Frost-free Air Source Heat Pump Research Articles

Three-dimensional model construction of a frost-free air source heat pump evaporator and its performance analysis

This study aims to design an efficient frost-free air source heat pump evaporator for use in cold regions. The challenges of low performance, slow heat transfer, and poor energy supply efficiency associated with air source heat pump evaporators are addressed. This study analyzed the benefits of combining solar energy and air source heat pumps for heat supply. A three-dimensional model of the evaporator was constructed in ANSYS software, and optimal operating parameters were determined through variable experiments. The experimental results showed that when other factors were controlled constant, the glass cover size increased from 1.7 m to 2.9 m, and the maximum value of heat absorption of the evaporator increased from 2155 W to 2418 W and then decreased to 2340 W. When the wind speed and wind pressure increased from 150Pa to 300Pa, the maximum value of heat absorption of the evaporator increased from 1620 W to 2110 W. In summary, the maximum value of heat absorption of the evaporator was increased from 1,620 W to 2,110 W when the size of the glass cover was 2.5 m*2.5 m*2.5 m. When the wind speed and pressure were 300Pa, the heat absorption efficiency of the evaporator was the highest. This finding not only provides a new idea for the performance optimization of air source heat pump evaporator, but also opens up new possibilities for energy efficiency improvement in cold regions, reflecting the innovative and application value of this study.

Energetic, economic and environmental analyses of frost-free air-source heat pump in multi-type buildings and different locations

Frost-free air-source heat pumps (FFASHPs) have gained increasing attention as an alternative to conventional air-source heat pumps (ASHPs), chillers and boilers (CBs), in the context of low-carbon space heating and cooling. Conducting energetic, economic, and environmental (3E) analyses on these systems in various building types and locations is crucial for system selection, yet relevant research is scarce. To address this issue, this paper presents a comprehensive framework for 3E analyses, including location selection, building assessment, load calculation, system sizing and modeling, annual simulations, and subsequent data visualization. The study includes seven building types and 353 locations with specific climate conditions. The results indicate that FFASHPs outperform ASHPs in terms of energy efficiency, lifecycle costs, and carbon emissions. Compared to CBs, FFASHPs exhibit superior energy efficiency across most regions and building types, except for severe cold, hot summer and warm winter regions. In central and coastal areas, the advantages are more prominent due to higher natural gas prices, making FFASHPs commercially viable. In hot summer and cold winter region with electricity carbon emission factors ranging from 0.3574 to 0.6829 kgCO2/kW, the FFASHP exhibits significant potential for decarbonization due to its high efficiency. In renewable energy-rich areas with low emission factors (0.1031–0.2602 kgCO2/kWh), FFASHPs can achieve significant carbon reductions, up to 90 %. This paper, for the first time, achieves a comprehensive 3E analyses of FFASHPs, ASHPs, and CBs. It offers a general methodology, extensive datasets, and visualized graphs for system selection, promoting low-carbon space heating and cooling.

Visualization experiment and numerical simulation on behaviors of gas-liquid falling film flow on the air-side of two-row plain finned-tube heat exchanger

The frost-free air-source heat pump (ASHP) with integrated liquid desiccant dehumidification is a more promising energy-saving alternative to the conventional ASHP unit. The performance of spray-type finned-tube heat exchanger (FTHE), as one of the key equipment of this frost-free ASHP system, is affected by a combination of various factors including wall wettability, liquid film flow pattern, liquid film thickness, velocity distribution in a liquid film, interface wave, interface velocity and so on. To provide a theoretical basis for the operation, design and optimization of high-performance spray-type FTHE, a visualization experiment system and a three-dimensional (3D) numerical model are employed to study the behaviors of counter-current gas-liquid falling film flow on the air-side of two-row plain FTHE under different parameters in the present paper. According to the experimental results, the flow pattern of liquid film on the finned-tube surface is a mixture flow composed of the rivulet flow and the thin falling film flow, demonstrating that the assumption of the uniform distribution for liquid film thickness proposed in previous studies is not completely consistent with the actual situation. Meanwhile, the numerical results indicate that the transient-average film thickness on the surface of two-row plain finned-tube is significantly 18.3% – 32.5% higher than that on the vertical plate calculated by Nusselt's model. After that, the critical gas and liquid Reynolds numbers of the flow pattern transition for the liquid film are obtained, and it is pointed out that the wall contact angle of 10° is more conducive to the formation of complete film flow on the air-side of the two-row plain FTHE within the fin pitch range of 4.8 – 6.0 mm. The changes of the falling liquid film and gas-liquid interface characteristics are fundamentally attributed to the drag effect of the interfacial shear force. Furthermore, there is a cubic curve relationship between average interface shear force and gas Reg. A quadratic curve relation exists between average wall shear stress and gas Reg. At the same time, the velocity of falling film fluid at the bottom region of laminar flow is reduced, resulting from the inhibiting effect of the interfacial shear force.

