Air Source Heat Pump Unit Research Articles

Simulation and optimization study on a solar space heating system combined with a low temperature ASHP for single family rural residential houses in Beijing

A pilot project of the solar water heating system combined with a low temperature air source heat pump (ASHP) unit was established in 2014 in a detached residential house in the rural region of Beijing, in order to investigate the system application prospect for single family houses via system optimization design and economic analysis. The established system was comprised of the glass heat-pipe based evacuated tube solar collectors with a gross area of 18.8m2 and an ASHP with a stated heating power of 8kW for the space heating of a single family rural house of 81.4m2. The dynamic thermal performance of the pilot system was measured for continuous 20days under typical cold climate conditions and the test data was used to validate the TRNSYS simulation model established. On the basis of model validation, system optimizations of both the existing pilot household and the typical rural house with good building insulation were undertaken to figure out the system economical efficiency in the rural regions of Beijing. The results show that the payback periods of the solar space heating system combined with the ASHP with the collector areas 15.04–22.56m2 are 17.3–22.4 years for the established pilot household on the current electricity price level of 0.5RMB/kWh, comparing with the reference condition of the fully ASHP space heating. It is further found that the equivalent solar heat price per kWh is too high under the current solar market cost price and collector technology. To put forward the integrated solar space heating for reducing carbon emission, it is suggested that the Beijing municipal government should offer some financial subsidy to compensate the equivalent solar heat price per kWh.

Experimental investigations on destroying surface tension of melted frost for defrosting performance improvement of a multi-circuit outdoor coil

When frost forms and accumulates over the outdoor coil’s surface in an air source heat pump (ASHP) unit, system operating performance will be adversely affected. Currently, reverse cycle defrosting is the most widely used standard defrosting method. A previous related study suggested that, for an ASHP unit with a multi-circuit outdoor coil, water collecting trays installed between circuits could eliminate the negative effects of downwards flowing melted frost due to gravity. However, for a vertical multi-circuit outdoor coil with separations, there is also a lot of melted frost remained on the downside surface of each circuit due to surface tension. The remained water would consume energy, and thus deteriorate system defrosting performance. To quantitatively study the negative effects of surface tension, in this paper, an experimental study changing the vertical multi-circuit outdoor coil into horizontally installed in an ASHP unit was conducted. Compared with the surface tension kept, defrosting duration could be shorted from 186s to 167s, or 20s less, and defrosting efficiency improved from 49.4% to 61.4%, or 12.0% more, when the remained water cleaned as the surface tension destroyed.

An experimental study on defrosting performance for an air source heat pump unit at different frosting evenness values with melted frost local drainage

A previous experimental study reported that the defrosting performance could be increased when the defrosting process started at a higher frosting evenness value (FEV) for an air source heat pump (ASHP) unit with a vertically installed multi-circuit outdoor coil. In that study, to calculate the FEVs and thus fix the opening degrees of stop valves, melted frost of each circuit was collected by using water collecting trays installed between circuits. Thereafter, these trays were taken away to model a regular multi-circuit outdoor coil. However, during defrosting, the downward flowing melted frost from up circuit(s) along the surface of coil due to gravity would adversely affect the defrosting performance of down circuit(s). Different negative effects of downward flowing melted frost would make the effects of FEV on defrosting performance hard to qualitatively and quantitatively confirmed. Therefore, in this study, an experimental study on defrosting performance for an ASHP unit at different FEVs with melted frost local drainage was carried out. Finally, experimental results indicated that the defrosting duration could be shortened by about 11.2% and defrosting efficiency increased by about 5.7% as the FEV increased from 79.4% to 96.6%.

