Multi-split Variable Refrigerant Flow Research Articles

Enhancing self-consumption for decarbonization: An optimization strategy based on a calibrated building energy model

To face the challenge of climate change and achieve the decarbonization target set by the European Union, the current trend is to electrify building services, replacing the use of fossil fuels for renewable energy sources. The installation of grid-connected photovoltaic (PV) systems is becoming a popular strategy. However, the widespread application of PV solutions carries certain concerns about grid-network security and stability, since intermittent renewable energy excess pouring into the grid may exceed voltage limits. Therefore, an optimization of the consumption of a building's own PV production (self-consumption) to reduce the excess output is vital. The following paper performs a demand side optimization strategy of the building's thermostatic controllable loads (heating and cooling), which represent at least 50% of the total energy consumed by the building. The process is applied in a previously calibrated building energy model (BEM) that describes a fully operational building under a typical Mediterranean climate (Greece). The site contains a PV plant and a multi-split Variable Refrigerant Flow (VRF) system dedicated to maintain indoor comfort conditions. The technology used is simple, able to perform 15 minute time-step yearly optimizations while saving a large amount of computational time. It performs a bi-dimensional optimization of both: indoor thermal-zone set-points and ventilation air supply temperature. The optimization process performed is based on 2019 data gathered from European Project SABINA, resulting in a self-consumption improvement of 11.6% for summer scenario (reaching 69.16%) and 78.7% for winter (reaching 57.47%) in comparison to a non-optimized “business as usual” base model.

Seasonal adaptation of VRF HVAC model calibration process to a mediterranean climate

Today, Building Energy Models (BEM) have become essential in regulatory compliance calculations, the correct assessment of it’s Air Conditioning (AC) systems is critical for the reduction of the performance gap between BEMs and reality and increase the accuracy of evaluating buildings energy performance and it’s systems efficiency. Given that multi-split Variable Refrigerant Flow (VRF) systems have grown in the market in recent years becoming a particular trending solution to achieve building indoor comfort; the present paper focus on technical issues when modelling such VRF systems inside EnergyPlus, a white-box simulation environment, especially regarding the effects weather conditions have on the behaviour of VRF systems and it’s correlation with the AC system performance curves. The study performs an empirical validation of an optimization-based calibration methodology assessing multiple levels: average interior temperature of the different building spaces and electric energy consumption from VRF outdoor unit. It is performed using fifteen minute time-step seasonal data obtained from a fully operational building located in a typical Mediterranean climate (Greece), adjusting the parameter and curve values of the VRF system using a genetic NGSA-II algorithm (Jeplus software) for both summer and winter conditions. The generated BEM captures the building’s hourly performance for summer conditions using 1717 hours to fit into international standards. Complying with the requirements of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Guidelines 14-2002 for hourly energy consumption, reaching an NMBE ⩽±10% ,Cv(RMSE) ⩽30% and R2 ⩾75% while keeping indoor temperatures on every room with a RMSE ⩽1 °C. The resulting BEM proved stable during the 2077 hours of it’s summer evaluation period, fitting into the new unseen weather and building operation conditions of 2020 which can be considered a step forward in the area of calibrating white box models. While for winter conditions the study demonstrates the value of the calibration methodology while presenting the importance of weather influence on VRF systems. Using a total of 802 hours the applied technology greatly improves the results from the baseline model, reaching a partially calibrated BEM model for winter. Which reinforces the fact that regardless of how good a baseline model is, building operating conditions and weather may will always generate a design/performance gap and therefore the calibration of a BEM is unavoidable.

Investigation on Current Control Defrosting Method of Multi-split Variable Refrigerant Flow System

There are many indoor units in the multi-split variable refrigerant flow (VRF) system. The continuous operation of heating in winter is longer than that of ordinary household air conditioning, so that the precise defrosting control is energy-saving and improves the comfort for users. In this paper, during the experiment, the indoor temperature was 20℃, the outdoor dry bulb temperature was 7℃, 0℃, -5℃ and -7℃, and the wet bulb temperature was 6℃, 0℃, -6℃ and -8℃ respectively. The defrosting temperature, liquid pipe temperature, defrosting time and the current value change rate of the condenser fan were compared when the heating capacity was reduced by 15%. The experiments showed that the current change rate was little affected by the outdoor temperature. It was only related to the frost mass, which could reflect the real frosting situation. Then through the maximum cycle heating capacity, air supply temperature and capacity decay rate, the most reasonable defrosting time could be seen when the fan current change rate was 15% - 20%. Compared with the traditional defrosting, the current control defrosting had a higher heating capacity. Under the four working conditions, the heating cycle capacity was increased by 2.3%, 4%, 2.8% and 5.8% in turn. At the same time, the control was precise. The phenomenon of thermosensor falling off and the uneven frost formation had little impact on the current control defrosting.

