Vapor Compression Heat Pump System Research Articles

Visualization Study on Oil Return Characteristics of Vapor Compression Heat Pump System

Vapor compression heat pump technology is a widely utilized method for energy conversion. Lubricating oil plays a crucial role in the heat pump system cycle by effectively reducing wear on the compressor’s moving parts and preventing refrigerant leakage. However, it can also create an oil film in the heat exchange equipment, which increases thermal resistance and diminishes heat transfer efficiency. This study utilizes a vapor compression heat pump system test bench to investigate factors influencing the system’s oil circulation rate, the two-phase flow patterns of refrigerant and lubricating oil, and the impact of oil circulation on system performance. The findings reveal that as the compressor speed increases, the oil circulation rate initially decreases before increasing again. Additionally, a decrease in the evaporator’s heat load leads to a reduction in oil circulation at high temperatures, while it increases at low temperatures. Furthermore, increasing the opening of the electronic expansion valve results in a gradual decrease in the oil circulation rate, whereas an increase in the refrigerant charge correlates with a rise in the oil circulation rate. The oil return flow pattern can primarily be categorized into three states: slow oil return, oil film flow, and high-speed oil return. These patterns are closely related to the degree of superheat, with lower superheat levels intensifying oil return.

Recent Advances in Ejector-Enhanced Vapor Compression Heat Pump and Refrigeration Systems—A Review

Recent Advances in Ejector-Enhanced Vapor Compression Heat Pump and Refrigeration Systems—A Review

Performance Analysis of an Ejector-Enhanced Heat Pump System for Low-Temperature Waste Heat Recovery Using UHVDC Converter Valves

This article proposes a heating method based on heat pump technology to address the large amount of low-grade waste heat generated by a certain type of ultra-high voltage direct current (UHVDC) converter valve. Thermal performance calculations for two systems, a basic vapor compression heat pump system (BVCHPS) based on thermal expansion valve throttling and an ejector-enhanced heat pump system (EEHPS) are analyzed. The research results show that the EEHPS exhibits superior COP and exergy efficiency when generating hot water above 80 °C using a heat source below 50 °C. Additionally, mathematical modeling analysis identifies optimal structural parameters such as nozzle throat diameter, throat area ratio, and nozzle outlet diameter for the ejector in its design state. The low-temperature waste heat recovered from the UHVDC converter valves can be further used in engineering applications such as heating, refrigeration, seawater desalination, and sewage treatment.

Research on the lubricating oil leakage characteristics in the vapor compression heat pump system under refrigerant leaking

Refrigerant leakage is a situation that often occurs in refrigeration equipment such as heat pumps. However, certain alternative refrigerants, while exhibiting superior environmental performance, possess varying levels of flammability. The leakage of flammable refrigerants into confined spaces can pose a significant risk of explosion. The amount of lubricant in the leaking refrigerant will undoubtedly affect the flammability characteristics of the refrigerant. In this paper, the content of PVE32 oil in the leaking refrigerant during the operation of a heat pump was investigated. The results showed that, under the five leakage rate scenarios set up in the experiment, the oil concentration was highest when the leakage occurred at the condenser inlet and lowest when the leakage was at the evaporator outlet. During the leakage process of the experiment, as the mass flow rate of the leakage increased, the oil content in the early stage of the leakage increased, but the oil content in the late stage of the leakage decreased. The reasons for the change in oil content were analyzed by the oil circulation rate and the degree of phase separation. The decrease of the oil circulation rate and the change of the degree of phase separation were the key factors affecting the magnitude of the oil content as the leakage proceeded. The results of the study reveal the leakage characteristics of lubricating oil with refrigerant leakage.

Performance Evaluation of R1224yd as Alternative to R123 and R245fa for Vapor Compression Heat Pump System

The search for environmentally friendly refrigerants for vapor compression systems has been a significant focus recently due to environmental concerns such as ozone depletion and global warming. In this study, the potential of R1224yd as an alternative refrigerant is investigated. A thermodynamic analysis of a 4-kW air conditioning system is conducted to assess the performance of R1224yd. The system is analyzed from a thermodynamic perspective, and key performance indicators such as the Coefficient of Performance and exergy efficiency. The results are then compared to R245fa and R123. Furthermore, a parametric study is performed to examine the impact of key parameters, such as evaporating and condensing temperatures, on the system's performance. This analysis provides insights into the sensitivity of the system's performance to variations in these parameters. The results indicate that R1224yd is a promising candidate as an environmentally friendly alternative refrigerant compared to R123 and R245fa. Because R1224yd has the lowest environmental impact. It has about 700 kg CO2 indirect emission, but about zero kgCO2 for direct emission. While, based on the thermodynamic results, R1224yd offers better performance compared to R245fa which has 1-3% higher in performance value and exergy efficiency, and has comparable performance to R123. This suggests that R1224yd can be a viable option for the systems, providing improved energy efficiency and lower environmental impact.

