Air Source Heat Pump Unit Research Articles

Enhancing demand response and heating performance of air source heat pump through optimal water temperature scheduling: Method and application

Air source heat pump (ASHP) is a key technology for the electrification of building heating, and its participation in demand response (DR) has important implications for reducing the grid peak load. However, current DR strategies mainly focus on changing the setpoint of indoor temperature, using the building’s thermal mass as a passive thermal storage, while the impacts of water temperature on the performance of ASHP units are often overlooked. Therefore, this study proposes an optimal water temperature scheduling (OWTS) method to respond time–of–use tariffs. It is developed based on a room thermal dynamic prediction model and an ASHP power consumption prediction model, optimizing supply water temperature schedule with objectives of maintaining indoor thermal comfort and reducing heating operating costs. The effectiveness of the OWTS method was tested on a field ASHP system in Beijing, and compared with two benchmark methods. Results demonstrate that the application of the OWTS method can enhance the DR capability of the ASHP heating system, with the average value of Flexibility Factor increasing from 0.134 to 0.728, and can reduce the heating operating costs by 20.8%–43.0%. This study thereby offers a promising method for ASHP heating systems to effectively participate in DR.

Performance of air source heat pump units with different wettability evaporators

The air source heat pumps (ASHPs) for winter heating have the advantages of high efficiency and environmental protection. However, the outside evaporator’s frosting negatively impacts the performance of heat pump systems in low-temperature and high-humidity environments. In this paper, we fabricated the superhydrophobic evaporator and mounted it on a commercial ASHP unit, expanding the use of superhydrophobic surface modification technology from heat exchangers in the laboratory to the ASHP units. Then we investigated the operating characteristics of two ASHP units equipped with hydrophilic and superhydrophobic evaporators respectively under six distinct outdoor and indoor environmental conditions experimentally. In the non-frosting conditions, film condensation formed on the surface of the hydrophilic evaporator and the evaporation temperature decreased with the formation of water film, resulting in a decrease in heat production and COP. In the frosting conditions, the time of forming the frost layers with the same thickness on the superhydrophobic surface was more than 4 times longer than that on the hydrophilic surface. The COP of the superhydrophobic evaporator unit was improved by 2.9% to 8.2% under the non-frosting conditions, by 11.5% under the continuous heating conditions and by 14.28% when the hydrophilic unit performed two defrosting cycles, respectively. During the period in which the hydrophilic evaporator unit completed two frosting/defrosting cycles, the energy loss coefficient caused by the defrosting process was 31.65% compared to the superhydrophobic evaporator unit. This work extended the experimental method of superhydrophobic heat exchanger, contributing to the application of superhydrophobic finned-tube evaporators in ASHP units.

Life cycle carbon emission calculation model of energy system in public buildings: A case study in Shanghai

Energy consumption and carbon emissions (CE) in the building sector have grown in importance in relation to global climate change for China's sustainable development. Relatively few research have addressed the embodied CE, despite the fact that many have examined the operational CE from energy system operational use. In order to determine the CE of the energy production, transportation, construction, operation, and recycling stages, we therefore put up an integrated model. An inventory analysis of the energy system's life cycle CE, a scenario forecast of various combinations of building energy system equipment, and a case study of the building energy system of a typical office building in Shanghai's service industry. According to the results, the embodied CE emits 38.01 % of its energy, and the operational CE emits 61.99 %. However, we should not underestimate the significance of the impact of the embodied CE. Among all projection scenarios, the suggested CE share is highest (18.40 %) when the system combination of air-source heat pump units, LED lights, and ground-source heat pump units is implemented. In the end, based on the structural analysis of the embodied CE, low-carbon growth strategies and policy recommendations for Shanghai's service sector will be made.

Applying neural network model to real-time frosting detection and intelligent defrosting control for air source heat pump

Periodic defrosting is essential for efficient operation of air source heat pump to avoid the attenuation of performance caused by frosting. However, the mal-defrosting phenomena in practice, which are no defrosting progress when necessary and unnecessary defrosting operation under no frosting, happen occasionally. To improve the accuracy of defrosting, a convolutional neural network model based on deep learning with YOLOv5 is introduced for frosting detection in this paper. Firstly, the evaluation method of frosting degree combining fractal dimension and heating capacity attenuation is constructed. Then, an image dataset is built based on frosting degree evaluation. The optimized YOLOv5-frost network is constructed, which improves the accuracy of frosting detection by iterative training. Thirdly, a novel defrosting control method based on YOLOv5-frost network is proposed, in which the defrosting initiation and termination points are investigated. Finally, comparative experiments are conducted between the proposed defrosting control method and traditional methods. The results indicate that the mal-defrosting phenomena are effectively avoided under the proposed method. The COP and total heating capacity are increased by 11.83 % and 7.42 % respectively, as well as the defrosting frequency and energy consumption are decreased by 75.00 % and 88.24 %, respectively. Additionally, the time consumption, memory overhead, and central processing unit overhead of the proposed method are 2.72 %, 71.58 %, and 74.07 % of those for the traditional methods, respectively. The performance of air source heat pump unit in real-time frosting detection and on-demand defrosting in complex conditions can be effectively improved with the proposed method.

