Real-time neural inverse optimal control for indoor air temperature and humidity in a direct expansion (DX) air conditioning (A/C) system

A real-time neural inverse optimal control for the simultaneous control of indoor air temperature and humidity using a direct expansion (DX) air conditioning (A/C) system has been developed and the development results are reported in this paper. A recurrent high order neural network (RHONN) was used to identify the plant model of an experimental DX A/C system. Based on this model, a discrete-time inverse optimal control strategy was developed and implemented to an experimental DX A/C system for simultaneously controlling indoor air temperature and humidity. The neural network learning was on-line performed by extended Kalman filtering (EKF). This control scheme was experimentally tested via implementation in real time using an experimental DX A/C system. The obtained results for trajectory tracking illustrated the effectiveness of the proposed control scheme.     

A DDC-based capacity controller of a direct expansion (DX) air conditioning (A/C) unit for simultaneous indoor air temperature and humidity control – Part I: Control algorithms and preliminary controllability tests

For residential buildings located in the subtropics, direct expansion (DX) air conditioning (A/C) units are commonly used. Most DX A/C units are currently equipped with single-speed compressors and supply fans, relying on on–off cycling compressors as a low-cost approach to maintain only indoor air dry-bulb temperature, resulting in either space overcooling or an uncontrolled equilibrium indoor relative humidity (RH) level. With the rapid development of A/C industry, the use of variable-speed compressor and supply fan has become more and more prevalent and practical. This paper, the first part of a two-part series, reports on the development of a novel direct digital control (DDC)-based capacity controller for a DX A/C unit having variable-speed compressor and supply fan to simultaneously control indoor air temperature and RH in a conditioned space served by the DX A/C unit. The controller is the first of its kind as a composite parameter, sensible heat ratio (SHR), is used as a controlled parameter. The core element of the capacity controller, a numerical calculation algorithm (NCA) is firstly presented. This is followed by reporting the results of preliminary controllability tests of the DDC-based capacity controller, which suggested that the controller developed could achieve a reasonable control accuracy, but with room for improvement with respect to control sensitivity. Part II of the two-part series reports on the further development of the controller to improve its control sensitivity, and the results of associated controllability tests.     

A DDC-based capacity controller of a direct expansion (DX) air conditioning (A/C) unit for simultaneous indoor air temperature and humidity control – Part II: Further development of the controller to improve control sensitivity

The development of the novel direct digital control (DDC)-based capacity controller for a direct expansion (DX) air conditioning (A/C) unit having variable-speed compressor and supply fan to simultaneously control indoor air temperature and relative humidity (RH) in a conditioned space served by the DX A/C unit has been reported in Part I of the two-part series. The results of preliminary controllability tests for the novel capacity controller presented in Part I, however, suggested that the controller developed was operational, with acceptable control accuracy but rooms for improvement with respect to control sensitivity. This paper, the second part of the two-part series, reports on the further development of the controller to improve its control sensitivity and the associated controllability test results. Both control accuracy and reasonable control sensitivity were achieved by incorporating a traditional Proportional–integral (PI) controller into the DDC-based capacity controller.     

A novel proportional-derivative (PD) law based fuzzy logic principles assisted controller for simultaneously controlling indoor temperature and humidity using a direct expansion (DX) air conditioning (A/C) system

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.     

A dynamic simulation model for transient absorption chiller performance. Part II: Numerical results and experimental verification

This paper is the second paper out of two which present the development of a dynamic model for single-effect LiBr/water absorption chillers. The first part describes the model in detail with respect to the heat and mass balances as well as the dynamic terms. This second part presents a more detailed investigation of the model performance, including performance analysis, sensitivity checks and a comparison to experimental data. General model functionality is demonstrated.A sensitivity analysis gives results which agree very well to fundamental expectations: it shows that an increase in both external and internal thermal mass results in a slower response to the step change but also in smaller heat flow oscillations during the transient period. Also, the thermal mass has been found to influence the heat flow transients more significantly if allocated internally. The time shift in the solution cycle has been found to influence both the time to reach steady-state and the transients and oscillations of the heat flow. A smaller time shift leads to significantly faster response.A comparison with experimental data shows that the dynamic agreement between experiment and simulation is very good with dynamic temperature deviations between 10 and 25 s. The total time to achieve a new steady-state in hot water temperature after a 10 K input temperature step amounts to approximately 15 min. Compared to this, the present dynamic deviations are in the magnitude of approximately 1–3%.     

