Two-stage Heat Pump Research Articles

Fluorobenzene as new working fluid for high-temperature heat pumps and organic Rankine cycles: Energy analysis and thermal stability test

Industrial high-temperature heat pumps and Organic Rankine Cycles play a pivotal role in reducing CO2 emissions of the industrial sector. While several eco-friendly refrigerants have been explored for subcritical heat pumps below 150 °C, above this threshold only a few fluids can be adopted.In this article, fluorobenzene (C6H5F) is proposed for the first time as a versatile working fluid suitable for both HTHP and ORC systems. Notably, it possesses a near-zero Global Warming Potential, null Ozone Depletion Potential, low cost, and low toxicity. The thermo-chemical stability of fluorobenzene is experimentally investigated with an advanced procedure, simulating the presence of the non-condensable-gases removal system in real plant operating conditions. The yearly rate of unimolecular decomposition is estimated less than 4 % at 350 °C, and even after 400 h of thermal stress no decomposition products have been detected in the liquid phase through Fourier Transform Infrared Spectroscopy.In a direct heat exchange case study, coupled with exhaust gases at 390 °C, fluorobenzene achieves a net power production higher than other commercial fluids adopted in high-temperature units. In subcritical two-stage throttling heat pump condensing at 180 °C fluorobenzene shows a good Coefficient of Performance of 3.25 at 100 °C temperature lift.

Optimal intermediate temperature of two-stage cascade heat pump with non-azeotropic refrigerants for simultaneous heating and cooling

This paper presents a model for finding the optimal intermediate temperature to achieve the highest coefficient of performance of the standard two-stage cascade heat pump with non-azeotropic refrigerants for simultaneous heating and cooling. The optimal intermediate temperature remains unaffected by the pinch point temperature at the cascade heat exchanger, though this temperature does impact heat pump performance. The optimal intermediate temperature rises with increasing high-side condensing and low-side evaporating temperatures and the value is deviating from the average temperature between the condensing and evaporating temperatures of the high and low-temperature sides of the cascade heat pump, respectively by a correction factor. The factor is found to be dependent on the critical temperature ratio of the working fluids in the low-temperature side and high-temperature side including the condenser temperature of the high-temperature side of the cascade heat pump cycle. A developed model consisting of the average temperature with the correction factor for estimating the optimal intermediate temperatures were examined with a set of non-azeotropic refrigerants with different compositions at different high-side condensing (65–95 °C) and low-side evaporating (5–15 °C) temperatures and pinch point (2–11 °C) temperatures. The results on the optimal intermediate temperatures and the maximum COPs agreed very well with those calculated from the enthalpy method within ±7 % and ±3 % deviations, respectively.

Comparative analysis of solar-powered heat pump systems for decarbonization of process heating and cooling applications: Case of milk pasteurization

The dairy processing industry is heavily dependent on fossil fuels for heating applications at different temperature levels and contributes to greenhouse gas (GHG) emissions. Besides, process cooling, product refrigeration, and storage are mainly reliant on vapour compression refrigeration technologies. This study investigates the performance of solar photovoltaic (PV) and solar thermal (ST) powered heat pumping technologies for simultaneous heating (at 75 °C) and cooling (at 2 °C) applications in a selected typical thermal process in a dairy processing plant (milk pasteurization). An ammonia/water absorption heat pump (AHP) and a two-stage R717 vapour compression heat pump with an open intercooler (VCHP) are used in the present study. The VCHP and AHP are powered by market-available solar PV modules and ST collector technologies. The electrical COP and exergy COP of the VCHP were about 4.710 and 0.535, respectively. The AHP operates at a thermal COP and exergy COP of 0.979 and 0.301 when the driving hot water and absorber cooling water inlet temperatures are 160 °C and 30 °C. The grid-connected PV-VCHP system and ST-AHP with natural gas-fired auxiliary heater operate at an annual solar fraction of 0.533 and 0.572, respectively, for a typical weather condition in Barcelona (Spain).