Solar-assisted biogas system with air-source heat pump for the energy-saving of indoor heating in north China

Abstract To address the challenges associated with winter heating in high-latitude regions of China, solar-assisted biogas heating systems have emerged as the predominant focus of research due to their cost-effectiveness, accessibility, and environmentally friendly attributes. However, traditional solar-assisted biogas heating systems encounter issues of low efficiency and limited practicality resulting from unstable solar radiation and extreme ambient temperatures during the heating season. To improve the economy and stability of the system, this study proposed a novel operational control method for a small-scale energy station system in rural regions of North China, named Solar-Biomass-Air Source Heat Pump Hybrid Heating System (SBHP-HHS). The integration of solar energy, biogas energy, and air-source heat pump (ASHP) systems in this proposed work has shown to create effective complementarity and enhances the production efficiency of the existing system. Test and simulation studies have been carried out for this system. The layout of buildings and equipment within a university campus in Beijing is reconfigured and redesigned, incorporating an ASHP into the existing heat source configuration. To begin with, a mathematical model is established for the complementary heating system that incorporates solar energy, a biogas digester, and an ASHP. Subsequently, a dynamic simulation model is developed using the TRNSYS platform, and a corresponding operational control strategy for the multi-energy complementary heating system is proposed. Dynamic simulation and analysis of the newly implemented system are performed using the TRNSYS platform, focusing on energy flow and thermodynamic performance. Throughout the heating season, the solar-biogas integrated system achieves a remarkable assurance rate of up to 79%. Additionally, ASHP maintains a relatively high heating efficiency, coefficient of performance (COP) reaches 3.02. Finally, an economic evaluation of the multi-energy complementary system was conducted based on the annual cost method. This was compared with the clean approach of using only an ASHP unit. The results indicate that the SBHP-HHS is more economical when the anticipated useful life is 6 years or longer. The results indicate that the proposed can achieve significant energy-saving and carbon-reduction benefits in rural areas, catering to their heating needs.

A visual defrosting control method for air source heat pump system based on machine vision

Frosting on the coil of the air source heat pump (ASHP) significantly affects its performance. An accurate defrosting control method is essential for the ASHP unit. Here, a novel visual defrosting control method based on machine vision is proposed. The frosting degree of the unit is accurately obtained by frosting image analysis with digital image processing to achieve on-demand defrosting. Fractal dimension, frosting degree coefficient, and single-circuit frosting evenness value are introduced to evaluate the frosting degree of coil images. The image acquisition point is determined with confusion matrix. Based on this, the logical framework of the proposed method is constructed, in which the defrosting starting and termination conditions are set as the moment when the frosting degree coefficient is equal to 0.9 and 0, respectively. A multi-variable model of fractal dimension is fitted, which improves the generalization of the proposed method. Finally, comparative experiments are conducted between the proposed method and the traditional temperature-time (T-T) method in typical cases. The results indicate that the COP and total heating capacity increase by 11.8 %, 10.6 % and 7.4 %, 6.6 %, respectively, and the defrosting frequency is reduced by 75 %, which demonstrates dominant advantages of the proposed method in accuracy, energy-saving, and indoor thermal comfort management.

The variable water temperature control strategy of the air-source heat pump compatible with floor heating system for an apartment

The adjustment of operating parameters and control strategies of an air-source heat pump (ASHP) heating system is of great significance for achieving system energy-saving. Ordinary constant supply water temperature setting could lead to higher energy consumption without concerning the changing of outdoor ambient temperature. In order to save the operational cost, we proposed a variable water temperature control strategy based on the hourly outdoor temperatures. In combination of TRNSYS simulation and experimental study, the system performance under variable supply water temperature control strategy were investigated and compared with four-period-adjusted supply water strategy and constant supply water strategy. The results showed that compared to operating at a constant supply water temperature of 35 °C, the daily power consumption of the system decreased by 56.7 % and 29.5 %, respectively under the typical high and low ambient temperature circumstances. Relative to operation with four-period-adjusted supply water temperature, the power consumption of the system decreased by 53.6 % and 24.6 %, respectively. Moreover, the variable water temperature control strategy could improve the coefficient of performance (COP) of the heating system. The control method proposed in this study would provide design reference for equipment manufacturers and help users to efficiently operate the ASHP unit.