An experimental study on defrosting performance for an air source heat pump unit with a horizontally installed multi-circuit outdoor coil

When frost forms and accumulates over the outdoor coil’s surface in an air source heat pump (ASHP) unit, system operating performance will be dramatically deteriorated. Reverse cycle defrosting is the most widely used standard defrosting method. A previous related study reported that downwards flowing of melted frost due to gravity over a vertical multi-circuit outdoor coil would decrease the reverse cycle defrosting performance. If the outdoor coil can be changed to horizontally installed, the flow path of melted frost over coil surface can be shortened, and the flow directions of refrigerant and melted frost changed from opposite to orthogonal. Consequently, a better defrosting performance is expected. In this paper, therefore, an experimental study on defrosting performance for an ASHP unit with a horizontally installed multi-circuit outdoor coil was conducted. Experimental results show that, when a vertical outdoor coil was changed to horizontally installed, the defrosting efficiency increased 9.8%, however, with the same defrosting duration at 186s. Furthermore, when the outdoor air fan was reversed to blowing the melted frost during defrosting, the total mass of the retained water collected decreased 222g. However, the defrosting efficiency was not increased, but decreased 6.6% because of the heat transfer enhanced between hot coil and cold ambient air.

Research on PV/T – Air Source Heat Pump Integrated Heating System in Severe Cold Region

Photovoltaic-thermal (PV/T) technique is applied to air source heat pump (ASHP) heating system in severe cold region, which can achieve high efficient heating of ASHP in low-temperature environment and maximize the utilization of renewable energy. The using limitations and common problems of ASHP heating system in severe cold region is studied in this paper. Based on the solar radiation heat transfer mechanism and characteristic of air--heat transfer media, PV/T--ASHP integrated heating system is developed. TRNSYS transient simulation software was used to study the integrated heating system. Through the simulation analysis, the outlet temperature of PV/T collector can reach 76.6°C and the average COP of ASHP unit reaches 4.1, which improves the heating efficiency of ASHP and provides a new idea for the future heating mode in severe cold region.

An experimental study on even frosting performance of an air source heat pump unit with a multi-circuit outdoor coil

When the surface temperature of the outdoor coil in an air source heat pump (ASHP) unit is lower than both the air dew point and freezing point of water, frost can be formed and accumulated over outdoor coil’s surface. On the other hand, an outdoor coil normally has multiple parallel circuits on its refrigerant side for minimized refrigerant pressure loss and enhanced heat transfer efficiency. Currently, most attentions were paid to the experimental and numerical studies on defrosting with different methods, without however paying attention to the distribution of frost accumulation on the multi-circuit outdoor coil. The phenomenon of uneven frosting exists, which also maybe the most important reason of uneven defrosting for an ASHP unit with a multi-circuit outdoor coil. Therefore, in this study, a comparative and experimental study on system frosting performance when frost accumulated on a three-circuit outdoor coil’s surface in an ASHP unit at different frosting evenness values (FEVs) has been carried out. Experimental results indicated that, when the FEV was increased from 75.7% to 90.5% for an ASHP unit with a three-circuit outdoor coil, the COP could be increased from 4.10 to 4.26 at a 3600s frosting process, and increased from 3.18 to 4.00 at its last 600s.

An experimental study on defrosting performance of an air source heat pump unit with a multi-circuit outdoor coil at different frosting evenness values

For an air source heat pump (ASHP) unit with a vertically installed multi-circuit outdoor coil, it is rarely possible to make the frost accumulations on each circuit's surface equal, leading to different frosting evenness values (FEVs) as a reverse cycle defrosting operation starts. On the other hand, uneven defrosting phenomenon was found and reported as many methods were developed to improve defrosting performance of ASHP systems, which may be attributable to the reverse cycle defrosting starting at an uneven frosting. Understanding of defrosting performance of an ASHP unit with a multi-circuit outdoor coil at different FEVs is of importance for ASHP units' application, but studies are scarce in the open literatures. In this paper, we report an experimental study on defrosting performance when frost accumulated on the surface of outdoor coil at different FEVs. This paper presents details of an experimental ASHP unit and the experimental conditions, followed by results, and conclusions. Finally, an increase of 6.8% in defrosting efficiency was confirmed when the FEV changed from 82.6% to 96.6%.

