Outdoor Coil Surface Research Articles

Experimental investigation on the space heating performance of air source heat pump with vapor injection under frosting condition

For air source heat pump with vapor injection (ASHPVI), the vapor injection can improve its heating performance of ASHPVI at non-frosting low temperature condition. However, its effect under the frosting condition is still unclear. To improve the heating performance of ASHPVI under frosting condition, a test rig for the frosting of ASHPVI was built in a psychrometer testing room. Then, a new control strategy for vapor injection was proposed to ensure its safety, and two groups of frosting experiments with and without vapor injection were conducted. Results showed that under frosting condition, vapor injection can still improve the heating capacity of ASHPVI by 11.31 %. It can also make the outdoor coil surface temperature decreased by 0.8–4.6 °C, and further result in a faster frosting speed. Compared with the frosting duration without vapor injection (64 min), the frosting duration with vapor injection is shortened by 14 min. Owing to the faster frosting speed, the average COP with vapor injection decreases by 0.17, with a decrease rate of 6.03 %. When the heating capacity of AHSPVI without vapor injection under frosting condition can meet the building heating load, the vapor injection should not be adopted.

Experimental study on the effects of melted frost icing on the operating performances of air source heat pump

Defrosting operation is always used to solve its frosting problem at a high humidity environment for air source heat pump units (ASHPs), and at the end of defrosting operation, the residual melted frost may exist on the surface of outdoor coil and freeze into ice in sequent frosting. However, till now, as the residual melted frost icing environment is difficult to create, its quantitative influences on the heating performance of ASHPs is still unknown, especially at different icing levels. To resolve this issue, a control strategy was firstly proposed to make it possible for the residual melted frost to ice. Then, based on the control strategy, the effect of five different icing levels on operating performances of an ASHP unit were investigated. Results indicated that ice can promote the frost formation, and the operating performances of ASHP unit were significantly affected by different icing levels. When the percentage of iced outdoor coil surface was increased from 0% to 15% and 35%, the output heating capacity of ASHP unit was decreased by 11.2% and 23.8%, and meanwhile the COP of ASHP unit was decreased by 7.7% and 16.1%, respectively. The results can assist with the defrosting control optimization of practical products.

A modeling study on developing the condensing-frosting performance maps for a variable speed air source heat pump

For a variable speed (VS) space heating air source heat pump (ASHP) unit, to comprehensive study its frosting suppression and heating performances, a condensing-frosting performance map has been proposed. An experimental study using an experimental VS ASHP unit to obtain condensing-frosting performance maps was previously reported. However, obtaining such performance maps experimentally was time consuming and costly, and it was therefore considered highly necessary to develop a mathematical model to obtain performance maps through modeling study. In this paper, therefore, a mathematical model for the experimental VS ASHP unit was firstly developed by referring to previously published mathematical models for VS air conditioners. The developed model was experimentally validated using reported experimental data. The average relative errors between the measured and predicted total output heating capacity and COP were at 3.7% and 3.6%, respectively, and the average error between the measured and predicted outdoor coil surface temperature was at 0.2 °C. A follow-up modeling study was then carried out using the validated model to obtain condensing-frosting performance maps for the experimental VS ASHP unit having different outdoor coil surface areas and at different outdoor operating conditions. The modeling study results suggested that by increasing outdoor coil surface area of the experimental VS ASHP unit from 50% to 150%, its surface temperature on average was increased by more than 1.37 °C, which was inducive to a better frost suppression performance, and the total output heating capacity by more than 11.07%, but at a higher initial manufacturing cost. The modeling study results also indicated that the developed model can be a useful tool in studying the characteristics of VS ASHPs during both frosting and frost-free operations, which was important for guiding the design and control of ASHPs for their efficient and reliable operations.