Development and Experimental Validation of a Two-stage Thermoelectric Heat Pump Computational Model for Heating Applications

The utilisation of thermoelectric technology as a heat pump in heating applications necessitates comprehensive investigation. The scalable nature of thermoelectric technology enables its operation at elevated temperatures without the requirement of refrigerants. In this work, an accurate computational model that can simulate one- and two-stage thermoelectric heat pumps is developed. This model uses the electric-thermal analogy and the finite difference method, including the thermoelectric effects, temperature dependent properties, thermal contact resistances and all heat exchangers, even the intermediate heat exchanger in the case of a two-stage configuration. Moreover, the model has been experimentally validated by built and tested prototypes, being the first time that a two-stage thermoelectric heat pump model is experimentally validated.The discrepancy between the simulated and experimental results is below the ± 10 % for COP, ± 8 % for generated heat and temperature lift in the airflow, and less than the ± 6 % for consumed power. Additonally, the model simulates real tendencies under different operating conditions, proving the reliability of the developed thermoelectric heat pump model.Finally, the model is used to optimise a thermoelectric system combining one- and two-stage thermoelectric heat pumps, and hybridising them with electric resistances. An airflow of 16.5 m3/h is heated from 25 °C to 160 °C, achieving a maximum COPtot of 1.21. Lastly, the importance of considering the thermal resistances of the heat exchangers is computationally modelled and demonstrated. Not taking them into account would overestimate the performance of the TEHP system by more than the 7 %.

Two-Stage Solar–NaOH Thermochemical Heat Pump Heating System for Building Heating: Operations Strategies and Theoretical Performance

Heating buildings with solar energy is challenged by the seasonal mismatch between solar availability and heating demand. Thermochemical energy storage is a promising technology to overcome this challenge because of its high energy density. In building applications, space requirement is also an important consideration. Therefore, both the storage space and collector areas are important considerations, with only the latter often being neglected in previous studies. This paper proposes a novel two-stage thermochemical heat pump heating system based on the working pair of NaOH/H2O. We demonstrate that this system can work with a concentration difference (70% wt–30% wt) for the climate in hot summer and cold winter regions in China. The energy storage density based on the discharged solution is 363 kWh/m3. With this solar-driven thermochemical heat pump heating system, 35.13 m2 of collectors and 10.48 tons of 70% wt NaOH solution are sufficient to complete a full charge–discharge cycle and meet the heating demand of a single-family house (winter space heating + DHW: 9370 kWh, summer DHW: 2280 kWh). The theoretical maximum storage for solution (discharged + water tank) is 32.47 m3. Compared with the sensible seasonal storage alternative, the collector area is reduced by 12.5% and the storage space is reduced by 59%, with a possible further reduction through optimization. With the potential to be further optimized for space saving, the two-stage solar–NaOH heat pump heating system is an energy-efficient and space-efficient heating system for buildings in the hot summer and cold winter regions of China.

Centrifugal compressor design and cycle analysis of large-scale high temperature heat pumps using hydrocarbons

Using natural refrigerants in heat pumps, including hydrocarbons, is becoming attractive because of the tightening regulations concerning synthetic refrigerants. In the literature there is lack of studies investigating detailed component design, including compressor design when using hydrocarbons. This study investigates numerically centrifugal compressor design with hydrocarbons for large-scale high-temperature heat pumps. The compressor analysis was combined with a thermodynamic analysis of a two-stage heat pump. The effect of temperature lift and flash intercooler temperature on compressor sizing, rotational speed, efficiency, axial force, and Mach number were investigated. Generally, the use of hydrocarbons with five carbon atoms resulted in higher coefficient of performance and required lower compressor rotational speeds and larger compressor dimensions when compared to the hydrocarbons with four carbon atoms. The simulated rotational speeds were ranging from 8 krpm to 30 krpm, impeller diameters from 0.15 m to over 0.7 m and compressor efficiencies from 77% to over 85%. The use of cyclic molecules, namely cyclopentane and cyclobutene resulted in the highest heat pump coefficient of performance. Also, the flash intercooler temperature had a notable effect on the compressor design and axial force which has to be taken into account in heat pump cycle design.