Towards sustainable process heating at 250 °C: Modeling and optimization of an R1336mzz(Z) transcritical High-Temperature heat pump

Fossil fuel boilers typically produce high-temperature process heating due to the impossibility of current renewable technologies. Vapour compression heat pump systems have been proposed up to 200 °C with subcritical cycles. To increase the heating production level, this paper proposes heating up to 250 °C through a transcritical high-temperature heat pump (THTHP) using R1336mzz(Z), which is not thermally degraded till this temperature, according to existing data. Computational models of a transcritical vapor compression cycle with and without an internal heat exchanger are developed for this work. The input cycle parameters, such as evaporation, production temperature, gas cooler output temperature, and the superheating degree, have been modified through iteration to consider possible scenarios in different industrial sectors. Moreover, the impact of using waste heat to increase evaporation temperature or superheating degree has been studied to optimize the cycle. The internal heat exchanger has also been considered for superheating the compressor suction. Other relevant parameters, such as compression ratio or volumetric mass flow rate, have also been assessed. The results show that a heat pump is feasible to reach 250 °C temperature using waste heat at 120 °C (evaporation at 100 °C and 20 °C of superheating degree), reaching a COP of 3.3. The variation of evaporator temperature from 80 °C to 140 °C increases COP from 2.5 to 5.9, while the increase of superheating degree from 20 °C to 100 °C increases COP from 3.2 to 7. Waste heat use optimization proves that it is more efficient to prioritize the increase of the evaporation temperature. If the industry requires lower production temperatures, the COP can reach a value of 7.0 at 180 °C. The decrease in gas cooler outlet temperature impacts less the COP than other input parameters. At baseline conditions, an optimum superheating degree with an internal heat exchanger of 45 K produces a maximum COP of 4.0. THTHP emissions are less than gas boilers for the same application, and in countries with greener electricity generation, these emissions can be 50 times lower.

The impact of a balanced humidification-dehumidification desalination system driven by a vapor-compression heat-pump system

This study adopts a numerical thermo-economic investigation of a novel desalination system. In this system, a vapor compression (VC) heat pump is used to drive a balanced humidification-dehumidification (HDH) desalination unit with multiple extractions. The mathematical model of the proposed VC-HDH system is validated experimentally. The system is examined for water production under a condenser temperature of 45 °C, evaporator temperature of −5°C, feed saline water temperature of 30 °C, feed saline water flow rate of 1 kg/s, and an energy effectiveness value of 0.85 for the HDH unit’s components. The results show that without air extraction, the system can produce 6.62 kg/h of desalinated water for 0.011 $/l with a gained-output-ratio (GOR) value of 9.1. On the other hand, when a single air extraction is applied, about 20 kg/h of desalination water is produced by the system for 0.0042 $/l and 29 GOR. Increasing the number of air extractions to two yields a freshwater production rate of approximately 40 kg/h for 0.0026 $/l and a GOR value of 57. A considerable positive effect of the HDH unit’s performance is found on the system’s performance on both thermodynamic and economic scales. For better desalination performance, it is found that the feed water temperature should be maintained between 25 °C and 30 °C. Finally, the theoretical ceiling of the proposed system’s GOR is estimated.

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.

