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The occupant-centric Part-Time-Local-Space heating mode holds paramount significance for the carbon neutralization of buildings in China. It ensures a harmonious match between heat supply and personal demand in both temporal and spatial dimensions, effectively reducing space heating consumption. However, current studies predominantly focus on the temporal-spatial heating demands of occupants at the room scale, but the zonal scale heating demands remain inadequately explored. Thus, we developed a temporal-spatial combined quantification method to analyze residents' indoor movement characteristics. It aims to identify the localized subzones in a room where residents have prolonged stays, with spatial boundaries of subzones entirely defined based on high-resolution positioning data. Using this approach, we analyzed the temporal-spatial heating demands of four households under single- and multiple-resident scenarios in typical rooms. Furthermore, we discussed the differences in demand between different heating modes. Results revealed distinctive "local space" demand features in living room and "part time" demand features in dining room. The areas of sofa subzones generally occupied less than 15 % of a room (within 4 m2), and stay durations in dining-table subzones ranged from 0.3 h to 1.2 h. Considering the entire monitored rooms, residents occupied only 4.1–12.1 % in space with 10.7–47.9 % in time per day, highlighting the significant energy-saving potential of the occupant-centric Part-Time-Local-Space heating mode. This evaluation provides a reference for the design and implementation of personalized environmental control system. • A quantification method on personal temporal-spatial heating demand was proposed. • High-resolution indoor positioning data of four households was monitored. • Temporal-spatial heating demand of residents in typical days and rooms were analyzed. • Differences in heating demand of single and multiple resident scenarios were compared. • Ideal energy-saving effects of Part-Time-Local-Space heating mode was analyzed. [ABSTRACT FROM AUTHOR]

Predicting the energy performance of household variable refrigerant flow (VRF) systems in the hot summer and cold winter (HSCW) region in China accurately is particularly challenging due to the frequent defrost operations of the VRF systems during winter. Hence, an improved energy model was developed to determine the energy performance of household VRF systems, taking into account the defrost operations. The energy model was validated by experiments and the energy performance of the VRF system for typical residential buildings in Chongqing, Changsha, Nanjing, and Shanghai was determined based on representative operation modes (full-time-full-space and part-time-part-space modes). The results indicate that the efficiency of the VRF system increases as the outdoor temperature decreases. However, the efficiency of the VRF system decreases as the part-load ratio decreases, which leads to variations in the cooling seasonal performance factor for the representative operation modes and cities. The heating seasonal performance factor is not only influenced by the outdoor temperature and part-load ratio distribution, but also defrost operations. Compared with the part-time-part-space mode of C1 and C2, the annual energy consumption for the C4 mode performs lower from 26.3% to 51.7% for typical residential buildings in the four representative cities. [ABSTRACT FROM AUTHOR]

Chiller consumes a significant power ratio in HVAC system. In order to minimize the power consumption of chiller, many efforts have been made on load predictions and optimal control strategies. The performance of chiller (e.g., COP) and dedicated devices (e.g., operation variables) play very important roles in chiller power consumption. However, it is difficult to find proper model to predict accurate chiller COP chiller in practice. Moreover, how the operating variables of the chiller system affects the over power consumption need to be identified. This paper therefore aims to find out a simple and accurate COP prediction model for chiller and investigates the optimal values of operating variables related to the chiller system. By com-paring kinds of models (e.g., SL, BQ, MP, GNU, QHP and BP-ANN), BP-ANN is considered as the proper model in predicting COP of chiller accurately. By adopting GA-based operating variables identification, optimal values of the variables can be found in specified ranges as well. This study evaluates the performance of different COP prediction models by using actual operation data of chiller plant in a commercial building. Furthermore, optimal settings for operating variables have also been identified in resulting of minimum power consumption of chiller plant. [ABSTRACT FROM AUTHOR]