Performance simulation study of multi-stage compression-throttle water working medium high-temperature heat pump system

In order to meet the demands of industrial high temperature heat and waste heat recovery in an economical and low-carbon manner, there has been a growing interest in the high-temperature heat pump using water as working medium. The study aims to address the issue of higher exhaust temperature in water working medium heat pumps resulting from a substantial increase in temperature, especially for the large temperature difference system which needs to adopt multi-stage compression. This will be accomplished by proposing a new and energy-efficient high-temperature heat pump system that utilizes water as the working medium. The optimized system combining with self-circulating water spraying and multi-stage compression, which can significantly reduce exhaust superheat and steadily improve system performance. A thermodynamic analysis was conducted on both two-stage and three-stage high-temperature heat pumps, taking into account the improved structure. The results indicate that the three-stage compression-throttle type can achieve the highest maximum water spraying ratio and the lowest exergy loss. This can effectively decrease the superheat of the compressor exhaust and enhance the overall system performance. Furthermore, in the three-stage compression-throttle system, when the evaporation temperature is set at 100 °C with a temperature increase of 40 °C, a mere 0.01 % augmentation in the water spraying ratio results in approximately a 0.0776 uplift in the COP of the water working medium system. This notable observation suggests that the implementation of self-circulating water spraying considerably enhances the system’s overall performance. Subsequently, an analysis of the corresponding design and operating parameters was conducted. It is important to note that the temperature of the waste heat source and the rotational speed of the compressor are the two key parameters that significantly impact the operation of the system. By this study, it provides some theoretical guidance for practical engineering applications.

Performance analysis of a two-stage evaporation heat pump drying system for graded cooling/dehumidification

Single-stage evaporation heat pumps are widely used in drying fields, but they suffer from relatively low efficiency. To enhance the performance of conventional heat pump drying systems with single-stage evaporating, a two-stage evaporation heat pump drying (TSE-HPD) system is proposed in this paper for graded cooling/dehumidification. In this innovative system, the quasi dual-stage compression, matching a double temperature level evaporation process, achieves graded cooling and dehumidification of air. Its performance is investigated using energy and exergy analysis methods and compared with that of a single-stage compression heat pump drying (SSC-PHD) system. The results indicate that the TSE-HPD system exhibits 15.5 %–19.3 % improvement in the coefficient of performance (COP) and 16.3 %–18.9 % enhancement in the specific moisture extraction rate (SMER) compared to those of the SSC-PHD system when the condensing temperature varies between 50 °C and 70 °C. The parametric analysis reveals that the highest COP, SMER, and exergy efficiency (ηex) occur when the optimum first-stage evaporating temperatures are 16 °C, 25 °C, and 34 °C, respectively, under three standard operating conditions. At this time, the first-stage evaporator handles 36 %–40 % of the cooling/dehumidification tasks, the refrigerant mass flow ratio at the intermediate-pressure suction port of the compressor ranges from 47 %–58 %, and the optimum first-stage and second-stage compression ratios are 1.49–1.52 and 2.45–2.61, respectively. The research results provide guidance and reference for the determination of matching design parameters and efficient operation control parameters of drying systems.

Energy and exergy analysis of various ejector-assisted two-stage compression heat pumps applied in various conditions

Energy and exergy analysis of various ejector-assisted two-stage compression heat pumps applied in various conditions

Novel two-stage adsorption heat pump with high efficiency and energy storage density, powered by non-concentrated sunlight

Low-cost solar-powered air-conditioning systems with energy storage have the potential to reduce fossil fuel consumption. Herein, a two-stage adsorption heat pump (AHP) system with high efficiency and high energy storage density, powered by non-concentrated sunlight, is proposed. In this system, solar energy is converted into heat at a selective solar absorber with transparent thermal insulation and is stored as heat of desorption for adsorbent 1. When air conditioning is required, the desorbed adsorbent 1 is used to desorb refrigerant (water) from adsorbent 2, and adsorbent 2 is used to cause the water in the evaporator to evaporate; both the heat of desorption and latent heat of evaporation are then extracted as cooling energy. When the aforementioned cooling operations are performed cyclically, the proposed AHP system can substantially increase the cooling output compared with that of a single-stage AHP even if the amount of adsorbent 2 is small compared with the amount of adsorbent 1. Simulations of solar energy storage during an entire day of sun exposure showed that this system is capable of an energy storage density per unit area of 5.3 kWh m−2 day−1 with a solar-energy-to-air-conditioning-energy conversion efficiency of 70 %. The prototype two-stage AHP was also demonstrated to repeatedly extract both the heat of desorption and the latent heat of evaporation. In outdoor experiments, the desorption of water in zeolite 13X (used as adsorbent 1) under non-concentrated sunlight was achieved.

