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• Fast and accurate prediction of high-resolution 3D air distribution are achieved. • The two ANNs respectively use one-step prediction and two-step prediction strategies. • Two-step prediction ANN could be helpful for improving prediction accuracy. • The method can be used for real-time HVAC control targeting occupant areas. To meet the thermal comfort requirements of room occupants, a fast and accurate method for predicting indoor high-resolution 3D airflow distribution is necessary, which can be combined with heating, ventilation, and air conditioning (HVAC) systems to adjust the indoor environment. Artificial neural networks (ANN) can establish complex mappings between variables with nonlinear relationships. The aim of this study was to verify the feasibility of an ANN for the fast and accurate prediction of indoor 3D airflow distribution. Considering two prediction strategies, ANN A and B were constructed. The inputs of both ANNs were the air supply velocity, air supply direction, and section location. ANN A output the airflow distribution of the entire room directly, whereas ANN B output the airflow distribution of the characteristic sections first and then the airflow distribution of other sections according to the contribution of the characteristic sections. The two ANNs' performances on training and testing datasets were compared. Both ANNs performed well in the training and interpolation cases. However, their performances decreased rapidly in the extrapolation cases. In the case of a large gap between the input and output dimensions of the ANNs, the two-step prediction strategy improved the prediction accuracy, accelerated convergence, and alleviated data imbalance problems. The prediction time of ANN A and B was 0.89 s and 12.38 s, respectively. The ANN can predict indoor high-resolution 3D airflow distribution quickly and accurately, and can add functions that include identifying internal heat sources and air supply inlet position changes. ANN combined with HVAC could allow real-time control to regulate indoor airflow. [ABSTRACT FROM AUTHOR]

Liquid desiccant dehumidification, a clean energy technique for creating comfortable environments has received a lot of interest. Several dynamic models have been proposed to predict the outlet air humidity ratio and temperature, however, a study on presenting variations of heat and mass transfer coefficients and thermal mass of desiccant during the dynamic dehumidification process was not encountered according to authors' best knowledge. This gap is primarily due to the complex nature of the coupled heat and mass transfer processes, which make it challenging to capture the evolution of heat and mass transfer coefficients over time. In this study, variations of these parameters are analyzed at first. A theoretical study is conducted to show the thermal mass of the desiccant can be considered constant when changing the inlet air humidity ratio or desiccant temperature, simplifying the dynamic modeling process. While the variation of heat and mass transfer coefficients show significant impact in such conditions. Subsequently, a new method based on mass conservation is provided to determine the thermal mass of desiccant. By examining the cause-and-effect relationships between heat and mass transfer coefficients and outlet air humidity ratio, the evolution of heat and mass transfer coefficients following exponential laws is deduced by inversion. Then a dynamic model of packed dehumidifiers can be completed easily based on above derivations, and its viability is confirmed by experimental results. Compared to the existing method, the prediction accuracy can improve by 50% based on the proposed methods. This study characterizes evolution rules of heat and mass transfer coefficients over time, and with this, evolutions of outlet air parameters during the dynamic dehumidification process can be acquired conveniently based on the existing steady dehumidification model. • Variations of heat and mass transfer coefficients and thermal mass of desiccant are analyzed. • Simple methods are proposed to figure out the evolutions of thermal masses, and heat and mass transfer coefficients. • The prediction accuracy of air outlet humidity ratio could improve by 50% based on the proposed methods. [ABSTRACT FROM AUTHOR]

Humidity is an important factor that influences both thermal comfort and indoor air quality. Air-conditioning (A/C) systems in large-scale buildings can employ different equipment to control temperature and humidity independently. However, A/C systems commonly seen in small- and medium-sized buildings have no specific device to deal with moisture due to space limitations, which may leave humidity in these buildings without regulation. This paper summarizes two methods applied to control temperature and humidity in these buildings using A/C systems. One is to modify the structure of A/C systems or to introduce an additional dehumidifying equipment that is adaptive in small- and medium-sized buildings to decouple temperature and humidity control loops. The other is to develop advanced control algorithms to realize simultaneous temperature and humidity control in a variable speed direct expansion air-conditioning unit. Principles, merits, and obstacles of these two methods are presented. Since the paths for development of the two methods were totally separated before, potential benefits of energy saving and better control performance brought by combining the two methods are discussed. It is expected that this paper could promote relevant studies and applications. [ABSTRACT FROM AUTHOR]

