Direct Expansion Air Conditioning Research Articles

Direct expansion (DX) air conditioning (A/C) system control: Hybrid BOANFIS technique

AbstractThis paper describes a novel hybrid technique with fractional order PID controller (FOPID) for simultaneously controlling the humidity of indoor air temperature and the direct expansion (DX) air conditioning (A/C) system. The proposed hybrid system is a joint performance of the butterfly optimization algorithm (BOA) and adaptive network fuzzy inference system (ANFIS), hence forth it is called BOANFIS Technique (BOANFIST). The purpose of the proposed system is to disconnect the temperature and humidity control circuits. The proposed control is modeled and replicated on MATLAB platform and is assessed using existing systems. The statistical performance of the proposed and existing systems of mean, median and standard deviation is also evaluated. It reduces computational time up to 1.01 s and also reduces energy consumption to around 16.42 KWh/day. Furthermore, the simulation outcomes suggest that the proposed technique may efficiently and accurately obtain the optimal global solutions of the proposed technique compared to existing systems.

Model predictive control of indoor thermal environment conditioned by a direct expansion air conditioning system

Model predictive control of indoor thermal environment conditioned by a direct expansion air conditioning system

An adaptive controller based dynamic simulation of household air-conditioner with indirect evaporative cooler as dedicated outdoor air system

The lack of controllable ventilation design for acceptable indoor air quality is a major problem facing small-scale direct expansion air conditioning (DXAC) systems for household applications. The DXAC-IEC hybrid cooling system, consisting of DXAC systems and indirect evaporative cooler (IEC) ventilators, can provide an energy-efficient solution to this problem. An adaptive controller is proposed in this study to respond to changeable indoor and outdoor disturbances to provide standard-compliant fresh air demand, guarantee indoor thermal comfort and reduce energy consumption at the same time. In the adaptive control scheme, different operation modes are applied by combining three modes of compressor speed (high-low, low-off, off) and three modes of IEC fan speed (high, middle, low) responding to outdoor and indoor conditions. The dynamic performance of DXAC-IEC system on typical days and annual performance were numerically investigated. The results show that considerable energy saving (22.1 % to 34.1 % in Xi’an) can be achieved in transition seasons by DXAC-IEC system for providing standard-compliant ventilation rate and guaranteeing indoor thermal comfort simultaneously compared to a standalone DXAC system. However, extra energy (10.0 % to 25.4 % in Xi’an) will be consumed on summer days. On a typical summer day, the indoor relative humidity will increase by 10 % to 15 % because of extra latent load brought from IEC ventilator compared to the standalone DXAC system, but indoor temperature can be precisely controlled and CO2 concentration is mostly below 1500 ppm.

Optimal Operation for Reduced Energy Consumption of an Air Conditioning System Using Neural Inverse Optimal Control

For a comfortable thermal environment, the main parameters are indoor air humidity and temperature. These parameters are strongly coupled, causing the need to search for multivariable control alternatives that allow efficient results. Therefore, in order to control both the indoor air humidity and temperature for direct expansion (DX) air conditioning (A/C) systems, different controllers have been designed. In this paper, a discrete-time neural inverse optimal control scheme for trajectories tracking and reduced energy consumption of a DX A/C system is presented. The dynamic model of the plant is approximated by a recurrent high-order neural network (RHONN) identifier. Using this model, a discrete-time neural inverse optimal controller is designed. Unscented Kalman filter (UKF) is used online for the neural network learning. Via simulation the scheme is tested. The proposed approach effectiveness is illustrated with the obtained results and the control proposal performance against disturbances is validated.

A Steady-State Modeling Method for Direct Expansion Air Conditioning Systems

To figure out the operating features of Direct-Expansion Air-Conditioning (DX-AC) system, this paper employed the BP training algorithm and built a two-in/two-out steady-state model based on artificial neural network (ANN) and connect the DX-AC system’s Totally output Cooling Capacity (TCC) and Sensible Heat Ratio (SHR) under different combinations of rotational velocities of compressor and blower; then, the paper verified the proposed model by comparing the prediction results of the proposed model with experimental data, and the numerical analysis of the steady-state model and bilinear interpolation indicated that the ANN-based model proposed in the paper showed a high prediction accuracy and can well exhibit its operating performance.