A modeling study on alleviating uneven defrosting for a vertical three-circuit outdoor coil in an air source heat pump unit during reverse cycle defrosting

Reverse cycle defrosting is the most widely used standard defrosting method for air source heat pump (ASHP) units. It was suggested in previous experimental studies that downwards flowing of the melted frost over a vertical multi-circuit outdoor coil in an ASHP unit has negative effects on reverse cycle defrosting performance. To quantitatively study the negative effects, an experimental study and a modeling study on draining away locally the melted frost for an experimental ASHP unit with a three-circuit outdoor coil were carried out and separately reported. However, for exiting ASHP units, it is hardly possible to install water collecting trays between circuits. To alleviate uneven defrosting for a vertical multi-circuit outdoor coil in an existing ASHP unit, an effective alternative is to vary the heat supply to each refrigerant circuit by varying the opening values of modulating valves installed at an inlet pipe to each circuit. In this paper, a modeling study on varying heat (via refrigerant) supply to each refrigerant circuit in a three-circuit outdoor coil to alleviate uneven defrosting is reported. Finally, in the designed three study cases, defrosting energy use could be decreased to 94.6%, as well as a reduction of 7s in defrosting duration by fully closing the modulating valve on the top circuit when its defrosting terminated.

An experimental study on the uneven refrigerant distribution over a vertically installed multi-circuit outdoor coil in an air source heat pump unit during reverse cycle defrosting

For an air source heat pump (ASHP) unit with a vertically installed multi-circuit outdoor coil, many parameters would affect system defrosting performance, especially the uneven distribution of refrigerant for each circuit and downwards flowing of melted frost due to gravity from up circuit(s) to down circuit(s). Currently, the negative effects of melted frost have been demonstrated and quantitatively studied, without however giving a fundamental study on the effects of uneven refrigerant distribution due to gravity and tube internal resistance for an ASHP unit with a vertically installed multi-circuit outdoor coil. Therefore, an experimental study on system defrosting performance when refrigerant was evenly or unevenly distributed into each circuit has been carried out and a comparative and quantitative analysis conducted using the experimental data. In this paper, the negative effects of refrigerant uneven distribution on system defrosting performance were demonstrated and an increase of 6.9% in defrosting efficiency when refrigerant evenly distributed, compared with the case of refrigerant unevenly distributed, was reported.

Experimental investigation on the defrosting performance of a finned-tube evaporator using intermittent ultrasonic vibration

When an air-source heat pump (ASHP) was operated in heating mode under certain ambient conditions, frost always accumulated on the fin surface of its outdoor coil. Frosting may increase the energy consumption and deteriorate the performance of the ASHP, and hence, periodic defrosting is necessary. In this study, a new defrosting method using intermittent ultrasonic vibration was investigated. First, the vibration attenuation characteristics of a double-row outdoor coil and the frost growth characteristics under different ambient conditions were determined. Next, the average frost thickness with and without the application of intermittent ultrasonic vibration was calculated using MATLAB software. Finally, the decrease in defrosting energy consumption, the increase in heating capacity, and the increase in the coefficient of performance were analysed. The experimental results indicate that intermittent ultrasonic vibration could effectively remove the frost accumulated on the fin surface, and the effective defrosting area of an single ultrasonic transducer was 0.165m2 for a double-row finned-tube evaporator on which an ultrasonic transducer with a rated power of 50W and resonant frequency of 40kHz was applied. The defrosting energy consumption of the ASHP unit with ultrasonic vibration was 3.14–5.46% lower than that without ultrasonic vibration, whereas the heating capacity increased by 2.2–9.03% and the COP increased by 6.51–15.33%. In addition, the thermal comfort of the indoor side was clearly improved.

Developing a new frosting map to guide defrosting control for air-source heat pump units

The development of a new frosting map for air-source heat pumps (ASHPs) based on a temperature-humidity chart is reported in this paper. Through a parametric analysis, two boundary lines (a condensing line and a frosting line) were obtained, dividing the map into three regions: a non-frosting region, a condensing region and a frosting region. Using the simulation results from a generalized numerical model of frost growth, the frosting region was further divided into three indicative zones representing different frosting levels: a severe frosting zone, a moderate frosting zone (I, II) and a mild frosting zone (I, II). Lab and field tests were conducted in order to verify the correctness of the zoning and to obtain indicative defrosting intervals for ASHPs operated at different zones. Based on the test results and using the developed map, reference defrosting intervals were suggested for different frosting zones, and they can be used in conjunction with a conventional defrosting control method to avoid potential mal-defrosting.