Developing condensing-frosting performance maps for a variable speed air source heat pump (ASHP) for frosting suppression

For a space heating ASHP unit, frosting on its outdoor coil would adversely impact its normal operation, requiring frequent defrosting and resulting in low operating efficiency and poor indoor thermal comfort. It has been shown that varying the speeds of both compressor and outdoor air fan in a variable speed (VS) ASHP unit can lead to different outdoor coil surface temperatures and thus different frosting suppression performances. However, this can also result in different output heating capacities from the ASHP unit. To establish the comprehensive relationship between frosting suppression performances and the total output heating capacity for a VS ASHP unit, an experimental study on developing the condensing-frosting performance maps for a VS experimental ASHP unit under different speed combinations of its compressor and outdoor air fan at three Test Conditions, was carried out and the study results are presented in this paper. The obtained performance maps clearly demonstrated that for the experimental ASHP unit, varying the speeds of its compressor and outdoor air fan would result in not only different outdoor coil surface temperatures, but also different total output heating capacities. A higher outdoor coil surface temperature and thus a better frosting suppression performance may be achieved, but usually at the expenses of losing some of its total output heating capacity. On the other hand, the experimental results also suggested that when the experimental ASHP unit was operated at different speed combinations that can lead to the same or similar total output heating capacities, its operating characteristics in terms of outdoor coil surface states and the total output heating capacity can be remarkably different. The obtained condensing-frosting performance maps may be used as the guidance for the design, operation and control of ASHPs for frost-free operation.

A novel index model for defrosting initiating time point of air source heat pump based on the cusp catastrophe theory

In winter, the operating performance of air source heat pump (ASHP) units deteriorates due to the accumulation of frost on the outdoor coil surface. In order to reduce the impact of this disaster phenomenon, ensure stable operation and save energy, it is necessary to accurately determine the initiation of defrosting after a period of unit operation. Although some existing defrosting initiation control strategies are effective, too much data volume and accuracy are required. To address these issues, a different perspective on mutation from previous studies is taken. This study combines the cusp catastrophe theory with frost accumulation, and presents a defrosting initiation index model for ASHP. After a brief description of the cusp catastrophe theory, the discriminant index model based on the cusp catastrophe theory (DICC) is proposed, state variable and control variables of the index model are determined. Then, the unit was tested for 10-days. To solve and verify the DICC, 10-days experimental data was used and it was found that DICC can efficiently predict the heating capacity decreases. Consequently, the DICC-based defrosting initiation control strategy was compared with the existing strategies, and the unit’s performance improvement compared to the original strategy was evaluated, as the results show, a 3.9% improvement in coefficient of performance (COP) and a 4.9% increase in heating capacity. Therefore, the strategy proposed in this paper is suitable for ASHP units with limited data capacity and low data accuracy, also can ensure the accurate initiation of defrosting and efficient operation.

Applying image recognition to frost built-up detection in air source heat pumps

When a space heating ASHP unit is operated in frosting conditions, accurate defrosting is necessary to ensure its operating efficiency and output heating capacity. Therefore, to improve the accuracy of defrosting initiation of ASHPs under frosting-defrosting operations, a study on applying image recognition to frost built-up detection in ASHPs has been carried out and the study results are presented in this paper. Firstly, the principle of a novel frost built-up detection method based on image recognition is presented. Secondly, a series of experiments to examine the applicability of the novel frost built-up detection method with respect to shooting angle, imaging pixels, illumination level, and the temperature of outdoor coil surface are carried out. Thirdly, the novel frost built-up detection method was further modified based on the experimental result obtained, and the verification of the modified novel detection method is reported. It is shown that the use of the modified novel frost built-up detection method could properly account for the impact of changing illuminance level on the accuracy of image recognition, to ensure the accuracy of frost built-up detection when ASHPs are operated in frosting conditions.