Performance comparison between bypass cycle and injection cycle for sub-cooling methods in multi-split variable refrigerant flow (VRF) system in hot seasons

In this study, the performance of a multi-split VRF system using the bypass cycle and injection cycle is evaluated using the numerical simulation as a possible sub-cooling method to prevent flash gas generation in liquid pipelines. The simulation for the multi-split VRF system is developed by considering the applications of the bypass cycle and injection cycle, and is validated with experimental data. The bypass cycle and injection cycle in the multi-split VRF system yield improvements in their cooling capacities of the order of 3.22% and 13.43%, respectively, and energy efficient ratio (EER) of the order of 1.98% and 1.72%, respectively. The input power of the injection cycle is reduced by up to 4.45% when the performance of the multi-split VRF systems with bypass cycle and injection cycle is compared under the same cooling capacity conditions.

Performance analysis of multi-split variable refrigerant flow (VRF) system with vapor-injection in cold season

The performance of the variable refrigerant flow (VRF) system becomes poor at a very low ambient temperature because of the decrease in the suction mass flow rate. The vapor injection is applied to the heat pump system to enhance the performance of the systems in a cold season. In this study, the influence of the vapor injection under different intermediate pressures and outdoor ambient temperatures was investigated mathematically, and the results were validated with experimental data. In addition, the performance variation was observed with different indoor ambient temperatures in nine indoor units. The intermediate pressure affected the heating capacity, input power, coefficient of performance (COP), and isentropic efficiency of the compressor. The improvement of the COP with vapor injection was dominant at a low ambient temperature. The heating capacity at different indoor ambient temperatures was observed with different refrigerant mass flow rates in each indoor unit.

An efficient online wkNN diagnostic strategy for variable refrigerant flow system based on coupled feature selection method

The refrigerant leakage would occur in a multi-split variable refrigerant flow (VRF) system after years of operation, thus contributing to the inefficient operation and even increased energy consumption. Diagnosing the refrigerant charge amount (RCA) malfunction in time is of great necessity to ensure the normal operation of VRF system and avoid superfluous energy waste. This paper proposes an efficient online weighted k-Nearest-Neighbor model (wkNN) strategy to diagnose the RCA malfunction of the VRF system, which is based on the coupled feature selection method. The minimal-redundancy-maximal-relevance (mRMR) algorithm is first applied to derive the subset of features that have the maximum correlation with the target category and the minimum redundancy among each other. The ReliefF algorithm is utilized to rank variables in descending order. Final variables importance is ascertained on the basis of the average weighs of importance of variables and contributions of variables to the model's classification error rate. Correlation analysis (CA) is implemented to verify the rationality of the selected variables. Finally, a subset of six variables is served as the input variables to establish three kinds of models. Results indicate that the coupled feature selection method outperforms the single feature selection method, and the proposed wkNN model based on the coupled feature selection method is superior to the other two models and achieves desirable diagnostic performance on both experimental and practical data.

Simulation of recombined household multi-split variable refrigerant flow system with split-type air conditioners

In order to effectively utilize the outdoor units and minimize material waste and reinvestment in the replacement of split-type air conditioners by a new household multi-split variable refrigerant flow (VRF) system, a recombined household multi-split VRF system with original split-type air conditioners has been introduced in residential buildings. The equal frequency allocation principle and switching principle for three outdoor-unit operation modes have been determined for the recombined system. A universal model available for both the split-type air conditioner and the recombined household multi-split VRF system has been developed. The model was used for the design of the split-type air conditioners and the recombined system for a typical one-family household in three cities. Then, the cooling season energy performance of the split-type air conditioners and the recombined system were predicted by hourly calculation. According to the results, compared with split-type air conditioners, the recombined system can reduce the start-and-stop energy loss and work at higher compressor efficiency by optimizing the operation mode of outdoor units. The predicted energy saving ratio of the recombined household multi-split VRF system in the cooling season for the typical one-family household is 12.1%, 11.6% and 11.6% in Beijing, Shanghai and Guangzhou, respectively.

Simulation and optimization of multi-split variable refrigerant flow systems

Abstract The use of a variable refrigerant flow (VRF) is being specified as an alternative solution to centralize HVAC systems in domestic building. This manuscript presents a comprehensive description of the configuration for the indoor and outdoor units of a multi-split VRF system. A case study of a hotel in Amman, Jordan was implemented. The suitable indoor and outdoor units were selected according to heating and cooling load. A sensitivity analysis of the effect of some parameters on heating and cooling loads were analyzed. Finally, the thermal human comfort indices of PMV and PPD were investigated. This study showed that even the main shortcoming of using VRF system due to its high initial cost, but this system is encouraged especially in small places due to its energy efficiency appeal at partial loads, satisfying optimum thermal human comfort, and a good substitute where scarcity of water and achieving more thermal human comfort.