Advanced Exergy Assessment of an Air Source Heat Pump Unit

Conventional exergy-based analysis methods are used for evaluating the performance of the energy conversation systems. Conventional exergy-based analyses identify the sources, amounts, and reasons of irreversibilities (exergy destructions), costs and environmental effects, and provide a general direction for improvement. However, interactions between system components (endogenous/exogenous) and technical limitations (avoidable/unavoidable) cannot be identified with any conventional analysis. Hence, the real potential for improvement and optimization strategies can be misguided. Advanced exergy based analysis seeks to overcome this limitation. An air source heat pump unit was assessed applying conventional and advanced exergy analysis approaches respectively. Avoidable/unavoidable and endogenous/exogenous exergy destructions, modified exergy efficiencies and modified exergy losses ratios were calculated for every single component of the system. The results showed that while the evaporator and condenser efficiencies could be upgraded via constructional enhancements to the overall system and other system components, internal operating conditions were mainly responsible of the inefficiencies regarding with the compressor. The analysis demonstrated that while it was possible to improve evaporator and condenser efficiency by making constructive enhancements to whole system design, the efficiency of the compressor was mainly determined by the internal conditions in which the compressor operated.

Optimizing the selection and combined operation of multiple air-source heat pumps for sustainable heating systems

A common concern is that air-source heat pumps (ASHP) perform poorly when operating at partial load. This issue arises due to the disparity between the unit's heating capacity and the actual heating load of the building. This work introduces a new, generally applicable method to determine the optimal combination of air-source heat pump units and provides an optimum unit selection scheme. It proposes two definitions: the average unit load rate throughout heating season (ALR) and the comprehensive nominal heating capacity throughout heating season (Qao), and develops a formula to intuitively demonstrate how different unit selection schemes affect the ASHPs’ performance on an annual basis. The results of the final calculations demonstrate that the best performance is achieved by selecting five units with varying nominal heating capacity. Compared to conventional selection schemes with the identical units for their nominal heating capacity, the average unit load rate throughout entire heating season rises from 81% to more than 95%, and the yearly average coefficient of performance (COP) rises by 6%.

A simulation study on the heating characteristics of residential buildings using intermittent heating in Hot-Summer/Cold-Winter areas of China

Radiant air conditioning system is an important heating method with better thermal comfort, less noise and greater energy-saving potential. However, different intermittent heating modes may significantly impact both the heating efficiency and indoor thermal comfort, which has been neglected in existing studies. Therefore, to obtain optimal intermittent heating modes for achieving higher energy efficiency and better indoor thermal comfort for the residential buildings in Hot Summer and Cold Winter areas, a TRNSYS model of the radiant air conditioning system was developed and validated. The indoor thermal comfort and energy consumption characteristics during the coldest day and entire heating season were simulated and analyzed based on four different intermittent heating modes. The results demonstrated that the supply water temperature of the air source heat pump unit and the radiant terminals stabilized at the set temperature in about 1 h during the start-up stage, and the indoor air temperature stabilized at 24.0 °C after 7.5 h during the shutdown stage. The proportion of time that meets first-level thermal comfort under the four intermittent heating modes was above 90.1% and even higher at over 96.1% when considering the sleeping period. Compared to the continuous heating mode, the four intermittent heating modes saved 22.3%, 25.8%, 40.4% and 48.4% of energy. Overall, the preferred intermittent heating mode for residential buildings with the radiant air conditioning system in Hot Summer and Cold Winter areas was identified as switching on from 7:00 to 15:00 and 19:00 to 3:00, while switching off from 15:00 to 19:00 and 3:00 to 7:00. This model not only considered the immediate benefits of improved indoor thermal comfort and energy efficiency, but also showed great advantages in developing sustainable construction.

