Combined Air Conditioning System Research Articles

Optimization and operation control for the combined impinging jet ventilation and chilled ceiling system with different cooling loads

The matching relationship of operating parameters between the air supply and radiant systems has significant impact on the indoor thermal comfort and energy efficiency for combined air-conditioning system. The impinging jet ventilation (IJV) and chilled ceiling (CC) combined system (IJV/CC) overcomes the drawback of IJV which cannot be used in spaces with high cooling load. However, the matching relationship between the operating parameters of the IJV and CC are not clear due to limited research on IJV/CC at present. To ensure good thermal comfort and indoor air quality of the IJV/CC in an energy efficient way, this study mainly focuses on exploring the optimal combination of the operating parameters between the IJV and CC (including the supply temperature and supply velocity of IJV and internal surface temperature of CC (Tc)). First, the indoor thermal environment for IJV/CC operating with different combinations of design parameters is simulated by CFD. Second, response surface methodology (RSM) is utilized to establish predictive models for various ventilation performance indicators by collecting the simulation results. Then, the optimal combinations of operating parameters between the IJV and CC are determined by using the technique of order preference by similarity to ideal solution (TOPSIS) coupled with the developed RSM models. Finally, control strategies for the IJV and CC are discussed and it suggests setting the value of Tc in the range of 20–21 °C and a variable air volume system is preferred for the control of the air supply system to achieve efficient operation of IJV/CC.

Operation Pattern Recognition of the Refrigeration, Heating and Hot Water Combined Air-Conditioning System in Building Based on Clustering Method

Air-conditioning system operation pattern recognition plays an important role in the fault diagnosis and energy saving of the building. Most machine learning methods need labeled data to train the model. However, the difficulty of obtaining labeled data is much greater than that of unlabeled data. Therefore, unsupervised clustering models are proposed to study the operation pattern recognition of the refrigeration, heating and hot water combined air-conditioning (RHHAC) system. Clustering methods selected in this study include K-means, Gaussian mixture model clustering (GMMC) and spectral clustering. Further, correlation analysis is used to eliminate the redundant characteristic variables of the clustering model. The operating data of the RHHAC system are used to evaluate the performance of proposed clustering models. The results show that clustering models, after removing redundant variables by correlation analysis, can also identify the defrosting operation mode. Moreover, for the GMMC model, the running time is reduced from 27.80 s to 10.04 s when the clustering number is 5. The clustering performance of the original feature set model is the best when the number of clusters of the spectral clustering model is two and three. The clustering hit rate is 98.99%, the clustering error rate is 0.58% and the accuracy is 99.42%.

Improved Thermal Performance of Combined Convection and Radiation Using Room Air Conditioner

This study aims to evaluate the indoor thermal environment within a detached residence in summer by applying combined convection and radiation air conditioning system using the refrigerant gas of room air conditioners (RACs). First, field measurements were performed in a detached residence at Hiroshima, Japan, in July and August 2021. Consequently, PMV values were generally maintained within the comfortable range during air-conditioning operation. The amount of heat generated by convection (via the RAC indoor unit) Qc and that by radiation (via the radiation panel) Qr were determined as 426 and 259 W, respectively, and the overall heat generated Qtotal was determined as 685 W. Applying the CFD model, thermal environment was compared varying the ratio between Qc and Qr with a constant Qtotal of 685 W. Results showed that the synergistic operation of a RAC and a radiant panel presented more comfortable PMV values than that of a single RAC application. Furthermore, the airflow from the RAC decreased with an increase in Qr ratio, and the chilled radiant panel provided a cooling effect around the dining area, where the airflow of the RAC was inadequate, thereby improving the thermal comfort.

Techno-economic evaluation of a hybrid photovoltaic system with hot/cold water storage for poly-generation in a residential building

There is a growing effort towards the application of solar technologies to meet electrical and thermal demand. However, substantial energy is used in buildings around the world for electricity and thermal comfort. Here we evaluate a novel hybrid solar-powered system for reducing the load on the direct expansion and heat pump unit of a combined heating and air conditioning system while generating electricity. The system combines rooftop photovoltaic (PV) and building-integrated photovoltaic thermal (BIPV/T) systems for electricity generation, while any excess electricity is used to drive a hot and cold water storage system. A residential building in Tehran is used as a case study, and the results are computed via MATLAB, TRNSYS, and Carrier HAP software. The system is shown to outperform both rooftop PV and BIPV/T systems alone, generating at least 50% more electricity while reducing the heating and cooling loads on the machinery by at least 60%. The system also enjoys a payback time of 2.87 years. The impact of thermal comfort setpoints, in addition to inflation and discount rates, are also evaluated via a sensitivity analysis involving several key performance metrics, namely the hot and cold water storage volumes, the size of the desiccant wheel, and the payback time. The cooling load is found to have the greatest influence on the cold water storage volume and the desiccant size, with 80.1% and 41.2% variations for baseline variations of ±20%. With a 57.7% increase, the heating load is found to have the biggest influence on the hot water storage volume. With a 27.1% variation, inflation has the strongest effect on the payback time.