Indoor air humidity is an important parameter for building up a thermally appropriate artificial indoor environment. However, introducing air humidity as a controlled parameter in addition to air temperature would significantly increase the difficulty to develop a control-oriented model for building air conditioning systems. This is particularly true for direct expansion (DX) air conditioning (A/C) systems, whose operational parameters are highly coupled and behave non-linearly and influenced by the controlled parameters. Neither physical modeling approach nor artificial neural network (ANN) modeling approach could solely satisfy the requirement, in terms of accuracy and sensitivity, for simultaneous control of air temperature and humidity using a DX A/C system. In this paper, a hybrid modeling approach is proposed and validated, which uses the physical modeling approach to simulate the performance of evaporator for accurately catching the cooling and dehumidification processes under various working conditions and uses ANN to simulate all other components of a DX A/C system for reduced calculation efforts. By such a hybrid modeling approach, the advantage of simplicity of an ANN-based sub-model could be utilized and the disadvantage of it that do not allow to accurately extrapolate beyond the range of the data used for training/estimating the model parameters could be avoided. [ABSTRACT FROM AUTHOR]

Air source heat pump units have found their wide applications in recent decades due to their high efficiency and low environmental pollution. To solve their undesired frosting problem, reverse cycle defrosting is always employed. As a transient and nonlinear heat and mass transfer procession, defrosting performance directly affects the occupants’ thermal comfort. During defrosting, the metal energy storage values of indoor and outdoor coils are varied as their temperature fluctuations. It is therefore necessary to investigate the energy transfer procession in an air source heat pump unit and the effect of metal energy storage during defrosting. However, scarce of attentions were paid to this fundamental problem. In this study, two experimental cases with two-working-circuit and three-working-circuit outdoor coils were conducted basing on frost evenly accumulated on their surfaces. After four types of energy supply and five types of energy consumption during defrosting were calculated, a qualitative and quantitative evaluation on the metal energy storage effect was then given. As concluded, after the outdoor coil enlarged 50%, the metal energy storage effect can be changed from positive (0.33%) to negative (−2.18%). The percentages of energy consumed on melting frost and vaporizing retained water were both increased. Defrosting efficiency was improved about 6.08%, from 42.26% to 48.34%. Contributions of this study can effectively guide the design optimization of indoor and outdoor coils and promote the energy saving for air source heat pump units. [ABSTRACT FROM AUTHOR]

For a fishing vessel, its diesel engine’s energy efficiency is only at 35–40%, with more than half of the energy being wasted as exhaust heat taken away by jacket water, cooling air and exhaust gas. Fishing vessel refrigeration systems driven by the exhaust heat from engines can therefore help achieve energy saving. However, to improve the COP/EER of these heat driven refrigeration systems and to ensure their operational stability under severe conditions on ocean are of challenges. In this paper, the progress and prospect of utilizing three different kinds of heat driven fishing vessel refrigeration systems, i.e., adsorption refrigeration system, absorption refrigeration system and ejection refrigeration system, are reviewed with a special focus on the techniques for improving system efficiency and stability. A hybrid heat driven refrigeration system, which combines merits of different types of systems, is then introduced. A summary table is provided to summarize and compare the features of adsorption refrigeration systems, absorption refrigeration systems and ejection refrigeration systems used in fishing vessels, followed by conclusions and suggestions for future works. [ABSTRACT FROM AUTHOR]

Liquid desiccant dehumidification technology as an efficient way of handling moisture load has attracted much attention, while the performance of liquid desiccant dehumidifiers has characteristics of uncertainty and time-varying in actual runtime, limiting its application fields. Thus, it is necessary to develop dynamic control methods to achieve stable and efficient operation. As the liquid desiccant dehumidification process is a multi-parameter, time-varying and large delay nonlinear dynamic process, therefore it is difficult to achieve the dynamic control. In this paper, dynamic control methods of the liquid desiccant dehumidification process were studied experimentally for the first time and main difficulties in the dynamic control of the liquid desiccant dehumidification process were presented clearly, helping to learn more knowledge on dynamic control process and facilitate the development of control methods. PID control as a classic control method was applied to control the liquid desiccant dehumidification process firstly, the result showed that it requires long adjusting time to follow the command and is not suitable to control nonlinear dynamic process with large delay. Novel fuzzy logic control (FLC) as an advanced control method was implemented as well, the result demonstrated that it follows command quickly while is unable to maintain the new set point accurately. Then reasons for shortcomings of each control method were analyzed. Along with the above analysis, a new control method, combining the PID control and novel FLC was developed. The command following experiment and disturbance rejection experiment for the newly developed control method were conducted. Control performances are great and validate the feasibility of the new control method. Besides, the newly developed control method also provides a good reference for achieving the dynamic control of a multi-parameter, time-varying, nonlinear dynamic process with large delay. [ABSTRACT FROM AUTHOR]

