Performance Of Air Source Heat Pump Research Articles

Methodology to evaluate design modifications intended to eliminate frosting and high discharge temperatures in air-source heat pumps (ASHPs) in cold climates

Air-source heat pumps (ASHPs) operating in cold climates experience problems with frosting and high refrigerant temperatures. These problems increase energy consumption, and their severity depends on the climatic conditions. In the present paper, a methodology for identifying the prevailing problem between frosting and high discharge temperatures is presented. Three performance indices, the frosting index (FI), the discharge index (DI), and the total loss index (TLI), are proposed to quantify the impacts of frosting and high discharge temperatures on the annual performance of ASHPs in different climatic conditions. The FI and DI show which problem (frosting or high discharge temperature) dominates, and the TLI indicates the combined effect of frosting and high discharge temperatures on the performance of an ASHP. A thermodynamic model of an ASHP coupled with the TRNSYS building simulation tool is used to estimate the performance of an ASHP and the proposed loss indices to estimate the impact of both frosting and high discharge temperatures for 45 cities in Canada. The results can be extended to other parts of the world that experience similar climatic conditions The results reveal that in cities in ASHRAE climatic zones 5 and 6 (classified as cold regions) where the ambient air temperatures are predominantly between −15 °C to 6 °C, ASHPs are heavily impacted by frosting. The problem of high discharge temperatures in ASHPs is predominant in cities in climate zones 7 and 8 (classified as very cold and subarctic regions) where the temperatures are frequently below −20 °C in winter. Among the cities considered, St. John, NL has the highest fraction of heating hours experiencing frosting (90 %), where the annual increase in energy consumption due to frosting is 13.5 % of the annual heating energy consumption. The highest annual increase in energy consumption due to high discharge temperatures is in Isachsen, NU (zone 8), where the increase is 30 % of the annual heating energy consumption.Based on the proposed indices, another index called the performance gain index (PGI) is created, which can be used as a first step to assess the energy-saving potential of design modifications applied to ASHPs to solve the problems of frosting and high discharge temperatures. The PGI will aid in developing climate specific ASHPs. One possible design modificationis the use of a two-stage ASHP with an economizer. It is observed that the two-stage ASHP with economizer can mitigate high discharge temperatures and improve performance in very cold and subarctic regions (zones 7 and 8). However, it is not as beneficial in zones 5 and 6, where the impact of high discharge temperatures on performance is minimal and frosting dominates. Finally, a case study, using the PGI to evaluate the economic and environmental effectiveness of a two-stage ASHP with economizer is presented for the city of Saskatoon.

Investigating the efficiency of air-source heat pumps in the secondary school physics laboratory

This paper presents an experimental method for investigating the coefficient of performance of air-source heat pumps for secondary age children and links this to ideas about energy efficiency. It was developed and trialled in a secondary school in the West Midlands of England in response to literature that reveals climate anxiety to be increasing amongst young people around the world and their resulting desire to take action. Links are made to the topic of energy efficiency in the current physics school curriculum in England which is used, where possible, to explain the results of the investigation with suggestions for where extensions might be made in future to facilitate a more complete understanding by pupils. Links to wider societal change in the decarbonisation of home heating in the UK as part of the national response to climate change are explored and an argument is made for the viability and necessity of including the technology of air-source heats pumps in the physics curriculum for all children.

Experimental and theoretical assessments on the systematic performance of a single-stage air-source heat pump using ternary mixture in cold regions

The performance of air-source heat pump is strongly influenced by ambient temperature variation. Improving its stability and efficiency during wintertime is important for promoting its applications, while existing studies paid few attentions to flexible parameter regulation of heat pump using mixture. In this paper, a single-stage compound air-source heat pump using ternary mixture was theoretically simulated and experimentally tested, in which operating parameters including pressure and concentration could be regulated with the cooperation of multiple valves and reservoirs. Using CO2/R134a/R600, operating pressure and mixture concentration were successfully regulated at ambient temperature from −30 ℃ to 0 ℃, achieving a coefficient of performance from 1.834 to 2.546 when producing hot water at 75 ℃. Whether R134a inclusion was favorable to energy performance depended on its influence on the temperature matching in recuperative process, especially the heat transfer process in recuperator. The addition of R134a significantly alleviated the increase of operating pressure and thus enlarged the operation range of this heat pump. The flammability range of CO2/R134a/R600 was calculated, indicating the insignificant influence of R134a on flammability retardance in the studied range of this paper. The significance of this study is further revealed with the potential advances of heat transfer technique and new-generation refrigerant. The findings in this paper contribute to: 1) the modification of air-source heat pump for operation at low temperature; 2) the regulation method for mixed-refrigerant system; 3) the exploration of new mixed-refrigerant in thermodynamic cycle.