Research on the Performance on Frost-Free Air Source Heat Pump with Waste Heat Desorption

For the novel frost-free air source heat pump system (FASHP), the compressor waste heat is used to assist desorption for the desiccant on the surface of the desiccant coated heat exchange (DCHE). It can promote the desorption efficiency of the DCHE in the FASHP and reduce the energy loss, which means the improvement of the stability and performance. In this paper, a DCHE model is established, and its accuracy is verified through experimental data. The model is then used to analyze the effect of working conditions and structural parameters on the adsorption rate, desorption rate, heat exchange and proportion of latent heat in the adsorption and desorption processes. The results show that the latent heat ratio is higher, the heat exchange of the DCHE is lower and the system performance is better when the temperature of the analytical circulating air increases and the velocity decreases to 25 °C/0.5 m/s. In addition, the performance is greatly affected by the fin length and width. With the increase of parameters, the heat exchange and proportion of latent heat can be increased to 115.9 J and 69.0%.

Techno-economic evaluation of a frost-free air source heat pump water heater

In order to improve the performance of air source heat pumps (AHSPs) when it is operated in winter, a novel frost-free ASHP system has been developed and the thermodynamic characteristics was performed in the previous work. Before it is large-scale promoted and applied, an economic analysis should be conducted. However, there is little research on the economic analysis of the system, which limits the development and application of innovation technologies. Therefore, a techno-economic analysis of the system was evaluated in typical climate zones in China and compared with a conventional ASHP unit in this paper. The results showed that the running cost of the frost-free ASHP system reduced 23.2, 23.6 and 29.0 % in comparison with the conversional ASHP system in Xi’an, Shenyang and Hefei cities respectively, also, corresponding to the payback period of the initial cost were 4.2 years, 3.3 years and 4.03 years respectively. It shows that the frost-free ASHP system is a promising economical way for space heating. Conclusion from this work will provide key information for the user in decision making and determining the economic viability of the frost-free ASHP system has been evaluated before it is large-scale promoted and applied.

Exergy analysis of a frost-free air source heat pump system

The main focus of this paper is to analyze the exergetic characteristics of a frost-free air source heat pump (ASHP) integrated with solid dehumidification and phase change heat storage. The exergy loss, exergy loss ratio and exergy efficiency of the system have been calculated to investigate the influence of the operation modes on the system characteristics. In addition, a comprehensive exergy loss ratio is defined to evaluate the system performance. It is concluded that the comprehensive highest exergy loss ratio is obtained as 51.4 % for compressor, followed by the energy storage device (ESD) with 15.8 %, the extra heat exchanger coated with a solid desiccant (EHECSD) with 10.7 %. Also the variation of the exergy loss, exergy loss ratio and exergy efficiency of the system and the key components with the ambient temperature changes from −20 to 5 °C are discussed. This study can provide guidance for optimizing the structure and improving the frost-free ASHP system in the engineering applications.

Performance investigation on a frost-free air source heat pump system employing liquid desiccant dehumidification and compressor-assisted regeneration based on exergy and exergoeconomic analysis

Frost build-up on the evaporator of air source heat pump severely deteriorates the heat transfer and subsequently reduces the performance of the system. Combining liquid desiccant dehumidification with heat pump system is an efficient strategy to achieve frost-free operation by dehumidifying the inlet air during winter in humid areas. In this paper, detailed energy, exergy and exergoeconomic analyses are performed for a frost-free air-conditioning system with integrated liquid desiccant dehumidification and compressor-assisted regeneration, in which the diluted liquid desiccant can be efficiently regenerated under low temperature condition. Parametric studies are conducted to appraise the influence of the important operation parameters on the performance of the proposed system. Then the hybrid system is optimized and compared from the exergoeconomic perspective. It is found that, the first and second largest exergy destructions take place in the regenerator and dehumidifier, which can be improved by increasing the investment costs. When the proposed system is optimized based on the exergoeconomics, the coefficient of performance and exergy efficiency increase by 13.02% and 12.73%, respectively. The total product unit costs can decrease by 12.64% compared to that under the basic operation condition.