Life cycle analysis of multi-split Variable Refrigerant Flow (VRF) system: case study

Life cycle analysis of multi-split Variable Refrigerant Flow (VRF) system: case study

A novel efficient SVM-based fault diagnosis method for multi-split air conditioning system’s refrigerant charge fault amount

For the multi-split variable refrigerant flow (VRF) system, the key of efficient operation is to achieve the appropriate refrigerant charge amount (RCA). However, it is difficult to achieve because of the complexity of VRF systems. To overcome the difficulty, this paper presents a hybrid RCA fault diagnosis model combined support vector machine (SVM) with wavelet de-noising (WD) and improved max-relevance and min-redundancy (mRMR) algorithm. WD is responsible for improving the quality of collected VRF experimental data. In addition, mRMR is firstly used to rank all the variables in descending order in terms of their importance for identify RCA faults. After top-ranked variable is determined, correlation analysis of features is implemented for further feature selection removing the redundant variables in linkage to the variable at the top. Finally, a subset of seven features are selected to develop the SVM model. Results indicate that fault diagnosis accuracy of the seven-feature SVM model decreases only 2.14% compared with the initial eighteen-feature model. The proposed wavelet de-noising-max-relevance and min-redundancy-support vector machine (WD-mRMR-SVM) model shows good fault diagnosis performance for RCA faults.

Simulation on effects of subcooler on cooling performance of multi-split variable refrigerant flow systems with different lengths of refrigerant pipeline

Multi-split variable refrigerant flow (VRF) systems have wide applications in offices and commercial buildings. In the multi-split VRF system, a subcooler is usually adopted to avoid flash vapor generation in the liquid pipeline. At present, the effects of the refrigerant pipeline length of the multi-split VRF system with a subcooler have not yet been investigated, nor the optimization of the subcooler. In this paper, the model of a multi-split VRF system with a subcooler has been developed and validated. In nine cases with different lengths of the main pipelines and the subcooling heat exchanger (SCHX), the cooling performance of a multi-split VRF system has been studied by simulation. The results indicate that the subcooler has little benefit in a system with a short pipeline, while it can help improve the coefficient of performance (COP) of a system with a long pipeline by controlling the bypass mass flow rate ratio in a proper range, and a higher maximum COP can be obtained by increasing the length of the SCHX in the subcooler. Furthermore, it is recommended to control the outlet superheated degree of the bypass stream in the SCHX in order to maximize the COP of a multi-split VRF system.

Experimental investigation on cooling performance of multi-split variable refrigerant flow system with microchannel condenser under part load conditions

On the basis of a 12.5 kW multi-split variable refrigerant flow (VRF) system, the nominal cooling capacity ratio (θ) and the COP of the baseline system with fin-tube heat exchanger (FTHX) and the VRF system with microchannel heat exchanger (MCHX) were obtained for different part load ratios (PLRs). The key control strategies of the VRF system were presented. The charged amount of refrigerant ratio β and the pressure drop ΔP of the MCHXs were also achieved. The experimental results show that the β of the MCHXs was decreased by 26.67–46.67% and the optimum charged amount of refrigerant of a VRF system with an MCHX should obtain a discharge pressure between 3.0 MPa and 3.15 MPa for PLR round 100%. The VRF system with the MCHXs obtained large ΔTHX to avoid refrigerant flashing expansion in long pipe application and it could provide residents with better thermal comfort in hot seasons or regions.

Field performance measurements of a VRF system with sub-cooler in educational offices for the cooling season

The effects of the subcooling heat exchanger (SCHX) on the performance of the multi-split variable refrigerant flow (VRF) system with long pipe were investigated in a field test during the cooling season. By varying the refrigerant mass flow rate via the electronic expansion valve, the degree of subcooling was controlled. It was found that VRF system with SCHX improved the cooling performance factor (CPF) about 8.5% under similar outdoor temperature profiles, as compared to the baseline without SCHX. However, when the fraction of total refrigerant that passes through the SCHX was higher than 5.27%, the CPF starts to decrease due to the decreased refrigerant mass flow rate through the evaporators. Employing a SCHX in a VRF system with long pipe was found to be an effective method for improving the system performance and reliability.

Variable refrigerant flow systems: A review

This review study presents a detailed overview of the configurations of the outdoor and indoor units of a multi-split variable refrigerant flow (VRF) system, and its operations, applications, marketing and cost. Besides, a detailed review about the experimental and numerical studies associated with the VRF systems is provided. The aim is to put together all the diversified information about the VRF systems in a single source. According to detailed review, it is observed that the compressor frequency and the electronic expansion valve opening should be controlled simultaneously for the control strategies, and it is concluded that VRF system not only consumes less energy than the common air conditioning systems such as variable air volume, fan-coil plus fresh air under the same conditions, but also provides better indoor thermal comfort as long as it is operated in the individual control mode. It is found that even though the main drawback of the VRF system is the high initial cost compared to the common air conditioning systems, due to the energy saving potential of the VRF system, the estimated payback period of the VRF system compared to an air cooled chiller system in a generic commercial building could be about 1.5 year.