Frost suppression performance of an air source heat pump using sensible heat from indoor air to preheat outdoor air

Frost forms on the surfaces of air source heat pump (ASHP) outdoor heat exchangers under heating mode in winter, when the surface temperature of the heat exchanger is lower than both 0°C and the ambient dew point temperature. This can reduce the coefficient of performance (COP) and heating capacity of the ASHP unit. A novel ASHP was examined using two rooms with the same size, orientation, and structure. To test the frost suppression performance of the novel system, two ASHPs of the same size and specifications, but one including a sensible-heat preheating device, were placed in the test platform rooms. By comparing and analyzing field-measured parameters such as COP, heat loss, and frost weight of the two ASHP units, the frost suppression performance of the novel ASHP was evaluated in severe, moderate, and mild frost areas. The results show that when the air conditions were −3.9°C and 98.7% in the moderate frost area and 3.2°C and 96.7% in the severe frost area, the novel ASHP showed significantly improved performance. The frost formation time increased from 25 min to 38.5 min, frost-defrost heat loss decreased by 31.6%–43.7%, frost weight decreased 11.8%–17.4%, and COP increased 7.4%–13.1% compared with those of the conventional ASHP. In the low temperature and mild frost area, when the air temperature was −11.8°C and the relative humidity was 88.8%, the frost suppression effect of the novel ASHP was unapparent. Affected by the climate, the operation performance of ASHP will be different. Different ASHP can be used to adapt to the weather in different regions. The novel ASHP can significantly suppress frost and improve the heating performance of the unit in the mild climate and frost serious areas.

Energy saving potential pre-estimation approach for air-source heat pump usage in China based on thermodynamic irreversibility

Building sector accounts for a substantial part of energy consumption (EC) and carbon emissions, mainly serving for building services engineering systems. As a known advanced high-efficiency equipment, air source heat pump (ASHP) has been increasingly used for indoor heating ventilation and air conditioning (HVAC) for residential and public buildings. Thus how to pre-estimate the energy saving potential (ESP) for given ASHPs under different climatic conditions is of high value for practical engineering application. In this paper, upon thermodynamic perfectibility conception, a simplified ASHP model is proposed to simulate its dynamic thermal performance according to the disparity between real equipment and the ideal reverse Carnot cycle. Moreover, a novel thermodynamic approach is proposed to pre-estimate and compare the seasoning-based ESPs of ASHP used for both heating and cooling. For illustrative examples, a typical public building is chosen, and the AHSP-ESPs are obtained and analyzed for five cities of different climate zones in China (Qiqihar, Beijing, Chengdu, Kunming, and Guangzhou). Preliminary research results show that lower/higher ambient temperatures contribute to higher heating/cooling ESPs respectively. However, the correlation between building EC and thermodynamic perfectibility is in inverse proportion. At first, building EC decreases rapidly with the increase of thermodynamic perfectibility, but as the thermal performance of ASHP gets closer to the corresponding ideal ones, the impact of thermodynamic perfectibility on building EC gradually decreases. This work is of great significance in understanding the regional applicability of ASHP and building EC, and providing application references for real ASHP usage in building sectors.

Theoretical Study and Experimental Validation on the Applicable Refrigerant for Space Heating Air Source Heat Pump

The air source heat pump (ASHP) is developing rapidly and is widely used for space heating due to its potential for increasing energy efficiency and reducing greenhouse gas emissions. The choice of appropriate low global warming potential (GWP) alternative refrigerants is one of the challenges that ASHP systems face. Alternative refrigerants also affect the energy performance of these systems. Thus, it is essential to evaluate the performance of ASHP using environmentally friendly refrigerants to facilitate reasonable refrigerant selection. A theoretical model for simulating ASHP performance with different refrigerants is developed in this study. Experiments are conducted to validate the theoretical model. The simulation and the experimental results are found to be in good agreement. The ASHP performance indices, such as compression ratio (CR), discharging temperature (DT) and coefficients of performance (COP), are investigated using R22, R417A, R410A, R134a, R152a, R161 and R1234yf as working fluids. The results show that R152a has an average COP of 2.7% higher than R22, and R161 has an average COP of 1.4% higher than R22. R152a and R161 also have a higher CR but a lower DT than R22 under the same design conditions. In addition, R152a and R161 have ozone depletion potentials (ODP) of zero and extremely low GWPs; thus, they can be candidates to replace R22 in ASHP heating systems. This research provides a reference on refrigerant replacements for ASHP heating systems in North China.

