Simulation comparison of VAV and VRF air conditioning systems in an existing building for the cooling season

Performance of two widely used air conditioning (AC) systems, variable air volume (VAV) and variable refrigerant flow (VRF), in an existing office building environment under the same indoor and outdoor conditions for an entire cooling season is simulated by using two validated respective models and compared. It was observed that the indoor temperatures could not be maintained properly at the set temperature by the VAV no-reheat boxes. However, it could be maintained by the VAV boxes with reheat with a significant energy consumption penalty. It was found that the secondary components (indoor and ventilation units) of the VRF AC system promised 38.0-83.4% energy-saving potential depending on the system configuration, indoor and outdoor conditions, when compared to the secondary components (heaters and the supply fan) of the VAV AC system. Overall, it was found that the VRF AC system promised 27.1-57.9% energy-saving potentials depending on the system configuration, indoor and outdoor conditions, when compared to the VAV AC system.     

Integration of variable refrigerant flow and heat pump desiccant systems for the heating season

Integration of the variable refrigerant flow (VRF) and heat pump desiccant (HPD) systems was investigated in a field performance test for a heating season. The HPD systems use only the moisture in the outdoor air and return air to humidify the indoors during ventilation in the heating season. Three different operating modes: non-ventilated, HPD ventilation assisted and HPD ventilation-humidification assisted VRF systems were investigated. It was found that the VRF systems provided an average of 93.5% of the total heating energy for the HPD ventilation assisted mode. The remainder was the recovered heat by the HPD systems during ventilation. The VRF systems provided an average of 46.8% of the total heating energy for the HPD ventilation-humidification assisted mode. The remainder was covered by the HPD systems which provided additional sensible and latent heating. Overall, among the three operating modes, it is concluded that the HPD ventilation-humidification assisted VRF outdoor units consume less energy than the HPD ventilation assisted ones (about the same energy as the non-ventilated ones), while providing the best indoor thermal comfort and indoor quality conditions. For the total system, the HPD ventilation-humidification assisted VRF systems consume less energy than the HPD ventilation assisted ones.     

Energy simulation in the variable refrigerant flow air-conditioning system under cooling conditions

As a high-efficiency air-conditioning scheme, the variable refrigerant flow (VRF) air-conditioning system is finding its way in office buildings. However, there is no well-known energy simulation software available so far which can be used for the energy analysis of VRF. Based on the generic dynamic building energy simulation environment, EnergyPlus, a new VRF module is developed and the energy usage of the VRF system is investigated. This paper compares the energy consumption of the VRF system with that of two conventional air-conditioning systems, namely, variable air volume (VAV) system as well as fan-coil plus fresh air (FPFA) system. A generic office building is used to accommodate the different types of heating, ventilating, and air-conditioning (HVAC) systems. The work focuses on the energy consumption of the VRF system in the office buildings and helps the designer's evaluation and decision-making on the HVAC systems in the early stages of building design. Simulation results show that the energy-saving potentials of the VRF system are expected to achieve 22.2% and 11.7%, compared with the VAV system and the FPFA system, respectively. Energy-usage breakdown for the end-users in various systems is also presented.     

Comparison study of a novel solid desiccant heat pump system with EnergyPlus

The variable refrigerant flow (VRF) air conditioning system usually needs to be operated with a ventilation system, since the VRF system cannot provide fresh air. The commonly used ventilation unit with the VRF system is the heat recovery ventilation (HRV) unit due to its merits of energy saving. In this study, a novel solid desiccant heat pump unit (DESICA) is introduced and mathematical model of DESICA is developed based on the dynamic building energy simulation software—EnergyPlus. The mathematical model is validated with experimental results. Based on the model, performance comparison study is conducted among the novel joint DESICA and VRF (DES&VRF) system, the conventional joint HRV and VRF (HRV&VRF) system, and the original VRF standalone with ventilation (VRFSA) system in an office building in Shanghai. Simulation results show that, HRV&VRF and VRFSA can handle the sensible load, though both of them cannot well deal with the latent load. On the contrary, DES&VRF system can keep both indoor temperature and humidity ratio at the target value, resulting in the best indoor thermal comfort than the other two systems. In addition, through the whole year, DES&VRF consumes 5% more energy than VRFSA and 20% less energy than HRV&VRF.     

