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• Influence effect of supply water temperature on frosting performance is revealed. • Increasing supply water temperature can improve frosting suppression performance. • An increase of 102 % in frosting duration when water temperature increases to 50 ℃. • Indoor temperature drop is 45 % less during defrost by improving water temperature. During the space heating in winter, the air source heat pump (ASHP) often encounters frosting problem. In former studies, it was found that the frosting performance of ASHP and indoor thermal environment vary significantly when the supply water temperature changes. However, the influence mechanism of supply water temperature is still unknown. To solve this problem, the frosting performance variations of ASHP and its effect on indoor thermal environment variations at the supply water temperature of 41–50 ℃ were investigated in the psychrometric chamber and artificial environmental chamber, respectively. Results showed that increasing supply water temperature can effectively suppress the frosting speed and reduce the impact of frosting-defrosting on indoor thermal environment. When the supply water temperature raises from 30 ℃ to 50 ℃, the frosting duration prolongs from 50 min to 101 min. Meanwhile, the indoor temperature drop caused by frosting-defrosting decreased by about 45 % when supply water temperature rises from 41 ℃ to 50 ℃. Besides, with the increase of supply water temperature, the impact of frosting on the heating performance of the ASHP decreases. When it rises from 30 ℃ to 50 ℃, the attenuation degree of the average coefficient of performance during frosting-defrosting cycle drops from 9.85 % to 7.25 %, compared to those at non-frosting condition. However, although the frosting performance of ASHP and its effect on indoor thermal environment both get better with the increase of supply water temperature, the overall heating performance of the ASHP still declines. [Display omitted] [ABSTRACT FROM AUTHOR]

• An optimal water temperature scheduling method to response time–of–use tariffs is proposed. • Room model predicts indoor air temperature and return water temperature for the subsequent day. • Both demand response and heating performance of ASHP are improved. • Feild experiments are conducted on an ASHP-FCUs heating system. • Operating costs decrease by 20.8% to 43.0% with comfortable indoor temperatures maintained. 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. [ABSTRACT FROM AUTHOR]

Defrosting under unnecessary conditions is a typical and undesirable operating phenomenon for air source heat pump (ASHP). This paper aims to evaluate the ASHP performance under unnecessary defrosting phenomena in different regions of China. Firstly, energy loss for two kinds of unnecessary defrosting phenomena was quantified by 60 days field test. Secondly, a method was developed to predict unnecessary defrosting times. The prediction accuracy can be controlled within 10%. Furthermore, the ASHP performance of unnecessary defrosting was evaluated for nine typical cities in China. It was found that the average unnecessary defrosting rate was 53% for northern cities, 38% for mid-land cities, and 15% for southern cities during a heating season. Correspondingly, the total energy loss was 233 MJkW −1 , 161 MJkW −1 , and 44 MJkW −1 . These results indicate that unnecessary defrosting processes should not have been ignored. It is a valuable work to improve the defrosting control method. [ABSTRACT FROM AUTHOR]

• A dimensionless coefficient, COFS , to reflect the level of frosting was proposed. • A method to develop a frosting suppression performance map was introduced. • 50%-100% frosting areas on the maps for experimental ASHP units can be predicted. • The four-point method can reduce map development time by at least 67%. It has been known that for space heating ASHPs, their different configurations and operation may influence their frosting performances. However, there has been no comprehensive coefficient that can be conveniently and comprehensively used to characterize the frosting status, severeness level of frosting, and the potential for frosting suppression during the real-time operation of an ASHP unit. Therefore, in this paper, a novel coefficient of frosting suppression based on the operating characteristic for ASHPs has been proposed, and its development is reported. Firstly, a novel dimensionless coefficient, COFS , to reflect the severeness of frosting for ASHPs was defined. Secondly, an experimental setup with four variable speed experimental ASHP units and test conditions are described. Thirdly, using the experimental results, COFS -based frosting suppression performance maps for the four units were developed. It is shown that because the experimental four units had different frosting suppression abilities, their frosting areas in their frosting maps were 50%, 100%, 94%, and 58%, respectively. In addition, as the test conditions changed, the frosting area on a map area could also change. These have proved that the proposed COFS could effectively reflect the frosting status and the relative level of frosting suppression performances of ASHP in actual operation. [ABSTRACT FROM AUTHOR]