To prevent frosting of air source heat pump used for space heating in winter, heat-source tower heat pump has been developed for its advantages such as frost-free, stable and reliable. Due to the mass transfer process between air and solution, however, the solution can continue to absorb the moisture of air, which results in the decrease of solution concentration and the increase of freezing point. It’s very essential to regenerate the dilute solution to ensure the stability and reliability of the operation of heat-source tower heat pump. Therefore, in this paper, the experimental test of closed-type heat-source tower is built to study the effect of inlet parameters of three fluids on heat and mass transfer performance under low temperature condition. According to the experimental results, with the increase of air volume, flow rate of internally heating water, so does the solution regeneration performance of closed-type heat-source tower. [ABSTRACT FROM AUTHOR]

Conventional absorption refrigeration systems (ARSs), which are mainly driven by heat, have a competitive primary energy efficiency ( PEE ) compared with chillers driven by electricity. In the typical ARS, a large quantity of high-temperature heat is supplied into the generator, while a substantial amount of low-temperature heat is rejected to the environment from the condenser, it’s a huge waste. In order to decrease the input heat for generator and enhance the performance of conventional ARS, a kind of absorption-compression hybrid refrigeration system recovering condensation heat for generation (RCHG-ARS) was ever proposed. In the present study, the models of both RCHG-ARS and double-effect absorption refrigeration system (DEARS) are established, and the effects of different parameters on them are simulated and compared with each other. As a conclusion, the PEE of RCHG-ARS can be 29.0% higher than that of DEARS, and RCHG-ARS has a wider working conditions than DEARS due to the existence of the compressor. [ABSTRACT FROM AUTHOR]

Recent years have seen a significant increase in study on the thermal comfort offered by room air conditioners (RACs) when utilized as heaters, especially in the steady stage. Research indicates that convective terminals can form a comfortable environment using optimized control settings and strategies. Nevertheless, complaints about the thermal comfort experienced when using RACs persist, which is probably related to cognitive bias in psychology. The final seconds of an experience are given a lot of weight by the peak-end rule, which is typically characterized as the positive/negative peak. They were used to analyze how different heating experiences rated in terms of thermal comfort. Therefore, dynamic indoor temperature variations with six different terminals and control strategies were analyzed. Furthermore, an experiment with a thermal sensation, thermal comfort, thermal preference, and thermal adaptability vote (TSV, TCV, TPV and TAV) with different dynamic controlling characteristics was conducted in an artificial chamber. Three control methods with different peak moments were determined: slow start, interval downtime operation, and continuous heating. The comprehensive experience TCVs were 2.72, 2.28, and 1.12, and average-weighted TCVs were 1.94, 1.60, and 1.14, respectively. This exhibited a significant difference in terms of the heating process evaluation. In contrast, the TCVs based on the peak-end rule were 2.52, 2.31, and 1.11, respectively, which is more consistent with field evaluation. There is strong evidence that peak-end rule applies to TCV for air-conditioning systems. Other cognitive biases were also analyzed to evaluate the heating experience. Reasons for the thermal discomfort of RACs are also summarized. [Display omitted] • Cognitive bias existed in the comprehensive evaluation of the heating experience. • Peak-end rule was used to analyze experience thermal comfort of air conditioner. • Reasons for the thermal discomfort of RACs are summarized. [ABSTRACT FROM AUTHOR]

The flue gas of boiler contains large amount of waste latent heat, and traditional heat recovery device is difficult to recover the latent heat due to the limitation of the specific heat of supply air. A deep heat recovery device between flue gas and supply air of gas-fired boiler is proposed in the paper, and the thermodynamic model is established to calculate its performance. The energy efficiency of gas-fired boiler with the device is compared with other systems. The result shows that the boiler efficiency with new heat recovery device is 5% and 13% higher than that of condensing boiler and boiler without heat recovery system, respectively. When the ambient temperature is higher, the energy efficiency is closer to the boiler with absorption heat pump for heat recovery, and the initial investment is lower. Therefore, both the energy efficiency and economy of this system is acceptable. [ABSTRACT FROM AUTHOR]