Experimental investigation on a novel frost-free air source heat pump system combined with liquid desiccant dehumidification and closed-circuit regeneration

Frost build-up on the fins of the evaporator severely decreases heat transfer and subsequently reduces the performance of the heat pump system. In this paper, a novel frost-free air source heat pump system is proposed, which combines with liquid desiccant dehumidification and closed-circuit regeneration. A test setup for investigating the performance of the proposed system and the corresponding performance indexes are developed to evaluate the system performance under variable working conditions. The effects of key parameters including solution temperature, solution concentration, air flow rate and solution flow rate on the regeneration performance and heating performance of the system are analyzed experimentally. Results show that the optimal mass flow rate ratio of the solution to the air is 1.4 in the regeneration mode, while the ratio of 0.55 is recommended in the heating mode. Additionally, performance comparison between the proposed system and the conventional reverse-cycle defrosting system available in the literature is conducted. It is concluded that the proposed system is superior to the reverse-cycle defrosting system when the ambient temperature is less than 0 °C and the relative humidity is higher than 65%.

Frosting Phenomenon and Frost-Free Technology of Outdoor Air Heat Exchanger for an Air-Source Heat Pump System in China: An Analysis and Review

Frost layer on the outdoor air heat exchanger surface in an air-source heat pump (ASHP) can decrease the system coefficient of performance (COP). Although the common defrosting and anti-frosting methods can improve the COP, the periodic defrosting not only reduces the system energy efficiency but also deteriorates the indoor environment. To solve these problems, it is necessary to clearly understand the frosting phenomenon and to achieve the system frost-free operation. This paper focused firstly on the analyses of frosting pathways and frosting maps. Followed by summarizing the characteristics of frost-free technologies. And then the performances of two types of frost-free ASHP (FFASHP) systems were reviewed, and the exergy and economic analysis of a FFASHP heating system were carried out. Finally, the existing problems related to the FFASHP technologies were proposed. Results show that the existing frosting maps need to be further improved. The FFASHP systems can not only achieve continuous frost-free operation but reduce operating cost. And the total COP of the FFASHP heating system is approximately 30–64% higher than that of the conventional ASHP system under the same frosting conditions. However, the investment cost of the FFASHP system increases, and its reliability also needs further field test in a wider frosting environment. In the future, combined with a new frosting map, the control strategy for the FFASHP system should be optimized.

Experimental performance analysis and evaluation of a novel frost-free air source heat pump system

To tackle the problem of frosting occurred on the outdoor coil surface when an air source heat pump (ASHP) is operated in winter, a novel frost-free ASHP system has been developed. However, the dehumidification performance and the frost-free time of the system operated are highly dependent on the electronic expansion valve (EEV) opening of the 1st stage throttle. In addition, the coefficient of performance (COP) of the systems is controlled by the EEV opening of the 2nd stage throttle. Therefore, the objective of this study is to investigate the effect of the EEV opening on the system thermodynamic performance in terms of dehumidification efficiency, regeneration efficiency, compression ratio, heating capacity and COP. It was observed that the optimal performance was achieved when the EEV openings of the 1st stage throttle and 2nd stage throttle were 75% and 40% respectively. In addition, a correlation of COP with the EEV openings of the 1st stage throttle and 2nd stage throttle was developed to optimize the system performance. The results showed that the system thermodynamic characteristics were highly sensitive to the variation in the EEV openings of both the1st stage throttle and 2nd stage throttle.

Performance evaluation of a novel frost-free air-source heat pump integrated with phase change materials (PCMs) and dehumidification

Abstract: Air-source heat pump (ASHP) has been widely used in domestic and commercial buildings because of its energy savings, high efficiency and environmental friendliness. However, the heating capacity of an ASHP system is greatly influenced by the frost accumulation on the surface of the outdoor exchanger. To solve this problem, a novel frost-free ASHP system, integrating with Phase Change Materials (PCMs) and dehumidification, has been developed. In this paper, the schematic design of this system is first presented. The effect of the match relationship between the desiccant materials and PCM thermal energy storage on the performance of the novel system is then studied using a dynamic mathematical model. The simulation results showed that the dehumidification efficiency was increased from 31.8 % to 34.7 % with the increase of the solid desiccant mass from 1.9 kg to 3.5 kg when the volume of the PCM was 1000 ml. The system COP was 2.87 when the desiccant was 2.2 kg and the volume of PCM was 1100 ml at a relative humidity of 80 % and ambient temperature of 0 °C. In addition, the water temperature was heated to 55 °C in one cycle which decreased the irreversible loss. Lastly, a correlation of the system COP with the amount of the solid desiccant and the PCM was obtained through a multivariate linear regression. The results obtained can facilitate optimal design and dynamic behavior investigation of this system.