• A characteristic index of frosting suppression, CICO , for ASHPs was defined. • CICO was based on the configuration and operation of ASHPs. • An empirical numerical relationship for CICO was established. • CICO can be used to evaluate the frosting suppression performances of ASHPs. It has been known that for space heating ASHPs, their different configurations and operation may influence their frosting performances. However, there has been not a single index that can be used to comprehensively and realistically evaluate the frosting suppression performances for a space heating ASHP unit by considering all the influencing configurational and operational factors. Therefore, in this paper, a novel characteristic index for frosting suppression based on the configuration and operation of ASHPs has been proposed and its development is reported. Firstly, the characteristic index, CICO , was defined, a detailed analysis on the heat transfer taking place in the outdoor coil of an ASHP was carried out, and the relationship between CICO and temperature difference in the heat transfer between outdoor coil and ambient air (Δ T) was established. Secondly, the description of an experimental setup with four experimental ASHP units is given. The set was used to establish all empirical numerical relationship between CICO and Δ T. Thirdly, using the experimental results, the empirical numerical relationship between CICO and Δ T was established, which suggested that a higher CICO would lead to a smaller Δ T , thus a better frosting suppression performance. The development results suggested that the index of CICO could be used to evaluate the level of frosting suppression performance for ASHPs, and to help improve the frosting suppression performances for an ASHP during its design and operation, to ensure its best possible operation performance. [ABSTRACT FROM AUTHOR]

Highlights • The TEPS defrosting initiation strategy was applied to mild and humid regions. • Five short-term and two long-term field test cases were conducted. • The TEPS defrosting initiation strategy was compared to the TT initiation strategy. • The use of TEPS strategy led to more accurate and reliable defrosting initiation. • The use of TEPS strategy led to better operating performances of ASHPs. Abstract A defrosting initiation strategy based on tube-encircled photoelectric sensors (TEPS) was previously proven effective for enhancing the frosting-defrosting performances of air source heat pumps (ASHPs) in cold and dry regions. To verify its applicability to, and evaluate the energy-saving potential when using the TEPS-based strategy in mild and humid regions, field tests were conducted in Guiyang City, China, where the outdoor averaged air temperature and relative humidity were 4.5 °C and 80% in winter, respectively. Two ASHPs of the same-make, one with the TEPS-based strategy and the other with the commonly used temperature-time (TT) strategy, were tested. Totally, seven test cases were organised. Cases 1–5 were for short-term tests to verify the technical feasibility of TEPS-based strategy by comparing the defrosting accuracy of the two strategies. Cases 6–7 were for long-term tests to validate the reliability and effectiveness of the TEPS-based strategy by comparing two ASHPs' operating performances. The test results demonstrated that the use of TEPS-based strategy in mild and humid regions can initiate defrosting more accurately, reliably and efficiently, and thus improve the COP and heating capacity by 11.3% and 12.1%, respectively, compared to the use of the TT strategy. Therefore, it was proven that the TEPS-based strategy was more effective for the ASHPs used in mild and humid regions, for a more accurate defrosting initiation and higher energy efficiency. [ABSTRACT FROM AUTHOR]

• A sizing method considering the joint effect of T a and RH was developed. • A capacity design correction factor (K) was proposed and evaluated. • Frosting caused the outdoor air design temperature for space heating to drop by 1.8–3.5 °C. • Maximum decrease rates of output heating capacity caused by frosting was 43.8–50.0% • A cloud picture was given by using the values of K in 278 Chinese cities. In cities, there are thousands of wastewater treatment plants. The treated tail water drained from them is clean relatively, and has a stable flow rate. Extracting heat from it through tail water source heat pumps (TWSHP) for space heating can decrease the building heating energy consumption significantly. Besides, by integrating with thermal energy storage (TES), it can realize the goal of peak load shifting, especially in commercial buildings. However, there is few reports on its experimental investigations or engineering applications till now. To promote the application of TWSHP integrated with TES system, it was designed for the space heating of a technology park in Beijing, and a control method was proposed to decrease the operating cost by using the valley electricity. Then, utilizing its operating parameters in the heating season of 2021–2022, its heating performance was analyzed. Results indicate that the proposed system has good heating performance. The average COPs of TWSHP units and the system in the heating season are 7.08 and 4.41, respectively. Compared with gas-fired boiler, its carbon emission decreased by 62.53%. By using the TES system, the power consumption during valley period of power grid increases by 581.23%, realizing the goal of peak load shifting. [ABSTRACT FROM AUTHOR]