The absorption heat pump (AHP) heating system is an efficient alternative to the conventional fuel-based heating systems, with energy saving rate reaching 20–50%. However, the coefficient of performance (COP) and heating capacity of normal AHP decreased obviously as the available driving source temperature dropped. Compression-assisted AHP (CAHP) could operate efficiently under lower driving sources, and an experimental prototype was constructed for performance investigation. At an evaporator inlet of −10 °C, as the generator inlet decreases from 130 °C to 115 °C, the COP changes from 1.442 to 1.271, while the heating capacity drops from 68.21 kW to 31.28 kW. At an increased evaporator inlet of −5 °C, as the generator inlet decreases from 130 °C to 110 °C, the COP changes from 1.511 to 1.103, while the heating capacity drops from 78.72 kW to 21.71 kW. Comparisons between CAHP and normal AHP indicated that CAHP can extend the lower limit of generator inlet temperature from 125–130 °C to 110–115 °C. Besides, CAHP can enhance the heating capacity by 96.4% even when AHP can operate normally at a generator inlet of 130 °C. Moreover, performance improvement contributed by CAHP is greater under lower-temperature driving sources. [ABSTRACT FROM AUTHOR]

Seasonal cold storage using natural cold sources for cooling is a sustainable cooling technique. However, this technique suffers from limitations such as large storage space and poor reliability. Combining seasonal storage with short-term storage might be a promising solution while it is not explored sufficiently. This paper presents a compound cold storage system that combines a heat pipe-based seasonal ice storage system with a chilled water storage system. The seasonal ice storage system automatically charges winter cold energy in the form of ice. In summer, the stored ice is extracted for cooling, and then the melting ice is used as a chilling medium for chilled water storage. Design optimization of the seasonal ice storage system and the compound storage system is addressed, including the sizes of heat pipes, the configuration and volume of the cold storage tank and the chiller capacity. A case study is conducted to demonstrate the design and the application of the proposed system in a real building in Beijing. Results show that the appropriate combination of the two types of cold storage can greatly improve the applicability of the seasonal cold storage and reduce the life-cycle cost of a building cooling system by 40%. [ABSTRACT FROM AUTHOR]

Seasonal cold storage is a high-efficient and environmental-friendly technique that uses the stored natural cold energy in winter (e.g., snow, ice or cold ambient air) for free-cooling in summer. This paper presents a seasonal cold storage system that uses separate type heat pipes to charge the cold energy from ambient air in winter automatically, without consuming any energy. The charged cold energy is stored in the form of ice in an insulated tank and is extracted as chilled water for cooling supply in summer, which help to reduce the chiller running time and reduce the associated electricity consumption and greenhouse gas emission significantly. A quasi-steady two-dimensional mathematical model of the system is developed for characterizing the dynamic performance of ice growth (i.e., cold charging). The model is validated using the field measurement data from an ice charging experiment conducted in Beijing. The impacts of various affecting factors, including the weather data and the key parameters of heat pipes, on the charging performance of the cold storage system are analyzed. The effectiveness and sustainability of the proposed system for cooling are demonstrated through a case study of a kindergarten building in Beijing. [ABSTRACT FROM AUTHOR]

Building energy systems consume increasing amounts of energy and emit many pollutants (e.g., NO x , PM 2.5 ) and CO 2 into the atmosphere. The problems of air pollution are very serious in China due to the very rapid urbanization and economic development over the past few decades and the fact that most of the primary energy comes from coal in China. In addition, China is facing serious indoor air quality problems caused by various pollutants with both indoor and outdoor origins. These pollutants have been found to be associated with certain diseases. The questions of increasing the energy efficiency of building energy systems and of how to effectively control temperature, humidity and pollutant concentrations in indoor air in an energy efficient way are two great challenges in the field of building energy and environment in China. Although engineering thermodynamics and heat and mass transfer are fundamental concepts for addressing building energy and environment problems, there are still some limitations in both theories: (1) the former can be used to optimize heat and work conversion processes, but it fails when used to optimize various heat and mass transfer processes and the processes of simultaneous control of indoor temperature, humidity and various pollutants; (2) the latter (including the entransy-based theory) can only be used for analysing or optimizing pure transfer processes. Based upon the inverse problem and variation method, a new approach is put forward for solving building energy and environment problems. Through illustrative examples we show that this new approach can overcome the limitations outlined above when addressing some building energy and environment problems. [ABSTRACT FROM AUTHOR]