Development of the technology for creating air conditioning systems in an arid climate (Eritrea)

This study aimed to determine the initial data and develop a technology for creating air conditioning systems under the complicated climatic conditions of East Africa. A technique for processing the observational materials is proposed and the corresponding results are stated, which are subsequently used to determine the temperature and humidity of the outdoor and indoor air on the premises of the College of Marine Science and Technology located in the most complicated climatic region of Eritrea. The choice was substantiated, and the calculation and selection of equipment for a special combined environmentally-friendly air conditioning system were performed.

Method of System Identification for Air Conditioning Systems in Operation

This study aims to optimize the operation of an air conditioning (AC) system by tracking situational changes due to outside temperature, number of people and computers, and other factors. Therefore, we studied the accurate estimation of system parameters of an AC unit during operation. We modeled the AC system using the first-order plus dead time model and discretized it using the autoregressive with exogenous input model. We developed a technique to estimate the system parameters using Bayesian optimization. Here, the system parameters are values that determine the physical characteristics of the combined air conditioning system and room. Therefore, we determined that there are cases where the characteristics deteriorate after repeated estimation. By solving this problem, we were able to establish a practical system. Keywords—ARX time-series model, Bayesian optimization method, First-order plus dead time, Gaussian process, PID control

Design and Control Method of Combined Air Conditioning System Considering the Sharing of Outdoor Air and Room Load Processing

Design and Control Method of Combined Air Conditioning System Considering the Sharing of Outdoor Air and Room Load Processing

Study on Mathematical Model of Desiccant Wheel

In the process of dehumidification and regeneration, a complicated heat and mass transfer process occurred between air and hygroscopic materials in desiccant wheel. The performance of dehumidifier is affected by the air inlet state parameters, regeneration air inlet state, air flow, regeneration air flow, and wheel speed and many other factors.Based on the conservation of water mass and energy in the gas region of the desiccant wheel and the conservation of water mass and energy in the solid region, a differential equation describing the absorption and regeneration processes in the wheel is established. In addition to necessary boundary conditions and supplementary equations, a complete mathematical model of desiccant wheel is developed.Numerical solution of mathematical model can be obtained by using finite difference method, which can be used in calculation of actual process and optimization analysis of wheel dehumidification combined air conditioning system.

Experimental study on total heat transfer efficiency evaluation of an indirect evaporative cooler

Indirect evaporative cooler is applied as a device of the combined air conditioning system to precool the fresh air in hot and humid regions, which may cause condensation in the primary air channels. For this case, the trend of the existing evaluation indexes used to express the heat exchange efficiency is inconsistent with that of the total heat transfer. In this paper, a plate cross-flow indirect evaporative cooling experimental system was set up, and the inadaptability of the existing evaluation indexes was analyzed by applying the experimental data. A new index called enthalpy efficiency was introduced in this paper. It was found that the change curve of enthalpy efficiency was highly consistent with that of the total heat transfer. By utilizing the data in other literatures, it was proved that the enthalpy efficiency can be used to evaluate the heat transfer capacity of plate cross-flow and counter-flow indirect evaporative coolers. The evaluation index can be used for thermal calculation, equipment selection design and performance optimization of an indirect evaporative cooler.

ПІДВИЩЕННЯ ЕФЕКТИВНОСТІ КОНДИЦІЮВАННЯ ЗОВНІШНЬОГО ПОВІТРЯ СИСТЕМИ КОМБІНОВАНОГО ТИПУ

One of the most attractive reserves of enhancing the energetic efficiency of air conditioning systems is to provide the operation of refrigeration compressors in nominal or close to nominal modes by choosing rational design cooling loads (cooling capacities) and their distribution according to a cooling load behaviour within the overall design (installed) cooling load band to match current changeable climatic conditions and provide close to maximum annual cooling capacity generation according to cooling duties. The direction of increasing the efficiency of outdoor air conditioning in combined central-local type systems by rationally distributing the heat load - cooling capacity of the central air conditioner into zones of variable heat load in accordance with current climatic conditions and its relatively stable value, i.e. cooling capacity required for further air cooling at the entrance to the indoor recirculation air conditioning system is justified. By comparing the values of the excessive production of cold and its deficit within every 3 days for a rational design heat load of the air conditioning system (cooling capacity of the installed refrigeration machine), which provides close to maximum annual production of cold, and the corresponding values of the excess and deficit of cooling capacity in accordance with current climatic conditions during July substantiated the feasibility of accumulating the excess of cooling capacity of a central air conditioner at low current loads and its use for covering cooling deficit at elevated heat loads through pre-cooling the outdoor air. It is developed a scheme of a combined central-local air conditioning system, which includes the subsystems for the outdoor air conditioning in a central air conditioner and the local indoor recirculated air conditioning.