Performance analysis of small-scale direct expansion air conditioning system with indirect evaporative cooler as dedicated ventilator

Split-type and window-type air-conditioners are typical small-scale direct expansion air-conditioning (DXAC) system widely-used for household applications. However, lack of controllable ventilation can be a serious problem leading to health problems for such air-conditioning (AC) systems. To improve the indoor air quality of a home space equipped with small-scale DXAC system, an indirect evaporative cooler (IEC) is used as a dedicated ventilator, on one hand, to guarantee proper outdoor air supply and on the other hand, to provide sensible cooling under certain climatic conditions. Validated numerical component models of IEC, DXAC and indoor thermal balance were established and coupled in series to form a closed-loop model of DXAC with IEC ventilator (DXAC-IEC) system. The effects of ambient humidity, ambient temperature, sensible heat ratio and ventilation rate on the system performance were investigated, with comparison to the standalone DXAC system. The results show that the IEC can enhance the system performance under moderate hot and dry ambient conditions (tamb ≤ 33 °C, RHamb ≤ 60%) by reducing the cooling load of DXAC. But it would bring extra moisture from outdoor in humid conditions (tamb ≥ 30 °C, RHamb>50%) for sacrifice of improved indoor air quality. Optimal fresh air ratio exists for the DXAC-IEC system to achieve the maximum coefficient of performance. The findings will be beneficial for the design and optimization of DXAC-IEC system.

상변화물질을 이용한 이동식 개인 공조시스템 해석

This study aimed to estimate the performance of a storage cooling system for roving personal air conditioning where a phase change material (PCM) was used as a condenser. The modeling on direct expansion air conditioning requires two-phase analysis. We applied 1-D two phase flow method for refrigerant inside tubes and effective thermal conductivity model outside tubes for PCM. Results were compared with those of a previous experience model which assumed a fixed ratio of super-heating region (10%), two-phase region (80%), and sub-cooling region (10%). The desired system discharge power was 600 Wh. Results showed that the previous experience method had an over-estimated performance due to the assumption of constant temperature applied over whole tube length while the current model coupled with 1-D two phase model and effective thermal conductivity model could cover the detailed heat transfer involved in the roving personal air-conditioning including developing a two-phase region.

Energy-Efficient Optimal Guaranteed Cost Intermittent-Switch Control of a Direct Expansion Air Conditioning System

To improve the energy efficiency of a direct expansion air conditioning (DX A/C) system while guaranteeing occupancy comfort, a hierarchical controller for a DX A/C system with uncertain parameters is proposed. The control strategy consists of an open loop optimization controller and a closed-loop guaranteed cost periodically intermittent-switch controller (GCPISC). The error dynamics system of the closed-loop control is modelled based on the GCPISC principle. The difference, compared to the previous DX A/C system control methods, is that the controller designed in this paper performs control at discrete times. For the ease of designing the controller, a series of matrix inequalities are derived to be the sufficient conditions of the lower-layer closed-loop GCPISC controller. In this way, the DX A/C system output is derived to follow the optimal references obtained through the upper-layer open loop controller in exponential time, and the energy efficiency of the system is improved. Moreover, a static optimization problem is addressed for obtaining an optimal GCPISC law to ensure a minimum upper bound on the DX A/C system performance considering energy efficiency and output tracking error. The advantages of the designed hierarchical controller for a DXA/C system with uncertain parameters are demonstrated through some simulation results.