Condensate retention on a louver-fin-and-tube air cooling coil

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.     

System dynamic model and temperature control of a thermoelectric coolerModèle dynamique d'un système et régulation de la température d'un refroidisseur thermoélectrique

A linear dynamic model of the thermoelectric cooler including the heat sink and the cooling-load heat exchanger was derived using small-signal linearization method. It shows that the dynamic model of a thermoelectric cooler has two poles and one zero. The linear dynamic model is shown to vary with operating conditions. A linear feedback system is designed for the cold-end temperature control of a thermoelectric cooler using the average linear dynamic model of the thermoelectric cooler and a PDF controller structure. The step response tests show that the controller has a very satisfactory performance. Some tests under variable cooling load and ambient temperature are also performed to examine the disturbance-rejection property of the controller. Experimental results show that the cold-end temperature can be maintained at the fixed value within ±0.1°C irrespective of the variations of the cooling load and the ambient conditions.     

Modeling of a domestic frost-free refrigerator

In the present study, a comprehensive thermo-fluidic model is developed for a domestic frost-free refrigerator. The governing equations, coupled with pertinent boundary conditions, are solved by employing a conservative control volume formulation, in the environment of a three-dimensional unstructured mesh. Experiments are also conducted to validate the results predicted by the present computational model. It is found that the computational and experimental results qualitatively agree with each other, although certain discrepancies can be observed in terms of the exact numerical values obtained. For the freezer compartment, the computationally predicted temperatures are somewhat higher than the experimental ones, whereas for the refrigerating compartment, the computed temperatures are lower than the corresponding experimental observations. The difference between experimental and computational results may be attributed to the lack of precise data on the airflow rates and the unaccounted heat transfer rates through the door gaskets and the compressor. From the heat transfer and fluid flow analysis, certain modifications in the design are also suggested, so as to improve the performance of the refrigerator.     

Modeling and experimental investigation of accumulators for automotive air conditioning systems

A stream analysis model was developed to simulate the behavior of accumulators and their influence on the automotive air conditioning (A/C) systems. It allows a comprehensive steady state simulation with a set of input conditions such as refrigerant vapor mass flow rate and pressure at the inlet of an accumulator. In this study, the refrigerant/oil mixture is R134a/PAG oil which are totally miscible, but could be any air conditioning refrigerant/oil, including carbon dioxide (CO₂)/oil. The model accounts for all major effects inside the accumulator, such as friction, bends, sudden expansion, sudden contraction and heat exchange. The outputs are vapor quality, pressure and temperature at various positions of accumulator. In order to verify the mathematical model, experiments are performed in an experimental setup made up of real size automotive air conditioning components. The simulated results agree well with the experimental data. The simulation and experimental results show an important function of accumulators that is to determine the vapor quality into compressor, and thus has influence on the performance of whole automotive A/C systems.     

变速直膨式空调系统运行特性的稳态ANN模型

在假设系统输出显、潜冷量的相对值在不同的蒸发器入口空气状态下不发生明显变化的前提下,本文针对实验用变速直膨式空调系统建立了稳态人工神经网络(ANN)模型,预测其在不同压缩机、风机转速组合下的系统输出,利用输出显、潜冷量的相对值可以消除室内空气状态对系统输出的影响。通过稳态实验获得数据训练、检测并验证ANN模型预测变速直膨式系统运行特性的准确性,并通过非训练状态点下的稳态实验验证所提出假设与ANN模型的适用性。ANN模型的训练、检测以及验证实验结果的最大误差均小于5%,平均误差均小于3%,表明该稳态ANN模型可以在训练状态点以及非训练状态点较为准确地预测变速直膨式系统的运行特性。     

Simulation of a transcritical CO2 heat pump cycle for simultaneous cooling and heating applications

A steady state simulation model has been developed to evaluate the system performance of a transcritical carbon dioxide heat pump for simultaneous heating and cooling. The simulated results are found to be in reasonable agreement with experimental results reported in the literature. Such a system is suitable, for example, in dairy plants where simultaneous cooling at 4 °C and heating at 73 °C are required. The optimal COP was found to be a function of the compressor speed, the coolant inlet temperature to the evaporator and inlet temperature of the fluid to be heated in the gas cooler and compressor discharge pressure. An optimizing study for the best allocation of the fixed total heat exchanger inventory between the evaporator and the gas cooler based on the heat exchanger area has been carried out. Effect of heat transfer in the heat exchangers on system performance has been presented as well. Finally, a novel nomogram has been developed and it is expected to offer useful guidelines for system design and its optimisation.     

