Vapor Injection Heat Pump System Research Articles

Experimental study on performance and compressor characteristics of transcritical CO2 heat pump system

The accelerated advancement of the economy has led to a heightened focus on the issue of energy conservation and reduced consumption within heat pump systems. An experimental investigation was conducted to analyze the impact of discharge pressure and outlet water temperature on the performance of the transcritical CO2 single-stage heat pump system and vapor injection heat pump system. The experimental results show that the heating capacity first increases and then tends to be constant or even slightly decreases with the increase of discharge pressure in single-stage heat pump system. When subjected to the same operating conditions, the coefficient of performance of vapor injection heat pump system exceeds that of single-stage heat pump system. Compared with single-stage heat pump system, the performance coefficient of vapor injection heat pump system has increased by an average of 6.21% and the heating capacity has increased by an average of 9.54%. Additionally, the volumetric efficiency of the compressor substantially decreases when producing high-temperature hot water. Semi-empirical models for evaluating the compressor performance in both single-stage heat pump system and vapor injection pump system are established through the parameterization of the volumetric efficiency with respect to the pressure ratio and compressor speed. These models effectively forecast compressor performance.

Experimental study on low temperature heating performance of different vapor injection heat pump systems equipped with a flash tank and economizers for electric vehicle

One of the most promising solutions is applying vapor injection technology to improve the heating performance of air conditioning heat pump systems in electric vehicles operating at extremely low temperatures. This study examined the effects of employing flash tank vapor injection (FVI) and economizer vapor injection (EVI) with both upstream injection (UI) path and downstream injection (DI) path configurations on a vapor injection heat pump system with a small ratio vapor injection scroll compressor. Experiments were conducted to record the system performance at low temperatures, and typical heat transfer characteristics were obtained with different expansion valve opening degrees and compressor speeds. The influence of the injection pressure on the system performance was also investigated, and the optimal injection pressure was determined. The results for the upstream injection, downstream injection and flash tank vapor injection systems indicated maximum increases in the system’s heating capacity of 19.7 %, 22.6 %, and 92.3 %, and in its coefficient of performance (COP) of 24.2 %, 16.9 %, and 118 %, respectively; the corresponding discharge temperatures decreased by 12.4℃, 18.99 ℃, and 16.95 ℃, respectively. In addition, the optimal injection pressure was determined according to the maximum value of COP, and the optimal injection pressure range for the system is 0.16–0.37 MPa. Hence, a vapor injection system using an economizer with a downstream path could achieve better heating performance at relatively low temperatures. In contrast, a flash tank would offer some heating capacity advantages at temperatures below −15℃.

Experimental performance of a vapor-injection CO2 heat pump system for electric vehicles in −30 °C to 50 °C range

Efficient and wide-range heat pump system is essential to improve the driving range and comfort of electric vehicles. In this paper, a vapor-injection transcritical CO2 heat pump system with a flash tank for electric vehicles was built up, and the heating and cooling performance in a wide range from −30 °C to 50 °C were experimentally studied. The maximum coefficient of performance (COP) at −30 °C/ 20 °C was 1.45, and the maximum COP at 50 °C/ 27 °C was 1.08. By comparing with the basic system without injection, it was found that the heating performance of the vapor-injection system was significantly improved, and the lower the outdoor temperature, the greater the improvement. The COP and heating capacity of the vapor-injection system were increased by 45.5 % and 75.7 % respectively at −30 °C/ 20 °C. Under the cooling conditions, only when the outdoor temperature was lower than 40 °C, the vapor-injection system had an advantage, and the maximum increments of COP and cooling capacity at 35 °C/ 27 °C were 9.5 % and 16.8 %, respectively. Further, a relationship model between vapor injection and compressor structure and operating conditions was established according to the working principle of the twin rotary compressor. The experimental verification showed that the deviation was within 8 %, which could provide a basis for the construction of the CO2 vapor injection heat pump system model.