Performance analysis of a novel frost-free air-source heat pump with integrated membrane-based liquid desiccant dehumidification and humidification

A novel frost-free air source heat pump (ASHP) combined with membrane-based liquid desiccant dehumidification and humidification is proposed and investigated in this paper. Liquid-to-air membrane dehumidifier (LAMD) is employed to dehumidify the air before entering the outdoor coils in the proposed system so that frosting can be prevented. Diluted solution is regenerated to humidify the supply air via the liquid-to-air membrane regenerator (LAMR), which can overcome the droplets carry-over problem in the liquid-to-air energy exchanger. Model simulations are carried out to study the system performance under various operating parameters and climatic conditions, meanwhile the comparison between the proposed system and conventional defrosting system is performed. Results demonstrate that the COPsen and the COPtot of the novel system are at least 37.7% and 64.3% higher than that of the COPreverse of conventional reverse-cycle defrosting ASHP system in the variation ranges of the analyzed parameters,respectively.Humidification effects on the supply air depend on the ambient air properties. And air humidification by the regenerator can meet the needs of the air-conditioning room comfort when the ambient air is above 0°C and 70% RH.

Numerical study on the operating performances of a novel frost-free air-source heat pump unit using three different types of refrigerant

Air-source heat pump (ASHP) is a promising device using in residential buildings because of its energy-savings. Besides, R134a and R407C as environmental friendly refrigerant have advantage to apply into heat pump system. In this study, an investigation of a novel frost-free ASHP system, integrated with dehumidification and thermal energy storage, working with R134a and R407C as an R22 alternative, was presented. A mathematical model of the system is constructed and verified by comparison with experimental data that shows the measured results are in good accordance with the numerical ones. According to the mathematical model, the dynamic performance of the system is characterized by dehumidification and regeneration efficiency, suction and discharge pressure, COP and so on at ambient temperature of 0°C and RH (relatively humidity) of 85%. The results show that the time for keeping the evaporator frost-free operation for R22, R407C and R134a are 29, 34 and 35minutes, respectively. In addition, at the end of working time, the discharge pressure of compressor for R134a is 29 and 32% lower than that for R22 and R407C. What is more, at the given ambient temperature of −10°C and RH of 85%, the average COP for R134a is 3.3 and 8.6%, respectively, which is higher than that for R22 and R407C.

Performance investigation of a novel frost-free air-source heat pump water heater combined with energy storage and dehumidification

Air-source heat pump (ASHP) often operates with substantial frost formation on the outdoor heat exchanger at low ambient temperature in winter, it insulates the finned surface and also reduces heat transfer rate, leading to performance degradation or even shutdown of ASHP systems. Although several defrosting methods have been reported, the frosting and defrosting processes reduced energy efficiency and resulted in, in some cases, heat pump breakdown. To solve this problem, a novel frost-free air-source heat pump water heater (ASHPWH) system has been developed, which coupled with an extra heat exchanger coated by a solid desiccant (EHECSD) with an energy storage device (ESD). Based on the previous studies, a further analysis and comprehensive research on the novel frost-free ASHPWH system is presented in this paper. The dynamic characteristics of the novel system are investigated experimentally in different ambient conditions. An experimental setup and experimental procedures are described in detail. Thereafter, the dehumidification efficiency and regeneration efficiency of EHECSD, suction and discharge pressures of the compressor, the temperature of PCM are evaluated during the heating and regeneration modes respectively. Results indicate that the system can keep the evaporator frost-free for 32, 34, 36min during heating mode at the ambient temperatures of −3°C, 0°C and 3°C and 85% RH. Compared with the reverse-cycle defrosting (RCD), COP of the frost-free ASHPWH are 17.9% and 3.4% higher at the ambient temperature of −3°C and 3°C respectively. With this innovative technology, it has been proved that the new system can realize continuous heating and excellent performance at a low ambient temperature.

Experimental analysis on a novel frost-free air-source heat pump water heater system

Air-source heat pumps (ASHPs) are being widely used in residential and commercial buildings and have a huge potential market and development foreground for its energy-savings, high efficiency and environmental friendliness. However, at low temperatures, frost can accumulate on the surface of the finned outdoor coil which will decrease the heating capacity and coefficient of performance (COP). This paper proposes a novel frost-free air-source heat pump water heater (ASHPWH) system, which is coupled to an extra heat exchanger coated by a solid desiccant (EHECSD) with an energy storage device (ESD). To test the system performance, experiments are carried out and the results show that the relative humidity (RH) of the air can be reduced to 52% after dehumidification and the outdoor heat exchanger can be kept frost-free for 34 min at a temperature of 0 °C and relative humidity (RH) of 80%. In addition, the average COP of the system is 2.81 in a single period, which is an increase of 7.25% and 46.3% in comparison with hot-gas bypass defrosting (HGBD) and electric resistance heating (ERH) respectively. With this new technology, it has been proven that frost-free ASHPWH be achieved.

