Evaluation of four control strategies for building VAV air-conditioning systems

It is necessary to adopt appropriate control strategies to save energy and improve the indoor air quality (IAQ). On the validated TRNSYS simulation platform, four different control strategies are investigated to examine the indoor air temperature, energy consumption, CO₂ concentration and predicted mean vote (PMV) for the variable air volume (VAV) systems in an office building in Shanghai. As an original scheme, Strategy A using constant outdoor air intake fraction shows high energy consumption, low CO₂ concentration and acceptable thermal comfort. By using minimum outdoor air ventilation based on dynamic occupancy detection, Strategy B can provide more than 15% energy saving, acceptable PMV value but high CO₂ concentration in breathing zone. By using indoor air temperature reset, Strategy C presents the most energy savings beyond 20% reduction, low CO₂ concentration but poor thermal comfort. In mild seasons, combining enthalpy-based outdoor airflow economizer cycle with supply air temperature reset, Strategy D can achieve 9.4% energy savings and the lowest CO₂ concentration. Taken together, each strategy covers some strengths as well as some weaknesses. How to comprehensively assess a control strategy for all specific objectives should be considered in future studies.     

关键区温度重设定的自适应按需新风控制

针对多区域变风量空调系统,提出一种关键区温度重设定的自适应按需新风控制策略,以最少的能耗满足各区的室内空气质量要求。该策略实时检测各区人员负荷,识别关键区,充分考虑关键区的新风需求,在进行新风设定时充分考虑循环空气中没有使用完的新风。关键区的温度重设定可以加大输送到关键区的送风量从而加大关键区的新风量,新风比减小,从而达到节能目的。利用多区域变风量空调系统的动态仿真平台对该控制策略进行了测试,对其动态控制性能、能耗特性及空气质量特性进行了评估,同时将该控制策略与其他按需新风控制策略进行了比较。     

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.     

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).     

Ventilation of air-conditioned residential buildings: A case study in Hong Kong

More and more studies reported that there were insufficient ventilation and excessive CO_2 concentration in air-conditioned residential buildings, but few solutions were provided. This study investigates the overnight evolution of CO_2 concentration in air-conditioned residential buildings and then focuses mainly on the evaluation of three ventilation strategies, including overnight natural ventilation, short-term mechanical ventilation and short-term natural ventilation. On-site measurements were conducted in a typical residential bedroom in Hong Kong in September. The indoor and outdoor CO_2 concentration, air temperature and relative humidity as well as the outdoor wind speed during the measurements were analysed. Ventilation rates were calculated based on the time series of CO_2 concentration. This study confirms that additional ventilation is usually needed in air-conditioned residential buildings. Overnight natural ventilation with even a small opening is associated with excessive energy consumption and deteriorated indoor thermal environment. Short-term natural ventilation strategies are inefficient and uncontrollable. Compared to the best short-term natural ventilation strategy, a reasonably designed short-term mechanical ventilation strategy requires only a 41% of ventilation period to complete one full replacement of indoor air and to reach a lower indoor CO_2 concentration. Nighttime case studies and a theoretical analysis suggest that a few several-minute mechanical ventilation periods could potentially maintain an acceptable indoor air quality for a normal sleeping period of 8 h.     

Evaluation of simplified ventilation system with direct air supply through the facade in a school in a cold climate

Many educational buildings in industrialised countries have poor indoor climate, according to today’s knowledge about the impact of indoor climate on well-being and productivity. Budget restrictions and practical limitations such as lack of space for central air handling units and ventilation ducts, have motivated the application of simplified ventilation systems in some schools, such as taking unconditioned supply air directly from the facade. One such school was recently evaluated in Norway.On cold days, thermal comfort in the classroom deteriorated due to cold downdraught from the supply outlet. In addition, moist and fertile conditions for microbiological growth were observed in the air supply ductwork. On the other hand the same pupils are more satisfied with the school and have less sick building syndrome (SBS) symptoms during winter than summer. An improved control strategy with a temperature-compensated CO₂ set-point for controlling the airflow is suggested. This could improve thermal comfort and reduce energy use without compromising perceived air quality (PAQ) during cold weather. Furthermore it could improve indoor air quality (IAQ) during warm weather with only a slight increase of energy use. Further evaluation of an improved solution is needed before such a ventilation concept can be recommended in cold climates.     

Fault-tolerant control for outdoor ventilation air flow rate in buildings based on neural network

This paper describes a supervisory control scheme that adapts to the presence of the measurement faults in outdoor air flow rate control using sensor-based demand-controlled ventilation, maintains an adequate indoor air quality and minimizes the resulting increase in energy consumption. A strategy, which is based on neural network models, is employed to diagnose the measurement faults of outdoor and supply flow sensor, and accomplishes the fault-tolerant control of outdoor air flow when faults occur. The neural network models are trained using the data collected under various normal conditions. The residuals between the measurements of flow sensors and the outputs of the neural network models are used to diagnose the faults. When the fault of outdoor or supply air flow sensor occurs, the recovered estimate of outdoor or supply air flow rate obtained on the basis of the neural network models is used in the feedback control loop to regain the control of outdoor air flow. Tests using dynamic system simulation are conducted to validate the strategy. The control, IAQ and energy performances of the system under fault-tolerant control strategy in the presence of the faults in air flow sensor are also presented.     