Energy, exergy, economic and environmental analysis of a novel direct-expansion solar-assisted flash tank vapor injection heat pump for water heater

Energy, exergy, economic and environmental analysis of a novel direct-expansion solar-assisted flash tank vapor injection heat pump for water heater is conducted by simulation method. Two working modes are designed for the presented system. The solar-assisted mode is recommended for most solar radiation conditions, and the air-source mode maintains more superior performance under extremely low or no solar radiation situations. By utilizing solar energy, the system heating efficiency and capacity can be respectively improved by up to 29.6% and 25.9% under the considered condition. The performance analysis shows that the presented system can respectively enhance the average heating capacity and efficiency by 11.7% and 10.6% in a typical winter day of Lanzhou City, compared with the conventional flash tank vapor injection heat pump system. The exergy analysis expresses that the heat exergy output increases with the solar radiation intensity, while the system exergy efficiency presents an opposite variation trend because of the greater increment of the exergy destruction in the solar collector. The economic analysis indicates that the presented system achieves a payback period of 8 years under the given assumptions. Benefiting from the electricity consumption reduction, the presented system can remarkably reduce the carbon dioxide and air pollution emissions.

Theoretical performance assessment of low-GWP refrigerant R1233zd(E) applied in high temperature heat pump system

With the enactment of the Kigali amendment regarding the phase-down of substances with Global Warming Potential (GWP), refrigerants based on HFCs are falling into disuse and low-GWP refrigerants are proposed as replacements. R1233zd(E) is a promising new working fluid for High-temperature Heat Pumps (HTHPs) with low GWP which has yet to be further developed. A proper model needs to be imperatively constructed to predict the characteristics of R1233zd(E) applied in HTHPs. This paper proposes a mathematical model of two-stage compression and vapor injection heat pump system, and analyzes the compression ratio, injection mass flow rate, power consumption, discharge temperature, heating capacity and Coefficient of Performance (COP) under variable working conditions. When recovering 50 °C waste heat and supplying 60–130 °C hot water, the heating capacity increases from 2.09 to 2.22 MW and the COP decreases from 23 to 2.93. When recovering 80 °C waste heat and supplying 90–160 °C hot water, the heating capacity increases from 4.11 to 4.23 MW and the COP decreases from 24.4 to 2.7. The detailed model of heat exchangers can demonstrate the inner heat transfer process clearly, such as refrigerant temperature and gas content ratio, which would be instructive for component optimization. The above results indicate that the studied R1233zd(E)-based HTHP has excellent performance and broad application prospects, which plays a positive role in further development of HTHPs.

Performance improvement of vapor-injection heat pump system by employing PVT collector/evaporator for residential heating in cold climate region

The adoption of vapor injection (VI) cycle could overcome the defects of the conventional one-stage air source heat pump (ASHP) system under extremely low ambient temperature conditions. Nevertheless, the ASHP system with VI cycle still could not operate efficiently due to the low evaporating temperature in the fin-tube evaporator. For such a situation, the PVT (Photovoltaic/Thermal) collector/evaporator could reach higher evaporating temperature attributed to its physical structure, thereby improving the system COP. This study proposes the detailed mathematical model of the vapor-injection heat pump system by incorporating PVT collector/evaporator and verifies the effectiveness of the proposed system. Parametric studies have been then conducted. The proposed system can operate off-grid and its COP can reach 4.0 at the ambient temperature of −10 °C and the solar irradiation of 500 W/m2. The practical electrical efficiency of PV panels achieves at 15.1%, whereas the thermal efficiency of the system is 44.8%. A hybrid control method including three modes has been also proposed based on the results for improving the system performance. The levelized cost of heat (LCOH) of this system is 0.054 $/kWh, which is 51.5% lower than that of electric heating system (0.111 $/kWh).