Recently, the photovoltaic-driven variable frequency air source heat pump (PV-VFASHP) space heating system has been widely used in low-carbon buildings. However, owing to the lack of performance prediction model for VFASHPs, the self-consumption rate of PV power generation is far lower than expected. To solve this problem, a simulation model and a test rig for VFASHP were built, and its heating performance under different conditions was investigated. A performance prediction model for heating capacity and COP was developed considering the joint effect of outdoor temperature, supply water temperature, and compressor speed. Then, this model was applied to a PV-VFASHP heating system in a low-carbon building in Beijing, and the application effect on sunny, cloudy, and rainy days was analyzed. Results show that the performance prediction model can predict the heating capacity and COP accurately. Experimental results of 13 VFASHPs from 8 manufacturers show that the relative errors are all within ±10 %. When regulating the PV-VFASHP system based on this performance prediction model, the self-consumption rate of PV power reaches 100 % on all the three days, contributing 77.26 %, 73.77 %, and 67.84 % of total power consumption. Additionally, the VFASHP operates efficiently, achieving an average COP of 3.59. [ABSTRACT FROM AUTHOR]

• The theory and methodology of T opt is proposed for ASHPs with different CICO s. • As the CICO increases from 8.28 to 20.31, the ε NLmin decreases from 11.53 % to 7.43 %. • As the CICO increases from 3.14 to 22.44, the T opt increases from 22 min to 148 min. • A prediction model for the optimal defrosting initiating time of ASHPs is developed. Frosting is a critical challenge that affects the energy efficiency of air source heat pumps (ASHPs). Defrosting periodically is the most widely used method to solve this problem. However, the rate of frosting varies considerably for ASHPs due to their configurations and operations (CICO) being different. This leads to the obvious differences in their defrosting initiating time. Defrosting too early or too late will both cause energy loss. To ensure the efficient operation of ASHPs, the variations of energy loss coefficient caused by frosting-defrosting (ε NL) and the defrosting initiating time for ASHPs with different CICO values were investigated. Based on the theory of optimal defrosting initiating time (T opt) and experimental results under different frosting durations, the T opt for different ASHPs is found. Then, a prediction model for ASHPs with different CICO values is established. Results show that there is a T opt for ASHPs with different CICO values, respectively. Under the frosting condition of 2/1 ℃, the T opt increases with the rise of CICO values. When the CICO values increase from 3.14 to 22.44, the corresponding T opt increases from 22 min to 148 min. By fitting the T opt and CICO values, a cubic mathematical model for the prediction of the optimal defrosting time of ASHPs has been developed. [ABSTRACT FROM AUTHOR]

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. • Applying image recognition to frost built-up detection in ASHPs was studied. • A characteristic parameter of F was proposed to quantify the frosting degree on ASHPs. • The applicability of the novel frost built-up detection method was examined. • The novel method was further modified account for the impact of varying illuminance levels. [ABSTRACT FROM AUTHOR]