Urban heating energy consumption in northern China accounts for 40% of total building energy consumption. In traditional central heating system, heat transfers from primary network (130°C/70°C) to secondary network (60°C/45°C) through heat exchangers in substations. In this paper, by introducing heat engine, heat pump and considering obtaining heat from environment, a new heat adaptor is built and the best system can be deduced out after thermodynamic optimization through extremum principle. The preliminary results indicate that when the return water temperature of primary network equals the ambient temperature, the heating capacity reaches the maximal value, which increases by 40% compared to absorption heat exchange system. Thus the thermal performance of existing heating systems can be evaluated. [ABSTRACT FROM AUTHOR]

The compression-assisted air source absorption heat pump (CASAHP) is a promising alternative heating system in severe operating conditions. In this research, parameter studies on the annual performance under various compression ratios (CRs) and source temperatures are performed to achieve the maximum energy saving rates (ESRs). Economic analyses of the CASAHP under different CRs and partial-design ratios are conducted to obtain an optimal design that considers both energy savings and economy improvements. The results show that the optimal CR becomes higher in colder regions and with lower heat source temperatures. For a source temperature of 130 °C, the optimal CR values in all of the cities are within 2.0. For source temperatures from 100 to 130 °C, the maximum ESR is in the range of 17.7–29.2% in the studied cities. The efficiency improvement rate (EIR) caused by compression in a severe source condition can reach 10.0–20.0%. From the viewpoint of economy, the relative investment of CASAHP is reduced to 30–60% with a CR of 2.0–3.0. With a 2–6% sacrifice in ESR, the payback period can be reduced from 12–32 to 5–9 years using compression. Partial-design of the CASAHP can further reduce the payback period to 3–6 years with a partial-design ratio of 50% and a CR of 2.8. Additionally, CRs and partial-design ratios are designed comprehensively by seeking the maximum ESR for a given acceptable payback period. [ABSTRACT FROM AUTHOR]

The low-temperature district heating system based on the air source absorption heat pump (ASAHP) was assessed to have great energy saving potential. However, this system may require smaller temperature drop leading to higher pump consumption for long-distance distribution. Therefore, the applicability of ASAHP-based district heating system is analyzed for different primary return temperatures, pipeline distances, pipeline resistances, supplied water temperatures, application regions, and working fluids. The energy saving rate (ESR) under different conditions are calculated, considering both the ASAHP efficiency and the distribution consumption. Results show that ASAHP system is more suitable for short-distance district heating, while for longer-distance heating, lower supplied hot water temperature is preferred. In addition, the advantages of NH 3 /H 2 O are inferior to those of NH 3 /LiNO 3 , and the advantages for warmer regions and lower pipeline resistance are more obvious. The primary return temperatures are optimized to obtain maximum ESRs, after which the suitable distances under different acceptable ESRs are summarized. To improve the applicability of ASAHP, the integration of cascaded heat exchanger (CHX) and compression-assisted ASAHP (CASAHP) are proposed, which can reduce the primary return temperature. The integration of CHX can effectively improve the applicability of ASAHP under higher supplied water temperatures. As for the utilization of CASAHP, higher compression ratio (CR) is better in longer distance, while lower CR is more advantageous in shorter distance. For a distance of 50 km, the maximum ESR is improved from 13.6% to 20.4–25.6% under different CRs. [ABSTRACT FROM AUTHOR]