Experimental study on frost unevenly distributed and melted frost downwards flowing during defrosting for ASHPs

For an air source heat pump (ASHP) unit, uneven frost distribution would result in mal-defrosting, or adversely degrade system defrosting efficiency. Previous investigations demonstrated that, for an ASHP unit with a multi-circuit outdoor coil, when the frosting evenness value (FEV) on its surface of outdoor coil is higher, the defrosting efficiency would also be higher. On the other hand, as reported, the downwards flowing melted frost during defrosting would also play negative effects on defrosting performance. Although the comprehensive influence of the two factors, uneven frost distribution and the downwards flowing melted frost, had been experimentally investigated, the influence of uneven frost distribution or the FEV on negative effect of downwards flowing melted frost was still unknown. As a fundamental and interesting problem, it is meaningful to be explored. It was therefore quantitatively analyzed here basing on six experimental cases. Water collecting trays were used to take away the melted frost during defrosting in three cases, and the other three ones without trays installed stood for the traditional status. Meanwhile, their FEVs of two groups were separately adjusted to be at three different levels. Results clearly showed the positive effects of increasing FEV and taking away the melted frost on system defrosting efficiency. After the melted frost was taken away, the FEV effects was increased. The increase of defrosting efficiency seems unchanged, but the reduction of defrosting duration was obviously increased from 17 s to 22 s. However, the differences of defrosting duration and efficiency changed were too small when melted frost was locally drained or not. More experimental cases with lower FEVs should be carried out.

Heating and energy storage characteristics of multi-split air source heat pump based on energy storage defrosting

In recent decades, multi-split air source heat pump (M-ASHP) unit has been widely used for space heating. Similar to the split-ASHP unit, frost would accumulate on its outdoor coil surface under frosting condition, which significantly deteriorates its heating performance. In the previous study, a novel reverse-cycle defrosting method based on phase change energy storage was developed and investigated, and the study demonstrated that the novel defrosting method could significantly improve defrosting performance. To provide guidelines for designing the M-ASHP units with phase change energy storage, it is necessary to investigate the characteristics of heating and energy storage for M-ASHP units, which is presented in this paper. Experimental results showed that there was no frost on outdoor coil when only the indoor unit with heating capacity of 7.1 kW was turned on under frosting condition. In addition, the energy storage period increased as the number of operating indoor unit increased, namely the load rate. Furthermore, the optimal starting time of the energy storage operation was around 10 min after the start-up of M-ASHP unit. Even though, compared with the conventional heating mode with three turned-on indoor units, the energy storage mode shortened the heating period by 8 min and decreased the average heating capacity and COP by 4.4% and 1.9%, respectively. The above negative impacts of energy storage operation on the heating performance of the M-ASHP unit could be neglected by considering its positive impacts on defrosting performance, for a periodic heating-defrosting condition.

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.

An experimental study on energy saving analysis in the defrost cycle of residential heat pumps with the use of reverse air flow during defrost

Defrosting outdoor coils of residential heat pumps in an energy efficient manner remains an active area of research in the HVAC industry. Inverting the cycle to reject heat from the outdoor coils is the most common method used currently. However, this method is energy intensive. The current study proposes a novel method that can substantially reduce the energy consumed during defrost cycles. It involves the controlled use of reverse airflow on the outdoor coil surface during the defrost cycle, which cleans the coil surface more thoroughly by improving the draining of the melted frost. It also removes portion of frost without needing to melt it. This process results in decreased number of defrost cycles needed for a given duration of heat pump operation. Energy savings of 56% and 31% were demonstrated experimentally for the above-freezing and below-freezing environments, respectively, as compared to a baseline that represents the ASHRAE recommended operating conditions.

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 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.

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.

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.

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.