• Regional performance evaluation indexes based on ETD method were developed. • All the ambient conditions over an entire heating season were considered. • The regional performances of ASHPs operated in any regions may be evaluated. • The key factors affecting operating performances of ASHP units were specified. • The regional performances of ASHPs operated in 278 Chinese cities were assessed. During the actual operation of an ASHP unit installed in different climate regions, ambient operating conditions can vary constantly over an entire heating season, thus the actual output heating capacity of ASHP units may vary significantly from region to region. To enable a convenient evaluation of output heating capacity of a space heating ASHP unit installed in different climate regions, regional performance evaluation indexes including T a ′ ¯ , HERR (T a') , β Ta and β RH based on ETD method have been developed. These evaluation indexes are generalized and can be used to evaluate the regional performance of ASHP units operated in any regions. It can also be used to specify the key factors affecting the operating performances of ASHP units in different regions. With the developed evaluation indexes, the regional operating performances of ASHP units installed in 278 Chinese cities were evaluated. The results demonstrated that the total output heating energy for ASHP units operated in Southern Chinese provinces may be lower than those in Northern China. In Shanghai, T a ′ ¯ at −6.0 °C is lower than that at −3.0 °C in Beijing, suggesting the total output heating energy from an ASHP units operated in Shanghai over an entire heating season would be lower than that in Beijing. Furthermore, in southern Xinjiang and northern Fujian Provinces, the total output heating energy from ASHP units may be higher than those operated at the nominal condition. In addition, in Tibet, Beijing, Liaoning, etc., the total output heating energy from the ASHP units would be affected more by T a , thus ASHPs' manufacturers are recommended to develop ASHP units suitable for operation in low temperature ambient. In most regions in southern China, RH would affect more on the total output heating energy from the ASHP units, thus ASHPs' manufacturers are recommended to develop ASHP units having frosting suppression provisions and accurate defrosting measures. [ABSTRACT FROM AUTHOR]

• A novel defrosting initiating method for ASHPs has been developed. • The method is based on the optimal defrosting initiating time point. • Field tests to verify the applicability of the method were conducted. • The method can ensure the best possible operating performances of ASHPs. Air source heat pumps (ASHPs) have been widely used for space heating in recent years. When a space heating ASHP unit is operated in frosting conditions, periodic defrosting is necessary to ensure its safe and efficient operation. Therefore, it is important to develop appropriate defrosting initiating method for best possible operating performances and energy efficiency of ASHPs during frosting-defrosting operations. As a result, a study on the development of a novel defrosting initiating method for ASHPs based on the optimal defrosting initiating time point has been carried out and the study results are presented in this paper. Firstly, various existing defrosting methods for ASHPs are briefly reviewed. Secondly, the development of a novel defrosting initiating method based on the optimal defrosting initiating time point is detailed. Finally, field tests with a field ASHP unit using the novel defrosting initiating method have been carried out, and the tests results demonstrated that the novel defrosting initiating method can respond to the changes in frosting environmental conditions and correctly evaluate the optimal defrosting initiating time point of the field ASHP unit. It was further shown that the use of novel defrosting initiating method can help greatly improve the output heating capacity and COP of the field ASHP unit, by 36.79% and 27.85%, respectively, as compared with the use of T-T defrosting method, the most commonly used defrosting initiating method currently, during frosting-defrosting operations. [ABSTRACT FROM AUTHOR]

When an air source heat pump (ASHP) is operated at frosting conditions, controlling its defrosting initiation is very important, which affects its operating efficiency and output heating capacity. Therefore, to improve the accuracy of defrosting initiation of ASHPs under frosting-defrosting operations, an investigation on determination of an optimal defrosting initiating time point (t opt) for an ASHP unit based on the minimum loss coefficient in the nominal output heating energy (ε NLmin) has been carried out and the investigation results are presented in this paper. Firstly, the existence of t opt was verified through a field study. Secondly, the development of multiple-variable-nonlinear models for predicting ε NLmin and t opt is detailed. Finally, the results of a related modeling study using the developed multiple-variable-nonlinear model for t opt is presented. The results from the investigation suggested that initiating defrosting for an ASHP unit at t opt could result in the best possible operating performances under its frosting-defrosting operations. It was further demonstrated through the modeling study that at a fixed ambient relative humidity, an increase in ambient air temperature would lead to a decreasing-increasing variation trend in t opt , and at a fixed ambient air temperature, t opt was decreased with an increase in relative humidity. • Determining the optimal defrosting initiating time point, t opt, for ASHPs was studied. • Initiating defrosting for an ASHP at t opt leads to its best operating performances. • A multiple-variable-nonlinear model for predicting t opt for ASHPs was developed. • t opt is influenced by an ASHP's operating ambient conditions. [ABSTRACT FROM AUTHOR]