Modeling and optimization of two-stage compression heat pump system for cold climate applications

When an air source heat pump runs at very low ambient temperatures, several issues could arise and thus limit its applications. These issues include lower coefficient of performance (COP), deteriorated heat output and higher compressor discharge temperature resulting from an increased pressure ratio. Therefore, there is a need to develop more efficient heat pump systems for low ambient temperature (∼-30 °C) applications. Under this circumstance, it becomes advantageous to use devices in which the compression of refrigerant vapor occurs in two or more stages. Two-stage vapor compression heat pumps can operate more efficiently at a greater difference in temperatures between evaporator and condenser and improve their performance under cold climate conditions. In this study, a model for a two-stage compression heat pump system is developed with the system configuration containing a closed economizer cycle and inter-stage refrigerant injection. In the two-stage heat pump, the intermediate temperature and injection pressure (or displacement ratio) between low-pressure compressor and high-pressure compressor are important parameters that influence the operating performance. In this paper, the optimal intermediate parameters corresponding to the maximum possible COP have been determined. The performance of the two-stage heat pump using R410A and R290 are modeled and compared. It is shown that the two-stage heat pump achieves a COP of 2.85 and a COP of 2.7 at an ambient temperature of −30 °C using R290 and R410A, respectively. In addition, the optimized total fin surface areas of the evaporator and the condenser heat exchangers are calculated from the developed model.

Performance investigation of a novel low-carbon solar-assisted multi-source heat pump heating system demonstrated in a public building in Hull

Global climate change has raised great attention from governments and prompted a wave of application of low-carbon heating technologies. However, solar-assisted heat pump, as an attractive technology, has challenges of unstable performance and complex structure and control strategy in practical application. Following the carbon-neutral strategy, a novel low-carbon solar-assisted multi-source heat pump heating system (LSMHS) is therefore proposed to improve the application potential of SAHP, which is demonstrated in Hull Central Library by replacing the library's original gas boiler heating system (GBHS). The LSMHS integrates eight novel multi-throughout-flowing solar collector arrays with an innovative two-stage heat recovery heat pump which can automatically switch different operation modes according to weather conditions. The practical operation results revealed that the LSMHS maximized the advantages of each component and achieved a high monthly average system COPsys ranging from 2.12 to 2.68 in the three-month demonstration. Eventually, the LSMHS provided a bill saving of 0.73 % with a significant carbon reduction of 63.69 % when compared to the GBHS in practice, achieving an equivalent bill saving of £6.7 for every tone of carbon reduction. The remarkable demonstration results showcased the application potential of the novel LSMHS and gave valuable guidance for low-carbon building heating.

Performance evaluation and multi-objective optimization of an innovative double-stage thermoelectric heat storage system for electricity generation

As the need for renewable energy continues to grow, there is an increasing requirement to utilize systems that offer enhanced performance and efficiency. Consequently, a comprehensive investigation is conducted on a two-stage thermoelectric heat pump. The primary objective is to assess the impact of various influential factors on the overall effectiveness of the system. This exhaustive parametric study aims to provide valuable insights into the system's performance and efficiency. Subsequently, the multi-objective optimization approach considers both technical and economic goal functions. By incorporating these two aspects, the optimization process seeks to achieve the most favorable balance between technical performance and economic feasibility. This allows for a holistic assessment that takes into consideration not only the system's efficiency and effectiveness but also its economic viability in real-world applications. The best optimal solution is discovered using a variety of methods for multi-objective optimization. The optimized system is examined from both an exergy and an exergoeconomic vantage point after the best optimal solution has been identified by comparing the results of various methodologies. The outcomes demonstrate that for the multidimensional analysis of Linmap, the combination of multi-objective particle swarm optimization (MOPSO) and the linear programming technique yields the best optimal solution. The objective functions for this final optimal solution are unit cost of heating power and exergy efficiency, which are defined as 1.91 and 51.28 USD/(kWh), respectively. The results showed that the optimal current for temperature changes of 10, 20, and 30 K is 2.84, 5.53, and 8.1 respectively. Also, the optimal length and number of thermocouples were 0.0055 m, 30, and 15 m respectively in all techniques. The optimal current changes from 23.30 to 27.60 A, which indicates that the optimization technique prefers to adjust the current over other effective parameters. When the thermocouples in the first and second stages are 30 and 15 pairs, respectively, according to the design parameters of the ideal point, the system will work at its peak efficiency.