Experimental study on the operating performance of the air source heat pump (ASHP) with variable outdoor airflow rate under the standard frosting condition

Frosting is a common phenomenon of the ASHP under the heating mode in winter, and the outdoor air flow rate flowing through the evaporator of the ASHP was always thought to be a major contributor. In order to validate its contribution, effects of outdoor fan airflow rate on the performance of air source heat pumps (ASHPs) were investigated under the winter heating condition. The experiment was conducted in a laboratory at the standard 2 °C air dry bulb temperature (DB)/ 1 °C air wet bulb temperature (WB) frosting condition, which enabled the analysis of the operating performance, frosting performance, and heating performance of the ASHP unit by changing the airflow rate of the outdoor fan. Results showed that as the airflow rate of the outdoor fan reduced from 100% to 36%, the operating performance decline and the elevated frosting-defrosting loss were observed. Meanwhile, both the frosting rate and the operating efficiency during frosting-defrosting cycles showed an increasing trend then followed by decreasing tendency. The maximum frosting rate and operating efficiency were 0.92 g/m2.min and 2.92, respectively, which were observed at 74% airflow rate of the outdoor fan of the ASHP unit. The observation implied the existence of the “minimum frosting suppression airflow rate”. At 36% airflow rate of the outdoor fan of the ASHP unit, however, the performance of the ASHP unit was attenuated greatly, with the frosting-defrosting efficiency loss coefficient of 0.47, the heating capacity and COP reduction by 51.5 and 38.8%, respectively. These findings provided significant references to the optimization of ASHPs performance with variable airflow rate of the outdoor fan under frosting conditions.

Energy-efficient defrosting of heat exchanger fins with embedded negative stiffness structures

Frost formation in heat pump fin surfaces during cold climate conditions can significantly degrade the performance of air-source heat pumps. Typical thermal defrosting methods are energetically expensive and can cause thermal discomfort. This research investigates a novel mechanical defrosting solution for heat exchanger fins using multistable shape morphing cells integrated into the fins. We use bending (metalworking) to manufacture fins exhibiting multistable behavior (bistable and metastable) out of three materials by plastically deforming targeted regions of a laser-cut flat fin precursor. These structures undergo snap-through and suddenly release energy upon actuation, resulting in out-of-plane displacements and vibrations that can be used to break and shed frost. We experimentally characterize the bistable behavior (actuating forces, stored and released strain energy) of the three sets of bistable and metastable fins to evaluate their mechanical defrosting potential. Using finite element analysis, we characterize the dynamic response of the fins under various boundary conditions. We experimentally investigate the influence of different boundary conditions on the percentage of ice shed from the fin surface, and measure and compare the power and energy consumption due to mechanical and thermal defrosting. We demonstrate the technological feasibility of the energetically efficient mechanical defrosting approach and make recommendations on fin design and boundary conditions to maximize the defrosting performance. The mechanical defrosting strategy significantly outperforms the thermal strategy. We find that the shape morphing mechanical approach on average consumes 81.09 times less energy, requires 10.84 times less power, and is 8.26 times faster. For certain fins, the shape morphing approach sheds 100 percent of ice from the fin surface, and on average sheds 57.44 percent of the ice. The majority of ice is shed during the first few defrosting cycles. As a result, there is an optimal number of defrost cycles for each combination of morphing fin, boundary condition, and actuation method.