Carnot cycle and theorem are major contributions of classical thermodynamics since proposed in 1824, while Carnot failed to prove it because of the influence of Caloric theory of heat. Then in 1850, Clausius confirmed Carnot theorem via reduction to absurdity and put forward the second law of thermodynamics. From then on, the theory that Carnot cycle is the one with the highest efficiency is generally accepted by scholars and regarded as the key premise or basis of thermodynamic analysis. However, few researchers care about how Carnot proposed the theory with the absence of the second law of thermodynamics. Can we get and prove Carnot theorem without the help of the second law of thermodynamics? In this paper, based on the polytropic process equations and p-v diagram, an analytical method of thermodynamics is developed and the cycle with the highest efficiency is deduced out only through extremum principle, instead of the second law of thermodynamics. The preliminary results indicate that Carnot cycle just serves as one form of the best cycles and only if the polytropic exponents ( n ) of the expansion and compression processes equal to each other, the efficiency of that cycle reaches the highest value, just that of Carnot cycle. This work gives a new insight into Carnot theorem and classical thermodynamics, which is of great importance in enlightenment for applying the new analysis method to address the complicated problems which are very difficult to address by using the second law of thermodynamics. [ABSTRACT FROM AUTHOR]

Abstract: Data centers and mobile phone base stations (MBS) are consuming more and more energy due to the development of the information communication technology (ICT) industry. The integrated air conditioner with thermosyphon (IACT) was proposed as an efficient cooling device for these spaces, combining vapor compression (VC) refrigeration with a thermosyphon (TS). In this paper, a model to simulate the annual energy consumption of IACT is built and compared with the traditional air conditioner (TAC) in four standard MBSs located in the typical climate zones of China. The results show that the IACT saves 19.1%–28.2% more energy than the TAC, and in hotter regions has a larger energy saving amount but a smaller energy saving rate (ESR). The TS mode works effectively when the mean monthly temperature is 5.4 °C–24.5 °C. The monthly ESR of IACT in the MBSs of Beijing and Guangzhou is 12.2%–48.6% and 12.1%–45.2% respectively. [Copyright &y& Elsevier]

Highlights: [•] We propose a new concept, heat adaptor, to optimize thermal system performance. [•] It is a combination of heat engine, heat pump and conventional heat exchangers. [•] The most suitable device and process can easily be identified using the concept. [•] A typical problem is used to demonstrate the advantages of applying the concept. [Copyright &y& Elsevier]

Highlights: [•] A double-stage coupled air source absorption heat pump (ASAHP) is proposed. [•] The coupled ASAHP exhibits stable and high performance in very cold regions. [•] Energy-saving rate of the coupled ASAHP in all the typical cities is above 20%. [ABSTRACT FROM AUTHOR]

In recent years, it has been observed that room air conditioners require considerable amounts of electricity. In this study, the annual performance and applicability of a hybrid air conditioner combining a fresh air ventilator and direct evaporative cooling water spray system were evaluated. The direct ventilation temperature degree-hour (TDH), evaporative cooling enthalpy degree-hour (IDH), and hybrid temperature degree-hour (TIDH) were defined to represent the applicability of natural energy. Meteorological data in China was used to determine the natural source utilization potential for direct ventilation and evaporative cooling. An applicability analysis was conducted in 334 cities in China. To further analyze the energy-saving potential, a simulation model was established and verified for the hybrid air conditioner based on laboratory tests. The operation periods with different work modes, energy-saving rates, and energy efficiency values were considered for three typical cities, i.e., Nanjing, Beijing, and Lanzhou, representing wet, semi-wet, dry, and semi-dry zones, respectively. Considering the extra energy consumption of the ventilator, the total energy consumption was reduced in these cities by 6.4%, 22.6%, and 50.2%, respectively, and the system seasonal energy efficiency ratios (SEERs) improved by 6.9%, 29.3%, and 98.9%, respectively. Finally, the energy-saving potential, operation ratios, and absolute energy-saving values were calculated and analyzed for 31 provinces in different working zones. The results indicate that the proposed system has the best applicability and energy-saving potential in northern China. • A simulation model was established for a hybrid air conditioner. • An applicability analysis was conducted in 334 Chinese cities. • Energy consumption was reduced by 6.4, 22.6, and 50.2% in three typical cities. • The energy-saving potential in 31 provinces was calculated and analyzed. [ABSTRACT FROM AUTHOR]