Performance optimization of an air source heat pump water heater using mathematical modelling

In South Africa, there is an ongoing constraint on the electricity supply at the national grid to meet the demand. Eskom is implementing various measures such as the Integrated Demand Management and the promotion and encouragement of the use of energy efficient devices like an Air Source Heat pump (ASHP) water heater to replace the high electrical energy consuming conventional geysers for sanitary hot water production. The ASHP water heater market is fast gaining maturity. A critical mathematical model can lead to performance optimization of the systems that will further result in the conservation of energy and significant reduction in global warming potential. The ASHP water heater comprises of an ASHP unit and a hot water storage tank. In this study, a data acquisition system (DAS) was designed and built which monitored the energy used by the geyser and the whole building, the temperature at the evaporator, condenser, tank outlet hot water, tank inlet cold water, the ambient temperature and relative humidity in the vicinity of the ASHP evaporator. It is also worthy to mention that the DAS also included to a flow meter and two additional temperature sensors that measured the volume of water heated and inlet and outlet water temperature of the ASHP. This work focused on using the mathematical equation for the Coefficient of Performance (COP) of an ideal Carnot’s heat pump (CHP) water heater to develop basic computation in M-file of MATLAB software in order to model the system based on two reservoir temperatures: evaporator temperatures (Tevp) of 0°C to 40°C (approximated to ambient temperature, Ta) and condenser temperatures (TCon) set at 50°C, 55°C and 60°C (approximated to the hot water set temperature of 50°C, 55°C and 60°C) respectively. Finally, an analytical comparison of a CHP water heater to the practical ASHP water heater was conducted on a hot water set point temperature of 55°C. From the modelling results, it can be deduced that at 0°C Tevp, the COP was 5.96 and 2.63 for CHP and ASHP water heater respectively, at a hot water set temperature of 55°C. Above 20°C Tevp, the rate of change of COP increased exponentially for the ideal CHP system, but was constant at 0.01/°C for the practically modelled ASHP water heater.

A novel Temperature–Humidity–Time defrosting control method based on a frosting map for air-source heat pumps

To improve the defrosting accuracy and the energy efficiency of the air-source heat pump (ASHP) under frosting and defrosting conditions, a novel Temperature–Humidity–Time (T–H–T) defrosting control method, based on a frosting map for the ASHP unit, is proposed in this paper. A field test was conducted for two heating seasons, to verify the feasibility and applicability of the T–H–T method. The advantages of the T–H–T method, compared to the conventional Temperature–Time (T–T) defrosting control method, are presented. In total, eight cases are shown in this paper. Cases 1 to 4 were chosen to reveal the T–H–T performance under different frosting conditions. It was found that no matter what kind of frosting conditions, defrosting was always initiated in a similar situation: ∼90% of the outdoor coil surface was covered by frost; the temperature difference between the compressor suction and discharge increased by ∼20%; and the heating capacity decreased by ∼30%. These results indicate that the T–H–T method can make an accurate decision under different frosting conditions. Cases 5a, 5b and Cases 6a, 6b were two groups of cases to compare the advantages of the T–H–T method against the conventional T–T method. Cases 5a and 5b were chosen for the non-frosting condition. It was found that the T–T method initiated the defrosting operation 31 times within 24 h. However, none of the defrosting operations was conducted for the T–H–T method. Cases 6a and 6b were chosen to compare these two methods under consecutive and variable frosting conditions. For the T–T method, 63% of the defrosting processes were found to be executed under conditions where defrosting was not necessary. However, for the T–H–T method, all the defrosting controls were found to be accurate and reasonable. These results indicated that the novel T–H–T method is suitable for the defrosting control of the ASHP, and has a more competitive performance than the conventional T–T method.