An experimental study on defrosting heat supplies and energy consumptions during a reverse cycle defrost operation for an air source heat pump

Abstract For a space heating air source heat pump (ASHP) unit, when its outdoor coil surface temperature is below both the air dew point temperature and the freezing point of water, frost will form on its outdoor coil surface. Frosting affects its operational performance and energy efficiency. Therefore, periodic defrosting is necessary. Currently, the most widely used standard defrosting method for ASHP units is reverse cycle defrost. The energy that should have been used for space heating is used to melt frost, vaporize the melted frost off outdoor coil surface and heat ambient air during defrosting. It is therefore necessary to study the sources of heat supplies and the end-uses of the heat supplied during a reverse cycle defrost operation. In this paper, firstly, an experimental setup is described and experimental procedures are detailed. This is followed by reporting the experimental results and the evaluation of defrosting efficiency for the experimental ASHP unit. Finally, an evaluation of defrosting heat supplies and energy consumptions during a revere cycle defrost operation for the experimental ASHP unit is presented. The experimental and evaluation results indicated that the heat supply from indoor air contributed to 71.8% of the total heat supplied for defrosting and 59.4% of the supplied energy was used for melting frost. The maximum defrosting efficiency could be up to 60.1%.

Operating performance of novel reverse-cycle defrosting method based on thermal energy storage for air source heat pump

To solve the fundamental problem of insufficient heat available during defrosting while ensuring the efficient and safe system operation for air-source heat pumps (ASHPs). A novel reverse-cycle defrosting (NRCD) method based on thermal energy storage to eliminate frost off the outdoor coil surface was developed. Comparative experiments using both the stand reverse cycle defrosting (SRCD) method and the NRCD method were carried out on an experimental ASHP unit with a nominal 2.5 kW heating capacity. The results indicate that during defrosting operation, using the NRCD method improves discharge and suction pressures by 0.24 MPa and 0.19 MPa, respectively, shortens defrosting duration by 60%, and reduces the defrosting energy consumption by 48.1% in the experimental environment, compared with those by the use of SRCD method. Therefore, using the NRCD method can shorten the defrosting duration, improve the indoor thermal comfort, and reduce the defrosting energy consumption in defrosting.

A study of the reverse cycle defrosting performance on a multi-circuit outdoor coil unit in an air source heat pump – Part II: Modeling analysis

The experimental part of the study of the reverse cycle defrosting performance on a four-circuit outdoor coil unit in an experimental 6.5 kW heating capacity residential ASHP has been reported in Part I of a two-part series. The experimental results indicated that downwards flowing of the melted frost along a multi-circuit outdoor coil surface would have negative effects on defrosting performance. In order to quantitatively analyze the effects, this paper, the second part of the two-part series, reports on a modeling analysis where a semi-empirical model of the airside defrosting in the four-circuit outdoor coil unit was developed. The model was validated by comparing the predicted defrosting duration and temperature variation of collected melted frost with the corresponding experimental data presented in Part I of the two-part series. Using the validated model, the frost-melting rate and the temperature of melted frost on the surface of each circuit were predicted. The effects of the downwards flowing melted frost along the entire outdoor coil surface on defrosting performance were quantitatively analyzed.

A study of the reverse cycle defrosting performance on a multi-circuit outdoor coil unit in an air source heat pump – Part I: Experiments

When an air source heat pump (ASHP) unit operates in heating mode, frost can be accumulated on the surface of its finned outdoor coil which normally has multiple parallel circuits on its refrigerant side for minimized refrigerant pressure loss and enhanced heat transfer efficiency. On its airside, however, there is usually no segmentation corresponding to the number of refrigerant circuit. Frosting deteriorates the operation and energy efficiency of the ASHP unit and periodic defrosting becomes necessary. Currently the most widely used standard defrosting method for ASHPs is reverse cycle defrost. This paper, the first part of a two-part series, reports on the experimental part of a study of the reverse cycle defrosting performance on a multi-circuit outdoor coil unit in an experimental 6.5 kW heating capacity residential ASHP unit. Firstly the experimental ASHP unit is described and experimental procedures detailed. Secondly, the experimental results are reported. This is followed by the discussion on the effects of downwards flowing of the melted frost along a multi-circuit outdoor coil surface on defrosting performance. Finally, the evaluation of the defrosting efficiency for the experimental ASHP unit is provided. In the second part of the series, a modeling analysis on the effects of downwards flowing of the melted frost along the multi-circuit outdoor coil surface on defrosting performance of the experimental ASHP unit will be presented.