Numerical simulation of an advanced energy storage system using H2O–LiBr as working fluid, Part 2: System simulation and analysis

This paper is the second part of our study on the advanced energy storage system using H₂O–LiBr as working fluid. In the first part, the system working principle has been introduced, and the system dynamic models in the operation process have also been developed. Based on the previous research, this paper focuses on the numerical simulation to investigate the system dynamic characteristics and performances when it works to provide combined air-conditioning and hot water supplying for a hotel located near by Yangzi River in China. The system operation conditions were set as follows: the outdoor temperature was between 29 °C and 38 °C, the maximum air-conditioning load was 1450 kW, the total air-conditioning capacity was 19,890 kWh and the 50 °C hot water capacity for showering was 20 tons which needed heat about 721 kWh on a given day. Under these conditions, the system operation characteristics were simulated under the full- and partial-storage strategies. The simulation results predicted the dynamic characteristics and performances of the system, including the temperature and concentration of the working fluid, the mass and energy in the storage tanks, the compressor intake mass or volume flow rate, discharge pressure, compression ratio, power and consumption work, the heat loads of heat exchanger devices in the system and so on. The results also showed that the integrated coefficient of performances (COP_int) of the system were 3.09 and 3.26, respectively, under the two storage strategies while the isentropic efficiency of water vapor compressor was 0.6. The simulation results are very helpful for understanding and evaluating the system as well as for system design, operation and control, and device design or selection in detail.     

A design method of thermoelectric coolerConception d'un refroidisseur thermoélectrique

A system design method of thermoelectric cooler is developed in the present study. The design calculation utilizes the performance curve of the thermoelectric module that is determined experimentally. An automatic test apparatus was designed and built to illustrate the testing. The performance test results of the module are used to determine the physical properties and derive an empirical relation for the performance of thermoelectric module. These results are then used in the system analysis of a thermoelectric cooler using a thermal network model. The thermal resistance of heat sink is chosen as one of the key parameters in the design of a thermoelectric cooler. The system simulation shows that there exists a cheapest heat sink for the design of a thermoelectric cooler. It is also shown that the system simulation coincides with experimental data of a thermoelectric cooler using an air-cooled heat sink with thermal resistance 0.2515°C/W. An optimal design of thermoelectric cooler at the conditions of optimal COP is also studied. The optimal design can be made either on the basis of the maximum value of the optimal cooling capacity, or on the basis of the best heat sink technology available.     

A mathematical model of variable displacement swash plate compressor for automotive air conditioning system

A mathematical model of control mechanism used in the variable displacement swash plate compressor (VDSC) is developed firstly based on the force balance equation, mass and energy conservation equation. The model of moving components dynamics is developed then by analyzing the forces and force moments acting on the pistons and the swash plate. The compression process model is obtained by fitting the data from our experiments. And finally, the steady-state mathematical model of VDSC is developed by combining the three sub-models above. In order to verify the mathematical model, a test bench for control mechanism and the test system for VDSC have been established, and the simulated results agree well with the experimental data. The simulation results show that, like the variable displacement wobble plate compressor, there are four operation modes for the VDSC, i.e. constant rotary speed and constant piston stroke length (PSL), variable rotary speed and constant PSL, constant rotary speed and variable PSL, variable rotary speed and variable PSL, which have included almost all operation modes of the refrigeration compressor in common use. And there is a hysteresis zone and multiple-valued relationship between the compressor parameters when PSL changes.     