A novel Proportional-Derivative (PD)law based Fuzzy Logic Controller (PFC) for a variable speed (VS) direct expansion (DX) air conditioning (A/C) system has been developed. There were two coupled control loops in this controller, i.e., varying supply fan speed to control indoor dry-bulb temperature ( T db ), and compressor speed indoor wet-bulb temperature ( T wb ). To weaken the coupling effect between the two loops, fuzzy logic principles were deployed. Furthermore, a PD law was used instead of a Proportional-Integral-Derivative (PID) law, in the PFC, which helped simplify not only calculation but also the structure of the PFC. The controller developed was validated by carrying out the controllability tests with the experimental conditions covering the normal operational range of a VS DX A/C system. The experimental results of the controllability tests suggested that the novel PFC developed is capable of realizing the simultaneous control of indoor temperature and humidity satisfactorily, in terms of control accuracy and sensitivity. [ABSTRACT FROM AUTHOR]

Researches on packed liquid desiccant dehumidifiers are abundant. However, only a few works concentrate on modeling the dynamic dehumidification process as it requires detailed heat and mass transfer coefficients along the dehumidifier, which are impacted by many factors such as flow pattern of liquid desiccant and hard to be obtained, hindering the development of dynamic models. Data driven modeling method could eliminate the requirement for detailed heat and mass transfer coefficient along the dehumidifier. A time delay neural network (TDNN) model is a general way to describe the dynamic process without the requirement of much knowledge on physical mechanism, yet the number of time steps for each inlet parameter is hard to be determined. In this paper, an improved time delay neural network (iTDNN) model was proposed. Through analyzing characteristics of dehumidification process, model inputs were simplified, which is the core of improvement of TDNN model. Besides, a new combined algorithm including back propagation (BP) algorithm and improved genetic algorithm (GA) was proposed to acquire the optimal weight values globally. The predicted results agree well with the experimental data for validation part. The method presented in the paper provides a reference for modeling multi-variables and large delay nonlinear dynamic systems. [ABSTRACT FROM AUTHOR]

Highlights: [•] A state steady model based controller (SSMBC) was developed for a DX A/C system. [•] The SSMBC could simultaneously control indoor air temperature and humidity. [•] A theoretical analysis of the control process was carried out to reveal a new control mechanism. [•] The SSMBC was combined with a stabilizing controller to form a hybrid controller. [•] The accuracy and sensitivity of the novel hybrid SSMBC are experimentally validated. [Copyright &y& Elsevier]

Abstract: A modified McQuiston model for evaluating the wet fin efficiency of cooling and dehumidification coils has been developed by modifying the existing popular McQuiston model and is reported in this paper. The condensate film moving on fin surfaces and its impacts on heat transfer have been taken into account in deriving the governing equation for fin temperature, and consequently, the enthalpy change of the moving condensate film has been included in the governing equation for fin temperature. The modified McQuiston model was validated by comparing its predictions with those using the McQuiston model under the same operating and boundary conditions. It is expected that the modified McQuiston model developed can be applied not only to the commonly seen air cooling and dehumidification but also to certain special operations such as industrial steam condensation. [Copyright &y& Elsevier]

Abstract: This paper presents an experimental study to develop a new control algorithm to replace the traditional on–off control algorithm widely used by direct expansion (DX) air conditioning (A/C) systems, which are widely used in small to medium sized buildings. The new control algorithm, which enables both the compressor and the supply air fan in a DX A/C system to operate at high speeds when the indoor air dry bulb temperature setting is not satisfied and at low speeds otherwise, is termed the H–L control. Extensive experiments have been conducted with an experimental DX A/C system at simulated different climates under both H–L and on–off control. The results suggest that the use of H–L control would result in better control performance in terms of an improved indoor humidity level and a higher energy efficiency of the DX A/C system when compared to the use of the traditional on–off control. The H–L control is also simpler, and its associated hardware cost is much lower than that of the control algorithms based on variable speed technology. [Copyright &y& Elsevier]

