Multi-evaporator Air Conditioning System Research Articles

Energy-efficient decentralized control method with enhanced robustness for multi-evaporator air conditioning systems

Multi-evaporator air conditioning system has found increasing applications in buildings. Energy-efficient control for this kind of system presents significant challenge because of strong coupling effects and various operation modes. Existing control methods are most likely to encounter uncontrollable problems and lack robustness under uncertain operation conditions. This paper proposes an energy-efficient decentralized control method with enhanced robustness for multi-evaporator air conditioning system. The proposed method is featured by a self-optimizing control strategy and a decentralized proportional-integral (PI) control algorithm based on the effective open-loop transfer function (EOTF). Different from most of previous studies that regulated compressor speed to control the evaporating pressure by using a supervisory optimizer, the proposed self-optimizing control strategy regulates compressor speed to maintain constant suction superheat and achieves energy-efficient operation in a simple and robust manner. Meanwhile, the EOTF-based decentralized PI algorithm overcomes the weakness of decentralized controllers in dealing with coupling effects, while retaining robustness against model errors and component faults. Control performance of the proposed method is tested using a validated system dynamic model. Through extensive controllability tests, it is found that the proposed control method significantly improves temperature control performance of the conventional strategy. Up to 8.6% energy savings are achieved with the proposed control strategy. Moreover, the proposed EOTF-based PI controller demonstrates enhanced robustness in comparison with an advanced model predictive controller.

Distributed control for a multi-evaporator air conditioning system

An autonomous hierarchical distributed control (AHDC) strategy is proposed for a building multi-evaporator air conditioning (ME A/C) system in this paper. The objectives are to minimize peak demand and energy costs, and to reduce communication resources, computational complexity and conservativeness while maintaining both thermal comfort and indoor air quality (IAQ) in acceptable ranges. The building consists of multiple connected rooms and zones. The proposed control strategy consists of two layers. The upper layer is an open loop optimizer, which only collects local measurement information and solves a distributed steady state resource allocation problem to autonomously and adaptively generate reference points, for low layer controllers. This is achieved by optimizing the demand and energy costs of a multi-zone building ME A/C system under a time-of-use (TOU) rate structure, while meeting the requirements of each zone’s thermal comfort and IAQ within comfortable ranges. The lower layer also uses local information to track the trajectory references, which are calculated by the upper layer, via a distributed model predictive control (DMPC) algorithm. The control strategy is distributed at both layers because they use only local information from the working zone and its neighbors. Simulation results are provided to illustrate the advantages of the designed control schemes.

Sleeping evaporator and refrigerant maldistribution: An experimental investigation in an automotive multi-evaporator air-conditioning and battery cooling system

With the arrival of electrified vehicles, the air-conditioning system has to be reconsidered. Battery cooling management system and high level of comfort for passengers make the single evaporator air-conditioning system a multi-evaporator one. In a multi-evaporator air-conditioning system, evaporating pressures are equal in each evaporator so that evaporators are coupled dynamically. However, the demand in cooling capacity and temperature target can vary from each other. For an operating point with a first evaporator working at high load and a second at low partial load, thermal interaction can occur from the superheated refrigerant coming from a first evaporator to the outlet of a second evaporator. This phenomenon makes the second evaporator sleeping since its expansion valve bulb misreads the superheat and closes. Furthermore, sleeping evaporator looks like an extreme case of refrigerant maldistribution in the evaporator. Refrigerant maldistribution is then investigated to show some drawbacks and advantages multi-evaporator air-conditioning systems (MEAC) have to face or can benefit by comparing two types of expansion valve: thermostatic and electronic ones. In this paper, sleeping evaporator and refrigerant maldistribution phenomena are experimentally investigated in order to propose in the future a robust control of an automotive MEAC. A test bench was built to compare two types of expansion valves (thermostatic/electronic) and study their behaviours in steady and transient state to tackle sleeping evaporators and benefit from refrigerant maldistribution.

Simulation study on a three-evaporator air conditioning system for simultaneous indoor air temperature and humidity control

A multi-evaporator air conditioning (MEAC) system, which may also be called a multi-split air conditioning system or variable refrigerant flow system in the open literature, may be regarded as one of the energy conscious air conditioning applications. However, most MEAC systems currently focus on air temperature control only, so that the potential of using MEAC system in achieving energy saving has not yet been fully utilized. In this paper, based on the previous extensive related research on modeling and controlling both single evaporator air conditioning systems and MEAC systems, a capacity controller for a three-evaporator air conditioning (TEAC) system was developed. Results of simulative controllability tests for the capacity controller demonstrated that, indoor air temperature could be directly controlled using the capacity controller, but indoor relative humidity can be improved and maintained at acceptable levels. Based on the operating characteristics of the TEAC system, the capacity controller was further improved and thus an improved controller was developed. Two tests to verify the performances of the improved controller were conducted. The test results showed that simultaneous indoor air temperature and humidity control of a TEAC system could be achieved using the improved controller.