Start-up behaviour of a combined air-conditioning system in cooling and heating operating modes

The energy demand of the commercial building sector is largely for cooling and heating and will eventually result in energy and environmental crises; therefore, enhancing the performance of the cooling and heating systems used in buildings is paramount. In response to this need, focus was set on the gas engine-driven heat pump-combined system (GHPC)—consisting of a gas engine, single-stage vapour compression system, and single-effect absorption system— that saves more energy as compared to a conventional gas engine-driven heat pump. However, so far, only the performance and transient characteristics of a fabricated GHPC have been investigated. Therefore, to ascertain the characteristics of the GHPC start-up dynamics under unsteady state operations so as to achieve efficient and stable start-up operation, experiments and simulations were attempted in this study. The transient behaviour from the start-up of this combined system in cooling and heating operating modes was also scrutinized. Accordingly, it was found that simulation results agreed well with experimental data. It was concluded that there were no large delayed responses in the absorption system during start-up operation when start-up of the absorption system is considered based on the system cooling capacity response after the solution pump starts.

Numerical Study and Analysis on Combined Ventilation System in House Room

Indoor air quality is studied using numerical simulation method in house room based on the tracer gas. A comprehensive discussion of indoor air temperature and contaminant concentration distribution is given by the comparing analysis results. The results show that the cooperation of combined ventilation system and air conditioning system has preferable effect on decreasing indoor contamination concentration and improving indoor air quality.

Experimental analysis of an evaporative–vapour compression based combined air conditioning system for required comfort conditions

An evaporative–vapour compression based combined air conditioning system for providing required human comfort conditions at comparatively low cost has been presented. The combined system has been experimentally analysed in a particular experimental setup at Bhopal, India for a wide range of ambient conditions in different modes of operation according to the climatic condition. System has also compared from conventional vapour compression air conditioner working at same fresh air ratio (FAR) and air flow rate for similar inside set temperature. Maximum energy saving in the combined system was found 23.8% at 43.3°C ambient temperature and 18.1% relative humidity, while it was run at no profit–no loss condition for humid climate. The system is working well with an average monthly power saving of 58.5kWh for moderate hot and dry, and 163.5kWh for hot and dry conditions. Therefore it could be a better alternative to conventional air conditioning system for dry and humid climate with a payback period of 8.1years.

Parametric analysis of a combined dew point evaporative-vapour compression based air conditioning system

A dew point evaporative-vapour compression based combined air conditioning system for providing good human comfort conditions at a low cost has been proposed in this paper. The proposed system has been parametrically analysed for a wide range of ambient temperatures and specific humidity under some reasonable assumptions. The proposed system has also been compared from the conventional vapour compression air conditioner on the basis of cooling load on the cooling coil working on 100% fresh air assumption. The saving of cooling load on the coil was found to be maximum with a value of 60.93% at 46°C and 6g/kg specific humidity, while it was negative for very high humidity of ambient air, which indicates that proposed system is applicable for dry and moderate humid conditions but not for very humid conditions. The system is working well with an average net monthly power saving of 192.31kWh for hot and dry conditions and 124.38kWh for hot and moderate humid conditions. Therefore it could be a better alternative for dry and moderate humid climate with a payback period of 7.2years.

Thermodynamic analysis of the evaporative-vapour compression based combined air conditioning system for hot and dry climatic conditions

An evaporative-vapour compression based combined air conditioning system for providing good human comfort conditions at a low cost working under hot and dry climate is proposed in this paper. The proposed system is thermodynamically analysed for hot and dry climate of Bhopal, India under some reasonable assumptions. Analysis was done on various parameters for the data from March to June 2013 Bhopal, provided by Indian meteorological department, Bhopal. The proposed system was also compared on the basis of saving (%) in cooling load on the cooling coil for the same sensible cooling rate to the conditioned space from the conventional vapour compression air conditioner working on 100% fresh air assumption. The saving of cooling load on the coil was found maximum with a value of 64.19% in the month of March due to lower outside temperature and it is minimum for the month of May with a value of 27.36% due to higher outside temperature. Saving in the month of June is −51.21% due to higher relative humidity. Negative sign indicates that proposed system is applicable for hot and dry climate and worked well with a net power saving of 646.8kWh from March–May for a small capacity application. Therefore it could be a better alternative for hot and dry climate with a payback period of 6.6 years.