With the widespread use of variable refrigerant flow (VRF) systems in various commercial and residential buildings, the performance and efficiency of VRF systems have attracted considerable attention in recent years. Although the compressor set energy conservation (CSEC) method exhibits a high accuracy in most cases of VRF performance measurement, several problems limit its accuracy. To improve the accuracy of the CSEC method for performance measurement, two methods are proposed to resolve the measurement difficulty of two-phase suction. The accuracies of the proposed methods are experimentally validated using three VRF configurations. In the experiments, the relative errors for the compressor isentropic efficiency-based method range from ±15%, ±17%, and ±18% for the three configurations. For the compressor volumetric efficiency-based method, the relative errors of the three configurations range from ±15%, ±16%, and ±16%, respectively. The capacity distribution and heat loss of the VRF system are measured and analyzed in the experiment. Heat losses from closed indoor units range from 0% to 21.5% in different cases in the cooling mode and from 0% to 31.1% in the heating mode. Additionally, an estimation method for the temperature measurement deviation is proposed to improve the precision of performance measurement. • Two methods are proposed to resolve the measurement difficulty of two-phase suction. • The accuracies of the proposed methods are experimentally validated within 18% and 16%. • The heat losses of VRF are measured and analyzed. [ABSTRACT FROM AUTHOR]

The main factors affecting field thermal comfort of convective terminals during the heating season have been widely studied in recent years. Since previous studies were conducted separately, this investigation aimed to analyze the thermal comfort under typical settings and integrated control strategies. A questionnaire, based on a field operation, was conducted to compare and find the main characteristics of different terminals. The contradiction of draft and vertical temperature difference and the continuous/intermittent feature control were the key factors investigated. The indoor environment distribution under different conditions was measured in laboratory cabins based on the different settings and control strategies of convective terminals. The thermal sensation vote (TSV) and thermal comfort vote (TCV) were conducted in an artificial chamber to show the characteristics of an indoor environment under different conditions. The results showed that people were more tolerant of drafting than vertical temperature differences (the TCV score decreased from 0.44 to 1.42 with different temperature non-uniformity and airspeed). In particular, the subjects' perceived control significantly reduced the risk of discomfort. In addition, there was no significant difference between the different control strategies, including continuous operation and intermittent operation, except for a long-term control shutdown. The TCV result was 0.48 in the on-off control with a 5-min shutdown, showing statistical significance in paired-sample t -tests (p < 0.05). • Terminals are compared by questionnaire, field test, and artificial chamber. • People are more tolerant to draft than to vertical temperature differences. • There is no significant difference in environment by various control strategies. [ABSTRACT FROM AUTHOR]

Abstract: Spaces with high heat density such as mobile phone base station (MBS) and data center increase fast recently. Huge energy is consumed for the air conditioning because of large indoor heat rejection and year round cooling in these spaces. The integrated air conditioner with thermosyphon (IACT) combines the vapor compression (VC) refrigeration and thermosyphon (TS) in one body, which can save energy and space for these spaces. After analyzing the problems in the existing IACT, a new IACT is proposed by using the novel heat exchangers and the self-operated three-way valve (STV), which works well in both VC and TS modes. The 3HP air cooled IACT is developed and tested in laboratory. The COP of TS mode reaches 9.43 when outdoor temperature is 20 °C lower than indoor, and the COP of VC mode reaches 2.94 under standard condition. This IACT has been applied in MBSs of different cities in China and the energy consumption is tested on-site and compared with the traditional air conditioners (TAC). It is shown that the IACT can save more than 30% energy compared with TAC. [Copyright &y& Elsevier]

Abstract: Gas refrigerant injection has been proven as an effective method to improve the performance of the scroll compressor and its refrigeration system under high compression ratio working conditions. Much research on the injected scroll compressor and its system has been conducted, but the universal control and design method is still lacking. A model of the refrigeration system with a gas-injected scroll compressor is developed in this paper. With this model, the effects of gas injection on the system and component parameters are investigated. Based on the identified evaporator characteristics and thermodynamic analysis, a set of general principles for the design and operation of the refrigeration or heat pump system with a gas-injected scroll compressor is proposed. [Copyright &y& Elsevier]