Abstract: This paper presents a study of condensate retention on a louver-fin-and-tube air cooling coil, which is commonly used in air conditioning (A/C) systems. Compared to previously related work focusing on the influence of condensate retention on the heat and mass transfer between air and a cooling coil, the present study emphasizes the impacts of operating parameters on condensate retention on a cooling coil. A new method to describe the steady-state condensation has been suggested and a new mathematical model to represent the force balance of retained condensate developed. The mass of condensate retained has been measured experimentally under various operating conditions of a direct expansion (DX) air cooling and dehumidification system. The influences of air dry-bulb temperature, moisture content and Reynolds Number on condensate retention are discussed. The model developed relates the mass of condensate retained to condensing rate, and is successful in predicting the trends of condensate retention under normal operating conditions for air cooling applications. [Copyright &y& Elsevier]

Both temperature control and humidity control are important in buildings. However, usually only temperature is controlled in small and medium-sized buildings using direct expansion (DX) air-conditioning (A/C) systems, while dehumidification is a by-product of cooling, and indoor humidity is in an uncontrolled state. Weights-based fuzzy logic control algorithm (WBFLCA) is a novel fuzzy control algorithm that can be applied to DX A/C systems with good simultaneous temperature and humidity control performance. However, the setting of weight values for WBFLCA is still highly dependent on expert experience and lacks a systematic optimization theory, which hinders the application of WBFLCA. In order to establish a weights setting rule for WBFLCA used in DX A/C systems, a coupled control simulation model is built in this paper to simulate the effect of simultaneous control of temperature and humidity with all 16 basic weights distribution mode combinations. All the 16 combinations are also experimentally tested on an experimental DX A/C system to reveal the control performance of different combinations in a real system. Four of them with good control performance are found. The reasons why these four combinations are good are also analyzed in this paper. Based on the simulation and experimental results, the weights setting rule of WBFLCA applicable to the simultaneous temperature and humidity control of DX A/C systems are summarized. The approach proposed in this paper of finding appropriate pattern combinations from the basic weights distribution patterns has the potential to be applied to solve other decoupling control problems as well. [ABSTRACT FROM AUTHOR]

Liquid desiccant dehumidification systems show significant energy-saving potential in HVAC applications. The dynamic characteristics of a liquid desiccant dehumidifier attract increasing attentions in recent years, which are essential for designing appropriate controllers for it. However, due to the complicate coupling of heat and mass transfer inside the dehumidifier, there are few models have been developed for predicting the dynamic characteristics of a packed liquid desiccant dehumidifier. In this paper, an online model was proposed, which predicted the dynamic response of a packed dehumidifier mainly based on the air outlet humidity ratio rather than the inlet parameters of the dehumidifier. The online model was derived from the heat and mass transfer principle and can be described by an exponential function. The dynamic response can be divided into two stages taking the maximum air outlet humidity ratio changing rate as a dividing point. The two unknown parameters (the maximum air outlet humidity ratio changing rate and the time coefficient) in the online model were determined by the first stage of dynamic response, and then the online model predicted the dynamic response of the packed dehumidifier at the second stage. The online model agreed well with the experimental data, casting a new light on learning the internal relation of the dynamic processes of a packed dehumidifier. Furthermore, the relation between the variation of inlets and dynamic response of air outlet humidity ratio could be predicted quickly using the proposed model, contributing to control algorithm development. And a preliminary idea about how to construct an appropriate controller based on the proposed model is presented, which provides a reference for further study. [ABSTRACT FROM AUTHOR]

The applicability of common defrost technologies depends on how easy it is for air source heat pumps to frost under local climate conditions. At the same time, the degree of frost formation of the air source heat pumps outdoor units are mainly affected by local climatic conditions. Therefore, this study proposes the concept of frosting degree hours, which is used to specifically characterize the impact of climatic conditions on air source heat pumps frost. In order to make the analysis and calculation of the frosting degree hours more accurate, this study carried out the collection of the actual operation data of the air source heat pumps in winter in many cities in China. Through the selection and analysis of the actual operating data, the data fitting method was used to analyze the different variables and the characteristic temperature difference between the evaporation temperature and the outdoor dry bulb temperature of systems with different power types was obtained. Finally, according to the analysis of the frosting degree hours under different characteristic temperature differences, combined with the frost characteristics of air source heat pumps of different power types in 275 cities in China, the climatic division was performed. This can provide a reference for the climate-adaptive design of air source heat pumps and the development of defrost control strategies. [ABSTRACT FROM AUTHOR]