A new method for preventing air-source heat pump water heaters from frosting

This paper proposes a frost-free air-source heat pump water heater (ASHPWH) system with integrated solid desiccant, in which frosting can be retarded by dehumidifying air before it enters the ASHPWH evaporator. The system consists of a conventional heat pump water heater and an extra heat exchanger coated by a solid desiccant. During the desorption process of the desiccant, regeneration air is cycled between the desiccant-coated heat exchanger and the evaporator to recover the sensible heat and latent heat of the regeneration air. A numerical simulation is carried out at a dry-bulb temperature of −7 °C–5.5 °C and a relative humidity of 60%–80% depending on the frosting conditions. The results show that the coefficient of performance (COP) is within the range 3.3–3.8, which is 5–30% higher than that of the ASHPWH system using the hot-gas defrosting method.

Combination of air-source heat pumps with liquid desiccant dehumidification of air

This paper proposes a frost-free air source heat pump system with integrated liquid desiccant dehumidification, in which frosting can be retarded by dehumidifying air before entering an outdoor heat exchanger. And the water removed from the air is used to humidify a room. Simulation is carried out at a dry-bulb temperature of −7 to 5.5°C and a relative humidity of 80% depending on the frosting conditions. The results show that the coefficient of performance (COP) is in the range of 2.6–2.9, which is 30–40% higher than that of heat pump heating integrated with an electric heater humidifying system. And it is found that the optimum value of the concentration of lithium chloride aqueous solution is 37% for the frost-free operation mode. Experiments are conducted for liquid desiccant system under low air temperature and high relative humidity conditions. Experimental results show that the dew point of the dehumidified air is decreased by 8°C and the humidity ratio of the humidified air is kept at 8.1gkg−1, which ensures the frost-free operation of the heat pump evaporator and the comfortable level of room humidity simultaneously. The heating load of solution is 3–4.5 times larger than cooling load of solution, which agrees with the assumption given at the part of the simulation. Furthermore, the deviations between the calculated COPLHRU and the experimental results are within 33%.

Analysis on performance of a novel frost-free air-source heat pump system

Heating capacity of an air-source heat pump (ASHP) system often decreases due to frost on the air-side heat exchanger (evaporator) when the air temperature drops in winter. If the amount of frost accumulates to a certain value, performance of the system will degrade. Therefore, defrosting mode needs to be operated periodically. In order to solve the above problems, heat transfer enhancement or advanced defrosting methods should be adopted, but all these methods cannot solve the problems mentioned in essence. A novel frost-free air-source heat pump system is proposed, the new system can realize heating which does not need defrost in winter. In this new system, extracting heat process from environment includes two steps: the first one is extracting heat from the environment and then to the solution, the second one is releasing heat to the evaporator from the solution, avoiding frosting on the evaporator surface. A theoretical model is established to analyze the performance of the system. Results indicate that the novel system can operate more efficiently than the conventional air-source heat pump in winter. In addition, the new system does not need to run in defrosting mode periodically.

ASHP Midyear Meeting

A novel frost-free air source heat pump (ASHP) combined with membrane-based liquid desiccant dehumidification and humidification is proposed and investigated in this paper. Liquid-to-air membrane dehumidifier (LAMD) is employed to dehumidify the air before entering the outdoor coils in the proposed system so that frosting can be prevented. Diluted solution is regenerated to humidify the supply air via the liquid-to-air membrane regenerator (LAMR), which can overcome the droplets carry-over problem in the liquid-to-air energy exchanger. Model simulations are carried out to study the system performance under various operating parameters and climatic conditions, meanwhile the comparison between the proposed system and conventional defrosting system is performed. Results demonstrate that the COPsen and the COPtot of the novel system are at least 37.7% and 64.3% higher than that of the COPreverse of conventional reverse-cycle defrosting ASHP system in the variation ranges of the analyzed parameters,respectively.Humidification effects on the supply air depend on the ambient air properties. And air humidification by the regenerator can meet the needs of the air-conditioning room comfort when the ambient air is above 0 °C and 70% RH.