Experimental Investigation of Gravity Effect on a Vapor Compression Heat Pump System

Vapor compression heat pumps have the potential to meet the future development requirements of thermal management in aerospace due to the ability to achieve high heat flux dissipation, precise temperature control, and a more compact and lightweight structure. In this study, a variable-angle test stand was constructed to investigate the effect of gravity on the performance, temperature distribution, and oil circulation rate of a vapor compression heat pump system in a ground environment. The results showed that the system could operate stably at various inclination angles, with the compressor exhibiting the lowest power consumption at an inclination angle of 90° and the maximum coefficient of performance (COP) of the vapor compression heat pump reaching 5.5. The system flow rate exhibited a sinusoidal curve with a 10.7% variation relative to the initial flow rate as the inclination angle increased. The evaporating temperature, condensing temperature, and compressor discharge temperature of the system were approximately symmetrical relative to the inclination angle of 180°. At inclination angles ranging from 90 to 270°, the oil circulation rate was highly sensitive to gravity changes and remained at a high level. The oil circulation rate agreed well with the change in subcooling/superheating, yet a rise in the oil circulation rate resulted in a decrease in COP.

Experimental investigation on a compact fresh air humidification/dehumidification system based on desiccant coated heat pump

Widely used vapor-compression heat pump system can provide both heating and cooling capacity, considering it cannot realize humidification and efficiency of condensation dehumidification is relatively low. In this paper, a fresh air humidification and dehumidification system based on desiccant coated heat pump is established. In which the condenser and evaporator of the heat pump system are replaced by desiccant coated heat exchangers. By using the 4-ventilation valve, the system is very compact. The feasibility of humidification and dehumidification of the system has been theoretically analyzed. The performance of the system under GB Shanghai summer conditions and GB winter conditions is experimentally verified. The results show that the system can realize dehumidification and humidification in summer and winter efficiently and the COP under these two conditions is 5.16 and 5.06 respectively. The dehumidification and humidification rates of the system in the two chosen conditions are 2.99 L h−1 and 1.99 L h−1 respectively. The results also show that under Shanghai summer conditions, by changing the compressor frequency, the system can get a dehumidification of 2.40 L h−1 to 3.05 L h−1 with a COP of 7.80 to 3.56. The optimized switchover period is 6 min.

Analysis on temperature vacuum swing adsorption integrated with heat pump for efficient carbon capture

Carbon capture and storage (CCS) is gathering the momentum to achieve the ultimate target of carbon neutrality. Temperature vacuum swing adsorption (TVSA) has uncovered its superiority due to large working capacity, CO2 purity and recovery rate. However, high energy demand in the regeneration process of adsorbent leads to the loss of net efficiency of coal-fired power plant (CFPP) after retrofitted. This paper initially proposes and evaluates an integrated system which is composed of an adsorption carbon capture unit (ADCCU) and a single-effect absorption heat transformer (SAHT). Then a general concept of sorption carbon capture integrated with heat pump could be introduced. Results indicates that CO2 purity and recovery rate of the integrated system vary from 91.20 % to 92.75 % and from 95.62 % to 98.11 % when flowrate range from 48 NL·min−1 to 60 NL·min−1. Under the condition of 140 °C regeneration temperature of ADCCU and 80 °C generation temperature of SAHT, exergy efficiency of SAHT achieves the maximum value of 85.28 %. Moreover, a vapor compression heat pump (VCHP) system is coupled to provide moderate temperature heat for SAHT when waste heat of CFPP is inadequate. The effect of waste heat and electricity consumed on performance of CFPP is also taken into consideration. When recovery rate is lower than 27 %, performance of the proposed integrated system is more favorable than that using steam evacuated from the turbine. It is demonstrated that heat pump assisted adsorption capture for CFPP may be a promising solution to reduce energy consumption in the near future.

Assessment method of the integrated thermal management system for electric vehicles with related experimental validation

Concerns about the driving ranges of electric vehicles (EV) have led to higher requirements for vehicle thermal management systems. Efficient thermal management systems should be closely integrated with the vehicle to ensure passenger comfort and maintain adequate component operating ranges. The thermal management functions should thus accurately meet the demands of the EV and its components. Based on the driving conditions for EVs and the component thermal management requirements, this study applies different working modes for the cabin, power battery, electric motor, and electric motor controller systems under various conditions, which combines these modes into 27 feasible system working modes. Subsequently, an integrated thermal management system is designed with multi-modes adjusted by each system component to adapt environmental variations by changing the system's working mode. Also, the energy exchange between components under different modes was investigated to ensure the rationality of the designed system. The best working mode is to ensure the safety and comfort for EVs with fewer components and lower cost. It was proved to meet a wide range of adaptations to environmental changes that can be used as a method to evaluate the integrated thermal management system. The method is then applied to the target thermal management system and proves that the target system offers considerable advantages over two other benchmark thermal management systems that have been used in the mass production of electric vehicles. Finally, the cooling, heating, and defrosting performances are verified experimentally, and the vehicle mileage is calculated. The thermal management system working modes are summarized systematically. The designed thermal management system is a waste heat recovery steam compression heat pump system that can meet the functional and experimental test requirements. This study verifies that the heat management system evaluation method can evaluate the system’s applicability effectively and that the waste heat utilization vapor compression heat pump system can effectively improve electric vehicle driving mileage, which manifests the potential for mass production for electric vehicle applications.