Centrifugal compressor design for high-temperature heat pumps

Heat pumps are identified as crucial technology for decarbonising the heat production. Especially for large-scale, high-temperature heat pumps the use of centrifugal compressors has been seen as an attractive technological option. However, there are very few detailed studies available discussing the compressor designs with different refrigerants for high-temperature heat pumps. This study combines the analyses of a 500 kW two-stage heat pump cycle and centrifugal compressor designs. The study qualitatively assesses the feasibility of compressor designs and provides information on how the dimensions, efficiency, and rotational speed are influenced by cycle conditions and the refrigerant. Temperature lift and evaporation temperature were varied with eight working fluids, including hydrocarbons and hydrofluoro-olefins. The estimated COPs were 2.0–2.55 with the highest values observed with pentane and isopentane. The compressor rotational speeds were between 20 – 50 krpm with hydrocarbons requiring higher values than fluids containing fluorine. The predicted compressor efficiencies were between 77% to 84%, with low pressure compressor reaching higher efficiencies than the high-pressure compressor. However, high compressor Mach numbers were observed, and the Mach number seems to be the main obstacle for the increase of the heat pump temperature lift when centrifugal compressors are used.

Experimental analysis of one and two-stage thermoelectric heat pumps to enhance the performance of a thermal energy storage

This experimental study demonstrates the possibility to enhance the performance of a low-temperature thermal energy storage system (∼160 °C) based on airflow heating using electrical heaters by including thermoelectric technology. An improvement of the 17 % on COP is reached by using an optimized thermoelectric heat pump system to preheat the airflow, consisting of three one-stage and three pyramidal two-stage thermoelectric heat pumps sequentially installed along the airflow that is heating.This research experimentally analyses and compares the COP of three different configurations of thermoelectric heat pumps: one-stage, square two-stage, and pyramidal two-stage thermoelectric heat pumps. The experimental study aims to characterize the operation of each configuration for heating an airflow of 16.5 m³/h at 25 °C as ambient temperature. To that purpose, the airflow inlet temperature, voltage supply, and voltage ratio between stages have been modified.The experimental results show that for 25 °C as inlet temperature the one-stage thermoelectric heat pump has the best performance with a maximum generated heat of 78 W. Whereas, a two-stage thermoelectric heat pump is required when the inlet temperature increases. At 40 °C as inlet temperature, the square two-stage configuration provides the best performance with a voltage ratio of 2, which reaches a COP of 3.29 generating only 20 W of heat. However, the pyramidal two-stage configuration is able to achieve the maximum heat outputs with a voltage ratio of 1, generating 172; 161; 149 and 138 W, with corresponding COP values of 1.17; 1.16; 1.14 and 1.11 for inlet temperatures of 25; 40; 55 and 70 °C. This configuration is the one that achieves the greatest COP values with high inlet temperatures.

Carbon-Free Heat Production for High-Temperature Heating Systems

The article presents a new carbon-free heat production technology for district heating, which consists of a combined heat and power generation fuel cell (FC CHP) with CO2 capture and a two-stage cascade high-temperature heat pump (TCHHP). The FC generates heat and electricity, the latter being used to drive the compressors of the TCHHP. During the winter period, the water temperature achieved can occasionally be too low, so it would be heated up with hydrogen gas boilers. The hydrogen would be produced by reforming natural gas, synthetic methane, or biogas. The results are presented with natural gas utilization—the ratio between the obtained heat flow transferred directly to the water for district heating and the input heat flow of natural gas. In the case of a return water temperature of 60 °C and district heating temperature of 85 °C, the TCHHP, whose heat source is groundwater, achieves plant efficiency of 270.04% in relation to the higher heating value (HHV) and 241.74% in relation to the lower heating value (LHV) of natural gas. A case with a TCHHP whose heat source is low-temperature geothermal water achieves a plant efficiency of 361.36% in relation to the HHV and 323.49% in relation to the LHV.

Performance analysis of a two-stage vapor compression heat pump based on intercooling effect