As the requirement for Li-ion battery thermal management system (TMS) in electric vehicles (EVs) rises, an integrated direct cooling thermal management system with high compactness and energy efficiency has been developed. The integrated system is based on a vapor-compression refrigeration cycle, forming a dual-evaporator system, and adopts a direct cooling scheme on the battery side, which brings high coupling effects. A novel decoupling control strategy was proposed and a weight-based fuzzy logic control algorithm was applied to implement the strategy, achieving effective temperature control and reasonable refrigerant distribution between the battery side and cabin side. Experiments conducted under various scenarios demonstrated that the average temperature of both the battery pack and cabin could be stably controlled around different target values. Maximum fluctuations were effectively limited to less than 0.7 °C in different scenarios. The implementation of this decoupling control strategy has opened up new possibilities for the application of direct cooling TMS in multi-target systems. [ABSTRACT FROM AUTHOR]

In small to medium sized buildings, direct expansion (DX) A/C systems are extensively used. Multi-variable controllers (MVCs) have been developed for variable speed (VS) DX A/C systems to control indoor air temperature and humidity simultaneously. Based on the previous extensive research work on the inherent operational characteristics of DX A/C systems, an alternative simpler approach was applied, by developing a fuzzy logic controller (FLC) for simultaneous indoor air temperature and humidity control, using a VS DX A/C system. In this paper, the developments of the FLC are presented. Firstly, a detailed description of the algorithm for the FLC is given. This is followed by detailing an experimental VS DX A/C system where the FLC was tested. Finally, the results of controllability tests for the FLC are reported. Controllability tests results demonstrated that the FLC developed can achieve the simultaneous control over indoor air temperature and humidity, with a reasonable control accuracy and sensitivity. [ABSTRACT FROM AUTHOR]

Conventional chilled water based air conditioning systems use low temperature chilled water to remove both sensible load and latent load in conditioned space, and reheating devices are usually installed to warm the overcooled air, which leads to energy waste. Alternatively, this paper proposes a neural network (NN) model based predictive control strategy for simultaneous control of indoor air temperature and humidity by varying the speeds of compressor and supply air fan in a chilled water based air conditioning system. Firstly, a NN model has been developed to model the system dynamics, linking the variations of indoor air temperature and humidity with the variations of compressor speed and supply air fan speed. Subsequently, the NN model is experimentally validated and used as a predictor. Based on the NN model, a neural network predictive controller is proposed to control the indoor air temperature and humidity simultaneously. The experimental results demonstrate the effectiveness of the proposed scheme compared with conventional PID controllers. Moreover, it has been proven that it is practical to simultaneously control indoor air temperature and humidity by varying the compressor speed and the supply air fan speed without adding any other devices to the chilled water based air conditioning systems. [ABSTRACT FROM AUTHOR]

Conventional drying processes discharge high humidity gas to the atmosphere. The exhaust gas contains large amount of energy. The direct discharging would result in relatively large energy waste. In order to improve the thermal efficiency of drying process, in this paper, a new open absorption heat pump system was proposed, which aims at recovering the latent heat from exhausted moist gas and producing steam for reutilization. The working principle was discussed in detail and thermodynamic models were established to analyze the performance of the new system. The new system can work under both single-stage and double-stage modes. Simulation results showed that the new system could utilize a heat source with lower generation temperature compared with that utilized by a traditional open absorption system. The temperature range of heat source for the double-stage mode is 130–160 °C, and that for the single-stage mode is 160–175 °C. The new system also eliminates the limitation of traditional close absorption system, whose evaporation temperature has to be lower than the dew point temperature of discharged moist gas to recover the latent heat of water steam. Simulation results also indicated an improved COP h of the new system compared with that of double-stage close absorption heat pump system. The COP h of the new system varied from 1.52 to 1.97 and the efficiency of heat recovery varied from 15.1% to 54.8% when the temperature of heat source varied from 135 °C to 175 °C and saturated steam of 100 °C was produced. [ABSTRACT FROM AUTHOR]

Different climate characteristics put forward different requirements for the performance of air conditioning system. Climatic division has been proposed to facilitate the selection or design of HVAC systems to match indoor cooling/heating loads of buildings better. However, humidity parameter and distinction of sensible and latent loads are usually ignored, bringing problems to the operation and control of HVAC systems. Therefore, this study proposed a novel climatic division method for China during the cooling period taking the coupling effect of cooling and dehumidification of HVAC systems into account. The humidity-based cooling degree-day was firstly taken as an index to evaluate the influence of outdoor humidity parameter. Then, 96 cities, whose cooling loads are particularly affected by the humidity, were selected for further analysis. Indoor cooling loads were calculated using EnergyPlus and the climatic division based on sensible heat ratio and total cooling load was presented. Finally, on the basis of the novel climatic division, hourly sensible cooling load and latent cooling load of representative cities in each zone were analyzed to get the characteristics of load boundary and frequency distribution. A new design method was therefore proposed and discussed, paving a new way for the customized design of HVAC systems for each climate zone. [ABSTRACT FROM AUTHOR]