Simulation Study on a Three-evaporator Air Conditioning System for Improved Humidity Control

Abstract Multi-evaporator air conditioning (MEAC) may be regarded as one of the energy conscious air conditioning technologies. However, most MEAC systems currently focus on temperature control only, so that the potential of MEAC technology in achieving energy saving has not yet been fully explored. Therefore, in this paper, based on previous extensive related research by the investigators on modeling and controlling both single evaporator air conditioning (SEAC) systems and MEAC systems, a novel controller for a three evaporator air conditioning system will be developed. The novel controller was developed by integrating two previous control algorithms for a dual-evaporator air conditioning system for temperature control and for a SEAC system for improved indoor humidity control. Simulative controllability test results showed that, with the novel controller, improved control over indoor humidity levels could be obtained.

Operating characteristics of a three-evaporator air conditioning (TEAC) system

Multi-evaporator air conditioning (MEAC) systems become more and more popular, as they possess many advantages compared with chilled water based air conditioning systems. These include higher energy efficiency and system design and installation flexibility. However, although the coupling effects of the operating parameters in an MEAC system have been extensively modeled, the detailed operating characteristics of an MEAC system with more than two indoor units (IU) have not yet been experimentally investigated.In this paper, two Sets of experiments were specifically designed and carried out to study the operating characteristics of an experimental three-evaporator air conditioning (TEAC) system, with the changes in the operating parameters in one of the three IUs/indoor spaces (IS) in the first Set and that in two of the three IUs/ISs in the other. Similar coupling effects of mutual influences reported in previous dual-evaporator air conditioning systems may also be observed in the first Set. However, when the operating parameters in two of the IUs/ISs varied, the coupling effects became more complicated, with mutual influences being more remarkable than that in the first Set with even less variation magnitude. Furthermore, the changes on both refrigerant side and air side of each IU/IS in the experimental TEAC system were studied, which is the first of its kind in the open literature. The results reported in this paper can be used as an important reference for the optimization of system design and developments of appropriate capacity controllers for MEAC systems.

A novel capacity controller for a three-evaporator air conditioning (TEAC) system for improved indoor humidity control

Using a multi-evaporator air conditioning (MEAC) system to correctly control indoor air temperatures only in a multi-room application is already a challenging and difficult task, let alone the control of both indoor air temperature and humidity. This is because in an MEAC system, a number of indoor units are connected to a common condensing unit. Hence, the interferences among operation parameters of different indoor units would make the desired control of an MEAC system hard to realize. Limited capacity control algorithms for MEAC systems have been developed, with most of them focusing only on the control of indoor air temperature, and no previous studies involving control of indoor air humidity using MEAC systems can be identified.In this paper, the development of a novel capacity controller for a three-evaporator air conditioning (TEAC) system for improved indoor air humidity control is reported. The novel controller was developed by integrating two previous control algorithms for a dual-evaporator air conditioning system for temperature control and for a single-evaporator air conditioning system for improved indoor humidity control. Experimental controllability tests were carried out and the controllability test results showed that, with the novel controller, improved control over indoor humidity levels and better energy efficiency for a TEAC system could be obtained as compared to the traditional On–Off controllers extensively used by MEAC systems.

Decentralized model predictive control of a multi-evaporator air conditioning system

Temperature control of multiple zones with a multi-evaporator vapor compression system is a common problem in modern air conditioning. Due to the coupled system dynamics, standard decoupled controllers can interfere with each unit′s performance. This paper proposes an architecture that is decentralized and modular, avoiding competing controllers and the practical difficulty of implementing a centralized controller. A model predictive control (MPC) supervisor calculates evaporator cooling and pressure setpoints for each zone, balancing temperature regulation with energy efficiency; these setpoints are tracked by local level controllers, which rely upon MPC's ability to respect constraints in order to maintain safe, efficient operation.

Distribution of size in multi-evaporator air conditioning systems

SUMMARY This paper reports the thermodynamic optimization of air conditioning systems with one or more evaporators, for one or more rooms. First, the paper shows that the minimization of the total power requirement leads to the same results as the minimization of entropy generation in the whole system, which includes the insulation between the cold space and the ambient. The results also show that there is a trade-off between the power lost by fluid friction in the distribution network and the power required by the refrigerant. This trade-off determines the optimal allocation of hardware in the installation. The optimum diameters of the refrigerant pipelines are independent of their lengths. Copyright © 2013 John Wiley & Sons, Ltd.