Experimental Investigation on a Vapor Injection Heat Pump System with a Single-Stage Compressor

In this study, a heat pump of 10 kW with vapor injection using refrigerant of R410A was developed. A vapor injection pipe connecting a gas–liquid separator at the outlet of the main expansion valve and the suction of a single-stage rotary compressor was designed. The heating performance of this vapor injection heat pump was investigated and analyzed at different compressor frequencies and primary temperatures. The experimental results show that for the heat pump without vapor injection, the heating capacity increased linearly with the compressor frequency, while the heating coefficient of performance (COP) decreased linearly with the compressor frequency for each tested primary temperature. The developed vapor injection technique is able to increase the heat pump system’s heating capacity and heating COP when the injection ratio R falls into the range 0.16–0.17. The refrigerant mass flow rate can be increased in the vapor injection heat pump cycle due to the decreased specific volume of the suction refrigerant. The power consumption of vapor injection heat pump cycle almost remains the same with that of the conventional heat pump cycle because of the increased refrigerant mass flow rate and the decreased compression ratio. Finally, it was found that the developed vapor injection cycle is preferable to decreasing the compressor’s discharge temperature.

Upstream and downstream injection effects on R134a economized vapor injection heat pump system at low temperatures for electric vehicles

Abstract Vapor injection technology has been applied to improve the performance of heat pump systems at low ambient temperatures. However, little attention has been paid to the application of heat pump systems combined with the economized vapor injection for electric vehicles. In this study, the performances of two electric vehicle system configurations were compared, including the upstream injection and the downstream injection. The effects of the injection pressure, the compressor speed, the condenser and outdoor inlet temperatures on the overall system performance and specific operating parameters were measured and analyzed in detail. The results indicated that the downstream injection exhibited better system performance. At the compressor speed of 6500 rpm, the maximum heating capacity of the downstream injection was 18.6% and 8.9% higher than that of the upstream injection at the condenser inlet and outdoor temperatures of −2 °C and −12 °C, −7 °C and −18 °C, respectively. At the compressor speed of 4500 rpm, the COP of the downstream injection was 12.9% higher at the condenser inlet and outdoor temperatures of −7 °C and −18 °C.

Numerical investigation on the heating performance of a transcritical CO2 vapor-injection heat pump system

CO2 is a new type of friendly refrigerant. In order to investigate the effect of the operating parameters of the transcritical CO2 heat pump system on heating performance, the transcritical CO2 vapor-injection heat pump (VIHP) system simulation models were developed. The models were validated with the previous experimental results. All the deviations are within the acceptable range. The simulation results show that the heating capacity will meet the requirement of electric vehicle (EV) in the cold region when compressor revolving speed is greater than 4000 r min−1. The heating capacity of the VIHP system is greater than that of the transcritical CO2 single-stage heat pump (SSHP) system; besides, the coefficient of performance (COP) of VIHP system is 30.7–38.6% greater than that in SSHP system under the same condition. The VIHP system is more suitable for EV to run in the cold region. The COP reaches a peak at a certain injection pressure. The correlation of optimization injection pressure is developed. Therefore, the simulation data provide great guidance for designing a transcritical CO2 heat pump system and developing the controlling system of the heat pump system.

Dynamic modeling and characteristic analysis of a two-stage vapor injection heat pump system under frosting conditions

This paper presents a distributed-parameter dynamic heat exchanger model integrated with a detailed frost growth model to account for non-uniform frost formation on a fan-supplied finned-tube coil. A novel, iteration-free approach is proposed to solve the air flow redistribution by linearizing a system of non-linear air pressure drop equalization equations, resulting in a significant improvement in the computational efficiency. As a continuation and extension of our previous work, the developed models along with the component models described in Qiao et al. (2015a) are applied for the first time to explore the frosting dynamics of a two-stage flash tank vapor injection heat pump system. It is found that frost formation degrades the heating performance of the system substantially. Meanwhile, the simulation indicates that air and refrigerant flow maldistribution, resulting from non-uniform frost growth on the outdoor heat exchanger, can lead to unstable system hunting behavior. Comparisons between the simulation results and experimental data indicate that the proposed models can reasonably predict the time-dependent heat transfer and fluid flow phenomena of the system.