In this study, a hybrid air conditioner was proposed and investigated, based on combining a fresh air ventilator and direct evaporative cooling water spray system. Relative to a previously developed air conditioner using fresh air, the proposed air conditioner could be controlled more efficiently controlled through control strategies based on the outdoor environment. In this study, four zones with different operation strategies were considered, including direct ventilation (Zone 1), evaporative cooling ventilation (Zone 2), high efficiency direct ventilation (Zone 3), and off ventilation (Zone 4). A 1.5-HP room air conditioner was integrated with a ventilator, and the design principle and ventilator selection was illustrated. To compare the performances of conventional and hybrid air conditioners, a laboratory was used to simulate a typical day with an ambient environment and indoor cooling loads. One day at the beginning of the cooling season in three typical cities (Nanjing, Beijing, and Lanzhou, representing wet, semi-wet, and semi-dry/dry zones, respectively) were selected to conduct the field tests. The results show that the hybrid air conditioner exhibits high energy efficiency and energy-saving potential. Compared with conventional air conditioners, the total cooling capacity shows decreases ranging from 27.9% to 69.1% for the three cities. And the seasonal energy efficiency ratio test shows improvements of 12.3%, 38.5% and 221.9% in Nanjing, Beijing, and Lanzhou, respectively, indicating high operation efficiency in the cooling season. • A hybrid air conditioner combining ventilator and evaporative cooling is proposed. • Typical day and SEER tests were carried out under three typical conditions. • Operation period of mechanical refrigeration decreased from 32.5% to 56.1%. • Total cooling capacity of mechanical refrigeration decreased from 27.9% to 69.1%. • SEER improved by 12.3%, 38.5% and 221.9% respectively. [ABSTRACT FROM AUTHOR]

Abstract: A simulation model of complex refrigeration system based on two-phase fluid network is developed in part I of the paper [Shao, S.Q., Shi, W.X., Li, X.T., Yan, Q.S., 2008. Simulation model for complex heat pump systems based on two-phase fluid network – part I: model development. International Journal of Refrigeration 31 (3), 490–499.]. One same fluid network is used in the model to describe the refrigeration system in different operating modes with the method of the factual and fictitious branches. When there are many heat exchangers with concurrent cooling and heating, the traditional models are not able to define the flowing directions. The developed model, however, whether the initial flowing direction of the connecting branches is right or not, is able to predict the flow and heat transfer. Three typical complex refrigeration systems such as the multi-unit inverter air conditioner, heat pump with domestic hot water and multi-unit heat pump dehumidifier are simulated with the developed model as demonstrations on how to use it. The model can evaluate the influences of one or more parameters on the system performance, which can be used for the optimization of the system by all-condition performance analysis. It is shown that the two-phase fluid network model is effective and convenient for the simulation of different complex refrigeration system, especially for performance analysis, system evaluation, and optimization based on the performance prediction. [Copyright &y& Elsevier]

Abstract: Some complex refrigeration and heat pump systems with several condensers and evaporators have been developed for different kinds of application. Traditional simulation models were developed for systems in certain operating modes and they failed in modeling the complex refrigeration systems with uncertainties of heat exchangers function and refrigerant flowing direction. In order to predict the performance of complex refrigeration systems, a simulation model is presented based on the two-phase fluid network. The model is consisted of distributed-parameter model of heat exchangers and connecting tubes, map-based model of inverter compressor and electronic expansion valve (EEV). Based on the characteristic of refrigeration system and fluid network, the three conservation equations, i.e. energy, momentum and mass equations, are solved iteratively. This model can deal with the uncertainty of refrigerant flow direction by separating the solving process of the components and the fluid network model, and therefore can simulate different kinds of complex refrigeration systems in different operating modes and conditions. The model is validated by the experimental data of an inverter air conditioner in heating/cooling operating modes and it shows the error of the model is mainly determined by the error of submodels of components in calculating heat transfer and pressure loss. The model is applied for performance analysis of three kinds of complex refrigeration systems in the accompanying article [Shi W.X., Shao, S.Q., Li, X.T., Yan, Q.S., 2008. Simulation model for complex heat pump systems based on two-phase fluid network: part II – model applications, International Journal of Refrigeration 31 (3), 500–509.]. [Copyright &y& Elsevier]