Improving performance of direct expansion air conditioning systems while reducing electricity consumption through using hybrid energy

At the present time, operating hybrid air-conditioning systems that use solar energy to saving electrical energy while improving the performance has become necessary to protect the environment, reduce pollution and emissions caused by using fuels and gases. In Iraq, temperatures reach half the boiling point at summer, therefore the demand for air conditioning systems increases, air conditioning systems consume more than half of average electricity production which affects on reliability and stability of the electrical energy thus leads to a continuous power outage. So, the issue of using renewable energies becomes more attractive. Because of saving energy leads to ensuring the reliability of electricity and reduces the consumption of fuels and gases that pollute on the environment and negatively affect on the ozone layer. In the current research, the atmosphere of Baghdad city was used to collect solar thermal energy and convert it into thermal energy through an evacuated solar collector by water and combine it with a conventional air conditioner in the part that follows the compressor in order to reduce the electrical energy consumption on the compressor and increase coefficient of performance. Several tests were conducted on the proposed system to compare results with the conventional system and evaluate performance. The results showed that the coefficient of performance with the hybrid system became 8.97 more efficient instead of 4.27 compared to the conventional system, and the energy consumption decreased by 52%.

Steady-State Performance Prediction for a Variable Speed Direct Expansion Air Conditioning System Using a White-Box Based Modeling Approach

When using a certain type of Heating, Ventilation & Air Conditioning (HVAC) systems, it is primary to obtain their steady-state operating behaviors for achieving a better indoor thermal environment. This paper reports a development of a white-box-based dynamic model for a direct expansion (DX) air conditioning (A/C) system to predict its steady-state operating performance under variable speed operation. The established model consists of five sub-models, i.e., a compressor, an electronic expansion valve, an evaporator, a condenser and a conditioned space. Each sub-model was developed based on partial lumped parameter approach. Using the available data generated from an experimental DX A/C system, both transient and steady-state behaviors predictions agreed well with the experimental ones. With the help of the validated white-box model, the inherent steady-state operating performance expressed in terms of the relationship among total cooling capacity (TCC), equipment sensible heat ratio (E SHR) and coefficient of performance (COP) under various speed combinations of compressor and supply fan were further examined. The results show that a higher COP could be achieved when the DX A/C system was operated at a higher fan speed or a lower compressor speed for dealing with a larger required E SHR. This model could be helpful for A/C system design and controller development.

A modeling study on a direct expansion based air conditioner having a two-sectioned cooling coil

In small- to medium- scale buildings such as residences and retails located in hot and humid regions, direct expansion based air conditioning systems are widely used for the control of indoor thermal environment. However, to simultaneously control indoor air temperature and humidity using direct expansion air conditioning systems, additional and costly provisions/installation spaces are usually required. Therefore, to address the inadequacies encountered in previous related studies, based on multi-evaporator technology, a novel direct expansion based air conditioning system having a two-sectioned evaporator or cooling coil (TS-DXAC) was proposed to provide variable dehumidification capacity and improved indoor air distribution. In this paper, the development of a steady-state physics-based mathematical model for the novel TS-DXAC system is presented. The model was developed by referring to the existing sub-models for the key system components, such as a compressor, a condenser and an evaporator, which were currently available in open literature. The developed model was experimentally validated using a purposely established prototype experimental TS-DXAC system, with the differences between experimental and predicted results of less than 6%. Using the validated TS-DXAC model, a follow-up modeling study was carried out on optimizing the sizes of the two sections. The simulation results suggested that a lower surface area ratio for the two sections (Rs) can lead to a larger variation ranges of both output total cooling load (TCC) and equipment sensible heat ratio (E SHR). For example, at the inlet state of 26 °C and 50% relative humidity, when Rs was altered from 1:1 to 1:3, the variation range for TCC was increased by 33%, and that for E SHR by 51%, which was beneficial to better dehumidification.

Energetic and Exergetic Performance of Solar-Assisted Direct Expansion Air-Conditioning System with Low-GWP Refrigerants in Different Climate Locations

This study aims to assess an air-conditioning cycle designed to make use of hydrofluoroolefins (HFOs) within a direct expansion air-conditioning system combined with a solar energy source. An energy–exergy analysis was applied in order (i) to examine which HFO working fluid is more efficient and (ii) to draw a comparative analysis between the proposed cycle and the conventional air-conditioning cycle. To achieve this, a simulation has been devised to determine the coefficient of performance (COP), exergetic efficiency, and total irreversibility for three HFOs. These HFO fluids are R1234yf, R1234ze(E), and R1233zd(E) which were compared to R410A and R134a. System operating conditions, such as the impact of storage tank exit temperatures on compressor discharge temperatures and condenser surface augmentation, were also taken into consideration and are being investigated. Results of the study indicated that the use of evacuated tube solar collector within a vapor compression air-conditioning system can improve performance by 15–56% and save 5–40% in electrical energy consumption. The study also concludes that HFOs are excellent alternatives for R410A and R134a. Accordingly, the total irreversibility of the proposed cycle with R1234yf, R1234ze(E) and R1233zd(E) is less than that of R410A and R134a.