卷烟厂绿色工房项目中的建筑环境模拟

简要介绍了在几个卷烟厂绿色工房实践中涉及到的建筑环境模拟,包括建筑能耗模拟、室外新风利用的模拟、非空调状态下室内温湿度的模拟、室外风环境模拟、自然通风模拟、日照模拟和室内采光模拟.     

一次回风送风再热与新风预冷互补的节能研究

对于一次回风空调系统,夏季工况,当空调区湿负荷较大时,采用露点送风只能保证设定温度,而湿度将会偏大,影响人体热舒适;若增加再热,又会出现冷热抵消现象。提出了利用室外新风热量再热来改善室内热舒适状况的方案。通过工程案例计算分析,结果表明:利用新风再热可减少再热送风系统6%~15%的能耗,认为送风再热与新风预冷互补措施能在一定程度上降低空调能耗,提高室内热舒适。     

对热舒适、空气感觉质量及能耗的模拟研究

室内空调设计温度和新风量对热舒适，室内空气质量及能耗量有重要影响，然而对它们之间相互关系进行研究的文献却较少。通过计算机模拟空调系统在7种室内设计温度和7种新风量条件下的运行情况，得到不同的设计条件组合对热舒适、人体感觉空气质量及建筑能耗量的影响。基于这项分析，提出了此办公建筑合理的室内设计温度和新风量取值。     

Development of regression equations for predicting energy and hygrothermal performance of buildings

Regression equations can be used for predicting indoor air temperature, relative humidity and energy consumption in an easier and more rapid way than building energy simulation tools. The independent variables, that is, the input data, are heating, ventilation and air conditioning (HVAC) power, outdoor temperature, relative humidity and total solar radiation. The present methodology for obtaining the regression equations is based on defining a couple of linear Multiple-Input/Single-Output (MISO) models, since two main outputs are involved, that is, indoor temperature and relative humidity. The methodology has been tested for the low- and high-thermal mass cases of the BESTest model (cases 600 and 900) and the output data is generated by using a building hygrothermal simulation tool. Validation procedures have shown very good agreement between the regression equations and the simulation tool for both winter and summer periods.     

Optimal supply air temperature with respect to energy use in a variable air volume system

In a variable air volume (VAV) system with 100% outdoor air, the cooling need in the building is satisfied with a certain air flow at a certain supply air temperature. To minimize the system energy use, an optimal supply air temperature can be set dependent on the load, specific fan power (SFP), chiller coefficient of performance, outdoor temperature and the outdoor relative humidity. The theory for an optimal supply air temperature is presented and the heating, ventilation and air-conditioning (HVAC) energy use is calculated depending on supply air temperature control strategy, average U-value of the building envelope and two outdoor climates. The analyses show that controlling the supply air temperature optimally results in a significantly lower HVAC energy use than with a constant supply air temperature. The optimal average U-value of the building envelope is in practise mostly zero.     

A novel and dynamic demand-controlled ventilation strategy for CO2 control and energy saving in buildings

Although conventional CO₂-based demand-controlled ventilation strategies, such as proportional and exponential controls, can ensure buildings/spaces meeting the minimum requirements of outdoor air by industry standards, they are operated under the assumption of equilibrium condition which can hardly be reached in practice and therefore there is still much space to improve on conventional strategies in terms of energy saving. In this paper, a novel and dynamic control strategy was developed for hourly scheduled buildings. The strategy utilized schedules by setting a base ventilation rate for unoccupied periods and calculating ventilation rate dynamically at each occupied period by solving the CO₂ mass balance equation to keep indoor CO₂ near the set point during the occupied period. Experimental simulations were made over a sports training center using both simulated and experimental CO₂ generation rates. Results show that the new strategy can save +34% of energy related to ventilation air compared to proportional control. The new strategy was also extended to common buildings which are occupied for almost all opening hours. In the case of common buildings, the new strategy can save about +26% of energy related to ventilation air compared to proportional control. The new strategy is simple, dynamic, flexible and efficient.     

CO2-based demand controlled ventilation under new ASHRAE Standard 62.1-2010: a case study for a gymnasium of an elementary school at West Lafayette, Indiana

CO₂-based demand controlled ventilation had been tried and tested in the United States under the old ASHRAE Standard 62 “Ventilation for Acceptable Indoor Air Quality”, but this had since been replaced by ASHRAE Standard 62.1 and little is known on the field about the relative performances of CO₂-based demand controlled ventilation between the old and new ventilation standards. In view of that, this paper presents a case study for an American elementary school gymnasium in order to compare the implementation of CO₂-DCV under the old and new ventilation standards in terms of control strategies involved, the resulting energy savings, and indoor air quality associated with each strategy. The results indicate that, compared to the existing fixed ventilation rate strategy at which the ventilation rate is always 5% of the total supply air flow, a cooling coil energy savings of 0.03% and 1.86% can be achieved using an occupancy detection control strategy under the new ASHRAE 62.1 and old ASHRAE 62 respectively, while preserving thermal comfort and indoor air quality.     