A semi-empirical modeling study on the defrosting performance for an air source heat pump unit with local drainage of melted frost from its three-circuit outdoor coil

Reverse cycle defrosting is the most widely used standard defrosting method for air source heat pump (ASHP) units. It was suggested in previous experimental studies that during reverse cycle defrosting, downwards flowing of the melted frost due to gravity over a vertically installed multi-circuit outdoor coil in an ASHP unit has negative effects on defrosting performance. Therefore, an experimental study on draining away locally the melted frost for an experimental ASHP unit having three refrigerant circuits using water collecting trays was carried out and the study results were separately reported. To enable further quantitative analysis on the effects of local draining away the melted frost on reverse cycle defrosting performance of the ASHP unit, a modeling study on the defrosting process, at the two experimental settings of with and without the use of water collecting trays between circuits was carried out and is reported in this paper. Two semi-empirical mathematical models, corresponding to the two settings, were developed. In this paper, firstly the detailed development of the two semi-empirical models is presented. This is followed by reporting the validations of the two models using the experimental data previously reported. Finally, detailed discussions on the potential uses of the two models developed and the limitations of the modeling work reported are included.

An experimental study on the negative effects of downwards flow of the melted frost over a multi-circuit outdoor coil in an air source heat pump during reverse cycle defrosting

When the surface temperature of the outdoor coil in an air source heat pump (ASHP) unit is lower than both freezing point of water and the air dew point, frost can be formed and accumulated over outdoor coil surface. Frosting affects the energy efficiency, and periodic defrosting therefore is necessary. Reverse cycle defrosting is currently the most widely used defrosting method. A previous related study has indicated that during reverse cycle defrosting, downwards flow of the melted frost over a multi-circuit outdoor coil could affect the defrosting performance, without however giving detailed quantitative analysis of the effects. Therefore an experimental study on the effects has been carried out and a quantitative analysis conducted using the experimental data. In this paper, the detailed description of an experimental ASHP unit which was specifically built up is firstly reported. This is followed by presenting experimental results. Result analysis and conclusions are finally given.

Investigation of defrosting water retention on the surface of evaporator impacting the performance of air source heat pump during periodic frosting–defrosting cycles

To investigate the impact of defrosting-water retention on the surface of heat exchangers of the air source heat pump (ASHP) performance during frosting–defrosting cycles, an experimental study has been conducted. Testing was carried out in a controlled-environment chamber with ASHP operational data collected over 16 periodic frosting–defrosting cycles spanning a 360-min period. It was found that the defrosting-water retention undergoes three stages: single-phase water, a mixture of water and ice, followed by a predominance of ice. In this study, the third stage appears after the unit has been in operation for 256min. A “permafrost area” forms on the lower quadrant of the heat exchanger, which occupies over 20% of the total heat exchanger surface. This area remained continuously occupied regardless of whether the unit was in a frosting or defrosting cycle. The appearance of this “permafrost area” not only impacts the performance of the ASHP, it also indicates the beginning of repetitive “shutdown accidents” for the ASHP units. It is the first time that these phenomena, originally caused by the defrosting-water retention have been revealed. And a quantitative analysis is presented for the resulting performance degradation of the ASHP related to the “permafrost area” conditions and the related “repetitive shutdown accidents”.

A novel defrosting method using heat energy dissipated by the compressor of an air source heat pump