Abstract: In a direct expansion (DX) air conditioning (A/C) system having a variable-speed compressor and supply fan, the degree of refrigerant superheat regulated by a built-in conventional proportional–integral–derivative (PID) controller would experience fluctuation, after the speeds of both compressor and supply fan were regulated by a capacity controller. This would impact the operating efficiency and stability of the DX A/C system. Therefore a new DS controller, which was intended for improving the DS control performance when the fluctuation of operating DS was resulted from speed changes, has been developed and its controllability tested. The test results showed that this new DS controller, which replaced the conventional PID DS controller, can significantly improve DS control performance after speed changes while indoor air temperature and humidity were controlled to their respective settings. With the improved DS control performance, the operating efficiency and stability of the DX A/Csystem were also enhanced. [Copyright &y& Elsevier]

Abstract: The increased use of variable speed compressors (VSC) in refrigeration systems can potentially lead to the unstable operation when compressor speed is varied from time to time for capacity control. The causes of unstable operation may be classified into two groups, one relating to control algorithms and the other to the inherent characteristics of systems. This paper reports on a study on the operational stability of a VSC refrigeration system due to its inherent characteristics. Based on experimental results, a new modified minimal stable superheat (MSS) line having a maximum MSS value and a minimal MSS value has been proposed. Using the modified MSS line, and supported by a series of purposely designed experiments, a detailed analysis on the operational stability of a VSC refrigeration system due to its inherent characteristics when its compressor speed is changed for capacity control has been carried out and presented. [Copyright &y& Elsevier]

Liquid desiccant dehumidifiers (LDDs) has attracted much attention due to its advantages of energy-saving and environmental protection. However, dynamic characteristics of LDDs, which are essential to develop appropriate control methods, are still not clear at present. In this paper, the performance of a packed LDD was investigated under changes of desiccant inlet temperature and flow rate as well as air inlet humidity ratio, temperature and flow rate. Due to the random packings and interaction between air and desiccant, the air is normally distributed randomly during a dynamic process of LDDs and air outlet humidity ratio at the sampling point is serrated with time going. It was found that exponent function could be utilized to smooth the experimental result of air outlet humidity ratio, providing a simple way to determine the evaluation indexes like the response delay and response time as well as response rate, which could facilitate to evaluate dynamic characteristics of the dehumidifier. And through comparing those evaluation indexes corresponding to changes of different inlet parameters, it was revealed that air outlet humidity ratio was more sensitive to the change of desiccant or air flow rate, but not desiccant inlet temperature as indicated in a previous research getting from quasi-steady state experiments. Therefore, it would be more reasonable to select the desiccant or air flow rate as control variables instead of desiccant inlet temperature. Furthermore, preliminary control experiments were conducted to compare the performance of control algorithms developed based on the steady state characteristics and expert experience of dynamic characteristics. Results show that the control algorithm developed based on the expert experience of dynamic characteristics performs much better, which indicates that the obtained steady state characteristics in existing researches are unsuitable for developing control algorithms for a packed LDD. The dynamic characteristics revealed in current research paves the way for the future development of appropriate control algorithms. [ABSTRACT FROM AUTHOR]

In the field of fluid mechanics, it is a potential consensus that nonlinear dimensionality reduction (DR) techniques outperform linear methods. However, this conclusion has been obtained based on simple fluid phenomena and an incomplete evaluation system of dimensionality reduction algorithms. In this study, we use an improved evaluation system of DR methods to compare and evaluate the performance of four DR methods, including two linear techniques: Principal Component Analysis (PCA) and Independent Component Analysis (ICA), and two non-linear techniques: Isometric Mapping (ISOMAP) and Locally Linear Embedding (LLE). The four methods are applied to analyze a complex hydrodynamic flow field with cavitation by considering the joint features of multiple variables. Results show that LLE can capture redundant features that do not contribute to understanding the characteristics of flow fields, while ISOMAP is more suitable for handling datasets with multiple scales than LLE. PCA and ISOMAP can successfully capture the characteristics and evolution of flow fields. In addition, 3D supplementary information can assist ICA in improving the problem of identifying unstable flow field states. • A comprehensive evaluation of linear and nonlinear dimensionality reduction methods applied to cavitation flow fields. • The evaluation system for dimensionality reduction methods has been enhanced by measuring data structure preservation. • Considering the joint features of multiple variables by combining dimensionality reduction with joint representation. • Utilizing different dimensionality reduction algorithms to identify the states of cavitation flow fields. [ABSTRACT FROM AUTHOR]