Energy, emissions, economic analysis of air-source heat pump with radiant heating system in hot-summer and cold-winter zone in China

The energy savings and economic performance study of the air-source heat pump (ASHP) and wall-hanging gas boiler (WGB) heating systems in hot-summer and cold-winter (HSCW) zones of China is beneficial to the development and implementation of relevant policies under the carbon neutrality background. This research presented a comprehensive analysis of the heat load, primary energy consumption, carbon dioxide and other emissions, and running costs of both heating systems in HSCW zones. Theoretical mathematical models of the energy consumption, emission, and economic performance of the ASHP and WGB heating systems were developed. The calculation results showed that the ASHP system consumed 12.3 % less primary energy than the WGB system, emitted 36.9 % more carbon dioxide, and reduced running costs by 26.0 %, where the increase in carbon dioxide emissions was mainly due to the limited heat-to-electricity conversion efficiency of grid systems. The experimental results showed the excellent energy consumption saving and economic benefits of the ASHP system compared to the WGB system, which promoted the applications of ASHP on a larger scale in HSCW zones of China. The comprehensive research results on the energy-saving and carbon dioxide emission performance of ASHP and WGB can contribute to evaluating the necessity of replacing WGB with ASHP and provide references for relevant policy planning in HSCW zones in China. The replacement of WGB with ASHP for spacing heating systems is promising coupled with the power system reformation to reduce carbon dioxide emissions and accomplish China's carbon neutrality target.

Experimental Investigation of Heat Transfer Performance of a Finned-Tube Heat Exchanger under Frosting Conditions

• Experiments were conducted with flat fin and corrugated fin heat exchangers. • The corrugated fins enter a high deterioration stage resulting from frost formation 50 minutes earlier than the flat fin. • The comprehensive performance evaluation factor of flat fin heat exchanger is 24.2% higher than that of the corrugated fin. The impact of factors influencing the performance of air source heat pumps (ASHPs) under frost conditions is complicated. Despite this, many preexisting studies focus only on the impact of individual factors on the heating performance of ASHPs. This study conducted an experiment to compare the performance of two commonly used evaporator fin structures, namely the corrugated fin and the plain fin, under different outdoor thermal parameters, including a temperature range of 0–6°C, 60%–90% humidity, and 1.5–4.5 m/s wind speed. The results show that the performance of the flat fin heat exchanger is better than the corrugated fin heat exchanger under frost conditions. The fin characteristics have a great influence on the frost of the heat pump outdoor unit. As the inlet airspeed increases, the effect of wind speed on the heat transfer performance of the heat exchanger gradually decreases. In the hot summer and cold winter zones in China, it was discovered that the performance of a flat fin heat exchanger, applied as a heat pump outside an evaporator, was better than that of a corrugated fin heat exchanger.

Unsteady characteristics of sleeping thermal comfort during defrosting of a T-ASHP system

Defrosting is necessary for the normal operation of air source heat pump (ASHP) units to solve the frosting in the winter, which deteriorates the performance of ASHP. During this defrosting period, the ASHP cannot provide heat to the indoor space, leading to adverse thermal comfort, which may reduce the sleep quality. Hence, a task/ambient ASHP (T-ASHP) system was designed in this study, and the transient thermal environment was investigated through a computational fluid dynamic (CFD) method. The duration of t oz (average air temperature in the occupied zone) increase period was 4.33 times as long as t oz decrease period, although the duration of the phase 1 heating period was 1.9 times as that of defrosting period. During the defrosting procedure, the PMV value decreased to the lowest value of −0.83, and the period of feeling cool and slightly cool lasted for 45.4 min. The hand, foot and leg parts were much cooler than the whole body, and the non-uniformity of the PMV value on body parts was obviously enlarged during the defrosting period. Higher draught risk around 22% near the head and trunk regions was found during the defrosting period and was significantly reduced in the follow-up heating period. Practical application: The results from this study emphasize the sleeping thermal comfort during defrosting of air source heat pumps. The present study gave a map of transient thermal comfortable levels influenced by the operating of defrosting and re-heating. These results should serve as a recommendation to air source heat pump designers to determine suitable defrosting technology to maintain normal indoor thermal comfort.

Air-, ground- and dual-source heat pumps: a comparison between energy-efficient systems for a Swedish dwelling

Air-source heat pumps (ASHPs) and ground-source heat pumps (GSHPs) are the most spread heat pumps nowadays. However, they suffer from drawbacks that could be mostly overcome coupling the two heat sources, i.e., using dual-source heat pumps (DSHPs). In this study, we compared the techno-economic performance of ASHPs and GSHPs for a small Swedish single-family house. We also calculated the performance improvements achieved using dual-source heat pumps, and evaluated the economic conditions necessary to make the dual-source heat pumps economically competitive with the more traditional systems. We found that the GSHPs were more efficient than the ASHP during the first year of installation, but their performance decreased - sometimes dramatically - during their lifetime. However, investing in either systems was always beneficial and no significant differences tended to exist between the investments. The dual-source heat pumps had– sometimes significantly – higher performance than the conventional systems but they might not be economically competitive with the more conventional systems.