Dynamic simulation of a thermostatically controlled counter-flow evaporatorSimulation dynamique d'un évaporateur à détendeur thermostatique avec débit à contre-courant

A numerical model is developed to simulate the transient behaviour of a counter-flow water cooling evaporator controlled by a thermostatic expansion valve (TEV) in a vapour compression refrigeration system. The liquid–vapour slip in the two-phase region of the evaporator is accounted for by a void fraction model (VFM). The thermal capacitance of the TEV is included in the analysis. For the purpose of comparison with predictions of the model, experimental data available are filtered to obtain the best estimate of the mean variation of the liquid–vapour transition plane. The predictions are in good trend-wise agreement with the filtered experimental data. The results of the transient simulations demonstrate the dependence of the stability of the evaporator–TEV system on the characteristics of the TEV, the thermal capacitance of the bulb, thermal conductance between the bulb and wall and on the nature of the input disturbance.     

A theoretical and experimental study of a small-scale steam jet refrigerator

The paper provides the results of a theoretical and experimental study of a steam jet refrigerator. A small-capacity steam jet refrigerator has been tested with boiler temperatures in the range 120–140°C. The experimental data were found to be within 85% of the theoretical values. The experiments showed that choking of the secondary flow in the mixing chamber of the ejector plays an important role in the system performance. Maximum COP was obtained when the ejector was operated at its critical flow condition. Off-design performance characteristics of the system are provided.     

A novel special distributed method for dynamic refrigeration system simulation

A novel dynamic mathematical model based on spatially distributed approach has been developed and validated in this paper. This model gives good agreement in predicting the system COP and other parameters. The validated model has been used to enhance the prediction of the micro variations of superheat and sub-cooling. The novel spatial distributed model for the condenser and evaporator in refrigeration system, calculates the two-phase region in gas and liquid field separately since the gas and liquid in the two-phase region have different velocities. Previous researchers have used a pre-defined function of the void fraction in their spatially distributed model, based on experimental results. This approach results in the separate solution of the mass and energy equations, and less calculation is required. However, it is recognized that the mass and energy equations should be coupled during solving for more accurate solution. Based on the energy and mass balance, the spatial distribution model constructed here solves the velocity, pressure, refrigerant temperature, and wall temperature functions in heat exchangers simultaneously. A novel iteration method is developed and reduces the intensive calculations required. Furthermore, the condenser and evaporator models have shown a parametric distribution along the heat exchanger surface, therefore, the spatial distribution parameters in the two heat exchangers can be visualised numerically with a two-phase moving interface clearly shown.     

A dynamic simulation model for transient absorption chiller performance. Part I: The model

This paper is the first of two which presents the development of a dynamic model for single-effect LiBr/water absorption chillers. The model is based on external and internal steady-state enthalpy balances for each main component. Dynamic behaviour is implemented via mass storage terms in the absorber and generator, thermal heat storage terms in all vessels and a delay time in the solution cycle. A special feature is that the thermal capacity is partly connected to external and partly to internal process temperatures.In this paper, the model is presented in detail. For verification, the model has been compared to experimental data. The dynamic agreement between experiment and simulation is very good with dynamic deviations around 10 s. General functionality of the model and a more detailed comparison with experimental data are presented in Part II of this paper.     

Thermodynamic properties of HFC-32 (difluoromethane)

An 18-coefficient modified Benedict–Webb–Rubin equation of state of HFC-32 (difluoromethane) has been developed, based on the updated available PVT measurements, heat capacity measurements and speed of sound measurements. Correlations of vapor pressure and saturated liquid density are also presented. The correlations have been developed based on the reported experimental saturation properties data. This equation of state is effective both in the superheated gaseous phase and compressed liquid phase at pressures up to 70 MPa, densities to 1450 kg/m³, and temperatures from 150 to 475 K, respectively.     

Thermal performance of cold storage in thermal battery for air conditioning

This article studies, experimentally and theoretically, the thermal performance of cold storage in thermal battery for air conditioning. Thermal battery utilizes the superior heat transfer characteristics of heat pipe and eliminates drawbacks found in the conventional thermal storage tank. Experimental investigations are first conducted to study the cold storage thermal performance in two experimental systems: the ratio of distance between heat pipes to outer diameter of heat pipe W/D=6 and 2. Different heat transfer mechanisms including nucleate boiling, geyser boiling and natural convection are identified in different experimental systems with various liquid fills. A theoretical model to determine the thermal characteristics of the thermal battery has also been developed. Comparisons of this theory with experimental data show good agreements in the nucleate boiling stage of cold storage process.