关于冷媒变流量多联机空调系统设计的探讨

本文根据多联机空调系统的特性,分析了配管长度对多联机空调系统制冷量和能效比的影响,提出了合理使用多联机,提高空调系统制冷量和能效比的建议。     

Modeling and energy simulation of the variable refrigerant flow air conditioning system with water-cooled condenser under cooling conditions

As a new system, variable refrigerant flow system with water-cooled condenser (water-cooled VRF) can offer several interesting characteristics for potential users. However, at present, its dynamic simulation simultaneously in association with building and other equipments is not yet included in the energy simulation programs. Based on the EnergyPlus's codes, and using manufacturer's performance parameters and data, the special simulation module for water-cooled VRF is developed and embedded in the software of EnergyPlus. After modeling and testing the new module, on the basis of a typical office building in Shanghai with water-cooled VRF system, the monthly and seasonal cooling energy consumption and the breakdown of the total power consumption are analyzed. The simulation results show that, during the whole cooling period, the fan-coil plus fresh air (FPFA) system consumes about 20% more power than the water-cooled VRF system does. The power comparison between the water-cooled VRF system and the air-cooled VRF system is performed too. All of these can provide designers some ideas to analyze the energy features of this new system and then to determine a better scheme of the air conditioning system.     

浅谈多联机空调系统的设计

简单介绍了多联机空调系统的工作原理,针对多联机空调系统在工程设计中应注意的建筑节能,室内机型选择,室外机布置等问题进行了分析探讨,指出了设计要点及注意事项,以促进多联机空调系统的推广应用。     

Comparison of energy efficiency between variable refrigerant flow systems and ground source heat pump systems

With the current movement towards net zero energy buildings, many technologies are promoted with emphasis on their superior energy efficiency. The variable refrigerant flow (VRF) and ground source heat pump (GSHP) systems are probably the most competitive technologies among these. However, there are few studies reporting the energy efficiency of VRF systems compared with GSHP systems. In this article, a preliminary comparison of energy efficiency between the air-source VRF and GSHP systems is presented. The computer simulation results show that GSHP system is more energy efficient than the air-source VRF system for conditioning a small office building in two selected US climates. In general, GSHP system is more energy efficient than the air-source VRV system, especially when the building has significant heating loads. For buildings with less heating loads, the GSHP system could still perform better than the air-source VRF system in terms of energy efficiency, but the resulting energy savings may be marginal.     

变制冷剂流量空调系统在列车卧铺车厢的应用研究

针对传统的单元式空调机组难以满足列车内舒适性要求的问题,提出了列车卧铺车厢采用变制冷剂流量(VRF)空调系统的新思路。分析了VRF空调系统的特点,给出了VRF空调系统的系统设计及电气控制方案。采用CFD模拟和试验相结合的方法分析了卧铺车厢的气流组织特性。     

建筑节能与变冷媒流量多联机空调系统的适用性

《公共建筑节能设计标准》（GB50189—2005）实施以来，变冷媒流量多联机空调系统的应用引起了诸多非议。分析了变冷媒流量多联机空调系统的特点，结合实际工程论证，指出变冷媒流量多联机空调系统适用性的关键在于建筑本身的使用功能，而与建筑规模并无直接的关系。在大型公共建筑中，只要合理选用空调系统并正确设计，变冷媒流量多联机空调系统在建筑节能中可以起到非常重要的作用。     

A study on the energy penalty of various air-side system faults in buildings

Automatic fault detection and diagnosis (FDD) can help enhance building energy efficiency by facilitating early detection of occurrence of system faults, especially those of air-conditioning systems, thus enabling rectification of the faults before much energy is wasted due to such faults. However, building owners may not invest in FDD unless they are convinced of the energy cost savings that can be achieved. This paper presents the results of a study on the energy cost impacts of a range of common system faults in variable air volume (VAV) air-conditioning systems, which are widely adopted for their good part-load energy efficiency. The faults studied include room air temperature sensor offset, stuck VAV box damper, supply air temperature sensor offset, stuck outdoor air damper and stuck/leaking cooling coil valve. The simulation results indicate that some faults may significantly increase energy use in buildings, for example, negative room air temperature sensor offset, stuck open VAV box damper, negative supply air temperature sensor offset, stuck open outdoor air damper and stuck open and leaking cooling coil valve. Since building occupants may adapt to the symptoms of these faults, such as reduced room air temperature, and thus may not complain about them, the occurrence of such faults are not immediately apparent unless a FDD system is available. Some other faults, e.g. positive supply air temperature sensor offset, positive room air temperature sensor offset, stuck closed cooling coil valve and stuck closed VAV box damper, may allow less energy to be used but will lead to unbearable indoor environmental conditions, such as high indoor temperature. Such faults, therefore, can easily be detected even without a FDD system, as there will be feedback from the building occupants.     

一种新型直接膨胀式温湿度独立控制恒温恒湿空调系统

为了消除传统恒温恒湿空调系统中冷热抵消现象,提出了一种新型直接膨胀式温湿度独立控制恒温恒湿空调系统(下称新型空调系统).该空调系统通过组合三级直接膨胀式制冷单元、加热器、加湿器,并配备特有的控制策略,可以在满足恒温恒湿环境一定温湿度控制精度的前提下,实现温湿度解耦控制.该空调系统不会造成投资成本大幅增加,并具备结构简单...     

