Performance of combined displacement ventilation and cooled ceiling liquid desiccant membrane system in Beirut climate

The performance of the chilled ceiling (CC) displacement ventilation (DV) systems is constrained by latent load removal capacity and cost of supply air dehumidification to prevent condensation on the ceiling. In this study, a liquid desiccant dehumidification membrane cycle (LDMC) is mathematically modelled to replace the CC and remove directly latent and sensible load from indoor space through the membrane. The desiccant system is coupled with the DV system. An optimized operational strategy is adopted while allowing ceiling temperature to drop to lower values than conventional CC/DV. The optimized LDMC-C/DV system was implemented in an office space in Beirut climate. It was found that decreasing the membrane liquid desiccant temperature resulted in a significant decrease in the total cooling energy of the system, while increasing the solar heating energy of the desiccant regeneration. At optimal set points, a decrease of 49% in energy consumption was observed compared to the conventional CC/DV system.     

夏季湿热地区置换通风和冷却顶板复合系统节能潜力研究

根据上海一办公楼置换通风和冷却顶板复合系统的设计及运行特点,应用EnergyPlus软件模拟了采用转轮除湿方式的复合系统的供冷季能耗,并分别模拟了采用冷却除湿方式的复合系统、带热回收装置的混合通风系统和置换通风系统的供冷季能耗以进行对比。结果表明,在湿热地区采用转轮除湿方式可比冷却除湿方式节约制冷机冷量,空调季总能源费用比混合通风系统节约30%左右。     

香港地区应用干式风机盘管系统节能和室内空气品质分析

冷板辐射加置换通风空词系统在保证室内环境的前提下比常规全空气空调系统节能,但在天气湿热的香港地区应用会出现冷板凝露现象,且采用置换通风方式,风量有限从而影响热舒适性.本文将干式风机盘管系统与独立除湿通风系统组合应用于香港地区办公楼,干式风机盘管系统处理室内显热负荷,独立除湿通风系统承担室内湿负荷和室外全热负荷.采用EnergyPlus软件模拟分析了该空调系统在香港地区某办公楼中的使用性能,结果表明该系统能很好地控制室内温、湿度,特别是湿度,与常规全空气空调系统相比,全年节能达10.3%.     

Evaluation of distributed environmental control systems for improving IAQ and reducing energy consumption in office buildings

Conventional heating, ventilation, and air conditioning (HVAC) systems are incapable of providing control over individual environments or adjusting fresh air supply based on the dynamic occupancy of individual rooms in an office building. This paper introduces the concept of distributed environmental control systems (DECS) and shows that improvement in indoor air quality (IAQ) and energy efficiency can be achieved by providing required amounts of fresh air directly to the individual office spaces through distributed demand controlled ventilation (DDCV). In DDCV, fresh air is provided to each micro-environment (room or cubicle) based on input from distributed sensors (CO₂, VOC, occupancy, etc.) or intelligent scheduling techniques to provide acceptable IAQ for each occupant, rather than for groups or populations of occupants. In order to study DECS, a numerical model was developed that incorporates some of the best available models for studying building energy consumption, indoor air flow, contaminant transport and HVAC system performance. The developed model was applied to a DECS in a model office building equipped with a DDCV system. By implementing DECS/DDCV and intelligent scheduling techniques it is possible to achieve an improvement in IAQ along with a reduction in annual energy consumption compared to conventional ventilation systems.     

Ventilation and energy performance of partitioned indoor spaces under mixing and displacement ventilation

Large variation in indoor air quality (IAQ) and thermal comfort can occur in partitioned office spaces due to heterogeneous air mixing. However, few published studies examined IAQ, thermal comfort, and energy performance of partitioned occupied spaces, which are commonly found in today’s buildings. The objective of this study is to evaluate indoor environmental quality and air conditioning performance of a partitioned room under two typical ventilation modes: (1) mixing ventilation and (2) displacement ventilation. For a total of six representative air-conditioning scenarios, three-dimensional computational fluid dynamics (CFD) simulations are performed to examine temperature distribution, ventilation effectiveness, energy consumption, and local thermal comfort for two partitioned spaces. Simulation results indicate that temperature distribution in a partitioned room is a strong function of ventilation strategy (mixing vs. displacement), but marginally affected by diffuser arrangements. Local age-of-air (air freshness) significantly varies with both diffuser arrangement and ventilation strategy. Regarding energy consumption, displacement ventilation can achieve an indoor set-point temperature in the partitioned spaces about two times faster than mixing ventilation. Under mixing ventilation, the time to achieve a set-point temperature was notably reduced when each partitioned space is served by its own diffuser. For the same supply airflow rate, displacement ventilation can generate local draft risk at ankle level, while mixing ventilation may result in a draft sensation in wider areas around an occupant. Overall, the results suggest that mixing ventilation system can save energy if each partitioned zone is served by its own diffuser such as a multi-split air conditioning. However, when multiple partitioned zones are served by only one diffuser, displacement ventilation is more energy-efficient and can achieve higher ventilation effectiveness than mixing ventilation.     