Abstract A novel diffusion absorption heat transformer was proposed and built. The temperature lift performance of the novel system was experimentally tested with the diffusion gas supplied by an electric gas pump or a thermally driven pump, and the gross temperature lift was realized as high as 17.6 °C and 11.8 °C, respectively. Higher mass fraction of HCOOK in aqueous HCOOK solution, lower flow rate of aqueous HCOOK solution, and larger flow rate of refrigerant, were all beneficial to higher temperature lift. It is of remarkable significance for the utilization and upgrading of low grade thermal energy that a temperature lift can be realized by the diffusion absorption heat transformer with heat input but no need of electricity. Highlights • A novel diffusion absorption heat transformer was developed. • Aqueous HCOOK solution with little corrosivity was employed. • A gross temperature lift of 11.8 °C was realized with heat input. • A gross temperature lift of 17.6 °C was realized with an electric gas pump. [ABSTRACT FROM AUTHOR]

The large ownership of direct-expansion (DX) air-conditioning (A/C) systems in small and medium-sized buildings brings with it the need to reduce their energy consumption without damaging the thermal comfort of the occupants. Model predictive control (MPC) is an effective method to optimally control the operation of air-conditioners. However, most existing MPC methods require the investment of additional equipment and labor-intensive work, which greatly increases the cost of MPC and hinders its practical application. To solve the problem, this paper presents an economical and practical MPC algorithm for DX A/C systems, capable of achieving a balance between thermal comfort and energy saving. The proposed algorithm was experimentally validated on both an experimental DX A/C system and a market available split-type air-conditioner. Experimental results on the experimental DX A/C system show that temperature and humidity set-points selected at α = 1 saved 23.3% of energy consumption compared to those selected at α = 0, while keeping indoor thermal comfort within acceptable range. And results on the split-type air-conditioner demonstrate energy savings of up to more than 32% compared to the baseline and proved that the algorithm can be practically applied on market available D/X air-conditioners. • A practical application-oriented model predictive control algorithm is developed for DX A/C systems. • A cooling capacity prediction ANN model of the DX A/C system is used to predict the cooling load. • An ANN model for predicting the operational status of the DX A/C system is developed without increasing the training data. • A cost function is used to balance indoor thermal comfort levels and energy consumption. • The algorithm has been validated in both an experimental DX A/C system and a market available split-type air-conditioner. [ABSTRACT FROM AUTHOR]

The expectation of sustainable development of energy and environment has widely attracted public׳s attention, and the high-efficiency heat transfer device is greatly needed. As a new heat transfer device, pulsating heat pipe (PHP) has been considered have a bright prospect due to its obvious advantages – simple structure, low cost and excellent heat transfer capability. A number of experimental and theoretical researches have been conducted on PHP in the past decades since it was proposed by Akachi in 1990. However, due to the hydrodynamic and thermodynamic coupling effect, the operational mechanism of PHP is extremely complex and has not been completely revealed so far. With great expectation of applications for the PHP in the near future, this paper attempts to review the development of PHP on the basis of systematic summary of the latest results of both experimental and theoretical studies. Meanwhile, some promising and innovatory applications of the PHP are also reported. This paper is expected to provide basic reference for future researches. [ABSTRACT FROM AUTHOR]

The cooling capacity of a radiant ceiling panel (RCP) is highly limited in hot and humid climates as the panel surface temperature must be controlled higher than the air dew point to avoid condensation. Previous research suggested that condensation risks can be mitigated by using superhydrophobic surface materials, making ceiling panels applicable with the surface temperature below the air dew point. In this study, the enhanced cooling capacity of RCP due to latent heat transfer is estimated. Firstly, the condensation heat transfer of a superhydrophobic aluminum surface under natural convection in humid air with subcooling degrees between 3 °C and 12 °C was investigated by experiments. The data proved that the heat and mass transfer analogy method can be used to well predict the condensation heat transfer of the superhydrophobic surface with an error within 15%. Based on the finding, the heat and mass transfer analogy and traditional empirical methods were employed to predict the total heat flux of RCP with latent heat transfer. The results indicated an enhancement of 3% to 500% for the total heat flux compared with RCP with only sensible cooling, depending on the panel surface temperature, air temperature and humidity. This study draws a clear picture of how the cooling capacity of RCP can be enhanced via a latent heat transfer process, and this can be realized not necessarily with net moisture removal capacities with hydrophobic surfaces due to the droplet jumping phenomenon and the due re-evaporation process, showing a hidden potential of radiant cooling panels. [ABSTRACT FROM AUTHOR]