Designing Intelligent MIMO Nonlinear Controller Based on Fuzzy Cognitive Map Method for Energy Reduction of the Buildings

Designing a suitable controller for air-conditioning systems to reduce energy consumption and simultaneously meet the requirements of the system is very challenging. Important factors such as stability and performance of the designed controllers should be investigated to ensure the effectiveness of these controllers. In this article, the stability and performance of the fuzzy cognitive map (FCM) controller are investigated. The FCM method is used to control the direct expansion air conditioning system (DX A/C). The FCM controller has the ability to do online learning, and can achieve fast convergence thanks to its simple mathematical computation. The stability analysis of the controller was conducted using both fuzzy bidirectional associative memories (FBAMs) and the Lyapunov function. The performances of the controller were tested based on its ability for reference tracking and disturbance rejection. On the basis of the stability analysis using FBAMS and Lyapunov functions, the system is globally stable. The controller is able to track the set point faithfully, maintaining the temperature and humidity at the desired value. In order to simulate the disturbances, heat and moisture load changed to measure the ability of the controller to reject the disturbance. The results showed that the proposed controller can track the set point and has a good ability for disturbance rejection, making it an effective controller to be employed in the DX A/C system and suitable for a nonlinear robust control system.

Dynamic modeling of room temperature and thermodynamic efficiency for direct expansion air conditioning systems using Bayesian neural network

In this paper, dynamic performance identification for a direct expansion (DX) air conditioning (AC) system is proposed using Bayesian artificial neural network (ANN). The input and output datasets are generated by a dedicated AC simulator by varying the compressor speed in various signal amplitudes and including dynamic cooling load and ambient temperature. The exergy destruction, which represents the work potential losses in the system and room temperature indicating the thermal comfort are selected as the output variables. The key parameters of an ANN model, including the number of neurons and tapped delay lines, are optimized to improve the prediction accuracy. The results show that the dynamic response of the exergy destruction and room temperature can be predicted accurately by the optimized ANN model using three neurons, a Bayesian regularization algorithm, five delayed inputs for the compressor speed and room temperature, and six delayed inputs for the cooling load and ambient temperature. The validation of the multi-step-ahead prediction showed satisfying results with respect to the root mean squared errors (RMSEs) and coefficient of variations (CVs) of the room temperature (RMSE: 0.18 °C and CV: 0.85%) and exergy destruction (RMSE: 1.79 W and CV: 0.4%). Accordingly, the identification of the AC system behavior presented in this paper could be further implemented to control the DX AC system operation to achieve a desired thermal comfort with low exergy destruction.

Performance maps for an air-cooled air conditioning system as a preliminary instrument for the diagnosis of evaporator fouling

During the past few decades, there has been increased interest in the development of automated approaches for detecting and diagnosing faults in air conditioning systems. Among them, thermoeconomic diagnosis is a technique which involves the use of exergy analysis. As a first step towards improving its performance, this work is focused on the thermodynamic and exergy analysis of a direct expansion air conditioning system used in small commercial building applications. The analysis was carried out by means of experimental activities on a 17.5 kW rooftop unit installed at the Herrick Laboratories, Purdue University, Indiana (USA). The system under investigation was equipped with a variable-speed compressor and variable-speed fans, thus allowing the unit to meet different loads. A detailed mapping of the performance of this system was carried out by considering the effect of the outdoor temperature and the cooling load. In addition, the effect of evaporator fouling was investigated. Results showed that poor exergy performance were achieved due to the exergy destruction occurring in the evaporator. Also, when testing evaporator fouling, results showed that this fault contributed to increase the consumption of the mechanical exergy of the air, thus allowing for an easy detection of this fault.

