Combined Air Conditioning Research Articles

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.

Study on Thermal Performance of the Small-Scale Air Conditioning with Thermoelectric Cooling Module

This work presents the COP and EER of the combined air conditioning and thermoelectric cooling systems. The small-scale air conditioning (8000 BTU/hr) with thermoelectric cooling systems is set up in the closed room (2.0*2.0*2.0 m), and free energy from the photovoltaic cell is used for the thermoelectric cooling module. The COP and EER with the thermoelectric cooling module are higher than those without the thermoelectric cooling module. It is observed that the COP and EER increase as the axial fan speeds increase. In addition, there is good agreement from the comparison, giving an average error of 3.77%. The highest COP and EER for the system with the thermoelectric cooling module are 1.71 and 5.85 for an airflow rate of 4.7 m3/s, respectively. The results can be used as guidelines for developing the thermal performance of air conditioning by combining it with the thermoelectric cooling module.

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

Experimental investigation on novel heat pump system for combined drying and air conditioning for arid climate

In an open-loop heat pump dryer, moisture removal in the evaporator is not profitable for an arid climate and hence evaporator may be used for comfort cooling, which leads to an efficient integrated system yielding both heat pump drying and air conditioning with a single energy source. Hence, the present experimental analysis is focused on the design, fabrication, and experimental investigation of an integrated heat pump system for hot and dry atmospheric conditions using future refrigerant R1234yf in the heat pump. The effects of the drying time, air inlet velocity, and inlet temperature to the evaporator and condenser on the energy, exergy, and economic performance of the air conditioning and the drying are investigated. The coefficient of performance for the combined operation is found to be much higher than that for the individual operation (heat pump drying or air conditioning), with an average value of 7.456 for input temperature of 26–45 °C. The average specific moisture extraction rate (SMER) is found up to 2.8 kg/kWh. The total annual profit gain by using the combined system instead of the separate systems is estimated as $309. The cooling air temperature and humidity are in the comfort zone, which confirms the suitability of the system for studied conditions.

Solar Energy Compensation for Building Energy Saving with Thermal Comfort in a Cold Climate

This paper proposes an energy-saving strategy with assistance from solar thermal compensation for building energy systems. The target of the control strategy was to minimize energy consumption under thermal comfort constraints in buildings. First, the factors influential to indoor temperature in building environments were analyzed. Secondly, the internal and external factors, such as building materials; building orientation; window size; heating, ventilation, and air conditioning (HVAC) facilities; blinding device; solar irradiation; wind speed; and outdoor temperature were used to construct a building model on the platform ENERGYPLUS (E+). A controller aiming to regulate the amount of solar irradiation was developed with the Building Controls Virtual Test Bed (BCVTB) tool. Afterward, the building performance under different strategies was tested by co-simulation using both the computational platforms, E+ and BCVTB. The optimum scheme achieved 30.6% energy savings while meeting the same comfort criterion of its competition strategy. The study verified that the proposed strategy of combined heating, ventilation, and air conditioning and blind control could realize the energy savings and comfort satisfaction at the same time. The proposed method provides a reference to the development of low-/zero-energy building concepts in the field.

Development of a non-stationary thermal environment analysis method for combined convection and radiation air conditioning

Advanced thermal environment simulation technology has been developing rapidly in recent years. The thermal environment simulation tools used in thermal engineering can be broadly classified into energy simulations and computational fluid dynamics programs. The combination of these approaches allows the high-accuracy prediction of a non-stationary thermal environment in a continuous space. The main feature of the coupled analysis method is that the radiation is calculated using energy simulation, which reduces the analysis load and makes transient analysis possible. Here, a method was devised for predicting the thermal environment of air conditioners and radiant panels, and its accuracy was verified. By coupling the convective heat transfer coefficient, the maximum mean absolute error of the proposed method was calculated to be 0.674 [-], which is acceptable considering the variable environmental conditions. Although in the acceptable range, this error is attributable to computational fluid dynamics, which is a steady-state analysis and does not fully track the non-stationarity of the radiant panel because of its stop–start nature. The results of this analysis are comprehensive, with the convective heat transfer coefficient calculated using the input conditions for each wall. Regarding the energy simulation, when temperature stratification occurred near a surface, such as the ceiling, a small error was induced because the reference temperature was set to the bulk temperature. However, as the overall trend was captured by the analysis method, these minor errors were deemed acceptable. These results show that the proposed method can be applied to a range of applications, including office spaces.

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.

A thermo-economic analysis of a combined cooling system for air conditioning and low to medium temperature refrigeration

This work investigates an integrated dual temperature system for combined air conditioning and low to medium temperature refrigeration applications (−40 °C to −10 °C). The system combines a steam driven parallel flow triple effect H2O–LiBr absorption cooling cycle with a CO2 refrigeration cycle. In this integrated system, the generated cooling effect by the H2O–LiBr absorption cycle is utilized in two different evaporators connected in series: the first evaporator as a heat sink or condenser for the CO2 refrigeration cycle, and the second evaporator as a regular evaporator for the absorption cycle. The system performance was evaluated from thermodynamic and economic viewpoints, and also detailed parametric studies were conducted. The integration of cycles assisted to significantly decrease the power requirements and annual operating cost of the CO2 refrigeration cycle as well as to increase its coefficient of performance. On the other hand, integration required a larger size absorption cooling cycle with a higher steam requirement, leading to an increase of the total capital cost and annual operating cost as well as a decrease of coefficient of performance of the absorption cycle. The economic analysis showed that integration causes a significant reduction in the annual operating cost and the unit production cost compared to the separate construction of both cycles. For the specified design and operating conditions, the annual operating cost and unit production cost of the integrated system reduced between 35.5% and 48.5% as well as between 23.4% and 34.5%, respectively.

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.

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.