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.     

Applicability and optimum control strategy of energy recovery ventilators in different climatic conditions

Energy recovery ventilators (ERVs) transfer energy between the air exhausted from building and the outdoor supply air to reduce the energy consumption associated with the conditioning of ventilation air. In this paper, the applicability of ERVs with sensible and latent effectiveness values in a practical range is studied using TRNSYS simulation program. The impact of ERV on annual cooling and heating energy consumption is investigated by modeling a 10-storey office building in four American cities as representatives of major climatic conditions. The results show that heat and moisture recovery can lead to a significant reduction in the annual heating energy consumption (i.e., up to 40%, which is 5% higher than heat recovery). Also, an ERV with the capability of moisture recovery may reduce the annual cooling energy consumption by 20% provided the ERV is properly controlled. Since the un-controlled operation of ERVs during the summer may increase the cooling energy consumption, an optimum control strategy is developed and verified in the paper. This optimum control strategy depends on ERV's latent to sensible effectiveness ratio. For instance, an ERV with equal sensible and latent effectiveness should be operated when either the outdoor enthalpy or temperature is greater than that of the indoor air.     

Dynamic simulation of building VAV air-conditioning system and evaluation of EMCS on-line control strategies

Dynamic models are developed to simulate the thermal, hydraulic, environmental and mechanic characteristics and energy performance of a building and VAV air-conditioning system under the control of EMCS. Three on-line supervisory strategies and programs based on integrated EMCS stations are developed to optimise the VAV static pressure set-point, AHU outlet air temperature set-point and outdoor ventilation air flow set-point, The strategies and programs are commissioned and evaluated under the simulated ‘real-life’ environment. This paper presents the dynamic models, the control strategies and the simulation exercises for commissioning and evaluation of the strategies.     

Sensory Pollution and Microbial Contamination of Ventilation Filters

Abstract The sensory pollution load and microbial contamination of glass-fibre filters at high and low relative humidity were investigated in an experimental set-up in the laboratory. Dust and particles from the outdoor air were collected in two EU7 glass-fibre filters for a pre-conditioning period of 16–18 weeks during which there was a constant airflow with a velocity of 1.9 m/s through the filters. One of the filters was exposed to outdoor air of approximately 40% relative humidity and 10°C, the other to outdoor air of approximately 80% relative humidity and 5°C. The dust in ventilation filters can constitute a serious pollution source in the indoor environment, causing deterioration in the quality of the supply air even before it enters the ventilated spaces. The sensory pollution load from the used filters after the continuous operating time of 16–18 weeks was significantly higher than the sensory pollution load from new filters but the sensory load at 40% and 80% relative humidity did not differ. The microbial contamination of the supply air downstream of the filters, which on average had been exposed to outdoor air of 40% and 80% relative humidity, was negligible.     

Experimental study on a residential temperature-humidity separate control air-conditioner

According to the temperature and moisture characteristics and current problems experienced in the Yangtze River Area, a temperature-humidity separate control air conditioner was developed. This unit can remove indoor sensible heat and latent heat load separately, and adjust indoor temperature and humidity respectively, thus improve indoor comfort and reduce energy consumption. The air-conditioner consists of an air cooling evaporator and a water cooling evaporator. Orthogonal experiments were designed to study the influence of outdoor temperature, indoor temperature, indoor humidity, compressor frequency, and refrigerant distribution ratio in air cooling evaporator (RDRAE) on the unit performance. The results showed that the dehumidification capacity ranged from 0 to 4.02 kg/h; the EER ranged from 2.71 to 4.57; the cooling capacity ranged from 6822 to 13,080 W. The results can help to make the control logic of the unit, and be used as the basis of energy consumption calculation. Units with temperature and humidity separate control could save about 15.6% of the cooling energy consumption against traditional residential air-conditioner, and 47.8% against the traditional residential air-conditioner that could control both indoor temperature and humidity.     

Outdoor Air Contaminants and Indoor Air Quality under Transient Conditions

The indoor concentrations of contaminants originating from outdoor sources have been measured and calculated under transient conditions. The results show that contaminants that are supplied to an office building via the ventilation system can reach considerably high concentration levels. The indoor/outdoor concentration ratio and time lag are dependent on the air change rate. In buildings with low air change rates the indoor concentration variations are smoothed out compared to buildings with high air change rates. The results from the theoretical model are compared to the results from both laboratory and field measurements and the model is verified for well mixed conditions in a 20 m³ test chamber. The model can be used to simulate different control strategies for reduction of indoor contaminant concentrations related to outdoor sources. One such control strategy is based on reduction of the outdoor air change rate during periods with peak outdoor contaminant concentrations.