Low-temperature air source heat pump system for heating in severely cold area: Long-term applicability evaluation

Air source heat pumps (ASHPs) have gained popularity in cold regions without central heating owing to their higher energy efficiency when compared with fossil energy sources. Although studies have shown that the short-term performance of ASHPs may meet the heating demand at an ambient temperature of −20 °C, long-term performance under actual variable meteorological conditions needs further investigation. In this study, to comprehensively evaluate the long-term performance of an ASHP system in cold climates, a three-month field test was conducted in Daqing city, which is the coldest region in China. The ambient temperature fluctuated from −24.7 °C to 15 °C during the test. The test results showed that the system coefficient of performance (COP) was related to the outdoor temperature (r = 0.8955), and when the supply water temperature increased by approximately 6 °C under an outdoor temperature of −5 ± 1 °C, the corresponding COP decreased by 14.2%. Apart from the frost temperature of −5 ± 1 °C, the relative humidity (RH) had a slight effect on the system COP. Moreover, during the three coldest days when the ambient temperature ranged from −24.7 °C to −15.0 °C, the heat produced met the heating demand, and the system ran smoothly. Compared with other common heating methods, ASHPs have the advantages of high energy conversion efficiency and low pollutant emissions. From the field test results, it can be inferred that the low-temperature ASHP system is a clean and energy-efficient heating method that is well suited for severely cold regions.

Investigating heat transfer and frosting performance of air source heat pumps with the impact of particulate fouling

Due to superior energy efficiency and emission-reducing potential, air source heat pumps (ASHPs) have been widely used in space heating and domestic water heating. However, the heating seasons usually coincides with the serious air pollution, hence particulate fouling appears on ASHPs' outdoor coils repeatedly, despite being de-fouled periodically. In this study, four levels of fouling, namely none fouling (0 g/4 m2), mild fouling (15 g/4 m2), moderate fouling (40 g/4 m2) and severe fouling (70 g/4 m2), were concretized on a finned tube heat exchanger successively, followed by experimentally investigating the negative effects of fouling on the heat transfer performance of an air source heat pump water heater (ASHPWH). Results demonstrated that the effects of fouling on the performance of ASHPs in heating mode would be exacerbated with decreased air temperature. In addition, the frosting performances at different levels of fouling were evaluated quantitatively, by depicting the frosting evaluation plots. The frosting evaluating plots suggested that fouling would expand the air temperature and relative humidity (RH) ranges where frosting appears, and would hasten frosting process. As verification, signal factor experiments were carried at different levels of fouling. The experimental results confirmed that the increase of fouling level would result in faster frosting process, but smaller final frost accumulation mass.

Air source heat pump in-situ performance

Air Source Heat Pumps (ASHPs) are part of the solution to decarbonise the residential heating sector. The coefficient of performance (COP) is a measure of the instantaneous efficiency of a heat pump. The heat energy produced by an ASHP is deemed renewable if it meets a specified sustained COP over a period of time, e.g., a seasonal performance factor (SPF). Heat pump performance in situ often differs from laboratory test conditions. This paper explores the performance of ASHPs in a field trial of deeply retrofitted Irish houses. Analysis shows that all houses in the trial qualify as producing renewable heat but vary from the manufacturer’s laboratory test performance. ASHPs with ratings of 8.5 kW (11.2 kW) underperformed against the manufacturers COP values on average by 16 (24%) at outside temperatures of 7 °C, and 3 (11%) at outside temperatures of 2 °C. The paper evaluates statistical and machine learning models, including a random forest (RF) model, to predict COPs and compare the difference between the manufacturer’s laboratory COPs and the trial results.