夏热冬冷地区住宅建筑新风系统全年运行工况分析

根据夏热冬冷地区的空调期、除湿期和采暖期的划分方法,提出了住宅建筑新风系统全年运行的转换条件,以及新风热湿处理工况要求。并针对该地区气候潮湿、除湿期时间长的特点,认为节能住宅采用温湿度同时监控的独立新风系统,有利于改善室内热环境质量和室内空气质量。     

Energy analysis of the personalized ventilation system in hot and humid climates

Personalized ventilation (PV) is an individually controlled air distribution system aimed at improving the quality of inhaled air and the thermal comfort of each occupant. Numerous studies have shown that PV in comparison with traditional mechanical ventilation systems may improve occupants’ health, inhaled air quality, thermal comfort, and self-estimated productivity. Little is known about its energy performance.In this study, the energy consumption of a personalized ventilation system introduced in an office building located in a hot and humid climate (Singapore) has been investigated by means of simulations with the empirically tested IDA-ICE software. The results reveal that the use of PV may reduce the energy consumption substantially (up to 51%) compared to mixing ventilation when the following control strategies are applied: (a) reducing the airflow rate due to the higher ventilation effectiveness of PV; (b) increasing the maximum allowed room air temperature due to PV capacity to control the microclimate; (c) supplying the outdoor air only when the occupant is at the desk. The strategy to control the supply air temperature does not affect the energy consumption in a hot and humid climate.     

Assessing the effects of sliding doors on an operating theatre climate

Surgical site infections (SSIs) are the second to third most common site of health care associated infections (HAIs). It is very important to maintain good indoor air quality (IAQ) and the best ventilation system in the operating theatre (OT) to ensure health and safety for the patient and surgical team, also to reduce the risk of post-operative wound infection. Then a significant consideration in OTs is the control of aerosols, anesthesia gases and smoke. The present paper uses the airflow modeling based on computational fluid dynamics (CFD) to study the indoor climate of a standard ISO5 class OT with an ultraclean air filter system and a total ceiling unidirectional diffuser. The OT has a heating, ventilation, and air conditioning (HVAC) plant with a laminar airflow (LAF) or ultraclean ventilation. A simple method to analyze the effects of different sliding door conditions combined with crossing persons and persons with a stretcher crossing, on the OT climate, airflow patterns and the indoor pressures scheme, is provided. The proposed simulation method, that belongs to the indirect approach for a numerical simulation of solid object movements in a fluid, provides important knowledge on complex flow phenomena combined with multi-physical conditions. Results obtained by transient simulation show disruptions of the airflow inside the OT and different airflow displacement and distribution caused by surgery staff movements and sliding door opening and closing, but in particular static pressure changes in the HVAC plant system with important effects on ventilation system working conditions and its energy performances.     

Indoor air quality and thermal comfort studies of an under-floor air-conditioning system in the tropics

This paper reports thermal comfort and indoor air quality (IAQ) studies of an under-floor air-conditioning (UFAC) system in hot and humid climate. Thermal comfort parameters were measured at predetermined grid points within an imaginary plane to predict the airflow pattern of the supply air jet as well as to determine the occurrence of thermal stratification in the office space. Fanger’s [Thermal Comfort Analysis and Applications in Environmental Engineering, McGraw-Hill, New York, 1970] thermal comfort index was also computed to detect the occupants’ thermal sensation. Besides, the concentration levels of dust and carbon dioxide were recorded with the intention to examine the quality of the indoor air. Statistical methods were applied to derive the relationship between air velocity and the other parameters as mentioned earlier. The main findings from the study revealed reasonable level of acceptability of IAQ associated with the UFAC system. However, occupants are likely to experience localised thermal discomfort near the supply diffusers due to the existence of large temperature gradients. In addition, a stagnant zone is discovered at sedentary level, which is caused by the parabolic airflow nature of the primary air jet.     