A simulation study on the operational stability of an EEV-controlled direct expansion air conditioning system under variable speed operation

Hunting causes a poor operational safety and efficiency of a direct expansion (DX) air conditioning (A/C) system when it is variable speed (VS) operated to output different sensible and latent cooling capacities. An existing dynamic model was further developed by adding the equations for evaluating temperature sensor dynamics and air side operating parameters. After experimentally validated the model, the temperature sensor dynamics and VS operation on stability were numerically examined. Simulation results showed that due to the temperature sensor dynamics, actual system hunting could be much more serious than what was measured, and increasing the thermal resistance between temperature sensor and refrigerant pipeline would mitigate hunting. The mixture-vapor transition point in the DX evaporator would move closer to its exit at a smaller output equipment sensible heat ratio (E SHR) or a larger output total cooling capacity (TCC), consequently, leading to a higher chance to instability.

Development of a superheat controller for mitigating hunting in a direct expansion air conditioning system

This paper reports a development of a new degree of superheat (DS) controller which can slow down the rate of DS signal transfer in a direct expansion (DX) air conditioning (A/C) system for help mitigate operational instability. Through analyzing the heat transfer occurred at the outlet of an evaporator where the DS was evaluated, the dynamics of the DS signal transfer could be simply characterized by a first-order transfer function model (FOM). The new DS controller was thus developed by incorporating a FOM into a conventional proportional-integral (PI) controlled EEV-evaporator control loop, so that the rate of DS signal transfer could be regulated through varying the time constant of the FOM. Using an experimental DX A/C system, controllability tests for the new DS controller were carried out. Experimental results showed that the new DS controller would help mitigate possible system hunting when the experimental DX A/C system was unstably operated. The developed control strategy provides an alternative way to regulate the DS with an emphasis on operational stability.

An experimental study on a novel direct expansion based temperature and humidity independent control air conditioning system

Direct expansion (DX) air conditioning (A/C) system are widely used in various buildings because they are simpler and more energy efficient, and generally cost less to own and maintain. However, it is often problematical for a conventional On-Off controlled DX A/C system to simultaneously control indoor temperature and humidity, since the cooling and dehumidification process of air are highly coupled, unless costly extra equipment or complicated control method to decouple the cooling and dehumidification are provided. Therefore, a novel DX based temperature and humidity independent control air conditioning (DX-THIC) system with two parallel-connected condensers and two electronic expansion valves is presented for improved indoor environmental control and energy efficiency. This paper reports on an experimental study on the operating performances of a prototype experimental DX-THIC system. The experimental results demonstrated that the proposed DX-THIC system can provide independent control of indoor temperature and humidity. The experimental results also demonstrated that the indoor air humidity could influence the operating performances of the DX-THIC system.

Adaptive control for degree of refrigerant superheat in a direct expansion air conditioning system under variable speed operation

Direct expansion air conditioning systems are widely installed in small-to medium- scale buildings due to their advantages of simple configuration, high energy efficiency and low cost to own and maintain. The two main targets of air conditioning operation include improving the system operational efficiency and energy efficiency. Stable system operational efficiency is directly related with the operation of electronic expansion valves and degree of superheat at the outlet of evaporator. In this paper, an adaptive DS controller is developed to improve the system operational efficiency based the experimental performance of a DX A/C system and reported. The adaptive DS controller is designed to seasonably regulate the degree of superheat when the system is subjected to changes in operating conditions. Experimental controllability test results show that the controller has a very stable behavior, allowing an effective and fast control of the DS setting.

Dynamic State Feedback Decoupling of a DX A/C System

A temperature and humidity controller is designed for a direct expansion air conditioning (DX A/C) system making use of a state feedback decoupling approach of nonlinear control systems. It is shown that the nonlinear dynamics of a DX A/C system can be input-output decoupled by dynamic state feedback. The resulting decoupled system is of minimum phase. Thereafter, the decoupled model is used to design a pole placement controller with guaranteed stability. Unlike controllers based on approximate local linearization of the DX A/C model, the controller proposed is global in the sense that it can track temperature and humidity setpoints in the complete operating range of the DX A/C system. Effectiveness of the controller designed is demonstrated by simulation results.