When an air source heat pump (ASHP) unit is used for space heating at low ambient temperatures in winter, frost may form on its outdoor coil surface. Since the accumulated frost adversely affects its performance and energy efficiency, periodic defrosting of the outdoor coil is necessary. Currently, the reverse-cycle defrosting (RCD) method is widely used for the defrosting of ASHP. However, this operation interrupts space heating during the defrosting process. A time lag occurs to resume heating at end of the defrosting cycle. Moreover, frequent reversing of the 4-way valve may cause mass leakage of the refrigerant, even make the system unsafe. Furthermore, some amount of heat is dissipated to the atmosphere through the compressor casing. To improve the defrosting process and use this waste heat, a novel ASHP unit is developed. The space is heated during the defrosting process using the heat dissipated by the compressor. Experiments using both the RCD method and the novel reverse cycle defrosting (NRCD) method developed in this study are conducted on an ASHP unit of 8.9kW nominal heating capacity. The experimental results indicated that in the NRCD method, the discharge and suction pressures are increased by 0.33MPa and 0.14MPa, respectively, the defrosting time is shortened by 65% while the resuming heating period vanished with the NRCD method, and that the total energy consumption in comparison to RCD method is reduced by 27.9% during the period which is composed of defrosting period and resuming heating period. Moreover, the NRCD method ensured continuous heating during defrosting. The mean temperature difference between the air entering and leaving the indoor coil reaches 4.1°C during defrosting. Over a test period of 125min, compared to RCD method, the total heating capacity and input power are increased by 14.2% and 12.6%, respectively. The increase in the system COP is 1.4%.

An experimental study on the effects of downwards flowing of melted frost over a vertical multi-circuit outdoor coil in an air source heat pump on defrosting performance during reverse cycle defrosting

A previous related study has indicated that for an air source heat pump (ASHP) unit having a vertically installed multi-circuit outdoor coil, during reverse cycle defrosting, downwards flowing of melted frost over the outdoor coil surface could affect the defrosting performance, without however giving a detailed quantitative analysis. Therefore an experimental study has been carried out and the study results are reported in this paper. Firstly, a detailed description of a specially built experimental ASHP unit having a vertical multi-circuit outdoor coil is presented. This is followed by reporting the experimental results. Finally, a detailed quantitative analysis is presented. The experimental results and the corresponding analysis demonstrated that allowing downwards flowing of the melted frost over a vertical multi-circuit outdoor coil would result in a longer defrosting duration and more energy consumption. The study results also suggested that the use of water collecting trays can help mitigate these negative impacts on defrosting performance.

An Experimental Study on Performance During Reverse Cycle Defrosting of an Air Source Heat Pump with a Horizontal Three-circuit Outdoor Coil

When the surface temperature of the outdoor coil in an air source heat pump (ASHP) unit is below both the air dew point and freezing point of water, frost can be formed and accumulated over the surface of the outdoor coil, which is usually of multi-circuit structure in order to minimize its refrigerant pressure loss and enhance the heat transfer between refrigerant and ambient air. However, frosting adversely affects the operational performance and hence the energy efficiency of the ASHP unit, therefore periodic defrosting is necessary. Currently, the most widely standard defrosting method used for ASHP units is reverse cycle defrosting. A previous related study has suggested that during reverse cycle defrosting, downwards flowing of melted frost due to gravity over a vertical multi-circuit outdoor coil in an ASHP unit could deteriorate defrosting performance, by using more energy for defrosting and prolonging a defrosting process. If however an outdoor coil can be horizontally installed, the flow path for melted frost over coil surface can be shortened, and a better defrosting performance expected. In this paper, therefore, an experimental study on performance during reverse cycle defrosting of an ASHP unit having a horizontal three-circuit outdoor coil has been carried out. Firstly, a detailed description of the ASHP unit is presented. This is followed by reporting the experimental results. Finally, a comparative and quantitative analysis is given.

Experimental study on concentration change of spray solution used for a novel non-frosting air source heat pump system

Frost formation on the outdoor coil surface of an air source heat pump (ASHP) unit may occur when it is used for space heating at a low ambient temperature in winter. To reduce the impact of frosting on the operational performance and energy efficiency of ASHPs, a novel non-frosting air-source heat pump (NASHP) system was proposed. The detailed introduction of NASHP prototype was given, and its test rig was developed, as well as its operating principles and modes. Then the characteristics of some conventional dehumidification solutions were compared; the glycerol was selected as spray solution for NASHP system, due to its great influence on the operating performance of the system. The glycerol solutions with different concentration and different mass flow were investigated experimentally, and the best practice by which NASHP system has the most remarkable efficiency was found finally. Such results will provide helpful reference for the potential application of NASHP system and future research work.