太阳能液体除湿处理热湿地区冷却顶板新风湿负荷

以广州为例分析了热湿地区冷却顶板空调系统新风冷负荷的特点,显示夏季空调期新风湿负荷占新风总负荷的90%以上.提出了利用太阳能液体除湿处理冷却顶板空调系统新风湿负荷的方案,并将其与常规的冷却除湿方案进行了比较.结果显示,冷却顶板空调系统新风湿负荷的太阳能液体除湿方式比冷却除湿方式节能40%以上,静态投资回收年限为2.2年,认为冷却顶板空调系统新风湿负荷的太阳能液体除湿方式要优于冷却除湿方式.     

地板个性化送风系统动态能耗分析

地板个性化送风能够改善工作区微环境的空气品质和热舒适性,其送风温度较传统空调系统高,有较大的节能潜力。对送风采用冷冻除湿和转轮除湿时,节能与否或者节能程度大小分别取决于再热量和再生热量及再生风机能耗。本文中,利用能耗模拟分析软件Energyplus分别模拟计算了在中国各气候区使用混合通风、冷冻除湿的地板个性化送风及转轮除湿的地板个性化送风系统时,各系统的年能耗,并进行了比较分析,得出地板个性化送风系统在中国各气候区的能耗特性。     

Experiences with a gas driven,desiccant assisted air conditioning system with geothermal energy for an office building

Thermal driven desiccant assisted air conditioning systems make use of waste heat to dehumidify humid outside air in a desiccant wheel. Within the scope of a research project, an investigation of a desiccant assisted air conditioning system was carried out, and a demonstration plant was built in an office building in Hamburg, Germany. The HVAC system consists of a small CHP-plant, a desiccant assisted ventilation system and an earth energy system (borehole heat exchangers) for cooling instead of an electric driven compression chiller. The radiant floor heating system of the building is used for cooling. In this paper, measurement results and investigations of performance, energy demand and operating costs will be presented. It was found that considerable primary energy savings can be achieved (70%) using desiccant air conditioning with borehole heat exchangers. But even if electric chiller is used, savings of 30% in primary energy can be accomplished. Starting costs for the demonstration plant were not higher than for a conventional system, but running costs could be reduced drastically.     

Optimization of indoor environmental quality and ventilation load in office space by multilevel coupling of building energy simulation and computational fluid dynamics

The fundamentals, implementation, and application of an integrated simulation as an approach for predicting the indoor environmental quality for an open-type office and for quantifying energy saving potential under optimized ventilation are presented in this paper. An integrated simulation procedure based on a building energy simulation and computational fluid dynamics, incorporated with a conceptual model of a CO₂ demand controlled ventilation (DCV) system and proportional integral control of an air conditioning system as the optimization assessment of conceptual model in the occupied zone, was developed. This numerical model quantitatively exhibits energy conservation and represents the non-uniform distribution patterns of airflow properties and CO₂ concentration levels in terms of energy recovery and indoor thermal comfort. By means of an integrated simulation, the long-term energy consumption of heating, ventilation, and air conditioning systems are predicted precisely and dynamically. Relative to a ventilation system with a basic constant air volume supply rate characterized by a fixed outdoor air intake rate from the ceiling supply opening, the optimized CO₂-DCV system coupled with energy recovery ventilators reduced total energy consumption by 29.1% (in summer conditions) and 40.9% (winter).     

Predictive controllers for thermal comfort optimization and energy savings

The present work is focused on the study of indoor thermal comfort control problem in buildings equipped with HVAC (heating, ventilation and air conditioning) systems. The occupants’ thermal comfort sensation is addressed here by the well-known comfort index known as PMV (predicted mean vote) and by a comfort zone defined in a psychrometric chart. In this context, different strategies for the control algorithms are proposed by using an only-one-actuator system that can be associated to a cooling and/or heating system. The first set of strategies is related to the thermal comfort optimization and the second one includes energy consumption minimization, while maintaining the indoor thermal comfort criterion at an adequate level. The methods are based on the model predictive control scheme and simulation results are presented for two case studies. The results validate the proposed methodology in terms of both thermal comfort and energy savings.     

Membrane heat exchanger in HVAC energy recovery systems, systems energy analysis

The thermal performance of an enthalpy/membrane heat exchanger is experimentally investigated. The heat exchanger utilizes a 60gsm Kraft paper as the heat and moisture transfer surface for HVAC energy recovery. The heat exchanger sensible, latent and total effectiveness have been determined through temperature and moisture content measurements. The annual energy consumption of an air conditioner coupled with an enthalpy/membrane heat exchanger is also studied and compared with a conventional air conditioning cycle using in-house modified HPRate software. The heat exchanger effectiveness are used as thermal performance indicators and incorporated in the modified software. Energy analysis showed that an air conditioning system coupled with a membrane heat exchanger consumes less energy than a conventional air conditioning system in hot and humid climates where the latent load is high. It has been shown that in humid climate a saving of up to 8% in annual energy consumption can be achieved when membrane heat exchanger is used instead of a conventional HVAC system.     