Performance analysis of vapor injection heat pump system for electric vehicle in cold startup condition

In this study, a vapor-injection (VI) cycle designed for indoor heating of electric vehicle (EV) was investigated for low temperature heating purposes. The heating capacity variation was observed both in mathematical and experimental ways to verify the influence of vapor injection at different injecting positions and under different intermediate pressure. From this study, the optimal injection position of the scroll compressor and intermediate pressure ratio were evaluated that maximizes heating capacity and coefficient of performance (COP) of the cycle. To validate the numerical results, the experiment was also conducted in the prototype of a vapor-injection (VI) heating system for electric vehicle. The experiment was carried out under the steady-state condition and the same parameters as those of numerical analysis were employed. The comparison between the results of numerical analysis and that of experiments showed a good agreement. For the increase of heating capacity, the optimal injecting port position was observed in specific value which was close to 300°. As the opening of the main expansion valve was decreased, the performance of the VI system generally got better but the system had much restricted intermediate pressure ratio in which the performance was drastically decreased. As a result, the optimal intermediate pressure ratio occurred in specific value below 0.25 in startup condition.

Performance evaluation of a vapor injection heat pump system for electric vehicles

Battery-powered electric vehicles (EV) need an efficient electric heating system for extending the driving mileage. An air-source heat pump system offers an economical alternative for EV because it consumes less energy than a heating system using Joule heat and it can use the same components as an air conditioning system for cooling. However, its performance degradation is inevitable at very low ambient temperatures. Although vapor refrigerant injection is known as a good technology to overcome this problem in residential heat pump systems, the number of vapor injection heat pump studies for EV applications is quite limited. In this paper, considering the characteristics of EV application, the configuration of a vapor injection heat pump system for EV is introduced, and it was modeled, using a scroll compressor geometry-based thermodynamic analysis. The performance was estimated for cold ambient regions under the EV operational conditions.

Transient modeling of a flash tank vapor injection heat pump system – Part I: Model development

This two-part article explores the dynamic behavior of a flash tank vapor injection heat pump system from a numerical simulation perspective. Part I provides a first-principles model describing the transient heat transfer and flow phenomena of the system with detailed modeling techniques for each component. The vapor injection scroll compressor is analyzed with the internal heat transfer between the refrigerant and metallic parts taken into account. Lumped-parameter models are developed for the flash tank and expansion devices. Heat exchangers are modeled using a finite volume approach and accounting for the complex tube circuitry. The separated flow model without interfacial exchange is utilized for two-phase flows in order to incorporate an appropriate void fraction model so that a more accurate prediction for refrigerant mass distribution can be achieved. The modular nature of the component models allows flexibility in the system configuration. Transient simulations are carried out for start-up and shut-down operations. A detailed comparison of model predictions against experimental data is presented in the companion paper.

Transient modeling of a flash tank vapor injection heat pump system – Part II: Simulation results and experimental validation

This two-part article investigates the transient characteristics of a flash tank vapor injection system through modeling and experimental validation. The first part describes the detailed modeling techniques for each component, and the second part describes the transient simulations that are carried out for the overall system under start-up and shut-down operations and presents the results. After comparing the predictions of the proposed model with the experimental data, it was concluded that the proposed model can adequately capture the transient heat transfer and flow phenomena of the system. The dynamic system response when subjected to a step change in the opening of the upper-stage EEV (electronic expansion valve) was also investigated. It was found that EEV opening has a significant impact on the system performance and the liquid level in the flash tank, but exhibits little effect on the suction pressure. These findings were corroborated through experiments.

On-field measurement method of vapor injection heat pump system

This paper examines air-to-air injection heat pumps (HPs) and proposes a method to determine both heating capacity and coefficient of performance (COP) in situ, allowing analyses of already-installed setups and long-term observation. Due to the uncertain nature of air enthalpy measurements, this method instead uses refrigerant enthalpies calculated from pipe contact temperatures and steady-state energy balances to determine the COP. However, these energy balances are highly dependent on the hypothesis that the working fluid remains strictly monophasic in certain locations, which is not always true in practice. A parametric variable analysis is conducted on the refrigerant vapor quality to calculate the deviation in the COP due to diphasic conditions at the compressor inlet, injection port, and condenser and flash tank outlets. A final in situ COP uncertainty is presented, due to both the measurement and vapor quality uncertainties.