Variable speed control of compressors is one of the best methods to regulate the capacity of heat pumps and air conditioners. An analysis is conducted for modeling the variable speed compressor for simulation of inverter air conditioner and heat pump. Having scattered the real operation performance of inverter compressor into infinite operation performance of constant speed compressor, the map-based method is utilized to fit the performance curves of inverter compressor. The model is built at the basic frequency and the map condition as the second-order function of condensation temperature and evaporation temperature. Then it is corrected by the compressor frequency as the second-order function of frequency and by the actual operating condition as the actual specific volume of the suction gas. This method is used to set up simulation models of three different compressors. Compared with the data provided by the compressor manufacturers, the average relative errors are less than 2, 3 and 4% for refrigerant mass flow rate, compressor power input and coefficient of performance (COP), respectively. This model of variable speed compressor is suitable for the simulation of inverter air conditioner and heat pump systems. Based on the experimental data and simulation model, the frequency at zero mass flow rate and power input at zero frequency are discussed and the relation between COP and compressor frequency is analyzed. [Copyright &y& Elsevier]

This paper presents a new scheme of an inverter air cooling heat pump system with domestic hot water. A water reheater is placed between the compressor outlet and the four way valve inlet to utilize the sensible heat of the superheated gas exhausted from the compressor, and a water preheater is placed between the condenser and the throttling device to use the sensible heat of the subcooled liquid flowing out of the condenser. With these two parts of heat, the domestic hot water can be heated to a temperature high enough for domestic use. In order to maintain the system efficiency in the period of part load, an inverter compressor is adopted as the substitute for the constant speed one used in the conventional heat pump system. A hot water storage tank with a circulation pump is placed in the system to reduce the peak load of the system. Compared with the traditional system, this new design is able to reduce energy consumption by 31.1% and decrease thermal pollution to the environment. [Copyright &y& Elsevier]

This paper presents an experimental study of a novel household ventilator with exhaust air heat pump enhanced by indirect evaporative cooling. In this study, a prototype was made according to this theory and tested on the test rig. The test was carried out to evaluate the energy performance of the system during the cooling season. During the test, the thermal performances of the prototype under four different operating modes were comprehensively studied. In the typical summer conditions, the prototype operating as an indirect evaporative cooler followed by an evaporative condenser heat pump shows the highest cooling capacity at 4.27 kW and energy efficiency ratio (EER) at 5.12, which is a great performance improvement compared with that of conventional operating methods. The results of variable conditions tests show that this system can operate efficiently under different summer ambient conditions with the EER varying from 4.16 to 5.65. Besides, a comparison with other prototypes found in existing literature was also made in this study. Give consideration to compact and energy efficiency, the novel ventilation system shows a good academic value and application prospect in energy saving air-conditioning domain. [ABSTRACT FROM AUTHOR]

Recovering waste heat can improve the energy efficiency of a combined heat and power plant. The pinch method is widely used in heat recovery systems; however, the minimum energy grade of the remaining heat demand cannot be determined using this method. In this study, a method for discretizing thermal energy for the heat sources and demands is proposed and illustrated with a detailed description. The proposed method can maximize heat recovery and minimize energy grade requirements. Different heat transfer temperature differences could be considered in the proposed method. Two different optimized heat recovery systems for a combined heat and power plant are developed based on the novel and pinch methods. The amount of heat recovered using the optimized systems are the same, with values 2.08% higher than that of the conventional heat recovery systems. Compared with the pinch method based optimized system, the energy grade of the remaining heat demand could be reduced from 61.9 °C to 58.2 °C using the proposed method. • Method of discretizing thermal energy for heat sources and demands is proposed. • An optimization method to match heat sources and demands is established. • Two heat recorvery systems are developed based on the proposed method. • The proposed method has the same heat recorvery with pinch method. • Energy grade of remaining heat demand is lower than that with pinch method. [ABSTRACT FROM AUTHOR]