Study on the leakage in water-injected twin-screw steam compressor

Steam compressor shows great energy-saving potential in mechanical vapor compression and high-temperature heat pump systems. However, leakage remains a huge problem for compressor performance improvement and design optimization. In this article, a thermodynamic model with detailed leakage flow model was developed for the working process in water-injected twin-screw compressor. Altogether, leakage in different paths and heat and mass transfer between injected liquid and compressed vapor were considered. Compared with experimental results, the accuracy of predicted efficiency is within ± 5% for all tested operating conditions. In the validated model, leakage flow in five leakage paths and their effects on compressor efficiency was investigated. Results indicate that leakage can reduce volumetric efficiency by up to 50.2%, while the injected water liquid vaporization can improve volumetric efficiency by 7.9% at 5000 r·min−1. The leakage through contact line (CL), tip sealing (TS) line, and discharge end face (DE) show the most important impact on compressor efficiency, while the leakage through blow hole (BH) and suction end face (SE) has little impact on compressor efficiency. Compared with zero leakage under certain conditions, the reduction of volumetric efficiency caused by CL, TS line, and DE can reach up to 22.4%, 20.4%, and 12.7%, respectively. Reducing clearances of the above three leakage paths can efficiently improve the performance of steam compressor, especially for CL.

Design method for heating towers used in vapor-compression heat pump systems

Given that they are highly efficient in summer and frost-free in winter, heating tower heat pumps are evolving rapidly. However, lack of an appropriate design method of heating towers is complicating the design process and causing a mismatch between cooling and heating conditions. To address this problem, this study develops a total heat transfer model taking enthalpy difference as the driving force, then experiments are carried out to test the volumetric enthalpy transfer coefficients. Based on the model and the coefficients, a novel design method is proposed based on curves of required and available transfer units, involving four steps, namely: 1) determination of design conditions, 2) calculating tower thermal performance, 3) sizing x-y section of packing, 4) determination of air flow rate and packing size. Finally, a performance comparison and matching degree analysis of the designed tower are presented. The results show that the proposed design method is simple and clear by avoiding trial-and-error. The designed tower saves considerable power consumption of the tower fans and pumps and achieves good matching degree between its heating and cooling capacities by setting reasonable approaches and ranges. The model, the coefficients and the design method can be used accurately by researchers/designers.

Upper-limit of performance improvement by using (quasi) two-stage vapor compression

Both quasi two-stage and two-stage compressions are important ways for improving the energy efficiency of the vapor compression heat pump system, especially for high compression ratio conditions. Many experiments and simulations have been conducted to investigate the performance improvement of two-stage or quasi two-stage systems. In most occasions, positive results were obtained. However, the performance improvements reported by different studies vary significantly. Actually, the results obtained by different researchers are barely comparable, because operating conditions, refrigerants, cycles, and economizers heavily affect the performance of (quasi) two-stage system, which hinders the potential evaluation in cycle-selection period and perfectness assessment of developed (quasi) two-stage systems. In this study, a general analytical expression for the (quasi) two-stage vapor compression system is proposed, by which the upper-limit of performance improvement of (quasi) two-stage systems using different refrigerants under different operating conditions can be calculated quickly. Based on that, the potential of adopting (quasi) two-stage cycle and the maturation of the developed prototypes can be easily identified by comparing the test performance with the upper-limit improvement.

Энергетическое и эксергетическое исследование R1234yf, R1234ζε для парокомпрессионной теплонасосной установки

This paper has carried out the study of the energy and exergy effectiveness of vapor compression heat pump system for hot water supply. Specific of the study is in the comparative analysis of heat pump systems efficiency which operates on refrigerants of third (hydrofluorocarbons HFC) and fourth (hydrofluoroolefins HFO) generation. The study results have proven the effectiveness of the refrigerant agents of HFO group use.