Experimental study of a novel capacity control algorithm for a multi-evaporator air conditioning system

The use of a multi-evaporator air conditioning (MEAC) system is advantageous in terms of installation convenience, high design flexibility, being easy to maintain and commission, better indoor thermal comfort control and higher energy efficiency. While MEAC units worth billions of dollars are sold worldwide, the detailed accounts on compressor capacity control and refrigeration flow distribution amongst evaporators remain unavailable in public domain, mainly due to commercial confidentiality. Limited control algorithms for MEAC systems have been developed based on system simulation, and no experimental-based capacity controller developments and their controllability tests may be identified in open literature. In the study reported in this paper, a novel capacity control algorithm, which imitated On–Off control of a single evaporator air conditioning (A/C) system in each indoor unit of a MEAC system by using variable speed compressor and electronic expansion valves (EEVs), was developed. Controllability tests under various settings for experimentally validating the novel capacity control algorithm were carried out and the control algorithm was further improved based on the experimental results.

A modeling study on the effects of refrigerant pipeline length on the operational performance of a dual-evaporator air conditioning system

Abstract Multi-evaporator air conditioning (MEAC) systems are gaining greater and greater popularity because they offer building owners many advantageous over convenient chilled water based A/C systems, such as higher energy efficiency and flexibility in design and installation. However, while the coupling effects among operating parameters in evaporators of an MEAC system have been extensively studied, the influence of large pressure drop along the complicated refrigerant pipework of an MEAC system on the operating performance has not yet been investigated. This paper reports on a modeling study on the effects of refrigerant pipeline length on the operational performance of a dual-evaporator air conditioning (DEAC) system. To facilitate the intended modeling study, a physical-based steady-state mathematical model with a sub-module specifically devoted to accounting for the influence of refrigerant pipeline length on system operational performance has been developed. The model has been validated by comparing its prediction results with the experimental results previously reported by others. Using the model developed, the effects of refrigerant pipeline length on the operating performance of the DEAC system have been studied and are reported, and the layout optimization of a DEAC system was studied for the highest possible operational efficiency. Results of the study indicated that the DEAC system’s COP decreased with an increase in the refrigerant pipeline length. The results also suggested that for a DEAC system, its highest COP would be resulted in when the outdoor unit was located equally between the two indoor units and its lowest COP when the outdoor unit was located close to either of the indoor unit.

Control of multi-evaporator air-conditioning systems for flow distribution

Modern air-conditioners incorporate variable-speed compressors and variable-opening expansion valves as the actuators for improving cooling performance and energy efficiency. These actuators have to be properly feedback-controlled; otherwise the systems may exhibit even poorer performance than the conventional machines which use fixed-speed compressors and mechanical expansion valves. In this paper, a control strategy with flow distribution capability is proposed for multi-evaporator air-conditioners to accommodate different thermal demands in different rooms. The structure in the control strategy is based on a low-order, linear model obtained from system identification. To determine appropriate control parameters, theorems regarding stability of the closed-loop system are given. Moreover, by performing steady-state analysis on the control system and utilizing characteristics of the identified system parameters, one can analytically explain the mechanics of flow distribution. Experiments indicate that the proposed strategy can successfully regulate the indoor temperatures regardless that the reference settings for respective rooms are different and the settings are switched in the middle of the control process.

Identification and control of multi-evaporator air-conditioning systems

Modern multi-evaporator air-conditioners (MEACs) incorporate variable-speed compressors and variable-opening expansion valves as the actuators for improving cooling performance and energy efficiency. These actuators have to be properly feedback-controlled; otherwise the systems may exhibit even poorer performance than the conventional machines which use fixed-speed compressors and conventional expansion valves. In this paper, feedback controller design for the MEAC system is first addressed through experimental identification. The identification produces a low-order, linear model suitable for controller design. The feedback controller employed is multi-input–multi-output-based and possesses a cascade structure for dealing with the fast and slow dynamics in the system. To determine appropriate control parameters, conditions that establish the stability for the cascade design are given. Due to the deficiency in control inputs, the proposed control structure exhibits steady-state errors in the superheat responses which in turn can produce unacceptable steady-state superheats. To resolve this issue, the reference superheat settings are determined via an optimization procedure so that the resultant steady-state superheats become acceptable. The control experiments indicate that the proposed controller can successfully regulate the indoor temperatures and maintain the steady-state superheat temperatures at acceptable levels.