The two-stage compression technology based on intercooling effect effectively improves the low-temperature adaptability of vapor compression heat pump systems. When a two-stage compression system is applied to address a spectrum of low-temperature operating conditions, obvious variations in the capacity ratio and interrelationship between the two compressors will take place, which may result in an instability of system control and compressor burnout during actual equipment operation. Therefore, to refine the control strategy and enhance the practicality of the two-stage compression system, this study mainly focuses on the investigation of the subcooling (SC) parameter and the desuperheating (SH) parameter during the injection process. Both simulation models and experimental testing platform of a two-stage compression heat pump system with a subcooler structure are conducted to explore the effects of intercooling effect under variable operating conditions on various system performance parameters. The results show that as the injection ratio Rinj increases, the changes of SC and SH, ΔTSC and ΔTSH show a parallelogram-like variation, and the turning point of the variation trend is around Rinj = 0.14, stabilized at about 21.55 °C; ΔTSH first increased slowly from 0 °C to 6.04 °C and then rapidly increased to 33.55 °C; the system heat production Qhot and COP were closely related to the change of SC, but not sensitive to the change of SH. Besides, the optimal intermediate pressure calculation equations are determined for different evaporation temperatures, condensing temperatures and cylinder capacity ratios. This study may offer valuable guidance for improving the throttling valve and compressor capacity control strategies in two-stage compression systems.

Thermodynamic analysis of electric to thermal heating pathways coupled with thermal energy storage

The intricate energy conversion involving thermal energy introduces complexities in assessing, analyzing, and optimizing such systems. Recognizing the paramount role of thermal energy in overall energy consumption, there arises an imminent necessity to identify and implement thermodynamically efficient electric to thermal heating pathways. This study delves into the attributes and applicability of different thermodynamic evaluation methods. Through a combination of theoretical elucidation and case-based comparisons, comprehensive thermodynamic assessments on three distinct electric heating options are performed, including electric heaters integrated with user-end thermal energy storage, heat pumps integrated with user-end thermal energy storage, and two-stage compression heat pumps paired with intermediate thermal energy storage. Our findings reveal that two-stage compression heat pumps paired with intermediate thermal energy storage exhibit heightened energy conversion efficiency, exergy efficiency, and diminished entransy dissipation. Notably, greater temperature differences between ambient and heat supply temperatures contribute substantively to the realization of both thermodynamic and economic advantages along this pathway. Considering the time-sharing tariff, optimal economic outcomes are observed for Shanghai electric bills when the intermediate thermal energy storage output temperature is maintained at 15 °C. Leveraging intermediate storage temperatures for thermal energy storage enables both heat storage during winter and cold storage during summer.

Constructing year-round energy-efficient fresh air handling unit with exhaust air heat recovery using multi-condition design

Traditional fresh air handling units with exhaust air heat recovery insufficiently utilize exhaust air waste energy, and are designed under the design-load condition, which are unable to efficiently adapt to the varying fresh air conditions for both the energy consumptions of compressors and fans. To promote energy efficiency, a fresh air handling unit with exhaust air heat recovery is constructed in this study by grade matching of loads and waste and natural energies under different typical fresh air parameter conditions, and the improved year-round efficient system configuration is determined by combining the configurations obtained under each typical condition. In addition, numerical models are adopted to calculate the energy and economic performances of the system in an office building in Nanjing, China, and a traditional system is also analyzed for performance comparison. The main conclusions are as follows: 1) The improved system configuration is determined to be a heat-recovery device plus a two-stage heat pump that can operate close to a separately designed configuration under various cases, thereby ensuring efficient operation throughout the year; 2) Compared with a traditional fresh air handling unit for fresh air load treatment with a variable refrigerant flow system in Nanjing, China, the annual saving rate of the proposed system is 14.4 %; 3) Compared with the traditional system, the static payback period is 1.1 years.

A unified approach for the thermodynamic comparison of heat pump cycles

The flexible heat pump cycle introduces a heat storage device into the Evans-Perkins cycle to recover, store, and reuse part of the sensible heat carried by the hot liquid refrigerant from the condenser, achieving a higher coefficient of performance than the latter. In this paper, we develop a unified approach, namely cycle superposition to allow comparison of the flexible heat pump cycle with other performance-enhancing cycle layouts including two-stage cycles with intercooling, subcooling, flash gas removal, or their combinations. We show that under ideal conditions, the flexible heat pump cycle is thermodynamically similar to two-stage heat pump cycles with full subcooling or flash gas removal, but no intercooling. From the energy recovery perspective, the two-stage cycles recover and reuse some sensible heat carried by hot liquid refrigerant simultaneously using their high-stage compressor, whereas the flexible heat pump cycle decouples the recovery and reuse of such heat in time using a heat storage. However, the irreversible heat transfer via real heat exchangers during charging and discharging processes will reduce the benefits of the flexible heat pump cycle. The effectiveness of all these performance-enhancing methods strongly depends on the characteristics of refrigerants.