变风量空调系统的建模与控制

介绍了变风量空调系统的基本原理。结合变风量空调系统的特点,在基于被控房间数学模型的基础上,将模糊控制和常规PID控制相结合,提出了一种模糊PID控制方法并将其应用于变风量空调室温控制中。     

能量法变风量技术在某卷烟厂中的应用

介绍了某卷烟厂用能量法变风量的方式调节室内送风量。室内外焓差作为能量法变风量调节的主要参数,用以调节风机的频率,进而改变室内送风量以及室外新风量。建立能量法变风量的数学模型,对卷烟厂应用能量法变风量的经济性进行了分析,证明能量法变风量具有较好的节能效果。     

Energy simulation and analysis of the heat recovery variable refrigerant flow system in winter

Heat recovery variable refrigerant flow (HR-VRF) system can supply cooling and heating for the building simultaneously and make good use of the indoor cooling and heating capacity efficiently. This system has a good energy performance and can meet the emerging requirements of modern buildings. In order to evaluate the energy features of the system, a new energy simulation module is developed and embedded in the dynamic energy simulation program, EnergyPlus. Using the program with the newly developed module, the dynamic energy simulation is performed for a simplified typical commercial building. The indoor thermal comfort of the building in winter and the setting temperature of the system are analyzed. Based on the simulation results, the energy characteristics of the system are investigated, and it is indicated that different methods of the temperature control and the percentage of the heat recovery have influence on the relative ratio of the energy saving. If the HR-VRF system adopts the same temperature control method as the heat pump VRF (HP-VRF) system, the HR-VRF system promises 15-17% energy-saving potential, when compared to the HP-VRF system.     

定速及变频空调器节能技术的探讨

对如何提高空调器能效比进行了理论分析,指出压缩机喷气增焓技术在提高空调能效比方面有很大的潜力,对如何改进变容量空调SEER的测试方法提出了建议。     

Energy conservative building air conditioning system controlled and optimized using fuzzy-genetic algorithm

In this work, the combined effect of the energy conservative variable refrigerant volume (VRV) system and the variable air volume (VAV) system was experimentally investigated using genetic fuzzy optimization method that yielded better thermal comfort, indoor air quality (IAQ) requirements without compromising on the energy savings potential. The proposed system was tested using the demand controlled ventilation (DCV) combined with the economizer cycle ventilation (ECV) techniques and examined for a year-round building air conditioning (A/C) application. The supply air temperature (SAT) set points were varied under three distinct strategies and the optimal solutions obtained for the fuzzy systems designed resulted in an enhanced energy conservative potential. The test results of the proposed system were compared with the conventional fan coil A/C system. Based on the three strategies of the supply air temperature, the proposed system yielded an improved per day energy savings potential of 54% in summer and 61% in winter design conditions. Furthermore, for the strategies considered the proposed system achieved an annual energy conservative potential of 36% and exhibited more possible ways to achieve thermal comfort, IAQ and energy conservation.     

变风量空调系统全工况性能模拟

介绍了变风量空调系统软件模拟的方法,该软件可用于模拟采用定静压控制、变静压控制和总风量控制的变风量空调系统的性能和能耗。利用该软件对香港地区典型办公楼的空调系统进行了模拟,结果表明,当采用变静压控制时,全年风机能耗可比传统的定静压控制方法减少13％;在负荷较大的时候,风机节能不明显,而负荷较小的时候,节能非常显著。     

论多联式空调机组

在分析了多联式空调机组特点的基础上，阐述了以变频制冷压缩机为核心的两相流体网络模拟法，以解决多联式空调机组难以分析掌握的问题；讨论了系统稳定可靠运行问题，并提出最大容量限度的概念；提出必须进行整体系统的评价标定试验的观点，以及试验方法的设想。     

A development of easy-to-use tool for fault detection and diagnosis in building air-conditioning systems

There are many reports about faulty status in building air-conditioning systems recently. It becomes difficult to keep indoor air temperature appropriately as faults occur, and the faults cause waste of building energy consumption. The model-based fault detection and diagnosis (FDD) methods have been researched for specific parts of air-conditioning system such as chillers, coils, variable air volume units (VAV units), etc. It needs, however, much time and labor to monitor and check every single part because we cannot predict where and when the faults occur. The purpose of this study is to examine indoor air temperature changes and energy consumption increase when faults occur and to develop an easy-to-use FDD tool that helps to find out the faulty place through the whole building air-conditioning system. And then, we treat the reliability of the proposed FDD tool and effectiveness to control of indoor environment deterioration and energy consumption increase by the tool is evaluated based on building air-conditioning system simulation in this paper.