A field study on the impacts of cold island effect on operating performances of air source heat pump array for space heating

Connecting multiple air source heat pump units in parallel to form an air source heat pump array (ASHP-A) is an important heating approach for central heating in small areas. At a low wind speed, the colder air stream exhausted by outdoor coils of ASHP units cannot effectively mixed with the ambient air and repeatedly return to the outdoor coil for heat exchange, resulting in the local air temperature inside the ASHP-A lower and lower and forming the cold island effect (CIE). CIE may seriously deteriorate the heating efficiency of ASHP units in buildings. However, the impacts of CIE on the operating performances of an ASHP-A were insufficiently studied in existing studies but necessary for developing a reasonable installation standard and efficient control strategy. Therefore, to reveal the impacts of CIE on the operating performances of ASHP units in ASHP-A, the characteristics of CIE, and its impacts on the operating performances of the field ASHP-A were experimentally carried out. The results demonstrated that CIE significantly existed in the field ASHP-A and resulted in frequent defrosting operations and lower heating performances. The defrosting frequency of Unit 1 having a more severe CIE was increased by 243.5 %. The averaged output heating capacity and COP in Unit 1 declined by 27 % and 27.7 %, and these for Unit 2 by 13.6 % and 14.8 %, respectively. The frosting-defrosting loss coefficient for Unit 1 was increased by 519 % relative to Unit 2. This study is valuable to promote the development of installation standards by taking the CIE into consideration for improving heating efficiency of ASHP-A in buildings.

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.

An advanced exhausting airflow photovoltaic curtain wall system coupled with an air source heat pump for outdoor air treatment: Energy-saving performance assessment

Building integrated photovoltaic (BIPV) and air source heat pump (ASHP) technologies have emerged as promising solutions for building energy conservation. However, traditional solar buildings face limitations such as overheating, limited efficiency, and singular functionality; additionally, ASHPs suffer from a low coefficient of performance (COP) due to high condensing temperatures or low evaporating temperatures; the process of outdoor air handling proves energy-intensive. To address these challenges, this study proposes an innovative exhausting ventilation PV curtain wall system coupled with ASHP units (EVPV-HP) for outdoor air treatment. This system features a fine combination of PV cooling, supply air reheating, and heat recovery from both the PV facade and exhaust air. The mathematical model of the BIPV curtain wall, based on energy balance equations, is developed and solved using Matlab programming. This model is then combined with the ASHP system model established in TRNSYS to predict the year-round energy performance of the hybrid system. The findings demonstrate that the EVPV-HP system achieves seasonal energy efficiencies of 5.08 for cooling and 4.06 for heating operation modes. Compared to a conventional system, the EVPV-HP system enhances yearly energy efficiency by 17.05 % owing to increased PV production, reduced air-conditioning load, and improved COP. Furthermore, parametric analysis suggests that the system exhibits improved performance under higher solar irradiation and hotter weather conditions during summer. Conversely, in winter, the system performs better under lower ambient temperatures with incident radiation around 800 W/m2.

Experimental study of relative humidity effect on the edge effect of frosting characteristics on a vertical cold plate surface

As a common phenomenon, frosting often brings negative impacts in various fields. To gain more insight into the frosting process on a vertical cold plate considering the edge effect, a specific experimental system is designed and built to perform frosting experiments under forced convection conditions with relative humidity varying from 40% to 80%. The results show that as the relative humidity increases, the droplet solidification stage period in the unaffected region decreases, but the equivalent width of edge-affected region, area-average equivalent contact diameter of droplets, and coverage area ratio of droplets increases and then decreases. The area-average equivalent diameter of droplets in edge-affected region increases by 41.25%, 214.63%, 137.98%, 159.03%, and 30.30%, when compared to the unaffected region from Case 1 to Case 5, respectively. Besides, the difference between the droplet distribution density in the edge-affected and unaffected regions gradually decreases, but the average frost layer thickness in the edge-affected region increases significantly as the relative humidity increases. The frost layer growth rate decreases first and then increases as relative humidity increases. From Case 1 to Case 5, the frost growth rates at 1,800 s decrease by 90.05%, 70.19%, 56.08%, 62.28%, and 82.39%, respectively, compared to the initial ones. Results of this study can provide a reference for the anti-frosting and defrosting control technologies in the field such as refrigeration and air source heat pump units.

Study on the Performance of a New Ultra-Low Temperature Air Source Heat Pump (ASHP) Unit in Cold Regions

Low air temperature and frosting have been reported as the critical factors that greatly attenuate the efficiency and performance of the ASHP in cold regions. In order to ensure the potential prevalence of the ASHP in cold regions of China, a new ultra-low temperature ASHP unit was developed, and the field measurement was carried out in an office building where these ASHP units were installed in Shanxi Province. Results showed that a coefficient of performance (COP) of 1.83 was obtained at the ultra-low environmental temperature of −25 °C. Meanwhile, measured results indicated significant frosting suppression and improved heating performance under three typical frosting conditions. In addition, long-term measurement results revealed that the mean COP and COPsys reached up to 3.34 and 2.63, respectively, indicating a higher performance in the cold regions of China. Consequently, the corresponding CO2 emission reached 11.3 kg per year and per square meter, and the annual total cost on the unit reduced by 15.8% compared with the conventional ASHP, which meant that the total investment could be covered in the second year. The reduced CO2 emission and the annual cost implied that the ASHP unit could produce better environmental and economic benefits. Findings of this study revealed that this ultra-low temperature ASHP unit had a better performance under cold environment, which offered a possibility for the prevailing of the ASHP in cold or extremely cold regions, as well as could contribute to the carbon peaking and neutralization.