Coordinated optimization of the variable refrigerant flow and variable air volume combined air-conditioning system in heating conditions

Although variable refrigerant flow systems have become attractive due to good energy performance in part-load conditions, the shortcoming of the variable refrigerant flow system in outdoor air ventilation has not been well solved. A variable refrigerant flow and variable air volume combined air-conditioning system was proposed to solve this problem. The variable air volume part of the combined system consists of an outdoor air processing unit and air supply and distribution devices. The combined system was found having potential to obtain high-level energy efficiency without compromising thermal comfort by an intelligent control system. Therefore, a so-called global coordinated optimization strategy based on a local coordinated optimization strategy is proposed in this study. The local coordinated optimization strategy decreases energy consumption of the combined system through optimizing load allocation between the variable refrigerant flow unit and the outdoor air processing unit. The global coordinated optimization strategy has a much deeper coordination of the variable refrigerant flow and outdoor air processing units, as it combines the local coordinated optimization strategy with stopping and restarting of the outdoor air processing unit. Performances of the combined system in heating conditions under this strategy are evaluated in the simulation platform. Results show that the global coordinated optimization strategy can effectively reduce the energy consumption of the combined system.

Online optimal control of variable refrigerant flow and variable air volume combined air conditioning system for energy saving

The variable refrigerant flow (VRF) and variable air volume (VAV) combined air conditioning system can solve the problem of the VRF system in outdoor air ventilation while taking advantage of its high part load energy efficiency. Energy performance of the combined air conditioning system can also be optimized by joint control of both the VRF and the VAV parts. A model-based online optimal control strategy for the combined air conditioning system is presented. Simplified adaptive models of major components of the combined air conditioning system are firstly developed for predicting system performances. And a cost function in terms of energy consumption and thermal comfort is constructed. Genetic algorithm is used to search for the optimal control sets. The optimal control strategy is tested and evaluated through two case studies based on the simulation platform. Results show that the optimal strategy can effectively reduce energy consumption of the combined air conditioning system while maintaining acceptable thermal comfort.

Optimal control of combined air conditioning system with variable refrigerant flow and variable air volume for energy saving

An optimal control strategy for minimizing the energy consumption of variable refrigerant flow (VRF) and variable air volume (VAV) combined air conditioning systems was presented. The combined system was proposed to take advantages of VAV systems to solve the ventilation problem of VRF systems. The VAV part consists of an outdoor air processing unit and air supply and distribution devices. To determine set-point of the optimal control variable, i.e. outdoor air supply temperature, this paper proposed an optimal control strategy based on adaptive predictive model and recursive least squares estimation technique. Typical days in summer and winter were selected to test the strategy. Results indicated that the optimal control strategy reduces energy consumption of the combined system by 32.17% in summer and 2.47% in winter. The overall energy efficiency is enlarged by 12.18% in summer and 3.37% in winter, compared with the benchmark operation strategy.

Control and energy simulation of variable refrigerant flow air conditioning system combined with outdoor air processing unit

A variable refrigerant flow (VRF) unit and outdoor air (OA) processing unit combined air conditioning system is proposed as a solution for ventilation problems in VRF systems. System structure and control strategies are addressed. Simulation platform is established based on component and sub-system models developed. Control and energy performances analysis are then put forward under various conditions. It is found that the combined system could maintain all the zones at their specific set-points within small errors no matter set-points of the zones are the same or different. In addition, indoor air quality can be ensured. Energy efficiency characteristics of the combined system are greatly affected by the OA supply temperature of the OA processing unit, which opens up the opportunity of minimizing the energy consumption of the combined system through optimal control strategy. Results reveal that the best OA supply temperature can be obtained through optimizing part load ratio of the OA processing unit to a range in which the system operates with high efficiency.

Energy conservative air conditioning system using silver nano-based PCM thermal storage for modern buildings

This work aims at improving the thermal performance and energy efficiency of chilled water based variable air volume air conditioning system integrated with the silver nanoparticles embedded latent thermal energy storage system. The latent thermal energy storage air conditioning system incorporated with the demand controlled ventilation and the economizer cycle ventilation schemes were experimentally investigated for the year-round building air conditioning application. Phase change material embedded with silver nanoparticles enabled it to exhibit improved heat transfer mechanisms in charging and discharging cycles. Experimental results suggest that the proposed air conditioning system achieved an on-peak and per day average energy savings potential of 36–58% and 24–51%, respectively, for year round operation while compared to the conventional air conditioning system. Similarly, while compared with a basically similar variable air volume air conditioning system, the proposed air conditioning system yielded 7.5–18.6% and 7.9–17.8% of on-peak and per day average energy conservative potential, respectively. Furthermore, test results infer that the combined effects produced by the silver nanoparticles embedded latent thermal energy storage system with the ventilation techniques augmented the overall thermal performance of the system. In total, the combined air conditioning system would be beneficial in terms of accomplishing good thermal comfort, acceptable indoor air quality and energy redistribution needs in buildings without sacrificing energy efficiency.