Abstract: An artificial neural network (ANN)-based dynamic model for an experimental variable speed direct expansion (DX) air conditioning (A/C) system has been developed, linking the indoor air temperature and humidity controlled by the DX A/C system with the variations of compressor and supply fan speeds. The values of average relative error (ARE) and maximum relative error (MRE) when validating the ANN-based dynamic model developed under three different input patterns were 0.33%, 0.27%, 0.27% and 0.89%, 0.99%, 1.15%, respectively, indicating the high accuracy of the ANN-based dynamic model developed. An ANN-based controller was then developed for controlling the indoor air temperature and humidity simultaneously by varying the compressor speed and supply fan speed in a space served by the experimental DX A/C system. The controllability tests including command following test and disturbance rejection test were carried out using the experimental DX A/C system, and the test results showed that the ANN-based controller developed was able to track the changes in setpoints and to resist the disturbances. [Copyright &y& Elsevier]

Abstract: Because of their excellent performance in local thermal environmental control, task/ambient conditioning (TAC) systems have attracted increasingly more research attention. However, the current applications of TAC systems are mainly for daytime activities, but seldom for nighttime. In fact, TAC systems may be best applied to a sleeping environment, since the subjects to be served by a TAC system are immobile during sleep. Therefore, there is a strong need for studying the operational and energy saving performance of TAC systems when applied to sleeping environments. This paper reports on an experimental study on evaluating the operational and energy saving performance of a novel bed-based TAC system. The experimental results and their analysis demonstrated that the use of the novel bed-based TAC system can help achieve energy saving, compared with the use of a full volume air conditioning (FAC) system. Considering the experimental limitations, the energy saving potential of the novel bed-based TAC system if used in an actual bedroom was studied through a theoretical analysis. It demonstrated that the novel bed-based TAC system would help achieve significant energy saving when used in real buildings. [Copyright &y& Elsevier]

Abstract: Usually, a medium such as water and electrical current is transported through an energy distribution network for delivering energy. Driving the medium through the network will lead to power loss. In this paper, a fundamental study was conducted to find out the optimal/near-optimal shape of an energy distribution network for minimising the total power loss under the constraint of the network’s total flow volume. A general equation was developed for calculating the total power loss. A parameter was used to classify all media into three types. For Type I and II medium, it was found that a radial shape network was the optimal one. A global search method was proposed to find out the near-optimal shape of the network for Type III medium. A case study was carried out for finding out the optimal/near-optimal shape of the networks for electric current, refrigerant and water in laminar flow, respectively, with a supplier and six users. The power losses to drive these media through the networks with the optimal/near-optimal shape were compared to those through the networks with other shapes. The comparison results indicated the power losses could be reduced when using the optimal/near-optimal networks. [Copyright &y& Elsevier]

A simulation model using the first thermodynamic law is first proposed to estimate the performance of diffusion absorption heat transformer, which could potentially improve energy utilization efficiency and reduce CO 2 emission. Thermodynamic analyses are performed to investigate the cycle performance when H 2 O is employed as refrigerant, LiBr as absorbent for H 2 O, R134a as diffusion gas and TEGDME as absorbent for R134a. Analyses aim to optimize system COP under specified temperature of two generators, two absorbers, evaporator and condenser, and compare the influence of the six temperatures on COP. The results under design conditions show that larger generation efficiency of R134a and smaller mass fraction of LiBr in H 2 O–LiBr solution are beneficial to COP improvement, but they are limited by the operating conditions. There is an optimal ratio of H 2 O–LiBr solution mass flow rate to R134a mass flow rate for an optimal COP, and the optimal ratio decreases with the increase of LiBr mass fraction in H 2 O–LiBr solution. The highest COP of 0.1701 is reached with the optimal ratio of 1.743. In addition, under design-off conditions, the temperatures of refrigerant generator and evaporator have the greatest influence on COP while the temperature of diffusion gas generator has the least influence. The diffusion absorption heat transformer will provide a new way to lift temperature of low grade heat without electricity input which can further improve the energy utilization efficiency and reduce the CO2 emission. Image 1 • A diffusion AHT cycle without any electricity input is proposed and analyzed. • The influences of flow rates and concentrations of working fluids are studied. • The impacts of six temperatures on the performance are compared. • The maximum COP is found to be 0.1701 at optimal conditions. [ABSTRACT FROM AUTHOR]