A novel variable speed ASHP frosting suppression operation method based on the frosting suppression performance map

For variable speed (VS) space heating air source heat pump (ASHP) unit, its different operation speed combinations may significantly affect the frosting suppression and heating performances. However, currently there is no frosting suppression operation method for VS ASHP units, so that their frosting suppression ability using different speed combinations is not fully utilized. Therefore, a novel frosting suppression operation method based on the frosting suppression performance map for VS ASHPs has been proposed, and its development through experimental is reported in this paper. Firstly, a novel operation method to guide the frosting suppression operation for VS ASHPs is proposed. Secondly, an experimental setup with two VS experimental ASHP units, test conditions, and nine study cases are described. Thirdly, the speed combinations that led to different frosting suppression potentials but with the same output heating capacity were determined using the developed frosting suppression performance map, and comparative tests were carried out. It is shown that the use of the proposed novel frosting suppression operation method with the optimal coefficient of performance (COP), which can increase the total output heating capacity (Q) by 15% and the COP by 25%. The proposed novel operation method was straightforward and simple to implement, which could contribute to the further development of frosting suppression operation for ASHPs.

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.

Field studies on indoor thermal environment and start-stop characteristics of the radiant air conditioning system during intermittent heating

With the improvement of social economy, the heating demand of residents in Hot Summer and Cold Winter (HSCW) areas in China is increasing. The radiant air conditioning (A/C) system, is popular as a heating way due to its high comfort, low noise and high energy saving potential. There is still a lack of field research on the system under the intermittent heating mode in the HSCW area. To provide a guide for the operation mode of the radiant A/C system during intermittent heating in HSCW areas, this paper focused on the radiant A/C system with the fresh air system, and then studied system's practical operational parameters and dynamic characteristics in winter. The indoor and outdoor air temperatures and relative humidity, the water temperatures of inlet and outlet of the air source heat pump (ASHP) unit, the supply and return water temperatures of radiant terminals, as well as supply air temperature were analyzed in three stages, i.e., the initial start-up stage, the intermittent shutdown stage and the intermittent start-up stage. The results show that, it took 7 h and 50 min for the indoor air temperature to reach the set indoor temperature during the initial start-up stage. Compared to the initial start-up stage, the intermittent start-up stage reduced the time for the indoor air temperature to reach the set indoor temperature by 4 h and increased the indoor air temperature rise rate by 97%. The indoor air temperature rise rate was 0.46 oC/h and 0.91 oC/h, during the initial start-up and the intermittent start-up stage, respectively. The indoor air temperature decrease rate was 0.23 oC/h during the shutdown period. During the entire winter season, the ASHP unit switched on and off intermittently 10 times. The longest shutdown lasted for 111 h, and the indoor air temperature dropped to 17.2oC.

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.

A method for sizing air source heat pump considering the joint effect of outdoor air temperature and relative humidity

When operated in a highly humid environment, frost may accumulate on the surface of the outdoor coil in air source heat pump units (ASHPs), leading to a reduction in its output heating capacity. Currently, the existing methods for sizing ASHPs only considered the effect of outdoor air temperature, Ta, without taking into account the effect of relative humidity, RH, on output heating capacity. This often resulted in an insufficient heating capacity from an ASHP selected. To resolve this issue, a model for predicting the output heating capacity of ASHPs, Qh, was established firstly. Then, considering the variation in the rated output heating capacity of ASHPs, a method for sizing ASHPs considering the joint effect of Ta and RH has been developed. Besides, for practical application, a capacity design correction factor, K, was proposed, evaluated, and plotted as a cloud picture. Evaluating results indicated that K can be used for sizing ASHPs accurately. When the effect of frosting on Qh was considered, the maximum rates of decrease in the output heating capacity correction factor, KQ, was between 43.8% and 50.0%, and the outdoor air design temperatures for space heating in six typical Chinese cities were decreased by 1.8–3.5 °C, with the average of 2.8 °C. For highly humid cities, such as Qiannan area China, even if its outdoor air design temperature was 1.4 °C higher than that in Chongqing City, the K value was 0.24 lower.