The results show that compared with the BSV cycle, the heating coefficient of performance (COPh), the volumetric heating capacity and the exergy efficiency of the MSV cycle are increased by 10.1%, 6.3% and 14.1% under a typical operating condition, respectively. The COPh of the MSV cycle using three low-GWP substitutes (R1234yf, R1234ze(E) and R32) are compared with R134a. The results indicate that among them, R1234ze(E) is the most promising refrigerant for improving MSV cycle performance while replacing R134a. It can be concluded that the MSV cycle has a better heating performance by the introduction of the ejector, and thus could offer potential benefits for applications in air source heat pumps.

Multiobjective optimization based on mixed refrigerants and different two-stage vapor injection systems

Vapor injection two stage cycles proved to be an effective method for heat pumps to cope with extreme conditions. COP (coefficient of performance) and volumetric heating capacity usually contradictory as two important indicators. It is necessary to provide system designers a variety of solutions to select to adapt different working conditions. This paper proposed a method to obtain the Pareto front of COP and volumetric heating capacity by exhaustive method and non-dominated sorting. This paper sets a series of equivalent conditions and range conditions to evaluate the performance of different combinations. Variables include system form, types of mixed refrigerants, mixed refrigerants components, ratio of injection. The results show that IHX (internal heat exchanger) and ternary refrigerants have advantages under the set conditions. An example and an extended case study were used to illustrate the application method of a series of optimization schemes in the Pareto front. The method proposed in this paper is not limited to the set working conditions and can be applied to other working conditions. At the same time, the Pareto front is not limited to one solution, and the system designer can select the best solution according to the actual needs from a series of solutions.