Performance analysis of liquid desiccant based air-conditioning system under variable fresh air ratios

In conventional air-conditioning system, fresh air volume is always restricted to save energy, which sacrifices indoor air quality (IAQ) to some extent. However, removing the latent load of air by liquid desiccant rather than by cooling is an alternative way of reducing energy consumption. Therefore, IAQ can be improved by increasing the volume of fresh air introduced into an air-conditioning system. In this paper, a liquid desiccant based air-conditioning system is studied, with the system performance under various fresh air ratios analyzed using simulation tests. In addition, the proposed system and a conventional system are compared. In the proposed system, with the increase in fresh air ratio, the heating load for solution regeneration rises, the dehumidification efficiency increases and the regeneration efficiency drops. The coefficient of performance (COP) of the liquid desiccant based system decreases sharply when the fresh air ratio exceeds 60%. The results also show that the proposed system can save power notably. The maximum power saving ratio is 58.9% when the fresh air ratio is 20%; however, the ratio drops when the fresh air ratio increases. These findings will be beneficial in the selection of fresh air ventilation strategies for liquid desiccant based air-conditioning systems.     

Optimization of a liquid desiccant based dedicated outdoor air-chilled ceiling system serving multi-zone spaces

A liquid desiccant based dedicated outdoor air-chilled ceiling (DOAS-CC) system is proposed to serve a multi-zone space. The outdoor airflow rate and the supply air humidity ratio are two crucial variables in such a system, which significantly influence indoor thermal comfort, indoor air quality and energy consumption. Two strategies are presented to optimize these two variables in the study. They are the demand-controlled ventilation (DCV) strategy and the supply air humidity ratio set-point reset strategy. To evaluate the performances of these two strategies, a basic control strategy, i.e., the strategy adopting constant ventilation flow rate and constant supply air humidity ratio, is selected as the benchmark. Performances of the two strategies in terms of indoor air temperature, relative humidity and CO₂ concentration as well as energy consumption are analyzed using simulation tests. The results show that the supply air humidity ratio set-point reset strategy is effective for the indoor air humidity control. It can save about 19.4% of total energy consumption during the whole year. The DCV-based ventilation strategy can further reduce about 10.0% of energy consumption.     

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.     

Energy analysis for workshops with floor–supply displacement ventilation under the U.S. climates

Many studies have shown that floor–supply displacement ventilation systems are better than mixing ventilation systems. The benefits include indoor air quality, thermal comfort and reduced energy use. The energy benefits depend on the climate conditions. This research compared the energy use of a floor–supply displacement ventilation system in a large industrial workshop with that of a mixing ventilation system for five U.S. climate regions. It was found that the energy use and the system performance vary with the locations. The displacement ventilation system may use more fan and boiler energy but less chiller energy than the mixing ventilation system. The total energy used is slightly less with displacement ventilation, although the ventilation rate was increased in order to handle the high cooling loads found in U.S. buildings. Thus, the displacement ventilation system can save some energy in cooling mode. However, displacement ventilation system has a lower capacity of dehumidification. This system alone, thus, is not suggested for use in humid regions.     

Optimization of ventilation system design and operation in office environment,Part I: Methodology

Ventilation principles that integrate flexible and responsive elements have grown in popularity in office buildings due to increasing concerns about the impact of indoor environment quality on office workers' well-being and productivity, as well as concerns over the rising energy costs for space heating and cooling in the office building sector. Such advanced elements as underfloor air distribution (UFAD), passive swirl diffusers, and demand controlled ventilation have posed challenges to system design and operation. This paper is concerned with the development and implementation of a practical and robust optimization scheme, aiming to assist office building designers and operators to enhance thermal comfort and indoor air quality (IAQ) without sacrificing energy costs of ventilation. The objective function was constructed in a way attempting to aggregate and weight indices (for thermal comfort, IAQ, and ventilation energy usage assessment) into one indicator. The path taken was a simulation-based optimization approach by using computational fluid dynamics (CFD) techniques in conjunction with genetic algorithm (GA), with the integration of an artificial neural network (ANN) for response surface approximation (RSA) and for speeding up fitness evaluations inside GA loop.