Performance Analysis of Internal Heat Exchanger‐Based Quasi‐Two‐Stage Vapor Compression Heat Pump System for High‐Temperature Steam Production

The thermodynamic performance of a high‐temperature heat pump steam (HTHPS) system is determined mainly by its different cycle configurations. The present study involves improving the thermodynamic performance of a quasi‐two‐stage vapor compression high‐temperature heat pump based on a front internal heat exchanger (IHX) cycle and a rear, and analyzing the influence of an IHX. The present study establishes thermodynamic models for front reheating structure (FRS) and rear reheating structure (RRS) models on the basis of conservation of mass and energy. The system is compared to the traditional quasi‐two‐stage compression cycle system in terms of thermodynamic parameters. The results demonstrated that the RRS system exhibits an increase of 4.87% in the coefficient of performance when the exhaust superheats is maintained under 5 K. With an increase in the condensation temperature, the required mass flow of the refrigerant in the traditional system is 9.94–12.36% higher compared to the RRS system, the power consumption of the compressor is increased by 3.9–5.6%, and the steam output per unit refrigerant is decreased by 1.04–5.07%. Furthermore, the Aspen Plus simulation verifies that the reason for the change in the exhaust superheats is the different pressure ratios in the theoretical calculation.

Further analysis of the influence of interstage configurations on two-stage vapor compression heat pump systems

Two-stage vapor compression technology can effectively improve system performance, mainly due to the thermal effects of different interstage configurations. The thermal effects should be further analyzed to obtain quantitative results for different refrigerants. Based on mathematical analysis and numerical methods, the influences of the two thermal effects, i.e., subcooling and desuperheating, on the heating coefficient of performance (COPh) are analyzed. The results show that the subcooling promotes the COPh regardless of what refrigerant is employed; and the larger the molar specific heat capacity (evaluated by Cp,m0) of the refrigerant is, the greater the subcooling promotes the COPh. When Cp,m0 is less than 60 J mol−1 K−1, the desuperheating improves the COPh, whereas when Cp,m0 is larger than 70 J mol−1 K−1, it reduces the COPh. The maximum improvement in the COPh caused by interstage subcooling can reach up to 23%, while that caused by desuperheating is less than 6%.

Experimental study on a fresh air heat pump desiccant dehumidification system using rejected heat

Desiccant coated heat exchanger (DCHE) was proposed for highly efficient dehumidification process due to the adsorption heat could be eased by inner cooling during the sorption process. Meanwhile, heat pump is probably used with DCHE together in humidity control processes because cooling and heating generated by vapor compression heat pump system match with the energy demand of DCHE well. A fresh air heat pump desiccant dehumidification system using rejected heat with desiccant coated heat exchanger (DCHE) was set up and experimentally tested in this paper. Near isothermal dehumidification process could be realized due to the utilization of DCHE which could convert the sensible cooling provided by vapor compression heat pump subsystem to latent cooling to increase the moisture removal of the system. The optimal average moisture removal and COP are 7.02 g/kgDA and 4.10, respectively, under the near ARI humid condition. Most of the total cooling generated by vapor compression heat pump subsystem is used for dehumidification. The moisture removal per kilowatt hour electric power consumption of the system is 1.60–2.88 times that of conventional condensation dehumidification method.

Simulation on vapour compression heat pump system for rough rice drying

In this study, a simulation of a vapor compression heat pump system for bed drying of rough rice have been carried out for various thicknesses in the range of 20-50 cm and various drying air mass flow rates in the range of 0.06 - 0.22 kg/s-m2. The modelling used was grouped into two parts, i.e. the determination of temperature and humidity of the air coming out of the heat pump and the drying process of the rough rice using these air conditions. Parameters related to flow rate and energy were expressed in units of cross-sectional area of the drying bed. Simulation results showed significant differences in drying time and temperature for various thicknesses and air mass flow rates but their energy consumptions were similar. Under these conditions the drying temperature and drying time obtained ranged between 34-45°C and 8-15 hours, respectively while the specific energy consumption was relatively low i.e. in the range of 1.07-1.36 MJ/kg of evaporated water for all the scenarios tested. The specific drying air mass flow rate of 0.3 and 0.5 kg/s/m3 did not provide significant differences in specific energy consumption. For the field applications, the selection of conditions for thickness and the appropriate air mass flow rate will depend on the user preference including the ease of mixing, drying time or energy consumption.