A comparison of ground and air source heat pump performance for domestic applications: A case study in Perth, Australia

A comparison of ground and air source heat pump performance for domestic applications: A case study in Perth, Australia

Analysis and determination of a seasonal performance evaluation for air source heat pumps

Performance evaluations of heat pumps play an important role in regulating energy markets and promoting energy conservation technologies. The development of air source heat pumps (ASHP) standards, especially seasonal performance factors including the annual performance factor (APF) method and integrated part-load value (IPLV) method, are reviewed for applications worldwide and in China. Based on the reviews, this study introduces and analyzes the methods of evaluating the seasonal performance of ASHPs by analyzing and proposing principles and key factors of seasonal performance evaluations, including building load models, operational period distributions (bin hours), and ASHP performance models. Then, The similarities and differences between the APF and IPLV approaches were analyzed and compared, indicating that the APF method should be applied as a seasonal performance evaluation method for ASHPs (for both air–water and air–air heat pumps). Therefore, the rules for three key factors of seasonal performance evaluations were determined, which take China as an example. The specific results and parameters were calculated according to meteorological data and simulation. Besides, the related energy efficiency limitations and comparisons of the APF index, which greatly improve the ASHP industry, were discussed. This paper provides a basic reference for other countries to formulate their own ASHP standards.

Investigation on the regulating methods of air source heat pump system used for district heating: Considering the energy loss caused by frosting and on–off

Air source heat pump (ASHP) has been used in Northern China for district heating. However, few investigations on its regulating methods have been conducted till now. To increase the energy-saving effect of AHSPs, a simulation model, which considered the energy loss caused by frosting and on–off, was developed in this paper. Based on this model, three combination regulating modes suitable for district heating using ASHPs as heating source were proposed. Then the heating performance of ASHPs used for district heating in seven cities was investigated using this model. Results showed that both the energy loss corrections of frosting and on–off have evident impacts on the power consumption and seasonal coefficient of performance (SCOP). Owing to the energy loss correction of frosting, the power consumption of ASHP unit is increased by 3.85%–17.41%, and meanwhile SCOP is decreased by 3.72%–14.83%. The influence of the energy loss of on–off decreases gradually with the increase of heating area. In the three regulating modes, the power consumption of ASHP unit changes slightly, while the energy saving rate of water pump can reach 80.97%–86.46%. When the pitch of buried pipe was 300 mm, ASHP can meet the heating demand in all the three regulating modes.

Impact of Compressor Drive System Efficiency on Air Source Heat Pump Performance for Heating Hot Water

A standard Polish household with a central heating system powered by a solid fuel furnace was chosen as a case study. The modular Air Source Heat Pump (ASHP) was used to heat the hot water outside the heating season. In this article comparative studies of the impact of the compressor drive system used on the energy efficiency of the heat pump have been carried out in operating conditions. The ASHP heating capacity and coefficient of performance (COP) were determined for the outside air temperature in the range from 7 to 22 °C by heating the water in the tank to a temperature above 50 °C. For the case of a fixed speed compressor, average heating capacity in the range 2.7−3.1 kW and COP values in the range 3.2−4.6 depending on the evaporator supply air temperature were obtained. Similarly, for the inverter compressor, the average heating capacity in the range of 2.7−5.1 kW was obtained for the frequency in the range of 30–90 Hz and COP in the range 4.2−5.7, respectively. On cool days, the average heating capacity of the heat pump decreases by 12%. For the simultaneous operation of two compressors with comparable heating capacity, lower COP values were obtained by 20%.

Defrosting Performance Improvement of Air-Source Heat Pump Combined Refrigerant Direct-Condensation Radiant Floor Heating System with Phase Change Material

Traditional defrosting methods applied to solve frosting problems of air-source heat pumps operating in cold periods may reduce heat capacity of the system and decrease indoor thermal comfort. In order to improve the performance of air-source heat pump (ASHP) and maintain indoor temperature in defrosting conditions, an air-source heat pump combined with a refrigerant direct-condensation radiant floor heating system with phase change material is proposed and evaluated in this study. Two radiant floor heating terminals with and without composite phase change material modules were compared through experiments. A composite phase change material based on dodecanoic acid-tetradecanol-hexadecanol mixture and expanded graphite was investigated for this application. Experimental results indicate that both heat fluxes of two comparing terminals are higher than 70 W/m2 in heating condition. At the same time, the floor surface temperature, indoor air temperature, and heating capacity of the terminal with composite phase change material modules are higher than those without composite phase change material modules in defrosting condition. This suggests that the proposed system with composite phase change material modules can improve indoor thermal comfort in defrosting condition as well as satisfy the heating requirement in heating condition.